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Activated Sludge Sewage Treatment Plants with Extended Aeration Used for Community Wastewater Treatment

By Brenda Martinez


Activated sludge sewage treatment plants utilize the extended aeration method of biological waste reduction. These extended aeration plants are characterized by the introduction of raw wastewater directly into the aeration tank, long term aeration (usually 24hrs.), high mixed liquor suspended solids, high sludge return ratio, and low sludge wastage. This system design is generally used to serve populations of 20,000 people or less in applications including subdivisions, multiple family dwellings, mobile home parks, shopping centers, commercial offices, industrial facilities, motels, schools, and recreation areas. The long detention time in the aeration tank permits the plant to operate effectively even though flows and strengths vary widely. The plant can be divided into three main units of operation: aeration, clarification, and disinfection.

History of Extended Aeration Package Plants

Modern decentralized activated sludge wastewater treatment plants, also known as package plants, were developed in the 1960s. The process was developed to mimic the natural breakdown of domestic waste in nature. For many years, domestic wastewater was discharged into open waterways and streams.  As this waste entered the waterway, the dissolved oxygen in the stream decreased and microbiological activity increased near the area of discharge.  Waste material in domestic wastewater is generally organic, meaning it is biodegradable and serves as a food source for bacteria in the water.

Example of above grade double hopper clarifier for extended aeration package plant. Photo: Delta Treatment Systems

This food source coupled with dissolved oxygen present in the water and added nutrients causes a rise in microbiological population. The bacteria are quick to degrade the organics and essentially, treat the wastewater. As the waste moved downstream, the bacteria eventually consumed the organic material, which would lead to a decrease in population. In moderation, this process proved to be a good natural process for treatment. As these discharge points started to move closer and closer together, growing steady in discharge quantity and loading, the dissolved oxygen was not recovering quickly enough, leading to a decline in water quality. This negatively impacted both human and ecosystem health, leading to a need to for change in wastewater treatment practices.

Process Summary

There are three main processes in extended aeration technology: aeration, clarification, and disinfection. Each step is critical to the success of the treatment process, and each process should be designed based on influent concentration loading and effluent quality expectations. Detention time, aeration criteria, and equipment are all important design considerations.


The aeration basin mimics the point of the stream where raw wastewater first enters. In a natural water body, the amount of dissolved oxygen is sufficient and the consistent movement of water from the discharge pipe into the stream causes a “mixing” action; allowing constant contact between the microbes and the organics in the wastewater. In an extended aeration package plant, raw wastewater enters a basin, typically sized at 1x the average daily flow rate of the site it is servicing. It flows first through a bar screen, which allows the removal of large debris from the waste stream. The bar screen is manually cleaned by the operator during routine maintenance visits. The wastewater is then introduced into an oxygen rich environment, provided by an external air compressor and distributed evenly throughout the basin via air distribution system. This air distribution system serves two purposes: to provide the necessary oxygen to the natural microbiology and to provide sufficient movement in the chamber to keep the aerobic bacteria in constant contact with the organics for biodegradation. The goal of this chamber is to reduce organics in the wastewater, therefore lowering Biochemical Oxygen Demand (BOD); the main analysis used today to determine water quality.


The second step of the extended aeration process is clarification. Following treatment of the wastewater in the aeration basin, it flows into the clarifier. The clarifier is a quiescent zone with no agitation or aeration. The clarifier, whether circular or cone shaped, allows for solids to settle to the bottom of the tank, and a clear supernatant to form at the top. The tank is typically sized for four hours of detention time, which may vary based on local regulatory or project specific requirements. The purpose of this chamber is to reduce Total Suspended Solids (TSS), another parameter used to determine water quality.  Fats, oils, and greases are skimmed off the top of the clarifier chamber and settled solids are removed and returned to the aeration basin for further treatment. As sludge builds up at the bottom of this tank, it is periodically manually or automatically removed, and hauled off for disposal.


The final step of extended aeration is disinfection. The disinfection chamber follows clarification and is typically sized for 15-30 minutes of detention at 2.5 times the average daily flow for proper detention.  Disinfection can be accomplished either by liquid chlorine, tablet feed chlorinators, or ultraviolet light.  The goal of this chamber is to kill coliform bacteria that could be harmful to human and environmental health before it is finally discharged into the environment.

Technology in Action

Indiana Coal Fired Power Plant Extended Aeration Package Plant

A five-unit, 3,145 MW coal-fired power plant located in southern Indiana required a new wastewater treatment system. The former sewage treatment facility was installed in 1982 and required replacement. The plant is a zero-liquid discharge facility and all plant waste water streams are returned to the 3,000-acre cooling pond, which is an industrial surface impoundment. The plant was installed below grade at a design flow rate of 40,000 GPD. The system was designed to treat domestic wastewater to effluent levels of CBOD, TSS, and fecal coliform down to less than 25 mg/L, 30 mg/L and 125 cfu/100 mL respectively. Delta Treatment provided a sanitary lift station with duplex pump system, and an extended aeration package plant for wastewater treatment. This includes flow equalization, UV disinfection, and a control panel that would be able to communicate with the operations room located on-site. All equipment necessary for proper operation and function was supplied. The system was started up in the Spring of 2016 and has continued to meet effluent quality requirements to date.

Washington Vacation Resort Activated Sludge Wastewater Treatment Plant

A vacation resort facility located off the coast of Washington State was operating an on-site activated sludge wastewater treatment plant servicing 47 individual condominium units at the resort and several privately-owned lots in a nearby subdivision. The system was installed in the 1970s. It required a structural upgrade and the resort was issued a more stringent permit that required total nitrogen discharge levels at 10 mg/L or less monthly average. The system was designed with a flow equalization chamber to handle the variable flow that accompanies vacation rentals, as well as a tertiary sand filter to meet the required total nitrogen limit. The unit provided by Delta Treatment was sized at 16,000 GPD daily flow. The system was installed in late 2015 and has consistently met regulatory permit limits.


Activated sludge and extended aeration sewage treatment plants provide communities with populations of 20,000 or less an effective wastewater treatment solution. They are a particularly effective option where flows vary widely, such as in shopping centers, commercial offices, industrial facilities, motels, schools, and recreation areas. Where BOD and TSS levels are of concern, these package plants excel.

Brenda Martinez has been the Commercial Project Manager for Delta Treatment Systems (formerly Delta Environmental) for three years. Brenda assists developers and engineers in designing wastewater treatment systems that will optimize treatment performance for their projects. Prior to working for Delta, Brenda spent 15 years in the environmental testing industry, doing bench scale treatability studies for municipal wastewater plants as well as standard analytical testing for soils and waters under both Standard Methods and ASTM methodologies. Brenda holds a Bachelor’s Degree in Environmental Science from Texas A&M University, a Master’s Degree in Environmental Management from the University of Maryland University College, and is currently a PhD student at Louisiana State University in the Environmental Science program. bmartinez@deltatreatment.com.