Chlorine Tablets for Aerobic Septic Systems: A Homeowner Guide

If you live on acreage west of Austin, there is a good chance your home has an aerobic treatment unit, often called an ATU. Somewhere on that system there is a small container, usually a plastic tube or a box, that holds chlorine tablets. Keeping those tablets stocked and using the right kind matters for the health of your system, and it matters for safety.

This guide covers what chlorine tablets for aerobic septic systems do, which tablets to use, which to avoid, how to check them and when to leave the job to a professional.

Why aerobic systems use chlorine at all

An aerobic system treats wastewater in stages. The first tank is a regular septic tank that settles out solids. Wastewater then moves to a chamber where an air pump bubbles in oxygen. Oxygen-loving bacteria break down waste much faster than the bacteria in a standard tank. A clarifier lets what is left settle, and the clearer water moves on.

Even after all that, the water still can contain germs. Because many aerobic systems send treated water to spray heads on the surface of your yard, that last step matters. Disinfection kills most harmful organisms before the water is sprayed. The usual method is a chlorinator, a device that holds tablets and lets water flow over them so chlorine dissolves into the water.

If you want a refresher on how the stages differ from a conventional system, our post on aerobic vs conventional septic in Texas explains the layout.

Which tablets to use

Texas A&M AgriLife Extension’s onsite wastewater program, which produces the state’s homeowner education material, gives clear direction: use only tablets made for wastewater treatment. These are made of calcium hypochlorite. They are designed to dissolve slowly and steadily in the flow of treated effluent.

Look for these points on the label:

• Active ingredient: calcium hypochlorite.
• Marked for septic or wastewater disinfection. Many are sold as “septic chlorine tablets” or “ATU tablets.”
• The size your chlorinator is made for. Tablet sizes vary. Your chlorinator is built around a particular size, so check your manual or ask your maintenance provider.

You can buy these tablets from septic supply stores and some online sellers. If you are not sure which one fits, ask your maintenance provider. They will know which product your unit expects.

Which tablets not to use

The most common mistake is buying swimming pool tablets. They look similar and may be cheaper or easier to find. They are the wrong product for an aerobic septic system.

Pool tablets are usually trichloroisocyanuric acid, often shortened to trichlor. According to Texas A&M AgriLife’s publication on tablet chlorination, those tablets are not approved for wastewater disinfection and release chlorine too slowly in this setting to work properly. They also contain stabilizers that are meant for pool water, not for a treatment system.

There is a safety reason too. The same publication warns that mixing the two types of tablets, or letting wetted pool-type tablets contact other chlorine products, can create dangerous reactions, including the formation of an explosive compound. Never store or load different chlorine products together in the same container.

Other products to avoid:

• Liquid household bleach poured into the system. It is not the same as a metered disinfection step and can disrupt the bacteria in the aeration chamber.
• Powders or granules not designed for the chlorinator.
• Mixed or homemade products.

When in doubt, do not guess. Call your maintenance provider.

What the maintenance contract has to do with it

In Texas, aerobic systems must be covered by a maintenance contract with a TCEQ-licensed maintenance provider. According to TCEQ guidance, the contract must list what is covered and set a response time for owner complaints, and the provider reports to the permitting authority and the owner on a regular schedule. Hays County requires an initial two-year contract for new aerobic systems, and a new contract must be filed after a property sale. Check with your county for the details, since local rules can be stricter than the state’s.

The details on who stocks the chlorine depend on your contract. Some contracts cover the cost of tablets, and others have the provider check and refill the chlorinator at each visit while the homeowner pays for the product. Read yours. If it is not clear, ask. Our aerobic septic maintenance page describes how visits typically work.

Also keep in mind that Texas rules do not treat a homeowner as a replacement for a licensed provider. A homeowner can usually refill a chlorinator between visits, but the inspections, testing and reporting belong to the licensed provider. Whether and when a homeowner may do their own maintenance on the system is a local-rule question, and Hays County says a homeowner may maintain a system only after the initial two-year period.

How to check the chlorinator

You can do a basic check in a few minutes, without any tools. Safety comes first.

1. Find the chlorinator. It is usually a plastic tube or box in the line between the clarifier and the pump tank or the spray area. It often has a cap or a lid.
2. Wear gloves and eye protection. Calcium hypochlorite is a strong oxidizer. Do not touch tablets with bare hands.
3. Open the cap away from your face. Fumes can be strong.
4. Look at the tablets. You should see several tablets. If the tube is empty or nearly empty, it needs a refill.
5. Check for crust or clumping. Tablets that are fused together or coated with white crust may need cleaning.
6. Close the cap securely.

Many owners check once a month, and some check every time they mow. Tablet use depends on how much water the household uses. A busy household burns through tablets faster than a couple.

How to refill the chlorinator

If your maintenance provider has told you it is fine to refill between visits, follow these basics:

• Use only calcium hypochlorite tablets of the right size.
• Wear gloves and eye protection.
• Add tablets by dropping them in, not by forcing them.
• Do not overfill. Leave the space the manufacturer recommends.
• Keep other chemicals away. Do not add anything else to the chlorinator.
• Wash your hands after.

Store the tablets in their original container, in a cool, dry place, out of reach of children and pets. Keep the lid tight. Never store them next to pool chemicals, fertilizer, fuel or other chemicals. In a Texas garage that gets hot in summer, find the coolest spot you have, or store them in a shaded outbuilding.

Signs the chlorine step is not working

Here are warning signs to watch for:

• The chlorinator is empty. Most obvious.
• Tablets are stuck or clumped. Water cannot flow over them.
• A blocked line. If water cannot reach the chlorinator, it cannot disinfect.
• A broken pump or float. The system does not move water to the next stage.
• A strong chemical smell around the chlorinator, which could mean tablets are mixed with the wrong product.
• Alarm lights or sounds. Some systems alarm for pump problems, which can affect the chlorine step.

Another point: when a maintenance provider tests the water, they check the chlorine level. Testing needs the right equipment. If you suspect a problem, ask your provider to run a test rather than guessing.

Can you have too much chlorine?

Yes. Too much chlorine in treated water can harm grass and plants in the spray area and may kill some of the helpful bacteria if it flows back into the tank. Brown or dead patches under the spray heads can be a sign. If you see them, tell your provider so they can adjust the tablet feed.

Too little chlorine is the other problem. It means the disinfection step is not doing its job. Both cases can be fixed when you catch them early.

Chlorine and spray irrigation

Aerobic systems that spray treated water must keep that water away from people. Spray heads are often marked, and the spray area is generally kept away from property lines, wells and play areas. Check the rules with your county and the original permit for your property.

If your spray heads are clogged, or if water is pooling under them, the chlorine step may not be the problem. Our post on aerobic spray heads not spraying goes through the usual causes. If a part has failed, our aerobic septic repair page describes what service involves.

Hill Country notes

A few local points:

• Heat. Chlorine tablets store best cool and dry. Summer temperatures in the Hill Country can get high in sheds and cars. Do not leave a bucket in a hot truck bed.
• Hard water. The Hill Country has plenty of limestone, and many private wells and public systems produce hard water. Mineral scale can build up in chlorinators and around spray heads, so ask your provider to check them for scale.
• Wells. Many homeowners near Dripping Springs and Wimberley rely on wells, and the disinfection step helps protect water quality. Keep your spray area away from well heads as permitted.
• Power outages. Storms and heat can cut power. When power fails, the air pump stops. If the outage lasts long, call your provider.

Costs, in general terms

Tablet cost depends on the brand, bucket size and how fast your household uses them. General retail prices for a pail of septic chlorine tablets run from a modest amount for a small pail to a larger amount for a bulk pail, and prices change. Ask your maintenance provider what they recommend, and compare the price per tablet rather than per bucket.

Treat any price you see online as a rough guide, not a quote.

A simple monthly routine

A short routine keeps you on top of things:

1. Look at the control panel. Is any light on?
2. Check the chlorinator level.
3. Walk the spray area. Are the heads working? Any pooling?
4. Note anything odd in a small log.
5. Make sure the maintenance contract is current.

Our homeowner septic maintenance checklist can help you build that into a routine.

When to call a pro

Call your maintenance provider if:

• The chlorinator will not hold tablets or water is not flowing through it.
• You are unsure which tablet your unit needs.
• You see dead patches in the spray area.
• You smell a strong chemical odor.
• You are behind on your maintenance contract.
• Anything about the system looks different than before.

Do not try to fix the chlorinator with a pool product or a work-around. Wrong chemicals cost more in the end.

Getting help with your system

The right chlorine tablet is a small thing that protects a big investment. If you are not sure whether your system is using the correct product, or you need a licensed provider to service your unit, request a free estimate and tell us about your system. We can explain what your ATU needs in plain terms and help you keep it running the way it was permitted.