In the UK Drainage Professionals office, our team take hundreds of calls each week from people experiencing septic tank problems of some sort. Our team do love a challenge, and will find out as much as they can to work out what's going on at our customer's property, and what needs to be done to put things right.

So when we get calls from people buying a property with a septic tank who are unsure about what they might be getting themselves into, we hear all sorts of conversations that have happened between buyers and sellers. One particular call this week came from a concerned buyer, who had been told by the seller `We haven't emptied our septic tank in 20 years and never had a problem!'. Seemingly the rest of the discussion was quickly dismissed. The buyer was wondering whether they should believe this statement or not, and took the best decision to call the septic tank experts and find out!

So, is it possible for a septic tank not to be emptied for that long, and to still be functioning as it should?

This is the crux of the matter here. Because it's not really a question of whether or not the seller of the property is telling the truth. When it comes to septic tanks, there's a big difference between knowing that you have a problem, and there actually being one. After all, septic tanks sit out of sight underground, so it's easy to assume that everything is ticking along as it should be, but problems below ground can take quite some time to start to affect life above ground.

If you've the time and inclination, you can read our guide to how a septic tank works. If not, don't worry, here's a quick summary. A septic tank receives waste and water from the toilets, showers, sinks, washing machine, dishwasher etc. within a property. During periods of the day when the facilities aren't being used (usually at night or in the daytime while we're at work/school) separation of the waste within the tank naturally occurs. This is an important part of the process. The dense matter sinks to the bottom of the tank to form the sludge layer, the fat, oil and grease floats to the top and forms the crust layer and that leaves just the dirty water in the middle. Yes I know it's graphic, so apologies to anyone eating their lunch right now! The perils of Googling information about septic tanks!

So, once the contents of the tank have separated, the dirty water mid-layer is the only part which should ever be allowed to exit the tank and enter the soakaway system. The soakaway system, assuming it's working as it should, then allows the separated water to pass safely into the subsoils without causing a pollution, usually through perforations or slots in the pipework. A soakaway can only deal with dirty water, it can't deal with any sort of solid matter passing through it. Anything other than dirty water can block the routes through the substrata and, in time, will cause soakaway problems and for the entire system to fail.

Regular emptying is key to preventing this happening. When a septic tank isn't emptied, the sludge layer at the bottom builds up and will get physically higher inside the tank. The longer the period of no emptying, the higher the level of sludge. The higher the sludge level, the less capacity in the tank there is above it for the separation process to occur sufficiently. A septic tank works by displacement, as in, when 100 litres goes in, 100 litres gets pushed out, and if there's inadequate separation due to the high sludge layer, then solid matter (fat, oil, grease, solids) that must be retained inside the tank, will get pushed through the tank and into the soakaway.

Most septic tanks should be emptied on average once a year. Now, as this is an average there are reasons why some septic tanks can go a bit longer in between empties - for example, a big septic tank serving a property with only one occupant is likely to take longer to fill than a smaller tank serving a property with a big family using all the facilities. However, if you're going to go longer between empties, you should only do so on the advice of a good septic tank emptying company - it's always best to check with them first.

In the scenario where a septic tank hasn't been emptied in 20 years, its very likely that the soakaway would have already failed. But if there haven't been any blockages or problems with the property's facilities, then it's more likely the septic tank is either discharging to a surface water (ditch/stream etc.) instead of a soakaway, or there's a structural breach and matter is escaping to ground directly from the tank.

Either option is dreadful for the environment, in addition to being entirely against all rules and septic tank regulations and a new system must be installed to stop the pollution and make the system compliant.

This might not be the news you're hoping for once you've set your heart on a new property to buy - but going in to a purchase without all the information you need could cost you £10-£20,000 for a new drainage system in addition to risking significant fines - not to mention the local environment!