Turbidity

The American Public Health Association (APHA) defines turbidity as "the optical property of a water sample that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample." In simple terms, turbidity answers the question, "How cloudy is the water?"Light’s ability to pass through water depends on how much suspended material is present. Turbidity may be caused when light is blocked by large amounts of silt, microorganisms, plant fibers, sawdust, wood ashes, chemicals and coal dust. Any substance that makes water cloudy will cause turbidity. The most frequent causes of turbidity in lakes and rivers are plankton and soil erosion from logging, mining, and dredging operations. Measuring Turbidity The most accurate way to determine water’s turbidity is with an electronic turbidimeter . The turbidimeter has a light source and a photoelectric cell that accurately measures the light scattered by suspended particles in a water sample. The results are reported in units called Nephelometric Turbidity Units or NTUs. You also can measure turbidity by filtering a water sample and comparing the filter’s color (how light or dark it is) to a standard turbidity color chart. You’ll need the following equipment to do this: filter apparatus (Gelman or other manufacturer), some white membrane filters and a standard color chart to compare your findings. Your teacher will show you how to operate the filter equipment and will provide a color chart. The procedure for using the Gelman filter apparatus to determine the turbidity of a water sample is as follows: Place a white gridded filter on the filter apparatus. You may handle the filter with your fingers; it’s not necessary to use a sterilized tweezers. Use a plastic cup or bucket to take a water sample from the river, lake or stream. Be sure you scoop only the water, not the sediment on the bottom. Pour 100 milliliters (mL) of your water sample into the top of the filter apparatus. The unit is graduated in milliliters. Just fill it to the 100-mL mark. Filter the sample. You may need to use a hand-operated vacuum pump to pull your sample through the filter. Remove the filter from the machine and let it dry. Estimate the turbidity of your sample by comparing its color to the color chart furnished by your teacher. Refer to the information below for a discussion of what these values mean. Table 9. Turbidity level of water for industrial use Industrial Use Maximum Turbidity Units Beverages 1-2 Food products 10 Water used in boilers 1-20 (varies with type of boiler) Making high grade paper 5-25 Making rayon 1 Making cotton 25 Baking 10 Water used for cooling 50 Ice making 0.5 (same as drinking water) Tanning leather 20 References 2 Drinking Water Standards The APHA specifies drinking water turbidity shall not exceed 0.5 NTUs. However, some scientists think this standard is too generous. They want to see the value reduced to 0.1 NTUs. Turbidity Effects on Fish and Aquatic Life Turbidity effects fish and aquatic life by: Interference with sunlight penetration. Water plants need light for photosynthesis. If suspended particles block out light, photosynthesis—and the production of oxygen for fish and aquatic life—will be reduced. If light levels get too low, photosynthesis may stop altogether and algae will die. It’s important to realize conditions that reduce photosynthesis in plant result in lower oxygen concentrations and large carbon dioxide concentrations. Respiration is the opposite of photosynthesis. Large amounts of suspended matter may clog the gills of fish and shellfish and kill them directly. Suspended particles may provide a place for harmful microorganisms to lodge. Some suspended particles may provide a breeding ground for bacteria. Fish can’t see very well in turbid water and so may have difficulty finding food. On the other hand, turbid water may make it easier for fish to hide from predators. The table below shows the amount of plankton per acre which may be expected in ponds of different turbidities. Table 10. Plankton density as a function of water turbidity Factor measured Clear ponds Intermediate ponds Muddy ponds Average turbidity units: less than 25 25-100 over 100 Amount of fish in pounds per acre: 162 94 29 Comparative amount of plankton caught in nets 12.8 1.6 1 Note that the average amount of plankton in pristine (clear) water is almost 13 times that found in turbid (muddy) water. Turbidity in pristine water apparantly comes from the healthy plankton population itself, an excellent food source for many fish.

References:1. Quality Criteria for Water, U.S. Environmental Protection Agency, July 1976. 2. Water Quality Criteria, California Water Quality Resources Board, Publication No. 3-A, 1963. 3. Water Quality Criteria, Environmental Studies Board, National Academy of Sciences, 1972. 4. Study and Interpretation of the Chemical Characteristics of Natural Water, United States Geological Survey, Water Supply Paper 1473, 1970. 5. Water Pollution Microbiology, Ralph Mitchell ed., Wiley-Interscience, 1972. 6. Quality Criteria for Water, U.S. Environmental Protection Agency, EPA#440/5-86-001, 1986. 7. Ammonia Toxicity Levels and Nitrate Tolerance of Channel Catfish, The Progressive Fish-Culturist, 35: 221, Knepp and Arkin, 1973.