Total Solids Explained: Why the Drying Temperature Decides the Number You Report

Total solids, also called total residue on evaporation, is what is left in the dish when a water sample is evaporated to constant weight under specified conditions. It is reported as milligrams of residue per litre of water, and it is simply the sum of dissolved solids and suspended solids in the sample. That definition is easy to write down and surprisingly easy to get wrong, because the number depends on how aggressively you dry the residue.

The drying temperature is the variable

Drying temperature is the main condition in the determination of total solids, and there are usually two to choose from: 103-105°C and 180±2°C. They do not give the same answer, and neither is more correct in the abstract, which is why every reported figure has to carry its method with it.

Dried at 103-105°C, the residue retains crystal water and part of the adsorbed water. Little organic matter is lost, and bicarbonates convert to carbonates. Because adsorbed water is hard to drive off completely in this range, reaching constant weight takes a long time. Dried at 180±2°C, all adsorbed water evaporates and organic matter volatilises and escapes; some crystal water may remain, bicarbonates convert fully to carbonates, and some carbonates may decompose into oxides or basic salts. Certain chlorides and nitrates can be lost as well. Because the temperature is higher, weighing has to be done quickly before the residue picks up moisture from the air.

Two ways to cut the same total

Total solids (TS, Total Solids) can be split two different ways, and knowing which split you are looking at matters more than the total itself. By physical state: TS = dissolved solids (DS) + suspended solids (SS). By behaviour on ignition: TS = volatile solids (VS) + fixed solids (FS).

Dissolved solids

Dissolved solids are what you get by evaporating the filtrate after the sample has been filtered. In practice this is a proxy for salt content, and the salt content of a water body affects the osmotic pressure of biological cells and therefore the normal growth of organisms. A high DS reading is a biological problem before it is a scaling problem.

Suspended solids

Suspended solids are what remains after the filter residue is dewatered and dried; they indicate the insoluble solid matter in water. Suspended solids cause waterway siltation, which is why they are regulated separately from the dissolved fraction even though both contribute to the same TS value.

Volatile and fixed solids

Volatile solids are the portion that volatilises when total solids are ignited at 600°C, and fixed solids are the residue left after that ignition. The volatile fraction reflects the organic-component content of the solid residue and is an important source of organic pollution in water bodies. A sample with high TS and low VS is mostly mineral; the same TS with high VS points at organic loading, and the two call for completely different treatment.

Where the figure gets misused

TS is frequently quoted in reuse and reinjection studies as if it were a single contaminant, which leads to bad decisions in both directions. A high TS dominated by dissolved salts will pass through an oil-removal train completely unchanged, and no amount of extra flotation or filtration will touch it, because the salt is in solution. The same TS value dominated by suspended solids may fall by most of its mass across a single well-run separator. Reporting TS alongside its splits turns an ambiguous number into a diagnosis; reporting it alone invites a treatment train built for the wrong fraction.

How the determination is actually run

The procedure is simple enough that its simplicity is the hazard. A well-mixed sample is measured into a weighed dish, evaporated on a water bath or steam bath, dried in the oven at the chosen temperature, cooled in a desiccator and weighed, then returned to the oven and weighed again until two consecutive weighings agree. Constant weight is the whole point: stopping early inflates the result, and at 103-105°C the adsorbed water that refuses to leave is exactly what makes the endpoint slow. Sample volume should be chosen so the residue falls in a range that can be weighed accurately, too little residue and the balance error dominates, too much and the crust traps water underneath it.

Two operational details cause most of the scatter between laboratories. Floating matter and coarse settled material make a representative subsample difficult, so the sample has to be mixed aggressively and taken quickly. And dishes taken from a 180±2°C oven pick up moisture from the air within seconds, which is why the method insists on a desiccator and a fast balance, and why results drift when the laboratory is humid.

Why anyone still measures it

TS is a screening number, not a diagnosis. It will not tell you which salt is present or whether the organic fraction is biodegradable, but it is cheap, fast and comparable across decades of records, which is more than can be said for most individual constituent tests. Used properly, the TS figure plus its two splits tells you whether a water is dominated by salt, by silt or by organic matter, and therefore whether the treatment problem is osmotic, physical or biological. Used improperly, as a single number quoted without a drying temperature, it tells you very little at all.