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The properties of sludges from a pilot-scale submerged membrane bioreactor (SMBR) and two bench-scale complete-mix, activated sludge (CMAS) reactors treating municipal primary effluent were determined. Compared with the CMAS sludges, the SMBR sludge contained a higher amount of soluble microbial products (SMP) and colloidal material attributed to the use of a membrane for solid-liquid separation; a higher amount nocardioform bacteria, resulting from efficient foam trapping; and a lower amount of extracellular polymeric substances (EPS), possibly because there was no selective pressure for the sludge to settle. High aeration rates in both the CMAS and SMBR reactors produced sludges with higher numbers of smaller particles. Normalized capillary suction time values for the SMBR sludge were lower than for the CMAS sludges, possibly because of its lower EPS content. Water Environ. Res., 79, 320 (2007).

One of the most promising and innovative technologies in water reclamation today is the membrane bioreactor (MBR) process. (Adham et al., 1998) However, the limits of the MBR process for wastewater treatment are not very well understood to date. With funding provided by WERF and in-kind contributions provided by the City of San Diego, the project team investigated the MBR process for wastewater treatment. Two MBR pilot units were operated in parallel to effectively evaluate each test condition on domestic wastewater in San Diego, California. The project was designed to evaluate the limitations and the advantages associated with the MBR process. The first part of the study evaluated the MBR process under high organic loading rates while sustaining nitrification at relatively long sludge ages. The second part of the study evaluated the MBR process under non-nitrifying conditions and reduced suspended solids concentrations. Through out the entire period of testing, regardless of extreme operating conditions, the MBRs consistently produced water that resulted in non-detect BOD 5 values and an average COD removal of greater than 90%. The effluent turbidities averaged 0.1 NTU through out the testing period. In addition, the MBR effluents consistently exceeded the water quality of the full-scale tertiary plant that was treating the same wastewater.

A pilot-scale submerged membrane bioreactor (SMBR) and two bench-scale conventional activated sludge (CAS) reactors were operated on municipal primary effluent over a range of mean cell residence times (MCRTs) from 2-10 d. The reactors had different turbulence levels. The root mean square velocity gradient (G) of the SMBR was 632 s -1 ; for the CAS reactors the G values were 72 s -1 and 250 s -1 . The sludges from all systems were analyzed for particle size distribution (PSD), colloidal material, extracellular polymer substances (EPS) and filamentous microorganisms. Capillary suction time (CST) and time to filter (TTF) analyses were also performed. The SMBR sludges had the highest amount of small particles and higher levels of colloidal material than the CAS sludges. This was attributed to its higher G value and the use of a membrane for solids-liquid separation. The SMBR sludge contained the higher levels of total filamentous organisms attributable largely to its higher nocardioform level. This resulted from more efficient foam trapping by the SMBR. The normalized CST values of the SMBR sludge were lower than for the CAS sludges. This was attributed to its lower EPS content. There was no significant difference between the normalized TTF values of the SMBR and the CAS sludges. This was attributed to the offsetting effects of colloidal material and EPS contents. KEYWORDSSubmerged membrane bioreactor (SMBR), particle size distribution (PSD), extracellular polymeric substances (EPS), filamentous microorganisms, nocardioform organisms, capillary suction time (CST), time to filter (TTF) BACKGROUND