What quality of water is produced by a sewage treatment plant?

Modern treatment plants are designed to treat the wastewater from a building to a level of quality that will not pollute the land or rivers.

Water produced from a treatment plant is measured in 3 different ways as follows:


Simply put, this is a measure of how much organic material is in the water. It is expressed as mg/l (milligrams per litre).

BOD5 stands for ‘biological oxygen demand’.  Water contains oxygen, and when organic material is put into water, bacteria break it down by using the oxygen up. By testing the level of oxygen over a 5 day period, we can see how much organic material is present in the water. For example, if there was no organic material in the water, there would be no loss of oxygen. On the flip side, if there is lots of organic material, the oxygen levels would drop.  With a treatment plant, we are looking for a low figure. In the UK, a treatment plant should be capable of producing water that has 20mg/l BOD5 or lower.


Raw ammonia is poisonous to fish and plant life. It comes from urine. Bacteria in the treatment plant break down ammonia and convert it to nitrite (using the nitrogen cycle). Here we are looking for a level of no more than 20mg/l although many treatment plants achieve much better results during test conditions. On larger treatment plants, and if the discharge is going to a site of special scientific interest, the Environment Agency may ask for the ammonia to be 10mg/l or even 5 in some cases.

Suspended Solids

Suspended Solids (SS) or Total Suspended Solids (TSS) is a measure of fine particles that are suspended in the water after treatment. This should be less than 30mg/l.

Further Information

If you are environmentally conscious you can check the test results of any treatment plant in Europe. Each treatment plant must be tested under EN12566/3, which records each of these three elements during a 37 week testing process. You may read about how we should pay close attention to these figures but it must be pointed out that whilst the test tries to imitate normal conditions, waste varies from one building to another and so the results in the test may not be what you get at home. You can improve your water quality by limiting detergents and bleach, more can be found out about this by reading our guide on what not to be put in a treatment plant.

Last Update: September 11, 2018  

February 8, 2018   1885    Sewage Treatment  
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