A Sewage Treatment Plant is a packaged sewage-works that converts raw sewage into water (effluent) and solid waste (sludge). These ‘packaged’ plants come in all shapes and sizes but treat the waste using the same principles - combining oxygen and food (organic waste) to speed up a natural biological breakdown.
All treatment plants need routine maintenance to keep them working at their optimum levels but treat them right and they are an environmentally friendly way of solving an off-mains drainage problem when a pumping station isn’t possible.
Sewage treatment plants allow you to treat waste before discharging it into a local watercourse, and are often the best way to meet the discharge limits imposed by the Environment Agency. To run one, you'll need a permit from the Environment Agency, as well as an electricity supply.
Sewage treatment plants create an artificial environment to speed up the natural process of breaking down the pollutants in sewage. The treated effluent can then be safely discharged into a local watercourse or soakaway system.
There are three main process types:
- Rotating biological contactor (RBC)
- Aerated filter
- Submerged aerated media (SAM)
Although each works in different ways, they all have three core treatment stages:
Solid matter (sludge) separates from liquid waste and settles at the bottom of the primary settlement tanks, while lighter material forms a layer of scum on the top of the liquid. Sludge and scum need to be removed regularly by a vacuum tanker, and will then be disposed of at a large municipal wastewater treatment works.
If it's not removed, the sludge will build up and eventually carry over into the secondary treatment stage, where it could interfere with the biological process, block the biological filter or pollute the watercourse or soakaway. Putting this right can be really expensive, so don't wait until things go wrong before you give us a call.
The remaining liquid is broken down by live, naturally occurring micro-organisms (biomass) until the quality is high enough for it to be safely discharged to a watercourse or soakaway. The biomass is fed with oxygen to speed up the digestion process.
This final stage allows the contents of the humus tank to settle out. Where discharge consent is more stringent, tertiary treatment is likely. This adds significantly to overall process costs, e.g. nitrification units, sand filters, reed beds etc. Not forgetting the need for more frequent tanker de-sludging.
If a watercourse is available and you've been given permission by the Environment Agency, this is the simplest way to dispose of the treated effluent. At the moment the most common consent issued is:
- BOD (biological oxygen demand): 20mg/l
- SS (total suspended solids): 30mg/l
- Ammonia: 20mg/l