Anaerobic membrane bioreactor for the treatment of low strength wastewater
This research investigated the fundamentals of anaerobic membrane bioreactor (AMBR) operated at low trans-membrane pressure (TMP) and cross-flow velocity (CFV) using poly-tetrafluoroethylene (PTFE) membrane for the treatment of low strength wastewater.;Specific methanogenic activity (SMA) test was used to examine the methanogenic activity profiles of suspended and attached sludge in AMBR treating synthetic municipal wastewater at 25 and 15°C. It was hypothesized that accumulated biomass on the membrane surface could act as a secondary membrane as well as a biofilm which removed chemical oxygen demand (COD) biologically. The results showed that attached sludge on the membrane surface had lower activity than suspended sludge. Attached sludge on the membrane surface contained less extractable extracellular polymeric substance (EPS), especially protein content, than the suspended sludge, which could be related to the decreased methanogenic activity. Membrane in AMBR system is likely not only to retain all biomass in the reactor, but also complement decreased biological removal efficiency by rejecting soluble organics.;AMBR was operated at COD loading rates of 1-2 kg/m3˙d for 280 days. Permeate TCOD concentration was always less than 40 mg/L, and no noticeable volatile fatty acids (VFAs) were detected regardless of hydraulic retention time (HRT) variations, while soluble COD was accumulated in the reactor at lower HRT. The particle size reduction was relatively less than other studies reported even after long operation time due to the low operation CFV. Approximately 30% of COD was not recovered as methane irrespective of applied HRTs, due to the COD loss by dissolved methane, sulfate reduction. The observed x methane yield was 0.21 to 0.22 CH4/g CODremoved regardless of the applied HRTs due to the COD loss by dissolved methane and sulfate reduction.;The filtration characteristics of anaerobic sludge suspension containing different solid contents were investigated. The initial rapid flux decline was in good agreement with standard blocking filtration law, while the latter gentle flux decline was attributable to the cake filtration law, which represented a Class II type dynamic membrane. The highest pseudo-steady state flux and lowest normalized flux reduction were observed at TS concentration of 13-17 g/L, Particles are likely to act as agglomerated particles by a bridging effect through particle-particle interactions at concentrated TS levels. However, the lower particle concentration does not necessarily yield the higher flux due to the internal fouling by dispersed particles. Moreover, the higher particle concentration also caused a gradual deterioration in flux due to the severe cake fouling. The increased CFV influenced the pseudo-state flux more significantly at low or high TS concentration. Anaerobic sludge suspension which had been filtered previously at CFV of 0.1-0.7 m/s had a lower flux than fresh anaerobic sludge suspension at the same CFV, because the higher shear force increased the concentration of soluble microbial product (SMP) and decreased the mean particle size in anaerobic sludge suspension. The extractable EPS content in anaerobic sludge, however, was not changed regardless of applied CFV.