Water System Design Manual
Water quality can significantly influence the effectiveness of UV disinfection. Reductions in
effectiveness can result from direct absorbance of UV radiation by the water as well as the
various constituents in the water, by the shielding of organisms often associated with higher
turbidities, and by the formation of scales (fouling) on lamp sleeves. The following list identifies
several water quality parameters to be considered. Specific recommendations or requirements
regarding these parameters are shown in italics.
Iron and Manganese:
Ferric iron strongly absorbs UV radiation. It can negatively affect
a reactor's ability to inactivate microorganisms by "consuming" the UV before it can be
absorbed by microorganisms. Oxides of iron and manganese can cause scaling on the
quartz sleeve that would reduce the UV irradiance that enters the water column. Iron and
manganese concentrations as low as 0.1 mg/L and 0.02 mg/L can cause significant
fouling of quartz sleeves (Mackey et al. 2001; Chen 2009; Black and Hill 2009). Iron or
manganese exceeding the secondary contaminant levels of 0.3 mg/L, or 0.05 mg/L,
respectively, must be removed prior to UV application.
Hardness greater than 140 mg/L as CaCO
can cause scaling on the quartz
sleeve that would reduce the UV irradiance that enters the water column (Mackey et al.
2001; Black and Hill 2009). Solubility calculations for carbonates of calcium and
magnesium can be conducted to provide a preliminary screening of the likelihood of
precipitation. If it appears possible, then the utility should perform pilot testing using the
same lamp proposed for the full-scale application.
Total Organic Carbon:
Many naturally occurring organic materials in water strongly
absorb UV radiation. Like iron, they can negatively affect a reactor's ability to inactivate
microorganisms. The nature and amount of the specific organic carbon in the water
strongly affects UV disinfection effectiveness. This should become evident through UV
Turbidity is a measure of a solution's ability to scatter light, generally as a
result of particulate matter. Turbidity alone cannot be directly correlated to a predictable
effect on UV system effectiveness, and in some cases, turbidity in excess of 5 NTU has
not resulted in diminished UV inactivation performance (Passatino and Malley 2001).
However, turbidity control to less than 5 NTU must be provided. In groundwater,
turbidity is often a result of iron or manganese precipitation, and removal of these
inorganics may eliminate the turbidity problem.
(of the water): UV transmittance is a measure of water's ability to
transmit ultraviolet radiation, and is a function of the factors identified above, as well as
some water treatment chemicals (Cushing et al. 2001). The UV transmittance of raw
water directly affects the ability of UV light to adequately disinfect raw water. Validation
testing must be performed consistent with the UV transmittance of the water for which
treatment is proposed.