The Process and Water Quality Specialists
For an MBR facility, a design engineer must understand the importance of mixed liquor properties as membrane filtration is being directly applied to this medium. The biological conditions that create an low fouling sludge are impacted by the solids retention time, hydraulic retention time, the reactor hydraulics, the reactor shear environment, influent wastewater characteristics, location of solids wasting with drawl point, and electron acceptor conditions. MBRs have the advantage of the perfect solid-liquid separation provided by the membrane barrier, but MBRs also have the disadvantage that all treated wastewater flow must be filtered through the membranes before being discharged. As a result, MBRs treating high peak and wet weather flows need to be carefully designed and expertise on membrane flux rates, membrane operating conditions, mixed liquor properties, and controlling these parameters to maximize the hydraulic throughput requires diligent, knowledgeable and conservative engineering.
At high organic loading rates, steady-state membrane fouling rates increase due to rapid organic fouling. The organic fouling results from incomplete biodegradation of carbohydrate and protein soluble microbial products (SMP) and larger molecular weight organics present in the mixed liquor. Through his research, Dr. Shane Trussell proved that protein and carbohydrate concentrations increase on the membrane surface as loading increases and that the molecular weight of these organics increased with increasing loading rates. Finally, although the majority of fouling resulted from SMP, the cake properties of materials rejected at the membrane surface increased in adhesion, also known as a “sticky cake.” The total concentration of extracellular polymeric substances (EPS) increased with organic loading as did the molecular weight of the EPS. This work was performed on real wastewater with two different MBR manufacturers, one a MF membrane and the other UF.