MICROBIOLOGICAL HAZARDS IN WATER SUPPLIES
Report No FR0114
S Denny, P Broberg and T Whitmore
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
- 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.
- 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.
- Ozone and chlorine dioxide were the most effective of the
disinfectants studied for destroying heterotrophic plate count
- 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.
- For inactivation of F-specific coliphages, ozone was the most
effective of the disinfectants studied, followed by chlorine
dioxide, free chlorine and chloramine.
- 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.
- 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
- 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.
V RESUME OF CONTENTS
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
Copies of the Report are available from FWR, price £25.00 less 20% to FWR Members