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Biodigester FAQs

Technical questions

How wide is the range of Biodigester treatment plants for sewage and wastewater? Sizes start at a population of only 2 in the Biodigester HBAW range suitable for shallow dig or above ground installation and also houseboats. Sizes go up to 500 persons in a single Biodigester ‘Standard’ tank and larger for multiple tanks or the Biodigester ‘EK’ ‘Design and Build’ system for much larger projects, especially overseas applications.

What makes the Biodigester ‘T’ series treatment plants unique? The Biodigester ‘T’ series product is a unique hybrid design with two chambers and continuous recirculation. The total aeration process is guaranteed to work without odour, there is no stored septic sludge. Humus sludge that collects on the surface of the outer chamber is continuously further degraded so that many ‘T’ range Biodigesters do not need to be emptied at all providing a major cost saving for the end user.

How often does a Biodigester sewage treatment plant need to be emptied? The unique ‘T’ range of Biodigesters has the important feature that many do not need to be emptied at all with a major cost saving for the end user. Larger Biodigesters normally need to be emptied annually.

Are Biodigester sewage treatment plants suitable for variable loadings? BES’s whole range of Biodigester sewage treatment plants is recommended for variable loadings due to a number of design features including ‘Total Aeration’ and ‘Recycling’. Seasonal timers are available for caravan sites or other similar sites.

Can surface water from roofs and hardstandings be discharged into a Biodigester? In broad terms the answer is no, but small amounts are allowable subject to discussion with BES.

What cleaning products can be used with a Biodigester for sewage treatment including single house applications? Remember that the Biodigester is a natural system, but usually cleaning products can be used as normal without any adverse affect. However avoid abnormally large quantities of bleach all at once and significant quantities of products containing bactericides or fungicides.

Can Biodigester sewage treatment plants be installed where there is a very high water table or hard rock in the ground? The Biodigester HBAW range of 10 treatment plant sizes for 2-11 domestic residents can be used for ‘Shallow Dig’ applications. The Biodigester HBAW range and some other larger Biodigesters can also be installed above ground.

Why is the depth of the water table important? Firstly, because when a Biodigester sewage treatment plant is installed there are different installation methods dependent upon whether the ground is dry or likely to be wet at times. Secondly, regulations apply regarding the discharge of effluent close to or within the water table. Thirdly, if the ground is wet at times it may be essential to use a pumping system with a non-return valve for effluent dispersal rather than expecting a gravity discharge system to be satisfactory under all conditions.

Does a Biodigester sewage treatment plant require a grease trap? Grease traps are only required where there is a communal kitchen at sites such as pubs, hotels, leisure facilities and nursing homes. A ‘Total Retention’ BES grease trap should be fitted to the kitchen drainage only. Grease control systems using emulsifiers or other additives must not be used.

What is the ‘Inlet Invert’? This is the depth of the pipe where it goes into the Biodigester. It is measured from the base of the foul drain to ground level. The importance of the ‘Inlet Invert’ is to supply the correct Biodigester version so that the cover ends up at ground level rather than lower than this. Biodigesters typically come with ‘Inlet Invert’ choices of 600mm, 900mm, 1200mm or 1500mm. These versions are labelled S1, S2, S3 and S4 respectively.

What is a Total Aeration Biodigester treatment plant? The Biodigester HBAW and ’T’ range products both use ‘Total Aeration’ which means that all the sewage is aerated, including solids and not just the liquid fraction as in most other systems. These products consequently operate without odour as there is no stored smelly septic sludge. Emptying frequencies are also highly extended with some ‘T’ range Biodigesters not needing to be emptied at all, a major cost saving feature for the end user.

Can a Biodigester sewage treatment plant be used with a timer system to reduce power consumption and costs? Some HBAW treatment plants are initially supplied with timers for routine operation. ‘T’ range Biodigesters in particular can benefit from the use of a timer system if lightly loaded. Consult BES for advice. If using a timer the aeration must be little and often, long off periods are not allowable.

Do Biodigester sewage treatment plants include pumps to lift the treated effluent to a watercourse or dispersal system? Most Biodigesters have the option of an integral effluent pumping system with a non-return valve. Otherwise separate pumping systems are supplied

Is it better to pump untreated sewage or treated effluent? Where there needs to be a pumping stage in a scheme it is normally better to pump the treated effluent. This requires pumps that are less expensive and more reliable. Biodigesters can be supplied with deep Inlet Inverts and integral or external pumping systems for the treated effluent. If pumping untreated sewage is essential it is advisable to use ‘Cutter’ pumps or ‘Solids Handling’ pumps. ‘Macerator’ pumps can be troublesome.

Can Biodigester sewage treatment plants be used at sites without mains power? Unless a suitable low voltage air blower is available there needs to be a renewable energy system to run the mains air blower. The renewable energy system needs to include an inverter and battery storage.

Who designed the BES Biodigester sewage treatment plants? The Biodigester products HBAW, ‘T’ Range, Bonus Range, Standard Range and EK Range are all designed by BES. The Biodigester ‘PA’ was designed by another UK company. All these products are made in the UK. The Biodigester ‘PHR’ range was designed and is made in Eastern Europe.

Sizing questions

How is a Biodigester sized for domestic use? For single houses this depends on the number of bedrooms, in normal applications this is as follows based on guidance from British Water (Refer to British Water’s Table of Flows and Loads 4). Up to 4 bedrooms – Biodigester T6 for 6 persons Up to 7 bedrooms – Biodigester T9 for 9 persons Up to 10 bedrooms – Biodigester T12 for 12 persons For groups of houses refer to British Water’s Formula For properties used as ‘Holiday Lets’ size according to the maximum number of bed spaces. For large housing projects make a special assessment in each case.

What is the daily volume of domestic sewage? The present design figure for the volume of domestic sewage in the UK is 150 litres per person per day. For example for 5 people: 5 x 150 litres = 750 litres/day = 0.75 Cubic Metres (M³) For new dwellings lower target figures are required under Building Regulations. The anticipated daily volume of sewage relates to DEFRA’s General Binding Rules for Small Sewage Discharges In England criteria for whether or not a ‘Permit’ is required from the Environment Agency (EA) include a maximum flow of 5.0M³/Day into a watercourse or 2.0M³/Day into a soakaway trench system/drainage field. Always check the full requirements of local regulations. See DEFRA’s General Binding Rules.

What is a ‘Population Equivalent’ (PE)? A ‘PE’ is worked out for establishments other than domestic housing such as factories, pubs, hotels, leisure facilities, caravan sites and nursing or care homes. This procedure can become quite complicated especially for variable loadings. PE’s are best worked out by BES prior to quoting for the appropriate Biodigester, but also see the method for factories and offices below. When sizing a Biodigester, the PE’s for both flow and organic loading (BOD) need to be worked out sizing to the greater of the two. Where there is a limit for Ammoniacal Nitrogen lower than 20mg/l special consideration is required. BES can consider retained volumes and size towards average rather than peak values. How are Population Equivalents (PE’s) worked out for factories and offices? British Water’s guidance indicates that if there is no cooking of meals, other than in microwaves, then 2.4 workers are equivalent to one domestic resident or 1PE. Hence a 6 person Biodigester T6 will cater for 2.4 x 6 shifts per day = 14.4 shifts per day And similarly 2.4 x Biodigester T9 = 21.6 shifts per day 2.4 x Biodigester T12 = 28.8 shifts per day and so on for larger populations. If there is a canteen cooking meals for the staff 1.6 workers are equivalent to one domestic resident or 1PE. For example a Biodigester PA30 sewage treatment plant (30PE) is suitable for 2.4 x 30 = 72 )shifts without cooked meal/day 1.6 x 50 = 48 )shifts with cooked meal/day and so on for smaller or larger populations. If in any doubt consult BES. Population Equivalents (PE’s) should always be carefully checked.

Effluent dispersal questions

If there is no watercourse to receive the treated sewage effluent from a Biodigester sewage treatment plant how is the effluent dispersed? Subject to Porosity Tests (See Porosity Test info) an underground dispersal system of stone in trenches, nowadays known as a ‘Drainage Field’ is required. The size of base area of trench required is calculated from the Porosity Test results and the anticipated number and type of users. BES Typical Specifications for drainage field design are:- Invert/drain depth is site specific – normally best kept to a minimum. Trench width 600mm or 900mm. Depth of stone 400mm. Type of stone 25mm-40mm clean stone. Type of pipe 100mm diameter for small applications. Rigid with slots or holes. Cover sheet – required. Soakaway pits and drainage crates are not permitted for treated sewage effluent under present regulations.

What is a Porosity Test? Porosity tests are required when an underground effluent dispersal system needs to be constructed, a system of soakaway trenches/drainage field. The tests are carried out in specially sized sumps dug just below the depth of the intended pipes. The rate of water loss/soakage is measured and the dispersal system designed accordingly (Refer to Porosity Tests information). The tests should not be carried out when the ground is especially wet or dry as the results can be misleading.

The owner or user of each Biodigester is responsible for compliance with environmental 'permits' or 'consents'.

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The science

What effluent quality do Biodigester sewage treatment plants work to? Effluent qualities can vary but the most common one in the UK is 20mg/l BOD, 30mg/l Suspended Solids and 20mg Ammoniacal Nitrogen measured as Nitrogen. (see ‘What is the BOD test? for explanation). One milligram per litre (mg/l) is the same as one part per million (ppm), so a very small amount. Always check the effluent quality required for each application. ‘Descriptive’ permissions sometimes apply meaning that a sewage treatment plant must be correctly sized and maintained.

What is the BOD test? The BOD test is used to measure the organic strength of a liquid, whether untreated wastewater, treated wastewater or a watercourse, lake or pond. A sample is incubated for 5 days at 20°C and the amount of oxygen used up allows calculation of the BOD. This test has to be carried out in a suitably equipped laboratory. Typical ranges are:- Untreated sewage 300-500mg/l BOD Treated sewage effluent 10-20mg/l BOD River water 2-7mg/l BOD

Does a reed bed offer practical alternative sewage treatment? Reed beds for the main secondary sewage treatment process are not recommended due to practical constraints. However tertiary reed beds can be used for ‘Effluent Polishing’. BES can specify horizontal beds with gravel and ‘Phragmites Australis’.

Is sterilisation of treated sewage effluent required? Normal secondary sewage treatment only reduces the content of micro-organisms by about 90%. At some UK sites effluent sterilisation is required, particularly close to bathing water. Many overseas applications require sterilisation. BES offers 3 methods of sterilisation: 1. Ultra Violet (UV) light with an automatic cleaning system for the glass tubes. 2. Ozonisation. 3. Chlorination. Whilst chlorination is widely used in overseas applications it has the drawback of producing by-products that are quite environmentally harmful.

What is Nitrification? Untreated sewage/wastewater contains Nitrogen largely as an ammonium salt. Nitrification is the process whereby Ammoniacal Nitrogen is first oxidised to Nitrite and then to Nitrate. This occurs naturally in a wastewater treatment plant subject to appropriate design and capacity.

What is Denitrification? Denitrification is the process of losing Nitrogen as Nitrate from fully nitrified treated wastewater. Loss of the Nitrogen occurs naturally where there is a low level of dissolved oxygen known as ‘Anoxic’ conditions. The Nitrogen from Nitrate is lost to the atmosphere as Nitrogen gas. It should be noted that this process does not readily go to completion.

What are the current concerns about ‘Phosphates’ in watercourses and lakes? High levels of ‘Phosphates’ can cause what is know as Eutrophication (see ‘What is Eutrophication’) with a risk of damage to aquatic habitats.

What is Eutrophication? Eutrophication is the situation in a body of water where excess levels of Phosphates and Nitrates cause a problematic growth of algae or other water plants that adversely affects the habitat for both flora and fauna.

Regulations question

Are Building Regulations required for the installation of a Biodigester? Building Regulations are normally required in the UK. In some areas these may nowadays be obtained from a private local company rather than the Local Authority. Always check the appropriate regulations.

Is Planning Permission required for a Biodigester sewage/wastewater treatment plant? There is no hard and fast rule but Planning Permission is not normally required for small Biodigesters. Larger projects should be checked with the appropriate Local Authority.

How close may a Biodigester treatment plant be positioned to the inhabited part of dwellings? 7 metres is the standard figure for small treatment plants. As the Biodigester ‘T’ range sewage treatment plants operate without odour, BES allows a closer distance but proximity to foundations then becomes an important issue as far as installation is concerned.

What are DEFRA’s General Binding Rules for Small Sewage Discharges? They apply to small discharges of effluent in some parts of the UK. In England compliance with these rules for discharges into watercourses or soakaway trenches/drainage fields avoids the need to obtain a ‘Permit’ from the Environment Agency (EA).

What affects do Sites of Special Scientific Interest (SSSI) status, Ramsar and other special sites have on effluent discharge requirements? Even within 500M of an SSSI or similar sites special requirements may apply. Always check with the environmental regulator.

Should the need for permission from the local environmental regulator and its specific requirements be checked? Yes, in all cases.

Who are the UK Environmental Regulators? England The Environmental Agency (EA) Website: Wales Natural Resources Wales (NRW) Website: Scotland Scotland Environmental Protection Agency (SEPA) Website: Northern Ireland Northern Ireland Environment Agency Website:

What is British Water’s involvement in effluent treatment? British Water is a private organisation that companies may join. British Water’s main relevance to effluent treatment is production and maintenance of their tables for Flows and Loads.

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Your eco-friendly way to treat and dispose of your property's wastewater

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