Internal versus external submerged membrane bioreactor configurations for dairy wastewater treatment

Two submerged membrane bioreactor (SMBR) configurations, one with the membrane module internal (SMBRi) and the other one external (SMBRe) to the biological tank, were used for the treatment of dairy industry effluent and evaluated in terms of pollutant removal capacity and fouling, focusing on the production of soluble microbial products (SMP) and extracellular polymeric substances (EPS). Both the MBRs presented excellent chemical oxygen demand (COD) removal efficiency (98% average), color (98%), and nutrients (86% for total nitrogen and 86-89% for total phosphorus); however, it was shown that shearing caused by the sludge recirculation pumps in the SMBRe reduced biomass growth considerably. The SMBRe presented better performance in terms of fouling than the SMBRi, which was associated with the higher concentration of suspended solids and SMP and EPS in the SMBRi. The SMP concentrations (mgSMP/gMLVSS) were superior in the SMBRe, showing that the friction from recirculation pumps leads to the breakdown of flocs and/or cells and to the release of polymeric material into the mixed liquor. Since this effect was more intense for SMP quantified in terms of extracellular transparent polymers, the conclusion was that apparently these substances participate in cellular metabolism in a different way than the carbohydrates and proteins, and that these can be more associated with released substances due to shear stress.