Report No FR0114

S Denny, P Broberg and T Whitmore

June 1990



To determine the efficiency of different disinfectants towards faecal indicator bacteria, heterotrophic plate count bacteria, streptomycetes, bacteriophages and viruses. To determine the effect of the presence of organic matter on the efficiency of the different disinfectants. It is also expected to provide information which will allow more secure disinfection practices to be implemented.


Chlorine has been used as the principal disinfectant for potable water supplies over the past five decades. Current opinion suggests that alternative disinfectants such as chloramine, chlorine dioxide, ozone or uv may provide equivalent or better disinfection of potable waters.


  1. The inactivation rates of "natural" populations of thermotolerant coliforms (ThTC), heterotrophic plate count organisms (HPC), bacteriophage and poliovirus by the disinfectants ozone, chlorine dioxide, chlorine and chloramine were non-linear. An initial rapid kill was followed by a gradual slowing and "tailing off" of the rate. Curves such as this are caused by a number of factors, including different inherent resistance, aggregation and association with particulate materials.

  2. For inactivation of ThTC, the disinfectants ozone, chlorine and chlorine dioxide gave similar rates of kill. Using a 1.0 mg/1 residual concentration, 99.9% kill (representing four orders of magnitude reduction) was achieved between 1 minute 20 seconds and 2 minutes contact period.

  3. Ozone and chlorine dioxide were the most effective of the disinfectants studied for destroying heterotrophic plate count organisms.

  4. The presence of humic acid in water interferes with disinfection by producing a chlorine demand. However, once this demand has been satisfied and a free residual achieved, the rate of inactivation of ThTC and HPC organisms in water containing humic acid was the same as the response of these organisms in a "pure" bore hole water with minimal chlorine demand.

  5. For inactivation of F-specific coliphages, ozone was the most effective of the disinfectants studied, followed by chlorine dioxide, free chlorine and chloramine.

  6. The results obtained in studies on the disinfection of bacteriophages did not support the use of F-specific coliphage as a model for enterovirus inactivation.

  7. The rate of inactivation of poliovirus by a free chlorine residual of 1.0 mg/1 was initially slower than the rate of inactivation of thermotolerant coliforms. However, within 5 minutes contact period 100% of poliovirus and 99.98% of ThTC had been d estroyed.

  8. Both streptomycete spores and mycelia were sensitive to the disinfectants tested. Chlorine and chlorine dioxide were efficient in eliminating both spores and mycelia, whilst chloramine was less effective. Studies using ozone are still in progress.


These will be made when studies are complete.


This is a progress report in connection with the FWR contract Microbial Hazards in Water Supplies (Reference 3.2.2.a) which describes the response of "natural" populations of thermotolerant coliforms (ThTC) and heterotrophic plate count organisms (HPC), to a range of free chlorine, chloramine, chlorine dioxide and ozone residuals. The response of F-specific coliphage, poliovirus and streptomyces to the different disinfectants is also described. The effect of organic matter, in the form of humic acid, on the efficiency of disinfection by free chlorine has also been determined.

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