What Can and Cannot be Flushed Down a Sewage Treatment Plant?

A sewage treatment plant is a natural biological process that relies on receiving normal organic waste. That’s why it is important to be aware of the items that go down the drain to ensure the system doesn’t develop any unexpected blockages and issues. But can you put into a sewage treatment plant and what should you avoid flushing away?

What you can flush down the toilet

The basic rule for any toilet – whether linked to a sewage treatment plant or not – is to minimise the amount of waste that is flushed away.

Toilets offer an alluring method for disposing of items as they enable waste to be removed from sight. So, instead of placing the item into the bin which fills up a bag that needs to be manually taken out for collection, the toilet appears to offer a much faster and easier solution.

However, while toilets and plumbing systems are durable, the water in the system can only break down and flush away very few items. This means you should only dispose of the following down a toilet:

  • Human excretion
  • Human urine
  • Toilet paper (not anything thicker)

Everything else must be disposed of in the appropriate bin. Some products that are marketed as ‘flushable’ should also not be placed into the toilet as they will likely struggle to break down.

Forcing items into the toilet that shouldn’t be flushed away can lead to blockages and a variety of issues that will likely create more problems further down the line, costing you more money in the long run.

What can’t you flush down the toilet?

There are some items that you should avoid flushing down the toilet if you have a sewage treatment plant. To make things a little clearer, we’ve broken them down into individual categories:

Items that can lead to blockages

  • Wet wipes/face wipes
  • Sanitary materials
  • Excessive amounts of toilet roll
  • Large amounts of kitchen roll
  • Nappies
  • Rags/cloths
  • Soft toys

Things that could kill or harm bacteria in the plant

Bacteria can be harmed or even killed if it comes into contact with certain liquids or materials. The bacteria are needed to maintain the treatment quality of the sewage plant and its removal could lead to the introduction of bad odours. Things to avoid include:

  • Weed killers, fungicides, insecticides and other harmful gardening chemicals
  • White spirit, paint, thinners and turpentine
  • Grease, anti-freeze, motor oil etc.
  • Excessive amounts of bleach or washing powder
  • Medicines – over the counter medicines should be disposed of carefully via your nearest pharmacy

Smaller amounts of washing powder or bleach shouldn’t harm the system. However, it is a good idea to spread their usage over a few days rather than all in one go, as some systems may struggle if you are doing a lot of washing on the same day.

Biological overloads

Providing the system meets the size requirements of British Water Flows and Loads 4, it should be able to process house wastage without any problems. There are some organic materials that can affect the water and bacteria that need to be managed carefully, such as:

  • Food waste
  • Cooking oil and fat
  • Milk, wine, beer and other alcoholic beverages
  • Pet waste
  • Too many people using the system

Repeatedly disposing of food that hasn’t been broken down can cause the system to overload. It can lead to the formation of bad odours and smells in the system which will eventually start to spread around your home.

What could happen if you flush the wrong items down the toilet?

When the toilet is flushed, we tend not to think of what happens once we close the lid and wash our hands. It’s a part of everyday life and we expect the plumbing system to do its job by removing the waste far away from our home.

However, the system is made up of a network of pipes and if too many unwanted items end up being flushed away, debris can accumulate at any point, causing clogs and backups. There are even instances where items that are too large for the pipes never actually leave the system and instead become lodged in the pipes underneath your home. You may not know they are there for weeks and sometimes months, but if enough objects pile up around it, once the pipe becomes blocked, you’ll become aware of it pretty quickly.

Plumbing repairs can be expensive, especially if it is an issue that has been quietly developing for some time. While plumbing systems develop their own leaks and blockages through natural wear and tear, the chances of it happening are far less if you keep a close eye on the items being flushed away. 

Other things to be aware of

If you remain aware of what is flushed down the toilet, your sewage treatment plant will remain in much better condition. There are a few other things to be aware of to ensure the system can continue to dispose of your waste efficiently.

For example, ensure that groundwater, rainwater or large volumes of water (from a jacuzzi or swimming pool, for example) does not get into the plant. Chlorine will kill the bacteria and too much water exposure will speed up the plant’s flow rate, affecting the quality of the treatment.

You should also avoid installing a waste disposal unit below your kitchen sink. This will increase the organic load being processed by the treatment system, which will lead to bacterial overgrowth that affects the treatment process. A waste disposal can only be used if the plant has been designed and built to receive a higher organic load.

If you go away or on holiday do not turn off the system, instead leave it running as normal. Turning off the plant can damage the motor and other moving parts, while also killing biomass. The system will continue to run without any issue while you are away and won’t cause any damage to your home.

Callum Vallance-Poole


Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!