A domestic sewage treatment plant is an all-in-one wastewater treatment system that takes over the operation of a treatment plant, at the scale of the individual dwelling or even the allotment. Wastewater treatment in a domestic sewage treatment plant is based on the natural treatment of wastewater by bacteria present in the water.
There are three types of domestic sewage treatment plants which are all based on the same principle, but implement it in a different way:
- Domestic sewage treatment plant with fixed-film
- Domestic sewage treatment plant with suspended-growth
- SBR (Sequencing Batch Reactor) domestic sewage treatment plant
By 2020, septic tank regulations are going to change in the UK. If you own a property that uses a septic tank which discharges treated water to a watercourse, not a soakaway or drainage field, you must upgrade or replace the system by 1st January 2020.
One of the simplest ways to comply with the new regulations is by replacing your septic tank with a domestic sewage treatment plant. These plants produce a cleaner form of water which is considered clean enough to discharge straight to a watercourse.
The Operation of a Domestic sewage treatment plant
The exact operation of a domestic sewage treatment plant depends on the technology used. Whichever technology is used, the wastewater from the housing comes out of the domestic sewage treatment plant completely clean and treated, without the need for any other device.
A fixed-film domestic sewage treatment plant has three successive compartments, each of which handles one wastewater treatment step.
The first compartment, sometimes called a settling tank, is responsible for pre-treating the water. The solids suspended in the tank settle and form sludge, while the fats present rise to the surface. It is this tank that needs to be drained periodically.
this tank, the bacteria present around the filter transform and degrade the organic matter in the wastewater. In order to accelerate the bacteriological treatment, this tank is supplied with oxygen by a compressor, which requires a permanent supply of electricity to the device. The water is then discharged into the third compartment.
The third compartment, called the clarifier, is responsible for the final stage of water clarification, by settling the solids still present in the water. The sludge formed is pumped to the first tank. The treated water can then be discharged into the natural environment.
Suspended-Growth domestic treatment plant
The operating principle of a suspended-growth domestic sewage treatment plant is almost the same as that of fixed-film, except that there is no bacterial support: the bacteria are suspended in the tank.
SBR sewage treatment plant
SBR domestic sewage treatment plants work a little differently from the other two types of domestic sewage treatment plants. In effect, they have only two basins: the first for the pre-treatment of wastewater (settling of solids, separation of fats) and the second which alternates the oxygenation (water treatment) and clarification (final settling) phases.
The second tank therefore has an electronic system designed to supply the tank with oxygen on a regular basis. The frequency of cycles varies by model and manufacturer, from a few minutes to several hours.
The advantages of domestic sewage treatment plants
Domestic sewage treatment plants have significant advantages over other sanitation systems, particularly traditional ones.
Domestic sewage treatment plants are the most compact non-collective sanitation systems. A 5 EH domestic sewage treatment plant has a footprint of only about 5 square metres, about 10 times less than a traditional draining field.
They are therefore first choice for houses with little or no land, and can also be installed above ground, in a cellar or garage.
The operating principle of a domestic sewage treatment plant does not include prolonged fermentation, unlike a septic tank combined with draining field. In its normal operation, the device does not emit odours and therefore does not disturb the inhabitants of the dwelling.
The purification process is completely natural, and no chemicals are used in water purification, making it an eco-friendly process. The same can also be said of most modern domestic wastewater treatment technologies.
Easy to install
The installation of a domestic sewage treatment plant, due to its compactness, requires little earthworks. In the best case, the installation can be completed in just one day.
Easy to maintain
The theoretical emptying frequencies of the domestic sewage treatment plants are very important (a few months), but are not realistic, as they come only from laboratory tests. In the field, sanitation professionals most often observe a emptying interval of at least 2 years, which can be as long as 4 to 5 years in some cases.
The maintenance of a domestic sewage treatment plant is therefore easy to carry out, since the drains are spaced apart. However, an annual maintenance check is recommended: some professionals offer maintenance contracts, similar to boiler maintenance contracts.
The disadvantages of domestic sewage treatment plants
The disadvantages of domestic sewage treatment plants are for the most part inherent in their operation.
The very principle of operation of the domestic sewage treatment plants requires that the reactor be supplied with oxygen. This is done by means of a compressor, which requires a permanent power supply. The cost of this power supply is about 50€ per year maximum.
The other disadvantage of the power supply is the noise produced by the domestic sewage treatment plants. The sound volume differs between manufacturers and models.
Low resistance to load variations
Bacteria on the bacterial support or in tanks need a regular supply of effluent to be treated to survive – this is called “bacterial growth systems”. In case of prolonged absences (several months), the bacterial flora will have degraded, and the domestic sewage treatment plant will need time to recover its performance.
Problems with use in a second home
The two disadvantages above, combined, are why the use of domestic sewage treatment plants are not recommended for second homes. Several problems arise:
Excessive variations in load could lead to malfunction of the device, and therefore to the discharge into the environment of waste water that has not been properly treated or not treated at all. A prolonged power failure during a period of absence would have too great an impact on the bacterial environment and therefore on performance.
The main manufacturers of domestic sewage treatment plants
Sewage treatment plants have become increasingly popular in recent years, and more and more manufacturers are designing models.
Historically, manufacturers of domestic sewage treatment plants have been more concerned with tank manufacturers and roto moulding specialists (the technique used to manufacture plastic moulded tanks), who use their know-how to design high-performance and reliable devices.
Here is a non-exhaustive list of the main manufacturers of domestic sewage treatment plants marketing in the UK:
- Hutchinson Environmental
- Klargester Kingspan
- RA Dalton
- Turley Tanks