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Reviews in Environmental Science & Bio/Technology (2004) 3: 361–380 DOI: 10.1007/s11157-004-4627-9

 Springer 2005

Removal of inorganic anions from drinking water supplies by membrane bio/processes Svetlozar Velizarov*, Joa˜o G. Crespo & Maria A. Reis CQFB/REQUIMTE, Department of Chemistry, FCT, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal (*author for correspondence, e-mail: [email protected]) Received 29 June 2004; accepted 8 October 2004

Key words: Donnan dialysis, drinking water, electrodialysis, inorganic anionic pollutants, integrated processes, membrane bioreactors, nanofiltration, ultrafiltration, reverse osmosis Abstract This paper is designed to provide an overview of the main membrane-assisted processes that can be used for the removal of toxic inorganic anions from drinking water supplies. The emphasis has been placed on integrated process solutions, including the emerging issue of membrane bioreactors. An attempt is made to compare critically recently reported results, reveal the best existing membrane technologies and identify the most promising integrated membrane bio/processes currently being under investigation. Selected examples are discussed in each case with respect to their advantages and limitations compared to conventional methods for removal of anionic pollutants. The use of membranes is particularly attractive for separating ions between two liquid phases (purified and concentrated water streams) because many of the difficulties associated with precipitation, coagulation or adsorption and phase separation can be avoided. Therefore, membrane technologies are already successfully used on large-scale for removal of inorganic anions such as nitrate, fluoride, arsenic species, etc. The concentrated brine discharge and/or treatment, however, can be problematic in many cases. Membrane bioreactors allow for complete depollution but water quality, insufficiently stable process operation, and economical reasons still limit their wider application in drinking water treatment. The development of more efficient membranes, the design of cost-effective operating conditions, especially long-term operations without or with minimal membrane inorganic and/or biological fouling, and reduction of the specific energy consumption requirements are the major challenges. Abbreviations: D – dialysis; DD – Donnan dialysis; DMB – dialysis membrane bioreactor; ED – electrodialysis; IEMB – ion exchange membrane bioreactor; MCL – maximum contaminant level; MF – microfiltration; NF – nanofiltration; RO – reverse osmosis; TOC – total organic carbon; UF – ultrafiltration; US EPA – United States Environmental Protection Agency; WHO – World Health Organization

1. Introduction A number of inorganic anions have been found in potentially harmful concentrations in numerous drinking water sources (DeZuane 1997; Smith et al. 2002; Petrovic´ et al. 2003). The maximum allowed concentrations of these compounds are generally set by the drinking water quality regulatory standards in the relatively low lgÆl)1–mgÆl)1

range; therefore, the majority of them can be referred to as micropollutants. The internationally accepted standards and guidelines, regarding the maximum allowed levels of these compounds, are proposed by the World Health Organization (WHO). In addition, the European Union and the US Environmental Protection Agency have issued similar health and environmental standards and a considerable number of regulatory methods have

362 been published worldwide for the analysis of inorganic anions in drinking water (US EPA 1998; Jackson 2001). Table 1 lists the current status for potentially toxic inorganic anions along with some information about the main sources of pollution and the potential health risks, associated with their ingestion in drinking water Since there are usually no organoleptic changes in drinking water that can be attributed to the presence of toxic inorganic anions in trace levels, it is rather possible that some of them may remain undetected, thus increasing the possible health risks. An example of such recently found compound is perchlorate, an important ingredient of solid rocket propellants, which may interfere with the ability of the thyroid gland to utilize iodine in hormones production (Richardson 2003; Min et al. 2004). Since perchlorate is a serious problem in some US regions, a provisional drinking water goal of 1 lg/l has been suggested (US EPA 2002). A number of inorganic anionic contaminants can be present at the same time in rather different levels (e.g. nitrate and perchlorate), thus leading to the emerging issue of their control and simultaneous removal from drinking water supplies. Finally, water of defined ion composition is required in the manufacturing of a number of food products, pharmaceuticals and in the fresh water fisheries and sea aquariums. Several common treatment technologies are nowadays used for removal of inorganic contaminants from water supplies. Large-scale plants usually apply coagulation with aluminium or iron salts followed by filtration but a number of anions (e.g. nitrate) have very little tendency to coordinate with metal ions and low potential for co-precipitation (Duan & Gregory 2003). Smallscale treatment facilities often use ion exchange and/or adsorption due to their ease of handling and compactness; however, regeneration and additional costs, associated with the disposal of the regenerants used, represent serious problems. Moreover, release of undesirable organics (such as styrene, divinylbenzene, trimethylamine, etc.) from some synthetic resins to the treated water still hinders a larger application of ion exchange for drinking water production (Kapoor & Viraraghavan 1997). Membrane separation processes such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), dialysis (D), Donnan

dialysis (DD) and electrodialysis (ED), if properly selected, offer the advantage of producing high quality drinking water. In many cases, one membrane process can be integrated with another to produce water of even higher quality. In these processes, the membrane can be viewed as a barrier between contaminated and purified water streams. The separation of the two streams often allows for operation with no or minimal chemical water pre-treatment, which otherwise can form deleterious by-products (Bergman 1995; Jacangelo et al. 1997). However, in physical membrane processes, inorganic anions are not destroyed but normally concentrated and the concentrate disposal can be costly and difficult to permit in many cases; therefore, post-treatment of the concentrate stream or hybrid membrane-assisted technologies capable of converting anionic contaminants to harmless products are highly desirable. Chemical, electrochemical or biochemical treatment processes are able to deal efficiently with anions (Daub et al. 1999; Carraro et al. 2000; Centi & Perathoner 2003). Among them, biological reduction is especially appropriate since it offers selective removal of the target anion from water due to anoxic bacteria, which under appropriate conditions (pH, oxidation–reduction potential, temperature, etc.) can use anions as electron acceptors and organic (heterotrophic microorganisms) or inorganic (autotrophic microogranisms) compounds as electron donors for their growth. However, the main concern of using bioprocesses is the risk of secondary water pollution by cells, incompletely degraded nutrients and metabolic by-products, which can promote microbial growth in water distribution systems, thus requiring extensive post-treatment in order to produce safe and biologically stable water. These problems can be overcome, or at least reduced, by introducing a membrane unit as a pre- or posttreatment stage in the water production process. When membrane(s) are integrated with an appropriate bioprocess in a single process, the configuration is generally referred to as ‘‘membrane bioreactor’’. In this paper, discussion on useful applications of membrane processes for removal of inorganic anionic contaminants from drinking water, with a special emphasis on the emerging issue of membrane bioreactors, is presented. Since it is rather difficult to select which membrane applications

Natural sources; near some mining sites Natural sources; some foods

F)

NO3)

NO2)

MoO42)

SeO42)

HSeO3)

ClO4)

Fluoride

Nitrate

Nitrite

Molybdade

Selenate

Selenite

Perchlorated

Military operations; some fertilizers

Natural sources; leached from certain agricultural soils

Chloramination; nitrification in water distribution systems

Natural eutrophication; inorganic fertilizers; human and animal wastes

Natural sources; production of aluminium, phosphate fertilizers, which may contain up to 4% fluorine.

CN

Cyanide

N˜Natural sources; some foods; electroplating, steel and mining industries; certain fertilizers and pesticides

Metal-working and mining industries

Disinfection by-product in waters treated with chlorine dioxide b

(as Cr)

0.001b

0.01 (as Se)

0.01 (as Se)

0.07 (as Mo)

0.2b

50

1.5

0.07

0.05

0.2b

0.025b

0.01b (as As)

0.01b (as As)

Maximum acceptable standard or guidelineb level, mg/l

Inhibits iodide uptake in humans and animals

Skin, hair, and nail damage; can replace sulphur in biomolecules

Anaemia; Diarrhoea; High levels of uric acid in the blood

Methaemoglobinaemiac, which could be fatal to infants

Mainly attributable to its reduction to NO2)

Pitting of teeth, bone fluorosis

Weight loss; thyroid effects; Neurological effects

Liver damage; kidney damage

Nervous system effects; anemia

Possibly cancerogenic

Kidney damage; skin cancer

Kidney damage; skin cancer

Possible adverse health effects of higher levels

b

The prevailing form depends on pH. Provisional value (this term is used for constituents for which there is some evidence of a potential hazard but where the available information on health effects is limited). c Oxidation of normal haemoglobin (Hb) to methaemoglobin (metHb), which is unable to transport oxygen to the tissues. d Still not regulated by the WHO; this provisional value is issued by the US EPA (Environmental Protection Agency).

a

CrO42)

Chromate

)

ClO2)

Chlorite

Disinfection by-product of bromide (if naturally present) generated during ozonation

Dissolution of rocks and minerals (mostly present in groundwater)

H2AsO3) ; HAsO32) ; AsO33)

Arsenitea

BrO3)

Natural erosion (mostly present in surface water under aerobic conditions)

H2AsO4) ; HAsO42) ; AsO43)

Arsenatea

Bromate

Main sources of contamination

Chemical formula

Anion

Table 1. Standards and guidelines for inorganic anions in drinking water (WHO 2003), main sources of contamination and possible health risks

363

364 might be referred as specifically anion removal ones (other water constituents are often also removed), the authors’ opinion, in some instances might be subjective. In most cases, only recent examples are referred to since the articles selected often familiarize the interested reader with the general history of a specific problem. Where it is appropriate, the discussion starts with a given membrane separation used as a single process and then moves on to its integration in membrane bioprocesses. An attempt is made to identify the advantages, limitations and future research needs in each case. Some of the processes discussed at the present time could be considered still far from practical implementation, but nevertheless, they might provide useful information, on which future developments could be based.

2. Pressure-driven membrane processes and membrane bioreactors Pressure-driven membrane processes use pressure difference between the water to be treated and a permeate side as the driving force to transport water across the membrane. These membrane processes include RO, NF, UF and MF. The operating trans-membrane pressure ranges vary significantly but are usually: 20–100 bar for RO, 5–20 bar for NF, 2–5 bar for UF, and 0.1–2 bar for MF. It has to be mentioned that while MF and UF membranes have well-defined porous structure, for RO and NF membranes the term ‘‘pores’’ may be better associated with the intramolecular voids within the polymeric matrix. Another possible classification can be based on the molecular mass of the solute to be separated by a given membrane process that is usually of up to 100 Da (RO), 100–500 Da (NF) and 500– 10,0000 Da (UF). Obviously, UF and MF are not suitable for the direct removal of inorganic anions from solution; however they can be implemented in hybrid processes, which produce larger aggregates (subsequently filtered) (Han et al. 2002; Yoon et al. 2003) or in bioprocesses with the purpose to retain microbial cells, using anions as electron acceptors. On the other hand, RO and NF can be used for removal of inorganic anions as single processes. The advantages and limitations of each process will be discussed in the following sections.

2.1. Reverse osmosis Reverse osmosis is a well-established technology used for many years in water desalination. Reverse osmosis membranes are asymmetric, i.e., consist of a thin polymer (nowadays, mostly polyamide) layer combined with a porous support to guarantee the membrane mechanical stability. The RO membranes discriminate on the basis of molecular size and due to the dense properties of the separating layer very high (often close to 100%) retention of low-molecular mass compounds and ions (total desalination) can be achieved. Moreover, the process can be easily automated and controlled. The treated water stream, however, may lack the right balance of minerals and has unpleasing taste because of the retention of all ions (Nicoll 2001). Another disadvantage of RO is the high energy consumption needed to maintain the required pressure difference. Reverse osmosis membranes are very sensitive to polarization phenomena (ions accumulation) at the membrane surface contacting the concentrate (retentate) side. In addition, the solubility products of some salts in the retentate can be exceeded, forming precipitates (mineral fouling) besides possible biological fouling due to natural organic matter and microogranisms present. The presence of non-toxic components, such as hardness (Ca2+, Mg2+) and SO42) anions, can interfere with the separation of toxic anionic species due to problems that these components may cause with water recovery and ionic strength (osmotic pressure) (Ritchie & Bhattacharyya 2002). Therefore, the contaminated water usually requires pre-treatment (other membrane processes like MF and/or UF are nowadays more and more used) before entering the RO modules. Concerning the RO applications in the case of toxic inorganic anions, few studies have been recently performed mostly aiming at the removal of arsenic species and nitrate (Table 2). Brandhuber and Amy (1998) showed that if the main arsenic species are present as As(III) only RO membranes would be effective. In a pilot-scale study, removal efficiencies between 96–99% for As(V) and 46– 84% for As (III) have been reported (Ning 2002). Pre-oxidation of As(III) to As(V) could guarantee better removal. It was demonstrated that RO could be effectively applied for removal of nitrate along with

Schoeman and Steyn (2003) Bohdziewicz et al. (1999) Brandhuber and Amy (1998) Ning (2002) Urase et al. (1998) Sato et al. (2002) Vrijenhoek and Waypa (2000) Paugam et al. (2002) Thanuttamavong et al. (2002) Van der Brugen et al. (2001) Cohen and Conrad (1998) Diawara et al. (2003) Lhassani et al. (2001) Hafiane et al. (2000) Yoon et al. (2003) Han et al. (2002) Natural with 188 mg NO3)/l (South Africa) Tap water (Poland) Pilot studies at various sites (USA) Review on pilot studies at various sites (USA) Groundwater spiked with 0.6 mg As/l (Japan) Model water Model water Model water MF pre)treated surface water (Japan) Groudwater spiked with NO3) salts (Belgium) Groudwater (California, USA) Model water Model water Model water River water spiked with ClO4) (Colorado, USA) Natural source, 0.042 mg As/l (Colorado, USA) NO3) NO3) As species As species As species As species As species NO3) NO3) NO3) NO3), F) F) F) CrO42) ClO4) As species 4040-LHA-CPA2 (Hydranautics) Different membranes (Osmonics) Different membranes and manufacturers Different membranes and manufacturers ES-10 (Nitto-Denko) Different membranes (Nitto-Denko) Filmtec NF45 (Dow Chemical) Nanomax 50 (Millipore) Different membranes (Nitto)Denko) Filmtec NF70 (Dow Chemical) NF300 (Osmonics) Filmtec NF45 (Dow Chemical) Filmtec NF70 (Dow Chemical) TFCS (Fluid Systems) GM (Desal); 8 kDa cutt)off Cellulose acetate membrane; 0.22 lm pores RO RO, NF RO, NF, UF RO NF NF NF NF NF NF NF NF NF NF Surfactants + UF Flocculation + MF

Target anion(s) Water origin Membrane and manufacturer Process

Table 2. Pressure)driven membrane processes for removal of inorganic anions from drinking water

Reference

365 water desalination in a rural area (Schoeman & Steyn 2003). The nitrate removal efficiency was close to 98% and, although the total dissolved solids (TDS) in the treated water stream were strongly reduced (from 1292 mg/l in the source water to 24 mg/l in the RO permeate), the authors suggested that the water could be used directly for potable purposes. Preliminary estimates showed that for an approximately 2 m3/h output plant, the capital and operating cost were about USD 29,900 and 0.50/m3, respectively. It can be concluded that RO is a highly efficient process for removal of inorganic anions from drinking water, which guarantees a secure detoxification of the water supply. However, total desalination is undesired due to possible corrosive problems if water hardness is reduced to very low levels. Water with hardness values under 50 mg/l is expected to be corrosive (lead, copper, iron, zinc, etc.) (DeZuane 1997). Therefore, modifications, allowing for selective toxic anions removal along with a sufficient retention of water salinity are required. 2.2. Nanofiltration Nanofiltration (NF) uses membranes, which can provide selective desalination, and is usually applied to separate multi-valent ions from monovalent ones; however, it is also possible to achieve a certain separation of ions of the same valence by selecting the proper membrane and operating conditions (Lhassani et al. 2001). NF membranes are sometimes designated as ‘‘loose’’ RO membranes (Ho & Sirkar 1992), since they provide higher water fluxes at lower trans-membrane pressures. These membranes are usually asymmetric and negatively charged at neutral and alkaline drinking water pH. Therefore, separation of anions is based not only on different rates of their diffusion through the membrane (at low pressure), convection (at high pressure), but also on repulsion (Donnan exclusion) between anions in solution and the surface groups, which is obviously higher for multi-valent anions (Levenstein et al. 1996). The advantage of introducing this additional mechanism of ion exclusion (in addition to the size-based exclusion) is that high ion separation degrees (ion rejections) similar to those in RO can be achieved but at higher water fluxes through the membrane. On the other hand, the NF

366 process is much more sensitive than RO to the ionic strength and pH of source water. The membrane surface charge is mainly due to anion adsorption from water rather than to fixed charged groups (as in the case of ion exchange membranes), therefore it depends strongly on bulk anion concentration (Hagmeyer & Gimbel 1998). Furthermore, it changes from negative to zero net charge at the membrane isoelectric point and then to positive at lower pH values (usually HCO3) > SO4 2). The use of a mono-valent anion perm-selective membrane proved successful in a full-scale ED plant designed to remove nitrate from groundwater in Austria (Hell et al. 1998). The nitrate concentration in the raw water was 120 mg NO3)/l and the plant removal efficiency (66%) was adjusted to obtain a product concentration of 40 mg NO3)/l. Under these conditions, the total desalination degree was about 25%, therefore the nitrate selectivity was reasonably high. Sulfate was practically not removed and water pH changed by 0.1 units only (7.4–7.5). The concentrate brine discharge, however, represented a challenge. The authors suggested that irrigation would be a possible solution. Since the chloride concentration was 270 mg Cl)/l, the brine should be suitable for direct irrigation, at least for chloride tolerable plants, or blended with other waters, before use. The option tested in practice was a discharge to the local sewage plant, which required less effort. The results showed that the total nitrate load of the brine (889 mg/l) was degraded in the sewer. In summary, electrodialysis can provide an efficient removal of inorganic anions from drinking water. Since most known toxic anions are mono-valent (Table 1), the use of mono-valent anion perm-selective exchange membranes is especially attractive. Situations, in which ED appears to be less applicable are for waters of very low salinity (conductivity of less than 0.5 mS), for which Donnan dialysis can be a better solution, and, in cases when besides ions, removal of low-molecular mass non-charged compounds (to which ED is obviously ineffective) from the water is desired. In the latter case, pressure-driven membrane processes as RO or NF may be preferable. The brine discharge/treatment issue, however, remains important for all these separation processes.

5. Gas-transfer membrane bioreactors Originally, gas-permeable membranes were suggested to provide bubbleless aeration in aerobic bioprocesses (Cote et al. 1988; Semmens 1991).

375 The advantage is that if no gas bubbles are formed, medium foaming and gas stripping effects can be avoided. Gas mass transfer can be accomplished by gas-permeable dense membranes or by hydrophobic microporous membranes, in which the pores remain gas filled (Ahmed & Semmens 1992). Composite membranes made of different materials can provide even more flexible operation. Since inorganic anions serve as electron acceptors in anaerobic microbial processes, oxygenation (aeration) must be prevented. Hydrogen can serve as a non-polluting and non-toxic electron donor for reducing different oxy-anions by autotrophic microorganisms. However, hydrogen economy concept must overcome tremendous obstacles and challenges due to the high cost of H2 production from renewables, and more importantly, the need to develop the required H2 infrastructure (Simbeck 2004). Moreover, direct H2 injection into contaminated water supplies is not acceptable because of its low solubility and high flammability. In such applications (as in the case of oxygen mass transfer) gas-permeable membranes can offer the advantage of efficient hydrogen mass transfer without bubble formation, which prevents both the waste of excess H2 and the accumulation of explosive amounts of H2 in the headspace above groundwater. Since the anionic pollutant(s) and microbial culture are in contact, in this aspect the configuration is more similar to pressure-driven membrane bioreactors (Figure 1). A promising application is believed to be an in situ water remediation, i.e., at the contaminated site. Stimulation of naturally occurring anaerobic processes in situ can be an effective alternative treatment for inorganic contaminants in aquifers since they can be anaerobically transformed to less toxic and/or immobilized forms (Coates & Anderson 2000). Therefore, hydrogen gas-transfer membrane bioreactors, in which H2 is supplied to the lumen of hollow fibre membranes and diffuses to the shell side, where it is used by microrganisms as an electron donor for the reduction of inorganic anions, have received considerable attention (Table 6). Lee and Rittmann (2000) studied autohydrogenotrophic denitrification of drinking water in a composite hollow-fibre membrane bioreactor. The membrane used was made up of two materials.

The outer and inner layers were of microporous polyethylene and, in between, a 1-mm thick layer of non-porous polyurethane allowed the creation of a high driving force for gas dissolution without bubble formation. Accumulation of a biofilm (thicker near the hydrogen gas inlet) on the surface of the hollow fibres was observed due to the partial pressure drop inside the fibres. An almost total H2-utilization efficiency within the biofilm was achieved at a nitrate removal rate of 0.41 g/(m2 h) and a treated water production rate of about 8 l/ (m2 h). However, an increase of 0.9 mg C/l (as dissolved organic carbon) in the treated water effluent was detected. The authors pointed out that this was a level high enough to result in bacterial growth in a distribution system, therefore, a process removing this ‘‘biological instability’’, such as rapid filtration or activated carbon adsorption, was suggested as a post-treatment step. Later, it was shown that the most important factor for nitrogen removal efficiency is the hydrogen pressure, which controls the hydrogen flux into the biofilm (Lee & Rittmann 2002). Under hydrogen-limiting conditions, incomplete nitrate removal degrees between 39 and 92%, and possible appearance of nitrite in the treated water were observed. The optimum pH for autotrophic denitrification was found to be in the range 7.7– 8.6, with a maximum at 8.4 (Lee & Rittmann 2003). Increasing the pH above 8.6 caused a significant decrease in nitrate removal rate and a dramatic increase in nitrite accumulation; therefore it was suggested that the process might require pH control. A hydrogen gas-transfer membrane bioreactor containing hydrophobic polypropylene hollow fibres with a pore size 0.05 lm was tested for nitrate removal from contaminated water by Ergas and Reuss (2001), who also observed the build up of a biofilm layer on the surface of the membranes. A pressure difference (of about 30 kPa) between the shell (liquid) and lumen (H2 gas) was required to maintain the proper conditions for system operation. A bigger pressure difference resulted in water penetration into the membranes while a lower one led to bubble formation. The maximum nitrate removal rate was 0.46 g/(m2 h), which compares well with the values obtained by Lee and Rittmann (2000); however, a very large increase in the effluent dissolved organic carbon from 11 to 31 mg C/l was detected. The authors attributed

Rittmann et al. (2004) Groundwater (California, USA)

Nerenberg et al. (2002) Spiked groundwater (California, USA)

Haugen et al. (2002)

Composite hollow fibres (Pilot scale system)

ClO4

)

ClO4 , NO3 Composite hollow fibres

)

NO3 Silicone-coated hollow fibres

)

NO3 Silicone tube

)

NO3

)

Polypropylene hollow fibres (0.05 mm pore size)

)

Model water

Ho et al. (2001) Model water

Ergass and Reuss (2001) Groundwater (Massachusetts, USA)

Tap water supplemented with nitrate salts (USA) NO3) Composite hollow fibres

Lee and Rittmann (2000)

Water origin Target anion(s) Membrane configuration

Table 6. Hydrogen gas-transfer membrane bioreactors for removal of inorganic anions from drinking water

Reference

376 this to soluble microbial products, such as proteins and polysaccharides, leaking from the microbial cells and concluded that further treatment is necessary to remove biological products from the water prior to distribution. Haugen et al. (2002) developed a novel flowthrough reactor designed to simulate groundwater flowing through a nitrate-contaminated aquifer. This process guaranteed almost complete NO3) removal once H2 limitation was corrected. With a membrane module, made of silicone-coated reinforced fibreglass fibres, a nitrate removal rate of 0.8 g/(m2 h) and a treated water production rate of about 10 l/(m2 h) were achieved. Effluent water analysis indicated that the total organic carbon rose by only about 0.5 mg C/l. Since the test reactor contained aquarium rock media in order to mimic natural environment, filtration and adsorption of organic matter may had improved the water quality. Again, biofilm formation encouraged by the delivery of the electron donor (hydrogen) on the membrane surface was observed. Although, in a previous study, Semmens and Essila (2001) found that the biofim could increase the flux of a gaseous substrate out the membrane due to the driving force improvement effect, this phenomenon would impend gas transfer to the bulk liquid, thus decreasing the reaction zone around the membranes. This problem appears to be more important for in situ remediation since natural convection in groundwater may be poor. Furthermore, mineral precipitates on the membrane surface (e.g. ferric oxide) might decrease gas transfer both to the biofilm and to the bulk liquid (Roggy et al. 2002). Following the promising results obtained with nitrate, perchlorate removal in the gas-transfer membrane bioreactor developed by Lee and Rittmann (2000) was studied by Nerenberg et al. (2002). The perchlorate concentration was successfully reduced to below 4 lg/l when the influent perchlorate concentration was as high as 100 lg/l. Perchlorate reduction occurred immediately upon exposure to this pollutant in nitrate-reducing system, and no inoculation with specialized perchlorate reducing bacteria was necessary. However, nitrate in the water slowed perchlorate reduction rate, therefore, the authors suggested that pH control (at around pH 8) and/or complete nitrate removal may be required in order to increase the perchlorate reduction rates.

377 A pilot-scale system, consisting of two gastransfer membrane bioreactors in series, was tested to treat a perchlorate-contaminated groundwater (60 lg/l of perchlorate) in La Puente, California (Rittmann et al. 2004). The perchlorate removal was high (95%) and the treated water perchlorate concentration below 4 lg/l. The water flow rate was set to 66 l/h and effluent recycle at 1020 l/h was performed to improve mass transfer and reduce clumping of the fibres; however, periodical air scouring to remove excessive biofilm accumulation was still necessary. For a polluted groundwater, containing 30 lg/l of ClO4) and about 58 mg/l of NO3) at a water flow rate of 228 m3/h, the preliminary cost estimations are of USD 933,000 total capital cost and USD 0.21/m3 operating cost. The authors pointed out that the concentration of nitrate controls the operating cost since it is the dominant electron acceptor and its reduction is the largest H2 consumer. It may be summarised that the gas-transfer membrane bioreactor is a promising technology for the bioremediation of drinking water supplies. Until now, research has been carried out with ex situ laboratory-scale reactors, but the possibility for in situ implementation is rather attractive. The most serious concerns for future research seems to be prevention, or at least minimization, of the membrane inorganic and microbial fouling and treated water quality, which has still not reached acceptably low TOC levels.

6. Conclusions In this paper, we described some of the features of membrane processes, with an emphasis on membrane bioreactors, making their inclusion in drinking water production attractive. We also discussed some current problems that still provide a challenge for research. The use of membranes in drinking water treatment is a developing technology. NF and ED can provide more or less selective removal of the target pollutants, especially when separations between mono- and multi-valent anions are desired. In NF, this is a consequence of both, ion size and charge exclusion effects, while in ED is due to the use of ion exchange membranes with mono-anion perm-selectivity. Higher membrane affinity for a given anion in ED can additionally

increase the selectivity discriminating also between mono-anions (nitrate and chloride are good example). However, the concentrate brine discharge and/or treatment can be problematic in many cases. Combining the advantages of membrane separation with biological reactions for the treatment of polluted water supplies has resulted in the development of three major membrane bio/processes: pressure-driven membrane bioreactors, gastransfer membrane bioreactors, and ion exchange membrane bioreactors. In the first case, membranes are essentially regarded as micro/ultra porous barriers to promote high biomass for process intensification and avoid contamination of the treated water with microbial cells. However, secondary water pollution by an incompletely degraded organic carbon source and other lowmolecular mass compounds is possible. Hydrogen gas-transfer membrane bioreactors appear especially attractive for in situ water remediation, while ion exchange membrane bioreactors can provide a highly selective target ion removal and avoid secondary pollution of the treated water. What next? The immense potential for using perm-selective membranes – acting not exclusively based on size exclusion – has not been yet completely explored. The opportunity for improving the performance of the existing and development of novel types of membrane bioreactors is clear: the demand for selective removal of defined pollutants (not only ionic but polar and non-polar compounds as well) opens new directions with the ultimate goal of producing safe and high quality drinking water. Acknowledgements The financial support from Fundac¸a˜o para a Cieˆncia e a Tecnologia (Lisbon, Portugal) through projects POCTI/BIO/43625/2000, POCTI/EQU/ 39482/2001 and grant SFRH/BPD/9467/2002 is gratefully acknowledged. References Ahmed T & Semmens MJ (1992) The use of independently sealed microporous membranes for oxygenation of water: Model development. J. Membr. Sci. 69: 11–20 Amor Z, Bariou B, Mameri N, Taky M, Nicolas S & Elmidaoui A (2001) Fluoride removal from brackish water by electrodialysis. Desalination, 133: 215–223

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