What shouldn't go into a septic tank

Your septic tank is designed to treat domestic sewage and waste water. These pipes from your house should be connected to the inlet pipe to the tank.

Because the tank works by allowing bacteria to break down the contents, it is important to make sure that nothing enters the septic tank that could prevent that process working.

Too much water

Make sure that your downpipes and drains that carry rainwater are not connected to your septic tank. This could wash out the contents of the tank before it is broken down, and could cause pollution and health hazards.

When you use a washing machine, wait until the cycle is finished before you take a shower. This prevents too much water entering the tank at the same time and flushing out untreated wastes. Think about how much water is leaving your house at any one time, and keep it to a manageable flow.

Too little water

To maintain a healthy population of bacteria your septic tank should have a regular inflow of sewage or waste water. This adds air to the liquid and encourages bacteria to grow. Septic tanks in holiday homes for example can become less effective if left for a long time unused. If this happens then try to let frequent small amounts of waste water enter the tank at first to revive the system.

Chemicals that stop the bacteria working

There are many household chemicals that are designed to kill all bacteria. You should never allow these to enter a septic tank. Killing the bacteria in the tank will result in untreated sewage and waste water leaving the tank, clogging your pipes and creating a nuisance and potential health hazard. Things to avoid include bleach, paint, disinfectants, garden pesticides, medicines, solvents like white spirit and drain cleaning and unblocking liquids that contain caustic soda. There are cleaning products that have been specially designed for use with septic tanks.

Oil grease and fat

These will solidify and clog your pipes and drains. They do not break down in the septic tank so float at the top of the tank creating a thick scum. Fats and oils that enter the soakaway can block the outlets and prevent soil bacteria working.

Items that won’t break down

Make sure that nothing goes into the tank that won’t break down, such as paper (other than toilet paper which is designed to disintegrate) sanitary waste, cotton buds, nappies etc.

Detergents that contain phosphates

You can buy cleaning products that don’t contain phosphates, these are preferable to ones that do. Phosphates are not broken down in a tank so they pass through and end up in the soil and eventually in watercourses. Phosphate pollution is a significant problem in watercourses, ponds and lochs, so reducing the amount in your septic tank discharge will help to reduce this.

In this Guideline

How septic tanks work

What shouldn't go into a septic tank

Maintaining your septic tank

Register your septic tank

What can go wrong with a septic tank

New tanks - Planning waste water and sewage treatment

Treatment of septic tank discharge where no soakaway is possible

If a septic tank can't be installed on your site

Septic tank legislation