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

  • AEROBIC SEPTIC SYSTEMS - home - CONTENTS: 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.
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ATUs - Aerobic Septic Systems: 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.

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Aerobic Treatment Septic Systems: types, designs, maintenance instructions

Article Series Contents

AEROBIC Septic systems add oxygen 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.

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.

Aerobic Wastewater Treatment System: difference between aerobic wastewater treatment unit and sandfilter 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.


Continue reading at ATU CERTIFICATIONS or select a topic from the More Reading links shown below.

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Frequently Asked Questions (FAQs) about aerobic septic systems

Question: I have been told I need an expensive aerobic septic system. Can I build my own for less cost?

I have a question about aerobic system. I've purchased land and now I have learned that I need aerobic system. I found out this system is very expensive. I am handy and can do some work on my own... I wander if there is anyway I can have alternative system like that for less?
Rob 7/15/11

(June 5, 2014) Ryan said:

I am exploring A.T.U. septic systems. Currently I have a design for a raised bed system and would like to eliminate or significantly reduce the field size by implementing an alternative. This project is new construction and has been determined to have bedrock at 17" (site#1) and 23" (site#2).


The cost of installing an aerobic system may be at least as much in the excavation as in the equipment itself; While I agree that there are septic system "add-ons" that add an aeration feature to an existing septic tank to increase the treatment level, but unless it's a multi compartment tank that can handle the increased agitation of sewage in the main tank (caused by the aerator) I think your results may not be nearly as good as you'd hope, and without some extra steps (filtration, settling chambers) you risk ruining the drainfield.

Furthermore, if you are being told that you already need a new system, chances are your drainfield is already shot; I'm not clear where you would save by a system that went only part-way to what you are being told is needed.

Question: Someone stole parts off of our aerobic system, I don't know what I need to replace

i recently purchased a home and before we could move in someone stole parts from my aeration system i don't know what they took i need some sort of a parts list or basic diagram to figure out what they took and what i need to replace - Jeremy 8/2/11


Jeremy the very specific parts you are missing may be more detailed than I can guess.

I'd take a look at the basic aerobic components described here so that you have an idea how systems work, including the aeration components, then I'd call a local septic system repair company or installer and ask them to get the system working. Typically the contractor has all the small parts on the truck that you may need to hook up a missing aeration pump, tubing, connectors, etc. that otherwise will send you running back and forth to plumbing suppliers guessing at how to hook things back up.

If you can send along photos (see the CONTACT link at the "More Reading" links at the bottom of this article ) of your system I might be able to help with research or comment further.

Question: my site septic plan was denied due to wet conditions. Do aerobic systems require perc tests?

My site was denied due to wet conditions, do you have to pass a perk test to have a aerobic system installed? - Marylin 10/4/11


Marylin, the specifics of what a local building or health department official wants to see as the perc test procedure as well as perc test results are usually determined locally, as local soil conditions vary. An aerobic system can produce effluent treated to a higher level, making its disposal easier, but you'll still need to demonstrate (with help from your septic engineer) that the effluent will be disposed-of acceptably for your site and soil conditions.

Question: we have a blockage in our aerobic lines and never pumped the tank for 12 years. Is the blockage due to the pump?

We have semi-blocked lines and we have a two-chamber ATU and has never had it pumped. We've lived on this site for 12 years. What can be the problem? Is it a bad pump, or is it overflowing? - Cody 12/1/11


Short answer: Cody depending on the number of occupants you may have fallen behind in septic tank pumping frequency. See SEPTIC TANK PUMPING SCHEDULE.

If a septic tank is not pumped often enough the risk is that the level of treatment falls and solids are pushed into the effluent disposal system causing clogging and sometimes the need for costly repairs. But without opening your system for inspection, one can't accurately guess further.

Details: A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with the septic system. The inspector will need to open the tank, check the operation of the aerator, disinfection system if any, and possibly inspect the effluent disposal system as well.

That said, if you never pump the septic tank the treatment level drops, solids get pushed into the effluent disposal system, and the system will first, contaminate the environment with under-treated effluent and second, will become clogged - that's in general. Of course the specifics for your system will vary.

You'll want your septic contractor to show you the working parts of the system, the maintenance that it requires, the maintenance schedule, etc.

Question: does an aerobic septic system need to be winterized?

I have recently purchased a new home with an aerobic system that has 2 sprinklers. Do I need to winterize the system or turn off the sprinklers in the winter? If so, what happens to the water? - Stevi 12/5/11


Stevi an aerobic septic system would be expected to be functional year round. But I agree completely that if your system is installed in a freezing climate, and if it uses above-ground effluent spraying, that system cannot work properly in freezing weather. Something sounds wrong with the whole design.

Question: Nyadic aerobic septic system

I just moved to 4 acre horse farm with horses and well. I have a nayadic system. The effluent pipe runs about 100 ft to a depression 30x30 ft. into the pasture. Is there a recommendation for an alternate "drainfield" or pit? There is room outside the pasture. I also have a shallow well(35ft deep with 15 ft of water). It is about 50 ft from effluent line. no codes out here Bill I. 12/11/11

Reply: contact information for Nyadic Aerobic Septic Systems


Nayadic aerobic septic systems are produced by Nayadic, a division of Consolidated Treatment Systems, Inc. These systems are NSF tested and certified using a single tank for both the aeration of sewage wastewater and for solids separation.

While I could envision disposing of effluent in a more conventional drainfield than an above-ground depression (and I wonder who approved that), I would contact the company to ask for details about the intended means of disposal or dispersal of treated effluent from their system. Give Consolidated a call at 800-503-0163. The company's contact information is in the references at the end of this article.[8]

While you're at it, ask them for an owner's manual for your system.

Nayadic also produces and sells rotary vane air pumps for aerobic septic systems that support Nayadic 500, 600, 800, and 1000 gallon per day aerobic septic designs.

Question: my sewer smells really bad, it was pumped, I put in new yeast and chlorine, and my air pump is running

My sewer smell really bad i have had it pump out about 1 1/2 years ago new yeast chorine and my air pump is working what can i do - Mark 1/23/12


Mark, yeast is not a normal septic system. I would contact the manufacturer of your system and ask for their advice and for a copy of the instruction manual.

According to some expert sources [3] a slight odour from an aerobic septic system is normal but strong, persistent odors indicate that the system is not working normally.

I agree that a strong odor problem is unacceptable, not normal, and can be corrected.

Question: the chlorine tablets in our aerobic septic system haven't dissolved - is that OK?

The last 2 tablets we put in haven't dissolved. Is that a problem? Thanks! - Spike 2/14/12


Spike, if you have an aerobic system that requires disinfectant tablets, and they are not dissolving, that might be normal - they don't dissolve entirely immediately. Check back each day for a few days and let me know what you see. Also see AEROBIC SEPTIC DISINFECTANTS - Calcium Hypochlorite - make sure you are using the right kind of disinfectant tablets. Do not use swimming pool chlorine.

Question: how do I turn on my aerobic septic system?

How do I turn on my aerobic system. It has 3 switches, pump, comp, and alarm. Do I turn all 3 switches on to operate it? - Linda 4/23/12


Linda it sounds as if you have a

  • pump switch (that needs to be on for an aerobic system to be working)
  • compressor switch (that needs to be on for an aerobic system to be working - presuming this is an air compressor as part of an aerator system)
  • alarm switch - that tells you when the level in a tank is high enough that a pump is not working - or some other failure condition.

I would contact the manufacturer of your system and ask for their advice and for a copy of the instruction manual.

Also see AEROBIC EFFLUENT SPRAY Head Maintenance and see AEROBIC SEPTIC DISINFECTANTS - Calcium Hypochlorite

Question: My home's septic system is 47 years old. Is it working right?

My home is 47 years old, so it would make my 's&p' septic system equally as old. I'm not sure if that is the correct term for my septic design because it is so old. I cannot find a diagram on your website that is comparable to what I think I have.

I have 1 -1000g tank with 1 lid, the solid are pumped from this tank. It is connected to a 2nd (1000g) tank, 50 feet away which is a 2 story tank. You can stand in upper portion of the 2nd tank as it is made of concrete and has a crock hole to the lower part. The bottom portion of the 2 story tank accumulates liquids coming from the 1st tank.

Once the liquids in the 2nd lower tank reach a certain level, a sump pump (hanging in the crock) assists in the discharge through an underground pipe that is laying on the ground in the woods another 50ft away. Because of its age ("grand fathered"), its my septic doing what it is supposed to be doing? - Kim 4/23/12


Kim very few septic systems that are basically untouched for 47 years would lead me, even knowing nothing about them, to opine that the system is performing as it should.

The design sounds thoughtful however, as if you have a dosing system = that is, the second effluent tank accumulates effluent in some volume before sending it to an absorption field.

But a pipe laying on the ground? That does not sound right at all. You didn't describe an aerobic septic system (maybe I missed that) and you didn't say in what state you are located, but no where in the U.S. or Canada are we permitted to discharge effluent from a septic tank right onto the ground surface from a distribution pipe such as you describe.

In some U.S. states it is permitted to use an air spray to distribute effluent that is discharged from an aerobic septic design (tank, aerator, separation baffles, holding tank or pumping chamber) that is certified to treat effluent to perhaps 95% or better.

But you are not describing such a system. A conventional septic tank treats effluent to a maximum of about 45% of what's needed. So you may be discharging sewage onto the ground surface.

Question: the sprinklers on my aerobic system are leaking. I pumped the tank. Didn't help.

My aerobic septic system is not working right. The sprinklers basically just leak all the time instead of spraying. The sprinklers are downhill from the tanks. I had the tanks pumped out (1st time in 6 years) and the sprinklers are still not spraying. Any clues? - Kenneth Skoczlas 5/1/12


Kenneth, if an aerobic sprinkler is leaking, it's not going to be fixed one iota by pumping the septic tank. More likely fittings or gaskets or sprinkler heads need replacement. Also see AEROBIC EFFLUENT SPRAY Head Maintenance

Question: is it normal for the septic aerator pump to run continuously?

I have a 4 tank aerobic system that eventually sprays the treated water, should the aerator pump run 24/7? System serves our single family (just two of us) house. - S.R. 7/17/2013

Reply: normally the septic tank aerator pump should be left running 24/7

Yes, most aerator pumps are designed to run continuously and further are designed to be quiet and to run rather economically. In fact since an aerobic septic system depends on aeration to maintain both healthy aerobic bacteria in the septic tank and to assure adequate wastewater treatment to meet the system operating specifications, you should leave the aerator pump on at all times.

Usually these are small aerobic system aeration pumps - about 1/6 to 1/3 hp. Some example data about operating cost include:

  • the add-on septic tank aerator pump sold by draws just about 2 amps
  • the septic tank aerator sold by J Jet Wastewater Treatment Systems ( is a 1/3 hp motor. In discussing typical annual operating cost the company says:

    The yearly cost of running an aerator continuously, varies from state to state so depending on what region of the country you live in and the average retail price of your electricity provider you can expect to pay in the range of $58.78 - $217.25  [4]

Using the Watts = Amps x Volts formula (which is technically right but crude), if your pump is running at 120V AC and draws 2 amps, it's drawing 240 watts - about the same as a heat lamp, or a typical window fan. By contrast, an attic fan draws about 370 watts, and a well pump, about 2,200 watts; or better, comparing with stuff that people leave running 24/7, a 20 cu. ft. automatic-defrosting or frostless refrigerator may draw about 800 watts.

Using the constant for converting electrical horsepower or hp to watts, 1 hp (electrical, with some simplifying assumptions) = 746 watts.

  • a 1/3 hp 120V electric septic system aeration pump should draw about 240 W
  • a 1/6 hp 120V electric septic system aeration pump motor should draw about 120 W - about the same as a 100 watt incandescent light bulb

Watch out: But if your pump is noisy or rattling it may need repair or replacement. If you'd like to give us the brand and model of your aerobic system aerator pump we'd be glad to check with the manufacturer on this question.

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