3.7 Wastewater drainage
This guidance applies to wastewater systems that operate essentially under gravity. The guidance to this standard provides recommendations for the design, construction and installation of drains and sewers from a building to the point of connection to a public sewer or public sewage treatment works.
The guidance should also be used for all pipework connecting to a private wastewater treatment plant or septic tank.
Guidance on private wastewater treatment plants, septic tanks and infiltration fields is provided to Standards 3.8 and 3.9.
Combined sewers - some sewers, called combined sewers, carry wastewater and surface water in the same pipe. It may be appropriate to install a drainage system within the curtilage of a building as a separate system even when the final connection is to a combined sewer. This will facilitate the upgrading of the combined sewer at a later date.
Incorrect connections - the connection of wastewater drains to surface water drains is a common occurrence during conversions and extensions in urban areas served by separate drainage systems. Incorrect connections can cause chronic and severe pollution of watercourses and a careful check should be made before final connection is made to the appropriate drain.
Differences in plumbing within Europe have led to a variety of systems being developed. These have happened as a result of differences in the type and use of sanitary appliances in Member States. The European Standards describe the 4 main systems in use but it is expected that traditional practices will continue in the various countries. However care will need to be taken if different systems are used to ensure that the entire system operates satisfactorily and that the system designed and installed is compatible with, and suitable for, connection to existing wastewater systems.
Sanitary pipework should be constructed and installed in accordance with the recommendations in BS EN 12056-2: 2000. The BS EN describes 4 different systems as follows:
System III (single discharge stack system with full bore branch discharge pipes) as described in Clause 4.2 of BS EN 12056-2: 2000 is the traditional system in use in the UK.
However low water and energy consumption is now a major consideration in any design solution. With this in mind, System II (single discharge stack system with small bore discharge branch pipes) may be appropriate. Careful consideration should be given to the design of the system where a low flush cistern is connected to an existing drain to ensure that blockages do not occur as a result of reduced flow rates.
Systems I (single discharge stack system with partially filled branch discharge pipes) and IV (separate discharge stack system) have developed as a result of different types of sanitary appliances and technical traditions in the various European countries. These system types are unlikely to be appropriate for use in this country.
Reducing the bore of a pipe in the direction of flow may lead to blockages and should not be considered as an acceptable method of construction. However sanitary pipework may be reduced where it is connected to a pump installed in compliance with the conditions of certification by a notified body. These pumped systems are generally in use where the appliances are located in basement levels below the flood level of the drain. (see clause 3.7.2).
The basements of approximately 500 buildings in Scotland are flooded each year when the sewers surcharge (the effluent runs back up the pipes because they are too small to take the required flow). Wastewater from sanitary appliances and floor gullies below flood level should be drained by wastewater lifting plants or, where there is unlikely to be a risk to persons such as in a car park, via an anti-flooding device. Wastewater lifting plants should be constructed in accordance with BS EN 12056-4: 2000. Wastewater from sanitary appliances above flood level should not be drained through anti-flooding devices and only in special case, e.g. refurbishment, by a wastewater lifting plant.
Reducing the bore of a drain in the direction of flow may lead to blockages and is not recommended.
Health and safety legislation requires that manual entry to a drain or sewer system is only undertaken where no alternative exists. Therefore use of remotely operated equipment will become the normal method of access. As well as the traditional inspection chambers used for depths of up to 1m, remotely operated equipment is available for inspection, cleaning and removal of debris from deeper drains and sewers, without the need for personal entry.
Where a private drain discharges into a public sewer, normally at the curtilage of a building, some form of access should be provided for maintenance and to allow a satisfactory connection. The preferred method is by a disconnecting inspecting chamber for each house immediately inside the curtilage, although other methods and locations may be acceptable. Although access for maintenance purposes may be required by Scottish Water, design and construction of the chamber should be in accordance with the recommendations of BS EN 752: 2008. It is preferable that a chamber is provided for individual houses but where this is not practicable, a shared disconnecting chamber (or manhole where the depth is more than 1.2m) should be provided in accordance with the requirements of Scottish Water in whom it is likely to be vested.
The disconnecting chamber, or manhole, for a block of individually owned flats or maisonettes should be located as close to the building as is reasonably practicable as the drain will become a public sewer once it passes out with the footprint of the building.
Some sewers, called combined sewers, carry wastewater and surface water in the same pipe. These systems are not recommended today as they are more likely to surcharge during heavy rains. A separate drainage system carrying wastewater and surface water therefore should be constructed within the curtilage of a building even if it connects to a combined sewer to facilitate the upgrading of the combined sewer at a later date.
Incorrect drainage connections, mostly related to conversions and extensions, whether wastewater to surface water or vice versa, is a common occurrence and can cause severe pollution of watercourses or surcharging of drains also leading to pollution. A careful check should be made before breaking into an existing drain to ensure it is the correct one and a further test carried out after connection, such as a dye test, to confirm correct connection.
Where it is intended that a private sewer (a sewer connecting 2 or more buildings that are privately owned and maintained) will be vested in the Water Authority, construction and installation should be in accordance with the requirements of ‘Sewers for Scotland’.
A Wastewater drainage system serving a building should be ventilated to limit the pressure fluctuations within the system and minimise the possibility of foul air entering the building. A system should be installed in accordance with the guidance in Sections 4, 5, 6 and National Annex ND of BS EN 12056-2: 2000.
Air admittance valves are another method of ventilating a drainage system as they allow air to enter the drainage system, but not to escape, thus limiting pressure fluctuations within the system. Care should be taken when installing these valves that they are located where they will operate effectively. Air admittance valves should be installed:
in accordance with the recommendations in BS EN 12380: 2002, or
in compliance with the conditions of certification of a notified body.
A wastewater drainage system should be tested to ensure the system is laid and is functioning correctly. Testing should be carried out in accordance with the guidance in:
National Annex NG of BS EN 12056-2: 2000, for sanitary pipework
BS EN 1610: 1998, for a drainage system under and around a building.
A wastewater drainage system should discharge to a public sewer or public wastewater treatment plant provided under the Sewerage (Scotland) Act 1968, where it is reasonably practicable to do so. Where it is not possible to discharge to a public system, for example in the countryside where there is no public sewer, other options are available, as described in the guidance to Standards 3.8 and 3.9: Private wastewater treatment systems.