Sources of sewage and city impact

Sources of sanitary sewage

• Residential buildings (toilets, baths, kitchens, washing), the largest source.
• Commercial and institutional buildings: offices, shops, hotels, restaurants, schools, hospitals.
• Industries: sanitary waste of workers and process waste.
• Public places: public toilets, markets, bus parks.
• Infiltration of groundwater and unauthorised connections.

Impact of time and city development

With time

• Daily: the flow is minimum at night (about 1/3 of average), has a first peak in the morning (7-10 am) and a second peak in the evening; the maximum rate may be 2 to 3 times the average.
• Seasonal: flow is larger in summer (more water use) and in the rainy season (higher infiltration and illegal storm connections).
• Long term: as the population grows over the design period, flow increases, so sewers must be designed for the future population.

With development activities

• Better living standard, piped water, flush toilets, washing machines and hotels increase water use and sewage per head from, say, 60 lpcd to 135 lpcd or more.
• Growth of commercial and industrial areas, hospitals, hotels and institutions add heavy and sometimes harmful flows, and raise the pollution load.
• Urbanisation increases impervious area, so more storm water reaches sewers in a combined system.
• Densification (high-rise buildings) increases flow per hectare.
• New connections and expansion of sewer network raise infiltration if construction quality is poor.