The short answer
You increase good bacteria in a septic tank mainly by not killing them. Every flush brings in fresh bacteria, so a tank that is used normally keeps a healthy population on its own. Avoid large doses of bleach, drain cleaner and antibiotics, keep fat and rainwater out, and have the tank emptied every year. Yeast, sugar and shop-bought additives aren't needed.
Most advice on boosting bacteria starts from the idea that the tank is short of them. It rarely is.
Where the bacteria come from
The bacteria that break down sludge in a septic tank come from the household's own waste. Human gut bacteria are exactly the kind that thrive without oxygen, which is the condition inside the tank. As long as the house is lived in, the tank is topped up every day.
A second group of bacteria, the ones that need oxygen, do their work in the drainage field soil. They are already there and are fed by the effluent.
What harms tank bacteria
- Large doses of bleach and disinfectant. Normal cleaning is fine, but pouring bottles away, constant in-cistern bleach blocks and heavy disinfecting all add up.
- Chemical drain cleaners, which are strongly caustic or acidic.
- Antibiotics and other medicines poured down the sink or toilet. Return unused medicines to a pharmacy.
- Paint, solvents and white spirit.
- Large amounts of fat and oil, which form a thick scum layer and slow the process.
- Rainwater, which floods the tank, dilutes it and washes bacteria and solids out.
A household course of antibiotics taken as prescribed is not a concern. The tank's population recovers.
Habits that keep the bacteria healthy
Regular, steady use suits a septic tank. Spreading laundry across the week, rather than doing it all on one day, gives the tank time to settle. Fixing a running toilet stops a constant trickle washing through it.
Emptying once a year might look as if it strips the bacteria out. In practice it clears the inert material that builds up and gives the tank its full working volume back, and the bacteria return within days. Some operators leave a little of the old contents behind to help.
Do you need to add anything?
No. Yeast is often suggested, but it ferments sugars and does little to break down sewage. Old food, buttermilk and similar home remedies add solids and fat that the tank then has to deal with. Commercial bacterial products are covered in our page on whether septic tank tablets work, and the short version is that a working tank doesn't need them.
What about after the tank has been empty?
After emptying, or when a house has stood empty for a long time, the tank restarts with whatever comes in. Normal use is all it needs. There is no need to "seed" it.
Treatment plants are different
In a sewage treatment plant, most of the cleaning is done by bacteria that need oxygen, living in the aerated chamber. What keeps them healthy is the air blower running all the time, regular servicing and desludging on schedule. If the blower stops, those bacteria die back within days and the plant starts to smell. Our guide to common treatment plant problems explains what to check.
When a tank seems "dead"
If a tank smells strongly, drains slowly or seems not to be breaking anything down, the cause is almost never a shortage of bacteria. It is more likely to be overdue emptying, a blocked outlet, groundwater getting in, or a drainage field that has stopped working. Our guide to why a septic tank smells covers the causes.
If the problem does not settle with normal use, a septic tank inspection will find out why. For treatment plants, our servicing checks the biology as well as the machinery.
