<P> Chlorine in water is over three times more effective as a disinfectant against Escherichia coli than an equivalent concentration of bromine, and over six times more effective than an equivalent concentration of iodine . </P> <P> Water treated by filtration and home filtration may not need further disinfection; a very high proportion of pathogens are removed by materials in the filter bed . Filtered water must be used soon after it is filtered, as the low amount of remaining microbes may proliferate over time . In general, these home filters remove over 90% of the chlorine available to a glass of treated water . These filters must be periodically replaced otherwise the bacterial content of the water may actually increase due to the growth of bacteria within the filter unit . </P> <P> UV disinfection is gaining popularity . UV treatment leaves minimal residue in the water . In water UV generates ozone in situ and thus has many of the advantages of ozone disinfection . However, ultraviolet germicidal irradiation alone (as well as chlorination alone) will not remove toxins from bacteria, pesticides, heavy metals, etc. from water . </P> <P> Like UV, ionizing radiation (X-rays, gamma rays, and electron beams) has been used to sterilize water . </P>

When did chlorine start being used in pools