total solids content = dry matter (DM)

The total solids content includes both the suspended solids and dissolved salts. The total solids content is also used to determine a sludge dry weight (expressed as a %).

dry solids content at 105°C

The total solids content is expressed as a ratio of weights obtained before and after the drying process. The test protocol consists of placing a sludge sample (25 to 100 mL depending on the sludge concentration) in an oven at a temperature of 105 °C until a steady mass is obtained.

If M1 is the weight of the initial (wet) sample and M2 the weight following drying.

dry solids content at 175‑185°C

Drying at 175-185°C compared with drying at 105°C enables the evaluation of the salts content in crystallisation water, the water content related to the hydroxides, and some extremely volatile matters (oils…).

dry solids content at 550 °C and volatile matter (VM)

The residue from the 105°C drying process is heated to 550°C for two hours in a preheated and thermostatically controlled muffle oven. Silica capsules with approximately 10 to 20 g of finely crushed dry sludge are normally used.

The portion of volatile matters that gasifies at 550°C should not be confused with the organic matter content for a number of reasons:

• part of the mineral matter and salts can break down at temperatures between 105° and 550°C;
• some of the OM (especially organocalcium or organometallic complexes) may not be ‘gasified’ at temperatures of 550°C. A higher temperature of 650-700°C is required for those compounds.;
• in a sludge that has been treated with lime the CO₂ produced when the OM are incinerated is fixed by the lime to yield CaCO₃ that only breaks down at temperatures well above 550°C.

Nevertheless, for most sludge the VS is considered as an approximation of the organic matter content. The VS is usually expressed as a % of dry matter or suspended solids.

dry solids at 900°C

VS between 550 and 900°C mainly consists of CO₂ produced by the decomposition of carbonates contained in the sludge.