EFFICIENCY OF UV RADIATION AS A WATER DISINFECTANT
Report No FR0209
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- "Natural" populations of ThTC found in sewage and river water
are more resistant to UV disinfection than pure cultures of
- A UV dose of 25 mWs/cm**2 destroyed 99.99% of poliovirus.
- 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.
V. RESUME OF CONTENTS
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