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Sewage Treatment Plant, Septic Tanks and Cesspits Explained

We are often asked here at CM what the difference is between a septic tank and a sewage treatment plant.


A septic tank is effectively a holding tank for solids, so depending on usage, it will need emptying frequently and will require a drainage field (or soakaway) designing and installing in order to treat the foul water before it enters the ground. A septic tank does not require electricity to work, and septic tanks do not require constant flow in order to work. Installing a septic tank is a good solution where there is seasonal usage in a remote place such as a glamping site. The drainage field does need to be designed to sites' requirements following a percolation test in order to adhere to building regulations and Environment Agency binding rules.


A cesspit is basically a holding tank that needs to be emptied entirely of both liquids and solids when the tank is full. These can be a costly ongoing expense depending on usage.


With a sewage treatment plant however, the treatment takes place through a biological process within the unit, so can discharge directly into a ditch or watercourse safely. Like a septic tank, a treatment plant can also discharge to a drainage field, but this will be smaller than the one required for a septic tank because it is only draining ie no further treatment is required to adhere to EA standards. A sewage treatment plant runs using a compressor that pumps air into the system through diffusers, that keeps bacteria alive, so they require a constant electricity supply. A standard 6 population is a 60w compressor constantly running, so in terms of costs, it's like having a lightbulb running 24/7. The environment agency prefer sewage treatment plants so specify unless there is reason for a septic tank, then a sewage treatment plant should be specified and installed.


Any sites' requirements are very individual as to the best solution; we have to consider ground conditions, electricity access, regulations, locations, usage, levels... which is why a site visit is always advisable.


All of our CM engineers are British Waters accredited and all CM staff are fully equipped to answer questions about the Environment Agency's 2020 regulations regarding updating septic tank systems to sewage treatment plants, so please do give us a call if you need any further information.


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