Are SV30 & SVI important in MBR (membrane) wastewater treatment systems?

Image from www.ovivo.com - installed MBR unit.

MBR systems use membrane separation instead of a DAF or clarifier to remove biological solids from effluent wastewater. MBR systems are becoming more common as membrane technology has improved and overall costs are lower when compared with larger foot print activated sludge systems. When compared with extended aeration activated sludge units, MBR systems typically have the following differences:

• Higher MLSS/MLVSS 
• Smaller aeration basin & overall foot print
• Better control of F/M & MCRT
• Fewer problems with solids in effluent

The MBR is technologically superior to older clarifier systems, but they still have issues that require operator attention. First since a membrane separates solids from discharged water, settling rates and floc formation would not seem as critical - however biological polymers are just as important for good MBR function as they are in systems with conventional clarification. Why do I say that floc formation is just as important in an MBR with advanced membrane technology?

Pores in the membrane allow for water to pass with solids being retained. As pores become clogged with solids - both biological & particulate - the system goes into a backwash cycle to clear the pores and restore membrane function. The problem is that backwashing is not perfect, overtime pores deteriorate with accumulated solids. Even with cleaning technologies that include oxidants, acids, or enzymes - the pores will eventually become plugged. 

A leading cause of plugged pores in biological waste treatment units is the accumulation of high molecular weight and "sticky" biological polymers. Organisms that thrive in the low F/M conditions found in MBR systems also tend to excrete copious amounts of extracellular polymeric substances (EPS). While some EPS is good and improves separation potential, the wrong EPS and overabundance of EPS can both create blinding conditions in membrane pores. 

The easiest ways to monitor EPS in a biological treatment unit is to run SV30, SVI, and microscopic exam on at least a daily basis. Additional monitoring with molecular testing is equally important - including both Microbial Community Analysis (MCA) and qPCR for specific problem or good organisms which gives earlier warnings for EPS blinding potential. When you have a buildup of problematic EPS, do not just assume that it cannot be corrected. Multiple control methods exist, it is just best to address the EPS problem before membrane issues start to compromise unit performance.