What you cannot put down the drain when you are off mains

Why it matters more off the mains

On mains drainage, whatever you put down the drain becomes somebody else’s problem at the boundary. Off the mains, it stays on your property, in your tank, and eventually in your drainage field. The entire system is a slow biological process that you own, and it has no tolerance for being treated like a bin.

The practical consequence is that the household habits that are merely antisocial on a mains system are expensive on a septic tank or treatment plant.

Wipes — including the ones that say flushable

Wipes are the single biggest cause of avoidable call-outs. They do not break down, whatever the packet says. “Flushable” means it will leave your toilet, not that it will decompose. In a septic tank they sink and accumulate, taking up volume that should be holding liquid, which shortens your emptying interval and pushes solids towards the drainage field. In a treatment plant they wrap around floats, jam pumps and foul impellers.

The same applies to sanitary products, nappies, cotton buds, dental floss and kitchen roll. Kitchen roll surprises people: it is designed not to disintegrate when wet, which is the opposite of what toilet paper does.

The rule is the usual one and it is genuinely the whole rule: the only things that should be flushed are what came out of you and toilet paper.

Fats, oils and grease

Poured down the sink, fat cools and sets. In the tank it forms a crust on the surface that reduces the working capacity, and the lighter fraction carries over into the drainage field where it coats the gravel and the soil and stops them accepting water. It is one of the quieter ways a field is killed.

Let it cool and scrape it into the bin or a container. Do not chase it down with hot water — that just moves the problem to the first cold bend in the pipe, or to the tank.

Bleach and antibacterial cleaners

This one causes the most argument, so to be precise: ordinary household cleaning is fine. A toilet cleaned with bleach, a kitchen wiped down with an antibacterial spray — the volumes involved are small and dilute quickly.

What causes harm is quantity. A bottle of bleach down the toilet, heavy use of drain unblocker, large volumes of disinfectant from cleaning out a kennel or a dairy, or an automatic cistern block dosing bleach into every flush. A septic tank is a bacterial process. Sterilise it regularly and solids stop breaking down, which means more frequent emptying and worse treatment.

The rest of the list

• Coffee grounds, eggshells and food waste. They do not break down quickly and build up sludge. A food waste disposal unit is a bad idea on a septic tank for exactly this reason.
• Paint, solvents, white spirit and oil. Toxic to the bacteria and a pollution problem once they reach the ground.
• Medicines. Take them back to a pharmacy.
• Cat litter. Even the “flushable” kind sets like concrete in a tank.
• Large volumes of softened water or backwash from a water softener, which can upset a small system.
• Surface water. Not something you pour in, but worth checking: a roof or yard drain connected into the foul run does more damage than anything on this list.

What is fine

Normal toilet paper, normal washing up, normal laundry, normal household cleaning in normal quantities. Modern detergents are fine in sensible amounts; liquid detergents are marginally kinder than powders because they add no bulk. Dishwashers and washing machines are fine — just try not to run three loads back to back, because it is the volume arriving at once that disturbs the settlement in the tank.

You do not need to buy septic tank additives. A tank repopulates itself from normal use within days. The money is far better spent on having it emptied on time.

Telling other people

Most of the damage we see is done by people who did not know. If you have a holiday let, a rental, builders in, or family staying who have only ever lived on mains drainage, a short note somewhere visible does more good than anything else: what the system is, what must not be flushed, and who to call if something goes wrong. It is the cheapest piece of maintenance available.

If something has already gone down that should not have, the useful move is to have the tank looked at before it reaches the drainage field. Our emptying service covers Chesterfield, Derbyshire, Sheffield, Nottinghamshire and Yorkshire, and we will tell you honestly whether it needs doing. For the related question of how often yours should be emptied, we have written about working out your interval.