1. Total solids, also known as total residue on evaporation, is the residue remaining when a water sample is evaporated to constant weight under specified conditions. It is expressed as the number of milligrams of residue per liter of water. It is the sum of dissolved and suspended solids in water. The drying temperature is the main condition in the determination of total solids. There are usually two drying temperatures to choose from: 103–105°C and 180±2°C. When dried at 103–105°C, the residue retains crystal water and part of the adsorbed water, with very little loss of organic matter, and bicarbonates are converted to carbonates. Because it is difficult to drive off the adsorbed water completely in this temperature range, the time to reach constant weight is longer. When dried at 180±2°C, all adsorbed water evaporates, organic matter volatilizes and escapes, some crystal water may remain, bicarbonates are all converted to carbonates, and some carbonates may decompose into oxides or basic salts; certain chlorides and nitrates may be lost. Because the drying temperature is higher, weighing should be performed rapidly.

2. The sum of all residues in water is called total solids (TS, Total Solids). TS = dissolved solids (DS) + suspended solids (SS).

Dissolved solids (DS): the solids obtained by evaporating the filtrate after a water sample is filtered. (Indicates the content of salts.)

· The salt content of a water body will affect the osmotic pressure of biological cells and the normal growth of organisms.

Volatile solids (VS): the amount that volatilizes when total solids are ignited at 600°C.

Suspended solids will cause waterway siltation.

· Total solids residue (TS) = volatile solids (VS) + fixed solids (FS).

Suspended solids (SS): the solids obtained after the filter residue is dewatered and dried. (Indicates the insoluble solid matter in water.)

· Reflects the organic-component content in the solid residue, and is an important source of organic pollution in water bodies.

Fixed solids (FS): the residue after total solids are ignited.