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What MLSS should I run in my activated sludge system?

10/3/2017

 
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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.

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    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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