Frequently Asked Questions - Click on a question for the
answer
What is the difference between a septic tank, cess pit
and a Package sewage treatment plant?
How does a Biodigester work?
How long does it take to work?
I have a pond on my land, can I discharge the treated effluent
into it?
Do you install the Biodigester
What is BOD 5?
Can I use normal household cleaning chemicals?
Can I use a sink garbage grinder?
What is population equivalent?
What if I need a bigger tank than you have?
What is the difference between a septic
tank, cess pit and a Package sewage treatment plant?
There is often a serious lack of understanding of the different systems available. There are significant differences in the range of applications and a wide variety of types, makes and arrangements of most of them. The following should help to clarify the situation.
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. You may come across the term cesspit used to describe what is actually a septic tank – See below.Septic tanks
Provides minimum treatment and discharges to a soakaway . These are generally only used for smaller domestic developments and are nowadays less acceptable to planners in both the EU and the USA.Biological treatment plants i.e. Biodigesters
Provide a much higher level of treatment than septic tanks and may discharge to a water course. Modern packaged plants are the officially preferred option at present in both the EU and the USA.Septic Tanks and Sewage Treatment Plants
Soakaway's
..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 or tertiary 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.
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 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.
Do you install the Biodigester?
We have a number of recommended installers as well as offering a consultancy service should you already have a builder that you wish to use.
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. i.e. 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.
What if I need a bigger tank than you have?
Population Equivalent relates to non domestic sewage treatment plant applications. For example an office with 30 workers is equivalent to 10 domestic residents. i.e. 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.