Secondary Clarifiers

Secondary Clarifiers

The focus on this page is clarifiers that follow the activated sludge process typically referred to as secondary clarifiers. The overriding goal of the secondary clarifier is to settle mixed liquor suspended solids (MLSS) and in doing so, to concentrate the MLSS so that it can be efficiently returned to the inlet of the biological reactor. The result is a clear, low solids content, low COD content wastewater that overflows to the receiving stream. Having said this, numerous problems can occur with the operation of secondary clarifiers resulting in a greater concentration of solids carryover than is desired. Carryover of suspended solids in the secondary clarifier effluent can have several causes:

  • Floc shear due to high aeration basin power levels
  • Poor clarifier hydraulics including shallow sidewall depth
  • High wastewater TDS concentration
  • High MLSS concentration
  • Low or high mixed liquor temperature
  • Rapid change in mixed liquor temperature
  • Low mixed liquor surface tension
  • High (or increasing) sludge volume index (SVI)

There are two key parameters used to size a secondary clarifier and to evaluate the performance of a clarifier. The two key parameters are

    1) the hydraulic overflow rate, and

    2) the solids loading rate.

The operating ranges for these two parameters are summarized in Table 1.

Photograph of clarifiers

Table 1: Design and Operating Parameters for Secondary Clarifiers

The MLSS concentration is used to determine the clarifier solids loading rate (lb/day/ft2) at peak hourly and average flow rates, including the required return sludge flow. As the MLSS concentration increases, the rate at which the MLSS will settle decreases, as shown in Figure 1.

Figure 1: Settling Rate of MLSS at Different Concentrations

Settleability Test

The MLSS concentration is used to determine the clarifier solids loading rate (lb/day/ft2) at peak hourly and average flow rates, including the required return sludge flow.

  • The settleability test is a good indication of how solids will settle in the clarifier.
  • Simply use a one liter settleometer and record the rate of settling in an MLSS sample every two minutes for 30 minutes.
  • At the end of the 30 minutes you also have your settled sludge volume (SSV) number for use in the calculation of the sludge volume index (SVI).

In Figure 2 we can see how the MLSS settling rate indicated poor settling of the MLSS in the clarifier in early January 2011 at this particular plant. The settling rates in July and August 2010 were excellent. And we can see that in June of 2011 this plant had recovered from poor settling and the loss of solids in the clarifier effluent.

Figure 2: MLSS Settling Rate Comparison

MLSS settling concentration graph

Sludge Volume Index

Of all the process control formulas the SVI is the easiest and quickest for operators to perform. It only requires a one liter graduated beaker or settleometer. Some wastewater plants use a 1000 mL graduated cylinder, the use of which is discouraged. The depth to diameter ratio of a graduated cylinder does not nearly approximate the depth to diameter ratio of a 1000 mL beaker which much more closely approximates the dimensions of a secondary clarifier.

The frequency of calculating the SVI is based on how many problems there are with solids settling in the clarifier. If the clarifier effluent total suspended solids are always low, and sludge blankets are always low, good practice would dictate calculating the SVI at least once a week. The more data available to the operator, the better that operator will be in recognizing a change in plant performance before real problems start to occur.

Calculation of the SVI is easily done as shown in Equation 1.

Equation 1: Formula for Calculating the Sludge Volume Index

Sludge Bulking

Sludge bulking is the term used to describe the condition in which mixed liquor suspended solids settle at a very slow rate and compact in the clarifier to a limited extent. Severe sludge bulking will cause the sludge blanket to rise to the point where excessive solids are carried over the clarifier weirs. Bulking is observed when clouds of billowing sludge occur throughout the secondary clarifiers and the sludge is not settling well. Bulking is usually caused by filamentous bacteria. Low pH, low DO, low nutrient concentrations, and a high F:M ratio are also possible causes of sludge bulking.

When sludge bulking is caused by filamentous bacteria the following conditions will be evident:

  • Filamentous organisms predominant
  • The MLSS will show large, strong flocs
  • The filaments will interfere with settling and compaction in the clarifier
  • The supernatant in a settleometer will be clear
  • The SVI will be high

We want to have some filamentous bacteria in the biological reactor because the filaments are like the reinforcement bars in concrete: they give strength to the floc mass just as the bars give strength to a concrete slab. Figure 3 provides an illustration of when the presence of “some” filamentous bacteria is good and desirable.

Figure 3: A Desirable Quantity of Filamentous Bacteria

Figure 4 provides an illustration of having too many filamentous bacteria at which point they will hinder the settling of the MLSS in the clarifier.

Figure 4: An Undesirable Quantity of Filamentous Bacteria

Figure 5 is a photomicrograph of MLSS. You can see “some” filaments but the key is that they are not extending from floc to floc. These filaments contribute to making a strong floc particle or mass that will not break apart from too much turbulence. The MLSS in Figure 5 will settle well in the secondary clarifier.

Figure 5: A Desirable Quantity of Filamentous Bacteria

Photomicrograph of good filaments

Figure 6 is another photomicrograph of MLSS. You can see that there are a lot of filaments and here the key is that they are extending from floc to floc. These filaments contribute to making the floc particles slow-settling or buoyant. The MLSS in Figure 6 will not settle well in the secondary clarifier.

Figure 6: An Undesirable Quantity of Filamentous Bacteria

Solutions to a Bulking Sludge

You have several options in correcting an excess of filamentous bacteria. These include the following:

  • Application of polymer to the inlet of the clarifier
  • Increase the sludge age (reduce wasting rate)
  • Chlorinate the return activated sludge if the bulking problem is due to filamentous growth
  • Increase the DO level in the aeration tank
  • Increase aeration detention time by reducing the RAS flow rate
  • Increase or initiate nutrient addition (urea, phosphoric acid)

Rising Sludge

  • Not to be confused with sludge bulking
  • Sludge settles well, compacts on the bottom of the clarifier, then starts to rise in clumps and patches to the surface
  • Rising sludge will typically be brown in color
  • Results from denitrification, septicity, or excessive detention time in the clarifier


  • Most likely when the sludge age is high
  • Low DO level in the secondary clarifier
  • Nitrate (NO3) becomes an oxygen source
  • Nitrogen gas (N2) is released with the gas bubbles carrying settled sludge to the clarifier surface

Solutions to a Rising Sludge

  • Reduce the sludge age which will increase the F:M ratio
  • Increase the RAS flow rate
  • Conversely, a decrease in the RAS flow rate may be required

In Summary

  • Bulking sludge is usually caused by low DO levels which facilitate the growth of filamentous microorganisms
  • Rising sludge usually results from high DO levels and excessive detention time in the secondary clarifier which can cause denitrification to occur

In Figure 7 you can see an Excel spreadsheet that I use as a means to check that the secondary clarifiers in a given wastewater plant are operating within typical design limits. Before you can make any recommendations based on field observations and laboratory testing that you may do you need to know whether or not the clarifiers are operating within their design range. And several typical flow rate conversions, from metric to U.S. Customary, are shown in Figure 8.

Figure 7: Check on Secondary Clarifier Design Parameters

Basic secondary clarifier analysis

Figure 8: Several Typical Flow Rate Conversions, Metric to U.S. Customary