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3.8.0 Introduction
Although a septic tank is a basic form of treatment plant, it has been
specifically mentioned in the guidance to clarify requirements.
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 normally an infiltration field to which wastewater is released
to the ground. The infiltration field is critical for protecting the environment
from pollution. Biological treatment plants treat the wastewater
to a much higher standard than septic tanks prior to release thus reducing
pollution and permitting a smaller infiltration field. There are many
different types of treatment plants with varying degrees of efficiency.
Packaged treatment plants 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 under standard 3.7.
In the case of conversions, as defined by Regulation
3, the building as converted must meet the requirements
of this standard (Regulation 12, Schedule 6).
3.8.1 Treatment plants (M3.1a)
Where it is not 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. The guidance to standard 3.9 provides
recommendations on suitable ground conditions and infiltration systems
that should be constructed as an integral part of the treatment
plant or septic tank.
A large number of small sewage treatment works in a limited area is undesirable.
Greater efficiency of operation as well as economy of construction
can be achieved by collective drainage and treatment. The designer should
make provision, where appropriate, for unusual pollution loads. These
may arise from the use of waste disposal units and from the specialised
occupancy of premises, such as public houses. Domestic use of detergents
and disinfectants is not detrimental but excessive use may have a harmful
effect on the performance of the works.
A private wastewater plant and septic tank should be designed,
constructed and installed in accordance with:
a. the recommendations of BS EN 12566-1: 2000, for a prefabricated
septic tank; or
b. the recommendations of BS 6297: 1983; or
c. the conditions of certification by a notified body.
3.8.2 Treatment plant covers (M3.1b)
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 (M3.1c)
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.
3.8.4 Location of a treatment plant (M3.1d & e)
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 effluent 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 5 m from a dwelling.
Every part of a private wastewater plant and septic
tank should be located at least 5 m from a boundary in order that
an adjoining plot is not inhibited from its full development potential.
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