Turbidity

TURBIDITY−an expression of the optical properties of a liquid that causes light rays to be scattered and absorbed rather than transmitted in straight lines through a sample. — ASTM, 2003a (American Society for Testing Materials) Although turbidity is not an inherent property of water, as is temperature or pH (Davies-Colley and Smith, 2001), the recognition of turbidity as an indicator of the environmental health of water bodies has increased over the past decade, resulting in a growing demand for high-quality and objective turbidity measurements. (National Field Manual for the Collection of Water-Quality Data, 2015)

Turbidity is a measure of the amount of dissolved or suspended particles in a given solution. A turbidimeter is used to direct light through a solution and detect the amount of light scattered by that solution. This is compared to a standard that contains Formazin polymer suspended in water. The more light scattered, the higher the turbidity of the solution.

Turbidity in surface waters is often caused by suspended sediment or algae. Turbidity can cause increased costs for drinking water treatment and degraded water quality for wildlife and recreation.

At the NWQL turbidity is a physical characteristic of the quality of a solution, usually of water and water samples. There are several factors affecting the levels of turbidity such as particle size, content, whether its silt or sand and the light source for the measurement including calibration of the sensors used to take the reading. The Standard ISO (International Organization for Standardization) 7027 at 90-degree angle specifies that the quantitative method using optical turbidimeters or nephelometers for the determination of turbidity of water.

A variety of water properties can affect the measurement of turbidity. These include the color of dissolved constituents in the water matrix and particulate materials, particle size, and density. Sensor fouling, such as biological growth or scratches on the optical surface of the instrument, tends to produce a negative bias when light beams are blocked, but can produce a positive bias if scratches increase the scatter of the sensor's light beam. Likewise, bubbles or gases in the water can cause apparent turbidity (positive bias) and might require special sample preparation or handling to eliminate without changing the particle characteristics of the original sample.

To account for the effects of properties of water or interferences on turbidity, many types of instruments have been designed, many with multiple light beams or detectors. For example, although stray light can cause a positive bias in turbidity measurement because of apparent additional reflectance, many newer instruments, particularly those used for dynamic monitoring, are designed to minimize stray light.

of properties of water or interferences on turbidity, many types of instruments have been designed, many with multiple light beams or detectors. For example, although stray light can cause a positive bias in turbidity measurement because of apparent additional reflectance, many newer instruments, particularly those used for dynamic monitoring, are designed to minimize stray light.