Continuous flow suspended growth aerobic septic design Convert Conventional Septic to Aerobic Septic Systems

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How & why to convert a septic tank and drainfield system to an aerobic septic system:

What is required to convert a conventional septic tank and soakbed system to an aerobic treatment system or ATU? Is it possible to change over a regular septic tank and drainfield to an aerobic treatment unit design?

Will that cure a failed drainfield? Will that conversion work better, use less space, better dispose of wastewater? What about aerobic septic conversion costs and aerobic system operating costs?

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.

Convert from a Conventional Anaerobic Septic System (tank and drainfield) to an Aerobic ATU Septic System Design

Septic tank schematicWhy Convert from Conventional Septic Tank to an Aerobic System

Converting a traditional septic tank and leachfield to an aerobic septic system improves the level of wastewater treatment, reduces the risk of contaminating the environment, wells, lakes, streams and neighbours with sewage pathogens, and it might permit on-surface discharge of highly-treated septic wastewater effluent if that is permitted where you live.

[Click to enlarge any image]

You'll read in some of the reader questions later in this article that some folks are fooled into thinking that increasing the level of treatment of their septic wastewater will repair a failed septic soakbed or drainfield. It won't.

Our sketch shown at left illustrates two conventional septic tanks - a single chamber tank in the upper sketch and a two-compartment septic tank in the lower sketch. Forbes (1992) explained where the pressure to convert to higher level wastewater treatment systems arises:

Two issues are currently being addressed by many states regarding individual home wastewater treatment (onsite wastewater disposal) and its environmental impact.

The first is how to remediate those inadequate septic tank systems which are already in place.

The second is to assure that all onsite wastewater treatment systems installed in new construction provide adequate wastewater treatment.

Many states have addressed both of these issues in a single policy.

As a result thereof, many states now require, for both existing systems and new construction, either: a septic tank system where the tile drain field is installed in soils capable of percolating to state defined standards and the local groundwater table is at a sufficient depth; or, the modification to or replacement of the septic tank system with a system in which the effluent leaving the last sealed tankage of the system meets state defined water quality standards. - Forbes 1992

It might be easier (and cheaper) to convert the two-compartment tank shown in the lower sketch to an aerobic treatment unit or ATU system. And indeed there have been a number of patented designs

for converting a standard anaerobic septic tank system to an aerobic system where the effluent discharged has a high level of quality, sufficient to meet or exceed all known national and state standards. - Forbes 1992

While there were designs and patents for converting from an anaerobic (conventional) to aerobic (ATU) septic design for decades before, Forbes in 1992 intended to provide a conversion approach that overcame some of the costs and difficulties of older aerobic treatment conversion approaches. His criticism of his predecessors included

... they have several economic and aesthetic drawbacks.

When used to replace an existing septic tank system, or placed in series with an existing septic tank, they require excavation equipment and hoisting systems. This equipment is costly, and the cost is passed on to the owner through unit costs.

The equipment also can, and usually does, cause major damage to a yard, particularly when the ground is wet. - op.cit.

Differences Between a Conventional Anaerobic Septic Tank & Drainfield & an Aerobic Septic System

Taking a simplified view, a traditional private or onsite wastewater system consists of a septic tank and an absorption system also referred to as a soakbed, drainfield, leachfield, or soakaway bed. Sewage or blackwater and perhaps other building wastewater such as from a kitchen or laundry (graywater) flows into the septic tank.

Sludge settles to the tank bottom, floating grease and scum coagulates on the septic tank top, and clarified septic effluent flows out of the tank into a buried absorption system such as a distribution network of perforated pipes in gravel trenches.

We also call these traditional septic systems anaerobic because there is not a lot of oxygen and not a lot of aerobic bacteria active in the septic tank. Such systems perform about 45% of the necessary treatment of the sewage wastewater while the remaining 55% of treatment - bacterial action and filtering - occurs in the soils where the effluent is dispersed.

Below is a more specialised-septic tank designed for use with an aerator also referred to as an aspirated mixer pump used in aerobic septic system designs.

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

As we explain at AEROBIC SEPTIC SYSTEMS, ATUs - home , aerobic septic systems or ATUs (aerobic treatment units) add oxygen to the process of treating septic of sewage wastewater by 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.

Spratt (1989) provided a simple flow chart showing what's needed to convert to an aerobic septic treatment design.

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Steps in aerobic septic treatment - Spratt 1989

Aerobic septic designs use two or more separate tanks or chambers to treat the sewage and to separate solids so that the effluent that is finally to be discharged to the environment - under the soil or perhaps sprayed on top of the ground by a sprinkler system - is highly treated - perhaps to 95%.

In a four-tank aerobic septic system (or four-chamber aerobic) we'd find all of the chambers listed below, while in 2 or 3-chamber systems some of the steps below may be combined in one chamber or tank.

  1. Aerobic Septic Tank Sewage receiver: a compartment to receive sewage and collect sludge;

  2. Aerobic Treatment Unit Aeration Chamber: an aerobic chamber to pump air and thus oxygen through wastewater to assist in the aerobic treatment process;

  3. Aerobic Septic Effluent Settling Chamber: a clarifying or settling chamber which permits remaining solids to settle out of the wastewater; disinfection may take place in this chamber;

  4. Aerobic Septic Effluent Pumping Chamber: a pumping chamber to receive treated effluent for discharge to an absorption system or other destination.

Spratt's 1989 patent shows a rather complex-looking 4-chamber aerobic septic treatment unit.

Aerobic septic design using a full four-chamber septic tank - Spratt 1989

Watch out: You will see from our simplified description of aerobic septic conversions hat simply plopping an aerator or air pump into a conventional single-chamber septic tank is not going to produce a properly-working aerobic septic system.

Worse, simply agitating or bubbling your sewage to death in a single chamber septic tank is going to keep the wastewater agitated. When that water, containing a high level of floating solids, flows into the absorption system or soakbed, it's going to clog and ruin the bed in no time whatsoever.

So at a minimum if we want to convert a conventional one-chamber anaerobic-treatment septic tank into an aerobic or ATU septic system we need some means of separating sludge and floating solids from effluent to be discharged from the system: usually that means adding a settlement tank or effluent pumping tank or perhaps a combination of a septic filter and a pumping chamber.

Watch out: also to get a realistic picture of the costs involved in both converting a septic system to an aerobic treatment system and the cost of maintaining an aerobic septic system.

The cost of a completely new aerobic septic system installation is about double the cost of a conventional septic tank and drainfield. But the cost of converting a working but conventional tank and drainfield septic system to an ATU might be much less, depending on how many components you're adding.

The ATU will also require more frequent maintenance and will have higher annual operating costs than a conventional residential on-site septic system.


Reader Question: converting a conventional septic tank to an aerobic system

(June 3, 2015) Gene said:
I have a single 1000 gal 2 outlet fiberglass septic. To convert to an aerobic septic, I believe I would need an additional clarifier tank. Would it work to install a settling chamber inside the existing septic tank through the outlet manhole or would it be too small to be a clarifier?



Several manufacturers sell "add-on" aerators for conventional septic tanks.

Watch out: simply aerating a conventional septic tank is guaranteed to destroy the drainfield in short order as the added agitation will cause the system to push solids out into the drainfield. A conventional septic system needs some quiet settling time.


Most likely it's easier, safer, and preferable to add a second clarifying chamber / tank outside the original septic tank. In my opinion 1000g is already a modest tank size and given access, safety, complexity, I'd think it's easier to excavate the outlet end of the existing tank to add a decent size settling chamber, filters, or other controls needed.

Check with the manufacturer of your tank for their suggestions.

Reader Question: drainfield is plugged, steps to improve things by going to an ATU design

(Mar 11, 2015) Robert Gibson said:
I have a 900 gallon rectangular septic tank with no baffles or partitions. The field is mostly plugged up.

Step 1.Next week I will install a T baffle on the inlet, a filter/baffle on the outlet and a man way riser above the outlet. Plus a 9" diameter fine bubble diffuser on each side of the outlet about 18" off the bottom powered by a 2.8 GPM/80L air pump.

Step 2. I was thinking of installing a settling tank in front of the main tank but I am only 10 ' from the house and the inlet/outlets would be level. An outlet tank would be level also.

I can build a block wall in the tank, say 600 gal aeration/ 300 clarification, would need to cut another hole to install/service a different style diffuser. I intend to end up with a correctly working ATU. Any suggestions?



I'd look into a septic tank outlet filter at the same time that you replace the missing septic tank baffles. Or if your system is clogging with laundry lint I'd add a filter on the laundry drain as well. You'll be adding a regular maintenance task but that procedure can considerably improve the life of your new septic drainfield - which is likely to be needed given what you've described.|

Watch out: none of the steps you propose will fix a failed drainfield. You'll need either a new drainfield or to convert to an aerobic treatment unit design that sprays effluent onto the ground surface - permitted in only some jurisdictions.

Reader Question: pro-active steps for an old drainfield - change from anaerobic to aerobic design?

(May 8, 2015) Gene said:

We have a cabin in the mountains that has a 1000 gal fiberglass septic tank with no baffles except T's at inlet and outlet. There are 2 drain fields separated by a valve. We normally spend only 6 months at this location. There are only two of us, but we often have visitors, some stay for weeks.

We do have a septic system that is grandfathered but by current code it is too close to a stream and our well. Any major changes would probably require a complete system replacement to meet code (not practical both financially or physically)

There are no apparent major problems with the system.

However, the system is approximately 40 years old and we are trying to be proactive and keep the system functioning properly. Should we consider changing from the anaerobic to an aerobic system to potentially extend its life? Are single tank solutions possible using either a product such as a Bio-Brush (Aero-Stream) or an Air Particle Recirculator (Septic Solutions)in lieu of a clarifier tank?

We already have installed low flush toilets, low volume showers, and sock filters on the washing machine.

We pump the septic tank bi-annually and have a filter on the outlet. We have only owned the property for the last ten years so other than some pumping records, we do not know its early history. There are no other records. We would appreciate your help.


Gene thanks for the interesting question.

I'm sure there are other designs that we don't know about as the industry keeps evolving.

But generally no single tank septic tank that has added to it something that agitates the sewage will work without destroying the drainfield unless there is at the very least an effective and regularly maintained filter at the tank outlet. In my experience people put in the add-on features but the maintenance needed is more than anyone wants to remember so the system fails.

And no septic tank for two people that's 1000 gallons should need to be pumped as often as you are doing it. Something's wrong there.


(May 9, 2015) Gene said:

Thanks for the quick response. The pumper initially recommended the interval for pumping and as a septic system was new for me, I just continued it. I will review per your suggestion.
I agree that maintenance is the issue for filters. Currently, we have advised to clean it every 6 months of usage.
As an alternative to the single tank, could the interior of the septic tank be altered by a baffle to separate the agitation zone from the clarification zone? I may be grasping at the proverbial straw.
I do plan to follow up on the Bio-Brush and the Air Particle Recirculator and if I get anything useful, I will share.


I'm doubtful that redesigning an existing 1000g tank to make part of it a settlement chamber is feasible.

You can by inspection see objective data that will tell you if the filter needs cleaning more or less often. I'd check monthly looking both at the extent of filter clogging and whether or not the wastewater level in the tank is too high (above the outlet opening bottom surface).

What you find on Bio Brush and the Air Particle Recirculator will be helpful to other readers.

Conversion of Septic to Aerobic Sewage Treatment, Studies, Patents, Products

Aerobic septic conversion device, Forbes  patent 1992

Device to convert to an aerobic septic system treatment unit - Goodman 1971


Continue reading at AEROBIC CONTINUOUS FLOW, Suspended Growth for more about aerobic type septic designs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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