Biodigester Sewage Treatment

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Below are some questions that we often get asked.

If you have any other queries that you need answering then please contact us.


What is “Consent to Discharge”?

It is a legal document that is issued by:



Environment Agency




Natural Resources Wales




Scottish Environmental Protection Agency




Department of Environment Northern Ireland



Each authority may lay down specific measurable parameters which you will have to satisfy to enable you to discharge the treated sewage effluent from your premises into a watercourse, soakaway or sub irrigation system.

In some areas ‘Consent to Discharge’ is not required.


Why do I have to have “Consent to Discharge”?

In England and Wales ‘Consent to Discharge’ is a legal requirement under the Water Resources Act 1991. (Schedule 10) (As amended by the Environment Act 1995). However legislation and interpretation of the legislation alters from area to area. In some areas ‘Consent to Discharge’ is not required. Always consult the appropriate regulatory authority.


How long does it take to get “Consent to Discharge”?

The maximum period normally allowed is 4 months. We can undertake the application if you buy a BIODIGESTER.


Do I always need a “Consent to Discharge”?

Not always. It is wise to check with the Environment Agency or other regulatory authority. In some areas ‘Consent to Discharge’ is not required for new projects. In most areas ‘Consent to Discharge’ is not required if you are replacing an existing system.


What is the difference between a septic tank, cess pit and a Package sewage treatment plant?

There are significant differences in the range of applications and a wide variety of types, makes and arrangements of most of them.

There are three common types of holding or treatment system in use which can be supplied by a number of companies in various forms. Alternatively systems may have been constructed on site using local materials.


Cesspits or Cesspools

Do not provide any treatment at all, they are simply a holding tank which must be emptied by tanker on a regular basis. They are large  structures, unsuitable for domestic use due to operating costs and they are the least favoured option under present regulations. You may come across the term cesspit used to describe what is actually a septic tank – See below.


Septic tanks

Provide minimum treatment and must now discharge to a soakaway only. These are generally only used for smaller domestic developments and are nowadays less acceptable to the planners.


Biological treatment plants ie Biodigesters

Provide a much higher level of treatment than septic tanks and may discharge to a water course, provided a Consent to Discharge is in place. Modern packaged plants are the officially preferred option at present.


Septic Tanks and Sewage Treatment Plants



or sub-irrigation systems for dispersal into the ground. These are constructed in different ways according to location but the preferred method is now a system of interlinked trenches. Design of a new soakaway is subject to the results of porosity or percolation tests.


Package Sewage Treatment Plants.

Discharges to a water course may be direct or in-direct. Indirect refers to a soakaway ortertiary reed bed with an overflow to a watercourse. Systems such as this are now commonly used and are referred to as ‘Partial’ or ‘Seasonal’.


Reed beds, constructed wetlands and mounds.

These are often used where the ground conditions and water table are unsuitable for traditional methods of effluent dispersal. They can take up large areas of ground and may be expensive to build.


Other Systems.

A number of other systems have been used recently with varying degrees of success.


How does a Biodigester work?

Air is blown into the BIODIGESTER by an electrically powered compressor mounted normally within 10 metres of the sewage treatment plant. The air is diffused from the bottom of the central chamber. This increased oxygen supply accelerates the activity of the naturally occurring micro-organisms which degrade the sewage to a clear effluent and a non toxic sludge. The plastic media is used to provide a high surface area for the micro-organisms to adhere to and also, as it is mobile, to facilitate rapid degradation of solid matter. The diffused air also operates as an ‘Air Lift’ which recirculates solids from the outer ‘Settlement Chamber’ to the inner ‘Treatment Chamber’. This recirculation also ensures that both chambers remain aerobic. The process runs continuously 24 hours a day. The plant manufactured in the UK and is designed to confirm to the requirements of BS6297:1983.


How long does it take to work?

About six weeks from start up. You can accelerate this by seeding the unit with the sludge from an operational aerobic sewage treatment system.


I have a pond on my land, can I discharge the treated effluent into it?

The answer is dependent on the size of the pond, size of the discharge and whether there is a flow through the pond. ‘Consent to Discharge’ may be required. Please contact us.


Do you install the Biodigester?

In some areas we have independent contractors who specialise in installing sewage disposal systems. More contractors are required. Please ask for details.


What is BOD5?

BOD stands for Biochemical Oxygen Demand. ‘5’ stands for a test that takes 5 days to carry out.The test is essentially a measure of the ‘Organic’ or ‘Polluting’ strength of an effluent. A measured sample is prepared for incubation and the oxygen level is determined.The sample is than incubated for 5 days at 20°C and the oxygen level is measured again. The difference in oxygen levels is used to calculate the BOD5. This represents the level of activity by micro-organisms naturally present in the effluent. The level of activity is proportional to the ‘Organic’ or ‘Polluting’ strength.If an effluent is too strong for a receiving aquatic environment then oxygen will be naturally depleted in the same process. Mg/l stands for Milligrams per litre. One Mg/l is the same as one part per million. Untreated sewage typically has a BOD5 of 400-500 mg/l. The Biodigester reduces this to less than 20mg/l.


Can I use normal household cleaning chemicals?

All normal products can be used in sensible quantities. Don’t forget that the system works by accelerating the natural sewage degradation process. Some chemicals used are designed to destroy micro-organisms. So, overuse of bleach or antibacterial cleaners may upset the process. You should avoid products containing ammonia where the ‘Consent to Discharge’ has a limit for Ammoniacal Nitrogen. Avoid allowing significant quantities of grease to enter the system. Where there is a commercial kitchen (Hotels, Pubs etc) a grease trap must be fitted to the kitchen drainage only.


Can I use a sink garbage grinder?

Yes but it will decrease the capacity by 35% and increase the emptying frequency. It would be better to compost this waste.


What is population equivalent?

Population Equivalent relates to non domestic sewage treatment plant applications. For example an office with 30 workers is equivalent to 10 domestic residents. ie the PE is 10. This becomes more complicated for Pubs/Hotels etc. Please refer to the programme on this site for specific details. Pubs typically have a PE in the range 40-200.

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