Aerobic Treatment Units

Aerobic treatment units, also called activated sludge tanks, are a common method of biological treatment used in municipal wastewater treatment facilities.
Aerobic Treatment Units - Articles


According to Chapter 73 of the PA Rules and Regulations, traditional septic tanks can be replaced with aerobic treatment tanks. In some cases, where the treated wastewater must be discharged into shallow soils, an ATU may make it possible to eliminate further pretreatment units within the system. Aerobically treated wastewater permits the homeowner to use a smaller soil absorption system.

What is an Aerobic Treatment Unit (ATU)?

An ATU is a multi-chamber tank or a series of tanks that applies a series of treatment processes to the wastewater entering the first tank (left side of Fig. 1). The wastewater enters the first tank or chamber, which serves as a small gravity settling tank where large, heavy solids settle out of the water. The clarified water from the settling tank or chamber then passes into the aeration chamber where it is aerated to digest (or stabilize) the biological waste. By periodically stopping the aeration pump in the aeration tank much of the nitrogen in the wastewater can also be gassed off. Finally the effluent from the aeration tank is pumped into the clarifier where the biological solids settle to the floor and the treated water is pumped out of the tank to an absorption field.

Figure 1. Aerobic Treatment Tank

The middle, or aeration chamber, is where air is pumped into the chamber to provide oxygen for the aerobic bacteria. The air is brought into the aeration chamber at the bottom and either flows freely throughout the chamber "suspended growth" (Fig. 2.) or flows through a "fixed film media" (the black block in Fig. 1).

Figure 2. Aeration Chamber

The quality of effluent leaving an ATU is considerably better than that discharged from a septic tank. Therefore, owners of on-lot systems with an ATU may reduce the size of their absorption fields by one-third, thus requiring a smaller absorption field than would be required if a septic tank had been used.

Why Choose Aerobic Treatment?

There are many households for which a septic system may not be the best wastewater treatment option. Some homes may not have enough land area to accommodate a full-sized absorption field. In areas where soil limiting zones are closer to the soil surface than 48 inches, at-grade or drip systems maybe the only reasonable option for absorption fields. These shallow systems require the addition of a pretreatment (filter) unit between the septic tank and the absorption field. By replacing the septic tank with an ATU, the additional secondary filter unit can be eliminated. In addition, ATUs can be a reasonable alternative when an on-lot system needs to be replaced because the old septic system has failed.

Chapter 73 requirements

  • The rated capacity of an ATU must be specified by the manufacturer.
  • The unit must be able to treat the waterwater flow from the home (minimum flow = 400 gpd plus 100 gpd for each bedroom more than three) or other facility.
  • Every ATU must be equipped with a visual and audible alarm system designed to respond to either electrical or mechanical failure or malfunction of any tank component.

The Downside of Aerobic Treatment

A major reason that ATUs are not more widely used is concern about operation and maintenance by homeowners. ATUs are not passive and therefore require regular maintenance, and inspection. Abuse or neglect can easily lead to component failure or unsatisfactory performance.

ATUs are also more expensive to operate than a septic system. They require electricity and there are mechanical parts that can break down. These tanks are more subject to biological upsets when sudden concentrated loads of organic waste are introduced than septic tanks.

There have also been reports that the effluent from an ATU contains many fine suspended particles that are pumped to the absorption field with the treated wastewater. These fine particles of partially digested waste have been known to contribute to the formation of a biomat in the absorption area requiring maintenance or repair.

Suggestions for Maintaining your ATU

  • Extend the contract service arrangement offered by the manufacturer after the initial two-year period has expired. It is extremely important that an ATU receive regular maintenance.
  • Keep your system accessible for inspections and pumping, yet protected from unauthorized entrance. If access to your system is locked, make sure that your service provider has a key.
  • Call a service professional whenever you experience problems with your system, whenever the alarm is activated, or whenever there are any signs of unsatisfactory system performance.
  • Keep detailed records about your aerobic system, including a map of where it is, and general information, such as model name, capacity, state license, date installed, contract service agreement, records of service visits, and maintenance performed.
  • Conserve water to avoid overloading the system. Be sure to repair any leaky faucets or toilets.
  • Divert all non-wastewater sources of water, like roof drains, house footing drains, and sump pumps, away from the aerobic system.
  • Become familiar with how your own particular system operates, and the way it looks, sounds, and smells when it is working correctly. This way you may be able to identify problems before they become serious and alert your service provider to anything unusual.
  • Be sure to ask your service provider questions about how to know if your ATU is malfunctioning.
  • Don't allow anyone to drive over or park on any part of the system.
  • Don't use your toilet as a trash can or poison your treatment system and the groundwater by pouring harmful chemicals down the drain. Harsh chemicals can kill the beneficial bacteria that treat your wastewater.
  • Don't use a garbage disposal.
  • Don't attempt to clean or perform maintenance on any sealed ATU components.
  • Do not flush…
    • Coffee grounds
    • Dental floss
    • Disposable diapers
    • Paints
    • Pesticides
    • Kitty litter
    • Sanitary Napkins
    • Tampons
    • Wet wipes
    • Varnishes
    • Photographic solutions
    • Cigarette butts
    • Condoms
    • Gauze
    • Paint or stain thinners
    • Bandages
    • Fat, grease, or oil
    • Waste oils

These items can over tax or destroy the biological digestion taking place within your system.

Warning signs of ATU problems

  • Alarms or lights going off.
  • Any changes in the system's normal operating sound.
  • Any changes in the normal color of the wastewater in the aeration chamber (for example, if the color is grayish brown rather than chocolate brown, this can sometimes indicate problems).
  • Excessive solids, foam, or scum in the unit.
  • Plumbing backups.
  • Sewage odors in the house or yard.

For additional assistance contact your local Sewage Enforcement Officer or County Extension Educator.

Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle pike #268
Mechanicsburg, PA 17050
Telephone: 717-761-8648

Pennsylvania Septage Management Association (PSMA)
Box 144
Bethlehem, PA 18016
Phone: 717-763-PSMA