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What can cause changes in MLSS to MLVSS ratio in A Wastewater System

What can cause changes in MLSS to MLVSS ratio in A Wastewater System

Why a system was with a history of stable operations was seeing an increase in non-volatile solids which we view as a decrease in the “bug” portion of MLSS.  Effluent quality remains excellent, but the local engineers wanted to think about what was driving this change. To understand MLSS to MLVSS ratio changes, it helps to think about what is happening in the system.

MLSS is total dried solids and includes “everything” after removing the water. This includes living microorganisms, dead microorganisms, extracellular materials including biopolymers, adsorbed organics, organic particulates, inorganic particulates.

MLVSS is the portion of MLSS that is removed during a high heat (550 Deg C) muffle furnace step. The residual contains non-volatile solids – Often benchmarked at 75% of solids being volatile in domestic WW.

How much of the volatile portion is living microbes? Usually less than 15% – longer sludge ages with lower F/M drop the % downward even further.

If you see an increase in the non-volatile fraction – this can mean

  • More influent non-volatiles!
  • Insufficient wasting leading to buildup of non-biodegradable or inorganic solids. As sludge becomes “old”, you enter endogenous respiration where active living biomass as a % falls
  • Influent biodegradable organics drops so a lower microbial population is supported

What does this MLSS/MLVSS ratio change mean for operations:

  • Don’t use the ratio in a vacuum – use settling tests, turbidity, microscopic exam, respiration rates and other available tests to determine operations.
  • Monitor & measure
  • Look upstream to see if anything has changed
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