Can turbidity be measured in place of suspended solids?
Measuring turbidity in place of suspended solids
Although suspended solids will cause turbidity, a turbidity measurement is not the same as a measurement of suspended solids. A suspended solids measurement, as defined by the USEPA, determines the amount of solids in a sample by weight, where a turbidity measurement shows how the suspended solids scatter light. When the particulate makeup of the sample changes, the light scattering characteristics of the sample may change in an unpredictable way. If the particulate makeup of the sample is known to be consistent over time, it may be possible to use turbidity measurements to estimate the level of suspended solids. This would require making a calibration curve of suspended solids (determined gravimetrically) vs. measured turbidity values (NTU) for a series of samples with varying levels of suspended solids. Even in these cases, many samples do not exhibit a linear relationship between ppm suspended solids and turbidity value. This can be caused by interferences such as color, particle shape, distribution, and absorption. For example, a natural sample showing a turbidity of 500 NTU often shows a turbidiy of substantially more than 100 when diluted 5-to-1 with distilled water.