Ozone is a strong oxidizing agent which is applied to wastewater as a gas. It is equal to or better than chlorine in destroying organisms and it does not form halogenated organics. However, it does not produce a residual to prevent bacterial growth. It increases coagulation, removes iron and manganese, needs a short contact time, and can be used for taste and odor control. It is costlier than chlorine, and forms nitric oxides and nitric acid which are corrosive. Ozone is chemically unstable as a gas and difficult to transport. Low doses may not inactivate some viruses, spores, and cysts.
Ultraviolet disinfection occurs during application of light with a wavelength of 40-400 nm with the germicidal range being between 200 nm and 300 nm and 260 nm being most lethal. The disinfection occurs when the ultraviolet light penetrates the cell walls of the organisms and alters the DNA. Ultraviolet light does not alter the physical or chemical properties of water, but it is affected by the amount of suspended solids present which can reduce the effectiveness of the treatment. It is necessary to closely monitor the flow rate, level of suspended solids, initial and final coliform count, number of lamps and average output of the ultraviolet lamps, and the average transmissibility of the transmitting system.