When chlorine is added to water that contains some remaining organic matter it is possible for the organic matter to become chlorinated.
Such chlorinated organic compounds tend to accumulate in body fat and can disrupt the operation of the central nervous system or cause fatty degeneration of the liver.
Traces of chlorinated organic matter (and other organic matter) can be removed by passing the water through a bed of activated carbon.
Disinfection with ozone or ultra violet light can avoid the problem of the formation of chlorinated organic compounds.