Biodigester Sewage Treatment

Sewage Treatment Plants

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How they work? Performance Treatment options

Treatment options

The following is intended to give a brief overview of the sewage treatment options available to areas with no access to mains sewerage:-

Cesspits (or Cesspools) are large, sealed storage tanks which provide no biological treatment of the sewage they contain. As such, modern environmental regulations discourage or even ban their use (Cesspits are banned in Scotland).


Due to the fact that they do not discharge any treated effluent, the Cesspit must be emptied frequently by a licensed waste disposal company. This leads to considerable running costs.


A modern septic tank provides basic anaerobic treatment of incoming sewage. The tank usually consists of two chambers which allow settlement of any solids to form a sludge. Naturally occuring anaerobic bacteria within the tank are able to provide minimal treatment, and the liquid effluent is discharged via an outlet pipe. The effluent from a septic tank still contains approximately 70% of the original pollutants.


The liquid effluent must undergo further treatment before it can be discharged. Typically this is achieved via the use of a drainage field (soakaway), which distributes the effluent over a large area of land, allowing micro-organisms in the soil to break down the remaining organic matter. Siting of a soakaway is dependent upon successful completion of a Percolation Test. 


Septic Tanks cannot be used in Groundwater Source Protection Zones.

Septic Tank

Sewage Treatment Plants

The Biodigester range of package sewage treatment plants provide a much higher level of treatment than the previously mentioned options. By providing a constant flow of air, the treatment plant is able to utilise aerobic bacteria, which degrade the sewage to a high level.


The multi-chamber design allows re-circulation of Activated Sludge, ensuring that the plant remains aerobic. Inclusion of plastic media within the treatment plant provides a large surface area for the bacteria to adhere to, whilst ensuring mobility of the bacteria for rapid degradation of solid matter.


The Biodigester sewage treatment plant is designed to produce effluent with a BOD 5* of 20 mg/l. In comparison, a septic tank produces a BOD of 400 mg/l.


*BOD 5 = Biochemical Oxygen Demand measured over 5 days (a measure of the amount of organic matter present in the discharged effluent). Often used to quantify the effectiveness of a sewage treatment plant.

soakaway_profile1 Biodigester-Sewage-Treatment-Plant

Due to the high effluent quality produced by the sewage treatment plant, it may be discharged directly into a watercourse (with approval from relevant authorities). If no watercourse is available, a soakaway may be used to discharge the effluent. See Regulations for more information.