Turbidity is measured by nephelometric turbidity units (NTU) and can be initially
noticed by the naked eye above approximately 4.0 NTU. However, to ensure
effectiveness of disinfection, turbidity should be no more than 1 NTU and preferably
much lower. Large, well-run municipal supplies should be able to achieve less than 0.5
NTU before disinfection at all times and should be able to average 0.2 NTU or less.
Surface water (and groundwater under the infl uence of surface water) treatment systems
that achieve less than 0.3 NTU prior to disinfection will have demonstrated that
they have signifi cant barriers against pathogens that adsorb to particulate matter. Of
particular importance is the fact that this will be a good indicator that they are removing
chlorine-resistant pathogens such as Cryptosporidium.