Determining total, volatile, settleable solids

All the solids are determined by weighing the residue after evaporation, filtration, settling or ignition.

Total solids (TS)

A well-mixed sample of 100 mL is poured into a clean, dried, pre-weighed evaporating dish (weight $W_1$). It is evaporated on a steam bath and dried in an oven at 103-105°C for 1 hour, cooled in a desiccator and weighed ($W_2$).

$$TS\ (\text{mg/L})=\frac{(W_2-W_1)\times10^6}{\text{volume of sample (mL)}}$$

Total fixed and volatile solids

The same dish with the residue is ignited in a muffle furnace at 550 ± 50°C for 15-20 min, cooled and weighed ($W_3$). Organic matter burns away.

$$\text{Volatile solids}=\frac{(W_2-W_3)\times10^6}{V},\qquad \text{Fixed solids}=\frac{(W_3-W_1)\times10^6}{V}$$

so $TS = \text{fixed} + \text{volatile}$.

Suspended and dissolved solids

A measured volume of sample is filtered through a weighed glass-fibre filter paper (Whatman GF/C). The filter with the residue is dried at 103-105°C and weighed. The weight gain gives the suspended solids (SS). Then dissolved solids = total solids − suspended solids.

Settleable solids

One litre of well-mixed sample is poured into an Imhoff cone and allowed to stand for 1 hour (after 45 minutes the sides of the cone are gently scraped or rotated to release the trapped matter). The volume of the solids settled at the bottom is read in mL/L. Settleable solids can also be found by weight:

$$\text{Settleable solids}=SS_{\text{raw sample}}-SS_{\text{supernatant after 1 h settling}}$$

Non-settleable solids

These are the solids which do not settle in 1 hour (colloids and very fine suspended matter):

$$\text{Non-settleable solids}=\text{Suspended solids}-\text{Settleable solids}$$

(or equal to the suspended solids in the supernatant after settling; some books also include dissolved solids in this term.)

 Sample
   |
   +-- evaporate 103 C ---> Total solids (TS)
   |        '-- ignite 550 C -> Volatile / Fixed
   +-- filter ---> Suspended (SS) / Dissolved (DS)
   '-- Imhoff cone 1 h --> Settleable / Non-settleable