My SVI or SV30 is too low... what is the cause and fix

The SV30 test uses a dedicated settleometer or 2L graduated cylinder to measure settling velocity and compaction in wastewater treatment biomass. (Note - always use the same size cylinder or settleometer to run the tests). To account for changes in MLSS concentration, the SVI takes the SV30 number and divides by MLSS in grams (some facilities use MLVSS in grams) - just use the same divisor each time.

Now for the SVI or SV30 being too low. Usually compact sludge is a good thing - it means no bulking and clarifiers with not solids carryover. However, if sludge settles too quickly or compacts too much there can be problems including turbidity, recycle (RAS) system troubles, and occasionally floating fines causing effluent TSS problems. Usually the problem is "Old Sludge" where lower F/M conditions over the long term result in bacteria consuming the extracellular polymer substances (EPS). Once the EPS percentage drops:

• the floc size begins to decrease 
  
• floc density increases and settles rapidly
  
• "fines" or small solids chunks break off from the floc and enter solution
• Polymer at the secondary clarifier to reduce TSS or clear up turbidity

How do you investigate and fix the problem

• Perform microscopic exam - look for smaller floc sizes, high density of floc, presence of low F/M filaments, and increased numbers of multicellular indicator organisms such as rotifers, tardigrades (water bears), and even nematodes.
• See if SOUR or DOUR has decreased. Lower oxygen uptake rates along with very low SVI are indicative of "old sludge"
• Determine if low influent organic concentrations is going to be an ongoing phenomenon and adjust target MLSS/MLVSS concentrations
• Increase wasting in 10% steps while monitoring sludge quality to see if EPS increase is triggered by increasing F/M and lower sludge age (MCRT)