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Where Does Wastewater Come From


Where does wastewater come? This basic question shall be answered if we want to design a wastewater treatment system. Source of wastewater, its characteristics, and its flow rate, will influence the type of technology to treat the wastewater.

Components that make up the wastewater depend on the type of collection systems. Urban wastewater that contribute to a wastewater treatment is originated from the following three main resources:

  • Domestic (also called sanitary) wastewater. This type of wastewater is discharged from residential districts, commercial districts, institutional facilities, or recreational facilities.
  • Industrial wastewater. Wastewater which predominantly consists of industrial waste.
  • Infiltration/inflow.
    • Infiltration is water entering a sewer system including service connections, defective pipes, pipe joints or manhole walls.
    • Inflow is the water discharged into a sewer system from roof leader, cellar, yard and area drains, foundation drains, cooling water discharges, drains from springs and swampy area, manhole covers, cross connections from storm and combined sewers.

Where does wastewater come from
Where does wastewater come from

Domestic Wastewater

Main source of domestic wastewater are residential and commercial districts. Other important sources include institutional and recreational facilities.

Average wastewater from residential districts is usually expressed in liter per person per day. This is applicable for small residential districts. For large residential districts, it is advisable to develop flow rates on the basis of land use areas and anticipated population densities. If actual data are not present, about 70% of domestic-water withdrawal can be taken.

Average flow rate of wastewater from residential resources
Average flow rate of wastewater from residential resources

Several example of commercial districts are airport, bar, office, restaurant, shopping center, etc. Flows of commercial wastewater are generally expressed in cubic meters per hectare per day (m3/(ha.day)) and based on existing or anticipated future development or comparable data from other areas. Unit may vary from 14 to more than 1500 m3/(ha.day).

Average flow rate of wastewater from commercial resources
Average flow rate of wastewater from commercial resources

To determine wastewater flows in institutional facilities, the actual record of institutions are the best sources of flow data. However, if the records are not available, the flows from institutional facilities can be estimated by using the following table.

Average flow rate of wastewater from institutional resources
Average flow rate of wastewater from institutional resources

Wastewater flows for recreational facilities are highly seasonal.

Average flow rate of wastewater from recreational resources
Average flow rate of wastewater from recreational resources

Industrial Wastewater

Industrial wastewater flows with the type and size of the industry, the degree of water reuse, and the on-site wastewater treatment methods used, if any. A typical design value for estimating wastewater flows from industrial districts that have no wet-process industries is about 30 m3/(ha.day). If the water requirements for the industries are known, wastewater flow projections can be based on water-flow projections. For industries without internal reuse programs, about 85% to 95% of the water used in various operations and processes will probably become wastewater. For large industries with internal-water-reuse programs, separate estimates must be made. Average domestic (sanitary) wastewater contributed from industrial activities may vary from 30 to 95 L/(capita.day).

Infiltration

Infiltration/inflow is a variable part of wastewater flow, depending on the quality of the material and workmanship in the sewers and building connections, the character of the maintenance, and the elevation of the groundwater compared with that of the sewers.

Inflow Into Sewers

Inflow is usually subdivided into two categories.

First category includes cellar and foundation drainage, cooling-water discharges, and drainage from springs and swampy areas. This type of inflow causes a steady flow and so is included in the measured infiltration.

The second category consists of inflow that is related directly to storm-water runoff and, as a result of rainfall, causes an almost immediate increase in flows in sewers. Possible sources are roof leaders, yard and area way drains, manhole covers, cross connections from storm drains and catch basins, and combined sewers.

Manhole is one of source of inflow wastewater
Manhole is one of source of inflow wastewater

References:

  • Marcos von Sperling, 2007. Wastewater Characteristics, Treatment and Disposal. Iwa Publishing, New Delhi
  • Metcalf and Eddy, 1981. Wastewater Engineering: Collection and pumping of Wastewater. McGraw Hill Inc., New York.

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