Chemical cleaning of reverse osmosis membranes

DF~ALINATION Desalination 134 (2001) 77-82

ELSEVIER

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Chemical cleaning of reverse osmosis membranes .a*

Sayed Siavash Madaenl , Toraj Mohamamdi b, Mansour Kazemi Moghadam b aChemical Engineering Department, Razi University, Kermanshah, Iran Tel. +98 (831) 24750; Fax +98 (831) 831618; e-mail: [email protected] bSchool of Chemical Engineering, 1ran University of Science and Technology, Tehran, 1ran Tel. + 98 (21) 7896621; Fax +98 (21) 7896620; e-mail: [email protected]

Received 11 September 2000; accepted 25 September 2000

Abstract

Membrane technology has become of great interest for treatment of different water feeds including seawater. Depending on the membrane type, materials in the feed and process conditions the membrane will lose its performance during time. Fouling is the most important problem associated with the application of membranes. A slrategy for membrane regeneration is chemical cleaning of the fouled membranes. One of the major applications of reverse osmosis membranes is processing of water from different resources or for varions applications. This includes seawater desalination or ion removal for makeup water for boilers. In all eases fouling restricts membrane performance. In this work reverse osmosis membranes were fouled with water. Chemical cleaning of the RO membranes using acid, alkaline, surfactant and detergent solutions has been discussed. Cleaning efficiency depends on the type of the cleaning agent and its concentration. It has been shown that the efficiency increases with increasing the concenlration of the cleaning agent. The concentration that provides the highest cleaning efficiency can be considered as the optimum concentration. This depends on the type of the cleaning agent. Operating conditions such as crossflow velocity, turbulence in the vicinity of the membrane surface, temperature, pH and cleaning time also play a role in the cleaning process. Optimum membrane cleaning requires in depth understanding of the interactions between the foulants and the membrane as well as the effect of the cleaning procedure on deposit removal and membrane performance. In this paper the mechanism of deposit removal is also investigated. Keywords: Membranes; Ultrafiltration; Whey; Cleaning; Chemicals

1. Introduction Membrane technology has become of great interest for treatment o f different feeds. Depend*Corresponding author.

ing on the membrane type, materials in the feed and process conditions the membrane loses its performance during time. Flux - - a measure for membrane performance is controlled by two phenomena, concentration polarization and

Presented at the International Conference on Seawater Desalination Technologies on the Threshold of the New Millennium, Kuwait, 4-7 November 2000.

0011-9164/01/$-- See front matter © 2001 Elsevier ScienceB.V. All rights reserved PII: S0011-9164(0!)00117-5

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S.S. Madaeni et al. / Desalination 134 (2001) 77-82

fouling [1]. Based on the terminology introduced by IUPAC fouling is "a process resulting in loss of performance of a membrane due to the deposition of suspended or dissolved substances on its external surfaces, at its pore openings or within its pores" [2]. Fouling not only reduces the flux but also changes the retention. Numerous researches are carried out around the world for fouling reduction and cleaning of the fouled membranes. However it is unlikely to completely eliminate fouling [3]. Control of fouling is of utmost importance. Techniques involved are pretreatment of feed which reduces the particulate density, operating conditions, e.g. moderate pressure, crossflow and backwashing, membrane regeneration, i.e. cleaning the membrane. An important technique for membrane regeneration is chemical cleaning of fouled membranes. Cleaning is defined as "a process where material is relieved of a substance which is not an integral part of the material" [4]. Many substances mostly chemicals and different procedures are used for cleaning of membranes. Chemical cleaning means removing impurities by means of chemical agents. However cleaning consumes time and money. In general around 5-20% of the operating cost is the cost of cleaning [5]. This shows the importance of the continuos research in this field. Up to now there are some problems associated with membrane cleaning. Cleaning procedures need long operation time [6], consume chemicals [7], degrade some membranes [8] and may cause corrosion in the system [9]. One of the major applications of reverse osmosis membranes is processing of water from different resources or for various applications. This includes seawater desalination or ion removal for makeup water for boilers. In all cases fouling restricts membrane performance. In this paper the chemical cleaning of reverse osmosis membranes fouled with pretreated water is explained. The fouled membranes were washed with chemical agents such as acid, alkali and

surfactant. The type of chemical agent and process conditions i.e. concentration of the cleaning solution, cleaning time affect cleaning efficiency. The effects of these parameters on cleaning efficiency as well as cleaning mechanism are discussed.

2. Materials

and

methods

FilmTec polyamide FT30 reverse osmosis membrane was used for all experiments. The following chemicals were used as cleaning agents: - hydrochloride acid (HC1), nitric acid (I-INO3), sulphuric acid (H2SO4), phosphoric acid (I-I3PO4), citric acid C6I-hO7), sulphamic acid (HO-SO2 NH2) all from Merck - sodium hydroxide (NaOH), potassium hydroxide (KOH) from Panreac and ammonium hydroxide (NI-hOH) from Merck - sodium hypochlorite (NaC10) from Fluka - ammonium chloride (NI-hCI) from Merck EDTA, i.e ethylene diamine tetra acetic acid [CH2N(CH2COOH2)] which is a chelating agent from Merck sodium dodecyl sulphate (SDS) which is an anionic surfactant from Merck two commercial dish washing liquids named Yekta and Goli The pretreated water was used as the feed for reverse osmosis system. The permeate of this system may be used as deionized water for boilers. The analysis of the feed is shown in Table 1. All experiments were carried out in a reverse osmosis rig at ambient temperature (254- I°C). The rig consisted of feed tank, a centrifuge Grnnndfos pump, pressure valves, pressure gages and a stainless steel crossflow cell with the membrane area of 90 cmz. The crossflow velocity was 0.45 m/s. Flux was calculated by measuring the permeate weight. Pure water flux was obtained at 17 bar.

S.S. Madaeni et al. / Desalination 134 (2001) 77-82 Table 1 Analysis of the water used as the feed in the reverse osmosis system Specious

Concentration, mg/1

K+ Na+ Mg++ Ca++ Mn++ Fe++ NH4+ Ba++ Sr++

54.7 354.7 24.4 139.9 <0.1 <0.1 0.0 <0.06 <1.80 <0.1 18.3 6.2 538.9

A f -~+

HCO3NO3CIFS04 =

SiO2 Turbidity, NTU

0.05 489.6

8.0 < 1.0

(3) through the cake and the membrane may be described by Darcy's law: J=AP/g~.,R

(1)

in which AP is transmembrane pressure (driving force),/~ is viscosity of the fluid and ~ is sum of the resistances. Membrane resistance (Rm) can be estimated from the initial water flux: R. = AP / # Jw,

(2)

The resistance which appeared after fouling (R/) can be calculated from the water flux after washing with water:

R: :(ae/uJ.)-R.

(3)

The resistance which remained afar cleaning (Re) can be calculated from the water flux after chemical cleaning:

Rc Prior to the cleaning, membrane was fouled by filtration of feed water for 5 h at 17 bar. The fouled membranes were cleaned according to the protocol suggested by Fane and colleagues [10]. Before and after fouling the water flux of the membrane was measured by passing distilled water through the membrane (initial water flux = Jwi, water flux after fouling = Jwf). The fouled membrane was washed with distilled water for 20 min to remove unbound substances from the membrane surface. The water flux was measured after washing (Jww). This was followed by washing the membrane with a cleaning agent for a specific time without applying pressure. The water flux after chemical cleaning was determined

79

(4)

Resistance removal (RR) which is a tool for cleaning quantification can be estimated from:

RR(%)= [(Ry - R c ) / R / I x 100

(5)

Flux recovery (FR) is another method for quantification o f cleaning efficiency:

FR(%)=[(J~ - J ~ )/(J., - J _ ) ] × l O 0

(6)

Both parameters, i.e. resistance removal and flux recovery, have been used for demonstrating the cleaning efficiency [3,10].

(Jwc). Fouling can be quantified by the resistance appearing during the filtration and cleaning can be specified by the removal of this resistance. The resistance is due to the formation of a cake or gel layer on the membrane surface. The flux

3. Results and discussion 3.1. Comparison of cleaning agents The XRD tests were carried out to elucidate the composition of the fouling layer. The results

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S.S. Madaeni et al. / Desalination 134 (2001) 77-82

showed that most (around 90%) of the foulant on the membrane surface is calcium sulphate and the rest is mainly calcium phosphate. This is in agreement with the feed water analysis (see Table 1). The type of the fouling materials on the membrane surface was the basis for choosing the cleaning agent. Preliminary understandings about the nature of the precipitated layer indicated that the ability of acids is less than alkaline solutions for membrane cleaning. To compare the cleaning agents similar membranes were fouled with the same feed and the fouled membranes were cleaned with different chemicals. The concentrations of all the cleaning solutions were 0.05%. Table 2 shows the cleaning efficiency as flux recovery and resistance removal for various chemicals. Acids were the weakest cleaning agents for the experimental conditions. Alkali had moderate effect and the combinations of chelating agent, surfactant with alkali provide the best cleaning efficiency. EDTA, which is a chelating agent, has a good ability to combine with metals. It is used Table 2 The cleaningefficiencyfor differentchemicals Chemical

Flux recovery, Resistance % removal,%

HCI HNO3 H3PO4 Sulphamicacid Citric acid NH4OH NH4C1 KOH NaOH Yekta Goli SDS+NaOH EDTA+NaOH EDTA+SDS+NaOH EDTA+SDS+KOH

18 25 44 45 62 51 60 71 72 74 79 65 82 100 100

21 27 46 49 65 55 63 67 76 79 81 69 86 100 100

in special soaps to remove metallic contamination. The effect of SDS (surfactant) can be attributed to the cleaning strength of emulsifiers due to altering the interfacial tension of water. This results in better separation of fouling materials from the membrane surface. NaOH changes the pH of the solution and provide a better condition for the highest removal of foulants with EDTA and SDS.

3.2. Effect o f cleaning conditions

The cleaning efficiency depends on the cleaner concentration. Figs. 1 and 2 show the flux recovery and resistance removal as a function of cleaner agent concentration. Higher concentration causes higher resistance removal or flux recovery. However the effect is insignificant at high concentration. This is due to the limited ability of cake removal by any agent. The adsorbed layers or irreversible fouling materials can not be removed. The effects of pH (Figs. 3 and 4), cleaning time (Figs. 5 and 6) and temperature (Figs. 7 and 8) on cleaning efficiency have been investigated. These figures show that the operating conditions have dominants effects on cleaning efficiency.

3.3. Cleaning mechanism

The investigation of fouling and cleaning mechanisms lead to better understanding of cleaning process and provide a basis for tailormade chemicals and procedures. It seems that cleaning agent diffuses into the deposited cake layer on the membrane surface. Diffusion rate depends on different factors including turbuleney. A chemical reaction may occur between cleaning agent and the substances on the cake layer. Depending on their type the reaction may be hydrolysis, dissolution or dispersion. This results in removal of fouling materials from the membrane surface.

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References

[9]

[1]

[10]

[2]

G. Danfin, U. Merin, J.P. Labbe, A. Quemerais and F. L. Kerherve, Biotechnol. Bioeng, 38(1) (1991) 82. W.J. Koros, Y.H./Via and T. Shimidzu, J. Membr. Sci., 120 (1996) 149.

M. Bartlett, M.R. Bird and J.A. Howell, J. Membr. Sci., 105 (1995) 147. G. Trag~tdh, Desalination, 71 (1989) 325. A.G. Fane, Proc., Symposium on Characterization of Polymers with Surface, Lappeenranta, Finland, (1997) 51. R. Chortg, P. Jelen and W. Wong, Sep. Sei. Teehnol., 20 (1985) 393. DJ~. Kane and N.E. Middlerniss, In: D. Lurid, E. Plett and C. Sandu (F_,ds.), Fouling and Cleaning in Food Processing. University of Wisconsin, Madison, WI, 1985, p. 312. K.E. Smith and R.L. Bradley, J. Dairy Sei., (1986) 1232. H.F. Bohner and R.L. Bradley J. Dairy Sei., (1992) 718. M.J. Munoz-Aguado, D.E. Wiley and A.G. Fane, J. Membr. Sei., 117(1-2) (1996) 175.