- Email: [email protected]
Online monitoring of transparent exopolymer particles (TEP) by a novel membrane-based spectrophotometric method
Accepted Manuscript Online monitoring of transparent exopolymer particles (TEP) by a novel membranebased spectrophotometric method Lee Nuang Sim, Stanislaus Raditya Suwarno, Darren Yong Shern Lee, Emile R. Cornelissen, Anthony G. Fane, Tzyy Haur Chong PII:
S0045-6535(18)32388-9
DOI:
https://doi.org/10.1016/j.chemosphere.2018.12.066
Reference:
CHEM 22765
To appear in:
ECSN
Received Date: 6 August 2018 Revised Date:
30 November 2018
Accepted Date: 8 December 2018
Please cite this article as: Sim, L.N., Suwarno, S.R., Shern Lee, D.Y., Cornelissen, E.R., Fane, A.G., Chong, T.H., Online monitoring of transparent exopolymer particles (TEP) by a novel membrane-based spectrophotometric method, Chemosphere (2019), doi: https://doi.org/10.1016/ j.chemosphere.2018.12.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Alcian Blue Spike
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Colour Measurement on Membrane
Colour vs TEP Concentration
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TEP Deposition
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Colour [-Δb*]
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TEP Concentration
TEP
MEMBRANE
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Online monitoring of transparent exopolymer
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particles (TEP) by a novel membrane-based
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spectrophotometric method
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Lee Nuang Sima,+*, Stanislaus Raditya Suwarnoa,+, Darren Yong Shern Leeb, Emile R.
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Cornelissena,c,d, Anthony G. Fanea,e, Tzyy Haur Chonga,b*
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Singapore Membrane Technology Centre, Nanyang Environment and Water Research
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Institute, Nanyang Technological University, Singapore b
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School of Civil and Environmental Engineering, Nanyang Technological University,
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Singapore
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KWR Watercycle Research Institute, 3433 PE Nieuwegein, Netherlands
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c
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Particle and Interfacial Technology Group, Ghent University, Coupure Links 653, B-9000
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Ghent, Belgium
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UNESCO Centre for Membrane Science and Technology, University of New South Wales,
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Australia
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+
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*Corresponding Authors
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Email addresses: [email protected] (L.N. Sim), [email protected] (T.H. Chong)
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Postal address: 1 CleanTech Loop, CleanTech One #06-08, Singapore 637141
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These authors contributed equally to this work
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ACCEPTED MANUSCRIPT ABSTRACT
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The presence of transparent exopolymer particles (TEP) in water bodies has been related to
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several adverse impacts in various water treatment processes. In recent years, there have been
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an increasing number of publications relating to TEP. Unfortunately, this increased interest in
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TEP measurement has not been accompanied by significant improvement in the analysis
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method or TEP monitoring. Currently, the most common method to analyze and quantify
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TEP only allows offline, and often offsite measurement, causing delays and slow response
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times. This paper introduces an improved method for TEP monitoring using a membrane-
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based spectrophotometric technique to quantify TEP in various water bodies. The proposed
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TEP monitor involves a crossflow filtration unit, reagent injection and a spectrophotometer
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system. The TEP retained on the membrane surface is stained by Alcian blue and the amount
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deposited is quantified directly using an optic fibre reflectance probe coupled with a
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spectrophotometer. The novel method shows a linear relationship with various concentrations
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of Xanthan gum (a model representing TEP). When tested with various water samples, the
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proposed method was found to correlate well with the conventional method. Several
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advantages of this novel method are shorter analysis time, increased accuracy, and the
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potential to be further developed into an online system.
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KEYWORDS: Transparent exopolymer particles, Alcian blue, monitoring, algae, fouling
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Introduction
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Transparent exopolymer particles (TEPs) are ubiquitous in marine and fresh water
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environments and especially abundant during seasonal algal blooms events (Villacorte et al.,
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2015a). They are known to be transparent, highly sticky and gel like substances which
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comprise mainly of acidic polysaccharides (Alldredge et al., 1993), with sizes ranging from
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0.4 µm to 200 µm (Berman, 2013). TEPs mainly originate from phytoplankton and
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bacterioplankton excretions in marine environments (Berman and Parparova, 2010). Marine
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biologists operationally defined TEPs as particles larger than 0.4 µm which are stainable with
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Alcian blue (C56H68Cl4CuN16S4) (Alldredge et al., 1993). The sub-micron components (<0.4
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µm) which have similar chemical properties as TEP are known as TEP precursors (Passow,
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2000). Since the technique to visualize these transparent particles using Alcian blue was
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developed, the awareness of the important role they play in aquatic systems has increased
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rapidly. It has become an emerging topic in areas such as environmental science and
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engineering. Data from Scopus shows that the number of TEP related publications strongly
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increased since 1993, from 1 to over 30 publication in year 2017.
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In particular, TEP has been extensively studied over the past decades by aquatic
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microbiologists to investigate the link between TEP and algal blooms (Passow, 2000, 2002;
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Passow, 2012; Villacorte et al., 2015b; Chowdhury et al., 2016). Chowdhury et al. (2016)
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observed variations in TEP concentration, phytoplankton and chlorophyll over three years of
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study at Indian Sundarbans. TEP concentrations were found to be the highest during the post-
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monsoon period and were mainly a function of phytoplankton production. Villacorte and co-
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authors (2015b) observed that TEP were produced during the exponential growth phase and
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stationary/death phase of three different types of microalgae.
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ACCEPTED MANUSCRIPT Berman and Holenberg (2005) were the first to propose the role of TEP as a potential initiator
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of RO biofouling. It is also a probable foulant of the pre-treatment membranes. Villacorte et
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al (2009) investigated the efficiency of various pre-treatment systems (Microfiltration (MF),
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Ultrafiltration (UF), coagulation, sedimentation, rapid sand filtration) on TEP removal and
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observed that UF is the most effective in removing particulate TEP (> 0.4 µm) but not for
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colloidal TEP (< 0.4 µm). This is because TEP is flexible in shape and in size, and is able to
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transfer through tight UF membranes. However, in their study, they observed that one of the
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pre-treatment systems with relatively high coagulant dose can achieved more than 90% of
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total TEP removal. As such, adjusting the coagulant dosage could be an immediate response
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when a high seasonal level of TEP is encountered.
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TEP in source waters play a significant role in the early stages of aquatic biofilm formation
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(Berman and Holenberg, 2005; Berman, 2010; Berman et al., 2011) and hence it has been
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identified as a potential cause of biological fouling in reverse osmosis (RO) membranes
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(Villacorte et al., 2009; Berman, 2013; Lee et al., 2015; Li et al., 2016) and UF membrane
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systems (Fang et al., 2010; Villacorte et al., 2010; Berman et al., 2011; Alizadeh Tabatabai et
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al., 2014). In one study by Villacorte et al (2010), they observed a greater fouling rate of the
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UF membrane in a seawater UF-RO desalination plant when high TEP levels in the seawater
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were detected (Villacorte et al., 2010). Recently Li and co-authors (2016) have demonstrated
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that apart from algal species, marine bacteria can also produce TEP and TEP precursors. Due
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to its sticky nature, TEP/ TEP precursors serve as a conditioning layer for other bacteria to
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attach onto the membrane surface and promote biofouling and hence should not be
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overlooked (Berman and Holenberg, 2005; Berman, 2010).
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Over the years, different analytical techniques to quantify TEP concentrations have emerged.
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TEP can only be made visible by staining with Alcian Blue, a hydrophilic cationic dye that
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complexes with carboxylated (―COO-) and sulfated (―OSO3-)
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groups on a complex material, resulting in the formation of neutral precipitates (Alldredge et
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al., 1993). The quantification of TEP was first determined by enumerating the stained
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particles microscopically (Alldredge et al., 1993), which is labor-intensive and subjective as
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the shape of TEP is not uniform. In 1995, Passow and Alldredge introduced a simpler method
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to determine the concentration of TEP spectrophotometrically. In this method, TEP solution
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is first filtered through a 0.4 µm polycarbonate membrane at low constant vacuum of 150
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mmHg and subsequently stained with Alcian blue (0.02% aqueous solution of Alcian blue in
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0.06% acetic acid, pH 2.5). The precipitate on the membrane is then re-dissolved using 5 ml
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of 80% sulfuric acid which takes 2 hours and measured spectrophotometrically at a
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wavelength of 787 nm. Calibration is done using known amounts of the standard Xanthan
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Gum. As such, the concentration of TEP is typically reported in mg Xanthan equivalent per
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liter. Villacorte et al. (2009) further extended the method by measuring the TEP precursors
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(<0.2 µm) and reported it as colloidal TEP (c-TEP) using 0.05 µm and 0.2 µm polycarbonate
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filters. This method allowed quantification of smaller size TEP which had been overlooked in
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previous methods (Villacorte et al., 2009).
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In 2004, Arruda Fatibello and co-worker proposed another method which offers faster
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determination of TEP concentration, reducing the detection time to approximately 40 min.
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Instead of staining the TEPs on the filter surface, they mixed the Alcian Blue directly into the
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sample solution and then sent it for centrifugation for 30 min. The supernatant was then
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measured spectrophotometrically at a wavelength of 602 nm (Arruda Fatibello et al., 2004).
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In 2007, Thornton et al. attempted to improve Fatibello’s method. Instead of using
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centrifugation, they proposed to filter the Alcian blue stained sample solution through a 0.2
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µm membrane. The final 1ml of the filtrate was then measured spectrophotometrically at a
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which was inversely proportional to the concentration of polysaccharide in the sample. This
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relationship was found to be linear and was calibrated using Xanthan Gum and alginic acid.
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Although Arruda Fatibello’s and Thornton’s methods are considerably more rapid than the
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conventional method, they may not be easily applied to marine water samples unless the
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marine sample is initially desalted by a dialysis process, which may take up to 24 hours
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(Thornton et al., 2007). It was observed that when the stain is directly added to a marine
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sample, the salt content in the suspension may interfere with the stability of the Alcian blue
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and result in insoluble precipitates (Passow and Alldredge, 1995). Recently, Villacorte et al.
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(2015a) adopted Thornton’s method to determine both the TEP and their colloidal precursors
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concentrations without the interference of salinity. TEP and precursors were first retained on
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10 kDa membranes and were re-suspended in ultrapure water by sonication. Subsequently,
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the solution was stained with Alcian blue, and followed by a filtration in which the filtrate
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was measured spectrophotometrically. This method covers the TEP precursors which were
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not determined in the previous methods, but is a tedious procedure (Villacorte et al., 2015a).
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Table 1 provides a summary of different methods to detect TEP thus far.
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It is important to note that Alcian blue is not specific for TEP as it can complex with other
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acidic polysaccharides which are not involved in TEP formation (Winters et al., 2016; Li et
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al., 2018). Although concern has been raised over the stainability of Alcian blue, the methods
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developed for TEP quantification are still based on Alcian blue staining. This is because an
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alternative dye with a similar selectivity as Alcian blue which is high selectivity for acidic
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polysaccharides and non-selective for nucleic acids and proteins has not yet been found to
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date (Discart et al., 2015). The aim of this study is to improve the current TEP detection
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method rather than looking for an alternative dye or agent. Furthermore, most of the TEP
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quantification methods are offline methods, where the measurements have to be performed
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off-site.
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that allows TEP/TEP precursors to be determined in a relatively straightforward manner for
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both freshwater and marine samples. The TEP monitor can be plugged directly into the water
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process line and allows regular semi-continuous measurement. The TEP monitor uses a fibre
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optic spectrophotometer to quantify the amount of TEP and their precursors that are stainable
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by Alcian blue in a crossflow filtration cell. This method allows determination of the amount
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of TEP directly on the membrane surface without the need to re-dissolve it into suspension
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and hence reduces the analysis time to less than 1to 2 hours depending on the selection of the
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filtration flux and water quality. The purpose of operating the TEP monitor in crossflow
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mode is to allows the fraction of TEP most likely to deposit to be captured on the membrane
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surface as a result of flow fractionation and selective deposition. Furthermore, the method
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allows a wider range of membrane selection for different applications. For example, tighter
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membranes such as UF membranes can be used to capture smaller size TEP responsible for
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membrane fouling in RO applications. This study aims to investigate the viability of this
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method and to investigate the efficiency compared to existing TEP methods.
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In this study, we propose an in-situ “online” method, known as the TEP monitor,
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Table 1. Comparison of various TEP measurement methods. Passow & Aldredge
Villacorte et al.
Detection limit
1 Particle / filtered amount
2 µg XG/L
50 µg XG/L
Membrane type
Polycarbonate 0.4 µm
Polycarbonate 0.4 µm
Regenerated cellulose 10 kDa
Salt interferences
No
No
No
4 steps
4 steps
5 steps
• Filtration
• Filtration
• Staining with AB
• Staining with AB
• Particle transferred to a slide through FilterTransfer-Freeze method
• Soak in H2SO4 to elute TEP from membrane
• Re-suspend TEP in MQ water
• Microscopic enumeration
• UV Spectroscopy quantification
0.1 x 103 µg XG/L
1.0 x 103 µg XG/L
5 x103 µg XG/L
Not used
Surfactant free cellulose acetate 0.2 µm
Regenerated cellulose 30 kDa
Yes
Yes
No
4 steps
4 steps
3 steps
• Dialysis desalting for marine sample
• Dialysis desalting for marine sample
• Filtration
• Addition of AB to sample
• Addition of AB to sample
• Fibre optic spectrophotometer quantification
• Centrifugation • UV Spectroscopy quantification
• Filtration with syringe filter
• Staining with AB
• UV Spectroscopy quantification
Fast for fresh water, slow for marine sample (>24 hr)
Fast for fresh water, slow for marine sample (>24 hr)
Relatively fast: 30 mins-1hour
No
No
No
Yes
Vacuum flask, infrared Spectrophotometer (787 nm)
Vacuum flask, infrared Spectrophotometer (787 nm)
Centrifuge (300 rpm), light spectrophotometer (602 nm) dialysis (1000 Da,)
Vacuum flask, light spectrophotometer (602 nm), dialysis (1000 Da)
Crossflow filtration cell, fibre optic spectrophotometer
Particulate TEP
Particulate and precursor TEP
Particulate and precursor TEP
Particulate and precursor TEP
Particulate and precursor TEP
(Passow and Alldredge, 1995)
(Villacorte et al., 2015a; Villacorte et al., 2015b)
(Arruda Fatibello et al., 2004)
(Thornton et al., 2007)
N.A
Online
No
No
Main equipment
Standard light microscope (200x Magnification) Particulate TEP
Relatively fast: more than 1 hour (sample dependent)
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Speed
(Alldredge et al., 1993)
This work
• UV Spectroscopy quantification
Slow: more than 2 hours (Sample-dependent)
Selected Refs.
• Addition of AB to sample
• Filtration with syringe filter
Slow
TEP type
Thornton et al.
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Aruda-Fatibello et al.
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Materials and Methods
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2.1
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2.1.1
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In this study, xanthan gum and sodium alginate were used as the model TEP as they share the
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key characteristics of natural TEP. Xanthan gum has always been used as a reference value to
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indicate the amount of TEP in the water. Known concentrations (0 – 100 mg/L) of xanthan
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gum solutions were prepared by dissolving the powdered form of xanthan gum (Sigma
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Aldrich) in deionised water (Milli-Q, Merck). The stock solution was vigorously stirred for at
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least 1 hour until no visible xantham gum could be visually observed in the flask. The
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xanthan gum solution was freshly prepared for every experiment. Alginate solution (100
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mg/L) was prepared by dissolving sodium alginate (Sigma Aldrich) in Milli-Q water.
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2.1.2
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Besides model foulants, the proposed TEP monitor was also validated with various types of
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source waters (see Appendix Table A1) collected within the period of October – November
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2016. Prior to TEP measurement, the water was pre-filtered with a 5 µm cartridge filter
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(KAREI) to remove the suspended solids.
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2.1.3
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Thalassiosira pseudonana was grown in batch cultures to represent an algal bloom situation
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in seawater. The algae were cultured in 10 L of sterilized synthetic seawater (16.6 g/L
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artificial sea salt) spiked with a nutrient solution based on f/2 +Si medium at room
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temperature of 25±1 °C. The composition of the prepared medium is shown in the Appendix,
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Table A2 The algal culture was aerated with 0.22 µm filtered air and under continuous
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exposure of an artificial light source (fluorescent lamps). The average algal cell concentration
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in the batch cultures were monitored daily by measuring their optical density at 676nm using
Feed Water Preparation
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ACCEPTED MANUSCRIPT a UV spectrometer (Hach, USA). Before TEP measurements, the samples were filtered
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through cellulose filter paper (WhatmanTM, Grade 1 Circle) with pore size of 11 µm to
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remove the algal cells prior to the test. TEP which are larger than the filter pore size or are
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bounded to the algal cells were not included in the TEP analysis.
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2.2
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The Alcian blue staining solution was prepared with 0.02% of Alcian blue 8GX (Sigma
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Aldrich) in 0.06% acetic acid (pH 2.5). The solution was adjusted to pH 2.5 to ensure that
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both sulphated and carboxyl TEP were completely stained (Passow and Alldredge, 1995).
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The staining solution was kept in the dark and at 4 ºC for less than 4 weeks. The solution was
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pre-filtered with a 0.45 µm membrane before use.
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Membranes
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Commercial UF membranes with a MWCO of 30 kDa (Millipore) were used in the proposed
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TEP monitor for capturing smaller sized TEP. The selected membrane material was
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regenerated cellulose as it was found to be negligibly stained by the Alcian blue staining
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solution in this study when compared to other types of membrane such as polyester sulfone
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(PES) membrane. For conventional TEP measurement, a polycarbonate filter (Nucleopore,
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Whatman) with pore size of 0.1 µm was used instead of 0.4 µm membrane (Passow and
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Alldredge, 1995).
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2.4
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The conventional TEP measurement was performed based on the spectrophotometric method
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introduced by Passow and Alledredge in 1995. In this method, the sample was filtered
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through the polycarbonate membrane of 0.1 µm at constant suction pressure of 150 mmHg.
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After the filtration, the TEP on the membrane surface were stained with 1 ml of Alcian blue
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solution and rinsed with 1 ml Milli-Q water to remove excess stain. The filter was soaked
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Conventional Method for TEP Measurement
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afterwards in 5 ml of 80% H2SO4 solution for 2 hours to elute Alcian blue precipitate that was
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attached on the membrane surface. Subsequently, the absorbance of the solution was
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measured using UV-Vis spectrophotometer (UV1800, Shimadzu) at wavelength of 787 nm
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using a 1 cm cuvette.
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2.5
Proposed TEP Monitor
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2.5.1
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Quantification of the stained TEP is the key to the TEP monitor and this section provides a
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brief background to colorimetric quantification. The Commission Internationale de
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I’Eclairage (CIE), or International Commission on Illumination is an international body
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responsible for developing basic standards and procedures in the fields of photometry and
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colorimetery. In 1976, CIE recommended the L*a*b* or CIELAB colour scale (CIE
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International Commision on Illumination, 1976). When a colour is expressed in CIELAB, the
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L* represents lightness, a* colour coordinates represents the red/green component colour and
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the b* represents the yellow/blue component colours. The maximum for L is 100 indicating
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the brightest white, while the minimum value of L is 0 representing darkest black. The a* and
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b* axes have no specific numerical limits. Positive a* values indicate red, where negative a*
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values indicate green. On the other hand, positive b* values represent yellow while negative
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b* values indicate blue. The CIELAB colour space is schematically shown in Appendix, Fig.
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A1.
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The difference in colour between two samples can be expressed in term of delta values, ∆L*,
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∆a* and ∆b*. The total difference ∆E* is defined as:
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2.5.2
TEP monitor configuration
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and an optic fiber spectrophotometer, depicted in Fig. 1. The feed water is pumped at a
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constant flowrate by a gear pump into the filtration flow cell. Feed pressure is controlled by a
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pressure control valve at the retentate side. Permeation is adjusted and maintained constant by
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a peristaltic pump. The spectrometer probe is placed on top of the flow cell quartz window to
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provide direct reading of the colour where it is connected directly to a PC for analysis.
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Separate injection and drain ports are required during the colour determination stage in which
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the Alcian blue and Milli-Q water are injected for staining and rinsing, respectively. This is to
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ensure that the staining dye does not contaminate the feed water.
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The crossflow cell consists of a top-plate and a bottom-plate (Fig. 1b). The feed flows into
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the cell from the top plate, while the permeate flows inside the bottom plate. The bottom plate
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has a cavity to place a perforated stainless-steel disc and a membrane support. These are
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needed to ensure a uniform distribution of membrane permeation. The membrane channel is
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provided by the cavity in the top plate with a measured channel height of 1mm. The
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dimensions of the membrane are 2.8 cm x 5 cm. The quartz glass window is 2 mm thickness
256
and 2.5 cm x 2.5 cm in area, located at the center of the membrane area.
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used in this study. A reflection probe is connected to a D65 halogen light source and a
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spectrometer and placed at the top of the quartz glass window at 70o angle. This angle was
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suggested by the manufacturer to enable optimum amount of reflected light into the probe.
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The halogen light is sent through 6 illumination fibers to the surface to be measured and the
263
reflection is detected by the 7th fiber located at the center of the probe. The detection fiber is
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coupled to a spectrometer that is configured to wavelengths of 380 to780 nm. The percentage
265
of wavelength that is reflected back to the detector is plotted against each wavelength, giving
266
a spectral data set. It provides a visual representation of a colour fingerprint and can be fitted
267
with different colour space models for comparison purposes. In this application, the CIE
268
L*a*b colour space (ISO 11664-4:2008 (E)/CIE S 014-4/E:2007) representation was used
269
(CIE International Commision on Illumination, 1976). The structure of the L*a*b colour
270
space is based on the theory that a colour cannot be both green and red at the same time, nor
271
blue and yellow at the same time. As such, single values can be used to describe the
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contribution of the red or green (a*) and yellow or blue (b*) of an object. L* is defined as the
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brightness.
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In this paper, the relationship of ∆b* against different xanthan gum concentrations was
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investigated since the change in b* represents a shift in blue colour intensity which is
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strongly shown by the Alcian blue stain.
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2.5.3
TEP Measurement Protocol
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ACCEPTED MANUSCRIPT The experimental procedure is divided into two stages (see Appendix, Fig. A2): (1) fouling
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stage (sample collection) and (2) monitoring stage. During the fouling stage, the feed water
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was pumped to the crossflow cell at a crossflow velocity of 0.1 m/s, under constant flux mode.
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Different combination of permeation flux and filtration duration were investigated in this
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study, tabulated in Appendix Table A2. It should be noted that the filtration time and flux
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may be application specific. For example, for high concentration TEP sample, low flux
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operation is preferred.
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During the monitoring stage, the feed pump was stopped but the permeation flux was still
288
maintained to ensure that TEP remained on the membrane surface when Alcian blue was
289
injected to the filtration cell. During this period, the filtration was operated under dead-end
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filtration mode. Prior to taking any colour measurement, a white reflective standard tile was
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used to calibrate the colour spectrometer before each experiment.
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measurement was taken prior to injection of the Alcian blue solution to account for any
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possible colour change of the membrane during the fouling stage, denoted as b*reference. Note
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that the b* may differ for each experiment if the feed water is not transparent. This may result
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in some colour changes on the membrane after the fouling stage. As such, the proposed
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method is ideal for low turbidity feed water and may require pre-treatment (e.g. 5 µm filter)
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for turbid feed solution to minimize the interference.
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Before injecting Alcian blue into the filtration cell 20 mL of Milli-Q water was slowly fed to
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the flow cell to flush out any remaining salt that may still be present in the membrane channel.
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This is to minimize the interference of salt to the measurement. Subsequently, 10 mL of
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Alcian blue solution was slowly injected into the flow cell followed by another 20 mL of
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Milli-Q water to remove any excess Alcian blue from the membrane cell. A final colour
A reference colour
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reading was then taken, denoted as b*final. For a clean membranes, it was observed that b*final
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is not equal to b*reference. This indicates that Alcian blue can stain the membrane but not
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significantly. As such, in this study, ∆b*, which is defined as ∆b* = b*final – b*reference –
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∆b*clean
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hypothesized that when there is a significant amount of TEP deposited on the membrane
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surface, after staining with Alcian blue, the colour space value of ∆b* will appear to be more
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negative.
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2.5.4
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One of the important characteristics for a reliable monitoring system is the ability for multiple
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measurements with minimum replacements of membrane in the monitor. Therefore, in this
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study the proposed method was evaluated for multiple measurements after membrane
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regeneration. Sodium hypochlorite (NaOCl, 0.5 w/v%) was selected as the cleaning agent to
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restore the membrane permeability.
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Fig. 2 shows images of three different concentrations of xanthan gum taken after filtration
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followed by the Alcian blue staining. It is obvious that different concentrations of XG deposit
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on the membrane surface resulted in different visual levels of blueness after Alcian blue
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staining. The visual blueness of the membranes increased with increasing XG concentration.
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From the images, it can be seen that the XG formed a uniform cake layer on the membrane
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surface as the colour intensity of blue was equally distributed over the membrane surface.
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Fig. 2. Different concentrations of Xanthan Gum fouled on membrane surface, stained with Alcian blue.
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One way to quantify the degree of blueness can be achieved through image analysis software.
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Recently, Palencia et al. (2016) proposed a digital image analysis method to describe the
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membrane surface changes as a result of fouling. Digital images were taken offline after UF
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filtration with aqueous extracts from the plant. The colour intensity was subsequently
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quantified by the software and represented in terms of surface colour, an index which is based
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on the (Red Green Blue) RGB colour model. Their results showed that there is a correlation
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between the colour index, membrane permeability and fouling layer thickness (Palencia et al.,
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2016). Nevertheless, this method does not allow in-situ detection.
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composed of blue-green component (respectively negative a* and negative b* directions). As
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such, in this section, the parameters ∆b*, ∆a* and ∆E*, the combination of the three vectors
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(a*, b* and L*) at different XG concentrations are investigated. Fig. 3 shows the plots of ∆a*,
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∆b* and ∆E* at different concentrations of XG. ∆a*, ∆b* and ∆E* show good linear
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regressions with the XG concentration, with R2 of 0.90, 0.99 and 0.93 respectively. As the
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Alcian blue is dominant in the blue component, ∆b* shows a stronger linear relationship with
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XG concentration with the data points less scattered compared to ∆a* plot. ∆E* is the
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combined effect of a*, b* and L* components, therefore, this parameter might be more
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∆b* is used for discussion in the remainder of this paper.
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Fig. 3: Different concentrations of Xanthan Gum fouled on membrane surface, stained with Alcian blue (a) ∆a*; (b) ∆b*and (c) ∆E*. Conditions: Filtration duration 1.5 h at flux of 90L/m2.h.
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In the previous set of experiment (Fig. 3), the duration of the fouling stage was set at 1.5 h to
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ensure that a uniform cake layer was formed on the membrane surface. This would mean that
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a homogenous colour can be achieved. In this section, the responses of the TEP monitor
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operated at shorter fouling durations were studied. Fig. 4a shows the relationship of b*
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against the XG concentration when operated at 90L/m2.h for 0.5 h. Clearly, the b* values
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were rather scattered when the XG concentration is less than 20 ppm. This could be because
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the amount of XG retained on the membrane surface was not significant enough to form a
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uniform cake layer on the surface and not strongly attached on the membrane surface. One
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way to shorten the duration fouling stage without compromising the foulant amount would be
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by operating the TEP monitor at higher permeation flux.
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regression of b* against XG concentration when operated at a higher flux of 180 L/m2.h,
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especially at the low TEP range. With operating at this flux, better sensitivity can be achieved
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with detection limit of about 5 ppm, with R2 of 0.98. It is important to ensure that the TEP
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monitor is able to detect low concentrations of TEP as the TEP content in natural seawater is
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typically around 20 ppm. Nevertheless, for high concentrated TEP feed solutions, it is
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recommended to operate at lower flux to avoid severe and irreversible fouling on the
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membrane.
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Investigation of different combinations of permeation flux and filtration duration
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Fig. 4b shows better linear
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Fig. 4: Different concentrations of Xanthan Gum fouled on membrane surface, stained
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with Alcian blue at different operating condition: (a) Flux =90L/m2.h; filtration
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duration = 0.5 h; (b) Flux = 180L/m2.h; filtration duration = 0.5 h
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3.3
Comparison with Conventional Method and Natural Sample Analysis
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conventional filtration method proposed by Passow and Alldredge (1995). Therefore, in this
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study, the proposed method was compared with this conventional filtration method and the
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relationship was validated using various water sources, as shown in Fig. 5.
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Fig. 5 (a) shows the expected linear relationship between the conventional method (presented
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as absorbance at 787nm) and the TEP monitor using XG solution. Overall the trend for model
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and natural samples lies below the standard calibration line, indicating that the TEP monitor
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may have captured the smaller size TEP precursors (<0.1 µm) that were present in the water
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samples, resulting in higher values of TEP concentration (in terms of mg Xequi.L-1), as shown
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in Fig. 5b. The TEP monitor generally shows higher TEP concentration compared to
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conventional TEP. This could mean that TEP monitor is more sensitive compared to the
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conventional method as it is able to capture the colloidal TEP especially for low TEP water
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samples. Typically, this fraction in the water is the portion that is responsible for fouling on
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RO membranes.
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It is anticipated that if the membrane used in the conventional method is similar to the TEP
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monitor, the two lines would lie closer to each other but do not necessarily overlap due to the
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different modes of filtration (dead-end vs crossflow). However, since the conventional
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method is based on vacuum filtration which can only reach a maximum pressure of 1 bar, the
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use of a tighter membrane would result in a substantially increased measurement time.
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Fig. 5: (a) TEP standard curve measured with conventional method and TEP monitor (o) and plot of different natural samples ( ):a=tap water; b=seawater; c=seawater brine; d=pond water; e=aquaculture water; f=sodium alginate; (b) TEP concentration of each type of water obtained from TEP monitor and conventional TEP method.
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TEP production during algae growth
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Algal blooms are a threat to water quality and can impact desalination plant and aquaculture.
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To investigate the relation between the TEP production and the algae growth, a single strain
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of algae was batched cultured. The changes in cell optical density (O.D.) at UV 676 nm and
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TEP concentration (conventional method and TEP monitor) were monitored for 12 days and
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are presented in Fig. 6. The algae showed a 2-day lag phase, followed by exponential growth
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phase for the remaining monitoring period. The TEP measured using the conventional
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method only showed slight variation throughout the monitoring period with values range
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from 2 to 4 mg Xeq/L depicted in Fig. 6a. In contrast, the TEP monitor shows the increase in
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TEP concentration which coincided with the increase in O.D during the lag and exponential
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phase with values range from 40 to 100 mg Xeq/L. The concentration of the TEP monitor
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(membrane ~ 30 kDa) was found to be higher than that measured through conventional
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method (membrane >0.1 µm) and indicates that TEP precursor (<0.1 µm) is likely the
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dominant fraction among the algal-released TEP during the bloom. Villacorte et. al. (2015b)
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obtained similar findings where they observed TEP > 10 kDa increased dramatically over the
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algae growth period, whilst, TEP > 0.4 µm only varied slightly. These results indicate the
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need to use a tighter membrane to capture the dominant fraction of TEP. In summary, the
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TEP monitor is very sensitive to algal growth and could be used as an ‘online’ algal bloom
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warning device.
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Fig. 6: Changes in cell density, TEP concentration determined by (a) conventional method
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(TEP > 0.1 µm) and (b) TEP monitor (TEP > 30 kDa) during the incubation period of batch
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culture of Thalasiorra Pseudonana. TEP monitor: filtration duration of 1.5 h at flux of 90
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L/m2.h. Note the error bars for TEP monitor and O.D. are smaller than the plot symbols.
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Regeneration of Membrane
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To be eligible as a practical monitor, it is important that this method requires minimal
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maintenance especially postponing (or preventing) the need to replace the membrane.
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regenerated after each measurement. NaOCl is one of the most commonly used and effective
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cleaning reagents to restore the membrane permeability. Thus, it is selected to be used in this
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study. After a cycle of filtration and staining process, the membrane was regenerated using
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NaOCl (0.05 w/v%).
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Fig. 7a shows the changes of b*final during the cleaning process for three different
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concentrations of XG. During the cleaning process, the NaOCl solution was circulating in the
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system without permeation flux and at high crossflow (0.3 m/s). After being cleaned for 30
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minutes, the b*final value returned to almost its initial value, indicating a cleaning efficiency
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of close to 100%. At lower XG concentrations, the cleaning cycle can be completed within 20
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minutes. The results indicated that the XG and Alcian blue complexes that formed a cake on
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the membrane surface can be easily removed from the surface with NaOCl. This phenomenon
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might be attributed to the oxidation and deprotonation of carboxyl groups when the NaOCl
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solution was introduced into the system under alkaline conditions (pH = 10). It was reported
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that the increase in the pH of the fouling layer would cause the negative surface charge to
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increase and hence creating strong repulsive forces between the macromolecules (Wang et al.,
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2018). As such, the XG-Alcian blue matrix structures became looser and can be easily
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detached from the membrane surface by the hydrodynamic shear forces.
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Fig. 7. (a) bfinal* values as a function of cleaning time for different XG concentration
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fouled membrane; (b) ∆b* vs Xathan Gum concentration. The red asterisk (⁎ ) are the values obtained after each cycle of regeneration. The number beside the asterisk indicates the sequence of the test. Conditions: Flux =180L/m2.h; crossflow velocity = 0.15ms-1, filtration duration 0.5 h.
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concentration and its corresponding ∆b* value was plotted in the plot of b* vs XG concentration. As
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cleaning effectively returns the used membrane back to its initial performance, a random
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concentration testing sequence was performed so as to eliminate the possibility of a systematic error
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in the evaluation design. The filtration-stain-regenerate cycle was repeated for 9 times. It was
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observed that the ∆b* values (⁎) lie within the linear relationship of XG concentration and ∆b* value,
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shown in Fig. 7b. The results confirm that the TEP monitor can be regenerated without compromising
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the accuracy. It is important to note that other organic matters that could be present in real feed water
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may cause irreversible membrane fouling during the measurement.
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investigate the regeneration of the membrane using real waters and this will be presented in a future
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publication. Furthermore, the frequency of replacing the membrane depends on the feed quality and
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this should also be investigated using real feed water.
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Conclusions
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TEP has been identified in the feed water as one of the important parameters resulting in
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biofouling in membrane systems. This work focused on the development of a novel TEP
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monitoring method which enables online measurement with a shorter analysis duration
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compared to reported methods. The new method was successfully validated with the
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conventional method at both low and high TEP concentrations, for both model and natural
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TEP. Additionally, it was shown that the new method also offered a more sensitive detection
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for smaller size TEP. The robustness of the new method was also shown by the ability of
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membrane regeneration during multiple measurement cycles. In addition to monitoring for
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desalination plant fouling control the TEP monitor also has potential as an early warning
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system for protection of aqua culture against algal blooms.
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Acknowledgement
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Singapore Membrane Technology Centre (SMTC) is supported by the Economic
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Development Board (EDB) of Singapore.
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Alizadeh Tabatabai, S.A., Schippers, J.C., Kennedy, M.D., 2014. Effect of coagulation on fouling potential and removal of algal organic matter in ultrafiltration pretreatment to seawater reverse osmosis. Water Res. 59, 283-294. Alldredge, A.L., Passow, U., Logan, B.E., 1993. The abundance and significance of a class of large, transparent organic particles in the ocean. Deep-Sea Res. Pt I: Oceanographic Research Papers 40, 1131-1140. Arruda Fatibello, S.H.S., Henriques Vieira, A.A., Fatibello-Filho, O., 2004. A rapid spectrophotometric method for the determination of transparent exopolymer particles (TEP) in freshwater. Talanta 62, 81-85. Berman, T., 2010. Biofouling: TEP – a major challenge for water filtration. FiltrSeparat 47, 20-22. Berman, T., 2013. Transparent exopolymer particles as critical agents in aquatic biofilm formation: implications for desalination and water treatment. Desalin. and Water Treat. 51, 1014-1020. Berman, T., Holenberg, M., 2005. Don't fall foul of biofilm through high TEP levels. Filtr. Separat. 42, 30-32. Berman, T., Mizrahi, R., Dosoretz, C.G., 2011. Transparent exopolymer particles (TEP): A critical factor in aquatic biofilm initiation and fouling on filtration membranes. Desalination 276, 184-190. Berman, T., Parparova, R., 2010. Visualization of transparent exopolymer particles (TEP) in various source waters. Desalin. Water Treat. 21, 382-389. Chowdhury, C., Majumder, N., Jana, T.K., 2016. Seasonal distribution and correlates of transparent exopolymer particles (TEP) in the waters surrounding mangroves in the Sundarbans. J. Sea Res. 112, 65-74. CIE International Commision on Illumination, 1976. CIE 1976 L*a*b* Colour Space (ISO 116644:2008(E)/CIE S 014-4/E:2007). Discart, V., Bilad, M.R., Vankelecom, I.F.J., 2015. Critical evaluation of the determination methods for transparent exopolymer particles, agents of membrane fouling. Crit. Rev. Environ. Sci. Technol. 45, 167-192. Fang, J., Yang, X., Ma, J., Shang, C., Zhao, Q., 2010. Characterization of algal organic matter and formation of DBPs from chlor(am)ination. Water Res. 44, 5897-5906. Lee, H., Park, C., Kim, H., Park, H., Hong, S., 2015. Role of transparent exopolymer particles (TEP) in initial bacterial deposition and biofilm formation on reverse osmosis (RO) membrane. J. Membr. Sci. 494, 25-31. Li, S., Winters, H., Jeong, S., Emwas, A.-H., Vigneswaran, S., Amy, G.L., 2016. Marine bacterial transparent exopolymer particles (TEP) and TEP precursors: Characterization and RO fouling potential. Desalination 379, 68-74. Li, X., Skillman, L., Li, D., Ela, W.P., 2018. Comparison of Alcian blue and total carbohydrate assays for quantitation of transparent exopolymer particles (TEP) in biofouling studies. Water Res. 133, 6068. Palencia, M., Lerma, T., Palencia, V., 2016. Description of fouling, surface changes and heterogeneity of membranes by color-based digital image analysis. J. . Membr. Sci. 510, 229-237. Passow, U., 2000. Formation of transparent exopolymer particles, TEP, from dissolved precursor material. Mar.ecol.prog. ser 192, 1-11. Passow, U., 2002. Transparent exopolymer particles (TEP) in aquatic environments. Prog. Oceanogr. 55, 287-333. Passow, U., 2012. The abiotic formation of TEP under different ocean acidification scenarios. Mar. Chem. 128–129, 72-80. Passow, U., Alldredge, A.L., 1995. A dye-binding assay for the spectrophotometric measurement of transparent exopolymer particles (TEP). Limn. Oceanogr. 40, 1326-1335.
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Thornton, D.C.O., Fejes, E.M., DiMarco, S.F., Clancy, K.M., 2007. Measurement of acid polysaccharides in marine and freshwater samples using alcian blue. Limn. Oceanogr- Meth 5, 73-87. Villacorte, L.O., Ekowati, Y., Calix-Ponce, H.N., Schippers, J.C., Amy, G.L., Kennedy, M.D., 2015a. Improved method for measuring transparent exopolymer particles (TEP) and their precursors in fresh and saline water. Water Res. 70, 300-312. Villacorte, L.O., Ekowati, Y., Neu, T.R., Kleijn, J.M., Winters, H., Amy, G., Schippers, J.C., Kennedy, M.D., 2015b. Characterisation of algal organic matter produced by bloom-forming marine and freshwater algae. Water Res. 73, 216-230. Villacorte, L.O., Kennedy, M.D., Amy, G.L., Schippers, J.C., 2009. The fate of Transparent Exopolymer Particles (TEP) in integrated membrane systems: Removal through pre-treatment processes and deposition on reverse osmosis membranes. Water Res. 43, 5039-5052. Villacorte, L.O., Schurer, R., Kennedy, M.D., Amy, G.L., Schippers, J.C., 2010. Removal and deposition of transparent exopolymer particles in a seawater UF-RO system. IDA Journal of Desalination and Water Reuse 2, 45-55. Wang, X., Ma, J., Wang, Z., Chen, H., Liu, M., Wu, Z., 2018. Reinvestigation of membrane cleaning mechanisms using NaOCl: Role of reagent diffusion. J. Membr. Sci. 550, 278-285. Winters, H., Chong, T.H., Fane, A.G., Krantz, W., Rzechowicz, M., Saeidi, N., 2016. The involvement of lectins and lectin-like humic substances in biofilm formation on RO membranes - is TEP important? Desalination 399, 61-68.
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A novel method (TEP monitor) to determine TEP using a colour based technique has been developed. The TEP monitor enables more sensitive detection of smaller size TEP. The TEP monitor offers shorter measurement time and multiple uses of membrane.
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S0045-6535(18)32388-9
DOI:
https://doi.org/10.1016/j.chemosphere.2018.12.066
Reference:
CHEM 22765
To appear in:
ECSN
Received Date: 6 August 2018 Revised Date:
30 November 2018
Accepted Date: 8 December 2018
Please cite this article as: Sim, L.N., Suwarno, S.R., Shern Lee, D.Y., Cornelissen, E.R., Fane, A.G., Chong, T.H., Online monitoring of transparent exopolymer particles (TEP) by a novel membrane-based spectrophotometric method, Chemosphere (2019), doi: https://doi.org/10.1016/ j.chemosphere.2018.12.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Alcian Blue Spike
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Online monitoring of transparent exopolymer
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particles (TEP) by a novel membrane-based
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spectrophotometric method
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Lee Nuang Sima,+*, Stanislaus Raditya Suwarnoa,+, Darren Yong Shern Leeb, Emile R.
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Cornelissena,c,d, Anthony G. Fanea,e, Tzyy Haur Chonga,b*
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Singapore Membrane Technology Centre, Nanyang Environment and Water Research
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Institute, Nanyang Technological University, Singapore b
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School of Civil and Environmental Engineering, Nanyang Technological University,
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Singapore
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KWR Watercycle Research Institute, 3433 PE Nieuwegein, Netherlands
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Particle and Interfacial Technology Group, Ghent University, Coupure Links 653, B-9000
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Ghent, Belgium
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UNESCO Centre for Membrane Science and Technology, University of New South Wales,
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Australia
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+
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*Corresponding Authors
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Email addresses: [email protected] (L.N. Sim), [email protected] (T.H. Chong)
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Postal address: 1 CleanTech Loop, CleanTech One #06-08, Singapore 637141
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These authors contributed equally to this work
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ACCEPTED MANUSCRIPT ABSTRACT
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The presence of transparent exopolymer particles (TEP) in water bodies has been related to
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several adverse impacts in various water treatment processes. In recent years, there have been
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an increasing number of publications relating to TEP. Unfortunately, this increased interest in
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TEP measurement has not been accompanied by significant improvement in the analysis
29
method or TEP monitoring. Currently, the most common method to analyze and quantify
30
TEP only allows offline, and often offsite measurement, causing delays and slow response
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times. This paper introduces an improved method for TEP monitoring using a membrane-
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based spectrophotometric technique to quantify TEP in various water bodies. The proposed
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TEP monitor involves a crossflow filtration unit, reagent injection and a spectrophotometer
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system. The TEP retained on the membrane surface is stained by Alcian blue and the amount
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deposited is quantified directly using an optic fibre reflectance probe coupled with a
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spectrophotometer. The novel method shows a linear relationship with various concentrations
37
of Xanthan gum (a model representing TEP). When tested with various water samples, the
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proposed method was found to correlate well with the conventional method. Several
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advantages of this novel method are shorter analysis time, increased accuracy, and the
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potential to be further developed into an online system.
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KEYWORDS: Transparent exopolymer particles, Alcian blue, monitoring, algae, fouling
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Introduction
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Transparent exopolymer particles (TEPs) are ubiquitous in marine and fresh water
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environments and especially abundant during seasonal algal blooms events (Villacorte et al.,
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2015a). They are known to be transparent, highly sticky and gel like substances which
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comprise mainly of acidic polysaccharides (Alldredge et al., 1993), with sizes ranging from
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0.4 µm to 200 µm (Berman, 2013). TEPs mainly originate from phytoplankton and
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bacterioplankton excretions in marine environments (Berman and Parparova, 2010). Marine
53
biologists operationally defined TEPs as particles larger than 0.4 µm which are stainable with
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Alcian blue (C56H68Cl4CuN16S4) (Alldredge et al., 1993). The sub-micron components (<0.4
55
µm) which have similar chemical properties as TEP are known as TEP precursors (Passow,
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2000). Since the technique to visualize these transparent particles using Alcian blue was
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developed, the awareness of the important role they play in aquatic systems has increased
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rapidly. It has become an emerging topic in areas such as environmental science and
59
engineering. Data from Scopus shows that the number of TEP related publications strongly
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increased since 1993, from 1 to over 30 publication in year 2017.
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In particular, TEP has been extensively studied over the past decades by aquatic
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microbiologists to investigate the link between TEP and algal blooms (Passow, 2000, 2002;
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Passow, 2012; Villacorte et al., 2015b; Chowdhury et al., 2016). Chowdhury et al. (2016)
65
observed variations in TEP concentration, phytoplankton and chlorophyll over three years of
66
study at Indian Sundarbans. TEP concentrations were found to be the highest during the post-
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monsoon period and were mainly a function of phytoplankton production. Villacorte and co-
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authors (2015b) observed that TEP were produced during the exponential growth phase and
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stationary/death phase of three different types of microalgae.
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of RO biofouling. It is also a probable foulant of the pre-treatment membranes. Villacorte et
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al (2009) investigated the efficiency of various pre-treatment systems (Microfiltration (MF),
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Ultrafiltration (UF), coagulation, sedimentation, rapid sand filtration) on TEP removal and
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observed that UF is the most effective in removing particulate TEP (> 0.4 µm) but not for
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colloidal TEP (< 0.4 µm). This is because TEP is flexible in shape and in size, and is able to
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transfer through tight UF membranes. However, in their study, they observed that one of the
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pre-treatment systems with relatively high coagulant dose can achieved more than 90% of
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total TEP removal. As such, adjusting the coagulant dosage could be an immediate response
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when a high seasonal level of TEP is encountered.
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TEP in source waters play a significant role in the early stages of aquatic biofilm formation
82
(Berman and Holenberg, 2005; Berman, 2010; Berman et al., 2011) and hence it has been
83
identified as a potential cause of biological fouling in reverse osmosis (RO) membranes
84
(Villacorte et al., 2009; Berman, 2013; Lee et al., 2015; Li et al., 2016) and UF membrane
85
systems (Fang et al., 2010; Villacorte et al., 2010; Berman et al., 2011; Alizadeh Tabatabai et
86
al., 2014). In one study by Villacorte et al (2010), they observed a greater fouling rate of the
87
UF membrane in a seawater UF-RO desalination plant when high TEP levels in the seawater
88
were detected (Villacorte et al., 2010). Recently Li and co-authors (2016) have demonstrated
89
that apart from algal species, marine bacteria can also produce TEP and TEP precursors. Due
90
to its sticky nature, TEP/ TEP precursors serve as a conditioning layer for other bacteria to
91
attach onto the membrane surface and promote biofouling and hence should not be
92
overlooked (Berman and Holenberg, 2005; Berman, 2010).
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Over the years, different analytical techniques to quantify TEP concentrations have emerged.
95
TEP can only be made visible by staining with Alcian Blue, a hydrophilic cationic dye that
4
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complexes with carboxylated (―COO-) and sulfated (―OSO3-)
97
groups on a complex material, resulting in the formation of neutral precipitates (Alldredge et
98
al., 1993). The quantification of TEP was first determined by enumerating the stained
99
particles microscopically (Alldredge et al., 1993), which is labor-intensive and subjective as
100
the shape of TEP is not uniform. In 1995, Passow and Alldredge introduced a simpler method
101
to determine the concentration of TEP spectrophotometrically. In this method, TEP solution
102
is first filtered through a 0.4 µm polycarbonate membrane at low constant vacuum of 150
103
mmHg and subsequently stained with Alcian blue (0.02% aqueous solution of Alcian blue in
104
0.06% acetic acid, pH 2.5). The precipitate on the membrane is then re-dissolved using 5 ml
105
of 80% sulfuric acid which takes 2 hours and measured spectrophotometrically at a
106
wavelength of 787 nm. Calibration is done using known amounts of the standard Xanthan
107
Gum. As such, the concentration of TEP is typically reported in mg Xanthan equivalent per
108
liter. Villacorte et al. (2009) further extended the method by measuring the TEP precursors
109
(<0.2 µm) and reported it as colloidal TEP (c-TEP) using 0.05 µm and 0.2 µm polycarbonate
110
filters. This method allowed quantification of smaller size TEP which had been overlooked in
111
previous methods (Villacorte et al., 2009).
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polyanionic functional
In 2004, Arruda Fatibello and co-worker proposed another method which offers faster
114
determination of TEP concentration, reducing the detection time to approximately 40 min.
115
Instead of staining the TEPs on the filter surface, they mixed the Alcian Blue directly into the
116
sample solution and then sent it for centrifugation for 30 min. The supernatant was then
117
measured spectrophotometrically at a wavelength of 602 nm (Arruda Fatibello et al., 2004).
118
In 2007, Thornton et al. attempted to improve Fatibello’s method. Instead of using
119
centrifugation, they proposed to filter the Alcian blue stained sample solution through a 0.2
120
µm membrane. The final 1ml of the filtrate was then measured spectrophotometrically at a
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122
which was inversely proportional to the concentration of polysaccharide in the sample. This
123
relationship was found to be linear and was calibrated using Xanthan Gum and alginic acid.
124
Although Arruda Fatibello’s and Thornton’s methods are considerably more rapid than the
125
conventional method, they may not be easily applied to marine water samples unless the
126
marine sample is initially desalted by a dialysis process, which may take up to 24 hours
127
(Thornton et al., 2007). It was observed that when the stain is directly added to a marine
128
sample, the salt content in the suspension may interfere with the stability of the Alcian blue
129
and result in insoluble precipitates (Passow and Alldredge, 1995). Recently, Villacorte et al.
130
(2015a) adopted Thornton’s method to determine both the TEP and their colloidal precursors
131
concentrations without the interference of salinity. TEP and precursors were first retained on
132
10 kDa membranes and were re-suspended in ultrapure water by sonication. Subsequently,
133
the solution was stained with Alcian blue, and followed by a filtration in which the filtrate
134
was measured spectrophotometrically. This method covers the TEP precursors which were
135
not determined in the previous methods, but is a tedious procedure (Villacorte et al., 2015a).
136
Table 1 provides a summary of different methods to detect TEP thus far.
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It is important to note that Alcian blue is not specific for TEP as it can complex with other
139
acidic polysaccharides which are not involved in TEP formation (Winters et al., 2016; Li et
140
al., 2018). Although concern has been raised over the stainability of Alcian blue, the methods
141
developed for TEP quantification are still based on Alcian blue staining. This is because an
142
alternative dye with a similar selectivity as Alcian blue which is high selectivity for acidic
143
polysaccharides and non-selective for nucleic acids and proteins has not yet been found to
144
date (Discart et al., 2015). The aim of this study is to improve the current TEP detection
145
method rather than looking for an alternative dye or agent. Furthermore, most of the TEP
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quantification methods are offline methods, where the measurements have to be performed
147
off-site.
148
that allows TEP/TEP precursors to be determined in a relatively straightforward manner for
149
both freshwater and marine samples. The TEP monitor can be plugged directly into the water
150
process line and allows regular semi-continuous measurement. The TEP monitor uses a fibre
151
optic spectrophotometer to quantify the amount of TEP and their precursors that are stainable
152
by Alcian blue in a crossflow filtration cell. This method allows determination of the amount
153
of TEP directly on the membrane surface without the need to re-dissolve it into suspension
154
and hence reduces the analysis time to less than 1to 2 hours depending on the selection of the
155
filtration flux and water quality. The purpose of operating the TEP monitor in crossflow
156
mode is to allows the fraction of TEP most likely to deposit to be captured on the membrane
157
surface as a result of flow fractionation and selective deposition. Furthermore, the method
158
allows a wider range of membrane selection for different applications. For example, tighter
159
membranes such as UF membranes can be used to capture smaller size TEP responsible for
160
membrane fouling in RO applications. This study aims to investigate the viability of this
161
method and to investigate the efficiency compared to existing TEP methods.
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Table 1. Comparison of various TEP measurement methods. Passow & Aldredge
Villacorte et al.
Detection limit
1 Particle / filtered amount
2 µg XG/L
50 µg XG/L
Membrane type
Polycarbonate 0.4 µm
Polycarbonate 0.4 µm
Regenerated cellulose 10 kDa
Salt interferences
No
No
No
4 steps
4 steps
5 steps
• Filtration
• Filtration
• Staining with AB
• Staining with AB
• Particle transferred to a slide through FilterTransfer-Freeze method
• Soak in H2SO4 to elute TEP from membrane
• Re-suspend TEP in MQ water
• Microscopic enumeration
• UV Spectroscopy quantification
0.1 x 103 µg XG/L
1.0 x 103 µg XG/L
5 x103 µg XG/L
Not used
Surfactant free cellulose acetate 0.2 µm
Regenerated cellulose 30 kDa
Yes
Yes
No
4 steps
4 steps
3 steps
• Dialysis desalting for marine sample
• Dialysis desalting for marine sample
• Filtration
• Addition of AB to sample
• Addition of AB to sample
• Fibre optic spectrophotometer quantification
• Centrifugation • UV Spectroscopy quantification
• Filtration with syringe filter
• Staining with AB
• UV Spectroscopy quantification
Fast for fresh water, slow for marine sample (>24 hr)
Fast for fresh water, slow for marine sample (>24 hr)
Relatively fast: 30 mins-1hour
No
No
No
Yes
Vacuum flask, infrared Spectrophotometer (787 nm)
Vacuum flask, infrared Spectrophotometer (787 nm)
Centrifuge (300 rpm), light spectrophotometer (602 nm) dialysis (1000 Da,)
Vacuum flask, light spectrophotometer (602 nm), dialysis (1000 Da)
Crossflow filtration cell, fibre optic spectrophotometer
Particulate TEP
Particulate and precursor TEP
Particulate and precursor TEP
Particulate and precursor TEP
Particulate and precursor TEP
(Passow and Alldredge, 1995)
(Villacorte et al., 2015a; Villacorte et al., 2015b)
(Arruda Fatibello et al., 2004)
(Thornton et al., 2007)
N.A
Online
No
No
Main equipment
Standard light microscope (200x Magnification) Particulate TEP
Relatively fast: more than 1 hour (sample dependent)
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Speed
(Alldredge et al., 1993)
This work
• UV Spectroscopy quantification
Slow: more than 2 hours (Sample-dependent)
Selected Refs.
• Addition of AB to sample
• Filtration with syringe filter
Slow
TEP type
Thornton et al.
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2
Materials and Methods
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2.1
165
2.1.1
166
In this study, xanthan gum and sodium alginate were used as the model TEP as they share the
167
key characteristics of natural TEP. Xanthan gum has always been used as a reference value to
168
indicate the amount of TEP in the water. Known concentrations (0 – 100 mg/L) of xanthan
169
gum solutions were prepared by dissolving the powdered form of xanthan gum (Sigma
170
Aldrich) in deionised water (Milli-Q, Merck). The stock solution was vigorously stirred for at
171
least 1 hour until no visible xantham gum could be visually observed in the flask. The
172
xanthan gum solution was freshly prepared for every experiment. Alginate solution (100
173
mg/L) was prepared by dissolving sodium alginate (Sigma Aldrich) in Milli-Q water.
174
2.1.2
175
Besides model foulants, the proposed TEP monitor was also validated with various types of
176
source waters (see Appendix Table A1) collected within the period of October – November
177
2016. Prior to TEP measurement, the water was pre-filtered with a 5 µm cartridge filter
178
(KAREI) to remove the suspended solids.
179
2.1.3
180
Thalassiosira pseudonana was grown in batch cultures to represent an algal bloom situation
181
in seawater. The algae were cultured in 10 L of sterilized synthetic seawater (16.6 g/L
182
artificial sea salt) spiked with a nutrient solution based on f/2 +Si medium at room
183
temperature of 25±1 °C. The composition of the prepared medium is shown in the Appendix,
184
Table A2 The algal culture was aerated with 0.22 µm filtered air and under continuous
185
exposure of an artificial light source (fluorescent lamps). The average algal cell concentration
186
in the batch cultures were monitored daily by measuring their optical density at 676nm using
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188
through cellulose filter paper (WhatmanTM, Grade 1 Circle) with pore size of 11 µm to
189
remove the algal cells prior to the test. TEP which are larger than the filter pore size or are
190
bounded to the algal cells were not included in the TEP analysis.
191
2.2
192
The Alcian blue staining solution was prepared with 0.02% of Alcian blue 8GX (Sigma
193
Aldrich) in 0.06% acetic acid (pH 2.5). The solution was adjusted to pH 2.5 to ensure that
194
both sulphated and carboxyl TEP were completely stained (Passow and Alldredge, 1995).
195
The staining solution was kept in the dark and at 4 ºC for less than 4 weeks. The solution was
196
pre-filtered with a 0.45 µm membrane before use.
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Membranes
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Commercial UF membranes with a MWCO of 30 kDa (Millipore) were used in the proposed
200
TEP monitor for capturing smaller sized TEP. The selected membrane material was
201
regenerated cellulose as it was found to be negligibly stained by the Alcian blue staining
202
solution in this study when compared to other types of membrane such as polyester sulfone
203
(PES) membrane. For conventional TEP measurement, a polycarbonate filter (Nucleopore,
204
Whatman) with pore size of 0.1 µm was used instead of 0.4 µm membrane (Passow and
205
Alldredge, 1995).
206
2.4
207
The conventional TEP measurement was performed based on the spectrophotometric method
208
introduced by Passow and Alledredge in 1995. In this method, the sample was filtered
209
through the polycarbonate membrane of 0.1 µm at constant suction pressure of 150 mmHg.
210
After the filtration, the TEP on the membrane surface were stained with 1 ml of Alcian blue
211
solution and rinsed with 1 ml Milli-Q water to remove excess stain. The filter was soaked
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Conventional Method for TEP Measurement
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afterwards in 5 ml of 80% H2SO4 solution for 2 hours to elute Alcian blue precipitate that was
213
attached on the membrane surface. Subsequently, the absorbance of the solution was
214
measured using UV-Vis spectrophotometer (UV1800, Shimadzu) at wavelength of 787 nm
215
using a 1 cm cuvette.
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2.5
Proposed TEP Monitor
218
2.5.1
219
Quantification of the stained TEP is the key to the TEP monitor and this section provides a
220
brief background to colorimetric quantification. The Commission Internationale de
221
I’Eclairage (CIE), or International Commission on Illumination is an international body
222
responsible for developing basic standards and procedures in the fields of photometry and
223
colorimetery. In 1976, CIE recommended the L*a*b* or CIELAB colour scale (CIE
224
International Commision on Illumination, 1976). When a colour is expressed in CIELAB, the
225
L* represents lightness, a* colour coordinates represents the red/green component colour and
226
the b* represents the yellow/blue component colours. The maximum for L is 100 indicating
227
the brightest white, while the minimum value of L is 0 representing darkest black. The a* and
228
b* axes have no specific numerical limits. Positive a* values indicate red, where negative a*
229
values indicate green. On the other hand, positive b* values represent yellow while negative
230
b* values indicate blue. The CIELAB colour space is schematically shown in Appendix, Fig.
231
A1.
232
The difference in colour between two samples can be expressed in term of delta values, ∆L*,
233
∆a* and ∆b*. The total difference ∆E* is defined as:
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Background on L*a*b* Colour Space
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2.5.2
TEP monitor configuration
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237
and an optic fiber spectrophotometer, depicted in Fig. 1. The feed water is pumped at a
238
constant flowrate by a gear pump into the filtration flow cell. Feed pressure is controlled by a
239
pressure control valve at the retentate side. Permeation is adjusted and maintained constant by
240
a peristaltic pump. The spectrometer probe is placed on top of the flow cell quartz window to
241
provide direct reading of the colour where it is connected directly to a PC for analysis.
242
Separate injection and drain ports are required during the colour determination stage in which
243
the Alcian blue and Milli-Q water are injected for staining and rinsing, respectively. This is to
244
ensure that the staining dye does not contaminate the feed water.
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Fig. 1. (a) Main components of a TEP monitoring device. (b) Detailed diagram of the TEP monitoring cross-flow filtration cell. Inset: Tip of the refection probe
249 250
The crossflow cell consists of a top-plate and a bottom-plate (Fig. 1b). The feed flows into
251
the cell from the top plate, while the permeate flows inside the bottom plate. The bottom plate
252
has a cavity to place a perforated stainless-steel disc and a membrane support. These are
253
needed to ensure a uniform distribution of membrane permeation. The membrane channel is
254
provided by the cavity in the top plate with a measured channel height of 1mm. The
255
dimensions of the membrane are 2.8 cm x 5 cm. The quartz glass window is 2 mm thickness
256
and 2.5 cm x 2.5 cm in area, located at the center of the membrane area.
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259
used in this study. A reflection probe is connected to a D65 halogen light source and a
260
spectrometer and placed at the top of the quartz glass window at 70o angle. This angle was
261
suggested by the manufacturer to enable optimum amount of reflected light into the probe.
262
The halogen light is sent through 6 illumination fibers to the surface to be measured and the
263
reflection is detected by the 7th fiber located at the center of the probe. The detection fiber is
264
coupled to a spectrometer that is configured to wavelengths of 380 to780 nm. The percentage
265
of wavelength that is reflected back to the detector is plotted against each wavelength, giving
266
a spectral data set. It provides a visual representation of a colour fingerprint and can be fitted
267
with different colour space models for comparison purposes. In this application, the CIE
268
L*a*b colour space (ISO 11664-4:2008 (E)/CIE S 014-4/E:2007) representation was used
269
(CIE International Commision on Illumination, 1976). The structure of the L*a*b colour
270
space is based on the theory that a colour cannot be both green and red at the same time, nor
271
blue and yellow at the same time. As such, single values can be used to describe the
272
contribution of the red or green (a*) and yellow or blue (b*) of an object. L* is defined as the
273
brightness.
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In this paper, the relationship of ∆b* against different xanthan gum concentrations was
276
investigated since the change in b* represents a shift in blue colour intensity which is
277
strongly shown by the Alcian blue stain.
278 279
2.5.3
TEP Measurement Protocol
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281
stage (sample collection) and (2) monitoring stage. During the fouling stage, the feed water
282
was pumped to the crossflow cell at a crossflow velocity of 0.1 m/s, under constant flux mode.
283
Different combination of permeation flux and filtration duration were investigated in this
284
study, tabulated in Appendix Table A2. It should be noted that the filtration time and flux
285
may be application specific. For example, for high concentration TEP sample, low flux
286
operation is preferred.
287
During the monitoring stage, the feed pump was stopped but the permeation flux was still
288
maintained to ensure that TEP remained on the membrane surface when Alcian blue was
289
injected to the filtration cell. During this period, the filtration was operated under dead-end
290
filtration mode. Prior to taking any colour measurement, a white reflective standard tile was
291
used to calibrate the colour spectrometer before each experiment.
292
measurement was taken prior to injection of the Alcian blue solution to account for any
293
possible colour change of the membrane during the fouling stage, denoted as b*reference. Note
294
that the b* may differ for each experiment if the feed water is not transparent. This may result
295
in some colour changes on the membrane after the fouling stage. As such, the proposed
296
method is ideal for low turbidity feed water and may require pre-treatment (e.g. 5 µm filter)
297
for turbid feed solution to minimize the interference.
298
Before injecting Alcian blue into the filtration cell 20 mL of Milli-Q water was slowly fed to
299
the flow cell to flush out any remaining salt that may still be present in the membrane channel.
300
This is to minimize the interference of salt to the measurement. Subsequently, 10 mL of
301
Alcian blue solution was slowly injected into the flow cell followed by another 20 mL of
302
Milli-Q water to remove any excess Alcian blue from the membrane cell. A final colour
A reference colour
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reading was then taken, denoted as b*final. For a clean membranes, it was observed that b*final
304
is not equal to b*reference. This indicates that Alcian blue can stain the membrane but not
305
significantly. As such, in this study, ∆b*, which is defined as ∆b* = b*final – b*reference –
306
∆b*clean
307
hypothesized that when there is a significant amount of TEP deposited on the membrane
308
surface, after staining with Alcian blue, the colour space value of ∆b* will appear to be more
309
negative.
310
2.5.4
311
One of the important characteristics for a reliable monitoring system is the ability for multiple
312
measurements with minimum replacements of membrane in the monitor. Therefore, in this
313
study the proposed method was evaluated for multiple measurements after membrane
314
regeneration. Sodium hypochlorite (NaOCl, 0.5 w/v%) was selected as the cleaning agent to
315
restore the membrane permeability.
316
3
317
3.1
318
Fig. 2 shows images of three different concentrations of xanthan gum taken after filtration
319
followed by the Alcian blue staining. It is obvious that different concentrations of XG deposit
320
on the membrane surface resulted in different visual levels of blueness after Alcian blue
321
staining. The visual blueness of the membranes increased with increasing XG concentration.
322
From the images, it can be seen that the XG formed a uniform cake layer on the membrane
323
surface as the colour intensity of blue was equally distributed over the membrane surface.
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10 ppm
50 ppm
100 ppm
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Fig. 2. Different concentrations of Xanthan Gum fouled on membrane surface, stained with Alcian blue.
327
One way to quantify the degree of blueness can be achieved through image analysis software.
328
Recently, Palencia et al. (2016) proposed a digital image analysis method to describe the
329
membrane surface changes as a result of fouling. Digital images were taken offline after UF
330
filtration with aqueous extracts from the plant. The colour intensity was subsequently
331
quantified by the software and represented in terms of surface colour, an index which is based
332
on the (Red Green Blue) RGB colour model. Their results showed that there is a correlation
333
between the colour index, membrane permeability and fouling layer thickness (Palencia et al.,
334
2016). Nevertheless, this method does not allow in-situ detection.
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As can been seen Fig. 2, the colour composition for XG stained with Alcian blue is mainly
337
composed of blue-green component (respectively negative a* and negative b* directions). As
338
such, in this section, the parameters ∆b*, ∆a* and ∆E*, the combination of the three vectors
339
(a*, b* and L*) at different XG concentrations are investigated. Fig. 3 shows the plots of ∆a*,
340
∆b* and ∆E* at different concentrations of XG. ∆a*, ∆b* and ∆E* show good linear
341
regressions with the XG concentration, with R2 of 0.90, 0.99 and 0.93 respectively. As the
342
Alcian blue is dominant in the blue component, ∆b* shows a stronger linear relationship with
343
XG concentration with the data points less scattered compared to ∆a* plot. ∆E* is the
344
combined effect of a*, b* and L* components, therefore, this parameter might be more
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346
∆b* is used for discussion in the remainder of this paper.
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Fig. 3: Different concentrations of Xanthan Gum fouled on membrane surface, stained with Alcian blue (a) ∆a*; (b) ∆b*and (c) ∆E*. Conditions: Filtration duration 1.5 h at flux of 90L/m2.h.
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354
In the previous set of experiment (Fig. 3), the duration of the fouling stage was set at 1.5 h to
355
ensure that a uniform cake layer was formed on the membrane surface. This would mean that
356
a homogenous colour can be achieved. In this section, the responses of the TEP monitor
357
operated at shorter fouling durations were studied. Fig. 4a shows the relationship of b*
358
against the XG concentration when operated at 90L/m2.h for 0.5 h. Clearly, the b* values
359
were rather scattered when the XG concentration is less than 20 ppm. This could be because
360
the amount of XG retained on the membrane surface was not significant enough to form a
361
uniform cake layer on the surface and not strongly attached on the membrane surface. One
362
way to shorten the duration fouling stage without compromising the foulant amount would be
363
by operating the TEP monitor at higher permeation flux.
364
regression of b* against XG concentration when operated at a higher flux of 180 L/m2.h,
365
especially at the low TEP range. With operating at this flux, better sensitivity can be achieved
366
with detection limit of about 5 ppm, with R2 of 0.98. It is important to ensure that the TEP
367
monitor is able to detect low concentrations of TEP as the TEP content in natural seawater is
368
typically around 20 ppm. Nevertheless, for high concentrated TEP feed solutions, it is
369
recommended to operate at lower flux to avoid severe and irreversible fouling on the
370
membrane.
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Investigation of different combinations of permeation flux and filtration duration
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Fig. 4b shows better linear
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Fig. 4: Different concentrations of Xanthan Gum fouled on membrane surface, stained
374
with Alcian blue at different operating condition: (a) Flux =90L/m2.h; filtration
375
duration = 0.5 h; (b) Flux = 180L/m2.h; filtration duration = 0.5 h
376
3.3
Comparison with Conventional Method and Natural Sample Analysis
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378
conventional filtration method proposed by Passow and Alldredge (1995). Therefore, in this
379
study, the proposed method was compared with this conventional filtration method and the
380
relationship was validated using various water sources, as shown in Fig. 5.
381
Fig. 5 (a) shows the expected linear relationship between the conventional method (presented
382
as absorbance at 787nm) and the TEP monitor using XG solution. Overall the trend for model
383
and natural samples lies below the standard calibration line, indicating that the TEP monitor
384
may have captured the smaller size TEP precursors (<0.1 µm) that were present in the water
385
samples, resulting in higher values of TEP concentration (in terms of mg Xequi.L-1), as shown
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in Fig. 5b. The TEP monitor generally shows higher TEP concentration compared to
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conventional TEP. This could mean that TEP monitor is more sensitive compared to the
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conventional method as it is able to capture the colloidal TEP especially for low TEP water
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samples. Typically, this fraction in the water is the portion that is responsible for fouling on
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RO membranes.
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It is anticipated that if the membrane used in the conventional method is similar to the TEP
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monitor, the two lines would lie closer to each other but do not necessarily overlap due to the
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different modes of filtration (dead-end vs crossflow). However, since the conventional
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method is based on vacuum filtration which can only reach a maximum pressure of 1 bar, the
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use of a tighter membrane would result in a substantially increased measurement time.
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Fig. 5: (a) TEP standard curve measured with conventional method and TEP monitor (o) and plot of different natural samples ( ):a=tap water; b=seawater; c=seawater brine; d=pond water; e=aquaculture water; f=sodium alginate; (b) TEP concentration of each type of water obtained from TEP monitor and conventional TEP method.
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TEP production during algae growth
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Algal blooms are a threat to water quality and can impact desalination plant and aquaculture.
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To investigate the relation between the TEP production and the algae growth, a single strain
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of algae was batched cultured. The changes in cell optical density (O.D.) at UV 676 nm and
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TEP concentration (conventional method and TEP monitor) were monitored for 12 days and
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are presented in Fig. 6. The algae showed a 2-day lag phase, followed by exponential growth
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phase for the remaining monitoring period. The TEP measured using the conventional
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method only showed slight variation throughout the monitoring period with values range
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from 2 to 4 mg Xeq/L depicted in Fig. 6a. In contrast, the TEP monitor shows the increase in
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TEP concentration which coincided with the increase in O.D during the lag and exponential
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phase with values range from 40 to 100 mg Xeq/L. The concentration of the TEP monitor
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(membrane ~ 30 kDa) was found to be higher than that measured through conventional
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method (membrane >0.1 µm) and indicates that TEP precursor (<0.1 µm) is likely the
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dominant fraction among the algal-released TEP during the bloom. Villacorte et. al. (2015b)
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obtained similar findings where they observed TEP > 10 kDa increased dramatically over the
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algae growth period, whilst, TEP > 0.4 µm only varied slightly. These results indicate the
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need to use a tighter membrane to capture the dominant fraction of TEP. In summary, the
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TEP monitor is very sensitive to algal growth and could be used as an ‘online’ algal bloom
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warning device.
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Fig. 6: Changes in cell density, TEP concentration determined by (a) conventional method
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(TEP > 0.1 µm) and (b) TEP monitor (TEP > 30 kDa) during the incubation period of batch
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culture of Thalasiorra Pseudonana. TEP monitor: filtration duration of 1.5 h at flux of 90
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L/m2.h. Note the error bars for TEP monitor and O.D. are smaller than the plot symbols.
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3.5
Regeneration of Membrane
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To be eligible as a practical monitor, it is important that this method requires minimal
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maintenance especially postponing (or preventing) the need to replace the membrane.
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regenerated after each measurement. NaOCl is one of the most commonly used and effective
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cleaning reagents to restore the membrane permeability. Thus, it is selected to be used in this
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study. After a cycle of filtration and staining process, the membrane was regenerated using
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NaOCl (0.05 w/v%).
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Fig. 7a shows the changes of b*final during the cleaning process for three different
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concentrations of XG. During the cleaning process, the NaOCl solution was circulating in the
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system without permeation flux and at high crossflow (0.3 m/s). After being cleaned for 30
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minutes, the b*final value returned to almost its initial value, indicating a cleaning efficiency
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of close to 100%. At lower XG concentrations, the cleaning cycle can be completed within 20
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minutes. The results indicated that the XG and Alcian blue complexes that formed a cake on
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the membrane surface can be easily removed from the surface with NaOCl. This phenomenon
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might be attributed to the oxidation and deprotonation of carboxyl groups when the NaOCl
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solution was introduced into the system under alkaline conditions (pH = 10). It was reported
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that the increase in the pH of the fouling layer would cause the negative surface charge to
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increase and hence creating strong repulsive forces between the macromolecules (Wang et al.,
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2018). As such, the XG-Alcian blue matrix structures became looser and can be easily
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detached from the membrane surface by the hydrodynamic shear forces.
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Fig. 7. (a) bfinal* values as a function of cleaning time for different XG concentration
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fouled membrane; (b) ∆b* vs Xathan Gum concentration. The red asterisk (⁎ ) are the values obtained after each cycle of regeneration. The number beside the asterisk indicates the sequence of the test. Conditions: Flux =180L/m2.h; crossflow velocity = 0.15ms-1, filtration duration 0.5 h.
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concentration and its corresponding ∆b* value was plotted in the plot of b* vs XG concentration. As
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cleaning effectively returns the used membrane back to its initial performance, a random
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concentration testing sequence was performed so as to eliminate the possibility of a systematic error
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in the evaluation design. The filtration-stain-regenerate cycle was repeated for 9 times. It was
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observed that the ∆b* values (⁎) lie within the linear relationship of XG concentration and ∆b* value,
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shown in Fig. 7b. The results confirm that the TEP monitor can be regenerated without compromising
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the accuracy. It is important to note that other organic matters that could be present in real feed water
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may cause irreversible membrane fouling during the measurement.
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investigate the regeneration of the membrane using real waters and this will be presented in a future
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publication. Furthermore, the frequency of replacing the membrane depends on the feed quality and
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this should also be investigated using real feed water.
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As such it is planned to
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Conclusions
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TEP has been identified in the feed water as one of the important parameters resulting in
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biofouling in membrane systems. This work focused on the development of a novel TEP
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monitoring method which enables online measurement with a shorter analysis duration
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compared to reported methods. The new method was successfully validated with the
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conventional method at both low and high TEP concentrations, for both model and natural
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TEP. Additionally, it was shown that the new method also offered a more sensitive detection
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for smaller size TEP. The robustness of the new method was also shown by the ability of
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membrane regeneration during multiple measurement cycles. In addition to monitoring for
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desalination plant fouling control the TEP monitor also has potential as an early warning
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system for protection of aqua culture against algal blooms.
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Acknowledgement
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Singapore Membrane Technology Centre (SMTC) is supported by the Economic
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Development Board (EDB) of Singapore.
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Alizadeh Tabatabai, S.A., Schippers, J.C., Kennedy, M.D., 2014. Effect of coagulation on fouling potential and removal of algal organic matter in ultrafiltration pretreatment to seawater reverse osmosis. Water Res. 59, 283-294. Alldredge, A.L., Passow, U., Logan, B.E., 1993. The abundance and significance of a class of large, transparent organic particles in the ocean. Deep-Sea Res. Pt I: Oceanographic Research Papers 40, 1131-1140. Arruda Fatibello, S.H.S., Henriques Vieira, A.A., Fatibello-Filho, O., 2004. A rapid spectrophotometric method for the determination of transparent exopolymer particles (TEP) in freshwater. Talanta 62, 81-85. Berman, T., 2010. Biofouling: TEP – a major challenge for water filtration. FiltrSeparat 47, 20-22. Berman, T., 2013. Transparent exopolymer particles as critical agents in aquatic biofilm formation: implications for desalination and water treatment. Desalin. and Water Treat. 51, 1014-1020. Berman, T., Holenberg, M., 2005. Don't fall foul of biofilm through high TEP levels. Filtr. Separat. 42, 30-32. Berman, T., Mizrahi, R., Dosoretz, C.G., 2011. Transparent exopolymer particles (TEP): A critical factor in aquatic biofilm initiation and fouling on filtration membranes. Desalination 276, 184-190. Berman, T., Parparova, R., 2010. Visualization of transparent exopolymer particles (TEP) in various source waters. Desalin. Water Treat. 21, 382-389. Chowdhury, C., Majumder, N., Jana, T.K., 2016. Seasonal distribution and correlates of transparent exopolymer particles (TEP) in the waters surrounding mangroves in the Sundarbans. J. Sea Res. 112, 65-74. CIE International Commision on Illumination, 1976. CIE 1976 L*a*b* Colour Space (ISO 116644:2008(E)/CIE S 014-4/E:2007). Discart, V., Bilad, M.R., Vankelecom, I.F.J., 2015. Critical evaluation of the determination methods for transparent exopolymer particles, agents of membrane fouling. Crit. Rev. Environ. Sci. Technol. 45, 167-192. Fang, J., Yang, X., Ma, J., Shang, C., Zhao, Q., 2010. Characterization of algal organic matter and formation of DBPs from chlor(am)ination. Water Res. 44, 5897-5906. Lee, H., Park, C., Kim, H., Park, H., Hong, S., 2015. Role of transparent exopolymer particles (TEP) in initial bacterial deposition and biofilm formation on reverse osmosis (RO) membrane. J. Membr. Sci. 494, 25-31. Li, S., Winters, H., Jeong, S., Emwas, A.-H., Vigneswaran, S., Amy, G.L., 2016. Marine bacterial transparent exopolymer particles (TEP) and TEP precursors: Characterization and RO fouling potential. Desalination 379, 68-74. Li, X., Skillman, L., Li, D., Ela, W.P., 2018. Comparison of Alcian blue and total carbohydrate assays for quantitation of transparent exopolymer particles (TEP) in biofouling studies. Water Res. 133, 6068. Palencia, M., Lerma, T., Palencia, V., 2016. Description of fouling, surface changes and heterogeneity of membranes by color-based digital image analysis. J. . Membr. Sci. 510, 229-237. Passow, U., 2000. Formation of transparent exopolymer particles, TEP, from dissolved precursor material. Mar.ecol.prog. ser 192, 1-11. Passow, U., 2002. Transparent exopolymer particles (TEP) in aquatic environments. Prog. Oceanogr. 55, 287-333. Passow, U., 2012. The abiotic formation of TEP under different ocean acidification scenarios. Mar. Chem. 128–129, 72-80. Passow, U., Alldredge, A.L., 1995. A dye-binding assay for the spectrophotometric measurement of transparent exopolymer particles (TEP). Limn. Oceanogr. 40, 1326-1335.
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A novel method (TEP monitor) to determine TEP using a colour based technique has been developed. The TEP monitor enables more sensitive detection of smaller size TEP. The TEP monitor offers shorter measurement time and multiple uses of membrane.
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