State Point Analysis

Evaluating Clarifier Performance Using State Point Analysis

State point analysis is a mathematical model that predicts clarifier performance using actual operating data. The operating data required to perform state point analysis using an Excel spreadsheet consists of the following six parameters:

  1. Influent flow rate
  2. Return activated sludge (RAS) flow rate
  3. Mixed liquor suspended solids (MLSS) concentration
  4. Clarifier surface area
  5. Number of clarifiers in service
  6. Sludge volume index (SVI)

An example of these inputs is shown in the Excel spreadsheet below along with the state point analysis graph that is generated from the inputs. The “SVISN” acronym represents a sludge volume index value generated using a 2 liter settleometer without stirring.


An understanding and interpretation of the state point graph will now be provided.

The “state point” is the point where the solids underflow line and the surface overflow rate line intersect as shown on the next slide.

For stable operation of the secondary clarifier, loosely defined here as maintaining a sludge blanket of ≤3 feet, the state point must be located well below the settling flux curve. As the state point moves closer to this boundary line it can be expected that the sludge blanket in the clarifier is increasing. This will be further indicated by the solids underflow line moving closer, or becoming tangent to, the settling flux curve.

When the state point has moved outside, or above, the settling flux curve, significant loss of solids in the clarifier overflow will be observed.

Sstate point analysis graph explained

Critical Loading of the Clarifiers

The clarifier becomes “critically” loaded when the solids underflow line is tangent to the settling flux curve. This does not mean the clarifier is going to “fail,” resulting in significant loss of solids carrying over the weirs. Rather, it indicates that the sludge blanket level is increasing and steps must be taken before the clarifier becomes overloaded, which will result in solids loss.

Steps to be taken might include adjusting the polymer feed rate, increasing the sludge wasting rate, adjusting the RAS flow rate, and/or reducing the influent flow rate.


State point analysis is not a particularly easy subject to understand. To help you with your modeling efforts I highly recommend you download an excellent article entitled “Design and Operation of Final Clarifiers” written by Sam Jeyanayagam.

You can also download the “macro-enabled” Excel spreadsheet State Point Analysis Model here. This spreadsheet was developed by Eric J. Wahlberg, Ph.D., Brown and Caldwell.