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What is a Sludge Blanket?

What is a sludge blanket? How does it work and what is its purpose in wastewater treatment?

A sludge blanket is the sludge at the bottom of a tank in a wastewater treatment system. The sludge could be activated sludge with the general chemical formula C60H87O23N12P or C118H170O51N17P. It could also be an anaerobic sludge with the general formula C54H99O32N11P

In favorable conditions, the elements could undergo chemical reactions to form gases, namely carbon dioxide (CO2), methane (CH4), hydrogen gas (H2), nitrogen gas (N2), along with ammonium ions (NH4+). These gases are typically released.

Sludge blankets play an integral role in wastewater treatment processes as they remove inorganic matter and pollutants from the water. They are typically used in wastewater treatment plants in the United States. 

What is the purpose of sludge?

Sludge means any solid, semi-solid, or liquid waste generated from a municipal, commercial, or industrial wastewater treatment plant (40 CFR 260.10). Depending on its chemical content and on how the sludge is reclaimed, it may or may not be considered solid waste in the legal definition. When the sludge settles in a wastewater treatment tank, it is referred to as a sludge blanket.

What is sludge blanket level?

This refers to the level of the sludge blanket in the treatment tank. When it reaches a certain level, it is pumped out to prevent regulation breaches and issues in the wastewater treatment process. 

On the contrary if there is not much sludge, there might be a thin layer or no sludge blanket. As a result, the solids could remain suspended and untreated in the treatment tanks. There are sensors including ultra sensors to monitor the sludge blanket level and when needed, they can adjust the chemical parameters for the optimal level of sludge blanket formation to ensure the proper treatment of the pollutants in the wastewater.

How does a sludge blanket work? 

A sludge blanket basically contains microorganisms and via the biodegradation process, these microorganisms remove a variety of pollutants from the wastewater. The sludge blanket is a medium to which the microorganisms adhere. They also use this environment to grow in numbers, and use the pollutants in the wastewater around them as food. They break the pollutants into smaller, non-toxic chemicals and this break-down process also results in the formation of gases. The gases are removed from the tanks. This is the basic operation for any sludge blanket in a wastewater treatment plant. 

Upflow anaerobic sludge blanket (UASB)

In an upflow anaerobic sludge blanket (UASB) bioreactor, which is an anaerobic digester used in wastewater treatment, the microorganisms in the anaerobic sludge blanket break down the organic matter in the wastewater via the biodegradation process producing methane and other gases that are collected from the top. 

UASBs are mostly used for the treatment of wastewater effluents from the textile, pulp and paper, and petrochemical industries. The operating temperatures and the types of chemical pollutants in the wastewater affect the UASB operation. The sulfur chemicals produced in the process require collection as they could cause corrosion and odor issues in the wastewater treatment facility.

The effects of mixing

A number of factors affect biodegradation including the biodegradation process occurring in the sludge blanket. These include the chemical composition of the wastewater that the microorganisms use for food, the temperature, and the water flow velocity. 

Depending on the types of microorganisms present in the sludge blanket, a particular temperature range is vital for their growth and biodegradation performance. The water flow velocity affects the settling rate of the solid particles in the water and can also affect how fast or how slow the sludge blanket forms.

In addition to these factors, mixing of the wastewater also affects the performance of the sludge blanket. For example, lagoon sludge blankets do not mix well and as such, byproducts from the biodegradation process of pollutants are not efficiently delivered to the microorganisms. Therefore, mixing is important to reduce the accumulation of by-products by allowing them to be diluted in the water and to decrease the formation of CH4.


Overmixing, like the absence of mixing, also decreases the biodegradation process in the sludge blanket. Overmixing releases an abundance of nutrients and decreases the dissolved oxygen concentration. The nutrients, required by the microorganisms for the biodegradation of pollutants, are diluted in the water and thus not obtainable by them. Overmixing also increases the potential for permit violations due to the resulting poor performance of the biodegradation process and thus, poor wastewater treatment quality.

Examples of wastewater treatment involving sludge blankets

City of Marsing Idaho’s wastewater lagoon system

The aerated lagoon system built in 1970, contains 20 acres of cells that provide sewer treatment. The sludge blanket has about 4.23 feet of water above it. The average sludge depth increased over time from 20.8 feet in 2006 to 44 feet in 2021 which could be due to the increased sedimentation in the influent possibly due to floods and other factors. 

West Bay County Regional Wastewater Treatment Plant (Bay City, MI)

The wastewater flows through the screening channels and a coarse screening system that removes coarse solids from the wastewater. 

The mix of solids and wastewater forms mixed liquor suspended solids (MLSS) which settles to the bottom. The microbes in the MLSS use the pollutants from the wastewater as food. The wastewater is eventually disinfected before it leaves the wastewater treatment facility.

Santa Teresa Water Treatment Plant and Penitencia Water Treatment Plant

Aluminum sulfate is added to the approximately 20 feet deep clarifier tank. This is a coagulant to cause the solid particles to clump together. The heavy clump material which forms a sludge blanket, settles to the bottom. As it sinks, it works like a fine-mesh accumulating solid particles.

After the treatment of pollutants by the sludge blanket, the wastewater is subjected to ozonation followed by filtration using granular activated charcoal. Then a trace amounts of ammonia and chlorine (to make chloramine to keep water fresh in the pipes) are added to produce safe drinking water. 

Los Angeles County Sanitation Districts

In the primary treatment, the solids that settle at the bottom are collected along with the floating material and sent to the Dissolved Air Flotation System where a thick sludge is formed and used in the anaerobic digester. The biogas produced in the anaerobic digester is used to generate electricity. The water containing suspended solids in the primary treatment undergo further treatment followed by filtration and disinfection.

A sludge blanket is an integral component of a wastewater treatment operation. The microorganisms in it use the pollutants in the wastewater as food and break them down, via the process of biodegradation, into smaller chemicals. However, as this article shows, sludge blankets require maintenance such as removing the biogases produced during the biodegradation process and ensuring the correct conditions for their optimal performance.

So, what is better – not having the sludge blanket because they require maintenance? or keeping up with the maintenance? It depends on the wastewater treatment facility and the type of wastewater to be treated. Nevertheless, as shown in the examples in this article, sludge blankets are an integral and an effective component of wastewater treatment.

Additional Resources

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