What is a Waste Water Treatment Plant (Station d'Epuration Individuelle) and how does it work
This type of system is known by many names, Biological Treatment Plant, Biodigester, Continuous Aeration Plant (CAP), Sequential Batch Reactor (SBR), to name but a few. Small domestic sewage treatment plants are by far the most effective system of waste treatment. Unlike the limited waste treatment of a septic tank, the treatment plant is significantly more efficient. Its main difference is its use of natural biological aerobic (reliant on oxygen) action enhanced by pumping small air bubbles through the effluent. The result is clear water that is clean enough to discharge into any ditch, soak-away or water course. This system is particularly useful when there is a low porous subsoil. However, as they are relatively new to France they currently still require to be linked to a filter bed.
Basically, the system consists of two chambers, one inside the other, where fine air bubbles are pumped though the secondary inner chamber to promote aerobic bacterial action. In the example shown below, compressed air (supplied from a compressor sited outside the tank) is used for aeration and effluent flow is due to gravity where sometimes one or more pumps are employed for aeration and movement of the effluent.
Crude waste from the house enters the main outer chamber (1) where initial separation of the solids takes place, the heavier particles fall to the bottom of the tank whilst the lighter solids float to the surface. The remaining effluent is gradually transferred to the inner chamber (2) where it is continuously aerated by a fine bubble diffuser (3) supplied by a compressor (5) located in a water tight container (6). The central mass of the effluent is then removed through a calming pipe (4) allowing a secondary brief settlement before discharge. Access (7) to both chambers is available for de-sludging purposes which should happen annually.
Until recently, all waste systems had to be paired with a sand filter bed but the French authorities are now beginning to approve certain sewage treatment units to be installed without a filter bed, draining either into a land drain or nearby water course.
(Diagram courtesy of Balmoral Tanks www.balmoral-group.com)