Primary Clarifiers

Primary Clarification

Primary clarifiers are most commonly used in “municipal” wastewater treatment systems even though many municipal wastewater plants have a significant industrial flow contribution. In an “industrial” wastewater treatment plant primary clarifiers are used less frequently. The reason has to do with the generally higher suspended solids concentration in a municipal wastewater influent than what is often found in many industrial wastewater streams which makes gravity settling in relatively large primary clarifiers a very effective unit process. In a municipal wastewater plant, the primary clarifier also helps provide “equalization” to the potentially high-strength recycle flows that are generated from sludge thickening, sludge treatment, and sludge dewatering processes.

Figure 1 shows a simple schematic for a conventional municipal wastewater plant layout. Wasted sludge may go to the primary clarifiers for co-settling with the influent (primary) solids or it may go directly to a solids thickening unit (dissolved air flotation, gravity thickener) or to sludge treatment units such as an aerobic or anaerobic digester. When WAS solids are co-settled in a primary clarifier the underflow solids will be reduced somewhat.

Figure 1: Typical Municipal Wastewater Plant Configuration

Typical Municipal WWTP

The purpose of the primary clarifier is to slow the velocity of the wastewater down so that readily settleable solids can fall (settle) to the bottom of the clarifier and readily floatable materials, such as fats, oil, and grease, which is referred to as “scum”, can rise to the surface of the clarifier. Bottom scrapers remove settled solids to a central sump or hopper for transport to a thickening unit, such as a gravity thickener, or to additional treatment such (aerobic or anaerobic digesters). On the surface, the primary clarifier rake arm skims the floatable materials to a scum box or trough for further treatment which may include concentration of the material.

In addition to the removal of settleable solids and floatable materials, a primary clarifier will also remove a portion of the 5-day biochemical demand (BOD5) or chemical oxygen demand (COD), a measure of the organic strength commonly used in the industrial treatment sector. Soluble and colloidal BOD5 (soluble and colloidal COD) will move through the primary clarifier without reduction but particulate (and some colloidal) BOD5 (particulate and some colloidal COD) that adsorbs, or attaches to, settleable solids, will be removed. Most textbooks will state the suspended solids removal rate to be in the range of 50 to 65% and the BOD5 (COD) removal rate to be in the range of 20 to 35 percent. The removal rate for scum is both highly variable and typically unknown.

Two key parameters used to design and predict removal rates in a primary clarifier are the surface overflow rate (SOR) and the clarifier detention time. The primary clarifier surface area determines the surface overflow rate and the clarifier volume determines the detention time. As in all wastewater treatment processes, temperature always plays a role, with warmer water improving the rate at which solids will settle due to a reduction in the density and viscosity of the wastewater. But, as ambient temperatures increase, combined with long travel times in wastewater collection systems, the potential for creating septic wastewater conditions increases dramatically, enhancing gasification in the clarifier, which reduces the sludge settling rate. The settling rate of solids in a clarifier at 80F (27C) is 50% faster than it is when the wastewater temperature is 50F (10C).

The surface overflow rate is calculated as shown in Equation 1. The design range is 800 to 1,200 gpd/ft2 if waste activated sludge (WAS) is not returned to the primary clarifier inlet. When WAS is wasted to the primary clarifier the SOR is reduced to a range of 600 to 800 gpd/ft2.

Equation 1: Calculation of Surface Overflow Rate (SOR)

Primary clarifier surface overflow rate equation

The detention time is calculated as shown in Equation 2. The detention time is typically two hours on average. There is seldom very little benefit to be gained from an increase in primary clarifier detention time. The goal of primary clarification is the fairly rapid removal (settling) of settleable solids and adsorbed BOD5. An excessive detention time, given the lack of oxygen in the wastewater, can create septicity in the waste stream which then reduces settling efficiency in the primary clarifier and creates an odorous environment. Septic solids will be harder to thicken in a gravity thickener and they will be more difficult to dewater, requiring a higher polymer dosage rate. “Solids in stale (septic) wastewater settle less readily than those in fresh wastewater because biological degradation of the stale wastewater reduces the particle sizes and the gel generated by the biological reaction tends to float the particles.”1

Equation 2: Detention Time (DT)

In contrast to a municipal wastewater plant, an industrial plant will often use an equalization tank in place of a primary clarifier as shown in Figure 2. Many industrial treatment plants must contend with oily wastewater which requires treatment before equalization. In a refinery this treatment is an API separator. And a dissolved nitrogen flotation unit will often follow the API separator upstream of the equalization tank. In a non-refinery industrial wastewater system a dissolved air flotation unit or corrugated plate interceptor (CPI) will often be found before (or after) an equalization tank.

Figure 2: Typical Industrial (Refinery) Wastewater Plant Configuration

Typical Industrial WWTP

A more detailed look at a refinery wastewater system, including the chemical applications that are often required, is shown in Figure 3.

Figure 3: Refinery Wastewater Plant Block Diagram

Industrial WWTP Block Diagram

1 Water Environment Federation. “Operation of Municipal Wastewater Treatment Plants.” 5th ed. Alexandria, VA: Water Environment Federation, 1996. (p. 538)