To predict the outcome of effluent testing for turbidity or total suspended solids (TSS), operators of activated sludge systems will test the supernatant from a thirty minute settling test since the settling test is a simulation of what will happen in a secondary clarifier.
Turbidity in water is caused by suspended and colloidal matter such as organic and inorganic matter including microorganisms. Turbidity is an expression of the amount of light that is scattered or absorbed by a sample rather than transmitted through it. The higher the intensity of scattered light, the higher the turbidity will be. The turbidity analysis will be performed on the supernatant (top layer, usually clear) of the mixed liquor which is saved from the 30 minute settling analysis. It is used to evaluate how well the sample flocculates and whether or not dispersed bacteria are a problem. Turbidity is measured using a nephelometer/turbidimeter. Dirty glassware and the presence of air bubbles can give false turbidity results. Water color, due to dissolved substances that absorb light, causes measured turbidities to be low. Samples should be tested immediately to prevent temperature changes and particle flocculation.
The supernatant total suspended solids (TSS) test is the process of measuring the total amount of suspended material in the supernatant collected after 30 minute settling. In order to test the total suspended solids a well-mixed sample should be filtered through a weighed standard glass-fiber filter. The residue left on the filter is dried to a constant weight at a temperature between 103 °C and 105 °C. The increase in weight of the filter represents the total suspended solids of the sample. Large floating particles or submerged agglomerates of nonhomogenous materials from the sample may be excluded in the total suspended solids measurements if it is determined that their inclusion is not representative of the entire sample.
High total suspended solids (TSS) values in supernatant are often related to excessive solids generation due to an increase in BOD loading. High total suspended solids values can also be attributed to high flows or poor settling.
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