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MLSS Cannot be Understood from SVI

MLSS and SVI are separate measurements that describe different properties of your activated sludge.

Here's why you cannot directly interpret MLSS from SVI and how they should be used together for a complete picture:

Why You Cannot Directly Interpret MLSS from SVI
1. SVI is a Ratio, Not a Concentration:

  • The SVI formula is: SVI (mL/g)=MLSS (mg/L)Settled Sludge Volume (mL/L) ​× 1000

  • As you can see, SVI is a ratio of volume to mass. It tells you the volume that 1 gram of your sludge occupies after 30 minutes of settling.

  • Knowing the SVI value (e.g., 100 mL/g) tells you about the quality of your sludge (how dense or fluffy it is), but it doesn't tell you the total quantity of that sludge (MLSS) in your aeration tank.

2. Variability of SV30:

  • The SVI calculation uses the Settled Sludge Volume after 30 minutes (SV30). The SV30 value can vary widely depending on the MLSS concentration and the sludge settleability.

  • If you only know the SVI, you have an infinite number of combinations of SV30 and MLSS that could produce that SVI.


    • Example: An SVI of 100 mL/g could result from:

      • SV30 = 200 mL/L and MLSS = 2000 mg/L

      • SV30 = 300 mL/L and MLSS = 3000 mg/L

      • SV30 = 400 mL/L and MLSS = 4000 mg/L


    • In all these cases, the sludge "quality" (SVI) is the same, but the "quantity" (MLSS) is different.

3. Distinct Information:

  • MLSS tells you: "How many microorganisms (and other solids) are there in my aeration tank?" This relates to your plant's organic loading capacity and the sheer amount of active biomass.

  • SVI tells you: "How well is that biomass settling and compacting?" This relates directly to the performance of your secondary clarifier and potential effluent suspended solids issues.

How to Use MLSS and SVI Together (The Correct Interpretation)

Instead of trying to interpret MLSS from SVI, operators should interpret MLSS in conjunction with SVI to gain a comprehensive understanding of their activated sludge process:

1. MLSS as the Primary Control Parameter for Treatment Capacity:

  • You establish a target MLSS based on your plant's design, influent characteristics, and desired F/M ratio or sludge age.

  • This MLSS is actively managed by controlling the sludge wasting rate (Waste Activated Sludge - WAS) and the Return Activated Sludge (RAS) rate.

2. SVI as the Indicator of Sludge Health and Settling Performance:

  • Once your MLSS is established, you monitor the SVI daily to assess the "health" or "quality" of that biomass.

  • Low SVI (e.g., 50-150 mL/g): This is generally ideal. It means your high MLSS is settling well and compacting effectively. You have plenty of workers, and they are doing their job efficiently in terms of separation.


    • Exception: Very low SVI (<50 mL/g) could indicate an "old" sludge, potentially leading to pin floc (small, non-settling particles) and turbid effluent, even if the overall volume settled is low.

  • High SVI (e.g., >200 mL/g): This is a red flag. It means that, despite your MLSS concentration, the sludge is light, fluffy, and not settling well (bulking). This is typically due to:


    • Filamentous bacteria overgrowth: The most common cause, often due to low DO, nutrient deficiency, low F/M, or septic influent.


    • Dispersed growth or pin floc (severe case): Less common for "bulking," but can lead to high SVI if floc integrity is poor.


    • Consequence: Even with high MLSS in the aeration tank, a high SVI will lead to solids carryover in the clarifier, resulting in high effluent TSS and potentially permit violations.

Operational Decisions based on MLSS & SVI Together:

An experienced operator will use the pair to make informed decisions:

> Scenario 1: MLSS is optimal, SVI is low.

  • Interpretation: Process is stable, sludge is healthy and settling well. Maintain current operations.

> Scenario 2: MLSS is optimal, SVI starts to increase (trending towards bulking).

  • Interpretation: Sludge quality is deteriorating. This is an early warning of filamentous bulking.

  • Action: Investigate causes (microscope examination for filaments, check DO, F/M, nutrients, influent characteristics). Adjust aeration, F/M (waste more or less sludge), or consider chemical additions (e.g., polymer, iron salts).

> Scenario 3: MLSS is high, SVI is low.

  • Interpretation: Excellent condition! You have a high quantity of highly effective and settleable biomass. This is often seen in advanced systems like SBRs with AGS.

  • Action: Continue optimization, but be mindful of aeration energy costs.

> Scenario 4: MLSS is low, SVI is high.

  • Interpretation: Serious problem. Not enough biomass for treatment, and what little you have is settling poorly.

  • Action: Immediately identify the cause of poor sludge quality and insufficient MLSS (e.g., toxic shock, severe washout, operational error). Increase return sludge, reduce wasting, and address the root cause.

> Scenario 5: MLSS is low, SVI is low (<50 mL/g) with turbid supernatant.

  • Interpretation: Sludge is "old" or "underloaded," leading to pin floc. The sludge settles too quickly but leaves fine, poorly flocculated particles in the supernatant.

  • Action: Increase F/M (reduce wasting to increase MLSS), potentially adjust aeration patterns.

In Summary:

You can't derive MLSS from SVI because SVI is a measure of sludge quality (how well it settles per unit mass), not its total quantity. Both MLSS and SVI are essential daily monitoring parameters that, when interpreted together, provide a comprehensive picture of the activated sludge health and guide effective process control decisions at a wastewater treatment plant.