SKETCH of a typical aerobic treatment unit tank, aerator, chamber Aerobic Septic System Diagnostic FAQs
ATU Maintenance or Repair Questions & Answers

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Aerobic septic system repair FAQs:

Questions and answers about diagnosing and fixing problems that occur in aerobic septic systems, aeration septic systems or fine-bubble septic aerators.

At this website are designs and products for aerobic septic treatment units (ATUs) for onsite waste disposal, also called fine bubble aeration systems. We address aerobic septic system design, features, inspection, repair, and maintenance. Product sources are also listed.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Aerobic Treatment Unit Problems

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

Recently posted questions & answers about aerobic septic system troubleshooting & repair

On 2017-02-23 00:46:52.869942 by (mod) - Hoot Aerobic System Aerator Troubleshooting


Glad to hear you had nearly a decade of good Hoot Aerobic system performance; studies I've read say that the No. 1 cause of aerobic system failure is the lack of maintenance.

I too would expect to see agitation and bubbles in the aeration chamber when the pump is running. You may need to pull the aerator to check for clogging. But first I'd check that the compressor is not just running its motor, but that it's outputting air into the manifold.

Here is an example operating manual for Hoot Aerobic Septics (6MB PDF) that may be helpful.

Be sure to read in that manual starting on page 6 the importance of filling the aeration chamber (center tank) to within a foot of the top before starting the pump. If you re-started your system with the chamber level empty or too low, the aerator may not work properly due to lack of adequate backpressure. The instructions say to stop the system, fill the tank, and re-start it.

On 2017-02-22 23:12:26.609065 by Ed

Our aerobic system (Hoot) was installed in 1998 and overall it has performed perfectly. I've had to replace the air pump about every 5 years and when it fails - it just stops. Over the last 6 months we have been getting a slight smell that we've never had before. I started investigating and I found that roots had grown into the conical lid in the middle tank. This is the tank with the air pump pipes.

I removed as much of the roots as I could, and cut off the rest so the lid seats perfectly. We had all three tanks pumped. The smell returned when the tanks returned to normal levels.

The pump is still working (and the green light is on). Looking in the tank I don't see bubbles surfacing. Should I see bubbles or some agitation in the water surface?

I don't know:
1. Is the air pump producing enough volume?
2. How can I tell if the diffusers at the end of the pipe are clogged?
3. Can I use my air compressor and force air into the pipe to unclog the diffuser?

Thanks for any help.

On 2017-01-08 22:58:18.846050 by (mod) - high water alarm for aerobic system

Suggests a backup, clog, or frozen line.

On 2017-01-08 16:56:19.367104 by roland

high water alarm on what to look for aerobic system just had cleaned @ 2 years back

On 2016-12-11 04:22:19.381462 by (mod) - low pressure or aerobic system piping leaks or damaged pump keeps sprinkler heads from popping up

I would look for a leak in the piping system or a damaged pump or pump impeller, or low voltage to the pump

On 2016-12-10 23:03:21.000193 by Billie

My husband noticed our sprinkler heads weren't popping up and spraying water, it wasn't getting enough pressure to the sprinklers to work properly.. He replace the effluent pump and it is still doing the same thing. Any ideals on what might be wrong.

Our aerobic system is about ten years old and we have had it pumped twice and have regular maintenance done. Thank you

On 2016-11-08 19:18:47.253759 by (mod) - red light on septic alarm box


I don't know what brand and model of aerobic septic system you have installed, but all of the installation and operating manuals for aerobic septics include a table describing the meaning of various lights, warnings, alarms, and controls. That's what we need.

Typically a red light goes on to tell you that the tank is at abnormal sewage level and that service or repair is needed. Your system may be operating on a backup secondary pump but I'd call a local aerobic septic company for service.

Ask also for an outline or tour of just what you have installed, and if you don't know the brand and model you need help finding that so that you can find what the manufacturer says you're supposed to do to maintain the system working properly.

On 2016-11-06 23:28:35.575349 by Kim rosa

Bought house feb 2016. Now red light went off on control box. Silenced the alarm flip the 1st switch down causing the light to turn off. Does that mean the pump is working again.
I have an aerobic system. Just bought the house in february 2016. Red light went off. Silenced the alarm . check the tank on the outside where as there was a little bit of waste on the ground with a ful smell. The other tanks were dry on the outside. switch the first switch down which caused the light to turn off . does that mean the pump started running again?

On 2016-10-25 15:46:18.148482 by (mod) - drip rate setting for aerobic system

Good questions, Craig. I'd start by looking at the aerobic system documentation, manual, or labeling on the pump and equipment to identify the manufacturer as the specific settings are going to be equipment and design-specific. If you can find any identifying information and send it along (page bottom CONTACT link) I'll be glad to help with that research.

On 2016-10-25 04:50:51.300189 by Craig

I have an aerobic system with a liquid bleach chlorinator which was designed to drip at a fixed rate directly into the holding tank, from which effluent is pumped to the sprinkler system. It consists of a capped reservoir of bleach, which is suspended in the inspection/service riser of the holding tank. Screwed into the bottom of the reservoir is a two-part nozzle - tightening the two parts together reduces the rate at which bleach passes through them, to fall into the tank below.

This drip nozzle was clogged, and I have cleaned it. However, I now must re-set the drip rate. This design drips the same amount of bleach regardless of how often the discharge pumps cycle. It there a standard drip rate for such a system, or is there a formula based on tank size or discharge frequency?

Finally, there is no vent passage to allow air past the reservoir cap. It appears likely that tightly capping the reservoir would prevent any dripping at all. Is air supposed to enter via the drip nozzle?

On 2016-10-23 23:42:34.222889 by (mod) - age of an aerobic septic pump & effect of leaky spray heads


1. Age of septic pump: installation & maintenance records, a note on the pump manual, or the month and year of manufacture encoded in the pump motor data tag

2. Not repairing leaky spray heads: failure of the system to properly disperse septic effluent, system failure, wet areas of inadequately-treated sewage effluent, site contamination, inadequate system pressure and thus inadequate dispersal flow of sprayed effluent, system failure

3. Unclear question. Perhaps you refer to risk of electrical short circuit, risking fatal electrocution hazards

On 2016-10-23 23:06:08.365198 by Brenda

I have three questions.
1. How can you determine the age of a aerobic tank pump?

2. What is the effect of not repairing leaking spray heads?

3. What is the effect of not putting spliced wires when installing a new pump in the junction box?

On 2016-09-30 23:03:04.494755 by (mod) smell from aerobic sysetm


Changing the septic treatment and effluent disposal system ought not change what odors show up in a shower though like you I'm not a believer in coincidences. Perhaps the plumbing vent system is defective - leaking, incomplete, blocked - and perhaps the new system piping is allowing more sewer gases than previously to return up the sewer piping system.

I'd ask that an expert who knows the specific design requirements and maintenance requirements of your new aerobic system inspect that installation, and I'd ask for help from an experienced plumber to check out the drain and vent piping.

On 2016-09-30 01:59:36.807771 by Jennifer

I recently had an arobic system installed. It replaced a 35 year old conventional. Every since it was installed I get a septic smell in my bathroom after my shower. Smells like it is coming from toilet, but not sure.

I have called and had the installers come out a couple of times. They cannot find anything wrong. Something is not right. Please help.

On 2016-09-15 19:56:36.096082 by (mod)


I'd look for a tag, sticker label that identifies the brand and model of your system and find the local company who installed or maintains it; I'd ask them to review the system, its condition, needs, and to tell you what regular maintenance it requires. And yes the sprinkler head need to work; you may be about to see a backup if effluent can't be disposed-of.

On 2016-09-15 19:45:15.596715 by K

We just moved to a place with a aerobic system and dont know any thing about them if the sprinkler heads are broke will it cause the light to come on

On 2016-09-12 21:00:59.499272 by (mod) - Mo-Dad Aeribic system sludge layer


I would be concerned enough to talk with the manufacturer about the system, its evaluation, condition, and maintenance requirements.

I'd want to know that the aerator is working properly, and that any post-treatment or disinfection system is working properly.

This PDF gives some information about the operation of your system and it gives contact information for a Mo-Dad Aerobic Septic system company in Louisiana.

I would NOT just try any septic treatment or additive, though Mo-Dad does recommend two additive products: use one that they recommend.

On 2016-09-12 15:05:54.155830 by Dre

I have an aerobic treatment system. It is a Mo-Dad 1 500gpd system. I'll start by saying I have absolutely no problems or issues. System is 5 years old. Prior to 6 months ago, the few times I opened the cap, I noticed black water. About 6 months ago, I noticed a layer of scum/sludge and no visible water is seen. Forward to now, I still see the same sludgy layer on top and no visible water.

I'm in South Louisiana so no drain field, treated water flows directly out of a pipe into the drainage ditch along the road in front of the house. This is working properly. Is this layer of sludge normal on the top of the tank? Installer/seller of the system told me it should always look like black swampy water inside and no layer on top with these systems. My father's same system is 18 years old and has never been pumped out and he has never had any issues.

He sees the same layer on top of his tank. Should I be concerned? Should I try any enzymes in attempt to help treat this scum layer on top? Far as I know, the aerator pump is working.

On 2016-09-07 22:26:35.534137 by (mod) - pumping a "slow" aerobic septic system ?

Sorry but I don't understand your aerobic design and don't have enough detail to give an authoritative answer.
In general, a manufacturer, motivated by cost control, does not include features such as a pump that a system does not need to function properly.

Effluent pumps move clarified effluent out of a treatment tank and into a disposal field or soil absorption system. Without the pump to move the effluent I speculate that if your septic system doesn't actually back up into the home it may at least be on the brink of that messy condition - hence you see it as "slow".

An effluent pump normally runs on cycles, activated by the wastewater level in a discharge chamber or tank. If the pump empties its charge more quickly (20 gpm) than required (12 gpm) that probably has little effect on the operation of the system. It may simply mean that the pump has slightly longer OFF times.

The actual GPM flow rate from a pump, including effluent pumps, is not a fixed number. Instead it will vary depending on the "head" or lift height that the pump has to achieve.

For example

A low-range, low-head pump such as Liberty's 250 series effluent pumps will pump at a flow rate of 45 GPM if the pump lift is only 5 feet while that rate falls to 5 gpm at a 22 foot lift.

A 1/2HP Liberty FL50 pump will pump at about 82 GPM if it is only lifting the waste 10 feet, while its pumping rate falls to 10 gpm if it has to lift the waste 50 feet. This is considered a mid-range capacity pump.

On 2016-09-07 22:13:30.700603 by Danny

Our aerobic tank is 14 years old and we've never had any issues with back-ups into the house, etc. However, we have it pumped once every 1-2 years because it's "slow"- especially when the ground is saturated like after a week of rainy days. (There are only 2 of us and it's the type of system that distributes the water underground.)

We finally got an electrician out and he found that our effluent pump motor was burned out and not running at all (the aerator pump is working just fine however). We want to replace it but it's rated at 12 Gallons Per Minute, and the closest suitable replacement we can find online is 20 GPM.

Would this difference negatively impact the system in any way? BTW, I've also read that the effluent pump isn't critical for the system to work- it just helps it work more efficiently... so I'm curious- without a working pump, how does the clean water get distributed? By shear osmosis or something? Thanks for any info you can provide.

On 2016-08-11 19:08:33.798720 by (mod) - aerobic aerator pump diaphragm replacement


To have space for a detailed reply I repeat your question and answer it at


Please take a look and let me know what happens.

On 2016-08-11 09:19:53.838711 by Rick

Great site. Can you tell me how often the diaphragm need to be replaced on the aerator for a clear stream system? Currently we are having to replace it every two years.

Question: aerobic system fresh water tank is low

(Sept 2, 2015) phil said:
My clean water tank was really low. After I put fresh water from a hose up to top switch , and everything seemed to work perfectly. Why did fresh water tank get so low?

Reply: ATU disinfection systems?


Let's start by giving the brand, model, age of the system we are discussing. Otherwise I don't know what you're asking about.

On some aerobic systems a disinfectant is mixed with fresh water and then injected into the septic ATU effluent before it is discharged to the environment. IF that's what you are asking about suspect that an automatic fill-valve or float valve is sticking on your system.

Question: humming noise from the aeration pump

(Sept 23, 2015) Debbie said:

I have had my tank pumped and a new aeration pump installed recently. There is now a loud humming noise inside my house. I have always been able to hear the pump outside, but never inside. The company that did the install said they don't know why it is making the noise inside and that there is nothing they can do. Any suggestions?


"Nothing we can do" is a pretty maddening reply, I agree. You want to know at the very least if this sound is actually normal or does it indicate an operating problem? If it's normal, why didn't you hear it before? Is this a different brand or model of ATU pump?

Make a note of the pump brand and model, then give the manufacturer a call to ask an opinion.

Also search InspectApedia HUMMING NOISE DIAGNOSIS where you'll find some notes about noise transmission through piping, vibrating motors or mounts, or failing motors.

A more generous and helpful plumber can

- confirm that the pump is working properly - increased pump noise could indicate a bearing or mounting problem

- install a sound-isolating plumbing fitting or connection on piping connecting the pumping chamber or tank top the building

Question: Aerobic effluent sprinklers never coming on

(Oct 28, 2015) Jan said:

I have a new aerobic system just installed the pump has been on and two tanks are almost full but the tank that is suppose to set sprinklers off is only to the bottom ball. The sprinklers have never come on in 2 weeks.


Jan as virtually all packaged ATU systems include inspection, maintenance, and repair for the first two years, I'd give your installer a call to ask what's up with your system.

You need to be taught how to inspect and operate the system or at the very least how to know if it's working properly. If the installer didn't do that, insist that they do.

Keep me posted.

Question: lights flicker when aerobic system runs

(Mar 9, 2016) Yona said:

I have a timer aerobic system and it makes my lights flicker when it goes off. Many times it will shorten the circuit to my bedroom and the pump will stop and this is only when the sprinkler comes on. I will go out into the garage and flip the breaker and the sprinkler will resume spraying and sometimes shorten the breaker again. Sometimes I have to cut the switch off to the pump so that my pump will stay on. I have replaced the breaker and there is no improvement. I have replaced the pump. I think it may be with the timer. Do I need an electrician or someone to work on the system?

I meant to say I cut the switch to my pump so that my electricity to my bedroom will remain on,



I suspect a short or an overloaded / failing electric motor on the circuit that is causing lights to dim. I'd ask for help from a licensed electrician.

If you can find a local plumber who can scope your sewer lines with a camera I'd do so, as I suspect there is a collapsed or damaged sewer pipe. But before making that examination ask the plumber to check the septic tanks for signs of a backup at the tank or tank outlet.

Question: aerobic system was blocked

(Mar 14, 2016) Anonymous said:
We have an aerobic septic system with two tanks. Two days ago it was blocked completely. We
took a snake and pushed it down the pipe and unblocked it. as soon as someone flushed the commode it would completely clog again. What could be wrong?


A clogged or broken waste pipe. Search InspectApedia for CLOGGED DRAIN PIPE REPAIR to read details.

Question: red warning light on the aerator; septic alarm sounds

(Mar 18, 2016) Carlos said:
Red light on aeration problem what can be wrong

(July 2, 2016) Anonymous said:
In the on position, the alarm sounds. In the off position, the alarm is silent but the Red light is on. My
maintenance company does not know the reason for this because the toilet is not backing up. The system does not need pumping. Do you have any suggestions?



I agree that the light probably means something's wrong, but without the brand, model, and service manual and where the light is located, any guess would be wild speculation. Typically warning lights on aerobic systems tell you that the pump is not working or that the sewage level is abnormally high.


I think we need to start by identifying the brand and model and serial number of the septic alarm; that'll take us to the operation manual and answer the question more reliably than my mere arm-waving.

Question: aerobic pump keeps cycling on every half-hour

(Apr 24, 2016) Anonymous said:
pump comes on every 30 to 40 minutes and sprays for 10 minutes.



I don't know which pump you're discussing. Aerobic treatment units may use both an aerator pump (that is normally always on though some run by a timer), and an effluent dispersion pump that may be used to operate sprinklers that dispose of the effluent on to the ground (depending on your ATU system design and type).

If you are NOT running wastewater into the system and the effluent dispersion pump keeps cycling on I would ask for inspection and repair by your aerobic system service company. I suspect groundwater leaking into your system. But check first for a running toilet.

Question: Cottage north of Toronto: how to provide toilet function during freezing weather

(May 27, 2016) Glen said:

Great and informative site! At our cottage north of Toronto we have been using a central composting toilet system and lo-flow toilets for many years and it works OK, but occasionally leaks so we want to move to a septic system. All our gray water is separate from the toilets and goes to a gray water pit. We have a few challenges/questions -

1) Since our gray water is separate, we have very little water for the septic system - is this OK?

2) Our cottage site is rocky and steep so if a drainage bed is needed, runoff (or sewage?) would probably have to be pumped up to a flat spot behind the cottage.

3) Everything is shut off during the (very cold) winters so is there a system that can be left for the winter?


Thanks for the nice note, Glen.

A septic system does need some water for proper operation, lest it act more as an outhouse pit. Flush water from the toilets may be enough. The steep rocky site means you probably need an alternative design system, raised bed septic, mound, peat bed septic, or a mini wastewater treatment system.

Would you consider an incinerating toilet as an alternative? If so search InspectApedia for INCINERATING TOILET to read more.

Question: diaphragm replacement on an aerator pump for aerobic septic systems

2016/08/11 Rick said:
Great site. Can you tell me how often the diaphragm need to be replaced on the aerator for a clear stream system? Currently we are having to replace it every two years.



To have space for a detailed reply I repeat your question and answer it at


Please take a look and let me know what happens.


Aerobic Septic System Articles


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





Suggested citation for this web page

AEROBIC SEPTIC DIAGNOSTIC FAQs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about problems with the operation of aerobic septic systems

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman