Aerobic Treatment Unit
Also called: ATU, aerobic septic system, aerobic digester
An aerobic treatment unit (ATU) uses active aeration to accelerate biological treatment of wastewater before it reaches the dispersal system. Unlike a conventional septic tank - which relies on anaerobic (oxygen-free) bacterial activity - an ATU injects or mixes air into the treatment chamber, fostering aerobic bacteria that break down organic matter more rapidly and to a higher degree. ATUs typically produce cleaner effluent than a conventional tank, allowing them to be used on challenging sites or to meet more stringent treatment standards.
ATU designs, treatment standards, and maintenance requirements vary significantly by manufacturer and model. County rules may require ATU owners to maintain an O&M agreement with a service company meeting the local program requirements and to follow periodic inspection and reporting procedures. Consult your system records, the manufacturer's documentation, and your county health department for requirements specific to your unit.
How It Generally Works
1Pre-treatment chamber
Incoming wastewater first enters a pre-treatment or settling chamber where large solids settle out before the liquid moves to the aeration zone. This protects the aeration equipment from excessive solids loading.
2Aeration chamber
In the aeration chamber, air is continuously or intermittently injected into the wastewater by an air pump (compressor) or by a rotating mechanical aerator. The dissolved oxygen supports the growth of aerobic microorganisms that digest organic material rapidly and efficiently. The result is a cleaner effluent with lower biological oxygen demand than a conventional system produces.
3Clarification chamber
After the aeration stage, the treated wastewater moves to a clarification chamber where suspended solids settle out and are periodically returned to the pre-treatment chamber or removed during maintenance. The clarified effluent is then ready for final dispersal.
4Effluent dispersal
Treated effluent from the ATU is delivered to a dispersal system - which may be a smaller drain field than a conventional system would require, a pressure distribution field, drip emitters, or another county-approved method. The higher quality of ATU effluent may allow reduced setback distances on some sites.
Common Maintenance Considerations
Routine service - often part of the system plan
ATUs are mechanically active systems with compressors, aerators, float switches, alarms, and multiple chambers. Some counties require ATU owners to maintain an O&M arrangement with a service company that meets the local program requirements, inspect the unit on the county's schedule, and submit required reports.
Air pump and aeration equipment
The air pump or aerator is the core of the ATU's operation. It requires periodic inspection, filter cleaning (on air pumps with intake filters), and replacement when it reaches the end of its service life. An air pump that fails silently - without an alarm - can result in the system reverting to anaerobic conditions without the owner's knowledge.
Solids management
Despite the advanced treatment, solids still accumulate in the pre-treatment and clarification chambers. Pumping is required periodically to remove accumulated sludge - on a schedule that depends on the unit size and household use.
Alarm system monitoring
ATUs typically have multi-function alarms that can indicate air pump failure, high water level, or treatment quality issues. Keeping the alarm system functional and responding to alerts promptly is essential.
Observable Conditions to Share for Service Planning
The following are observable conditions, not remote diagnoses. General Septic can assess the system on site as part of the requested service.
Alarm activation
An active alarm on an ATU control panel can indicate a range of issues - air pump failure, high water level, or a treatment stage problem. Because ATUs have multiple potential failure modes, alarm conditions should be reported to General Septic promptly.
Unusual odor from the unit or dispersal area
A properly functioning ATU produces relatively odor-free effluent. A persistent sewage odor from the unit or dispersal area may indicate that aeration is not functioning, that the system is hydraulically overloaded, or that a treatment stage is failing.
Wet areas over the dispersal field
As with any system, surfacing effluent or saturated soil above the dispersal area may indicate a problem. Keep people and pets away, reduce water use, and describe the observation to General Septic or the applicable health authority.
System Details to Include in a Request
Having this information ready helps General Septic plan the requested service:
- The ATU brand and model number (usually on a label on the unit or control panel)
- Whether the alarm is currently active and what indicators are showing
- Whether the unit is under an active service contract and when it was last serviced
- When the unit was last pumped for solids removal
- Any change in household water use or occupancy
Related Services
Describe a request in the greater Seattle area
General Septic directly provides septic services across King, Pierce, and Snohomish counties with its staff and equipment. Describe your system and situation in the request form so the team can plan the work.