Percolation tests: how your ground decides what system you can have
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
Read the guideThe two look similar from the garden but work differently, need different maintenance and face different rules. How to tell which one you own.
If there is an electricity supply running to it, it is a sewage treatment plant. If there is no power, it is a septic tank. A plant treats the sewage with air and bacteria and can discharge to a watercourse; a septic tank only settles it and must discharge to ground through a drainage field.
Go and look for a cable. A sewage treatment plant needs power: an air blower, a rotating mechanism or a pump. There will be an armoured cable running to it, often with a small control box or an alarm on a wall or post nearby, and if you stand next to a working plant you will usually hear a low hum.
A septic tank is passive. No cable, no noise, nothing to go wrong mechanically. Two or three chambers of concrete, brick or GRP, with lids at ground level.
That is the difference that matters, and it determines everything else — what the maintenance is, what it costs to run, and what the rules say about where the outflow can go.
Sewage flows in and sits. Solids sink, fats and lighter material float, and the relatively clear liquid in the middle flows out of the far end. That liquid is not clean — it is settled sewage, nothing more. The actual treatment happens afterwards, in the ground, as the liquid soaks away through a drainage field and the soil and the organisms in it break down what is left.
Because the ground does the treatment, a septic tank cannot legally discharge to a ditch, stream or surface water drain. It needs a drainage field that works. If that field silts up or waterlogs, the system has nowhere to go and it backs up.
A treatment plant does the biological work inside the unit. Air is blown through the sewage (or the sewage is passed over a rotating surface) so that aerobic bacteria can grow and consume the waste. The result is an effluent clean enough, when the plant is working properly, to discharge to a watercourse or to a much smaller drainage field.
That performance comes at a price: moving parts, an electricity bill, and a service requirement. A plant that is not serviced stops performing long before it stops running, and the discharge quietly falls below standard while everything looks normal from the garden.
| Septic tank | Treatment plant | |
|---|---|---|
| Power | None | Mains electricity |
| Treatment | Settlement only — the ground finishes it | Aerobic treatment inside the unit |
| Discharge allowed to | Drainage field only | Watercourse or drainage field |
| Emptying | Full empty, roughly annually | De-sludge, typically 6–12 months |
| Servicing | None mechanical | Every 6–12 months |
| Running cost | Emptying only | Emptying, servicing and electricity |
If you are replacing a failed system or putting drainage into a new build, the honest answer is that the ground decides more than you do.
A drainage field needs suitable ground and space. A percolation test tells you how quickly the soil accepts water, and that figure sets the size of the field. Heavy clay, a high water table, solid rock near the surface or simply not enough garden can all rule it out. Where a field is viable, a septic tank with a properly built field is simple, cheap to run and has nothing to break.
Where a field is not viable — or where the only available outfall is a ditch or stream — a treatment plant is the answer, because its effluent is clean enough to discharge to surface water.
Load matters too. A plant sized for a three bedroom house and then asked to serve a holiday let sleeping ten will not perform, no matter how well it is maintained. Sizing is done from the population the property can hold, not the number of people living there this year.
Buyers’ solicitors now ask directly about off-mains drainage, and the answers differ sharply between the two systems. A treatment plant with a service history is straightforward. A septic tank discharging to a drainage field is straightforward. A septic tank discharging to a ditch is a problem that has to be resolved, and it tends to surface at the worst possible moment — which is why it is worth knowing what you have long before you market the house.
If you are not sure, we can trace the system, survey it and tell you plainly what it is and where it goes. If it needs replacing, here is what that involves; if it does not, we will say so.
People tend to compare the two on installation price alone, which is the wrong comparison. The septic tank is cheaper to buy, but it is only half the job — the drainage field is the expensive part, and on difficult ground it can cost more than the tank. A treatment plant costs more up front but often needs much less ground.
Over ten years the picture evens out further. A septic tank costs you an annual empty and nothing else. A treatment plant costs you a de-sludge, an annual service, the odd blower diaphragm or pump, and the electricity. Against that, a treatment plant is far more tolerant of poor ground and far less likely to leave you with a waterlogged garden in year eight.
The thing that genuinely does not pay is installing either system a size too small to save money. Both are sized from the population the property can support, and an undersized system never catches up.
With a septic tank, the whole job is keeping solids out of the drainage field: empty it on a sensible interval, keep wipes and fats out, and do not pour bleach down in quantity. With a treatment plant, add a service to that list and take the alarm seriously when it sounds.
Both appreciate the same thing: knowing where everything is. If you have never lifted the lids, a half hour spent finding the tank, the inspection chambers and the outfall is worth more than any product you can buy.
Conversion units exist, but whether one suits your tank depends on its condition, shape and size. On an old tank it is often better value to install a new plant than to adapt a tank that is near the end of its life. We will tell you which applies to yours.
The electricity for a domestic plant is modest — typically comparable to a fridge. The real running costs are the annual service and the de-sludge, which together are usually less than the cost of a failed drainage field.
A well-built concrete or GRP tank can last decades. Drainage fields are the part that fails, usually from solids carrying over because the tank was not emptied often enough.
If you are installing or replacing a drainage field, yes. It sets the size of the field. Skipping it is how undersized fields get built, and an undersized field will waterlog within a few years.
Septic tank emptying, installations, servicing and drainage across Derbyshire, Sheffield and Yorkshire — 24/7.
A percolation test measures how fast your soil accepts water, and that single figure sets the size of your drainage field — or rules one out entirely.
Read the guideA waterlogged drainage field is the most expensive part of an off-mains system to fix. What causes it, how to spot it early, and what a rebuild costs.
Read the guideReplacing a septic tank is usually permitted development, but building regulations still apply and some sites need consent. Here is what applies when.
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