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Solids: A Discussion of Terms


Article written by Wade Delong, Lab Manager, Analytical Products Group, Inc.

Complete article from Edition 25 APG eNewsletter

When the term "Solids" is discussed in terms of water quality two terms readily come to mind, suspended and dissolved, of course "Solids" are not limited to these types. There are fixed, volatile, settleable and total. There are many areas in which the amounts of these types of solids must be monitored, including drinking water, wastewater, industrial discharges and process control. Suspended solids are not desirable in water used for drinking and bathing. Dissolved solids are important to the quality of drinking water because if levels are high, the taste of the water is affected. Solids are an important parameter to monitor in the control of biological and physical treatment processes and for assessing compliance with regulatory agency wastewater effluent limitations.

The term "Total Solids" applies to the material residue remaining in a vessel after evaporation and drying in an oven at 103-105ºC. A well mixed sample is evaporated in a weighed dish and dried to a constant weight in an oven. The increase in weight over that of the empty vessel represents the total solids. The total solids value includes the combination of "Total Suspended Solids" and "Total Dissolved Solids".

The "Total Suspended Solids" portion is the solids retained on a filter of specified pore size as a sample is drawn through the filter after drying at 103-105ºC. A well mixed sample is vacuum filtered through a method specified glass-fiber filter and dried in an oven. The filter and filter support must be prepared according to method specifications. The increase in weight of the filter represents the suspended solids.

The "Total Dissolved Solids" are able to pass through the filter used for the total suspended solids and are left as residue after evaporation and drying at 180ºC. A well mixed sample is vacuum filtered through the same filter used in the total suspended solids procedure. The filtered liquid is then evaporated in a weighed vessel on a steam table and then dried in an oven. The increase in the weight of the vessel represents the dissolved solids. The values for total suspended and total dissolved solids are affected by the selection of filter and the preparation technique of the filtering apparatus.

The "Fixed Solids" is the term applied to the residue of any one of the following "Total Solids", "Total Suspended Solids", or "Total Dissolved Solids" after igniting at 500 ± 50ºC while the weight lost after ignition is the "Volatile Solids". If fixed and volatile solids are being determined from the same samples being used in the determination of total, suspended or dissolved then the drying vessels must be prepared in accordance with instructions for fixed and volatile solids.

The "Settleable Solids" is the term applied to the material settling out of suspension within a defined period of time. Settleable solids analyses are usually performed using the volumetric or Imhoff cone procedure. A well-mixed, one liter volume of sample is poured into the Imhoff cone and allowed to settle for 1hour with a gentle agitation at 45 minutes. The results are read from graduations on the Imhoff cone and expressed in milliliters per liter (ml/L).

Reference Methods by Parameter


Standard Methods


Total Solids

2540 B


Total Suspended Solids

2540 D


Total Dissolved Solids

2540 C


Fixed and Volatile Solids

2540 E


Settleable Solids

2540 F


Possible Sources of Error
The greatest source of potential error with any of the solids methods is obtaining a representative sample and maintaining sample homogeneity during subsampling for analysis. Other significant factors contributing to error tend to be matrix specific; therefore, it is important to know your matrix. For example, sample mixing may be best accomplished with a magnetic stirrer but not if you have magnetic particles in your sample. High levels of suspended solids may clog your filter before you have sufficient volume to accurately determine dissolved solids. If you have a thick layer of suspended material on your filter a crust may form that will inhibit or prevent drying. High levels of dissolved solids may form a mineral or crystalline layer trapping liquid during evaporation that is released by splattering during the drying step at 180ºC resulting in loss of material and contamination of other samples in the same oven. If your matrix contains sulfates some water of crystallization may be retained even at 180ºC. If organic material is present in the sample matrix then you may see a negative error in the volatile solids results caused by the drying step. If bicarbonates are present in your raw sample then a loss of CO 2 will be experienced from the conversion to carbonates during the drying at 103-105ºC or 180ºC.

For tips on sample handling proficiency testing (PT) samples for total suspended and total dissolved solids, see our July 2004 newsletter article "Simplifying Solids".

For further information on solids testing in general see:
Standard Methods for the Examination of Water and Wastewater -method 2540: A Introduction.

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