Suspended solids (TSS)

Suspended solids are the undissolved particles carried in wastewater — silt, fibres, organic debris, and process particulates — as distinct from dissolved matter. They are measured by filtering a known volume of effluent and weighing the dried residue retained on the filter (total suspended solids, TSS). The discharge limit varies sharply by disposal mode: 100 mg/L for inland surface water, 600 mg/L for public sewers, 200 mg/L for land irrigation, with marine coastal relaxed further; an additional rule requires that no particle should fail to pass an 850-micron IS sieve for surface-water discharge.

Suspended solids matter because they cause turbidity, smother aquatic habitats, carry adsorbed pollutants (metals and organics ride on particles), and settle as sludge in receiving waters. They are also the easiest pollution indicator to see — visibly dirty effluent is high in suspended solids — which makes TSS the parameter most likely to draw a complaint or an inspector's attention.

For recyclers, suspended solids are often the dominant effluent parameter, especially in plastic washing lines, where dirt, label fragments, fines and degraded polymer particles load the wash water heavily. Pre-processing washing of any contaminated waste, and the solids carried in CBG digestate liquor, are other major sources. Because TSS removal is the foundation of effluent treatment, getting it right is the first step of any ETP.

The control is straightforward and cost-effective: screening, settling (sedimentation), and clarification remove the bulk of suspended solids, often before any biological or chemical treatment, and the recovered solids can sometimes be reused or must be managed as sludge. For plastic recyclers, a well-designed solids-removal and water-recirculation front end both meets the TSS limit and cuts freshwater consumption — and the captured plastic fines may have recovery value. TSS is a routine, easy-to-monitor parameter and usually the first thing checked in effluent.