Back to Basics
Mains & Off-Mains Drainage
The majority of houses in the UK are linked up to a public sewer pipe
- You pull the plug, flush the toilet, pay the monthly bill and have
little to worry about. This is Mains Drainage.
In rural areas many homes & villages are self-contained; the combined
waste ending up in a local Cesspool, Septic Tank or Treatment Plant.
This has no connection to the public sewer system and is known as Off-Mains.
Cesspools
A Cesspool is simply a single-chamber storage tank with no outlet. The
tank is usually very large (as it has to contain all waste water & sewage)
and often made from concrete, brick or GRP. The only way to dispose of
the waste is by hiring a licensed sewage contractor to remove the sewage
for off-site disposal.
The use of Cesspools is no longer an option in most instances.
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Septic Tanks
A Septic Tank is a multi-chamber storage tank allowing liquid and solid
waste to separate: The liquid is allowed to flow out of the tank and
be disposed of separately.
Firstly the sewage enters a settlement chamber, allowing solid waste
(sludge) to sink and the liquid to rise to the surface. The surface liquid
makes contact with oxygen and the organic matter starts to breakdown
biologically. This liquid still contains sewage but in small enough particles
to be carried through the discharge outlet and into the ground (soakaway).
Septic Tanks only partially treat sewage. Many areas of the UK & Ireland
prohibit the installation of Septic Tanks.
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Treatment Plant
A Treatment Plant is a packaged miniature sewage-works that converts
raw sewage into water effluent and solid waste (sludge).
These ‘packaged’ plants come in all shapes and sizes but
all use the same science in their process of tackling sewage – Putting
bacteria into contact with oxygen and food (organic waste) results in
a natural biological breakdown (aerobic digestion).
The key to any Treatment Plant’s efficiency and success is how
its design puts these three things together in its operating motion.
The BioDisc range for example, acts by rotating the host ‘media’ through
the effluent. Airflow models move the effluent over the host media. Media
is simply bacteria growing on perforated plates designed to provide a
large contact area for this digestion to take place.
No Treatment Plant is a ‘fit & forget’ product as they
do require periodic maintenance and a degree of care to keep working
to optimum levels. What they do offer though is the most environmentally
and authority friendly way of solving an Off-Mains drainage problem.
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Reed Beds
A Reed Bed is a product often specified in order to obtain an even higher
level of water quality out of a Treatment Plant before it migrates out
into the surrounding watercourse. Reed Beds are also used in conjunction
with SUDS applications.
Reeds are grown in a GRP tank. Water exiting the Treatment Plant (or
SUDS system) passes through these reeds, which (by their nature) filtrate
the water removing further impurities.
Reed Beds are often used in situations with high-specification discharge
consents.
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Pump Stations
A Pump Station is a chamber with electrical pumps installed. They are
designed simply to move a body of water (or sewage) uphill or over a
distance where gravity is insufficient to provide the required flowrate.
Consisting of a chamber with either single or multiple electric pump
units, the Pump Station is tailored to site conditions, automatically
monitoring and maintaining a specific flow of waste water over a certain
length and height.
Alarms are often fitted to warn of any blockage or failure before a
situation may occur.
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Separators & Silt Traps
Designed for surface water applications, Separators remove pollutants
(of a certain specific gravity) from effluent and allow the cleaner water
to be passed into a surface water drainage system. Pollutants are retained
for removal and disposal.
Separators work by allowing the pollutants and water to naturally separate
out from one another - pollutants will float on the surface of water
and can then be drawn off. Silt will also settle within the unit and
can also be drawn off.
Numerous designs and sizes are available to cover applications such
as petrol stations, airports, car parks, scrap yards etc.
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Grease Traps
Grease & Silt Traps work in a more basic fashion but again, remove
unwanted contaminants from wastewater before it reaches the drainage
system. Often used in catering applications.
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