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Sewage flow is estimated as water supply times a return factor of 70-85%, the balance being consumptive use and garden watering. Infiltration through joints and inflow from illicit connections adds 10-30%. Sewers and pumping stations are sized on peak flow using a population-dependent peak factor.
Sewage flow
Dry weather flow = population x supply x return factor / 1000
Peak flow
Peak = average x (1 + 14 / (4 + sqrt(population/1000)))
It dilutes the sewage, reducing treatment efficiency, and consumes hydraulic capacity that could serve real growth. In old networks it can double dry weather flow in wet periods.
No. Arid regions with heavy garden irrigation return as little as 60%, while dense urban areas with little outdoor use can exceed 90%. Local flow monitoring is far better than a generic factor.