By Jordan Schmidt
Conventional activated sludge wastewater treatment plants (WWTP) operate with solids (mixed liquor suspended solids – MLSS) concentrations ranging from approximately 1500 to 5000 mg/L. In membrane bioreactors (MBRs), typical MLSS concentrations are even higher (10,000 to 12,000 mg/L). A higher concentration can be advantageous as they allow for smaller aeration tanks while performing the same level of treatment. Therefore, pushing MLSS higher, and potentially converting from conventional activated sludge to MBR, may allow a WWTP to deal with problems associated with insufficient capacity. However, raising the MLSS concentration also has some downfalls. As the MLSS concentration increases, aeration efficiency decreases. For instance, going from an MLSS of 300 mg/L to 4000 mg/L, results in aeration efficiency decreasing approximately 9%. Greater than 50% of a typical WWTPs energy demand is attributed to aeration; therefore, optimizing – rather than just raising – the MLSS concentration can result in significant energy savings. Aeration energy demand is related to the MLSS concentration through the alpha factor. The alpha factor is a ratio of aeration efficiency in wastewater versus clean water and is indirectly proportional to theoretical power consumption.