What are the major operating parameters influencing Transmembrane Pressure (TMP) increase in Membrane Bioreactors (MBR)?

In Membrane Bioreactors (MBRs), Transmembrane Pressure (TMP) increase, indicating membrane fouling, is influenced by several major operating parameters. TMP is the pressure difference across the membrane, which is used to drive water through the membrane. As the membrane fouls, the resistance to flow increases, leading to a higher TMP to maintain the same permeate flow rate. 1) Mixed Liquor Suspended Solids (MLSS) Concentration: Higher MLSS concentrations increase the solids loading on the membrane, leading to faster fouling. The higher the concentration of solids, the more material is available to deposit on the membrane surface and block the pores. 2) Hydraulic Retention Time (HRT) and Solids Retention Time (SRT): Shorter HRTs can lead to higher organic loading on the membrane, increasing fouling. Longer SRTs can improve sludge stability and reduce fouling, but very long SRTs can also lead to the accumulation of slowly biodegradable compounds that contribute to fouling. 3) Airflow Rate: Insufficient aeration for scouring can exacerbate fouling. Adequate aeration is needed to keep the membrane surface clean by removing accumulated solids. However, excessive aeration can also damage the membranes and increase energy consumption. 4) Flux Rate: Higher flux rates (permeate flow per unit membrane area) increase the rate at which foulants are transported to the membrane surface, accelerating fouling. Operating at lower flux rates can reduce fouling but requires a larger membrane area. 5) Backwashing Frequency and Duration: Inadequate backwashing can allow foulants to accumulate on the membrane, leading to a faster TMP increase. Regular and effective backwashing is crucial for removing accumulated solids. The backwash duration and pressure must be optimized to effectively clean the membrane without causing damage. 6) Chemical Cleaning Frequency and Intensity: Infrequent or ineffective chemical cleaning allows irreversible fouling to occur, leading to a permanent increase in TMP. Regular chemical cleaning with appropriate cleaning agents is necessary to remove strongly bound foulants. 7) Sludge Characteristics: The characteristics of the sludge, such as particle size distribution, extracellular polymeric substances (EPS) content, and hydrophobicity, can significantly influence fouling. Sludges with high EPS content tend to foul membranes more rapidly. For example, if an MBR system experiences a rapid TMP increase, operators should first check the aeration rate and backwashing frequency, and then evaluate the MLSS concentration and sludge characteristics to identify the root cause of the fouling.