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H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis concentrate
Accepted Manuscript Impact of coagulation as a pre-treatment for UVC/H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis concentrate Muhammad Umar, Felicity Roddick, Linhua Fan PII:
S0043-1354(15)30259-1
DOI:
10.1016/j.watres.2015.09.047
Reference:
WR 11555
To appear in:
Water Research
Received Date: 10 May 2015 Revised Date:
21 September 2015
Accepted Date: 28 September 2015
Please cite this article as: Umar, M., Roddick, F., Fan, L., Impact of coagulation as a pre-treatment for UVC/H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis concentrate, Water Research (2015), doi: 10.1016/j.watres.2015.09.047. 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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Impact of coagulation as a pre-treatment for UVC/H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis
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concentrate
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Muhammad Umar, Felicity Roddick*, Linhua Fan
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School of Civil, Environmental and Chemical Engineering, RMIT University,
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GPO Box 2476, Melbourne, 3001 Victoria, Australia
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Submitted to Water Research
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Corresponding author: Felicity Roddick
School of Civil, Environmental and Chemical Engineering RMIT University Email: [email protected] Tel: +613 9925 2080
Fax: +613 9639 0138
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ACCEPTED MANUSCRIPT Abstract
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After coagulation of high salinity reverse osmosis concentrate (ROC) with either alum or
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ferric chloride followed by UVC/H2O2 treatment, biological activated carbon (BAC) was
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investigated for the removal of DOC. BAC treatment mainly removed low molecular weight
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(LMW) neutral molecules indicating that biodegradation was the predominant mechanism of
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organic matter removal. Coagulation with ferric chloride gave greater DOC reductions than
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alum both as a stand-alone treatment and after the sequence of UVC/H2O2 and BAC
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treatment. However, overall reduction after the sequence of coagulation, UVC/H2O2 and
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BAC treatment was only marginally greater for ferric chloride (68%) than for alum (62%).
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Trihalomethane formation potential and N-Nitrosodimethylamine concentration decreased
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markedly after UVC/H2O2 treatment. UVC/H2O2 treatment of the ROC led to the generation
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of extreme toxicity according to the Microtox assay, but no toxicity was observed after BAC,
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demonstrating its advantage for enabling safe disposal of the treated ROC. Implementation of
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coagulation as a pre-treatment and BAC as a post-treatment markedly reduced (6-8 times) the
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electrical energy dose (EED) required for the UVC/H2O2 process. The sequence of
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coagulation, UVC/H2O2 and BAC treatment was demonstrated as a potential process for the
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removal of organic matter from high salinity municipal ROC.
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Keywords: Biological activated carbon; coagulation; UVC/H2O2; reverse osmosis
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concentrate; disinfection by-products
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ACCEPTED MANUSCRIPT 1.
Introduction
As reverse osmosis (RO) is widely used as a polishing treatment for secondary effluent in wastewater reclamation schemes, safe disposal or reuse of the resultant RO concentrate
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(ROC) is becoming increasingly important. The ROC comprises 15-20% of the volume of the feed stream and contains almost all the contaminants present in the original wastewater at elevated levels. These contaminants may be toxic and/or bio-accumulative, and so disposal of
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untreated ROC presents a potential environmental risk (Westerhoff et al., 2009). The addition of chemicals such as antiscalants, biocides and acids during the treatment process can change
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the character of the organic and inorganic compounds and can influence the chemical equilibrium of the dissolved constituents (van der Bruggen et al., 2003). Conventional practice has been to dispose of the ROC by direct discharge to surface water, sewer, evaporation ponds, and deep well injection (Lee et al., 2009), of these direct discharge and
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sewer disposal are the most widely used options (Khan et al., 2009). Treatment of ROC to decrease the overall DOC content and to remove or inactivate the toxic and bio-accumulative
safe disposal.
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contaminants is therefore desirable to increase the overall water recovery as well to enable its
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Advanced oxidation processes (AOPs) have been demonstrated to be one of the preferred treatments for ROC as they are effective for oxidising the organic matter and so improving the biodegradability of the organic matter which can then be removed by biological treatment (Westerhoff et al., 2009; Liu et al., 2012; Umar et al., 2014). UV/H2O2 (Justo et al., 2013), electrochemical oxidation (Pérez et al. 2010; Bagastyo et al., 2012), and ozonation (Lee et al., 2009; Zhou et al., 2011; Justo et al., 2013) are among the most investigated AOPs for the treatment of ROC. UVC/H2O2 is one of the most commonly used AOPs for the treatment of
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ACCEPTED MANUSCRIPT water and wastewater and is leading the way in commercial applications (Sarathy and Mohseni, 2010). It utilizes the highly oxidizing hydroxyl radical which non-selectively oxidises organic compounds, unlike ozone. Although electrochemical oxidation also utilizes radicals, in the presence of chloride ion, as in the present ROC, high concentrations of
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chlorinated DBPs may be produced (Pérez et al. 2010; Hurwitz et al., 2014) and chlorine gas may be generated. UV/H2O2 has been shown to oxidise and so remove ecotoxic contaminants such as endocrine disrupting chemicals and pharmaceuticals (Linden et al., 2007; Justo et al.,
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2013). However, a sequence of complementary processes may be required to ensure
sufficient removal of the highly heterogeneous organic content of ROC, and appropriate
effective and hence more economic.
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process selection can reduce the load on the subsequent process, thus making it more
As AOPs lead to increased biodegradability of the oxidised compounds, they are generally
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followed by biological treatment to further remove the DOC and to reduce the potential for microbial growth in subsequent processes or applications. BAC is the usual biological treatment after AOPs to enhance the removal of organic matter and improve the cost
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effectiveness (Toor and Mohseni, 2007). BAC treatment after the sequence of coagulation and UVC/H2O2 treatment may improve the removal of organic matter as it can effectively
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remove the low molecular weight breakdown products from the AOP (Takeuchi et al., 1997).
The simultaneous adsorption and biodegradation of organic matter during BAC filtration means that the activated carbon can be partially regenerated while the carbon bed is in operation (Rodman et al., 1978; Rice and Robson, 1982) and more recalcitrant organic matter can be removed by sorption onto the biofilm, and then slowly degraded by microorganisms (Rice and Robson, 1982; Carlson and Silverstein, 1998). As noted by Ying and Weber (1979), simultaneous adsorption and biodegradation of organic matter in a single reactor 4
ACCEPTED MANUSCRIPT eliminates the need for individual processes resulting in low capital cost, and as BAC requires less frequent regeneration of the carbon it has lower energy requirements and operating cost.
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For municipal wastewater ROC we have shown that coagulation is effective for the removal of high molecular weight compounds thus facilitating subsequent UVC/H2O2 treatment due to reduction in colour and increase in UV transmissivity (UVT). This results in enhanced
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biodegradability (as measured by biodegradable dissolved organic carbon assay) of the
remaining organics and thus reduces the overall energy consumption required for the removal
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of the organic matter (Umar et al., 2014).
Although some studies have utilised BAC as a post-treatment of AOP-treated ROC (Lee at al., 2009; Lu et al., 2013), no investigation of the impact of coagulation with and without
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intermediate UVC/H2O2 treatment on subsequent BAC treatment has been reported.
Therefore, the aim of this work was to compare two of the most commonly used coagulants,
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alum and ferric chloride, for the treatment of a high salinity ROC prior to UVC/H2O2 and
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BAC treatment. Alum was selected due to its low cost and non-corrosive nature, ferric chloride was selected as it is considered as the most feasible coagulant for high ionic strength water (Edzwald and Haarhoff, 2011). Furthermore, these coagulants had been shown to be effective for another high salinity ROC sample (Umar et al., 2015). The presence and formation of various disinfection by-products (DBPs) and the ecotoxicity of selected samples was investigated. A target residual DOC value of 15 mg/L was selected as this is close to values in secondary effluent, enabled comparison of the energy efficiency of the different treatments, and was acceptable to the water treatment plant. To the best of our knowledge,
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ACCEPTED MANUSCRIPT this is the first investigation of coagulation+UVC/H2O2+BAC for the reclamation and/or safe disposal of a high salinity ROC taking into account the formation of DBPs as well as
2.
Materials and Methodology
2.1.
Collection and characterization of ROC
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ecotoxicity.
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The ROC sample was collected from a wastewater reclamation facility at a local municipal wastewater treatment plant (WWTP), and stored at 4oC. In the treatment process at the
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WWTP, raw sewage is screened and de-gritted and sent to intermittently decanted extended aeration (IDEA) bioreactors where it is treated in a cycle of aeration, settling, and decant. The IDEA effluent is then treated using a combination of ultrafiltration and RO. The characteristics of the ROC sample, which was high in alkalinity and salinity, are given in
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Table 1.
Table 1
BAC treatment and reactor set-up
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2.2.
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Two BAC columns were operated as continuous down flow reactors with 60 min empty bed contact time (EBCT). The columns were packed with thoroughly pre-washed GAC (Acticarb BAC - GS1300 obtained from Activated Carbon Technologies, Victoria, Australia) to a bed height of 17 cm. The GAC had a Brunauer-Emmett-Teller (BET) surface area of 1200 m2/g, is manufactured for maximum biological activity and is normally used for BAC filters. A peristaltic pump was used to feed the ROC sample to the columns at 60 mL/h. An air gap was maintained at the top of the column to allow contact with oxygen. The BAC bed was
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ACCEPTED MANUSCRIPT operated under fully submerged conditions and backwashed with ROC every two weeks to avoid physical clogging. The DO concentration of the ROC feed was always more than 8 mg/L, sufficient to ensure microbial activity. The columns were equilibrated for 80 days to achieve consistent DOC, colour and A254 reduction before use. DOC reduction was compared
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with (pH 7-8) and without pH (pH 5.5) adjustment after the UVC/H2O2 treatment. The difference in the reduction of DOC, colour and A254 was insignificant and so no pH
adjustment was made before BAC treatment. Residual H2O2 was not removed prior to the
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BAC treatment as it was shown to not affect the DOC removal.
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2.3. Coagulation
Aluminium and ferric stock solutions were prepared using alum (Al2(SO4)3.18H2O) and ferric chloride (FeCl3.6H2O), respectively, (Chem-Supply Pty Ltd, Australia). Coagulation
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was carried out using a laboratory jar test apparatus (Phipps and Bird, PB-700) using 2 L samples. The samples were rapidly mixed for 2 min at 250 rpm followed by slow mixing for 30 min at 30 rpm and subsequent settling for 2 h before taking the supernatant for UVC/H2O2
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treatment. Coagulation (1 mM metal dose) was performed at pH 5 taking the results for a
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previous sample of the ROC into account (Umar et al., 2015). The most appropriate pH (pH 5) was verified by performing coagulation at pH 5-7 for alum and pH 4-7 for ferric chloride. Testing over a range of doses (1-3 mM) for both coagulants showed that 1 mM (i.e., 2.7 and 5.6 mgL Al3+ and Fe3+, respectively) was the most appropriate (Umar et al., 2015). The pH value was adjusted using 1 M H2SO4 or 1 M NaOH, as appropriate.
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ACCEPTED MANUSCRIPT 2.4. Irradiation conditions
Irradiation was conducted using an annular reactor with a centrally mounted UV lamp (Thomson et al., 2002). The working capacity of the reactor was 900 mL. The average
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irradiated area was 464 cm2, and the path length was 1.94 cm. UVC irradiation (253.7 nm) was provided by a 39 W UV lamp (Australian Ultra Violet Services, G36T15NU). The average fluence rate of the lamp was 8.91 mW/cm2, as determined by hydrogen peroxide
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actinometry (Beltrán et al., 1995). After the addition of H2O2 (3 mM), the sample was mixed and aerated by humidified air and irradiated with UV fluence of 16×103 mJ/cm2 based on
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previous results (Umar et al., 2014). Although not shown in this paper, a range of 2–6 mM H2O2 was tested for the ROC sample and 3 mM was selected taking into account chemical consumption and DOC reduction (Umar, 2014) and was used in this study. To enable comparison, the pH of the ROC samples not subjected to coagulation was adjusted to 5 prior
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2.4. Analytical methods
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to UVC/H2O2 treatment.
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All analyses were conducted in duplicate according to Standard Methods (APHA, 2005) and average results are reported. DOC was determined using a TOC analyser (Sievers model 5310C) in in-line mode to purge inorganic carbon. UV absorbance was determined using a double beam scanning UV/vis spectrophotometer (Unicam UV2). Colour was measured with a Hach DR 4000 spectrophotometer at 455 nm in Platinum Cobalt (Pt.Co) units. Fluorescence excitation-emission matrix spectra were determined with a Perkin Elmer LS-50B luminescence spectrometer. All samples were filtered (0.45 µm, PVDF Millipore) before these analyses.
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ACCEPTED MANUSCRIPT The concentration of residual H2O2 was indicated by Merckoquant® peroxide test strips. To avoid the interference of H2O2 in DOC measurement, the residual H2O2 was quenched using the enzyme catalase (from Aspergillus niger, Calbiochem®). Catalase (10 µL of 4000 U/mL 0.05 M phosphate buffer pH 7.0) was added to 25 mL ROC sample followed by 2 h of
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shaking at 100 rpm at room temperature or until the concentration of H2O2 was < 0.5 mg/L.
Molecular size distribution was determined using LC-OCD at the Water Research Centre of
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the University of New South Wales (Sydney, Australia) with a DOC-Labor LC-OCD Model 8, with a Toyopearl TSK HW-50S column, using a phosphate buffer of pH 6.4 as the mobile
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phase.
The analyses for disinfection by-products were undertaken at the Australia Water Quality Centre, Adelaide, Australia. Trihalomethanes (THMs) and THM formation potential
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(THMFP) were analysed by headspace gas chromatography with an electron capture detector (GC-ECD) as described by Buchanan et al. (2006). Haloaldehydes, haloacetonitriles, haloketones and halopicrins were measured by USEPA Method 551 using GC-ECD. For the
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analysis of NDMA, methylene chloride was used for solid phase extraction (SPE). The
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extracts were then analysed using chemical ionization (CI) gas chromatography and mass spectroscopy (GC/MS) running in selected ion monitoring mode (SIM).
Ecotoxicity analysis using the Microtox assay was conducted by ALS Laboratory Group (Scoresby, Victoria). The Microtox® test, which employs the luminescent marine bacterium Vibrio fischeri, was conducted according to the protocol provided with the Microtox 500 Analyzer. Inhibition of cellular activity due to toxicity results in a decrease in the rate of bacterial respiration which corresponds to a decrease in luminescence. A difference in
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ACCEPTED MANUSCRIPT luminescence between the sample and the control is attributed to the effect of the sample on the V. fischeri.
3.
Results
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3.1. Impact of pre-treatment by coagulation on UVC/H2O2 and BAC treatment
DOC reduction after the various treatments is shown in Fig. 1. The reduction of DOC was
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lower for alum than for ferric chloride both for coagulation alone and as pre-treatment to UVC/H2O2 and BAC treatment. After coagulation the reduction of DOC with alum and ferric
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chloride was 21% and 37%, respectively, and increased to 33% and 48% after the sequence of coagulation and UVC/H2O2 treatment.
The flocs formed after alum coagulation were weak and settled poorly compared with those
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for ferric chloride. High salinity affects chemical hydrolysis and metal-hydroxide solubility reactions and as aluminium-based coagulants are more soluble than ferric-based ones under high salinity conditions, the superior performance of ferric chloride could be attributed to its
conditions.
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lower solubility and better floc formation (Edzwald and Haarhoff, 2011) under these
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The reduction of DOC after UVC/H2O2 and downstream BAC treatment was 45% which was effectively equal to the sum of the reductions for UVC/H2O2 (14%) and BAC (30%) treatment. Hence the reduction of DOC was additive for these treatments, although the fractions removed by each process would be different. BAC treatment of the coagulated samples led to a further 18% and 14% reduction of DOC for alum and ferric chloride, respectively. The overall decrease in DOC after the sequence of coagulation, UVC/H2O2 and BAC treatment was 62% for alum and 68% for ferric chloride. The two-tailed t-test for unequal variances
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ACCEPTED MANUSCRIPT was applied to the results for four independent experimental runs, and showed that the p-value was 0.006, i.e., < 0.05, for 95% confidence levels, and thus there was a significant difference in DOC removal between these systems. The decrease in DOC after BAC was 7% lower than the sum
of DOC reductions after the individual treatments when ferric chloride was used. The ferric
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chloride and BAC combination led to a DOC concentration fairly close to the target residual of 15 mg/L, hence it could be an attractive combination depending on the treatment objective.
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Figure 1
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The decreases in A254 and colour followed trends similar to the reduction of DOC (Table 2). Greater A254 reduction was obtained for ferric chloride than alum. The reduction of A254 was the lowest after BAC treatment compared with the other individual treatments. UVC/H2O2 treatment of the coagulated samples improved the reduction of A254 markedly, i.e., 43% and
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29% for alum and ferric chloride, respectively. However, only small reductions (8-9%) of A254 were observed after BAC treatment of the coagulated samples. An appreciable difference (15%) in A254 reduction between the alum and ferric chloride coagulants was
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observed with and without BAC treatment, but reduced to be negligible when UVC/H2O2 treatment was incorporated in the treatment train. The reductions in colour were greater (by
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9-28%) than A254 for similar treatments. UVC/H2O2 treatment markedly improved the UVT, and a further improvement in UVT was observed after UVC/H2O2 treatment of the pretreated samples with only a small difference (6%) between the two coagulants. A marginally greater (5-6%) UVT improvement was observed for ferric chloride than alum after UVC/H2O2+BAC treatment.
Table 2 3.2.
Changes in molecular size 11
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The molecular size of the organic components of ROC after the various treatments were determined by LC-OCD which separates DOC into five different chromatographic fractions: biopolymers (≥20,000 Da), high MW humic and humic-like substances (1000–20,000 Da),
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building blocks (300–500 Da), low MW (LMW) acids and humic substances (<350 Da) and LMW neutrals (<350 Da) (Huber and Frimmel, 1996). Humic and humic-like substances were the major constituents of raw ROC, representing 42% of the DOC, followed by LMW
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neutrals (29%), building blocks (6%) and biopolymers (2%) as shown in Fig. 2a. Coagulation using ferric chloride removed more biopolymers (51%) and humics (63%) than did alum,
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15% and 29%, respectively (Fig. 2a). The decrease in building blocks was however slightly greater for alum (45%) than ferric chloride (36%). Some increase in LMW neutrals was observed for both coagulants (6-10%). UVC/H2O2 caused breakdown of the humics, leading to increased concentration of low MW components (LMW acids and HS) which is consistent
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with previous results. BAC treatment removed biopolymers (27%) and LMW compounds (47%), but was ineffective for removing humic-like compounds, and an increase (20%) in the concentration of building blocks was observed. These may be associated with metabolites
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produced by the microorganisms in the biofilm.
Fig. 2b shows the DOC concentration of various fractions after different combinations of the treatments. Combining UVC/H2O2 and BAC treatment showed an additive effect for the reduction of humics (47%) and LMW neutrals (56%) which was due to the breakdown of humics to LMW compounds which were removed by downstream biological treatment. Comparison of alum and ferric chloride pre-treated ROC after UVC/H2O2 treatment shows greater reduction of biopolymers (83%) and humics (46%) for ferric chloride than for alum,
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ACCEPTED MANUSCRIPT i.e., 70% and 27%, respectively. The concentration of remaining building blocks and LMW neutrals was fairly similar for both coagulants after UVC/H2O2 treatment.
Including BAC in the treatment scheme (coagulation+UVC/H2O2) gave greater removal of
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humics for the sample pre-treated by ferric chloride (69%) than alum (57%). However, the biopolymer content doubled for the alum pre-treated sample whereas it increased only 40% for ferric chloride. Generation of biopolymers in a BAC column after UVC/H2O2 treatment
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has been reported by Lu et al. (2013). The growth of the bacteria in the biofilm leads to
excretion of biopolymers such as extracellular polysaccharides, some of which will pass
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through the BAC column. This is consistent with the increase in biopolymers being observed only after the UVC/H2O2 treatment which led to increased concentration of smaller molecules which were a food source for the bacteria. The removal of building blocks was marginally greater for ferric chloride (52%) than alum (48%), whereas the removal of LMW neutrals
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was marginally greater for the alum (71%) than the ferric chloride (65%) pre-treated ROC.
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Figure 2
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3.3. DBP formation after treatment
Determination of DBPs was carried out for selected samples to investigate the potential health risks of the treated ROC. The concentration of total THMs (chloroform, bromoform, bromodichloromethane (BDCM) and dibromochloromethane (DBCM)) as well as the THM formation potential (THMFP) was determined. As the presence of ammonia and organic nitrogen in wastewater preferentially contribute to the formation of nitrogenous DBPs which were found to be more toxic than the regulated carbonaceous DBPs and hence pose a greater health risk (Krasner et al., 2006), nitrogenous DBPs were also determined. Since NDMA is
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ACCEPTED MANUSCRIPT the major nitrosamine, and others are generally present at markedly lower concentrations, about an order magnitude lower than NDMA (Bond et al., 2011), the formation of NDMA was investigated after the various treatments. The concentrations of a range of DBPs which have greater toxicity than the regulated DBPs were determined after the sequential treatment
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for both coagulants. These were species representing the major haloacetonitriles (HANs, trichloroacetonitrile, dichloroacetonitrile, bromochloroacetonitrile, dibromoacetonitrile), haloaldehydes (chloral hydrate), halopicrin (chloropicrin), dibromonitromethane, and
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haloketones (1,1-dichloroacetone, 1,1,1-trichloroacetone, 1,3-dichloroacetone, 1,3-
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dichloroacetone).
Raw ROC had a total THM of 16 µg/L, comprised of 1 µg/L BDCM, 7 µg/L for each of bromoform and chloroform, and 2 µg/L DBCM. UVC/H2O2 treatment removed all but chloroform (decreased to 4 µg/L) and thus reduced the total THM concentration to 4 µg/L.
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Consequently no THMs were detected after sequential coagulation, UVC/H2O2 and BAC treatment for either coagulant.
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An increase in the formation potential of bromoform and total THMs occurred after
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UVC/H2O2+BAC treatment whereas a decrease was noted when coagulation (using alum) was used as pre-treatment (Fig. 3).
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The NDMA concentration in raw ROC was 43 ng/L and was reduced to 6 ng/L after standalone UVC/H2O2 treatment. Similarly, the concentration of NDMA was markedly lower after UVC/H2O2 followed by BAC treatment with and without coagulation pre-treatment. A small
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ACCEPTED MANUSCRIPT increase in the concentration of NDMA was noted when BAC was used after UVC/H2O2 treatment with and without coagulation which may have been due to analytical error.
The concentrations of haloacetonitriles (HANs), chloral hydrate, chloropicrin,
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dibromonitromethane, 1,1-dichloroacetone, 1,1,1-trichloroacetone, 1,3-dichloroacetone and 1,3-dichloroacetone were below the detection limit in raw ROC and none was observed after
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treatment.
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3.4. Ecotoxicity
As partial oxidation of organic matter during UVC/H2O2 treatment could lead to the formation of toxic by-products, an assessment of the ecotoxicity of the ROC was made after UVC/H2O2 with and without BAC treatment and after sequential coagulation, UVC/H2O2 and
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BAC treatment. The Microtox® assay indicated that the raw ROC was non-toxic, but showed toxicity (EC50 value of 13% which is considered extremely toxic) after stand-alone UVC/H2O2 treatment (UV fluence of 16 ×103 mJ/cm2). However, no toxicity was noted after
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BAC treatment of the UVC/H2O2 treated ROC, indicating the removal of toxic by-products formed during the UVC/H2O2 treatment. Similarly, no toxicity was noted after sequential
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coagulation (using alum or ferric chloride), UVC/H2O2 and BAC treatment.
3.5. Discussion
Although ferric chloride gave a markedly greater DOC reduction as a stand-alone treatment, when followed by BAC or UVC/H2O2 treatment, overall reduction after the sequence of coagulation, UVC/H2O2 and BAC treatment was only marginally greater for ferric chloride
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ACCEPTED MANUSCRIPT than alum (68% for ferric chloride cf. 62% for alum). This was due to the greater DOC reduction by the UVC/H2O2+BAC steps for the alum pre-treated ROC. As the concentration of remaining organic matter was higher after alum than ferric chloride coagulation, subsequent UVC/H2O2 treatment led to a proportionally greater generation of biodegradable
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organic matter that was effectively removed during downstream BAC treatment. The
presence of ferric ion at pH 5 during coagulation with ferric chloride may have led to some removal of DOC due to the Fenton reaction and thus to greater DOC removal than for
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coagulation with alum. The extent of this reaction would likely be small since the optimum pH is 3-4. However, since there was similar removal of DOC in the UV/H2O2 treatment step
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for the ROC coagulated by ferric chloride and by alum (Fig. 1) it is clear that if the photoFenton reaction occurred, it did not lead to a net increase in DOC reduction after both the AOP and the subsequent BAC process. Furthermore, if this reaction did take place, it did not lead to a net enhancement in the biodegradability of the ROC, since the difference in
process.
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DOC reduction between the two coagulants decreased from 15-16% to 6% after the BAC
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As the flocs formed during alum coagulation were weak and needed long settling times
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and/or use of a flocculant aid to improve settling, and ferric chloride is expensive and corrosive in nature, a trade-off would need to be made with regard to cost and chemical use in relation to the desired treatment objectives.
LC-OCD results confirmed that ferric chloride removed a greater proportion of humic-like matter and LMW acids than alum. Enhanced reduction of organic matter after sequential coagulation and UVC/H2O2 treatment of ROC was attributed to the enhanced breakdown of humic-like matter after UVC/H2O2 treatment of the coagulated ROC. BAC treatment mainly
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ACCEPTED MANUSCRIPT removed LMW compounds and these results were consistent with the low A254 but high reduction of DOC after BAC treatment, implying that biodegradation was the predominant mechanism during BAC treatment.
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Similar to the results reported in our previous study (Umar et al., 2014), coagulation
predominantly removed humic-like matter, leading to a significant increase in the UVT. This then led to enhanced breakdown of some of the remaining large MW organic matter to LMW
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biodegradable products during UVC/H2O2 treatment which were then removed by the BAC treatment. DOC residuals of 14 mg/L and 12 mg/L (below the target residual DOC of 15
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mg/L) were only achieved after the sequence of coagulation, UVC/H2O2 and BAC treatment, with alum and ferric chloride, respectively.
The raw ROC had a markedly lower concentration of total THMs than the Australian
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Drinking Water Guideline (ADWG) value of 250 µg/L (ADWG, 2011) and the USEPA guideline value of 80 µg/L (USEPA, 1998). The UVC/H2O2 treatment led to the partial oxidation of THM precursors (humic-like matter) (Kleiser and Frimmel, 2000) as indicated
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by the decrease in the concentration of total THMs. The removal of partially oxidised organic
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matter after biological treatment led to further reduction in total THMs so that they were below the detection limit after the sequence of UVC/H2O2 and BAC treatment. However an increase in the THMFP was noted after UVC/H2O2 (16 ×103 mJ/cm2, 3 mM H2O2) and BAC which was attributed to the insertion of phenol groups in the aromatic structures of organic matter as a result of partial oxidation, which increased the reactivity of such compounds with chlorine (Kleiser and Frimmel, 2000). Kleiser and Frimmel reported an increase of 20% in the formation potential of THMs after UV/H2O2 treatment of surface water. Similarly, an increase in THMFP was reported following UV-based treatments of a ROC by Liu et al.
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due to partial oxidation of the organic content as described by Kleiser and Frimmel. The
NDMA concentration in the raw ROC was 43 ng/L, which is well below the ADWG of 100 ng/L (ADWG, 2011). The reduction in the NDMA concentration after UVC/H2O2 treatment
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was in agreement with literature reporting the great efficacy of UVC/H2O2 treatment for this compound (Mitch et al., 2003) which is due to the oxidation of organic compounds with
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dimethylamine or trimethylamine groups (Lee et al., 2007). Other DBPs (HANs, haloaldehydes, halopicrins, and haloketones) were all below the detection limit for both raw and treated ROC. The final concentration of DBPs compared favourably with those in the AWDG and the USEPA guidelines for drinking water and hence the treated ROC was
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deemed safe for reuse or discharge to the environment.
The increase in ecotoxicity after stand-alone UVC/H2O2 treatment differed from the earlier
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findings of Liu et al. (2012) who reported no toxicity after 30 min UVC/H2O2 treatment (UV
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fluence of 23 ×103 mJ/cm2) of a municipal ROC prepared in the laboratory. This difference may be attributed to several factors including the lower UV fluence, different nature of the organic content, and the presence of antiscalant and biocide in the ROC sample used in this study which could have been broken down to more toxic by-products. Downstream BAC treatment is therefore useful not only in terms of enhanced organic content removal, but also in removing potentially toxic by-products formed during UVC/H2O2 treatment.
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Reduction in EED by integrating pre- and post-treatment with the UVC/H2O2 process
Electrical energy dose (EED), the electrical energy required to reduce the concentration of
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DOC during the UVC/H2O2 treatment (kWh/m3) was calculated for various treatments. Generally, the energy needed for the production of H2O2 is not included in the EED
calculation which is most likely due to the EED being calculated at optimum H2O2 dosage.
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However, it must be considered, particularly when high concentrations of H2O2 are used. An average energy requirement of 10 kWh/kg for H2O2 production was assumed according to
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Rosenfeldt et al. (2006). As expected, the EED value to obtain a target residual DOC of 15 mg/L (60% reduction) was high for stand-alone UVC/H2O2 treatment due to low DOC mineralization (Fig. 4). The coagulation and BAC processes contributed significantly to DOC removal thus leading to a marked decrease in the EED for the sequential process. The largest
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decrease in EED occurred when both coagulation and BAC were used to obtain DOC reductions almost 4 times greater than stand-alone UVC/H2O2 treatment.
4. Conclusions
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Figure 4
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The sequence of coagulation, UVC/H2O2 and BAC treatment was demonstrated to be a potential process for the removal of organic matter from a high salinity municipal ROC to meet the target of residual DOC of 15 mg/L. Removal of humic-like matter by coagulation facilitated subsequent UVC/H2O2 breakdown of some of the remaining large MW compounds to generate LMW compounds that were readily removed by downstream BAC treatment. The sequential treatment enabled more than meeting the target residual DOC of 15 mg/L. Ferric chloride removed more than twice the humic-like matter than alum leading to a large
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A reduction in the THMs and NDMA levels was observed after UVC/H2O2 treatment but
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some increase in the formation potential of bromoform, DBCM and total THMs was noted after UVC/H2O2+BAC treatment, whereas a decrease was noted when coagulation was used as pre-treatment. Other DBPs, including haloaldehydes, halopicrins, and haloketones were
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below the detection limit in raw and treated ROC. The concentration of DBPs compared
favourably with Australian and USEPA drinking water guidelines, and therefore the treated
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ROC was safe for reuse or environmental discharge. An increase in the ecotoxicity was observed after UVC/H2O2 treatment but disappeared after UVC/H2O2+BAC treatment with and without coagulation.
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A large reduction (85-87%) in EED occurred when coagulation and BAC treatments were integrated with UVC/H2O2 treatment; ferric chloride was 18% more energy efficient than alum for a marginally greater (6%) reduction in DOC. Either of these coagulants could be
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utilised after taking the trade-offs and desired treatment objectives into account.
Acknowledgements:
The authors would like to thank the Commonwealth Government of Australia for providing Endeavour IPRS and APA scholarships, City West Water, Water Research Australia (Project #4054/12) and the Smart Water Fund (project #80S-8010) for providing support for this project.
References:
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APHA 2005. Standard Methods for the Examination of Water & Wastewater, American Public Health Association, Washington D.C. ADWG, 2011. Australian Drinking Water Guidelines NHMRC: Canberra.
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Bond, T., Huang, J., Templeton, M.R., Graham, N. 2011. Occurrence and control of nitrogenous disinfection by-products in drinking water: A review. Water Research, 45, 4341-4354.
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Beltrán, F.J., Ovejero, G., García-araya, J.F., Rivas, J. 1995. Oxidation of polynuclear aromatic hydrocarbons in water. 2. UV radiation and ozonation in the presence of UV radiation.
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Carlson, G., Silverstein, J. 1998. Effect of molecular size and charge on biofilm sorption of organic matter. Water Research, 32, 1580-1592. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K. 2003. Fluorescence excitation emission
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Edzwald, J.K. Haarhoff, J. 2011. Seawater pre-treatment for reverse osmosis: chemistry, contaminants, and coagulation, Water Research, 45, 5428–5440.
Huber, S.A., Frimmel, F.H. 1996. Size-exclusion chromatography with organic carbon detection (LC-OCD): a fast and reliable method for the characterization of hydrophilic organic carbon. Vom Wasser, 86, 277–290. Justo, O., González, J., Acena, S., Pérez, D., Barceló, C., Sans, S., Esplugas, S. 2013. Pharmaceuticals and organic pollution mitigation in reclamation osmosis brines by UV/H2O2 and ozone. Journal of Hazardous Materials, 263, 268–274. 21
ACCEPTED MANUSCRIPT Hurwitz, G., Hoek, E.M.V., Liu, K., Fan, F., Roddick, F.A. 2014. Photo-assisted electrochemical treatment of municipal wastewater reverse osmosis concentrate. Chemical Engineering Journal, 249, 180-188. Khan, J., Murchland, D, Rhodes, M, Waite, T. D. 2009. Management of concentrated
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waste streams from high-pressure membrane water treatment systems. Critical Reviews in Environmental Science and Technology, 39, 367-415.
Kleiser, G., Frimmel, F.H. 2000. Removal of precursors for disinfection by-products (DBPs)
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- differences between ozone- and OH-radical-induced oxidation. Science of the Total Environment, 256, 1-9.
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Krasner, S.W., Weinberg, H.S., Richardson, S.D., Pastor, S.J., Chinn, R., Sclimenti, M.J., Onstad, G.D., Thruston Jr. A.D. 2006. Occurrence of a new generation of disinfection byproducts. Environmental Science and Technology, 40, 7175–7185. Lee, C., Schmidt, C., Yoon, J., von Gunten, U. 2007. Oxidation of N-nitrosodimethylamine
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(NDMA) precursors with ozone and chlorine dioxide: kinetics and effect on NDMA formation potential. Environmental Science and Technology, 41, 2056−2063. Lee, L.Y., Ng, H.Y., Ong, S.L., Hu, J.Y., Tao, G., Kekre, K., Viswanath, B., Lay, W., Seah,
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H., 2009. Ozone-biological activated carbon as a pretreatment process for reverse
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osmosis brine treatment and recovery. Water Research, 43, 3948-3955. Linden, K.G., Rosenfeldt, E.J., Kullman, S.W. 2007. UV/H2O2 degradation of endocrinedisrupting chemicals in water evaluated via toxicity assays. Water Science and Technology, 55, 313-319.
Liu, K., Roddick, F.A., Fan, L. 2012. Impact of salinity and pH on the UVC/H2O2 treatment of reverse osmosis concentrate produced from municipal wastewater reclamation. Water Research, 46, 3229-3239.
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ACCEPTED MANUSCRIPT Lu, J., Fan, L., Roddick, F.A. 2013. Potential of BAC combined with UVC/H2O2 for reducing organic matter from highly saline reverse osmosis concentrate produced from municipal wastewater reclamation. Chemosphere, 93, 683-688. Mitch, W.A., Gerecke, A.C., Sedlak, D.L. 2003. A N-Nitrosodimethylamine (NDMA)
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precursor analysis for chlorination of water and wastewater. Water Research, 37, 3733-3741.
Pérez, G., Fernández-Alba, A.R., Urtiaga, A.M. & Ortiz, I. 2010. Electro-oxidation of reverse
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osmosis concentrates generated in tertiary water treatment. Water Research, 44, 27632772.
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Rice, R.G., Robson, C.M. 1982. Biological Activated Carbon: Enhanced Aerobic Biological Activity in GAC Systems, Ann Arbor Science Publisher, Ann Arbor, MI. Rodman, C.A., Shunney, E.L., Perrotti, A.E. 1978. Biological regeneration of activated carbon. In: Cheremisinoff, P.N., Ellerbusch, F. (eds.) Carbon Adsorption Handbook,
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Ann Arbor Science Publication, Ann Arbor, MI.
Rosenfeldt, E.J., Linden, K.G., Canonicaa, S., von Gunten, U. 2006. Comparison of the efficiency of •OH radical formation during ozonation and the advanced oxidation
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processes O3/H2O2 and UV/H2O2. Water Research, 3695–3704. Sarathy, S., Mohseni, M. 2010. Effects of UV/H2O2 advanced oxidation on chemical
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characteristics and chlorine reactivity of surface water natural organic matter. Water Research, 44, 4087-4096.
Takeuchi, Y., Mochidzuki, K., Matsunobu, N., Kojima, R., Motohashi, H., Yoshiimoto, S. 1997. Removal of organic substances from water by ozone treatment followed by biological activated carbon treatment. Water Science and Technology, 35, 171-178. Thomson, J., Roddick, F.A., Drikas, M. 2002. Natural organic matter removal by enhanced photo-oxidation using low pressure mercury vapour lamps, Water Science and Technology: Water Supply, 2, 435–443. 23
ACCEPTED MANUSCRIPT Thomson, J., Roddick, F.A., Drikas, M. 2004. Vacuum ultraviolet irradiation for natural organic matter removal. Journal of Water Supply: Research and Technology, AQUA, 53, 193-206. Umar, M. 2014. Treatment of Municipal Wastewater Reverse Osmosis Concentrate Using
and Chemical Engineering, RMIT University, Australia.
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Oxidation by UV-Mediated Processes, PhD thesis, School of Civil, Environmental
Umar, M., Roddick, F.A., Fan, L. (2014). Effect of coagulation on treatment of municipal
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wastewater reverse osmosis concentrate by UVC/H2O2. Journal of Hazardous Materials, 266, 10-18.
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Umar, M., Roddick, F.A., Fan, L. 2015. Comparison of coagulation efficiency of aluminium and ferric-based coagulants as pre-treatment for UVC/H2O2 treatment. Chemical Engineering Journal, doi:10.1016/j.cej.2015.08.109.
USEPA, 1998. Stage 1 disinfectants and disinfection byproducts: final rule. Federal Register,
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63 (241), 69389-69-476.
van der Bruggen, B., Lejon, L., Vandecasteele, C. 2003. Reuse, treatment, and discharge of the concentrate of pressure-driven membrane processes. Environmental Science and
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Technology, 37, 3733-3738.
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Westerhoff, P., Moon, H., Minakata, D., Crittenden, J. 2009. Oxidation of organics in retentates from reverse osmosis wastewater reuse facilities. Water Research, 43,
3992–3998.
Ying, W.C., Weber Jr., W.J. 1979. Bio-physicochemical adsorption model systems for wastewater treatment. Journal of the Water Pollution Control Federation, 51, 26612677.
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oxidation processes with/without pretreatment. Chemical Engineering Journal, 166, 932–939.
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Tables
Value
DOC (mg/L)
37
COD (mg/L)
105
pH
7.7
Colour (Pt.Co units)
156
Chloride (mg/L)
8,212
TDS (mg/L)
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Parameter
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Table 1 Characteristics of ROC
17,245 0.65
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A254 (/cm)
1.75
Alkalinity (as CaCO3, mg/L)
418
Conductivity (mS/cm)
23.5
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SUVA (L/mg.m)
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Table 2 Reduction of colour, A254 and improvement of UVT after various treatments (UVH denotes UV/H2O2, Al and Fe denote coagulation by alum and ferric chloride, respectively, values shown ± 1 SD)
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Colour
A254
UVH
86 ± 0.8
61 ± 1.1
57 ± 1.0
Al
65 ± 0.2
39 ± 0.1
40 ±1.3
Fe
82 ± 1.0
54 ± 1.1
50 ± 0.9
BAC only
29 ± 3.2
20 ± 3.2
31 ± 2.2
Al+BAC
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Treatment
75 ± 1.4
47 ± 1.1
45 ±2.3
86 ± 1.6
62 ± 1.9
58 ± 1.8
92 ± 2.8
74 ± 2.3
66 ± 3.0
94 ± 0.9
82 ± 1.4
67 ± 1.1
Fe+UVH
95 ± 0.5
83 ± 1.6
73 ± 1.1
Al+UVH+BAC
97 ± 2.7
85 ± 2.8
79 ± 2.4
Fe+UVH+BAC
97 ± 3.3
85 ± 2.4
85 ± 1.9
Fe+BAC UVH+BAC
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Al+UVH
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initial UVT was 22%
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*
% increase
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% reduction
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Figure Captions and Figures Figure 1 Comparison of DOC reduction after individual and combined treatments
samples)
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(UVH refers to UVC/H2O2, error bars represent ±1 standard deviation measured for 3
of fractions after various combined treatments
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Figure 2 (a) LC-OCD chromatograms after individual treatments, (b) DOC concentration
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Figure 3 Formation potential of individual and total THMs after various treatments
Figure 4 EED and corresponding DOC reduction for each treatment (line represents the
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% DOC reduction corresponding to the target residual DOC of 15 mg/L)
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100 90
70
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% reduction
80
60 50 40 30
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20 10
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0
Figure 1 Comparison of DOC reduction after individual and combined treatments (UVH refers to UVC/H2O2, error bars represent ±1 standard deviation measured for 3
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samples)
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(a)
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(b)
Figure 2 (a) LC-OCD chromatograms after individual treatments, (b) DOC concentration of fractions after various combined treatments
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Figure 3 Formation potential of individual and total THMs after various treatments
80
350
70
300
60
250
50
200 150 100
0 UVH
UVH+BAC
Al+UVH
Fe+UVH
30 20 10 0
Al+UVH+BAC Fe+UVH+BAC
% DOC reduction
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EED (kWh/m3)
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50
40
Figure 4 EED and corresponding DOC reduction for each treatment (line represents the
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% DOC reduction corresponding to the target residual DOC of 15 mg/L)
% DOC reduction
400
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EED (kWh/m3)
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ACCEPTED MANUSCRIPT Highlights
Coagulation+UV/H2O2+BAC treatment removed DOC from reverse osmosis concentrate Coagulation with ferric chloride gave greater DOC removal than with alum
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Ferric chloride coagulation plus BAC treatment is a possible option
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Decreased THMFP, NDMA and ecotoxicity levels were obtained after the treatment
S0043-1354(15)30259-1
DOI:
10.1016/j.watres.2015.09.047
Reference:
WR 11555
To appear in:
Water Research
Received Date: 10 May 2015 Revised Date:
21 September 2015
Accepted Date: 28 September 2015
Please cite this article as: Umar, M., Roddick, F., Fan, L., Impact of coagulation as a pre-treatment for UVC/H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis concentrate, Water Research (2015), doi: 10.1016/j.watres.2015.09.047. 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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Impact of coagulation as a pre-treatment for UVC/H2O2-biological activated carbon treatment of a municipal wastewater reverse osmosis
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concentrate
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Muhammad Umar, Felicity Roddick*, Linhua Fan
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School of Civil, Environmental and Chemical Engineering, RMIT University,
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GPO Box 2476, Melbourne, 3001 Victoria, Australia
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Submitted to Water Research
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Corresponding author: Felicity Roddick
School of Civil, Environmental and Chemical Engineering RMIT University Email: [email protected] Tel: +613 9925 2080
Fax: +613 9639 0138
1
ACCEPTED MANUSCRIPT Abstract
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After coagulation of high salinity reverse osmosis concentrate (ROC) with either alum or
3
ferric chloride followed by UVC/H2O2 treatment, biological activated carbon (BAC) was
4
investigated for the removal of DOC. BAC treatment mainly removed low molecular weight
5
(LMW) neutral molecules indicating that biodegradation was the predominant mechanism of
6
organic matter removal. Coagulation with ferric chloride gave greater DOC reductions than
7
alum both as a stand-alone treatment and after the sequence of UVC/H2O2 and BAC
8
treatment. However, overall reduction after the sequence of coagulation, UVC/H2O2 and
9
BAC treatment was only marginally greater for ferric chloride (68%) than for alum (62%).
10
Trihalomethane formation potential and N-Nitrosodimethylamine concentration decreased
11
markedly after UVC/H2O2 treatment. UVC/H2O2 treatment of the ROC led to the generation
12
of extreme toxicity according to the Microtox assay, but no toxicity was observed after BAC,
13
demonstrating its advantage for enabling safe disposal of the treated ROC. Implementation of
14
coagulation as a pre-treatment and BAC as a post-treatment markedly reduced (6-8 times) the
15
electrical energy dose (EED) required for the UVC/H2O2 process. The sequence of
16
coagulation, UVC/H2O2 and BAC treatment was demonstrated as a potential process for the
17
removal of organic matter from high salinity municipal ROC.
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Keywords: Biological activated carbon; coagulation; UVC/H2O2; reverse osmosis
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concentrate; disinfection by-products
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Introduction
As reverse osmosis (RO) is widely used as a polishing treatment for secondary effluent in wastewater reclamation schemes, safe disposal or reuse of the resultant RO concentrate
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(ROC) is becoming increasingly important. The ROC comprises 15-20% of the volume of the feed stream and contains almost all the contaminants present in the original wastewater at elevated levels. These contaminants may be toxic and/or bio-accumulative, and so disposal of
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untreated ROC presents a potential environmental risk (Westerhoff et al., 2009). The addition of chemicals such as antiscalants, biocides and acids during the treatment process can change
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the character of the organic and inorganic compounds and can influence the chemical equilibrium of the dissolved constituents (van der Bruggen et al., 2003). Conventional practice has been to dispose of the ROC by direct discharge to surface water, sewer, evaporation ponds, and deep well injection (Lee et al., 2009), of these direct discharge and
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sewer disposal are the most widely used options (Khan et al., 2009). Treatment of ROC to decrease the overall DOC content and to remove or inactivate the toxic and bio-accumulative
safe disposal.
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contaminants is therefore desirable to increase the overall water recovery as well to enable its
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Advanced oxidation processes (AOPs) have been demonstrated to be one of the preferred treatments for ROC as they are effective for oxidising the organic matter and so improving the biodegradability of the organic matter which can then be removed by biological treatment (Westerhoff et al., 2009; Liu et al., 2012; Umar et al., 2014). UV/H2O2 (Justo et al., 2013), electrochemical oxidation (Pérez et al. 2010; Bagastyo et al., 2012), and ozonation (Lee et al., 2009; Zhou et al., 2011; Justo et al., 2013) are among the most investigated AOPs for the treatment of ROC. UVC/H2O2 is one of the most commonly used AOPs for the treatment of
3
ACCEPTED MANUSCRIPT water and wastewater and is leading the way in commercial applications (Sarathy and Mohseni, 2010). It utilizes the highly oxidizing hydroxyl radical which non-selectively oxidises organic compounds, unlike ozone. Although electrochemical oxidation also utilizes radicals, in the presence of chloride ion, as in the present ROC, high concentrations of
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chlorinated DBPs may be produced (Pérez et al. 2010; Hurwitz et al., 2014) and chlorine gas may be generated. UV/H2O2 has been shown to oxidise and so remove ecotoxic contaminants such as endocrine disrupting chemicals and pharmaceuticals (Linden et al., 2007; Justo et al.,
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2013). However, a sequence of complementary processes may be required to ensure
sufficient removal of the highly heterogeneous organic content of ROC, and appropriate
effective and hence more economic.
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process selection can reduce the load on the subsequent process, thus making it more
As AOPs lead to increased biodegradability of the oxidised compounds, they are generally
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followed by biological treatment to further remove the DOC and to reduce the potential for microbial growth in subsequent processes or applications. BAC is the usual biological treatment after AOPs to enhance the removal of organic matter and improve the cost
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effectiveness (Toor and Mohseni, 2007). BAC treatment after the sequence of coagulation and UVC/H2O2 treatment may improve the removal of organic matter as it can effectively
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remove the low molecular weight breakdown products from the AOP (Takeuchi et al., 1997).
The simultaneous adsorption and biodegradation of organic matter during BAC filtration means that the activated carbon can be partially regenerated while the carbon bed is in operation (Rodman et al., 1978; Rice and Robson, 1982) and more recalcitrant organic matter can be removed by sorption onto the biofilm, and then slowly degraded by microorganisms (Rice and Robson, 1982; Carlson and Silverstein, 1998). As noted by Ying and Weber (1979), simultaneous adsorption and biodegradation of organic matter in a single reactor 4
ACCEPTED MANUSCRIPT eliminates the need for individual processes resulting in low capital cost, and as BAC requires less frequent regeneration of the carbon it has lower energy requirements and operating cost.
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For municipal wastewater ROC we have shown that coagulation is effective for the removal of high molecular weight compounds thus facilitating subsequent UVC/H2O2 treatment due to reduction in colour and increase in UV transmissivity (UVT). This results in enhanced
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biodegradability (as measured by biodegradable dissolved organic carbon assay) of the
remaining organics and thus reduces the overall energy consumption required for the removal
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of the organic matter (Umar et al., 2014).
Although some studies have utilised BAC as a post-treatment of AOP-treated ROC (Lee at al., 2009; Lu et al., 2013), no investigation of the impact of coagulation with and without
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intermediate UVC/H2O2 treatment on subsequent BAC treatment has been reported.
Therefore, the aim of this work was to compare two of the most commonly used coagulants,
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alum and ferric chloride, for the treatment of a high salinity ROC prior to UVC/H2O2 and
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BAC treatment. Alum was selected due to its low cost and non-corrosive nature, ferric chloride was selected as it is considered as the most feasible coagulant for high ionic strength water (Edzwald and Haarhoff, 2011). Furthermore, these coagulants had been shown to be effective for another high salinity ROC sample (Umar et al., 2015). The presence and formation of various disinfection by-products (DBPs) and the ecotoxicity of selected samples was investigated. A target residual DOC value of 15 mg/L was selected as this is close to values in secondary effluent, enabled comparison of the energy efficiency of the different treatments, and was acceptable to the water treatment plant. To the best of our knowledge,
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2.
Materials and Methodology
2.1.
Collection and characterization of ROC
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ecotoxicity.
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The ROC sample was collected from a wastewater reclamation facility at a local municipal wastewater treatment plant (WWTP), and stored at 4oC. In the treatment process at the
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WWTP, raw sewage is screened and de-gritted and sent to intermittently decanted extended aeration (IDEA) bioreactors where it is treated in a cycle of aeration, settling, and decant. The IDEA effluent is then treated using a combination of ultrafiltration and RO. The characteristics of the ROC sample, which was high in alkalinity and salinity, are given in
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Table 1.
Table 1
BAC treatment and reactor set-up
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2.2.
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Two BAC columns were operated as continuous down flow reactors with 60 min empty bed contact time (EBCT). The columns were packed with thoroughly pre-washed GAC (Acticarb BAC - GS1300 obtained from Activated Carbon Technologies, Victoria, Australia) to a bed height of 17 cm. The GAC had a Brunauer-Emmett-Teller (BET) surface area of 1200 m2/g, is manufactured for maximum biological activity and is normally used for BAC filters. A peristaltic pump was used to feed the ROC sample to the columns at 60 mL/h. An air gap was maintained at the top of the column to allow contact with oxygen. The BAC bed was
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with (pH 7-8) and without pH (pH 5.5) adjustment after the UVC/H2O2 treatment. The difference in the reduction of DOC, colour and A254 was insignificant and so no pH
adjustment was made before BAC treatment. Residual H2O2 was not removed prior to the
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BAC treatment as it was shown to not affect the DOC removal.
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2.3. Coagulation
Aluminium and ferric stock solutions were prepared using alum (Al2(SO4)3.18H2O) and ferric chloride (FeCl3.6H2O), respectively, (Chem-Supply Pty Ltd, Australia). Coagulation
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was carried out using a laboratory jar test apparatus (Phipps and Bird, PB-700) using 2 L samples. The samples were rapidly mixed for 2 min at 250 rpm followed by slow mixing for 30 min at 30 rpm and subsequent settling for 2 h before taking the supernatant for UVC/H2O2
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treatment. Coagulation (1 mM metal dose) was performed at pH 5 taking the results for a
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previous sample of the ROC into account (Umar et al., 2015). The most appropriate pH (pH 5) was verified by performing coagulation at pH 5-7 for alum and pH 4-7 for ferric chloride. Testing over a range of doses (1-3 mM) for both coagulants showed that 1 mM (i.e., 2.7 and 5.6 mgL Al3+ and Fe3+, respectively) was the most appropriate (Umar et al., 2015). The pH value was adjusted using 1 M H2SO4 or 1 M NaOH, as appropriate.
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Irradiation was conducted using an annular reactor with a centrally mounted UV lamp (Thomson et al., 2002). The working capacity of the reactor was 900 mL. The average
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irradiated area was 464 cm2, and the path length was 1.94 cm. UVC irradiation (253.7 nm) was provided by a 39 W UV lamp (Australian Ultra Violet Services, G36T15NU). The average fluence rate of the lamp was 8.91 mW/cm2, as determined by hydrogen peroxide
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actinometry (Beltrán et al., 1995). After the addition of H2O2 (3 mM), the sample was mixed and aerated by humidified air and irradiated with UV fluence of 16×103 mJ/cm2 based on
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previous results (Umar et al., 2014). Although not shown in this paper, a range of 2–6 mM H2O2 was tested for the ROC sample and 3 mM was selected taking into account chemical consumption and DOC reduction (Umar, 2014) and was used in this study. To enable comparison, the pH of the ROC samples not subjected to coagulation was adjusted to 5 prior
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2.4. Analytical methods
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to UVC/H2O2 treatment.
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All analyses were conducted in duplicate according to Standard Methods (APHA, 2005) and average results are reported. DOC was determined using a TOC analyser (Sievers model 5310C) in in-line mode to purge inorganic carbon. UV absorbance was determined using a double beam scanning UV/vis spectrophotometer (Unicam UV2). Colour was measured with a Hach DR 4000 spectrophotometer at 455 nm in Platinum Cobalt (Pt.Co) units. Fluorescence excitation-emission matrix spectra were determined with a Perkin Elmer LS-50B luminescence spectrometer. All samples were filtered (0.45 µm, PVDF Millipore) before these analyses.
8
ACCEPTED MANUSCRIPT The concentration of residual H2O2 was indicated by Merckoquant® peroxide test strips. To avoid the interference of H2O2 in DOC measurement, the residual H2O2 was quenched using the enzyme catalase (from Aspergillus niger, Calbiochem®). Catalase (10 µL of 4000 U/mL 0.05 M phosphate buffer pH 7.0) was added to 25 mL ROC sample followed by 2 h of
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shaking at 100 rpm at room temperature or until the concentration of H2O2 was < 0.5 mg/L.
Molecular size distribution was determined using LC-OCD at the Water Research Centre of
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the University of New South Wales (Sydney, Australia) with a DOC-Labor LC-OCD Model 8, with a Toyopearl TSK HW-50S column, using a phosphate buffer of pH 6.4 as the mobile
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phase.
The analyses for disinfection by-products were undertaken at the Australia Water Quality Centre, Adelaide, Australia. Trihalomethanes (THMs) and THM formation potential
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(THMFP) were analysed by headspace gas chromatography with an electron capture detector (GC-ECD) as described by Buchanan et al. (2006). Haloaldehydes, haloacetonitriles, haloketones and halopicrins were measured by USEPA Method 551 using GC-ECD. For the
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analysis of NDMA, methylene chloride was used for solid phase extraction (SPE). The
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extracts were then analysed using chemical ionization (CI) gas chromatography and mass spectroscopy (GC/MS) running in selected ion monitoring mode (SIM).
Ecotoxicity analysis using the Microtox assay was conducted by ALS Laboratory Group (Scoresby, Victoria). The Microtox® test, which employs the luminescent marine bacterium Vibrio fischeri, was conducted according to the protocol provided with the Microtox 500 Analyzer. Inhibition of cellular activity due to toxicity results in a decrease in the rate of bacterial respiration which corresponds to a decrease in luminescence. A difference in
9
ACCEPTED MANUSCRIPT luminescence between the sample and the control is attributed to the effect of the sample on the V. fischeri.
3.
Results
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3.1. Impact of pre-treatment by coagulation on UVC/H2O2 and BAC treatment
DOC reduction after the various treatments is shown in Fig. 1. The reduction of DOC was
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lower for alum than for ferric chloride both for coagulation alone and as pre-treatment to UVC/H2O2 and BAC treatment. After coagulation the reduction of DOC with alum and ferric
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chloride was 21% and 37%, respectively, and increased to 33% and 48% after the sequence of coagulation and UVC/H2O2 treatment.
The flocs formed after alum coagulation were weak and settled poorly compared with those
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for ferric chloride. High salinity affects chemical hydrolysis and metal-hydroxide solubility reactions and as aluminium-based coagulants are more soluble than ferric-based ones under high salinity conditions, the superior performance of ferric chloride could be attributed to its
conditions.
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lower solubility and better floc formation (Edzwald and Haarhoff, 2011) under these
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The reduction of DOC after UVC/H2O2 and downstream BAC treatment was 45% which was effectively equal to the sum of the reductions for UVC/H2O2 (14%) and BAC (30%) treatment. Hence the reduction of DOC was additive for these treatments, although the fractions removed by each process would be different. BAC treatment of the coagulated samples led to a further 18% and 14% reduction of DOC for alum and ferric chloride, respectively. The overall decrease in DOC after the sequence of coagulation, UVC/H2O2 and BAC treatment was 62% for alum and 68% for ferric chloride. The two-tailed t-test for unequal variances
10
ACCEPTED MANUSCRIPT was applied to the results for four independent experimental runs, and showed that the p-value was 0.006, i.e., < 0.05, for 95% confidence levels, and thus there was a significant difference in DOC removal between these systems. The decrease in DOC after BAC was 7% lower than the sum
of DOC reductions after the individual treatments when ferric chloride was used. The ferric
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chloride and BAC combination led to a DOC concentration fairly close to the target residual of 15 mg/L, hence it could be an attractive combination depending on the treatment objective.
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Figure 1
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The decreases in A254 and colour followed trends similar to the reduction of DOC (Table 2). Greater A254 reduction was obtained for ferric chloride than alum. The reduction of A254 was the lowest after BAC treatment compared with the other individual treatments. UVC/H2O2 treatment of the coagulated samples improved the reduction of A254 markedly, i.e., 43% and
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29% for alum and ferric chloride, respectively. However, only small reductions (8-9%) of A254 were observed after BAC treatment of the coagulated samples. An appreciable difference (15%) in A254 reduction between the alum and ferric chloride coagulants was
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observed with and without BAC treatment, but reduced to be negligible when UVC/H2O2 treatment was incorporated in the treatment train. The reductions in colour were greater (by
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9-28%) than A254 for similar treatments. UVC/H2O2 treatment markedly improved the UVT, and a further improvement in UVT was observed after UVC/H2O2 treatment of the pretreated samples with only a small difference (6%) between the two coagulants. A marginally greater (5-6%) UVT improvement was observed for ferric chloride than alum after UVC/H2O2+BAC treatment.
Table 2 3.2.
Changes in molecular size 11
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The molecular size of the organic components of ROC after the various treatments were determined by LC-OCD which separates DOC into five different chromatographic fractions: biopolymers (≥20,000 Da), high MW humic and humic-like substances (1000–20,000 Da),
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building blocks (300–500 Da), low MW (LMW) acids and humic substances (<350 Da) and LMW neutrals (<350 Da) (Huber and Frimmel, 1996). Humic and humic-like substances were the major constituents of raw ROC, representing 42% of the DOC, followed by LMW
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neutrals (29%), building blocks (6%) and biopolymers (2%) as shown in Fig. 2a. Coagulation using ferric chloride removed more biopolymers (51%) and humics (63%) than did alum,
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15% and 29%, respectively (Fig. 2a). The decrease in building blocks was however slightly greater for alum (45%) than ferric chloride (36%). Some increase in LMW neutrals was observed for both coagulants (6-10%). UVC/H2O2 caused breakdown of the humics, leading to increased concentration of low MW components (LMW acids and HS) which is consistent
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with previous results. BAC treatment removed biopolymers (27%) and LMW compounds (47%), but was ineffective for removing humic-like compounds, and an increase (20%) in the concentration of building blocks was observed. These may be associated with metabolites
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produced by the microorganisms in the biofilm.
Fig. 2b shows the DOC concentration of various fractions after different combinations of the treatments. Combining UVC/H2O2 and BAC treatment showed an additive effect for the reduction of humics (47%) and LMW neutrals (56%) which was due to the breakdown of humics to LMW compounds which were removed by downstream biological treatment. Comparison of alum and ferric chloride pre-treated ROC after UVC/H2O2 treatment shows greater reduction of biopolymers (83%) and humics (46%) for ferric chloride than for alum,
12
ACCEPTED MANUSCRIPT i.e., 70% and 27%, respectively. The concentration of remaining building blocks and LMW neutrals was fairly similar for both coagulants after UVC/H2O2 treatment.
Including BAC in the treatment scheme (coagulation+UVC/H2O2) gave greater removal of
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humics for the sample pre-treated by ferric chloride (69%) than alum (57%). However, the biopolymer content doubled for the alum pre-treated sample whereas it increased only 40% for ferric chloride. Generation of biopolymers in a BAC column after UVC/H2O2 treatment
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has been reported by Lu et al. (2013). The growth of the bacteria in the biofilm leads to
excretion of biopolymers such as extracellular polysaccharides, some of which will pass
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through the BAC column. This is consistent with the increase in biopolymers being observed only after the UVC/H2O2 treatment which led to increased concentration of smaller molecules which were a food source for the bacteria. The removal of building blocks was marginally greater for ferric chloride (52%) than alum (48%), whereas the removal of LMW neutrals
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was marginally greater for the alum (71%) than the ferric chloride (65%) pre-treated ROC.
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Figure 2
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3.3. DBP formation after treatment
Determination of DBPs was carried out for selected samples to investigate the potential health risks of the treated ROC. The concentration of total THMs (chloroform, bromoform, bromodichloromethane (BDCM) and dibromochloromethane (DBCM)) as well as the THM formation potential (THMFP) was determined. As the presence of ammonia and organic nitrogen in wastewater preferentially contribute to the formation of nitrogenous DBPs which were found to be more toxic than the regulated carbonaceous DBPs and hence pose a greater health risk (Krasner et al., 2006), nitrogenous DBPs were also determined. Since NDMA is
13
ACCEPTED MANUSCRIPT the major nitrosamine, and others are generally present at markedly lower concentrations, about an order magnitude lower than NDMA (Bond et al., 2011), the formation of NDMA was investigated after the various treatments. The concentrations of a range of DBPs which have greater toxicity than the regulated DBPs were determined after the sequential treatment
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for both coagulants. These were species representing the major haloacetonitriles (HANs, trichloroacetonitrile, dichloroacetonitrile, bromochloroacetonitrile, dibromoacetonitrile), haloaldehydes (chloral hydrate), halopicrin (chloropicrin), dibromonitromethane, and
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haloketones (1,1-dichloroacetone, 1,1,1-trichloroacetone, 1,3-dichloroacetone, 1,3-
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dichloroacetone).
Raw ROC had a total THM of 16 µg/L, comprised of 1 µg/L BDCM, 7 µg/L for each of bromoform and chloroform, and 2 µg/L DBCM. UVC/H2O2 treatment removed all but chloroform (decreased to 4 µg/L) and thus reduced the total THM concentration to 4 µg/L.
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Consequently no THMs were detected after sequential coagulation, UVC/H2O2 and BAC treatment for either coagulant.
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An increase in the formation potential of bromoform and total THMs occurred after
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UVC/H2O2+BAC treatment whereas a decrease was noted when coagulation (using alum) was used as pre-treatment (Fig. 3).
Figure 3
The NDMA concentration in raw ROC was 43 ng/L and was reduced to 6 ng/L after standalone UVC/H2O2 treatment. Similarly, the concentration of NDMA was markedly lower after UVC/H2O2 followed by BAC treatment with and without coagulation pre-treatment. A small
14
ACCEPTED MANUSCRIPT increase in the concentration of NDMA was noted when BAC was used after UVC/H2O2 treatment with and without coagulation which may have been due to analytical error.
The concentrations of haloacetonitriles (HANs), chloral hydrate, chloropicrin,
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dibromonitromethane, 1,1-dichloroacetone, 1,1,1-trichloroacetone, 1,3-dichloroacetone and 1,3-dichloroacetone were below the detection limit in raw ROC and none was observed after
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treatment.
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3.4. Ecotoxicity
As partial oxidation of organic matter during UVC/H2O2 treatment could lead to the formation of toxic by-products, an assessment of the ecotoxicity of the ROC was made after UVC/H2O2 with and without BAC treatment and after sequential coagulation, UVC/H2O2 and
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BAC treatment. The Microtox® assay indicated that the raw ROC was non-toxic, but showed toxicity (EC50 value of 13% which is considered extremely toxic) after stand-alone UVC/H2O2 treatment (UV fluence of 16 ×103 mJ/cm2). However, no toxicity was noted after
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BAC treatment of the UVC/H2O2 treated ROC, indicating the removal of toxic by-products formed during the UVC/H2O2 treatment. Similarly, no toxicity was noted after sequential
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coagulation (using alum or ferric chloride), UVC/H2O2 and BAC treatment.
3.5. Discussion
Although ferric chloride gave a markedly greater DOC reduction as a stand-alone treatment, when followed by BAC or UVC/H2O2 treatment, overall reduction after the sequence of coagulation, UVC/H2O2 and BAC treatment was only marginally greater for ferric chloride
15
ACCEPTED MANUSCRIPT than alum (68% for ferric chloride cf. 62% for alum). This was due to the greater DOC reduction by the UVC/H2O2+BAC steps for the alum pre-treated ROC. As the concentration of remaining organic matter was higher after alum than ferric chloride coagulation, subsequent UVC/H2O2 treatment led to a proportionally greater generation of biodegradable
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organic matter that was effectively removed during downstream BAC treatment. The
presence of ferric ion at pH 5 during coagulation with ferric chloride may have led to some removal of DOC due to the Fenton reaction and thus to greater DOC removal than for
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coagulation with alum. The extent of this reaction would likely be small since the optimum pH is 3-4. However, since there was similar removal of DOC in the UV/H2O2 treatment step
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for the ROC coagulated by ferric chloride and by alum (Fig. 1) it is clear that if the photoFenton reaction occurred, it did not lead to a net increase in DOC reduction after both the AOP and the subsequent BAC process. Furthermore, if this reaction did take place, it did not lead to a net enhancement in the biodegradability of the ROC, since the difference in
process.
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DOC reduction between the two coagulants decreased from 15-16% to 6% after the BAC
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As the flocs formed during alum coagulation were weak and needed long settling times
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and/or use of a flocculant aid to improve settling, and ferric chloride is expensive and corrosive in nature, a trade-off would need to be made with regard to cost and chemical use in relation to the desired treatment objectives.
LC-OCD results confirmed that ferric chloride removed a greater proportion of humic-like matter and LMW acids than alum. Enhanced reduction of organic matter after sequential coagulation and UVC/H2O2 treatment of ROC was attributed to the enhanced breakdown of humic-like matter after UVC/H2O2 treatment of the coagulated ROC. BAC treatment mainly
16
ACCEPTED MANUSCRIPT removed LMW compounds and these results were consistent with the low A254 but high reduction of DOC after BAC treatment, implying that biodegradation was the predominant mechanism during BAC treatment.
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Similar to the results reported in our previous study (Umar et al., 2014), coagulation
predominantly removed humic-like matter, leading to a significant increase in the UVT. This then led to enhanced breakdown of some of the remaining large MW organic matter to LMW
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biodegradable products during UVC/H2O2 treatment which were then removed by the BAC treatment. DOC residuals of 14 mg/L and 12 mg/L (below the target residual DOC of 15
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mg/L) were only achieved after the sequence of coagulation, UVC/H2O2 and BAC treatment, with alum and ferric chloride, respectively.
The raw ROC had a markedly lower concentration of total THMs than the Australian
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Drinking Water Guideline (ADWG) value of 250 µg/L (ADWG, 2011) and the USEPA guideline value of 80 µg/L (USEPA, 1998). The UVC/H2O2 treatment led to the partial oxidation of THM precursors (humic-like matter) (Kleiser and Frimmel, 2000) as indicated
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by the decrease in the concentration of total THMs. The removal of partially oxidised organic
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matter after biological treatment led to further reduction in total THMs so that they were below the detection limit after the sequence of UVC/H2O2 and BAC treatment. However an increase in the THMFP was noted after UVC/H2O2 (16 ×103 mJ/cm2, 3 mM H2O2) and BAC which was attributed to the insertion of phenol groups in the aromatic structures of organic matter as a result of partial oxidation, which increased the reactivity of such compounds with chlorine (Kleiser and Frimmel, 2000). Kleiser and Frimmel reported an increase of 20% in the formation potential of THMs after UV/H2O2 treatment of surface water. Similarly, an increase in THMFP was reported following UV-based treatments of a ROC by Liu et al.
17
ACCEPTED MANUSCRIPT (2012) (16 ×103 mJ/cm2, 3 mM H2O2) and surface water by Thomson et al. (2004) and Buchanan et al. (2006), however they noted that increasing the UV fluence led to reduction of THMFP indicating the loss of THM precursors. Although the conditions and source waters were different in the foregoing studies, the increase in THMFP in the present study is likely
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due to partial oxidation of the organic content as described by Kleiser and Frimmel. The
NDMA concentration in the raw ROC was 43 ng/L, which is well below the ADWG of 100 ng/L (ADWG, 2011). The reduction in the NDMA concentration after UVC/H2O2 treatment
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was in agreement with literature reporting the great efficacy of UVC/H2O2 treatment for this compound (Mitch et al., 2003) which is due to the oxidation of organic compounds with
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dimethylamine or trimethylamine groups (Lee et al., 2007). Other DBPs (HANs, haloaldehydes, halopicrins, and haloketones) were all below the detection limit for both raw and treated ROC. The final concentration of DBPs compared favourably with those in the AWDG and the USEPA guidelines for drinking water and hence the treated ROC was
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deemed safe for reuse or discharge to the environment.
The increase in ecotoxicity after stand-alone UVC/H2O2 treatment differed from the earlier
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findings of Liu et al. (2012) who reported no toxicity after 30 min UVC/H2O2 treatment (UV
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fluence of 23 ×103 mJ/cm2) of a municipal ROC prepared in the laboratory. This difference may be attributed to several factors including the lower UV fluence, different nature of the organic content, and the presence of antiscalant and biocide in the ROC sample used in this study which could have been broken down to more toxic by-products. Downstream BAC treatment is therefore useful not only in terms of enhanced organic content removal, but also in removing potentially toxic by-products formed during UVC/H2O2 treatment.
18
ACCEPTED MANUSCRIPT 3.6.
Reduction in EED by integrating pre- and post-treatment with the UVC/H2O2 process
Electrical energy dose (EED), the electrical energy required to reduce the concentration of
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DOC during the UVC/H2O2 treatment (kWh/m3) was calculated for various treatments. Generally, the energy needed for the production of H2O2 is not included in the EED
calculation which is most likely due to the EED being calculated at optimum H2O2 dosage.
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However, it must be considered, particularly when high concentrations of H2O2 are used. An average energy requirement of 10 kWh/kg for H2O2 production was assumed according to
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Rosenfeldt et al. (2006). As expected, the EED value to obtain a target residual DOC of 15 mg/L (60% reduction) was high for stand-alone UVC/H2O2 treatment due to low DOC mineralization (Fig. 4). The coagulation and BAC processes contributed significantly to DOC removal thus leading to a marked decrease in the EED for the sequential process. The largest
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decrease in EED occurred when both coagulation and BAC were used to obtain DOC reductions almost 4 times greater than stand-alone UVC/H2O2 treatment.
4. Conclusions
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Figure 4
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The sequence of coagulation, UVC/H2O2 and BAC treatment was demonstrated to be a potential process for the removal of organic matter from a high salinity municipal ROC to meet the target of residual DOC of 15 mg/L. Removal of humic-like matter by coagulation facilitated subsequent UVC/H2O2 breakdown of some of the remaining large MW compounds to generate LMW compounds that were readily removed by downstream BAC treatment. The sequential treatment enabled more than meeting the target residual DOC of 15 mg/L. Ferric chloride removed more than twice the humic-like matter than alum leading to a large
19
ACCEPTED MANUSCRIPT difference after coagulation and coagulation+UVC/H2O2 treatment. However, total DOC reduction after BAC treatment was only marginally greater (6%) for ferric chloride.
A reduction in the THMs and NDMA levels was observed after UVC/H2O2 treatment but
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some increase in the formation potential of bromoform, DBCM and total THMs was noted after UVC/H2O2+BAC treatment, whereas a decrease was noted when coagulation was used as pre-treatment. Other DBPs, including haloaldehydes, halopicrins, and haloketones were
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below the detection limit in raw and treated ROC. The concentration of DBPs compared
favourably with Australian and USEPA drinking water guidelines, and therefore the treated
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ROC was safe for reuse or environmental discharge. An increase in the ecotoxicity was observed after UVC/H2O2 treatment but disappeared after UVC/H2O2+BAC treatment with and without coagulation.
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A large reduction (85-87%) in EED occurred when coagulation and BAC treatments were integrated with UVC/H2O2 treatment; ferric chloride was 18% more energy efficient than alum for a marginally greater (6%) reduction in DOC. Either of these coagulants could be
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utilised after taking the trade-offs and desired treatment objectives into account.
Acknowledgements:
The authors would like to thank the Commonwealth Government of Australia for providing Endeavour IPRS and APA scholarships, City West Water, Water Research Australia (Project #4054/12) and the Smart Water Fund (project #80S-8010) for providing support for this project.
References:
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oxidation processes with/without pretreatment. Chemical Engineering Journal, 166, 932–939.
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Tables
Value
DOC (mg/L)
37
COD (mg/L)
105
pH
7.7
Colour (Pt.Co units)
156
Chloride (mg/L)
8,212
TDS (mg/L)
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Parameter
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Table 1 Characteristics of ROC
17,245 0.65
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A254 (/cm)
1.75
Alkalinity (as CaCO3, mg/L)
418
Conductivity (mS/cm)
23.5
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SUVA (L/mg.m)
ACCEPTED MANUSCRIPT
Table 2 Reduction of colour, A254 and improvement of UVT after various treatments (UVH denotes UV/H2O2, Al and Fe denote coagulation by alum and ferric chloride, respectively, values shown ± 1 SD)
RI PT UVT*
Colour
A254
UVH
86 ± 0.8
61 ± 1.1
57 ± 1.0
Al
65 ± 0.2
39 ± 0.1
40 ±1.3
Fe
82 ± 1.0
54 ± 1.1
50 ± 0.9
BAC only
29 ± 3.2
20 ± 3.2
31 ± 2.2
Al+BAC
SC
Treatment
75 ± 1.4
47 ± 1.1
45 ±2.3
86 ± 1.6
62 ± 1.9
58 ± 1.8
92 ± 2.8
74 ± 2.3
66 ± 3.0
94 ± 0.9
82 ± 1.4
67 ± 1.1
Fe+UVH
95 ± 0.5
83 ± 1.6
73 ± 1.1
Al+UVH+BAC
97 ± 2.7
85 ± 2.8
79 ± 2.4
Fe+UVH+BAC
97 ± 3.3
85 ± 2.4
85 ± 1.9
Fe+BAC UVH+BAC
TE D
Al+UVH
EP
initial UVT was 22%
AC C
*
% increase
M AN U
% reduction
ACCEPTED MANUSCRIPT
Figure Captions and Figures Figure 1 Comparison of DOC reduction after individual and combined treatments
samples)
RI PT
(UVH refers to UVC/H2O2, error bars represent ±1 standard deviation measured for 3
of fractions after various combined treatments
SC
Figure 2 (a) LC-OCD chromatograms after individual treatments, (b) DOC concentration
M AN U
Figure 3 Formation potential of individual and total THMs after various treatments
Figure 4 EED and corresponding DOC reduction for each treatment (line represents the
AC C
EP
TE D
% DOC reduction corresponding to the target residual DOC of 15 mg/L)
ACCEPTED MANUSCRIPT
100 90
70
RI PT
% reduction
80
60 50 40 30
SC
20 10
M AN U
0
Figure 1 Comparison of DOC reduction after individual and combined treatments (UVH refers to UVC/H2O2, error bars represent ±1 standard deviation measured for 3
AC C
EP
TE D
samples)
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
(a)
AC C
EP
TE D
(b)
Figure 2 (a) LC-OCD chromatograms after individual treatments, (b) DOC concentration of fractions after various combined treatments
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Figure 3 Formation potential of individual and total THMs after various treatments
80
350
70
300
60
250
50
200 150 100
0 UVH
UVH+BAC
Al+UVH
Fe+UVH
30 20 10 0
Al+UVH+BAC Fe+UVH+BAC
% DOC reduction
M AN U
EED (kWh/m3)
SC
50
40
Figure 4 EED and corresponding DOC reduction for each treatment (line represents the
AC C
EP
TE D
% DOC reduction corresponding to the target residual DOC of 15 mg/L)
% DOC reduction
400
RI PT
EED (kWh/m3)
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Highlights
Coagulation+UV/H2O2+BAC treatment removed DOC from reverse osmosis concentrate Coagulation with ferric chloride gave greater DOC removal than with alum
AC C
EP
TE D
M AN U
SC
Ferric chloride coagulation plus BAC treatment is a possible option
RI PT
Decreased THMFP, NDMA and ecotoxicity levels were obtained after the treatment