A septic tank system consists of the tank itself and drainage field or a water course discharge point. Raw sewage is fed to the tank, the tank performs primary filtering, then liquor leaves the tank and enters the drainfield (AKA soakaway) where naturally occurring bacteria in the soil perform secondary treatment. All septic tank outlets should have consent to discharge granted by the EA/SEPA. Key points about septic tanks are:
- Cheaper to buy than sewage treatment plants (2.5x)
- MUST outlet to a drainfield (AKA soakaway), NOT surface water
- Drainfields can be expensive to install
- Partial treatment only of sewage, bacteria in the drainfield do the rest
- Do not require power usually, unless an uplift pump is necessary
- New installs MUST have EN12566 – Part 1 Certification or you’re wasting your money
- Require Consent to Discharge (usually free) from the EA
- Two main types, old brick tanks (usually 3 chambers) or newer plastic onion bulb tanks
- Bacteria in the tank do not require oxygen to process sludge
- 95% of material entering the tank exits the tank, just the sludge is collected
- Require emptying annually usually
- Suitable for upto 50 persons
- Running costs approx. £300pa for de-sludging and a service for a 3 bed-house
It is essential that septic tanks are properly maintained if they are to work properly. Often this is not the case and owners leave it until they have an emergency (eg. toilets backing up, or the EA pays a visit). It is much better to have a regular maintenance plan to avoid this sort of problem in the first place.
There are many different types of septic tank systems on the market, but generally they all undertake the same task. Commercially manufactured septic tanks are usually made from GRP or other suitable stable materials. The tanks are commonly spherical in shape with a narrow shaft at the top to a manhole ground level. These tanks have built into them several baffles which perform the same function as dip pipes and separate out the heavier solids to the bottom and let the greases / scum, and effluent rise to the top.