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Aerobic Treatment

Aerobic treatment systems are on-site sewage treatment systems that use forced air to treat wastewater and  surface application or drip irrigation to disperse the treated wastewater. They may also be used to replace subsurface systems that are failing due to high groundwater and/or poor soil. While aerobic systems may be  designed without performing a soil test, it is recommended that a soil profile be performed at the site. Based on the results of the soil profile, you may be able to reduce the size of the dispersal field or install a  subsurface system.

Aerobic systems treat wastewater using oxygen and bacteria that thrive in an oxygen-rich environment. The bacteria work to break down and digest the wastewater inside the aerobic treatment unit. Like most onsite systems, the wastewater is treated in stages. Some units include a pretreatment step to reduce the amount of solids (greases, oils, toilet paper, etc.) which can clog the unit and prevent effective treatment. The next stage is the actual  wastewater treatment. Two aerobic primary systems have been adapted for onsite use: suspended growth and fixed film. Methods for final treatment include discharge to a soil absorption field, a sand filter, an evapo transpiration bed, or disinfection. Aerobic wastewater treatment may be a good option when: the soil quality is not appropriate for a septic system; there is high groundwater or shallow bedrock; a higher level of treatment is required; a septic system has failed; and/or there is not enough land available for a septic system. 

Aerobic Treatment System Description

An aerobic wastewater treatment system is specifically designed and engineered for the treatment of wastewater. The treatment is achieved by microbes which oxidize and decompose the organic compounds in the presence of oxygen. There are different types of treatment systems and processes that are used to accomplish this treatment which include, but may not be limited to:

  1. Fixed Activated Sludge Treatment: This system is an aerobic wastewater treatment system that utilizes an aerobic fixed film process that is a combination of the conventional trickling filter and activated sludge processes. The system is designed to be installed within a two-compartment tank or into a two tank system, where the first compartment or tank provides a primary settling zone for incoming sewage and the second houses the actual treatment units. The technology contains submerged media specific to the application, which provides surface area for microbial growth. Aeration and circulation are provided by a blower that pumps air into a draft tube that extends down the center of the tank.
  2. Fixed Film Trickling Filter: is a treatment system which follows a primary septic tank in which the solids are settled and partially digested. The septic tank effluent flows to the trickling filter unit where microorganisms present in the wastewater attach to filter media and use the nutrients and organic materials provided by the constant supply of fresh wastewater to form new cell mass. The open spaces within the media allow air to freely pass through, providing oxygen to support the microorganisms. The system may be equipped with a recycle line for pumping of recycled solids from trickling filter, below the media, back to the primary tank. Final effluent may be recycled back to the head of the treatment to provided additional treatment (denitrification) or out to final discharge.
  3. Packed Bed Filters: Raw sewage enters a septic tank or multi-compartment tank through an inlet tee. In the septic tank or first compartment of a multi-compartment tank, the raw sewage separates into three distinct zones: a scum layer, a sludge layer, and a clear layer. Wastewater from the clear layer flows into the second compartment of the tank or a separate treatment tank. In this second compartment or tank, pumps convey filtered effluent to a distribution system in the treatment device. Effluent is treated by the device which contains a media of some type (textile, sand, foam, etc.) and is collected. In a single pass treatment system the effluent is directed to the final discharge. In a multi-pass or recirculating treatment system the effluent is diverted either back to the tank to be recirculated or out to final discharge.
  4. Sequencing Batch Reactors (SBR): This is a sequential process in which all major process steps occur in the same tank, in sequential order. Wastewater flows to a single reactor, is treated and then discharged. In this process aeration and clarification can all be achieved in a single reactor. All discharge from the system is by pumps activated by programmed controls. The treatment process uses activated sludge for treating raw wastewater by providing oxygen to reduce contaminants. Wastewater flow is accepted by the system as it occurs and is treated and a predetermined volume is discharged in batches. Excess flow is stored and discharged as programmed. System design provides surge capacity to provide storage at times of high flows.
  5. Submerged Attached Growth Bioreactors (SAGB): is a biological reactor in which the media is submerged in the process flow. Many SAGB configurations have been conceived and utilized for the oxidation of soluble organic matter and for biological nitrogen removal. The main components of a SAGB are the media for biofilm growth and the underdrain system for even distribution of air and water. The media in a SAGB has a high specific surface area which allows for a high biomass concentration leading to a short hydraulic retention time and, thus, a significantly reduced reactor volume. Some SAGBs are operated without downstream clarification. The media in such reactors is fine enough to provide physical filtration for solids separation and therefore, has high specific surface area. In such reactors the hydraulic retention is less than the minimum solids retention time required for microbial growth. Therefore, the growth of suspended microorganisms is minimized and the growth of attached microorganisms is maximized.


Size of an Aerobic Treatment Unit:

Aerobic units should be large enough to allow enough time for the solids to settle and for the wastewater to be treated. The size of most units range from 300 to 1,500 gallons per day, but local regulations often require that the unit be at least large enough to handle 500 gallons of wastewater per day. The needed size of an aerobic unit is often estimated the same way the size of a septic tank is estimated, by the number of bedrooms (not bathrooms) in the house. It is assumed that each person will use approximately 50 to 100 gallons of water per day, and that each bedroom can accommodate two people. When calculated this way, a three-bedroom house will require a unit with a capacity of 300 to 600 gallons per day. Some health departments require  that  aerobic units be sized at least as large as a septic tank in case the aerobic unit malfunctions and oxygen doesn’t mix with the wastewater. In such cases, the aerobic unit will work as a septic tank—which will, at least, provide partial treatment for the wastewater.


• Can provide a higher level of treatment than a septic tank
• Provides an alternative for sites not suited for septic systems
• May extend the life of a drain field and reduce its size

• More expensive to operate than a septic system
• Requires electricity
• Includes mechanical parts that can break down
• Requires more frequent maintenance than a septic tank
• May release more nitrates to groundwater than a septic system

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