Building standards technical handbook 2019: domestic

The building standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004. This handbook applies to a building warrant submitted on or after 1 October 2019 and to building work which does not require a warrant commenced from that date.

3.8 Private wastewater treatment systems – treatment plants

Mandatory Standard

Standard 3.8

Every private wastewater treatment plant or septic tank serving a building must be designed and constructed in such a way that it will ensure the safe temporary storage and treatment of wastewater prior to discharge.

3.8.0 Introduction

A wastewater treatment system is an effective, economical way of treating wastewater from buildings. It consists of 2 main components, a watertight underground tank into which raw sewage is fed and a system designed to discharge the wastewater safely to the environment without pollution. This is normally an infiltration field through which wastewater is released to the ground, but when ground conditions are not suitable, a discharge to a watercourse or coastal waters may be permitted. The infiltration field provides secondary treatment and is often critical for protecting the environment from pollution. Not all treatment plants treat to the same standard, for example, biological treatment plants treat the wastewater to a much higher standard than septic tanks prior to release of the wastewater thus reducing pollution and permitting a smaller infiltration field. As there are many different types of treatment plants with varying degrees of efficiency, then the type chosen should be selected for the effluent standard required to protect the environment.

Although a septic tank is a basic form of treatment plant, it has been specifically mentioned in the guidance to clarify the recommendations.

Package treatment plant is the term applied to a range of systems engineered to treat a given hydraulic and organic load using prefabricated components that can be installed with minimal site work.

Guidance on the construction and installation of drains discharging into private wastewater treatment plants or septic tanks is covered by Standard 3.7.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard (regulation 12, schedule 6).

3.8.1 Treatment plants

Where it is not reasonably practicable to connect to a public sewer or a public wastewater treatment plant then discharge should be to a private wastewater treatment plant or septic tank.

Treatment plants provide suitable conditions for the settlement, storage and partial decomposition of solids that need to be removed at regular intervals. The discharge can however still be harmful and will require great care when discharging to ground to ensure a suitable level of protection of the environment is achieved. A large number of small sewage treatment works in a limited area is undesirable. The guidance to Standard 3.9 deals with the infiltration system that should be constructed as an integral part of the treatment plant or septic tank.

When sizing and selecting the wastewater treatment plant the designer should take full account of all population loadings, effluent treatment standard, and also where appropriate, any unusual pollution loads such as waste disposal units.

Domestic use of detergents and disinfectants is not detrimental but excessive use may have a harmful effect on the performance of the sewage treatment works. The British Water Code of Practice, Flows and Loads – 2, provides guidance on sizing criteria and treatment capacity for small (maximum 1000 population equivalent) wastewater treatment systems.

A private wastewater treatment plant and septic tank should be designed, constructed and installed in accordance with:

  1. the recommendations of BS EN 12566-1: 2000, for a prefabricated septic tank, or

  2. the recommendations of BS 6297: 1983, or

  3. the conditions of certification by a notified body.

3.8.2 Treatment plant covers

The settlement tank of a private wastewater plant and a septic tank should have a securely sealed, solid cover that is capable of being opened by 1 person using standard operating keys.

3.8.3 Inspection and sampling

A private wastewater plant and septic tank should be provided with a chamber for the inspection and sampling of the wastewater discharged from the tank. The owner should carry out inspection at regular intervals and SEPA can sample the effluent to ensure compliance with any discharge consent. A chamber should be provided in accordance with the following diagram.

Figure 3.8. Section through inspection chamber

Section through inspection chamber

3.8.4 Location of a treatment plant

Research has shown that there are no health issues that dictate a safe location of a treatment plant or septic tank relative to a dwelling. However damage to the foundations of a dwelling has been shown to occur where leakage from the tank has occurred. In the unlikely event of there being leakage, it is sensible to ensure that any water bearing strata directs any liquid away from the dwelling. To prevent any such damage therefore, every part of a private wastewater plant and septic tank should be located at least 5m from a dwelling.

Every part of a private wastewater plant and septic tank should be located at least 5m from a boundary in order that an adjoining plot is not inhibited from its full development potential.

3.8.5 Discharges from septic tanks and treatment plants

Where mains drainage is not available, it may be possible to discharge treated wastewater to ground via an infiltration system, as described in clause 3.9.2, or to a water course, loch or coastal waters.

SEPA will require an authorisation, under the terms of the Water Environment (Controlled Activities)(Scotland) Regulations 2005 to be applied for all discharges of sewage effluent whether to ground via an infiltration system or to a watercourse.

3.8.6 Access for desludging

Wastewater treatment plants should be inspected monthly to check they are working correctly. The effluent in the outlet from the tank should be free flowing. The frequency of desludging will depend upon the capacity of the tank and the amount of waste draining to it from the dwelling but further advice on desludging frequencies should be obtained from the tank manufacturer or the desludging contractor.

A private wastewater treatment plant and septic tank should be provided with an access for desludging. The desludging tanker should be provided with access to a working area that:

  • will provide a clear route for the suction hose from the tanker to the tank, and

  • is not more than 25m from the tank where it is not more than 4m higher than the invert level of the tank, and

  • is sufficient to support a vehicle axle load of 14 tonnes.

3.8.7 Labelling

Every dwelling with a drainage system discharging to a private wastewater treatment plant or septic tank should be provided with a label to alert the occupiers to such an arrangement. The label should describe the recommended maintenance necessary for the system and should include the following:

‘The drainage system from this property discharges to a wastewater treatment plant (or septic tank, as appropriate). The owner is legally responsible for routine maintenance and to ensure that the system complies with any discharge consent issued by SEPA and that it does not present a health hazard or a nuisance’.

The label should be located adjacent to the gas or electricity consumer unit or the water stopcock.

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