Turbidity is the measure of a liquids clarity or its ability to transmit light. We use the measurement every day, but probably think little about it. When we pour a glass of water, before taking a drink, we often look into the cup, or through the glass to assure ourselves the water is clear, and thus at a very basic level, good to drink.
Cloudiness in potable water is a sign of heightened sediment content, which can be a sign that something isn’t quite right in the filtration or delivery system. As a general rule, water systems chlorinate or disinfect the water they supply to the system. This disinfection is a means of assuring the water posses no health concerns to the public.
Modern turbidity instrumentation is used in surface water treatment plants to assure a continuous supply of safe, clean water is being produced. It can be used as a parameter for automating the feed of coagulants and flocculants to a water plant as well as meeting regulatory requirements that water plants must provide a safe water supply at all times.
Just as clear water is important to consumers for drinking purposes, it is important to the environment into which wastewater is discharged. This environment is often a wetland, river, stream, or lake. An excessive level of turbidity can mean that wastewater effluent loads a stream with sediment and places a heightened demand on oxygen in the stream. This increase demand for oxygen to break down the sediment means there is less available for aquatic species present in the environment.
Wastewater is almost always disinfected before it leaves a treatment plant. Historically this was done through a chlorination/dechlorination process where chlorine was added to the waste water, allowed to react with bacteria for a set period of time and then the residual chlorine was destroyed most commonly through addition of sulfur dioxide. A chemical free way of disinfecting is becoming much more common, and that is the use of ultraviolet light (UV). UV treatment carries no residual, so no further treatment of the wastewater stream is required. For UV treatment to be effective however, the water stream must be able to transmit light, and thus the turbidity of the wastewater is a critical factor in attaining an effective bacteriological removal.