First flush

[1][2]: 216 The term "first flush effect" refers to rapid changes in water quality (pollutant concentration or load) that occur after early season rains.

Inflow may produce a foul flush effect in sanitary sewers if flows peak during wet weather.

Combined sewers sized for peak runoff events expected once a decade can carry up to 1,000 times the average sanitary flow.

In large combined sewers it may be impossible to attain sanitary sewage velocities generating sufficient turbulence to keep solids suspended during dry weather.

[7] The high pollution load in wastewater at the beginning of a runoff event occurs when increased flow rate erodes accumulated sewer sediment.

[8] Erosion of sediments in sewers can release pollutants in concentrations exceeding levels found in contributing sources.

[6] Pollutant concentration levels are influenced by the age and condition of the collection system and the amount of infiltration/inflow in comparison to the sanitary flow.