Turbidity measurementIntroductionTurbidity is the amount of cloudiness in the water.Turbidity is a principal physicalcharacteristic of water and is an expression of the optical property that causes lightto be scattered and absorbed by particles and molecules rather than transmitted instraight lines through a water sample.This can vary from a river full of mud and silt where it would be impossible to seethrough the water (high turbidity), to a spring water which appears to be completelyclear (low turbidity).Turbidity can be caused by following: silt, sand and mud bacteria and other germs chemical precipitatesIt is very important to measure the turbidity of domestic water supplies, as thesesupplies often undergo some type of water treatment which can be affected byturbidity. For example, during the rainy season when mud and silt are washed intorivers and streams, high turbidity can quickly block filters and stop them fromworkingeffectively. High turbidity will also fill tanks and pipes with mud and silt, andcan damage valves and taps.Where chlorination of water is practised, even quite low turbidity will prevent thechlorine killing the germs in the water efficiently.Some treatment systems, such as sedimentors, coagulators and gravel prefilters aredesigned to remove turbidity. It is important for operators of both large and smalltreatment systems to know how well these systems are working.Measuring the turbidity of the water before and after each part of the system cantell the operator where maintenance or cleaning is needed.Measuring turbidityTurbidity can be measured using either an electronic turbidity meter or a turbidity tube.Turbidity is usually measured in nephelometric turbidity units (NTU) or Jackson turbidity units (JTLJ),depending on the method used for measurement. The two units are roughly equal.
TurbidimetersAn instrument for measuring the turbidity of water or other fluids is called turbiditymeter.An instrument for measuring and comparing the turbidity of liquids by viewing lightthrough them and determining how much light is cut off.Advantages very accurate useful for measuring very low turbidities (less than 5 TU)Disadvantages high cost need power supply (mains or battery) easily damagedTypes of TurbidimetersThere are three common types of turbidimeters employed today.Bench top Bench top units are typically used as stationary laboratory instruments and are not intended to be portable.Portable Bench top and portable turbidimeters are used to analyze grab samples.On-line instruments On-line instruments are typically installed in the field and continuously analyze a sample stream spilt off from a unit process.
Nephelometric principle of turbidity measurementGenerally most of the Portable and Bench type Turbidimeter operates on thenephelometric principle of turbidity measurement.The optical system includes a tungsten-filament lamp, a 90° detector to monitorscattered light and a transmitted light detector. The instrument’s microprocessorcalculates the ratio of the signals from the 90° and transmitted light detectors. Thisratio technique corrects for interferences from color and/or light absorbing materials(such as activated carbon) and compensates for fluctuations in lamp intensity,providing long-term calibration stability.Measuring principles of turbidimeterThe turbidimeter uses following methods to measure turbidity. Surface light scattering method Light scattering/transmission method Light transmission method Integrating sphere method 1) Surface light scattering method The sample is sent to the measurement chamber, and light is radiated to the surface of the liquid being measured. The light scattered from that surface is used to measure the turbidity.
The intensity of the scattered light is in proportion to the concentration of suspended substances in the sample. This relation is used to obtain the turbidity. Since this method requires no window exposed to the sample to be measured, there is no error which would result from a contaminated window.2) Light scattering/transmission method Light is radiated to the sample cell. The scattered light resulting from the suspended substances in the sample and the light which passes through the sample are measured. The difference between the two values is in proportion to the concentration of suspended substances in the sample. Using this relation, the turbidity is obtained. In this method, the ratio between the scattered light and the transmitted light is obtained. Therefore, the effects of fluctuations of the power source, deterioration of the lamp, and the color of the sample are minimized as they offset each other. Thus, stable measurements of the sample can be obtained.
3) Light scattering method In the Light scattering method, light is radiated to the sample cell. The radiated light is attenuated by the suspended substances in the sample. The transmitted light attenuates according to the amount of the suspended substances, and is used to measure the turbidity. 4) Integrating sphere method In the integrating sphere method, parallel beams are radiated to the cell. The light scattered with the suspended substances is focused with the integrating sphere. The ratio between the scattered light and the transmitted light is used to measure the turbidity.Applications of turbidimeters as Analyzers and Monitors Turbidity meters monitor and in some cases analyze the condition of the liquid in the process. The advanced systems available today transmit this data in real time for true real-time automated process control, and offer an immediate capability to react to changing process conditions. Reliable, repeatable turbidity analyzers can then be further utilized to baseline process parameters and system performance across multiple process lines, even across multiple processing facilities. Well-placed turbidity measurement techniques will not only respond to changing process conditions, but can provide useful information to assist in troubleshooting any turbidity related process issues. Turbidity meter applications include turbidity detection as in a solids break- out, or a carry-over of an undesirable constituent, separation and filtration monitoring and control, solids concentration, for dosing or biomass growth, interface between product phases, water quality, wastewater BOD, blending control of cloudifier agents, haze control, concentration, clarity and stability assessments.Precautionary measures Measurement with these units requires strict adherence to the manufacturer’s sampling procedure to reduce errors from dirty glassware, air bubbles in the sample, and particle settling. Power supply should be ensured. Proper lightening should be provided. Light source should neccessitate proper straight beam. Lens should be stationary.