A sanitary inspection is an on-site inspection of a water supply to ...

Fact Sheet 2.33Turbidity measurementThe import<strong>an</strong>ce of measuring turbidityTurbidity <strong>is</strong> the amount of cloudiness in the water. Th<strong>is</strong> c<strong>an</strong> vary from a river fullof mud <strong>an</strong>d silt where it would be impossible to see through the water (highturbidity), to a spring water which appears to be completely clear (low turbidity).Turbidityc<strong>an</strong> be caused by :• silt, s<strong>an</strong>d <strong>an</strong>d mud ;• bacteria <strong>an</strong>d other germs ;• chemical precipitates.It <strong>is</strong> very import<strong>an</strong>t to measure the turbidity of domestic water supplies, as these suppliesoften undergo some type of water treatment which c<strong>an</strong> be affected by turbidity. Forexample, during the rainy season when mud <strong>an</strong>d silt are washed into rivers <strong>an</strong>d streams,high turbidity c<strong>an</strong> quickly block filters <strong>an</strong>d stop them from workingeffectively. Highturbidity will also fill t<strong>an</strong>ks <strong>an</strong>d pipes with mud <strong>an</strong>d silt, <strong>an</strong>d c<strong>an</strong> damage valves <strong>an</strong>d taps.Where chlorination of water <strong>is</strong> pract<strong>is</strong>ed, even quite low turbidity will prevent thechlorine killing the germs in the water efficiently.Some treatment systems, such as sedimentors, coagulators <strong>an</strong>d gravel prefilters aredesigned to remove turbidity. It <strong>is</strong> import<strong>an</strong>t for operators of both large <strong>an</strong>d smalltreatment systems to know how well these systems are working. Measuring the turbidityof the water before <strong>an</strong>d after each part of the system c<strong>an</strong> tell the operator wheremainten<strong>an</strong>ce or cle<strong>an</strong>ing <strong>is</strong> needed.Measuring turbidityTurbidity c<strong>an</strong> be measured using either <strong>an</strong> electronic turbidity meter or a turbidity tube.Both methods have adv<strong>an</strong>tages <strong>an</strong>d d<strong>is</strong>adv<strong>an</strong>tages, as shown below. Turbidity <strong>is</strong> usuallymeasured in nephelometric turbidity units (NTU) or Jackson turbidity units (JTLJ),depending on the method used for measurement. The two units are roughly equal.

Turbidity meterFact Sheet 2.33There are m<strong>an</strong>y different types of electronic turbidity meter available. Their adv<strong>an</strong>tages<strong>an</strong>d d<strong>is</strong>adv<strong>an</strong>tages are as follows :Adv<strong>an</strong>tages• very accurate, <strong>an</strong>d especially useful for measuring very low turbidities (less th<strong>an</strong> 5TU)D<strong>is</strong>adv<strong>an</strong>tages• high cost• need power supply (mains or battery)• easily damagedIt <strong>is</strong> impossible to give general guidelines on their use here.You should refer tom<strong>an</strong>ufacturers' instructions for use <strong>an</strong>d mainten<strong>an</strong>ce of these meters. Figure 1shows <strong>an</strong> example of <strong>an</strong> electronic turbidity meter.Figure 1. Turbidity meter

Fact Sheet 2.33Turbidity tubeThe adv<strong>an</strong>tages <strong>an</strong>d d<strong>is</strong>adv<strong>an</strong>tages of the turbidity tube are asfollows :Adv<strong>an</strong>tages• simple design• low cost• not easily damagedD<strong>is</strong>adv<strong>an</strong>tages• c<strong>an</strong>not measure very low turbidities (usual minimum <strong>is</strong> 5TU)• less prec<strong>is</strong>eSince the turbidity tube <strong>is</strong> simple to use, instructions are givenbelow. It <strong>is</strong>, however, recommended that you refer to theinstructions provided with the tube that you are using.A turbiditytube <strong>is</strong> shown in Figure 2.Figure 2. A turbidity tubeTo measure the turbidity of a water sample using a turbidity tube :• Where the tube <strong>is</strong> in two parts, push the two parts together; making sure that theyfit squarely.• Take a sample of water from the water source..• Hold the tube in one h<strong>an</strong>d near the bottom <strong>an</strong>d look into the open end with yourhead about 10 to 20 centimetres above the tube, so that you c<strong>an</strong> clearly see theblack circle,

Fact Sheet 2.33• cross or other murk on the bottom of the tube.• Slowly pour the water sample into the tube, waiting for air bubbles to r<strong>is</strong>e ifnecessary, until the mark on the bottom of the tube just d<strong>is</strong>appears.• Stop pouring the water sample into the tube <strong>an</strong>d look at the level of water in thetube. For turbidity tubes which have a turbidity scale marked on the side, read thenumber on the nearest line to the water level. Th<strong>is</strong> <strong>is</strong> the turbidity o f the water. I fthe tube does not have a scale marked, measure the d<strong>is</strong>t<strong>an</strong>ce from the bottonz o fthe tube to the water level with a tape measure <strong>an</strong>d look up or calculate theturbidity o f the water sample using the instructions provided with the tube.• After use, wash the tube in cle<strong>an</strong> water <strong>an</strong>d store the two parts of the tube where they c<strong>an</strong>notbe damaged.Turbidity guidelines for drinking waterFor drinking water supplies, the following guidelines should be taken into consideration :• Drinking water should have a turbidity of 5 NTU/JTU or less. Turbidity of moreth<strong>an</strong> 5 NTU/JTU would be noticed by users <strong>an</strong>d may cause rejection of the supply.• Where water <strong>is</strong> chlorinated, turbidity should be less th<strong>an</strong> 5 NTU/JTU <strong>an</strong>dpreferably less th<strong>an</strong> 1 NTU/JTU for chlorination to be effective.