Secondary clarifier capacity changes in step with how well the mixed liquor settles.  So, having insight into that settleability helps us understand how much flow the clarifiers can handle before bad things happen.

How do we measure settleability?

Sludge Volume Index (SVI) is how we measure settleability.  SVI quantifies the volume (in mL) occupied by 1 gram of mixed liquor suspended solids (MLSS) after 30 minutes of settling.  In short, it tells us how well sludge settles and compacts. 

How do we calculate SVI?

Follow these steps:

• Step 1 – Measure MLSS concentration. Grab a sample from your aeration basin and analyze for TSS as mg/L.
• Step 2 - Measure settleable solids with a settleometer. Grab a sample from the aeration basin and perform the 30-min test.  But don’t just track the final value at the end of 30 minutes.  Track:
  • SSV5 (5-min volume) to get insight into settling speed
  • SSV30 (30-min volume) to get insight into compaction
  • “Pop” time (the time when the settled material rises to the top) to get insight into denitrification speed in the sludge blanket
• Step 3 – Calculate SVI with the equation SVI = 30 min settable solids / MLSS x 1000. The figure above illustrates common sludge settleability categories. Keep these benchmarks in mind: <80 mL/g = excellent, 80–150 mL/g = good, >150 mL/g = poor

How to interpret results?

Traditionally, clarifiers are designed with an SVI = 150 mL/g.  A good rule of thumb is an additional 5% of capacity is gained for every 10 unit drop in SVI.   For example, if your clarifiers are designed for 10 MGD @ SVI = 150, then an SVI = 120 would increase capacity by 15% to 11.5 MGD.  This increased capacity provides more operational reliability and flexibility WITHOUT more tankage.

Conclusions

Sludge settleability is a critical factor in clarifier capacity and performance, and the SVI test gives us direct insight into this that can be used to make better-informed decisions. Most of the time, clarifier capacity is influenced by two key parameters: 1) settleability and 2) MLSS concentration.  Both parameters change daily, and this means your clarifier capacity changes daily.  Wouldn’t it be great to know your clarifier capacity each day?  Consider downloading a tool I created to do just that – download it here.