Mechanisms of SMP production in membrane bioreactors: choosing an appropriate mathematical model structure

Being able to predict the soluble microbial product (SMP) concentration, an important foulant in membrane bioreactors (MBRs), with mathematical models provides the opportunity to use foulant production as an MBR design and optimization parameter. This study examined the ability of three mathematical model structures to describe two distinct mechanisms of SMP production. The production mechanisms evaluated are (1) the erosion or hydrolysis of floc-associated extracellular polymeric substance (EPS) and (2) decay of active cells. The models were compared based on their ability to predict SMP concentrations observed in an MBR system during a period of increased SMP and floc-associated EPS production due to increased predation. Predation was an important contributor to overall biomass decay. Short-term batch experiments were also preformed to examine model assumptions related to the (1) production of SMP due to decay of active cells, (2) production of SMP due to erosion of floc-associated EPS, (3) degradability of SMP present in the MBR mixed liquor during increased predation and (4) degradability of eroded floc-associated EPS. Both erosion of floc-associated EPS and decay of active cells were shown to be important independent mechanisms of SMP production. Therefore, a mathematical model used to predict SMP concentrations should provide the ability to capture both mechanisms independently. SMP produced during increased predation were slowly degradable while eroded floc-associated EPS was rapidly degradable. Model results demonstrate that the slowly biodegradable SMP fraction will dominate the bulk phase SMP concentration.