Helminth egg concentration in wastewater: Influence of rainwater

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 i n the sewer system until the next heavy rainfall. The concentration of para site eggs varies according to the water flow rates in the urban system. Hea vy rain cleans both the urban surfaces and the drains. In particular, it re moves particles less than 100 mu m diameter which are laid down in the pipe s. These particles are present in high concentration in storm water wheneve r heavy rain is preceded by days with low rainfall. Variations in parasite concentration are thus dependent upon the precipitation of the previous day s making it difficult to predict the concentrations of parasites in raw was tewater at input to the treatment plant. Many factors may affect parasite c oncentration including the presence of sediment in the sewer system, the in tensity 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 so lids in rainwater. However, they represent a major health hazard in case of overflow of storm water associated with wastewater into the natural enviro nment. (C) 1998 IAWQ Published by Elsevier Science Ltd. All rights reserved .