SKETCH of a typical aerobic treatment unit tank, aerator, chamberGuide to Aerobic Septic Systems - A Design Alternative for Difficult Sites

  • POST a QUESTION or READ FAQs about aerobic treatment units (ATUs) and aerobic septic systems: design, installation, inspection, maintenance, & repair

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

ATUs - Aerobic Septic Systems:

ATUs aerobic treatment units - Design, installation, and maintenance of aerobic septic systems. Specifications for ATU septic tank sizes, final treatment steps, certifications, and failures or problems. Suggestions for aerobic septic system maitnenance, choices of ATU disinfectants, and sources of Aerobic Septic System Supplies. ATU spray head maintenance.

This article series explains the designs and products, installation, maintenance, and repair of aerobic septic treatment units (ATUs) for onsite waste disposal, also called fine bubble aeration systems. We describe how aerobic treatment units work, what are the components of aerobic septic systems, now they are maintained or repaired, and where to find replacement parts, owners manuals, and operating instructions.

We address aerobic septic system design, features, inspection, repair, and maintenance. Product sources are also listed.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Aerobic Treatment Septic Systems: types, designs, maintenance instructions

Aerobic Septic systems add oxygen to the process of treating septic of sewage wastewater by using any of several types aeration or "fine air bubble" systems to increase the level of effluent treatment in the septic tank by encouraging aerobic bacteria. Aerobic systems produce a better-quality wastewater effluent for discharge into the absorption system for final treatment and disposal.

Article Series Contents

What is an Aerobic Treatment Unit for Septic Effluent?

In residential use, aerobic treatment units (ATUs, also called "home aeration systems" or "septic tank aeration systems") are pre-packaged septic treatment systems which are in essence a mini-wastewater treatment plant for home use. "Aerobic" refers to the use of an air pump to add oxygen to the treatment tank to increase the level of treatment by the system.

AEROBIC Septic systems thus require electrical power and cost more to install and operate (more frequent tank pumping) than a traditional gravity septic tank and drainfield. Aerobic treatment, which can produce very high quality treated effluent, is used at sites where a conventional septic drainfield simply wont' work, perhaps because of wet soils or very rocky conditions.

ATUs are also used to restore a working septic system where a traditional septic system has failed and is difficult to repair. Other common reasons for installing aerobic septic treatment units include lots close to lakes and streams or lots which are too small to fit a conventional septic system.

Aerobic treatment may not entirely eliminate the requirement for a drainfield, but it can substantially reduce the drainfield area and capacity required. This is not a "new" idea. Aeration of wastewater as a means of septic effluent treatment has been in use for more than 100 years (using media filters according to Jantrania).

How do Aerobic Septic Treatment Units Work?

An aerobic treatment unit is basically an "oxidizer" which uses extra oxygen dissolved in the wastewater to support aerobic microorganisms which in turn decompose dissolved organic and nitrogen compounds into simple CO2 or into inorganic compounds.

As microorganisms die off they accumulate as a sludge of biological material, some of which supports the development of new cells or microorganisms to keep the system working. ATU's separate solid waste first in the "trash tank" and later, additional solids are separated in the clarifier or settlement tank from which they may be returned to the primary tank for more treatment. (See the sketch at the top of this page.)

"Typical organic materials that are found in residential strength wastewater include carbohydrates, fats, proteins, urea, soaps and detergents. All of these compounds contain carbon, hydrogen, and oxygen. Domestic wastewater also includes organically bound nitrogen, sulfur and phosphorus.

During biochemical degradation, these three elements are biologically transformed from organic forms to mineralized forms (i.e., NH3, NH4, NO3, SO4, and PO4)." -- Reference #3 at page bottom. In contrast, an example of an anaerobic process involved in the breakdown of wastewater would be fermentation, the exothermic, enzymatic breakdown of soluble organic compounds which does not depend on the presence of dissolved oxygen. Methane and CO2 are both products of wastewater fermentation.

Simplest Two-Chamber Aerobic Treatment Unit Design

Waste from the occupied building is fed into a septic tank primary treatment chamber where it is kept agitated and aerated (oxygenated) by an air pump and rotor or mixer. By increasing the oxygen level in the effluent, we increase activity by the tank's aerobic bacteria and other naturally occurring microorganisms such as fungi, protozoa, rotifers, and other microbes.

Effluent passes out of the primary treatment tank into a settlement chamber where sludge settles out for recycling into the primary treatment tank. Clarified effluent passes to an absorption or further treatment system. An alarm system is usually installed to tell the building owner if the equipment has stopped working.

Three-Chamber Aerobic Septic System ATU Design

  1. AEROBIC Septic Trash Tank: Waste from the occupied building is fed into a "trash tank" (similar to a septic tank); septic solid waste and scum are retained in the "trash tank" and as with a conventional septic tank, must be periodically removed by a septic pumping company. (ATU's require more frequent septic tank pumping than a conventional septic system.)

    The ATU tank works like a septic tank but can be smaller because the system does not depend on a long "settlement time" to remove solids and grease as occurs in a conventional septic tank.

  2. AEROBIC Septic System Aeration Chamber & Aeration Pump: An aerator or air pump, normally installed in a chamber atop or close to the septic tank, pumps air into the septic tank's aeration compartment using any of several methods to aerate the wastewater.

    A mixing device or rotor may be used to further agitate the wastewater in the aerobic treatment tank to increase the oxygen level in the effluent and to support treatment by aerobic bacteria in the tank. Speaking slightly more technically, the aerobic process in the treatment tank provides for biochemical oxidation of the soluble organic compounds found in domestic wastewater.

    AEROBIC Septic Aeration Chamber: Septic effluent moves out of the "trash tank" to a separate aeration chamber. In the aeration chamber air (oxygen) is pumped through the system to provide oxidation and waste treatment using a variety of designs. The added level of oxygen permits a variety of microbial life forms (bacteria, fungi, protozoa, and others) to oxidize or otherwise process pathogens and nitrogen compounds in the discharged septic effluent. The aerated, or oxygenated wastewater is called the "mixed liquor".

  3. AEROBIC Septic System Clarification Chamber: After having been aerated and mixed in the aeration chamber, the effluent flows to a clarification chamber. Solids settle out of the effluent and stay inside the ATU. In some designs the sludge is recycled to the aeration chamber.

    The settled sludge and solids support the formation of additional microbial growth which in turn is used to process pathogens as we just described. The ATU may, depending on its design, also remove nutrients, solids which were not retained in the trash tank, and pathogens.

Suspended Growth vs. Attached Growth Septic Systems: Oxygen-supported (aerobic) bacteria in the mixed liquor perform the primary treatment in the system. As the bacteria themselves die off they remain suspended in the mixed liquor - a "suspended growth aerobic treatment system". Alternatively, a media, such as synthetic fabrics, may be suspended in the treatment tank, permitting the bacteria to attach to the media surfaces - an "attached growth aerobic treatment system".

Saturated vs. Non-Saturated Wastewater Treatment Systems: An aerobic treatment unit (ATU), because it involves a tank filled with wastewater and forced oxygenation of that wastewater, is a type of saturated wastewater treatment system. Other non-saturated wastewater treatment systems such as trickling filter beds use passively-infused air to support their oxygen-supported microorganisms. Unlike ATUs, non-saturated systems allow passive air contact with effluent as it moves through the media. Air is not being pumped. Both types of systems make use of aerobic microorganisms.

Four-Chamber Aerobic Systems - 4-chamber ATU Design

Four-chamber Aerobic Treatment Units are also designed for and used in some areas, though how we count chambers may be confusing - some designers may not design or count a separate aeration chamber. A four-chamber aerobic tank uses

  1. AEROBIC Septic Tank Sewage receiver:  a compartment to receive sewage and collect sludge;

  2. Aerobic Treatment Unit Aeration Chamber: an aerobic chamber to pump air and thus oxygen through wastewater to assist in the aerobic treatment process;

  3. AEROBIC Septic Effluent Settling Chamber: a clarifying or settling chamber which permits remaining solids to settle out of the wastewater; disinfection may take place in this chamber;

  4. AEROBIC Septic Effluent Pumping Chamber: a pumping chamber to receive treated effluent for discharge to an absorption system or other destination.

Also see BAT MEDIA SEPTIC PLANTS for an example of a three or four chamber septic tank that combines aerobic treatment with a septic media.

Aerobic Wastewater Treatment System: difference between aerobic wastewater treatment unit and sand filter treatment design

Be sure to check with the manufacturer of your specific ATU or WTU for its maintenance requirements as the details vary considerably among aerobic treatment unit designs. But in general, as various experts point out [3], there are two basic types of ATU systems:

  1. Aerobic wastewater treament units - the focus of information in the article above
  2. Aerobic sandfilter treatment systems. Sand filter ATUs have the same treatment level as the type 1 aerobic systems above, but work by filtering effluent through a sand layer to provide natural aeration rather than using a compressor or air pump inside of the treatment tank. Treatment of wastewater in the sand bed occurs through natural (no pump) aeration and biological oxidation through the action of aerobic bacteria and nitrifying organisms.

Following increased tretment of wastewater in the aerobic treatment unit tank (using higher levels of oxygen provided by aeration) the effluent is further processed by allowing for settlement out of solids, disinfection, and then pumping to a disposal location.

The effluent discharged from a properly working ATU is sufficiently sanitary that it should be able to be used for surface irrigation within the site. That's why we see, for example in the Southwestern U.S., wide use of spray diffusers that in dry areas may discharge treated effluent onto lawns as a watering system.

Typical Aerobic Septic System Costs

Reader Question:

(Nov 12, 2015) Todd Barclay said:
It does not help to have all this and no prices. Lists could be made avail. It is hard enough to make good choices but without some reference point on pricing there is only data. Ty but I need more without sales pressure.


Prices for the equipment, ranging from hundreds of dollars to a few thousand U.S., are typically the smaller part of the septic system installation, Todd as site work, excavation, and even ongoing maintenance as well as anticipated septic system life are key cost factors in any septic system design and installation.

That said

See AEROBIC SEPTIC SYSTEM SUPPLIERS for suppliers, contact information, more prices

Regarding your concern about sales pressure, I agree that that's an interference.

To protect the trust of our readers, does not sell anything, no product, no service. We have no business nor financial relationship with companies or their products and services that may be discussed at this website.


Continue reading at AEROBIC SEPTIC ATU CERTIFICATIONS, the next article in this aerobic treatment unit (ATU) design & maintenance series, or see the complete list of aerobic septic system articles near the top of this page, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BAT MEDIA SEPTIC PLANTS that combine aerobic treatment with a media for biologically accelerated treatment septic systems


Or see HOOT Aerobic Systems Drip Disposal Design & Installation Guide [PDF]

Pr see "Guidance For Design, Installation And Operation Of Subsurface Drip Distribution Systems As A Replacement For Conventional Title 5 Soil Absorption Systems For Disposal Of Septic Tank Effluent", [PDF] Massachusetts Department of Environmental Protection (2006) refrencing MA regulations 310 CMR 15.240, 15.242, 15.247, and 15.280-15.289

Suggested citation for this web page

AEROBIC SEPTIC SYSTEMS, ATUs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about aerobic treatment units (ATUs) and aerobic septic systems: design, installation, inspection, maintenance, & repair.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher - Daniel Friedman