total solids content = dry matter (DM)

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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.

Formula: total solids contente dry matter - dry solids content

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.

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