Other than gases, all contaminants of water contribute to the solids content. Solids typically include inorganic matter such as silt, sand, gravel, and clay, and organic matter such as plant fibres and microorganisms from natural and man made sources. Classified by their size and state, chemical characteristics, and size distribution, solids can be dispersed in water in both suspended and dissolved forms. In regards to size, solids in wastewater can be classified as suspended, settleable, colloidal, or dissolved. They are also characterised as being volatile or non-volatile.

Photo 1: Separated solids (source: Stadtentwässerung Göttingen)

There are different analytical procedures (see figure 3) for analysing solids in wastewater such as settling, filtration, and evaporation; because of their different particle sizes (see figure 2).

Figure 2: Particle sizes and their scanning method (Dunn, 2003)

Total solids (TS) in wastewater is the amount of all solids, which are determined by drying a known volume of the sample in a preweighed crucible dish at 105 °C. After cooling in an exsiccator, the crucible dish is again weighed. TS is determined by using the following formula:

TS = (M₁–M₂)/V

with

M₁ : mass of crucible dish after drying at 105 °C (mg)

M₂ : mass of initial crucible dish (mg)

V : Volume of sample (L))

Volatile solids (VS) are the amount of solid that volatilises when heated at 550 °C. This is a useful estimation for organic matter present in wastewater and is determined by burning the total solid at 550°C for about 2 hours in a muffle furnace. After cooling in an exsiccator to room temperature, it is weighed. VS is determined by using the following formula:

VS = (M₁–M₃)/V

with

M₁ : mass of crucible dish after drying at 105 °C (mg)

M₃ : Mass of crucible dish after ignition at 550 °C (mg))

V : Volume of sample (L))

It can be divided in a suspended and a filterable fraction.

Fixed solids (FS) are the amount of solid that does not volatilise at 550 °C. This measure is used to gauge the amount of mineral matter in wastewater. It is the difference between TS and VS. It can be divided in a suspended and a filterable fraction.

Suspended solids (SS) are the solids retaining in a filter and is usually determined by filtration using glass fibre filters. In all analytical procedures for determination of suspended solids, weighed filters are used for sample filtration, the filters are dried at about 105°C after filtration, cooled in an exsiccator to room temperature and the weight of the loaded filter is determined. SS is determined by using the following formula:

SS= (M₄–M₅)/V

with

M₄ : mass of filter after drying at 105 °C (mg)

M₅ : mass of initial filter (mg)

V : Volume of sample (L))

Volatile suspended solids (VSS) are, as indicated in figure 3, one portion of SS which are defined as that part of SS which can be removed by heating the solids at 550°C in a muffle furnace. The suspended solids is burned at 550°C for 2 hours in a muffle furnace and weighed after cooling in an exsiccator to room temperature. VSS is determined by using the following formula:

VSS=(M₄–M₆)/V

M₄ : mass of filter after drying at 105 °C (mg)

M₆ : mass of filter after ignition at 550 °C (mg)

V : Volume of sample (L))

Fixed suspended solids (FSS) are the solid that are unburnable at 550 °C and is determined by subtracting VSS from SS.

Dissolve solids (DS) or filterable solids can be determined by subtracting SS from TS. The solids passing through the filter consist of colloidal and dissolved solids.

Settable solids are those solids that will settle to the bottom of an Imhoff cone (a cone-shaped container) in one hour and determined by allowing a wastewater sample to stand for one hour in an Imhoff cone which enables to read the volume of the settled solids. It is expressed as mL/L and is important, because it is related to the efficiency of sedimentation tanks.

Figure 3: Interrelationships of solids found in water and wastewater