Domestic waste water: Characteristics of raw sewage

Domestic wastewater (raw sewage) is mainly composed of water (> 99.9%), solid wastes (most of it of organic nature), some inorganic matter and heavy metals , grit , sand and floatable debris.
Wastewater also contains  bacteria. Although non-pathogenic bacteria are essential for the decomposition (treatment) of the organic waste load in the wastewater, some may be pathogenic or disease-causing.  Micro-organisms, such as faecal coliforms and E.Coli, are abundant in sewage and serves as important indicators to identify sewage pollution in the environment.

A typical sewage composition according to WRc (1990) is shown below:

Constituent Concentration (mg/l)

Suspended solids 250 - 400
BOD 300 - 500
Ammoniacal nitrogen 25 - 50
Total phosphorous 15 - 25
Chloride 60 - 100
Fats 100 - 200
Chromium 0.1 - 0.5
Copper 0.2 - 0.5
Lead 0.08 - 0.4
Zinc 0.4 - 0.7
Faecal coliforms 2 - 30 x 106 /100ml

Raw sewage inflow to a treatment works

Concentrations for specific areas will vary, depending on the housing types and the commercial/industrial activities in an area.
WATER RESEARCH CENTRE (WRc) (1990)  Design guide for marine treatment schemes.  Volume I:  Introduction, Volume II:  Environmental design and data collection, Volume III:  Materials, construction and structural design, and Volume IV:  Operations and maintenance and cost functions.  Report No. UM 1009.  Swindon, UK.

Diurnal variations (over 24 hours) of sewage from a specific area does not only relate to flows, but as well to the composition of the effluent. For the optimum operation of a waste water treatment plant, the diurnal flow and load variations must be taken into account.
In certain areas (for example coastal resorts), seasonal variations (flows and loads) have a significant impact on the performance of a treatment works and the subsequent quality of the effluent, due to temporary population increases and activities related to visitors.

Back to top