Aerated Tanks (Aerobic Units)...


Aerobic units, or small extended aeration package plants, utilize a suspended growth wastewater treatment process, and may be used to remove substantial amounts of BOD and suspended solids which are not removed by simple sedimentation (as occurs in septic tanks). Under appropriate conditions, aerobic units may also provide for nitrification of ammonia, as well as significant pathogen reduction. Some type of primary treatment usually precedes the aerated tank. The aerated tanks contain an aeration chamber, with either mechanical aerators or blowers, or air diffusers, and an area for final clarification (settling). Aerobic units may be designed as either continuous flow or batch flow systems, with most commercially available units being the continuous flow type. Effluent from the aerated tank is conveyed either by gravity flow or pumping to either further treatment / pretreatment processes, or final treatment and disposal in a subsurface soil disposal system.

Common Modifications...

Various types of pretreatment may be employed ahead of the aerobic units, including septic tanks, trash traps, and comminutors. Septic tanks or trash traps are most commonly used for pretreatment for smaller onsite systems. Aerobic units may be of either the continuous flow, or batch type. The batch (fill and draw) flow system collects and treats wastewater over a period of time (usually one day), then discharges the settled effluent at the end of the cycle. Some proprietary package treatment units are equipped with filters for providing further treatment following the extended aeration activated sludge process. This system modification may provide for additional TSS and BOD. A modified type of proprietary aerobic treatment unit has been undergoing research and demonstration during the past few years. The "biofilter" unit consists of a covered tank (usually concrete) containing foamed plastic media packing. The foamed plastic is very porous, so flow paths through and around the media is possible. Septic tank, or "trash trap", effluent uniformly distributed over the surface of the media. A fan (or blower) is used to simultaneously circulate air through the media via vent pipes in the tank. The system appears to provide very effective removal of BOD, TSS, as well as nitrification.

Technology Status...

Aerobic units have been commercially available for approximately 25 years.


Aerobic units may be used by individual or clustered residences and establishments for treating wastewater prior to (1) further treatment/pretreatment, or (2) final onsite subsurface treatment and disposal. They are particularly applicable where enhanced pretreatment is important, and where there is limited availability of land which is suitable for final onsite disposal of wastewater effluent. Due to the need for routine maintenance of these systems in order to ensure proper operation and performance, aerobic units may be well-suited for multiple-home or commercial applications, where economies of scale tend to reduce maintenance and/or repair costs per user. The lower organic and suspended solids content of the effluent may allow a reduction of land area requirements for subsurface disposal systems.


The rate of sludge production for aerobic units is much greater than for septic tanks, necessitating more frequent sludge removal by a licensed transporter. To ensure proper performance of the units, it may be necessary in at least some cases to require a maintenance contract. Electrical power is required for aerobic units. Current Austin-Travis County Health and Human Services rules require that this type of system be designed by a licensed professional engineer.

"Diffuser Installation"...


Numerous studies have been conducted during the past 20 to 25 years to evaluate the performance of aerobic treatment units. The results of a 4 - year study conducted in Wisconsin appear to be representative of, and consistent with other studies conducted during that same general time period (late 1970's and early 1980's). Mean effluent values for various wastewater parameters measured during that study are presented in a table given below, included as the last page of this fact sheet.

Effluent quality parameter Mean No of samples 95 % confidence interval Range of data
BOD5 (mg / L) 37 112 32 - 42 0 - 208
Soluble BOD5 (mg / L) 15 94 12 - 18 0 - 120
COD (mg / L) 108 116 100 - 116 20 - 349
TSS (mg / L) 39 117 33 - 46 3 - 252
VSS (mg / L) 27 118 23 - 32 1 - 144
Total N (mg / L) 36 87 34 - 38 15 - 78
NH3 - N (mg / L) 0.9 92 0.1 - 1.7 0 - 60
NO2 + NO3 (mg / L) 30 95 27 - 33 0.3 - 72
Total P (mg / L) 26 80 22 - 30 6 - 140
PO4 - P (mg / L) 21 78 18 - 24 6 - 51
Fecal coliform (Log no / L) 5 115 4.7 - 5.3 2.8 - 7.3
Fecal streptococci (Log no / L) 4.3 113 3.9 - 4.7 2.0 - 6.3

Although the nitrification (ammonia removal) reported in the table is very high, levels of nitrification in aerobic units will be very dependent on a variety of factors including loading rates for key wastewater constituents, detention times, oxygen transfer, and temperature. More recent testing of certain aerobic unit models has been performed by NSF International. Those results indicate that there may have been some design and performance improvements for those models as compared with the systems tested in the earlier studies. Operation and maintenance practices could however be responsible for the different performance reported from those studies. NSF studies on several units showed the following effluent quality for TSS and BOD.

Parameter Average concentration (mg / L)
BOD5 5 - 20
TSS 7 - 22

Residuals Generation...

US EPA literature generally recommends that aerobic units are pumped out at least about once every year.

Overall Reliability...

Several studies conducted to evaluate the performance of aerobic units have shown that, if properly designed, installed, and maintained for a particular site's application, these units can perform reliably. Those same studies have also found that home owner neglect, or in general, failures to maintain or replace system components as needed can result in the failure of systems using these units. The acceptable operation of aerobic units has been found to be a function of (1) home owners' understanding of the limitations of the unit, (2) a dependable power supply, and (3) sufficient maintenance.

Operation and Maintenance Requirements...

Pretreatment Units : If septic tanks or "trash traps" are used as a pretreatment unit prior to an aerobic unit, as discussed under "Residuals Generation" in the Septic Tanks fact sheet, septic tanks should be pumped at an average frequency of 2 to 5 years, depending on their size relative to the system's capacity and use. Communitors or other pretreatment units with mechanical or electrical components must occasionally be serviced or replaced.
Aerobic Units : Sludge must be removed from these units, on the average, about once every eight to twelve months, based upon studies conducted to evaluate their performance. EPA recommendations include performing certain maintenance activities on a monthly basis. These include: (1) Checking for foaming and uneven air distribution in the aeration tank; (2) Checking the air distribution system components, including filters, seals, oil level, pressure (for diffused air systems), vibration or overheating (for mechanical air systems), and any other manufacturer's required or recommended maintenance procedures; (3) Check the clarifier for floating scum, appearance of effluent, location of sludge blanket, and any mechanical equipment in accordance with manufacturer's recommendations; and (4) Check controls, alarms, and controls box. Results of several studies on aerobic units have consistently indicated that home owners usually have neither the expertise nor the incentive to properly maintain their own treatment units. In order to provide assurance to regulatory authorities that these systems will be properly maintained and operated, and ensure adequate environmental and public health protection, it may be essential to require that home owners enter into maintenance agreements with either a local utility district or private service provider.

Potential Environmental Impacts...

Properly designed, installed, and maintained aerobic units can, on the average, produce effluent of the quality indicated on the attached table. In addition, the effluent from aerobic treatment processes tends to have much less odor than does effluent from septic tanks. If environmental conditions for a given site for onsite disposal are such that enhanced total nitrogen and pathogen reduction are needed prior to final land disposal, then adverse environmental impacts might result from the use of these units alone, without an additional pretreatment process(es).

Energy Consumption...

Studies show that the energy consumption for aerobic units averages between 2.5 to 10 KWh / day, depending upon the motor design and time of operation.


Estimated initial aerobic unit costs, installed, and including septic tank pretreatment unit $ 6,000
Equipment repair / replacement costs, estimated at $ 50 / year $ 4.17 / month
O & M, with a maintenance contract of $ 360 / year (est. 12 hrs. @ $ 15 / h * 2.0, including taxes, overhead, and profit) $ 30 / month
Septage and sludge pumping once annually (it is assumed that the septic tank is pumped simultaneously, as needed, so as to eliminate separate costs for that) $ 14.58 / month
Energy costs (using 6 KWh / day energy use) $ 14.60 / month
20 - year NPW (not including design & permitting costs) $ 13,573.76

Aesthetic Considerations...

Both septic tanks and aerobic units are typically buried below grade, and usually do not have significant visual impacts on the site. Some minor background sound may be associated with the operation of the aerobic unit. A properly functioning aerobic unit should produce an effluent with far less odor than would be characteristic of septic tank effluent, should that be of concern for the particular site.