Properties affecting the solids processing of sludges from the submerged membrane bioreactor and activated sludge process

Physical, chemical and biological properties were determined for sludges from a pilot-scale submerged membrane bioreactor (SMBR) and two bench-scale complete-mix activated sludge (CMAS) reactors treating municipal primary effluent wastewater at mean cell residence times (MCRT) from 2 to 10 d. One CMAS reactor (CMAS Hi) was operated at high shear, and the other (CMAS Lo) was operated at low shear. Sludge properties were compared using laboratory methods for predicting solid-liquid separation. In general, SMBR sludge had higher colloidal material content, lower extracellular polymeric substance (EPS) content, and higher levels of filamentous microorganisms than either CMAS sludge. Higher colloidal material content was due to the more efficient solids retention by the membrane. The EPS, necessary for bioflocculation, was lower because there was no selective pressure for sludge to settle. The higher filamentous microorganism content was due to high levels of nocardioform bacteria resulting from the "trapping environment" created by the membrane. The CMAS Hi and SMBR sludges had similar particle size distributions (PSD) with a majority of particles in the 2--4 mum range due to the higher shearing conditions. The CMAS Lo sludge PSD was distinctly bimodal with peaks in the 2--4 mum and 10--20 mum ranges. Lower settling rates of SMBR sludge were attributed to higher filamentous microorganism content and possibly to smaller particles. Lower solids recovery and float concentration in batch flotation tests for SMBR sludge was related to higher levels of colloidal material and nocardioform bacteria. Low compaction of SMBR sludge in centrifuge tests corresponded to higher levels of filamentous microorganisms. Normalized capillary suction time values for SMBR sludge were lower than for CMAS sludges, likely caused by lower EPS content. There was no significant difference in normalized time to filter values between reactors at each MCRT because of the offsetting effects of EPS and colloidal material; SMBR sludge had high colloidal material content and low EPS, and CMAS sludge had high EPS and low colloidal material content. Chemical conditioning of SMBR sludge improved settling, flotation and filterability but not compaction. Results were used to assess the performance of SMBR sludge type in full-scale thickening and dewatering processes.