Describe the mechanism by which membrane fouling in MBRs affects flux and transmembrane pressure (TMP).

Membrane fouling in Membrane Bioreactors (MBRs) directly impacts flux and transmembrane pressure (TMP) by increasing resistance to water flow across the membrane. MBRs use membranes to separate solids from treated wastewater. Membrane fouling refers to the accumulation of materials on the membrane surface or within the membrane pores. These materials can include organic matter, inorganic colloids, microbial biomass, and extracellular polymeric substances (EPS) produced by bacteria. This accumulation creates a physical barrier that hinders water passage. Flux is the rate of water flow per unit area of membrane surface, typically measured in liters per square meter per hour (LMH). As fouling increases, the effective pore size of the membrane decreases, reducing the amount of water that can pass through per unit area in a given time. Therefore, membrane fouling causes a reduction in flux. To maintain the desired flux despite fouling, the transmembrane pressure (TMP) must be increased. TMP is the pressure difference between the feed side (mixed liquor) and the permeate side (filtered water) of the membrane. A higher TMP forces water through the fouled membrane. However, increasing TMP has its limits. Excessive TMP can compress the fouling layer, making it more difficult to remove during cleaning and accelerating the fouling process. Furthermore, exceeding the membrane's pressure tolerance can cause irreversible damage. For example, if organic matter deposition significantly restricts water flow, the TMP will increase to compensate. If the TMP reaches its maximum allowable limit and the desired flux cannot be maintained, the membrane requires cleaning. Chemical cleaning typically involves using agents like sodium hypochlorite or citric acid to remove the accumulated foulants. Therefore, membrane fouling reduces flux and increases TMP, requiring careful monitoring and management to optimize MBR performance and prevent membrane damage.