A sewage treatment plant is simply an off-mains drainage system that deals with a property’s waste on site. It’s job is to treat the wastewater to quality that it can be safely discharged into a local watercourse or groundwater reserves without damaging the environment or eco-system.
For a domestic property easy to size the right treatment plant for you, as per the British Water’s flows and loads, which is the standard that every sewage treatment plant must meet in the UK, the smallest size treatment plant in the UK is a 5 population (P, PE or Pop) system. This size system is designed to cater for a 3 bedroom house as if every bedroom in the property was at max occupancy the system would in theory be able to deal with the daily flow coming from the property.
The rule of thumb in the UK is to take the number of bedrooms within the property and plus 2 to the figure to give you the required population your treatment plant would need to cater for. Therefore if your property had 4 bedrooms you would need a 6P system, if your had property had 5 bedrooms you would need a 7P system and so on.
Once a systems ‘P’ exceeds 12 then a reduction is required to balance the effects of the daily flow. So as per British Water’s guidelines, when the total number of bedrooms that a plant must cater for is between 13-25P you will need to multiply the total by 0.9 to give you the adjusted P value. For example if there are four four-bedroom houses the total P will be 24 P (4 x 6) and the adjusted P will be 22 P (24 x 0.9 = 21.6) therefore you would need a 22P system.
There is a further adjustment made once the ‘P’ exceeds 25. When the total is 26-50P multiply the total by 0.8 to give you an adjusted P value. For example, if there are four three-bedroom houses and three four-bedroom houses the total P will be 38 P (4 x 5 and 3 x 6) and the adjusted P will be 31 P (38 x 0.8 = 30.4) therefore you need a 31P system.
Once the ‘P’ exceeds 50 you would need to contact the manufacturer as they would have to size a system specially for your project, this is the same if you’re installing a system for commercial or non-domestic such as; offices, schools, hotels, golf courses, camp sites, caravan sites, etc. You would need to contact the manufacturer as different applications require different calculations for the daily flow of each person into the system.
Simply put a sewage treatment plant isn’t the same as a septic tank but both do the same job. A sewage treatment plant is more advanced solution than a septic tank as they use more advanced technology to break down the suspended solids inside the waste from a property so a point where it can be discharged safely back into the water cycle.
Due to a change in legislation septic tanks can no longer discharge into a watercourse only into a soakaway, whereas the effluent quality from a sewage treatment plant is high enough to be safely discharged into a watercourse, but you will need a permit from building control to do so.