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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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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).
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.)
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.
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 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
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 , there are two basic types of ATU systems:
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.
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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?
(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.
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.
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.
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  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
I would contact the manufacturer of your system and ask for their advice and for a copy of the instruction manual.
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.
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:
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.
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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Technical Reviewers & References
Continuous-Flow, Suspended-Growth & Aerobic Septic Systems References, Manufacturers, Contact Information
Design Manuals for Septic Systems
Onsite Wastewater Disposal Books