Sludge Thickening

Sludge thickening reduces water from the collected solids prior to further treatment. Thickening of primary solids releases soluble carbon for nutrient reduction in the secondary process.

Two separate sludge streams are generated during treatment: primary sludge and waste activated sludge (WAS). The thickening process’ two primary objectives are:

1) production of soluble carbon for nutrient removal in the secondary treatment process.

2) reduction of water in the sludge to reduce the amount of infrastructure required for downstream treatment.

Different thickening technologies are typically employed for each stream. 

Dissolved Air Flotation Thickener at RWHTF

Robert W. Hite Treatment Facility

Sludge removed during primary clarification is pumped to the Gravity Thickener (GVT) Building. Gravity thickening involves using specially designed circular tanks that utilize gravity to concentrate thin sludges to create soluble carbon and generate a thicker sludge. The sludge from this process is thickened from one percent total solids to between four and six percent total solids. 

Waste Activated Sludge (WAS) removed from the secondary clarifiers is pumped to the Dissolved Air Flotation Thickener (DAF) building. The WAS is conditioned by dosing polymer and dissolving air. The conditioning allows the sludge to float to the top of the tank. The floating solids are known as concentrated WAS (CONWAS) and are skimmed off the top of the tank while the remaining liquid (called subnatant) is returned to the secondary treatment process. 

The end-product from the GVTs, thickened primary sludge (TPS), and the CONWAS from the DAF are each pumped to the sludge holding tanks to mix the two streams and provide equalization prior to downstream anaerobic digestion. 

Northern Treatment Plant

NTP also uses gravity thickeners for primary sludge thickening, but the process is slightly modified to enhance beneficial carbon recovery. Using the unified fermentation and thickening (UFAT) process, NTP operates two gravity thickeners in series, in which the first tank holds the sludge to promote fermentation, which is biologically generating special carbon molecules that are ideal for feeding the denitrification and biological phosphorus removal processes in the secondary treatment process. 

The sludge and liquid are then mixed together to release the carbon into the liquid. Then they are sent to the second gravity thickener tank to utilize gravity to separate the solids to be sent to the digester while the carbon-enhanced liquid is dosed to the secondary process in specific zones to turbo-boost biological treatment. 

For secondary solids thickening, the NTP uses rotary drum thickeners (RDTs) to separate the solids from the liquids. This process is very similar to the RWHTF DAF process in that polymer is mixed with the sludge, but instead of floating the sludge with tiny bubbles, the RDTs allow water to fall out while slowly rotating the sludge through a mesh cylinder. 

Rotary drum thickeners at NTP

Robert W. Hite Treatment Facility

Capital Investments

PAR 1404 – Solids Improvement Evaluation (RWHTF)

2021-2023 Investment: $1M

An evaluation of new technologies and innovations in solids processing will be performed to determine a master plan for solids treatment at the RWHTF.


A thickening centrifuge pilot was conducted in summer 2021 to determine the anticipated chemical polymer reduction and other operating conditions in support of a future replacement of the existing DAF system at the RWHTF. The pilot also studied the thickening performance on densified WAS.

Capital Investments (15 years)

Projects are based on the current anticipated regulatory requirements but are subject to change without notice. Projects are planned and are subject to change without notice including the cost estimates.

Gravity Thickener and Primary Sludge Screening Expansion and Replacements (RWHTF)

2026-2030 Investment: $37M

An expansion of the gravity thickener complex is anticipated to increase treatment capacity and performance. The project will also study improvements to the primary sludge screening to help with ragging issues in the downstream treatment processes.

WAS Thickening Facility (RWHTF)

2028-2032 Investment: $68M

As DAF reaches the end of its useful life, a project is anticipated to replace the system with an alternative thickening technology.

Upcoming Innovations

Further studies and pilots are anticipated to support technology selection and design considerations for thickening improvements.

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