Water Journal : Water Journal April 2011
membranes & desalination 104 APRIL 2011 water technical features This report has been drafted by the Editor based on the platform presentation by Professor Fane at the AWA Membranes and Desalination Conference. Introduction Membranes and microbiology have complex relationships in water and wastewater treatment. One is planned and beneficial, where it enables cheap and effective reduction of organics in an effluent, mainly by the activated sludge process coupled with low pressure membranes, as in the Membrane Bioreactor (MBR). However, the other relationship is unplanned, where biological growth causes biofouling of membrane processes, such as in MBRs and Reverse Osmosis (RO) systems. The question is, how best to manage the relationships between membrane processes and microbiology. This paper reports on some of the projects underway at the Singapore Membrane Technology Centre at Nanyang Technological University, dealing with membranes and microbiology. Membrane Bioreactors MBRs applied to activated sludge processes have greatly increased efficiency, mainly by increasing the MLSS by their filtration mechanism rather than relying on sedimentation, as well as retention by the membrane of macromolecular organics. There is no doubt that MBRs have come of age. Many thousands are in operation worldwide and they typically achieve 99% removal of most organics. However, with conventional MBRs some 25% of the low molecular weight compounds may pass through the membrane, so there is scope for development of high-retention (HR) MBR systems. We are investigating two such HRMBRs, which operate at atmospheric pressure and can be driven by low-grade waste (or solar) energy, as outlined in Figure 1. These systems offer high-quality product water for recycling, but also have a low carbon footprint. They rely on the application of specially acclimatised microorganisms coupled with non- conventional membranes. One HRMBR option is the Forward Osmosis Bioreactor, which uses a 'tight' FO membrane (a development of the more common RO membrane) and necessitates an effective system for regenerating the draw solution. Since salt is retained by such membranes, and only discharged with the waste excess sludge, its concentration in the mixed liquor will spiral up to, say, 9-10g/L. In our studies this seems to exert only a mild effect on the activity of the micro-organisms, and in a pilot plant very high removals of TOC were achieved, > 99% for a run of 45 days. At that point, a shock dose of pharmaceuticals (25μg/L Ibuprofen, Naproxen, Carbamazepine, Diclofenac) reduced TOC removal to 97%, but within a few days the system had completely recovered. We are also investigating the Membrane Distillation Bioreactor (MDBR), using thermo-tolerant organisms at 55°C. The MD membranes allow transmission of volatiles, such as water, and retain all other species. In the lab the MDBR achieved > 99.5% organics removal, until the membrane was accidentally wetted. However, this was reversed by simple procedures and normal T Fane and co-workers (see Acknowledgements, p. 106) Initiatives at the Singapore Membrane Technology Centre WATER, MEMBRANES AND MICROBIOLOGY: THE GOOD NEWS AND THE BAD NEWS M W/W S A M W/W S A X H2O H2O QH QH Membrane Distillation Bioreactor (MDBR) Forward Osmosis Bioreactor (FOMBR) Driving force : waste or solar heat Draw solute regeneration 550C Figure 1: Options for high retention MBRs. 95.0% 95.5% 96.0% 96.5% 97.0% 97.5% 98.0% 98.5% 99.0% 99.5% 100.0% 0 5 10 15 20 25 30 35 1 6 11 16 21 26 31 36 Organic removal eﬃciency Permeate TOC (mg/L), Flux (LMH) Time (days) MDBR: Flux and organic removal eﬃciency Permeate ToC Flux Overall organic removal eﬃciency Organic removal > 99.5% , except for membrane wetting event. Figure 2: MDBR performance: MD (55C) + thermophilic/ halotolerant biomass. Applications: (i) Petrochemical water reuse (pilot) (ii) Anaerobic MDBR.
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