Effects of microbial degradation of biofoulants on microfiltration membrane performance in a membrane bioreactor.

In membrane bioreactors (MBRs) for wastewater treatment, membrane fouling, particularly biofouling caused by soluble microbial products (SMP), is a nuisance problem causing decreases in permeation flux. In a previous study, we identified primary biofoulants of microfiltration (MF) membranes in SMP as polysaccharides containing uronic acids that undergo inter- and intramolecular ionic cross-linking by polyvalent cations, forming a gelatinous mass that clogs membrane pores. In the present study, we therefore attempted to isolate biofoulant-degrading microorganisms from activated sludge on a polygalacturonic acid-overlaid agar medium and evaluate their efficiency for preventing biofouling of MF membranes. Among the isolates, the fungal strain HO1 identified as Phialemonium curvatum degraded 30% of polysaccharides containing uronic acids into smaller molecules in a SMP solution containing a high concentration of saccharides after 30 days of cultivation. Microfiltration tests using a laboratory-scale submerged MBR indicated that the filtration resistance of this degraded SMP solution was lower than that of the control SMP solution without fungal inoculation. Importantly, accumulation of gelatinous mass on the membrane responsible for biofouling was avoided in the SMP solution augmented with P. curvatum HO1 during the microfiltration test. This is the first report to describe a new method for avoiding biofouling of MBRs by microbial degradation of primary biofoulants.

Pilot study of a submerged membrane bioreactor for water reclamation.

The objective of this pilot study was to investigate the operational conditions of newly developed MBR modules for water reclamation under tropical conditions. MUDC-620A MBR modules with hollow fibre PVDF membranes from Asahi-Kasei Chemicals were used in the study. The pilot plant with capacity of 50 m(3)/d was operated continuously (24-hour) over four months on site of Ulu Pandan Water Reclamation Plant (UPWRP) in Singapore. During the study, the MLSS in membrane tank was in the range of 6,840 approximately 9,540 mg/L. Filtration operation mode of the membrane unit was 9 minutes on production and 1 minute backwash. The air scouring for the membranes was 0.18-0.30 Nm(3)/h per m(2) membrane area all of the time. Trials on different membrane fluxes were conducted to obtain the sustainable flux. The analytical results showed that COD, TOC, T-N and NH4-N of the treated water were <30 mg/L, 5-7 mg/L, <13 mg/L and <0.1 mg/L, respectively, which met the requirement of Industrial Water for reuse. TMP was in the range of 12-40 kPa and could be recovered after cleaning with 2,000 mg/L sodium hypochlorite solution. Sludge clogging could be a challenge for long-term operation with the current module design. It was concluded that it was feasible for MUDC-620A MBR to operate at a net flux of 25-29 LMH (or 0.6-0.7 m/d) for treating the municipal wastewater at UPWRP.

Identification of biofoulant of membrane bioreactors in soluble microbial products.

To reveal primary biofoulant in soluble microbial products (SMP) and/or soluble extracellular polymeric substances (EPS), after removal of sludge particles, activated sludge samples were subjected to microfiltration tests in a submerged MBR. Filtration resistance directly correlates with the saccharide concentration. Saccharides in wastewater from several sources contained uronic acids, which increased the filtration resistance. When the microfiltration test liquids contained saccharides over 80mg l(-1), a gelatinous mass remained on the membrane surface after filtration and contained concentrations of saccharides and uronic acids 50 times higher than the original test liquid while only trace amounts of these substances were contained in the filtrate. The gelatinous mass contained high molecular weight substances of 10(6)-10(8)Da, suggesting the presence of polysaccharides. However, molecules of this size were calculated to be much smaller than the pore size of the membrane. Ethylenediaminetetraacetic acid decreased filtration resistance, suggesting that polysaccharides containing uronic acid units could undergo intermolecular or intramolecular ionic cross-linking by polyvalent cations and form the gel, thus clogging the membrane pores as an actual biofoulant.