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:
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.
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
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.
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
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 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:
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
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
You have several options in correcting an excess of filamentous bacteria. These include the following:
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
Figure 8: Several Typical Flow Rate Conversions, Metric to U.S. Customary