Membrane Bioreactor for Water Reuse

Desalination 203 (2007) 15–19

Membrane bioreactor for water reuse

Christelle Wisniewski Laboratoire de Génie des Procédés d’Elaboration de Bioproduits, UMR CIRAD 016, CC 005, Université de Montpellier II, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France Tel. +33 (4) 67 14 46 24; Fax +33 (4) 67 14 47 97; email: [email protected]

Received 22 May 2006; accepted 26 May 2006

Abstract The membrane bioreactor (MBR) is based on the combination of a suspended biomass reactor and a separation step on porous membrane filtration. It presents several advantages in terms of water resource protection because of the great quality of the treated water that can be reused as well for irrigation, cleaning or cooling water on industrial site, domestic purposes (e.g. toilet flushing). The quality of the water, notably the absence of solids in suspension and turbidity, allows considering the MBR as a perfect tool for pre-treatment before desalination or preparation of water of very high quality. Moreover, because of its modular property, the MBR may also be favourable for developing a new wastewater network configuration in an urban area. Keywords: Membrane bioreactor; Water reuse; Sludge production; Membrane fouling

Wastewater treatment is based on the abate- must now be removed to comply with the effluent ment of (i) solids in suspension by physical pro- quality standards. cesses (settling and/or filtration) and (ii) soluble For biodegradable compounds, biological pro- compounds mainly by biological processes when cess combinations are proposed to ensure the compounds are biodegradable or physical or appropriate water quality and the environmental thermal specific processes (chemical oxidation, legislation in regards to organic carbon, nitrogen ion exchange, desorption, etc.) when they are not or phosphorus removal. Different biological tech- biodegradable. However, since the 1990s, it is no nologies are developed: extensive processes for longer sufficient to eliminate settable and soluble small communities such as constructing wetlands organic compounds, and phosphorus and nitrogen or intensive processes, such as trickling beds and

Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.

0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.05.002 16 C. Wisniewski / Desalination 203 (2007) 15–19 activated sludge, for large industrial plants or large aesthetics, environmental impact (odour and cities. noise), Because the biological process is easier to con- (iii) reliability notwithstanding the influent trol in a suspended culture reactor, activated characteristic variation, sludge processes are often prevalent in wastewater (iv) standards regarding sustainability (energy, treatment when an intensive procedure is looked chemicals and waste production). for. This process consists in biodegradation of the pre-treated influent by microorganisms in a con- The membrane bioreactor (MBR) holds the tinuous tank where oxygen supply is controlled potential to become one of the new generation according to the type of reaction (aerobic, anoxic treatment processes. This system is based on the or anaerobic conditions). Following bioreactions, combination of a suspended biomass reactor and a clarification is operated in a settling tank to en- a separation step on porous membrane filtration sure the separation of the treated water from the [1]. Low-pressure membranes, either microfilters biomass. Although this conventional technique (MF) or ultrafilters (UF), are used for the solid/ may appear robust and safe, the activated sludge liquid separation step. The membrane retains not process displays three major disadvantages: only solids but also high-molecular weight com- • The treated water quality is dependant on the pounds that generally escape into the treated water settling properties of the biological suspension. when settling is operated as the final separation Indeed, bad flocculation of the biological sus- step. Thus, the retention capacity of the porous pension, resulting from dysfunction (e.g. varia- membrane involves a very high treatment degree tion of influent characteristics, oxygen short- of the treated water in terms of solids retention age, etc.), induces bad settling with (i) a result- including germs and viruses. ing presence of suspended solids in the treated The membrane configurations are based on water and (ii) a progressive washing out of the either plane or cylindrical geometry by using biomass from the bioreactor. plate-and-frame membranes, tubular and hollow • According to the variation of the biomass settl- fibres. The membrane bundle can be configured ing rates in regards to biomass concentration, in a sidestream position or directly immersed in the clarification is generally carried on with the bioreactor (Fig. 1). biomass concentration lower than 5 kgTSS/m3 In sidestream configuration, the operation of to be in favourable settling conditions. filtration is generally carried out under high • The insufficient germ removal or persistent pressure (from 1 to 4 bar) and consists in a tan- organic pollutants presence limit the reuse of gential recirculation at hogh speed of the bacterial treated waters except (i) for specific condition suspension in the membrane module (from 0.5 to of irrigation or (ii) when using tertiary treat- 4 m/s). In immersed configuration, the membranes ment including extensive processes or some are even inside the biological reactor, in total im- drastic oxidation step. mersion. Filtration is carried out under low trans- membrane pressure (from 0.05 to 0.5 bar) and of Nowadays, because of the shortage of water more important membrane surfaces must be so resources and continuous quality degradation, it operated in order to ensure the desired pro- is clearly essential that new generations of treat- ductivity. ment processes have to be defined to achieve: Because the membrane separation is not (i) disinfection without any oxidation step that dependant on the influent characteristics or the induces carcinogen molecule formation, flocculation state of the biological suspension, the (ii) possibility of compactness to optimize biomass concentration can be higher than in C. Wisniewski / Desalination 203 (2007) 15–19 17

In Out In

Out Bioreactor Bioreactor Clarif ier Membrane Membrane

Sludge Sludge a) Sludge b) Sludge extraction extraction

Fig. 1. Configurations of MBRs: (a) sidestream and (b) submerged configuration. conventional activated sludge (2–5 times higher) in the sidestream configuration or to the air injec- so the reaction rates can be more successful and tion in the submerged configuration), relaxation the quality of the treated water is generally very period and/or back-flushing. high from a sanitary point of view. In comparison with the conventional activated The limiting factor remains the membrane sludge process, this technology presents several fouling that reduces the membrane permeability advantages: during the MBR operation. This membrane foul- (i) the fact that the biomass can be 2–5 times ing is dependant on various parameters concerning more concentrated induces a reduction of the the suspension characteristics, the membrane cha- reactor size and/or a reduction of the sludge racteristics and the operating conditions (Fig. 2). production, Three families of compounds (particular com- (ii) the effluent quality is significantly better as pounds, colloidal and soluble compounds) take the suspended and colloidal materials are re- part in membrane fouling that can be considered moved as well as all the associated pollutants, to be either reversible or irreversible. A long-term such as heavy metals, micro-pollutants, bac- diminution in flux which is not recovered by teria, viruses even colour, simple hydrodynamical techniques is indicative (iii) flexible and phased upgrading of existing of irreversible fouling, and this is often attributable wastewater treatment plants is possible. to colloidal deposition or soluble adsorption onto the membrane [2,3]. Chemical cleanings are nece- Table 1 gives the results of comparison of the ssary to eliminate such fouling. The deposition of MBR performances with the performances of particular compounds is considered as reversible conventional treatment processes [4,5]. fouling and can be avoided by suitable filtration conditions. Indeed, this deposition of solids and Conclusion high-molecular weight compounds can be controlled during the operation by achieving specific The association of a biological reaction and cleaning procedures by means of high shear stress separation by porous membrane displays several at the membrane surface (due to the fluid velocity advantages compared to the traditional activated 18 C. Wisniewski / Desalination 203 (2007) 15–19

Suspension characteristics

Total Suspended Solids concentration

Soluble compound concentration (SMP) …

MEMBRANE FOULING

Membrane configuration Membrane characteristics Operating conditions

Fibre density Porosity pH, Temperature

Module type Surface charge Hydraulic retention time

Hydrodynamic conditions Hydrophobic aspect Solids retention time … … …

Fig. 2. Key parameters in membrane fouling.

Table 1 MBR performances vs. conventional processes

Raw water Treated water TSS COD Turbidity Germs TSS COD Turbidity Germs (kg/m3) (kg/m3) (NTU) (/100ml) (kg/m3) (kg/m3) (NTU) (/100ml) Trickling bed 0.2 0.7 120 108 0.035 0.125 10 106 Activated sludge 0.2 0.7 120 108 0.030 0.080 5 106 Physico-chemical process 0.2 0.7 120 108 0.060 0.130 20 107 MBR process 0.2 0.2 120 108 0 0.020 <2 <102

sludge process, such as high effluent quality and water could be considered for surface water flush- space requirement. The high quality of effluent, ing or for domestic purposes (e.g. toilet flushing). free from solids and germs, meets many of the In addition, the effluent of an MBR plant would current quality standards throughout the world and be very suitable for further upgrading with nano- will meet the increasingly stringent standards of filtration/reverse osmosis up to drinking water tomorrow [1]. A recycling or reuse of the treated quality. Moreover, because of its modular property C. Wisniewski / Desalination 203 (2007) 15–19 19 the MBR may also be favourable for developing tion of membrane configurations and the reduction a new wastewater network configuration in an of the sludge production. urban area. If the main disadvantages of the MBR process result from membrane costs and membrane foul- References ing, the recent reduction of membrane costs and the implementation of fouling control strategies [1] T. Stephenson, S. Judd, B. Jefferson and K. Brindle, should guarantee the development of this process. Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London, 2000. In this context, there is recent interest in MBR [2] A. Massé, Bioréacteurs à membranes immergées technology for domestic wastewater treatment, pour le traitement des eaux résiduaires urbaines due to an increasing number of water repurifica- spécificités physico-chimiques du milieu biologique tion/reclamation projects and continuing advance- et colmatage, Thèse INSA Tououse, N°ordre 759, ment in the membrane technology. For municipal 2004. application, the submerged configuration appears [3] S. Ognier, C. Wisniewski and A. Grasmick, Influ- to be more cost-effective, and preliminary cost ence of macromolecule adsorption during filtration evaluation shows that the MBR-process is cost- of a membrane bioreactor mixed liquor suspension, J. Membr. Sci., 209 (2002) 27–37. competitive in relation to conventional wastewater [4] E. Tardieu, A. Grasmick, V. Geaugey, J. Manem, treatment processes including MF/UF to produce C.H. Xing and Y. Qian, Pilot study of domestic equivalent effluent quality [6]. wastewater tretment by ceramic membrane bio- Whether or not the MBR-process holds the reactor. Treatment performances and fouling charac- potential to become the new generation of waste- terisation, Wefuec ASIA, Singapore, 8–11 March water treatment plant, research needs to continue 1998. in order to optimise the synergistic relationship [5] M.F. Pouet, A. Grasmick, F. Homer, F. Nauleau and between the biological process and the membrane J.C. Cornier, Tertiary treatment of urban wastewater by cross-flow microfiltration, 17th Biennial Int. process. New operational methods and strategies Conf., Budapest, 24–29 July 1997. need to be developed and some aspects still need [6] S. Adham, San Diego trials show MBR competes to be studied, such as the influence of high biomass on cost and quality, Desalination and Water Reuse, concentrations on oxygen transfers, the optimisa- 12(3) (2002) 24–29.