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must ensure a maximum dose to cover this wide variation of UV sensitivities (eg.) 99.9% reduction of E. coli requires a dose of 7,000 microwatts cmq, whereas a similar reduction of protozoa cysts requires a dose of approximately 105,000 microwatts cmq. UV Dose is a product of intensity multiplied by contact time. Intensity   is the amount of UV energy per uni area measured in microwatts per square centimeter. The contact time is the amount of time the solution is exposed to UV in the reactor (measured in seconds). Therefore UV Dose is expressed in microwatt seconds per square centimeter. Survival curves demonstrate the susceptibility of a specific organism to different doses of UV. Effective UV dosage is reduced by a loss of UV in passing through water.

Transmittance is a measure of the amount of UV energy, that will pass through 1 centimeter of water. This is measured using a UV spectrophotometer. The National Sanitation Foundation (NSF) UV Disinfection Committee has suggested that drinking water should have a UV transmittance greater than 75% at a wavelength of 254 nm. Appropriate water treatment devices can be used to reach the required level of transmittance. Water quality influences the effectiveness of all types of disinfections. including UV. The three major concerns affecting UV performance are suspended solids, absorption of UV and coating of quartz sleeves. All reduce the intensity of UV energy reaching the microbes, Suspended solids create shadows that prevent UV energy from reaching microbes. This can be overcome by designing the reactor to create turbulent flow patterns bringing microbes out of shadowed positions, but microbes within particles cannot be reached without adequate intensity to penetrat the particles. Absorption of UV light by various soluble molecules reduces the amount of UV energy available to penetrate through the water layer to reach the nucleic acids, Humic acids, tannins and iron, commonly found in drinking water are examples of UV absorbing compounds.