SKETCH of a typical aerobic treatment unit tank, aerator, chamber AEROBIC Septic System Tanks

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Aerobic Treatment Unit Septic Tank Sizes:

Here we provide the size and design requirements for aerobic septic tanks or ATUs. Here are designs and products for aerobic septic treatment units (ATUs, or in the UK, Australia, New Zealand, Aerobic Wastewater Treatment Units or AWTs) for onsite waste disposal, also called fine bubble aeration systems.

We address aerobic septic system design, features, inspection, repair, and maintenance.

Aerobic septic system product sources are also listed. Qualified aerobic septic tank designers, producers, or service providers may be listed at this website at no fee. The page top illustration of a four-chamber aerobic wastewater treatment unit is adapted from the New Zealand Taranaki Regional Council's sewerage system guide.[5]

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Aerobic Treatment Unit Tank Size Requirements

SKETCH of a typical aerobic treatment unit tank, aerator, chamber

Typically a residential aerobic septic treatment unit does not run continuously, just as inflow to it is not continuous.

ATU size or capacity is determined as for a conventional septic system, by allowing 50 to 100 gallons of wastewater per day per building occupant.

Some jurisdictions require that the system minimum capacity accept 500 gallons of wastewater daily.

Safe aerobic treatment tank size advice

A "safe" design size for an aerobic treatment tank that allows for temporary power outage (and thus loss of the aeration pump) is an aerobic treatment tank at least as large as the conventional tank that would have been required for a given site.

Aerobic treatment tanks in cold climates

If your ATU tank is being installed in a cold climate, it may be necessary to insulate the treatment tank (use 2" solid foam panels) to keep the tank temperature high enough (above 54 degF) for the aerobic processing to continue in cold weather. (Some thermophilic microorganisms require temperatures as high as 131 to 149 degF.)

ATU Flow: Continuous or Intermittent Design Alternatives for Aerobic Septic Systems

SKETCH of a typical aerobic treatment unit tank, aerator, chamber

Two basic ATU designs provide for intermittent or continuous flow through the system.

Intermittent flow aerobic septic systems

Just as most residential septic systems do not receive wastewater continuously but rather intermittently, the ATU may be designed to turn the aerator and agitator on and off as needed. Intermittent flow septic systems, including aerobic or aeration treatment designs are also called a "batch system" septic design approach.

Continuous flow aerobic treatment system designs

Other aerobic or aeration septic treatment systems, particularly aerobic systems which have to serve a larger facility where wastewater inflow may be continuous, may themselves operate continuously. Obviously these designs are called "continuous flow" aerobic treatment systems.

How to Select & Where to Buy an Aerobic Septic Tank Aerator Pump

Question: How do I size the correct aerator pump for my ATU ?

11/5/2015 Josh said:

How do I size the correct aerator pump for my ATU ? My systems holds 600 gallons . There are 5 people in our house , with vistors sometimes 10 ?
Do I need a dual pump or can I run pumps alternately ?



Perfectly reasonable question - I don't know. The proper aerator size depends on the specifics of how your particular system was designed, which varies by manufacturer and model; if you cannot find any manufacturer name then we quickly fall back to generic advice:

The aerator pump size (and thus its capacity to bubble) is chosen to match the average wastewater flow into the septic tank per day.

In the article above we suggest

ATU size or capacity is determined as for a conventional septic system, by allowing 50 to 100 gallons of wastewater per day per building occupant.

Or you can estimate your daily wastewater usage more accurately from SEWAGE FLOW & DESIGN FLOW ESTIMATES and use your browser "back" button to return here.

Also see AEROBIC ATU SEPTIC TANK SIZES to check the size of your aerobic treatment unit (septic tank)

Below I've cobbled up an aerobic septic tank aerator pump sizing table.

Aerobic Septic System Aerator Pump Sizing Table for Residential-sized Septic Systems

Average Daily Wastewater Flow in Gallons or Liters Aerobic Septic Aerator Pump Delivery of Air per Minute Comments
500 - 600 gpd / 1900 - 2300 lpd 4-5 cfm / 80 - 100 lpm

Example: Hoot® Troy Air Alternative this air pump fits Hoot septic system models H450, H500, H600 and LA 500.

Older Hoot septic systems use a different blower type - a "regenerative blower" so check your system requirements.

750 gpd / 2800 lpd 5.2 cfm / 120 lpm Example: Hoot Troy 750 GPD septic air pumps work with Hoot Troy Air models H750 and LA 750.
900 - 1000 gpd / 3400 - 3800 lpd 7 cfm / 150 lpm Example: Hoot Troy Air Alternative 1000 GPD septic pump works with Hoot Troy Air Models H1000 and LA 1000
1200 - 1500 gpd / 4500 - 5700 lpd 8.4 cfm / 200 - 1000 lpm Example: Hoot Troy Air Alternative septic air pump works with the Hoot Troy Air models H1600 and LA 1500.


cfm = cubic feet per minute

gpd = U.S. gallons per day

lpd = litres per day

lpm - liters per minute or in the U.K. litres per minute

We recommend checking to assure that the aerobic pump you select is approved by the NSF (National Small Flows) for your system.

Watch out: A review of companies selling aerobic septic system aerator pumps (November 2015) found both OEM as well as alternative pump models and brands widely available. Check with the designer or manufacturer of your specific aerobic septic system as manufacturer's specifications may vary from this general table.

Aerobic Septic Pump Sources

  • Cyclone High Performance Air Pump Specifications [PDF], retrieved 2017/09/21, original source
  • Gast septic aerator pumps, Gast Manufacturing, Inc. 2300 M-139 Highway Benton Harbor, MI 49023-0097 Office Phone: (269) 926-6171, E-mail: - USA or
    Gast is owned by IDEX corporation.

    Gast Asia: Gast Group Limited c/o IDEX TRADING(SHANGHAI) CO.,LTD Room 3502-3505, No. 1027 Chang Ning Road, Zhaofeng Plaza, Shanghai, China 200050 Phone: +86-21-52415599 ext 302 Fax: +86-21-52418339 ext. 302 E-mail:;

    Gast Europe: Gast Group Ltd Unit 11, The IO Centre Nash Road Redditch, B98 7AS United Kingdom Phone: +44 (0) 1527 50 40 40 Fax: +44 (0) 1527 52 52 62 Email:
  • HiBlow septic aerator pumps, Tel: 1-877-925-5132 Website: Email:
  • Septic Solutions, Inc., 314 W. Center St., Dieterich IL 62424 USA, TEl: 1-877-925-5132 Website:

    Distributes Alita Air pumps, Cyclone air pumps, HiBlow and HiBlow Dual port air pumps, Medio Piston type air pumps, Regenerative Blowers, Secoh Air Pumps, Thomas Air Pumps,
  • Spunair septic tank aerators Website: - see our warning just below
  •, 111 Vernon Rd Aledo, TX 76008, Website:

Watch out: do not simply add-on an aerator or agitator to a conventional septic tank to "convert your septic tank to an aerobic system". Making this change alone without other critical design changes such as outlet filters or a settlement chamber may agitate the sewage in the septic tank causing solids to flow into the septic drainfield or absorption bed - causing its rapid desctruction through clogging.

Aerobic Septic System Air Pump Specifications

Question: How do you check the air pressure on an aerobic septic air pump?

2017/09/21 Dexter said:

How do you check the air pressure on a pump to make sure your diffuser is not clogged what should the air pressure reading be

Reply: aerobic septic air pumps are rated as open flow in CFM or LPM but air pressure readings can be diagnostic

hanks for asking, Dexter.

There is not a single "right answer" to the question of what should be the air output for an aerobic septic system air pump (also referred to as septic aerator pump or septic diffuser pump, or septic air pump or septic "compressor" pump). That's because depending on the septic tank size and daily waste flow, the pump model will be chosen to match with a particular output rate.

Septic air pumps are typically rated not by an air pressure - because the pump is designed so that its output end is open to water and ultimately to the atmosphere. It isn't designed to be pumping into a closed container - which is what you'd need to measure air pressure.

Put another way, though the aerobic air pump has a pump operating air pressure capacity, the pump output is normally going to be described by the manufacturer as "open flow" capacity in CFM (or LPM).

When a septic air pump is running in open flow its output is actually at 0 psi.

That's why I said you can't easily measure it.

Types of Septic Air Pumps

Typical AerobicSeptic Air Pump Ratings

Aerobic septic air pumps are rated in output as cubic feet per minute (CFM) or liters per minute (LPM) of air. A typical septic aerator pump might run at 5 CFM or 80 LPM of air output. The typical range of septic air pumps for residential septic systems is about 2 to 8 cfm or from about 40 to 200 LPM of air.

Indeed there is something tricky to understand about these ratings. A septic air pump rated for 100 LPM air output at "open flow" (or un-restricted output) will actually deliver a lower flow rate when the output is restricted such as by having its outlet six feet deep in a septic tank.

SO: if you had a tee in the pump's output air line that runs between the pump's output port and the aerobic septic tank diffuser, you WOULD see some air pressure if you put a gauge on that tee (with appropriate fittings of course - you can't just hold the gauge there).

What Air Pressure Could be Seen at a Septic Air Pump?

Then , using a Hiblow HP-80 aerobic septic air pump as an example, that pump's maximum airflow is rated by the manufacturer as 4.2 cfm (119 LPM) WHEN THE PUMP IS OPERATING AT at 0 p.s.i., and the rated air flow will be LOWER - about 80 LPM (2.83 cfm) when under actual installed-conditions the pump is operating at the rated pressure of 2.13 p.s.i.

Because individual septic air pump specifications will be different among manufacturers' brands and models even before the pump is installed, you want to make a note of your pump's specific brand and model.

Then from the manufacturer you (or we) could obtain their expected pump air flow rates at open conditions and their expected pump air flow rates at typical installed conditions.

Most likely the range will be between 1.5 and 5 psi. Why? Well if we went under water (I don't want to swim under sewage) down to 30 feet, we would be at one ATM of pressure or about an additional 14.6 psi of pressure from being on the ground at sea level.

If we figure that your septic tank is maybe 4 or 5 feet deep (of sewage) then we are at 5/30 or 1/6 of 1 ATM. 1/6 of 14.6 psi = 2.4 psi.

So you can see that we don't need huge pressures to push air down to about five feet of depth.

IF at our magic Tee that we installed in the air line we saw pressures lower than specified by the manufacturer the pump is probably defective (or there's an air leak). If we see pressures creeping UP from what the manufacturer specifies then the diffuser may be clogged.

But it's easier to skip the tee and the pressure gauge and look for bubbles.

Other aerobic septic air pump checks you can make

Check that

1. that the pump is running

2. that it is delivering air to the septic tank - air bubbles are appearing at the tank top inspection port.

3. Some septic air pump models include an alarm feature to let you know if the pump is off or not working.

That's a nice feature since if you leave your aerobic system without its aerator it is not working, it is not treating sewage adequately, and in as little as a few days the system can fail.


Continue reading at AEROBIC SEPTIC DISINFECTANTS - Calcium Hypochlorite or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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Aerobic Septic System Articles

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