Report No FR0209

June 1991



To determine the efficiency of UV disinfection towards faecal indicator bacteria, heterotrophic plate count bacteria, streptomycetes and viruses. To determine the effect of the presence of colour, suspended solids and turbidity on the efficiency of UV disinfection. To provide information which will allow more secure disinfection practices to be implemented.


Chlorine has been used as the principle disinfectant for potable water supplies over the past five decades. Current opinion suggests that alternative disinfectants such as UV irradiation may provide equivalent or better disinfection of potable waters.


  1. A UV dose of 15 mWs/cm**2 reduced Thermotolerant coliform (ThTC) numbers by 99.99% (four orders of magnitude) in bore hole water inoculated with sewage and rapid sand filtered water from a water treatment works. In slow sand filtered water containing 1 ThTC/100 ml, analysis of 100 1 volumes after UV treatment demonstrated that greater than 99% reduction had been achieved. This level of kill is comparable to chlorination at a dose to give a 1.0 mg/1 free chlorine residual after a 15 minute contact period.

  2. A UV dose of 25-30 mWs/cm**2 reduced the Heterotrophic plate count (HPC) of bore hole water inoculated with sewage and rapid and slow sand filtered water from a water treatment works by 99%. This level of kill is comparable to chlorination at a dose to give a 1.0 mg/1 free chlorine residual after a 30 minute contact time.

  3. The UV doses required to reduce streptomycete spores and mycelial propagules by 99% were 16 mWs/cm**2 and 29 mWs/cm**2 respectively. There is thus a considerable difference between the sensitivities of spores and mycelia towards UV radiation. This level of inactivation is equivalent to a free residual level of chlorine of 1 mg/1 for contact times of 0.6 min and 1.3 min for spores and mycelium respectively. The semi-log inactivation plots revealed that the log survivors were not a simple function of the UV dose for mycelia, but a linear plot was obtained for spores.

  4. A concentration of suspended solids and turbidity of 3.5 mg/1 and 1.2 NTU respectively, found in the rapid sand filtered water (these were the highest figures encountered during the sampling period) did not appear to reduce the efficiency of UV disinfection.

  5. Sewage effluent samples with a suspended solids content of 5-35 mg/1 and a turbidity of 2-16 NTU reduced the level of kill of ThTC and HPC by UV disinfection by 1 order of magnitude.

  6. "Natural" populations of ThTC found in sewage and river water are more resistant to UV disinfection than pure cultures of Escherichia Coli.

  7. A UV dose of 25 mWs/cm**2 destroyed 99.99% of poliovirus.

  8. Colour of up to 100°H does not reduce the bactericidal efficiency of UV disinfection.


The maximum admissible concentration given in the EC directive (Directive 1980) for colour and turbidity in potable water is 20°Hazen and 0.4 NTU respectively. The results of the studies reported here suggest that in terms of colour and turbidity, any water suitable for potable use is suitable for UV disinfection.


This is a progress report in connection with the FWR contract Microbial Hazards in Water Supplies which describes the response of "natural" populations of thermotolerant coliforms (ThTC), heterotrophic plate count organisms (HPC), streptomycetes and poliovirus to a range of UV doses. The response of "natural" populations of ThTC and pure cultures of E. coli have been compared. The effect of colour, turbidity and suspended solids on the efficiency of UV disinfection has been determined.

Copies of the Report are available from FWR, price £25.00 less 20% to FWR Members