There are various different types of off mains systems on the market. Generally, they fall into three categories.
The first is cesspool, which is a storage tank only; that is generally used when there is no other means of discharge available. So you can’t treat it, there’s no soakaway and there’s no water course to discharge into. So, that’s just a holding vessel that requires emptying on a regular basis.
A septic tank is a system that does overflow. So, it produces an effluent that is partially treated, but it can only be discharged into a soakaway, if the ground conditions permit it. And the Environment Agency also lay down particular legislation in terms of areas you can use a septic tank.
The most common system that is now used is a package sewage treatment plant, which – like a septic tank – produces a effluent, but is actually treated to a higher standard. So it could actually be discharged into a water course, a stream. or indeed into a soakaway, subject to soil porosity tests.