What is the purpose of measuring both VS (volatile solid) and TS (total solid)?
Total solids (TS): The part of the sludge that remain after drying at 105$^{\circ}$C for 20 hours:

$$ TS(\%) = \frac{\textrm{weight dried at } 105^{\circ}C}{\textrm{wet weight} }\times 100 $$

Volatile solids (VS): The part of the sludge that is combusted at 550$^{\circ}$C after 2 hours:

$$ VS(\% \textrm{ of TS}) = \frac{\textrm{dried weight at }105^{\circ}C -
 \textrm{dried weight at }550^{\circ}C}{\textrm{dried weight at }105^{\circ}C}\times100$$