The solids present in wastewater can be broken down into three general categories:
- Settleable Solids
- Suspended Solids
- Dissolved Solids
Settleable solids are those solids which will settle to the bottom of an Imhoff cone in a given time period. In the lab analysis, the mixed wastewater sample is quickly poured into an Imhoff cone and allowed to stand undisturbed for the desired time, usually after 60 minutes. At the end of the given time period the amount of solid material that has settled to the bottom of the Imhoff cone is measured in mL/Liter.
The settleable solids analysis is normally used to estimate the efficiency of primary clarifiers and other treatment processes by comparing the results obtained from samples collected before and after the treatment unit.
In measuring for settleable solids the samples can be grab or composite. The samples must be chilled or iced at or during collection. There is a 24 hour holding time which for grab samples starts at the time of collection and for composite samples starts at the time the last portion of the composite is collected.
Suspended solids are undissolved material in wastewater which will not pass through a glass fiber filter. In lab analysis, a portion of a well mixed sample is filtered through a glass fiber filter of known weight.
After the filtration process, the filter with the residue is oven-dried until a constant weight, for the filter and residue, is obtained. By subtracting the initial weight of the filter from the weight of the filter plus residue and knowing the volume of sample filtered, it is possible to obtain the concentration of the suspended solids present in the original sample.
The data generated by the suspended solids analyses can be used to:
It is possible to determine the volatile portion of the suspended solids (or that which can be burned) in a sample by "firing" the dried, weighed sample in a muffle furnace at 550°C until a constant weight is obtained. However, the glass fiber filter must be put through the "firing" process before the start of the suspended solids analysis. This will insure that any material on the filter will be "burned" off before and not during the analysis. Many operators feel the volatile suspended solids analysis is a better operational control "tool" than total suspended solids. The volatile suspended solids test gives a more accurate measure of the portion of the sample that is composed of organic material. For suspended solids determinations, the samples can be either grab or composite and should be chilled on ice during collection. There is a 7 day holding time. For grab samples, the holding time starts at the time of collection and for composite samples, starts at the time the last portion of the composite sample is collected.
Dissolved solids are the material contained in wastewater which will pass through a glass fiber filter. After passing the filter a portion of the liquid is measured and placed in a pre-weighted evaporating dish. The sample is then placed on a steam bath and the liquid evaporated. The dish with the solid materials is then placed in an oven and dried to a constant weight. From the weight of the empty dish and the weight of the dish plus dissolved solid residue, it is possible to determine the dissolved solids concentration.
To determine the volatile dissolved solids in a sample, "fire" the dried, weighted sample as mentioned before, in a muffle furnace until a constant weight is obtained. However, the evaporating dish must be put through the "firing" process before the start of the analysis. This will insure that any material in the dish will be "burned" off before and not during the analysis.
Dissolved solids are generally of little or no concern to wastewater treatment plant operators. However, they can become of importance should the receiving waters be sensitive to changes in salinity, as might be the case of a treatment plant or a small stream that is chlorinating and then dechlorinating its effluent.
Total solids is the combined sum of the suspended and the dissolved material in wastewater. It can be calculated by adding the results of the analyses of the suspended solids and dissolved solids tests.
To determine to total solids directly a well mixed portion of sample is measured, put into a clean pre-weighed evaporation dish. The dish and liquid sample are placed in a steam bath with the liquid portion being evaporated. The dish with the remaining residue is over dried to a constant weight. Subtracting the weight of the dish from the weight of the dish plus the solids yields the weight of the solids in the sample. From these results the total solids concentration in mg/L can be calculated.
Total volatile solids can be determined by "firing" the dried, weighed sample in a muffle furnace until a constant weight is obtained. However, the evaporating dish must be first put through the "firing" process before the start of the total solids analysis and then weighed. This will insure that any material on the dish will be "burned" off before and not during the analysis.
The direct total volatile solids analysis is usually performed on sludge solids. The determination is used to estimate digester feeding and withdrawal as well as digester efficiency.