Control of turbidity in drinking-water systems is important for both health and aesthetic reasons. The substances and particles that cause turbidity can be responsible for significant interference with disinfection, can be a source of disease-causing organisms and can shield pathogenic organisms from the disinfection process.

Turbidity is an important indicator of treatment efficiency and the efficiency of filters in particular. A significant relationship has been demonstrated between turbidity increases and the number of Giardia cysts and Cryptosporidium oocysts breaking through filters. Operational Guidelines for turbidity as an indicator of the efficiency of filters in relation to credits for Giardia cysts and Cryptosporidium oocysts removal have been provided in the “Procedure for Disinfection of Drinking Water in Ontario”.

The effect of turbidity on disinfection efficiency, including potential for disinfection by-products, is related to the type and nature of the particles in the water. A raw water supply which is surface water or ground water under direct influence of surface water is likely to contain organic particles that cause turbidity and adversely affect disinfection efficiency. A significant factor in the formation of disinfection by-products is the organic or humic component of turbidity.

Raw water supply which is ground water with very low organic content may contain inorganic-based turbidity, which may not seriously hinder disinfection. For such waters, an Operational Guideline for turbidity is not established. Since ground water quality is inherently stable, any significant variation in turbidity, excluding pump startup, should be investigated and analyzed immediately for the potential of surface water influence and the presence of organic particles.

Inorganic turbidity formed during the disinfection process or post-disinfection treatment processes through oxidation and chemical participation would not likely interfere with disinfection effectiveness. Therefore the most meaningful location for taking a turbidity sample is before the disinfection process and where applicable after filtration.

Turbidity in excess of 5.0 NTU becomes visible to the naked eye and as such a majority of consumers may object to its presence. Therefore, an aesthetic objective of 5.0 NTU has been set for all waters at the point of consumption.