The Treatment of Domestic Sewage
The design of a sewage treatment works will be dependent on the quality and quantity of the waste to be treated.
The following are some of the standards and standard tests which may be carried out :-
Biochemical Oxygen Demand (BOD 5)
This test is the measurement of the weight of dissolved oxygen consumed by chemical and micro-biological action during the incubation of a sample for a definite period at a defined temperature. In the UK the period is 5 days and the temperature is 20 degrees C. The average 5 day BOD strength of domestic crude sewage is about 300 mg/l. to 400 mg/l., although this can very easily vary between 250 mg/l. (weak sewage) to 500 mg/l. (strong sewage).
Suspended Solids (S.S.)
The amount of total suspended solids will indicate the insoluble content of the sewage. The average strength of domestic crude sewage in the UK is about 300 mg/l to 400 mg/l.
Ammoniacal Nitrogen (N or NH3)
The nitrogen in sewage is assessed as ammoniacal nitrogen. This indicates the amount of nitrogenous organic matter which has been converted to ammonia. The average strength of crude domestic sewage will have a combined nitrogen content of 40 mg/l to 60 mg/l. pH VALUES. The pH of a liquid is a measurement of its acidity ( low figures of 3 to 5.) or alkalinity (high figures of 9 to 12.) with a neutral of 7.0. The average strength of crude domestic sewage is slightly alkaline at 7.2.
Chlorides. (C1)
Chlorides are always present in domestic sewage and natural unpolluted water (at levels averaging 10 to 20 mg/l. ) The average strength of chlorides in crude domestic sewage is about 70 mg/l to 100 mg/l.
Phosphorous (P)
Phosphorous occurs in domestic sewage as organic phosphorous derived from food. The average strength of phosphorous in crude domestic sewage is between 3 mg/l to 22 mg/l.
Please Note - The above figures are for the U.K. and may vary throughout the world.
For further impartial advice on any problems associated with all types of sewage and effluent treatment contact Johnston Smith Consulting - The Independent Specialists.