December 2, 2016 10:25 pm
Turbidity is the haziness of a fluid caused by large numbers of particles that are generally invisible to the naked eye.
Definition
It is based on comparison of the intensity of light scattered by the sample under defined conditions with the intensity of light scattered by a standard reference suspension under the same conditions. The higher the intensity of scattered light, the higher is the turbidity. Nephelometric turbidity units (NTU).
Range
| Drinking Water Standards of BIS (IS: 10500: 2012) | |||
| Parameters | Required (Acceptable Limit) | Permissible Limit in Absence of Alternate Source | |
| Turbidity (NTU), Max | 1 | 5 | |
Interferences
Coloured solutes cause low turbidity values.
Sample Handling and Preservation
Preservation of sample is not practical. Analysis should begin as soon as possible. Refrigeration or chilling to 4°C is recommended to minimize microbiological reaction.
Procedure
Turbidimeter Calibration—Follow the manufacturer’s operating instructions. Measure the standards on turbidimeter covering the range of interest. If the instrument is already calibrated in standard turbidity units, this procedure will check the accuracy of calibration.
Turbidity Less Than 40 Units—Shake the sample to disperse the solids. Wait until air bubbles disappear. Pour sample into turbidimeter tube and read turbidity directly from the instrument scale or from calibration curve.
Turbidity Greater Than 40 Units—In case turbidity values are greater than 40 units, dilute the sample with turbidity-free water to bring the values within range. Take readings of diluted sample. Compute the turbidity of the original sample from the turbidity of the diluted sample and the dilution factor.
Calculation
Environmental Significance
Aesthetic
Consumers of public water supplies expect and have a right to demand turbidity-free water. Laymen are aware that domestic wastewater is highly turbid. Any turbidity in the drinking water is automatically associated with possible wastewater pollution and the health hazards occasioned by it. This fear has sound basis historically, as anyone knows who is familiar with the water-borne epidemics that formerly plagued the water works industry.
Filterability
Filtration of water is rendered more difficult and costly when turbidity increases. The use of slow sand filters has become impractical in most areas because high turbidity shortens filter runs and increases cleaning costs. Satisfactory operation of rapid sand filters depends upon effective removal of turbidity by chemical coagulation before the water is admitted to the filters.
Disinfection
Disinfection of public water supplies is usually accomplished by means of chlorine or ozone. To be effective, there must be contact between the agent and the organisms that the disinfectant is to kill.
In turbid waters, most of the harmful organisms are exposed to the action of the disinfectant. However in cases in which turbidity is caused by sewage solids, many of the pathogenic organisms may be encased in the particles and protected from the disinfectant. For this and aesthetic reasons the U.S Environmental Protection Agency has placed a limit of 1 unit of turbidity as the maximum amount allowable in public water supplies.
Domestic and Industrial Waste Treatment
The suspended solids determination is usually employed in waste treatment plants to determine the effectiveness of suspended solids removal. The determination is slow and time consuming and in plants employing chemical treatment, changes in chemical dosages have to be made rather frequently. Turbidity measurements can be used to advantage, because of the speed with which they can be made, to gain the necessary information. By their use, chemical dosages can be adjusted to use the minimum amount of chemical while producing a high quality effluent.
Conclusion
The turbidity of sample is the reduction of transparency due to the presence of particulate matter such as clay or silt, finely divided organic matter, plankton or other microscopic organisms. These cause light to be scattered and absorbed rather than transmitted in straight lines through the sample. The method is applicable to drinking, surface and saline waters in the range of turbidity 0-40 NTU. Higher values may be obtained by dilution of the sample. The values are expressed in nephelometric turbidity units (NTU).
Disclaimer
The details given are for general understanding. References should be taken from respective authorized agencies.