What is primary clarifier in wastewater treatment?

In a wastewater treatment plant, the influent exiting the screening and grit removal treatment still has suspended solids in it. To prevent these solids from accumulating and becoming a septic and odorous sludge, the influent is routed to the primary clarifier.

Essential for maintaining the efficiency of wastewater treatment, the primary clarifier removes more total suspended solids (TSS) and thus more biological oxygen demand (BOD) than any other treatment processes in a wastewater treatment plant. However, it is important to understand the operating principles and design factors that affect the primary clarifier to prevent inefficient treatment and costly mechanical removal of the solids.

Primary clarifier and primary treatment

Occurring in primary settling tanks, primary clarifier treatment may be inclusive in primary treatment depending on the state definition. The following examples demonstrate this inclusivity:

• Massachusetts state, 314 CMR § 5.02 Definitions: Primary Treatment - the process or group of processes capable of removing from sewage a minimum of 25% of the five day biochemical oxygen demand, 55% of the suspended solids, and 85% of the floating and settleable solids.
• Texas, Title 30, 30 Tex. Admin Code § 217.2 – Definitions: Primary treatment - A physical treatment process, other than preliminary treatment, that removes biologically available wastewater constituents primarily by settling or floatation.
• California Code of Regulations, Title 23, Section 3671 – Definitions: "Primary treatment" means a wastewater treatment process that allows substances in wastewater that readily settle or float to be separated from the water being treated.

The above states’ definition for primary treatment is inclusive of primary clarifiers. Numerical values for reducing the BOD and suspended solids are specifically in the Massachusetts state’s definition – both reduced in a primary clarifier.

Settling is mentioned as a treatment other than preliminary treatment for Texas and for California. The concentrations of TSS and BOD are reduced in a primary clarifier by the settling mechanism.

When is a primary clarifier required?

A primary clarifier is required if the concentration of TSS and the BOD in the influent is high. Due to federal regulations on the maximum allowable amount of TSS and BOD in wastewater specified in secondary treatment standards in 40 CFR Part 133, is imperative to treat the influent for high concentrations of TSS and BOD and practically, this is done using a primary clarifier.

However, it should be noted that primary treatment is not always necessary if a sequencing batch reactor (SBR) is used. Because a SBR also reduces the TSS and BOD concentrations, there is no need for a primary clarifier if a SBR is used.

Considering that TSS and BOD are conventional pollutants designated in CWA, section 304 (a) (4) and primary clarification of raw wastewater is a Best Practicable Technology (BPT), primary clarification is widely used at wastewater treatment facilities. Its use is also due to stringent federal limits on the concentrations of TSS and BOD in discharged water.

The EPA secondary treatment standards require general municipal discharge to meet 30-day average effluent concentration limits of 30 mg/L for TSS and BOD5. Alternatively in lieu of BOD5 limit, a 30-day average limit of 35 mg/L for cBOD5 also apply to many wastewater facilities.

There are also state requirements for the amount of TSS and BOD in the effluent from primary treatment. For example:

• Virginia law 9VAC2-790-460 requires the TSS range of 1 to 150 mg/L and BOD5 range of 100 to 180 mg/L in the effluent from primary treatment. Primary treatment includes primary clarifiers unless the state law specifies otherwise.
• Washington state law WAC 173-221-050 requires that the TSS and BOD should not be less than 65% of the influent concentration or up to a 30-day average of 45 mg/L for both the TSS and BOD.

What is the function of a primary clarifier?

Located upstream from the screening and the grit removal systems, the main function of a primary clarifier is to reduce the concentrations of TSS and BOD. In top conditions, primary clarifiers can remove up to 65% of TSS and 35% of BOD.

How is a primary clarifier different from a secondary clarifier?

A primary clarifier is different from a secondary clarifier in its location, although both are based on the principle of sedimentation. Primary clarifiers are located after the screening and grit removal stage while secondary clarifiers are located after the biological treatment stage.

After biological treatment, the solids in the secondary clarifiers settles in the activated sludge at the bottom and the treated water then passes to tertiary treatment, also known as disinfection treatment. The elimination of harmful microbes in the disinfection stage also further reduces the BOD before the treated water is discharged.

How does a primary clarifier work?

A primary clarifier works differently depending on whether it is rectangular or circular shaped with a universal purpose: to reduce the concentrations of TSS and the BOD so that the treated water is in compliance with regulations. There are two types of primary clarifiers.

In a rectangular-shaped primary clarifier, the influent wastewater enters from one side and the settleable solids in the water are collected by the rakes before they drop to the bottom of the tank. The slumped floor in this clarifier allows for the settleable solids to be collected and pumped out.

A round-shaped primary clarifier basically looks like a funnel from sideways and this shape allows the sludge blanket, made of settled solids, to form at the bottom. Entering through a central feed pipe, the influent wastewater is evenly distributed by a baffle wall and as it flows, the solids in it settle to the bottom and they are eventually removed.

To enter the downstream processes, the treated water flows over the weirs in a rectangular-shaped clarifier while in a round-shaped clarifier, the treated water flows into the effluent pipe located on the side. The downstream processes vary depending on the wastewater composition and the wastewater treatment facility.