Sources and factors of sanitary sewage

Sanitary sewage is the wastewater from the sanitary fixtures and activities of homes, offices, institutions and (to some extent) industries. It consists of human excreta, bath and kitchen water and washing water.

Potential sources

1. Residential areas: toilets, bathrooms, kitchens, washing; the largest part.
2. Commercial buildings: offices, hotels, restaurants, shops, markets.
3. Institutions and public places: schools, colleges, hospitals, bus parks, temples, public toilets.
4. Industries: sanitary wastes of workers and (after pretreatment) process waste.
5. Infiltration: groundwater entering through defective joints and cracks.
6. Other: illegal roof or yard drain connections and leakage from the water supply.

Factors affecting the quantity

• Population and its growth: more people, more sewage. The sewer is designed for the future population at the end of the design period.
• Rate of water supply: sewage is about 75-80% of the water supplied; the rest is lost in lawn watering, evaporation, leakage and consumption.
• Type of area: residential, commercial or industrial areas produce different flows; commercial and industrial areas have high per-hectare flows.
• Standard of living and habits: in rich households, the use of flush toilets, washing machines and showers increases the flow.
• Private water supplies: water from wells entering the sewer adds to the sewage that is not recorded by the public supply.
• Groundwater infiltration: depends on the water table, sewer material, quality of joints and age; it can be 5,000-15,000 litres per day per km of sewer per cm diameter.
• Unauthorised connections: storm water or other waters connected to the sewer.
• Climate and time variation: hourly, daily and seasonal peaks; summer use is higher than winter use.
• Water meters and tariff: metering reduces wastage, and so reduces sewage.