Why examine wastewater; solids procedure

Why examination of wastewater is necessary

1. To know the nature and strength (organic load, solids, nutrients, bacteria) of the wastewater.
2. To design the treatment plant and select the treatment process.
3. To control the operation and check the efficiency of each unit.
4. To decide the degree of treatment needed before disposal to a river or the land, or for reuse.
5. To check the standards (effluent limits) and to prevent water pollution and diseases.
6. To study industrial wastes, infiltration and illegal connections.

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

Dissolved and suspended solids (for completeness)

A filtered sample (glass fibre filter) is dried at 103°C to give suspended solids, and total dissolved solids = TS − suspended solids. Fixed and volatile solids are found in the same way for each fraction.

Typical values for raw domestic sewage: total solids about 700-1000 mg/L, of which about 50% are volatile.