Turbidity and Cloudiness of Drinking Water
Cloudy or murky drinking water is unappealing, and for good reason. Many different types of materials can cause water to lose its clarity, such as clay, silt, organic matter, algae, plankton and other microorganisms, and dissolved organics. Although cloudiness is not itself a health concern, it could indicate problems with the water supply that may lead to health risks.
Turbidity is a measurement of how cloudy water is. The higher the turbidity value is, the cloudier (murkier) the water is. Turbidity is usually measured by shining a light through the water and detecting the amount that passes through. Turbidity is measured in nephelometric turbidity units (NTUs).
“In well water, high turbidity may be caused by an incorrectly sized well screen or a drop in the water table, causing fine particles from the aquifer to become mixed in with the water.”
High turbidity levels can be caused by a number of different reasons. River water often develops a muddy brown color after heavy rains because of eroded soil particles carried in the runoff. In well water, high turbidity may be caused by an incorrectly sized well screen or a drop in the water table, causing fine particles from the aquifer to become mixed in with the water. In new wells, turbidity may initially be high, but it will tend to clear up over time.
Health Effects Associated with Turbidity in Drinking Water
“Suspended particles in the water can help protect pathogens from disinfectants in the water distribution system.”
Not only is cloudy water ascetically unappealing, it can pose a health risk by providing food and shelter to microorganisms in the water. The suspended particles in the water can help protect pathogens from disinfectants in the water distribution system, and they can even promote their regrowth after the disinfectants are used up. Because it interferes with disinfection of drinking water, excessive turbidity levels have been associated with gastrointestinal illness. Removal of turbidity in drinking water has been shown to correlate with the removal of protozoa, and it should ideally be eliminated for effective disinfection of drinking water.
Contaminants such as heavy metals, toxic organic compounds and pesticides can become attached to suspended particles, giving them a free ride through the water distribution system. Bacteria, viruses and parasites can also attach themselves to suspended particles in water.
For drinking water, the U.S. Environmental Protection Agency (USEPA) has set a secondary drinking water standard for turbidity of 5 nephelometric turbidity units (NTUs). Secondary drinking water standards are for factors that may cause cosmetic or aesthetic effects rather than health concerns. However, the World Health Organization (WHO) recommends that turbidity should ideally be below 1 NTU.
In Canada, municipal water suppliers will issue a warning if the turbidity is greater than 1 NTU. Children, the elderly, and people with weakened immune systems are advised to boil water before consumption if the turbidity is greater than 1 NTU, and all users are advised to boil water if the turbidity is greater than 5 NTUs. Water intended for drinking, washing fruits and vegetables, making juice or ice and brushing teeth should be boiled for 1 minute before use.
Interesting fact: High turbidity levels in lakes can cause thermal pollution because the suspended particles absorb heat from sunlight.
How to Treat Drinking Water for Turbidity
If a private well is used, treatment should start with making sure that the well screen is properly sized and in good condition. Not only can turbidity cause problems with water quality, but grit can also damage water pumps.
In a household, turbidity in water can be treated using sand filters and cartridge filters. A sand filter consists of a tank with fine sand on top of gravel. The sand effectively removes all but the finest suspended particles. These filters must be backwashed periodically to remove the accumulated sediment from the filter.
Cartridge filters are small enough to be used to treat water coming to a specific tap. They are effective at removing extremely fine particles, but they are designed to be replaced when they become clogged.
Sand filters and cartridge filters are often used in combination, with a sand filter used to remove the bulk of the suspended particles, and a cartridge filter takes care of the remaining fine particles. However, if the turbidity is relatively low, a cartridge filter may be all that is needed.