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What does ‘final effluent quality’ mean?

The treated effluent from a sewage treatment system will generally be measured using three parameters:

Biological Oxygen Demand (BOD)
Suspended Solids (SS)
Ammoniacal Nitrogen (NH4-N)

Each of the above are measured in mg/l (milligrams per litre), and modern sewage treatment systems are designed to achieve performance levels of 20:30:20 (BOD:SS:NH4-N) as a minimum.

These performance levels would meet the requirements set out in the adopted European standard BSEN12566/3.

To meet this standard, each plant is independently tested over a 38-week period, and the average results are shown on the performance certificate. Each plant is issued with a certificate from the independent test house, which gives percentage reductions of each indicator. The higher the percentage reduction, the better the performance of the sewage treatment system.

In order to test the effluent, a sample should be taken and tested at a certified laboratory. It is very difficult to fully assess the effluent quality without a laboratory test, however, a good indicator is a clear liquid free of solids and without any pungent odour.