Aerobic Treatment Unit: 11 Things (2024) You Should Know

Aerobic treatment units (ATU) consist of several processes — gross solids (trash) removal, aeration, clarification, and sludge return — that create a high-quality effluent.

These processes are generally contained within separate chambers of a single tank.

If you’re considering an ATU for your land, keep reading.

They’re a particularly good option in the following circumstances:

bulletYour soil quality isn’t appropriate for a septic system

bulletYour land has high groundwater or shallow bedrock

bulletYou require a higher level of wastewater treatment

bulletYour septic system has failed

bulletYou don’t have enough land available for a septic system

We’ll tell you everything else you need to know!

1. What is an aerobic treatment unit?

An aerobic treatment unit (ATU) — also called aerobic treatment system (ATS) or aerobic septic system — is a small-scale sewage system that’s similar to a septic tank system.

Instead of using an anaerobic process like septic systems, aerobic treatment units use aerobic processes (one that requires oxygen) for digestion.

You’ll often find these systems in rural areas where public sewers are not available.

They’re used for single residences or a small group of homes.

These treatment systems produce a high-quality secondary effluent that can be sterilized and used for surface irrigation.

2. How does an aerobic treatment unit work?

There are numerous types of aerobic treatment units.

The most common household aerobic treatment unit utilizes a process called suspended growth.

In these units, there is a main compartment called an aeration chamber.

Air is forced through an aeration chamber and mixed with wastewater, creating an environment where bacteria are free-floating in the liquid and growing as they digest the solids (suspended growth).

Some ATUs also have a second chamber where solids that the bacteria are unable to digest settle.

These two chambers are connected, which allows undigested solids to be returned to the aeration chamber (either by gravity or a pump).

Both the process of return and mixing are critical for effective operation.

3. What phases occur in the ATU process?

An aerobic treatment unit generally operates in the following phases.

bulletPre-treatment: In this stage, the unit removes large solids and other undesirable substances.

bulletAeration: In this stage, aerobic bacteria digest biological wastes.

bulletSettling: In this stage, undigested solids will settle to form a sludge that must be periodically removed from the system.

bulletDisinfection: In this stage, chlorine or another disinfectant is mixed with the water to produce an antiseptic output.

Another option is UV disinfection where the water is exposed to UV light inside of the UV disinfection unit.

This stage is optional.

4. How do you maintain an aerobic treatment unit?

ATUs are more complex than conventional septic systems.

Thus, you’ll need to stay on top of maintenance to ensure optimal performance.

There are numerous brands of ATUs.

We suggest that you follow the requirements specified by the manufacturer to guarantee the life of the system.

These requirements often include:

bulletOperational Certificate: This is a certificate that is renewable each year with the county’s environmental health department

bulletMaintenance Contract: A current and continuous service contract with a county approved manufacturer’s certified monitoring specialist (proof of which is annually required with the renewal of the Operational Certificate)

bulletMonitoring and Maintenance: The monitoring and maintenance of your system are critical to its performance.

Here are some of the aspects of your system your manufacturer will suggest you check:

    1. Check your electrical panel for functioning alarms
    2. Check and clean filters on your aerator
    3. Check for odor (it should be “musty”)
    4. Check the clarity of effluent (it should be a clear, brownish color — if it’s gray/black then it means proper oxygenation is not occurring)
    5. Record the sludge level in the pretreatment tank and pump and refill water as needed
    6. Record the sludge level in the aeration tank — pump as needed

If your ATU has a disinfection unit, then quarterly fecal coliform testing is required.

You can find the specific requirements for this on your Operational Certificate.

Additionally, you should maintain an active population of bacteria in the system to break down solids.

To do this, homeowners must:

  1. Keep electricity to the ATU going as the aeration system requires a continuous supply of power
  2. Watch the amount of water or wastewater in the system as too much will dilute the bacteria’s food source
  3. Avoid adding too much organic matter (i.e., fat, grease, or garbage disposal debris) as it will result in more solids than the bacteria can consume (Note: The more organic matter, the more frequently the system must be pumped)
  4. Avoid using too many toxic or cleaning products that can kill microbes and reduce the solids that are consumed
  5. Use the system consistently as infrequent use (such as vacation homes) may not keep enough waste in the system to give the bacteria enough food to sustain themselves
  6. Use the toilet a few times to allow the microbes enough time to rebuild themselves before doing laundry or other high-water usage activities

5. What is the cost of aerobic treatment?

The cost of aerobic treatment depends on factors like design, size, location, operation, and maintenance requirements.

Here are some of the factors that affect the cost of aerobic treatment:

bulletUnit price

bulletCost of unit installation and electricians’ fees

bulletCost of construction of the drain field or cost of other methods of additional treatment if required

bulletCost of electricity (per year)

bulletMaintenance service contract fee (per year)

bulletCost of disinfection (if applicable)

Additionally, some factors like unit price can be negatively impacted by the lack of demand for an ATU in your area.

Installation costs may be higher for aerobic units than septic tanks because electrical work is required.

It’s wise to determine whether an ATU is a cost-effective route compared to other methods.

Consider these factors and ask your local health official to help you evaluate your options.

6. What are the advantages of an aerobic treatment unit?

Using an ATU has numerous advantages according to the U.S. Environmental Protection Agency.

Here’s why you might consider one for your land.

bulletAn ATU performs a higher level of treatment than a septic tank

bulletAn ATU helps protect water resources in areas of failing septic systems

bulletAn ATU provides an alternative for sites unsuitable for septic systems

bulletAn ATU may extend the life of a drain field

bulletAn ATU will reduce ammonia discharged to receiving waters

bulletAn ATU is useable on poor soil types

bulletAn ATU can be used in challenging lots of space and restrictive topography

bulletAn ATU allows for more green spaces as it utilizes smaller drain field footprints

bulletAn ATU is used in setback constrictions to property lines, open water sources, and aquifers

bulletAn ATU reduces nitrogen

bulletAn ATU offsets initial costs by extending overall system performance and longevity

7. What are the disadvantages of an aerobic treatment unit?

While these treatment units have a lot of benefits compared to septic systems, they’re not perfect.

Here’s why you may avoid them depending on your situation.

bulletAn ATU is more expensive to operate than a septic system

bulletAn ATU requires electricity

bulletAn ATU requires more maintenance than a septic tank system

bulletAn ATU includes mechanical parts that can break down

bulletAn ATU is subject to upsets under heavy loads or when neglected

bulletAn ATU may release more nitrates into groundwater than a septic system

bulletAn ATU can be noisy

bulletAn ATU can be smelly if not vented properly

bulletAn ATU can experience adverse effects in cold weather so it must be insulated

8. What are the different types of aerobic treatment units?

There are several types of ATUs.

Small-scale aerobic systems are generally one of two designs: fixed-film systems or continuous flow, suspended growth aerobic systems (CFSGAS).

Below, we’ll talk about those as well as a few others.

bulletFixed film systems: This system uses a porous medium that provides a bed to support the biomass film that digests the waste material in the wastewater.

Designs for fixed film systems vary widely but fall into two categories.

The first category is a system where the media moves relative to the wastewater.

It alternatively immerses and exposes the film to air.

The second category uses a stationary media and varies the wastewater flow so the film is alternately submerged and exposed to air.

In both of these cases, the biomass must be exposed to wastewater and air for aerobic digestion to occur.

The film itself must be made of suitable porous material like formed plastic or peat moss.

bulletCFSGAS: This system is designed to handle a continuous flow.

It does not have a bed for a bacterial film and rather relies on bacteria suspended in the wastewater.

The suspension and aeration are normally provided by an air pump which pumps air through the aeration chamber and constantly stirs the wastewater in addition to the oxygenation.

Sometimes, a medium is added to promote fixed film bacterial growth in systems designed to handle higher than normal levels of biomass in the wastewater.

bulletRetrofit or portable aerobic systems: This is a type of aerobic treatment that is becoming more and more common, especially for failing or failed anaerobic septic systems.

This system is designed to remediate biologically failed and failing anaerobic distribution systems by significantly reducing the biochemical oxygen demand and total suspended solids of the effluent.

bulletComposting toilets: These systems are designed to treat only toilet waste rather than general residential wastewater.

They are typically used with water-free toilets rather than the flush toilets associated with the above types of aerobic treatment systems.

Composting toilets treat the waste as a moist solid rather than in-liquid suspension — thus separating urine from feces during treatment to maintain the correct moisture content in the system.

9. How do ATUs compare to traditional septic systems?

The primary differences between the aerobic treatment unit and the traditional septic systems are the aeration and disinfection stages.

An aerobic treatment system can be used as a secondary treatment for septic tank effluent.

These stages increase the initial cost of the aerobic system as well as the maintenance requirements over the passive septic system.

With that, aerobic treatment systems are unlike many other biofilters.

They require a constant supply of electricity to drive the air pump increasing overall system costs.

Additionally, the disinfectant tablets must be periodically replaced like the electrical and mechanical components (air compressor and air diffuser, respectively).

An aerobic system produces a higher quality effluent than a septic tank.

As such, the leach field may be smaller than that of a conventional septic system.

The output can be discharged in areas too environmentally sensitive for septic system output.

Some aerobic systems will recycle the effluent through a sprinkler system and use it to water the lawn where regulations approve.

10. What are some warning signs your aerobic system is experiencing problems?

If you’re aerobic treatment unit isn’t performing well, here are some signs you’ll begin to see:

bulletAlarms or lights going off

bulletChanges in the system’s normal operating sound

bulletChanges in the normal color of the wastewater in the aeration chamber (Ex: If the color is greyish brown rather than chocolate brown)

bulletExcessive solids, foam, or scum in the unit

bulletPlumbing backups

bulletSewage odors in the house or yard

11. What type of records should you keep about your ATU?

Homeowners and service providers should keep detailed records of the system and service visits.

Your manufacturer should include a user’s manual that describes its recommendations for the unit, including the system design, how to maintain it, and how to tell if the system’s working properly.

Other information you’ll want to keep in these records include the state permitting license, the date the system was installed, the type of disinfection used, and any modifications made to the system.

Final Thoughts

If your land isn’t quite right for a septic system, you may consider an aerobic treatment unit.

By installing an ATU, you can create a small-scale sewage system that is ideal for areas where public sewage isn’t available.

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Erika Gokce Capital

Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.


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