TL;DR
Off-mains properties around Bath drain in one of three ways: a septic tank settles solids and lets the liquid soak away through a drainage field; a cesspit is a sealed tank that holds everything with no outlet at all; and a package sewage treatment plant mechanically treats the effluent to a higher standard before it's discharged, often where a drainage field isn't practical. Which one you have — or need — depends on your site and, since January 2020, on where the discharge actually goes. If you'd like your system checked, request a quote online.
The three systems compared
A septic tank is the simplest of the three. Solids settle to the bottom as sludge, lighter material floats as scum, and the liquid in between flows out to a drainage field, where the surrounding soil finishes the job. It needs a suitably sized, permeable drainage field to work properly — something that isn't always straightforward on the heavier clay soils found around parts of Bath and Somerset.
A cesspit is the opposite of clever: a sealed, watertight tank with an inlet and no outlet whatsoever. Nothing leaves except by tanker. Because there's no discharge, there's no risk of it polluting anything nearby, but the tank fills with the full volume of everything that goes down the drain, which is why cesspits need emptying so much more often than septic tanks.
A package sewage treatment plant sits a step above a septic tank. It typically uses an aerator to encourage bacteria to break the effluent down more thoroughly than passive settling alone can manage, then lets the treated water settle further before it's discharged — usually to a soakaway or, where conditions allow, directly to a watercourse. Because the effluent leaving a treatment plant is cleaner than from a septic tank, it's often the route chosen where a site can't support a large enough drainage field, or where regulation requires a higher standard of treatment.
How the maintenance pattern differs
The three systems ask very different things of an owner. A septic tank needs its sludge and scum removed periodically — an average household should expect to arrange this roughly once a year, though tank size and how many people use the property will move that either way. Beyond emptying, it's worth keeping an eye on the drainage field itself: boggy ground or a lingering smell over the field can mean it's struggling to cope, and no amount of emptying the tank fixes a soakaway that's failing.
A cesspit needs far more frequent attention, because the entire volume has to be removed every time rather than just the settled solids — for a busy household, that can mean visits every few weeks rather than once a year.
A package treatment plant adds a second dimension: alongside occasional desludging of the primary chamber, the mechanical parts — the aerator and any pump — need periodic servicing to keep them running, in addition to whatever the manufacturer's own guidance for that specific model recommends.
When the 2020 rules push a septic tank towards a treatment plant
The Environment Agency's General Binding Rules, in force since January 2020, apply where a septic tank discharges its liquid directly to a watercourse — a ditch, stream or a river such as the Avon — rather than to a drainage field. Where that's the case, the tank needs to be brought up to a higher standard, and a package treatment plant is the option most commonly chosen because it's built specifically to treat effluent to that level before release. A drainage field can sometimes achieve the same result, but where there isn't the space or the ground conditions for one — not unusual on some of the smaller or more constrained plots around Bath — a treatment plant tends to be the practical route.
What this business actually offers
This article is meant as a comparison to help you understand what you have, not a pitch for a new installation — fitting a new septic tank, cesspit or treatment plant isn't a service on offer here. What is on offer is emptying and maintenance for existing systems, along with CCTV inspection of tanks and their inlet and outlet pipework, graded to the WRc MSCC5 standard, to check condition and flag anything that needs attention. That covers all three systems — whichever one turns out to be on your property.
Common in the villages south of Bath
Off-mains drainage of one kind or another is the norm rather than the exception in villages like Peasedown St John and the hamlets around it, where the mains sewer network simply doesn't extend. Our guide to septic tank emptying costs in Somerset goes into more detail on what actually drives the cost of keeping any of these systems maintained.
Frequently Asked Questions
**What is a package sewage treatment plant?**
It's a mechanical system that treats wastewater more thoroughly than a septic tank, typically using an aerator to help bacteria break down the effluent before it's discharged. It's often used where a drainage field isn't practical or where the discharge needs to meet a higher standard.
**What's the difference between a septic tank, a cesspit and a treatment plant?**
A septic tank settles solids and lets the liquid soak away through a drainage field. A cesspit is sealed with no outlet at all, so it holds everything until it's pumped out. A treatment plant mechanically treats the effluent to a cleaner standard before discharging it, usually where a drainage field isn't suitable.
**Do I need a treatment plant instead of a septic tank?**
Only if your current septic tank discharges to a watercourse without adequate treatment, which the January 2020 General Binding Rules require to be resolved, or if your site doesn't have room for a compliant drainage field. A site assessment is the only reliable way to know which applies to you.
**Does a package treatment plant need more maintenance than a septic tank?**
It needs a different kind of attention rather than simply more. Alongside occasional desludging, similar to a septic tank, it also has mechanical parts that need periodic servicing to keep working properly — something a passive septic tank doesn't have to worry about.
