A sewage treatment plant is a system used to effectively collect and treat domestic wastewater on site. Having a sewage treatment plant installed on your property offers a number of great benefits. This includes - being more environmentally-friendly by reducing the risk of pollution, minimising the risk of disease and harm to health, and allowing you to make your home more self-sufficient without relying on the public sewage system.
What does a sewage treatment plant do?
A sewage treatment plant works in a similar way to other wastewater management systems. However, it uses mechanical components to help break down harmful contaminants to provide a cleaner, safer, and more environmentally-friendly effluent. Wastewater enters the primary tank where the solids and liquids separate. The remaining liquid then passes into a secondary tank and a pump is used to aerate the chamber. The additional oxygen helps speed up the treatment process by encouraging the growth of friendly bacteria that breaks down the contaminants and bacteria found in wastewater. The resulting liquid is of high enough quality to then be safely disposed into water courses like canals and rivers. The mechanical components found in sewage treatment plants means that they require a constant source of electricity to operate.
How to work out the size sewage treatment plant needed
Sewage treatment plants come in a variety of different sizes and styles. It is important to find the best system for you and your needs, so that your wastewater can be treated onsite efficiently. In order to determine which sewage treatment plant is suitable for your property, you must follow the British Water flows and loads guide. Under this guide, the size of plant needed is determined by looking at the occupation of the property.
Single domestic households
For a single domestic household with up to three bedrooms, the following applies:
- The sewage treatment plant should be installed for a minimum of five people (5P)
- Households containing more than three bedrooms must add 1P for every extra bedroom. For example, a household containing five bedrooms would need a 7P sewage treatment plant.
Groups of households
For groups of households, the following applies:
- When a sewage treatment plant is required for a group of houses with more than 12 occupants, the calculation is different to the above. The guidelines allow some reduction to account for the balancing effects on daily flow of the group of houses.
- When the total number of occupants is between 13 and 25, you multiply it by 0.9 to give an adjusted P value. So, for seven three-bedroom homes, the total P based on the original calculation would be 35P (7 x 5). Taking the discount into account, the adjusted P would be 31.5 (35 x 0.9). The number is a minimum requirement, so should always be rounded up. In this scenario, the required sewage treatment plant would be for 32P.
- When the total number of occupants is 26 to 50 people, you multiply it by 0.8. For instance, if there are three four-bedroom houses and two six-bedroom houses, the total P would be 34 (3 x 6 and 2 x 8). The adjusted P would be 27.2 (24 x 0.8). This would then be rounded up to 28P.
- Where the population exceeds 50 persons, the guidelines will require that the design of the package sewage treatment plant should be carried out by a suitably qualified engineer.
Sizing and regulations
It is important to select a sewage treatment plant that is the correct size, in order for it to function efficiently. Make sure that you follow the above guidelines to determine the size tank that you require on your property. If you need help working out the best size sewage treatment plant for you property, then it is best to get advice from wastewater management professionals.
In addition to choosing the correct size sewage treatment plant, careful consideration should also be taken to the method used to dispose of the resulting effluent. A permit is not always required, by your disposal must meet certain criteria. For example - the drainage field must be located at least 10 metres from a watercourse, 15 metres from a building, and 50 metres from a spring. It is always best to contact the Environment Agency to ensure that you sewage treatment plant meets the required criteria. This will protect you from fines, or having to pay for costly upgrades to improve your system to the required standard.
