Rainwater runoff in urban areas removes helminth eggs into the sewer system where they are carried to the treatment plant by the rainwater or remain in the sewer system until the next heavy rainfall. The concentration of parasite eggs varies according to the water flow rates in the urban system. Heavy rain cleans both the urban surfaces and the drains. In particular, it removes particles less than 100 μm diameter which are laid down in the pipes. These particles are present in high concentration in storm water whenever heavy rain is preceded by days with low rainfall. Variations in parasite concentration are thus dependent upon the precipitation of the previous days making it difficult to predict the concentrations of parasites in raw wastewater at input to the treatment plant. Many factors may affect parasite concentration including the presence of sediment in the sewer system, the intensity of rainfall and the amount of rain that has fallen on previous days. Parasites account for only a small proportion of the mass of suspended solids in rainwater. However, they represent a major health hazard in case of overflow of storm water associated with wastewater into the natural environment.

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