Primary and Secondary Clarifiers
This page provides information on the expected performance of primary and secondary clarifiers. Removal efficiencies, detention times, and other parameters have been summarized to provide a handy reference. Of course, every wastewater plant is different, with potentially vast differences between municipal and industrial wastewater systems. The data presented here tends to cover some relatively broad ranges, but the information will still give you a good idea of how your clarifiers are performing.
Before beginning with the data I want to share this excellent passage from yet another textbook I highly recommend. In the “Clarifier Design” textbook, published by the Water Environment Federation (Manual of Practice No. FD-8), the following is stated:
Design of primary clarifiers has historically been done more empirically than rationally. The main reason for this is a lack of understanding of what pollutants primary clarifiers are capable of removing. For example, it is not uncommon to see in many wastewater treatment plant master or facilities plans a statement such as “The primary clarifiers are designed to remove 60% of the total suspended solids.” Never is any basis given for such statements. In reality, 60% removal is assumed, not designed for. The more appropriate statement would be “The primary clarifiers are designed to remove all of the settleable total suspended solids during average dry weather flow conditions.” As the settleable total suspended solids concentration is a characteristic of the wastewater, good primary clarifier design begins with a characterization of the wastewater.
Okay, so there is no real scientific basis for the total suspended solids removal rate in a primary clarifier. Now that we have that important point out of the way I will proceed to provide you with removal rates for several parameters.
Clarifier efficiencies are affected by many factors, including:
The nature of solids in the wastewater and their source. A large industrial contribution to a municipal wastewater plant will have solids with very different characteristics compared to the solids from a “conventional” municipal plant.
The transit time and temperature of the wastewater stream will have a large, negative impact on the solids. Higher wastewater temperatures and long transit times increase the likelihood of the wastewater becoming septic. Septic wastewater reduces the settling rate of solids due to gas bubble attachment, increasing the buoyancy of the solids, keeping them in suspension.
Hydraulic loading rate on the clarifier. Higher loading rates will reduce solids settling.
Maintenance status of the clarifier. Performance differences between well-maintained and poorly-maintained clarifiers can be significant.
Recycle streams, such as waste activated sludge, digester supernatant, dewatering centrate/filtrate streams, will have a negative impact on the settling of solids. In addition, as solids are recycled around a treatment plant, the particle size continues to reduce, increasing surface area, slowing the rate at which the small particles will settle.
Detention time in the clarifier needs to be in the range of 2.0 to 3.0 hours. Too short a detention time will cause solids carryover and too long a detention time will increase septicity. In my experience 3 hours of detention is too much and will likely increase septicity in the sludge during warmer months. Different sources will show different values for all of the operating parameters shown in the table below.
Temperature can also be a factor during winter when the wastewater temperature drops and long detention times in the clarifier add to the cooling. As the temperature drops the density of the wastewater will increase, slowly the rate at which solids will settle.
In the primary clarifier graphic below you will not the detention time is shown as 1 to 2 hours. BOD and TSS removal rates are also shown for the "conventional" operation of a primary clarifiers in contrast to the increased removal rates for chemically enhanced primary treatment (CEPT) where the addition of chemicals, such as a metal salt like ferric chloride followed by an anionic polymer, are used to improve the operation of the clarifier.
Primary Clarifier Formulas
Where the volume of a circular clarifier is calculated as follows:
And the volume of a rectangular clarifier is calculated as:
Weir Overflow Rate
The weir overflow rate (WOR) parameter is used to determine both the potential for short-circuiting in the clarifier and excessive velocities over the weir which would increase the quantity of solids carried out of the clarifier. The weir overflow rate is the number of gallons of wastewater that flow over one lineal foot of weir per day. The typical WOR range for primary clarifiers is 10,000 to 20,000 gallons per day per lineal foot of weir.
Where GFD represents gallons per day.
Surface Settling Rate or Surface Loading Rate
The recommended surface loading rate for primary clarifiers is 300 to 1,200 gallons per day (GFD)/square foot. Loading rates are sometimes varied in response to wastewater temperature, being decreased, by putting more clarifiers in service, during the colder season. During summertime conditions, when the wastewater temperature is elevated, having fewer clarifiers in service will reduce detention time, reducing the potential for septic sludge, though the solids loading rate increases.
The area and circumference of a circle is calculated as follows:
In case you are wondering, the factor 0.785 used in the equation above for calculating the area the area of a circle using the diameter is derived as follows:
Secondary clarifiers handle a large concentration and volume of solids due to the mixed liquor suspended solids (MLSS) leaving the activated sludge process. The typical range for MLSS concentrations in the activated sludge process is between 1,800 and 4,000 mg/L. This range applies fairly well to municipal wastewater plants but often does not nearly match conditions in an industrial wastewater system.
On many occasions I have seen MLSS concentrations exceeding 10,000 mg/L in chemical, petrochemical, and food & beverage wastewater plants. These excessive MLSS concentrations not only place a heavy burden on the secondary clarifiers, which are often loaded with solids floating on the surface, but these extreme concentrations also overload the biological treatment system, reducing oxygen transfer efficiency, and causing an overall low dissolved oxygen (DO) condition throughout the bioreactor. Oxygen in the bioreactor is one of several critical "nutrients" required by bacteria. When the DO is low a stressed condition is created in the bioreactor and BOD/COD reduction suffers.
The picture below, of a secondary clarifier, was taken at a food plant which carries an MLSS concentration greater than 14,000 mg/L!! Sadly, this is not an exaggeration.
A lot more analysis goes into the design and evaluation of secondary clarifiers compared to primary clarifiers, where four types of settling are evaluated as follows:
1. Discrete settling, controlled by the overflow rate.
2. Flocculent settling, controlled by the overflow rate and depth of the clarifier.
3. Zone settling, controlled by the solids flux (see the page on State Point Analysis).
4. Compression settling, controlled by the solids retention time and the sludge depth.
What is provided here is a very general review of secondary clarifier operation. If you really want to know more about clarifier design I recommend you purchase the “Clarifier Design” textbook, published by the Water Environment Federation (Manual of Practice No. FD-8). This excellent reference will provide you with all the detailed information you need to truly understand clarifier design.
To Be Noted
What you should note in the table above is that the recommended detention time of 2.0 to 3.0 hours is the same for both primary and secondary clarifiers, as is the surface settling rate. Where this table differs from the primary clarifier table is with the reduction in the weir overflow rate for secondary clarifiers. In addition, a range for the solids loading rate is shown for secondary clarifiers, a parameter typically not used in the design of a new primary clarifier or the evaluation of an existing primary clarifier.
Solids Loading Rate
The solids loading rate (SLR) is the quantity of solids that can be removed by a secondary clarifier per square foot of surface area. An increase above the design SLR will likely result in an increase in solids leaving the clarifier. For secondary clarifiers that follow an activated sludge system the solids loading rate should fall in the range of 12 to 30 pounds of solids per day per square foot of clarifier surface area. Depending on the textbook you reference, you will see a somewhat different range for the SLR.
The formula for calculating the solids loading rate for a secondary clarifier is:
Where the solids applied represents the pounds of mixed liquor suspended solids flowing to the clarifier, calculated as follows:
State Point Analysis
A simple but comprehensive method to evaluate the performance of your secondary clarifiers is the use of State Point Analysis (SPA). You can find a detailed review of SPA here.
Excel Spreadsheet Download
If you have read this far in this rather lengthy post you may find you have benefited from being able to download an Excel spreadsheet that assists in determining key wastewater secondary clarifier calculations. This spreadsheet is shown in the screenshot below. Required inputs to the spreadsheet are:
1. The influent flow rate,
2. The MLSS concentration,
3. The return activated sludge flow rate, and
4. The number of clarifiers in service.
The spreadsheet then calculates the:
1. Surface overflow rate,
2. Weir overflow rate,
3. Solids loading rate, and
4. The detention time, taking into account the appropriate flow rate.
Click here or on the image to download the spreadsheet.