State Point Clarifier and RAS Control Tool

Check whether your secondary clarifiers can thicken the solids you send them, and find a RAS range that keeps the blanket under control. This is the same flux analysis consultants use, simplified for daily operations.

State point calculator

Load a White River WRRF (fictional) scenario:

1. Settling characteristics

Estimate from SVI Settling test data Vesilind V0 and k

SVI

mL/g

Uses the Daigger correlation. Best for SVI about 50 to 400 mL/g.

Enter zone settling velocities measured at several MLSS concentrations (dilute mixed liquor with effluent). At least 3 rows.

Test	MLSS (mg/L)	Initial settling velocity (ft/h)
1	MLSS	Velocity
2	MLSS	Velocity
3	MLSS	Velocity
4	MLSS	Velocity
5	MLSS	Velocity
6	MLSS	Velocity

2. Operating conditions

Influent flow to clarifiers (Q)

MGD

RAS flow

MGD

MLSS

mg/L

3. Clarifiers in service

Clarifier diameter

Number in service

Or total surface area (overrides diameter)

ft²

Use for rectangular clarifiers.

How the method works

State point analysis is based on solids flux theory. A clarifier can thicken only so many pounds of solids per square foot per day. That limit depends on how fast the sludge settles at each concentration and on how fast you pull sludge out of the bottom (the underflow, or RAS rate).

Settling model. Zone settling velocity drops as concentration rises. The Vesilind equation describes it: Vs = V0 x e^(-k X), with X in g/L. You can enter V0 and k directly, fit them from a set of settling tests, or estimate them from SVI with the Daigger correlation: ln(Vs, m/h) = 1.871 - (0.1646 + 0.001586 x SVI) x X.
Gravity flux curve. Gravity flux = Vs x X. It rises, peaks and falls as concentration increases.
Overflow line. Starts at zero with slope Q/A (the surface overflow rate). Where it crosses the MLSS is the state point.
Underflow line. Starts at the applied solids flux on the vertical axis, passes through the state point, and slopes down at RAS/A. It meets the horizontal axis at the predicted RAS concentration.
Limiting flux. For a given underflow rate u, the limiting flux is the minimum of total flux (Vs X + u X) on the thickening limb (where kX is greater than 2). If u/V0 is at least e^-2, no limiting flux exists and thickening does not control.
Status. Applied flux at or below 90% of limiting flux: underloaded. 90 to 100%: critically loaded. Above 100%: thickening overloaded. If the overflow rate exceeds the settling velocity at the MLSS, clarification fails no matter what the RAS rate is.

The tool searches RAS flows to find the minimum RAS that just reaches the limit and the RAS that gives 80% and 90% of the limit.

Limits of the method

It is a one dimensional, steady state model. Real clarifiers have inlet effects, density currents and short circuiting. Many designers apply a safety factor (often using 75 to 80% of the predicted capacity).
The Daigger SVI correlation is a population average. Your sludge may settle better or worse. Site settling tests are far better, and stirred SVI tests are better than unstirred.
It does not predict effluent TSS from poor flocculation, pin floc, denitrification or hydraulic problems.
It assumes all clarifiers share flow and solids equally.
It describes steady state. During a storm, the blanket takes time to rise, so a short overload may be absorbed; a long one will not.
Use it as a screening tool alongside blanket depth, SVI trends and effluent TSS.

Running your own settling tests

Dilute mixed liquor with clarified effluent to make four to six concentrations, from about 1,500 to 7,000 mg/L. Settle each in a tall column (ideally 6 ft, at least a 2 L settleometer with gentle stirring at about 1 rpm), record the interface height every minute, and take the slope of the straight early part as the zone settling velocity. Enter MLSS and velocity pairs in the test table.

This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.