Sewage Treatment Options
Package sewage treatment plants, septic tanks or cesspits - options for rural properties which conform to the current UK regulations
There are several different types of sewage treatment systems - package treatment units, septic tanks and cesspool, or cesspit tanks, where NO biological wastewater treatment takes place.
Cesspools and cesspits
Cesspools and Cesspits offer no sewage treatment. NO biological treatment occurs and the tank has no outlet. They are simply huge sealed underground tanks where all the wastewater from a property is stored until it is emptied by a tanker.
The MINIMUM size allowed for a one bedroom house is 18,000 litres.
Cesspools need emptying by a licensed waste disposal tanker contractor, and must have adequate capacity to store a minimum of 45 days sewage production. Approval to install a cesspool is required from the Local Authority under the Building Regulations. They will generally only allow one as a temporary measure, i.e. if mains drainage is planned for your village fairly soon. They are NOT allowed in Scotland or France.
Septic Tanks settle the solids out of the sewage and the septic effluent discharges into a soakaway drainage field.
They were the traditional method of sewage disposal in rural areas, but they are no longer allowed for over 60% of UK sites.
A septic tank is only allowed for up to a maximum population of 15 people under the Environment Agency PPG4 rules.
Septic tanks are not allowed in Zone 1 of the Groundwater Source Protection Zones at all and even existing ones must be decommissioned. .
Before you choose a septic tank as your method of sewage disposal, there are 3 mandatory tests that must be done to make sure that you are allowed to have one at all.
- You must find out if you are in a Groundwater Source Protection Zone.
- A Trial Site Assessment Hole must be dug to decide the maximum height of the winter water table. Septic tank systems are not allowed if the water table ever gets within 2.1 metres of ground level.
- A Percolation test will be required to determine whether the ground will absorb the liquid efficiently and remove the pollutants. If the ground cannot accept the liquids properly (i.e. CLAY ground is NOT suitable) then it is probable that the soakaway will become blocked very quickly and the system will fail. In any case, septic tank soakaway drainfields do not usually last longer than 10 years as the porous ground becomes blocked with slimy black biomatt and made waterproof by sodium binding. The septic tank should be emptied at least annually.
Package Sewage Treatment Plants
These type of domestic sewage treatment systems comprise of a small electrically driven packaged plant, usually all contained in a single tank. The sewage is first settled to remove the gross solids which provide a 'larder' that keeps the bacteria fed during periods of low, or, with some plants no flow. The liquid effluent passes into the biozone where it is biologically treated in a filter bed, or by rotating disc media, or by air injection, with a final settlement stage to remove the fine solids. The treated wastewater effluent can be discharged to a land drainage system, ditch or watercourse, (subject to Local Authority and Environment Agency Consents, if applicable). Maintenance, (which can be expensive, particularly for some non-electric package sewage treatment systems) and emptying of the plant to remove the settled gross solids, will be required at regular intervals. This is usually annually for a single dwelling domestic sewage treatment plant and 2 to 6 monthly for a multiple dwelling unit, as the settlement tank is comparatively smaller in multiple house units.
There are many variants of package wastewater treatment plants available on the market, as these are the most well established types.
Three stage plants cope very well with fluctuating flow rates and underloading, which is a problem with many residential sites, campsites at the beginning and end of the season and Pub trade during weekdays.
Some also keep the bacteria trickle-fed during holiday absences, See Falcon sewage treatment plants.
Maintenance tends to be 12 monthly and electrical consumption is low.
These sewage systems recycle their own waste products along with live, settled, beneficial bacteria back to the digestion chamber to activate and speed up the digestion of the raw sewage.
They don't need any extra bacteria adding during their lifetime as they constantly 'top-up' the digestion chamber with active bacterial sludge from the final settlement chamber.
They are a simple, reliable wastewater treatment solution and tend to be cheaper than 3 stage plants, with a similar low electricity cost.
Only the Vortex ASP is adjustable, up or down, for different numbers of people using it.
These biological sewage treatment plants try to digest the gross solids and liquids together in the central compartment via air injection. The sewage is aerated for a longer period of time than with a three stage plant, hence the name, 'extended aeration'. There is no 'larder' as with a three stage plant, so 'Feast and Famine' restricts their application for most plants. However, the Vortex sewage treatment system, has overcome this problem by incorporating an air regulator valve to 'turn them down' for low populations. The effluent can be discharged into a soakaway drainfield, or a watercourse, subject to the necessary consents. Click for How the VORTEX™ sewage treatment plant is so energy-efficient.
Servicing of these treatment plants is 6 monthly and the plants need emptying about every 1 to 2 years, (depending on how many persons are using them), to remove sludge.
The first extended aeration plants were brought into the UK from America in the mid 1990's. It was said at the time by the original importer, that they did not cope very well with underloading situations, but this information seems to be no longer passed on to the customer. They work better when reasonably loaded.
Electrical consumption of most plants tends to be higher than that of three stage sewage treatment plants, due to the longer aeration time needed by the process.
This sewage treatment system uses no electricity for the process and only needs an annual service. They are the most sustainable plants in the UK.
The latest non-electric sewage treatment plant is the FilterPod. It is a unique UK design and uses inert RDX media for the filtration. This has the advantage of being non bio-degradable, unlike some of the others of this type.
The liquid is split from the solid waste in the pre-filter tank (or existing traditional septic tank if one is available) and filtered by the media in the second chamber. Bacteria grow in the media and digest the filtered pollutants.
The effluent quality is superb; far better than from most electric treatment plants.
Some of these systems have high running costs, however, as maintenance and servicing is expensive due to the high cost of replacement filter material. CHECK before you buy.
In our experience, you should only consider the Vertical flow reed beds when following a septic tank. We have seen many cases where the effluent coming out of a horizontal flow reed bed, (designed by a firm of ‘expert reed bed designers’) following a septic tank was considerably WORSE than the effluent which went in! As our local Environment Agency officer told me - a reed bed was his first successful pollution prosecution! Click here for more information about reed bed sewage treatment
WTE can design and build all types of reed beds for both domestic and commercial wastewater.
We have stopped our reed bed design work due to the very cold weather during the last 2 winters and the near death of a customer due to bacteria in his reed bed. Many reed beds froze solid for weeks and the resulting pollution and problems due to the complete lack of sanitation for the owners, has made us re-evaluate their suitability for the current UK climate.
The vertical flow beds nearly always need electric pumps which are expensive to run, although the system can be operated without power, if an adequate fall is available across the site.
Horizontal flow reeds beds can successfully follow sewage treatment units as a tertiary treatment but should never be considered as an answer to cleaning septic tank effluent.
Reed Beds are NOT a ‘fit and forget’ item and the wastewater treatment system requires a lot of maintenance, e.g. harvesting and composting the reeds in the Autumn, re-planting, weeding, etc. and annual emptying of the septic tank. They can also smell and act as a breeding ground for flies and mosquitoes, so must not be situated close to dwellings. Children and pets must be kept away from reed beds as they will be paddling in dangerous sewage effluent. All reed bed areas must be fenced. A Permit to Discharge is required from the Environment Agency.
Contact us for a correct sewage treatment design according to your site.