What MLSS should I run in my activated sludge system?

MLSS or MLVSS contains all "mass" suspended in the wastewater system. An aggregate bound by biological polymers, MLSS contains:

• Living microbes - what "does the work"
• Dead microbes
• Biological polymers - called Extracellular Polymeric Substances (EPS)
• Inorganics (dirt, sand, etc)
• Insoluble organics (Fats, long chain fatty acids, larger solids)
  
  

So why would you want to increase MLSS in an activated sludge unit?

• Reduce solids going to wasting - extended aeration produces fewer solids when compared to conventional ASU. In effect, you are moving aerobic digestion into the activated sludge unit.
• Increase resistance to shock loadings by offering more MLSS to adsorb influent organics

Why would you not want to increase MLSS?

• More energy required to maintain DO residuals. While standardized respiration rates (SOUR) does drop, the biological solids still use oxygen during endogenous respiration.
• Secondary clarifier solids handling - clarifiers are designed for a target amount of solids. Run MLSS too high and you get problems with carryover, rake torque issues, and anaerobic solids.
• Filaments can increase with low F/M and low DO - end result is solids problems in the clarifier.
  
  

At what MLSS should I operate?

• Look at design manual for F/M and MCRT ranges
• Monitor respiration rates, settling rates (SV30), microscopic exam, and treatment efficiency to keep sludge in proper growth stage. Normally this means decline phase growth extending into endogenous respiration.