Just because something is apparently suitable for disposing of down the toilet or the sink doesn’t mean it belongs in your septic tank. When the wrong things enter your septic system they don’t break down the way they’re supposed to. Over time, that build-up can lead to blockages, unpleasant smells, or a system that simply stops working as it should.
The good news is that protecting your septic tank doesn’t take any specialist knowledge. It mostly comes down to cultivating a few sensible everyday habits.
This guide covers what should and shouldn’t go into your system, why certain items cause problems, and how to tell when it’s time to call in a professional to deal with issues.
Before looking at what to avoid, it helps to know what your septic tank is actually designed to deal with.
What can you safely flush?
Your septic system is built to handle two things from the toilet: human waste, and toilet paper that’s designed to break down quickly in water. That’s it. Everything else needs a moment’s thought before it goes anywhere near the pan.
What can go down household sinks and drains?
Normal household wastewater from sinks, showers, baths, washing machines and dishwashers is exactly what a septic system is designed to receive and treat. Problems tend to start when solids, fats or strong chemicals are added into that mix.
The simplest rule to remember
If you take one thing from this guide, make it this: don’t treat your toilet as a general waste bin. If you wouldn’t put it in a normal bin without thinking twice, it’s worth pausing before you flush it.
This is the area that causes the most problems for septic tank owners, so it’s worth going through the “no-nos” in a bit of detail rather than just offering a list:
Wet wipes
Baby wipes, cleaning wipes and personal hygiene wipes are among the most common causes of septic tank blockages we come across. Just because something looks like it is biodegradable and can be flushed away like toilet paper doesn’t mean it belongs in a septic system. Even wipes labelled “flushable” are generally far slower to break down than toilet paper, and they can accumulate in the tank or pipework over time.
Kitchen roll and facial tissues
Kitchen roll and facial tissues are made to a different standard to toilet paper. They’re designed to be strong and absorbent rather than to disintegrate quickly, which means they behave more like wipes than toilet tissue once they’re in your system. The safer habit is to put them in the bin rather than the toilet.
Sanitary products and tampons
Tampons, sanitary towels and applicators shouldn’t go into a septic tank. These products are designed to absorb liquid, not break apart in it, so they’re one of the more likely causes of a blockage if they’re flushed regularly.
Nappies, cotton buds and dental floss
Nappies, cotton buds and dental floss all fall under the heading of products that aren’t designed to break down like toilet paper, and shouldn’t be treated as though they are. Cotton buds and floss are small enough to seem harmless, but they don’t degrade, and they can contribute to blockages further down the line.
Condoms and other non-biodegradable items
Condoms and similar non-biodegradable items should always go in the bin. They don’t break down in a septic system and can contribute to blockages in the same way as other non-degradable materials.
Your toilet isn’t the only route into your septic system. What goes down the kitchen sink matters just as much, and it’s a part of septic tank care that’s easy to overlook.
Fats, oils and grease
Cooking oil, fat and grease are some of the most common culprits behind septic tank problems. Poured down the sink while still warm, they seem to disappear easily enough. Once they cool inside the pipework or the tank, they solidify and cling to surfaces, gradually narrowing pipes and contributing to blockages. Let fat and oil cool in a container, then dispose of it in the bin rather than down the drain.
Food waste and coffee grounds
Food scraps, leftovers and coffee grounds add unnecessary solids to a system that’s designed to deal with wastewater, not food disposal. A small amount now and then is unlikely to cause an immediate issue, but it all adds to the level of solids building up in the tank over time. A food waste caddy or compost bin is a far better repository for it.
Paint, solvents and chemicals
Paint, solvents and similar chemicals shouldn’t go down any drain connected to a septic tank. Beyond the risk to your system, they need to be disposed of responsibly through your local household waste site, rather than poured away down a plughole.
Medicines and pharmaceuticals
Unused medicines and pharmaceuticals don’t belong in a septic tank either. Most pharmacies will take back unwanted or out-of-date medication for safe disposal, which is a far better route than flushing it with the danger of contaminating the groundwater.
Yes, ordinary toilet paper is designed to be flushed and is exactly what a septic system expects to receive. Toilet paper and wet wipes might feel similar when you’re using them, but what happens after they’re flushed is where the real difference lies. Toilet paper is engineered to break apart quickly once it’s wet, while wipes and similar products are built to stay strong and intact.
Standard, single or two-ply toilet paper is generally the safest choice for a septic system. Thicker, quilted or “extra soft” varieties can take a little longer to break down, so if you’ve had drainage issues before, it’s worth switching to a simpler product.
It’s best avoided. Facial tissues are made to be more durable than toilet paper so they hold together when you use them, which is exactly the opposite of what a septic system needs. Popping used tissues in a bin next to the toilet is a small habit that makes a real difference. It’s worth not assuming that all paper products behave the same way once they’re flushed.
Food scraps, coffee grounds and other kitchen waste shouldn’t go down the toilet or the kitchen sink if you’re on a septic system. If it belongs in the food waste bin, it’s best not to make your septic tank deal with it instead. Even where a kitchen sink has a waste disposal unit fitted, using it regularly can send far more solid material into the tank than the system is designed to cope with.
No. Pouring leftover cooking fat down the sink might be a quick and convenient way of getting rid of it, but dealing with the drainage problems it causes is certainly no easy process. Cooking oils, fats and grease solidify once they cool, coating pipes and gradually restricting flow. Letting them cool in a container and then binning them takes only a little extra effort and avoids a much bigger job later.
Washing machines and dishwashers are two more sources of wastewater being fed into your septic system, and it’s worth giving them a moment’s thought too.
Standard laundry detergents used at the recommended dose are generally fine for septic systems. Problems tend to come from using far more than necessary, or from running loads of laundry unnecessarily often.
The same principle applies to dishwasher tablets and rinse aids: use them as directed rather than adding extra “for good measure”.
Spacing out laundry and dishwasher loads across the week, rather than running several large loads back to back, gives your septic system a steadier, more manageable flow of wastewater to deal with.
It isn’t simply a case of knowing that something is “bad for septic tanks” in a vague sense. Different products can create very real and diverse problems once they’re inside the drainage system. Issues include things like:
Materials that don’t break down as expected
Wipes, sanitary products and similar items are built to hold their shape, so instead of dispersing the way toilet paper does, they tend to sit in the tank or pipework.
Build-up and blockages
Fats, oils and solid materials gradually accumulate on pipe walls and within the tank itself. Individually, one incident is rarely a problem. Repeated over months and years, it becomes one.
Disrupting the normal operation of the system
Harsh chemicals can reduce the bacteria your septic tank relies on to break waste down naturally, making the whole system less effective at doing its job.
Warning Signs Your Septic System May Have a Problem
Changing what goes down your drains can help prevent future problems, but it won’t necessarily remove something that’s already causing one. Look out for:
Toilets or sinks draining slowly
A gradual slowdown, rather than a sudden blockage, often points to a build-up developing somewhere in the system.
Water backing up
Water appearing in an unexpected place, such as a shower or an outside gully, when you flush the toilet or run a tap elsewhere in the house.
Unpleasant smells
Persistent odours around drains, manholes or the garden area above the tank or drainage field.
Recurring drainage problems
The same issue coming back again and again, even after it seems to clear up on its own.
If any of this sounds familiar, a septic tank inspection can help identify exactly what’s happening inside the system before it develops into something more serious. The Drain Specialists engineers regularly encounter septic tanks in the countryside in Wiltshire and Somerset affected by exactly the items covered in this guide. We look for the early signs of a developing problem, not just the obvious ones.
Most of this guide applies just as much to commercial premises as it does to domestic properties, but the scale is different. At home, one person putting the wrong thing down the toilet can cause a problem. On a site used by dozens or hundreds of people a week, controlling what enters the system becomes an operational issue.
Pubs and restaurants
Kitchens generate far higher volumes of fats, oils and food waste than a domestic household, so a proper grease trap and clear staff procedures make a real difference.
Hotels and holiday accommodation
Guests won’t necessarily know your property is on a septic system, so clear, simple signage in bathrooms is one of the most effective ways to prevent avoidable problems.
Campsites and caravan sites
Shared washroom and shower blocks see heavy, varied use, which makes routine checks and well-placed guidance for guests particularly worthwhile.
Other commercial properties
Any business off mains drainage benefits from the same basics: staff awareness, clear guidance for visitors, and routine inspection so that a developing issue is caught before it causes a shutdown. Our commercial septic tank and sewage treatment plant services are built around exactly this kind of proactive care.
Here are the main points made in this guide, which it would be wise to remember if you have a septic tank:
Toilet: human waste and toilet paper only.
Sink and drains: normal household wastewater.
Bin, or the appropriate disposal route: wipes, sanitary products, food waste, fats and oils, non-biodegradable items, and any leftover chemicals or medicines.
Keeping to that simple split does more to protect your septic tank than any amount of technical knowledge.
Alongside watching what goes into your system, it’s worth building in some routine care. Our guide on how to look after your septic tank and our step-by-step advice on checking your septic tank levels are both good next steps if you’d like to stay on top of things.
If you’re already experiencing slow drainage, water backing up, or recurring problems, it’s worth having it investigated rather than waiting for it to get worse. With over 15 years of drainage industry experience across Bath, Bristol and north-west Wiltshire, our team can carry out a full septic tank inspection or investigate a blocked drain or sink and let you know exactly what’s going on. Get in touch with the Drain Specialists team on 01225 632468 or 07703 893688, or email info@thedrainspecialists.co.uk.
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