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Part H Drainage

Section 2: Wastewater Treatment Systems


Wastewater Treatment Systems H2 Requirements

(1) A wastewater treatment system shall be so designed, sited and constructed that:

(a) it is not prejudicial to the health of any person,

(b) it does not cause a risk to public health or the environment,

(c) it prevents unauthorised access but allows adequate means of access for emptying and maintenance,

(d) it will function to a sufficient standard for the protection of health in the event of a system failure,

(e) it has adequate capacity,

(f) it is impermeable to liquids, and

(g) it is adequately ventilated.

(2) Information on the wastewater treatment system and any continuing maintenance required to avoid risks to health and the environment shall be provided to the owner.

2.1 Guidance

2.1.1 A wastewater treatment system is a septic tank system or a packaged wastewater system. In general a septic tank system is a wastewater treatment system that includes a septic tank mainly for primary treatment, followed by a percolation system in the soil providing secondary and tertiary treatment. A packaged wastewater treatment system, generally uses media and mechanical parts to enhance the treatment of the domestic wastewater and is followed by a polishing filter.

2.1.2 In general, the design and installation of wastewater treatment systems for single houses should comply with the relevant parts of the Environmental Protection Agency's (EPA) Code of Practice 2009 Wastewater Treatment and Disposal Systems Serving Single Houses. This publication also includes guidance relating to site assessment, test holes, the appropriate distances between the wastewater treatment systems and critical targets including dwellings, wells and site boundaries. It also gives design criteria for filters and wetlands. This Code of Practice has superseded previous guidance given in SR 6: 1991 Septic tank systems - Recommendations for domestic effluent treatment and disposal from a single dwelling house. Where the proposed system requires energy for operation, consideration should be given to the energy consumption and energy efficiency of the system.

2.1.3 Guidance on the design and installation of wastewater treatment systems capable of serving larger buildings or more than one house is contained in I.S. EN 12566 Parts 1-4, BS 6297: 2007 Code of Practice for the design and installation of drainage fields for use in wastewater treatment and the EPA's Wastewater treatment manual - Treatment systems for small communities, business, leisure centres and hotels: 1999.

2.1.4 The design, installation and commissioning of wastewater treatment systems should be carried out and / or supervised by a suitably qualified person or persons.

2.1.5 The owner of the building should be provided with sufficient, clear and comprehensive information on any continuing maintenance required to facilitate the effective operation of the system in order to protect health and the environment.

2.2 Septic tanks

2.2.1 Septic tanks provide suitable conditions for the settlement, storage and partial decomposition of solids which need to be removed at regular intervals. The effluent discharge from the septic tank can still be harmful and further treatment from either a percolation area, percolation mound, constructed wetland or intermittent filter will be required. In general, where a site is suitable for the development of a septic tank system, these systems designed and constructed in accordance with this guidance, and properly maintained, have been found, through national research, to achieve satisfactory effluent quality in terms of BOD, SS and NH4 as N, equivalent to Table 18. In nutrient-sensitive areas, the local authority may require further treatment of the effluent.

2.2.2 An effluent screen on the outlet of the tank is optional but is recommended as it gives extra protection to the percolation / filter system and gives early warning of the need for de-sludging. The screen may incorporate an alarm system. Septic Tanks must conform to I.S. EN 12566 Part 1 or 4 including their national annexes and comply with the EPA Code of Practice - Section 7.

2.2.3 The tank should rest on a uniform surface capable of bearing the weight of the tank and its contents. After setting the tank, leveling and joining the drains from the house and the tank outlet to the distribution box, the joints and seals should be tested for water tightness before backfilling. The backfilling material should be free flowing and carried out in lifts to prevent disturbance of the tank. Precautions should be taken to prevent flotation of the tank either during or after construction.

2.2.4 The distribution device, which divides the effluent evenly between the percolation pipes where a percolation area or mound is used, should be laid on a stable foundation and be leveled accurately to ensure the incoming effluent is evenly diverted to the outlet percolation pipes. Access / inspection covers should be visible, watertight and flush with ground level. This distribution device requires on-going maintenance.

2.3 Percolation areas and mounds

2.3.1 A percolation area typically consists of a system of sub-surface irrigation pipes which allow the effluent to percolate gradually into the surrounding soil. Biological treatment takes place naturally in the aerated layers of soil.

2.3.2 Percolation mounds are essentially percolation areas placed above the natural surface of the ground providing an aerated layer of soil to treat the effluent. Percolation areas should comply with the EPA Code of Practice - Section 7 and I.S. CEN/TR 12566 Part 2 Soil infiltration systems.

2.3.3 Land drainage pipes should not be used in a percolation trench. Access / inspection pipes should be fitted to the end of the percolation pipes to facilitate the assessment of the proper functioning of the percolation area and to facilitate rodding or scouring of the pipes. There should be a maximum of five trenches attached to each distribution box when designing a gravity system for a percolation area. On sloping sites the pipework should be installed parallel to the contour to aid distribution of the effluent. The infiltration pipes should be inspected before installation to ensure a clean and smooth finish on all cuts and drill holes.

2.4 Constructed wetlands

Constructed wetland is the generic term used to describe both (gravel- and sand-based) horizontal and vertical flow reed bed systems and soil-based constructed wetlands. The main difference between a constructed wetland and other filter systems is the planting of vegetation in the media where the thick root mass acts as a pathway for the transfer of oxygen from the atmosphere to the root zone (rhizosphere). Pumping may or may not be required for constructed wetland systems depending on the slope of the site and the wetland configuration. They can be used to provide secondary or tertiary treatment to effluent from septic tank or tertiary treatment to packaged wastewater treatment systems. The design and construction of wetlands should comply with the EPA Code of Practice - Section 8 or Section 10 for secondary or tertiary treatment respectively and / or SR CEN TR 12566 Part 5 - Pre-treated effluent filtration systems.

2.5 Intermittent filter systems

2.5.1 This comprises a pump chamber which transfers the partially treated effluent, (generally from a septic tank), onto the filter at regular intervals. The filter may consist of soil, sand, peat or other media.

A range of configurations may be considered:

a) an intermittent soil filter system (soil olishing filter is built in);

b) an intermittent sand filter followed by a polishing filter (may be inbuilt or offset);

c) an intermittent peat filter followed by a polishing filter;

d) an intermittent plastic or media filter followed by a polishing filter;

e) a constructed wetland or reed bed followed by a polishing filter.

2.5.2 Where a pumped distribution system is used an alarm should be fitted to alert the user to any malfunction. Intermittent Filter Systems should comply with the EPA Code of Practice - Section 8 and / or SR CEN TR 12566 Part 5 - Pre-treated effluent filtration systems. Packaged filter systems should conform to prEN 12566 Part 6 - Prefabricated treatment units for septic tank effluent (when available). A polishing filter should be used in conjunction with all intermittent filters.

2.6 Polishing filters

A polishing filter is a filter system the purpose of which is to provide additional treatment of the effluent and to reduce pollutants such as micro-organisms, phosphorous and in certain cases nitrate nitrogen. It also provides for the hydraulic conveyance of the treated effluent to the ground. Polishing filters should comply with the EPA Code of Practice - Section 10 and prEN 12566 Part 7 - Prefabricated tertiary treatment units (when available).

2.7 Packaged wastewater treatment

2.7.1 Packaged wastewater treatment systems may be used to treat wastewater from a building where the site is unsuitable for a septic tank system or they may be used as an alternative to septic tank systems. These systems should conform to I.S. EN 12566-3: 2005/A1: 2009 Packaged and / or site assembled domestic wastewater treatment plants, and its National Annex.

Table HH18 - Minimum performance standards - Extract from TGD H
Table HH18 - Minimum performance standards - Extract from TGD H

2.7.2 The system should be designed for a minimum hydraulic daily load of 150 l/person/day based on the number and size of bedrooms and a minimum organic daily load of 60g BOD/person/day to ensure adequate treatment is provided.

2.7.3 All such systems must have a treatment efficiency capable of meeting the minimum performance effluent standards set out in Table 18 below. Monitoring and maintenance of these systems is required to ensure that the effluent is treated to this standard. In nutrient sensitive areas, more stringent performance standards for nitrogen and phosphorous may be necessary.

2.7.4 The sludge storage capacity should be checked with the manufacturer to establish the necessary frequency of de-sludging. All package wastewater treatment systems should be provided with an alarm to indicate operation failure in line with the requirements of I.S. EN 12566-3: 2005/A1: 2009 Packaged and / or site assembled domestic wastewater treatment plants.

Many systems are available including: -

(a) Biofilm Aerated Filter (BAF) systems;

(b) Rotating Biological Contactor (RBC) systems;

(c) Sequencing Batch Reactors (SBR) systems;

(d) Membrane filtration systems.

Packaged Wastewater Treatment Systems should comply with the EPA Code of Practice - Section 9 and be installed in accordance with the manufacturer's instructions.

2.7.5 The effluent from all packaged wastewater treatment systems should discharge to a polishing filter to allow for further treatment of the wastewater and to convey the treated wastewater to groundwater (refer to sub-section 2.6).

2.8 Tertiary treatment systems

The term tertiary treatment system includes polishing filters, constructed wet lands and packaged tertiary treatment systems. They provide additional treatment to wastewater from secondary treatment systems. Tertiary treatment systems should comply with the EPA Code of Practice - Section 10 and prEN 12566 Part 7 - Prefabricated tertiary treatment units (when available).

2.9 General recommendations

2.9.1 Household garbage grinders / sink macerators can increase the BOD loading rate of wastewater by up to 30%. Their use is not recommended for buildings where wastewater treatment systems are used, unless specifically designed to do so, as they result in additional maintenance requirements due to the increased solids. Excessive amounts of waste fats, oils and grease (FOGs) should be avoided as they impair the treatment process and require desludging more frequently. Where this is not practicable (e.g. commercial kitchens etc.) a grease trap should be used.

2.9.2 Under no circumstances should rainwater or surface water be discharged to wastewater treatment systems.

2.9.3 The siting of systems should allow for access for a sludge tanker and maintenance equipment to de-sludge the tank. (A maximum distance of 30 m from a hardstand with 3 m invert level is recommended.) Storm water drains, water mains, service pipes, soakaways, access roads, driveways, paved areas or land drains should not be located within or around the infiltration area.

The minimum separation distances should be in accordance with the EPA Code of Practice - Section 6.

2.9.4 In areas of low permeability soils, shallow intercepter drains should cut off all surface run-off and seepage from the surrounding soil. The intercepter drain should be 2 m distant from the upgradient side and parallel to the side edges of the infiltration area. These drains would comprise land drainage pipes overlain to ground surface with permeable gravel or broken stone aggregate. These intercepter drains should be brought to the nearest water course or stream into which they outfall.

2.9.5 Under the Water Pollution Act 1977 a Discharge Licence is required where the effluent is being discharged to a surface water course and this will dictate the final effluent quality which may require the use of specific tertiary treatment in accordance with the EPA Code of Practice - Section 10.

2.10 Holiday homes. When choosing a wastewater treatment system for holiday homes, consideration
should be given to the selection of a system that can adequately deal with periods of inactivity (i.e. when the house is unoccupied for prolonged periods). Systems that are capable of re-circulating the effluent may be appropriate.

Energy Foundation Trench Damp proof membrane Damp proof course Underfloor heating pipes Air to water heat pump Air to air heat pump Air to ground heat pump IS 440 Battery storage Time and temperature Zone control Drainage Septic tanlk Percoltion area Ground conditions Air tightness Air tightness tape Tongue and groove Public liability Energy grants Building energy rating Energy performance certificate Retrofit assessment Home energy assessment Water pump Water tank Pumped insulation Pump cavity Eaves box Mechanical ventilation Natural ventilation Air tight membrane Water membrane Water vapour membrane Vapour control layer