Effects of solid concentrations and cross-flow hydrodynamics on microfiltration of anaerobic sludge

The filtration characteristics and rheological properties of anaerobic sludge suspension containing different solid content were investigated using poly-tetrafluoroethylene (PTFE) microfiltration membrane with a pore size of 1 μm. The initial rapid flux decline was in good agreement with standard blocking filtration law, while the latter gentle flux decline is 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 total solids (TS) concentration of 13–18 g/L. Initial standard blocking significantly affects the flux decline mechanism at TS concentrations of 10 g/L or below and cake formation dominates the flux decline at higher TS concentrations above 25 g/L. TS concentration significantly affected rheological properties as well as flux decline in microfiltration of anaerobic sludge. TS concentration of 5 g/L or less characterizes the transition from Newtonian to non-Newtonian behavior. Both Casson and power-law equations were fit for rheological modeling of anaerobic sludge with TS concentration of 10 g/L or above. The minimum cross-flow velocity (CFV) to produce turbulent flow (Reynolds number ∼ 2100) increased almost linearly to 0.5–0.8 m/s at TS concentration of 10–20 g/L. The concentration of soluble microbial products (SMP) was increased and the mean particle size was decreased with increase of CFV. Extracellular polymeric substances (EPS) even decreased after 6 h filtration runs. Some of EPS might contribute to the increased SMP due to cell lysis.