Sludge Concentration in Water: Pollutant Classification and Regulatory Standards

Sludge concentration is a key indicator reflecting the content of suspended solids in water bodies, and it holds a specific position in water quality monitoring and wastewater treatment. In essence, sludge concentration is a form of suspended solids concentration. Suspended solids in water refer to solid substances retained on a 0.45 μm filter membrane and dried to constant weight at 103–105°C, serving as an important measure of water pollution. 

In wastewater treatment, sludge concentration is typically represented by mixed liquor suspended solids (MLSS), which refers to the total weight of activated sludge solids per unit volume of mixed liquor in an aeration tank. Its composition includes metabolically active microbial communities, microbial endogenous residue, inert organic matter difficult for bacteria to degrade, and inorganic matter carried in by the influent. 

Therefore, sludge concentration belongs to the category of suspended solids pollutants and is classified as a physical pollution indicator, reflecting the overall content of insoluble solid substances in water. In sanitary classification, physical pollution refers to wastewater altering the physical characteristics of a water body, increasing turbidity and suspended solids; the main measurement indicators include turbidity, color, and suspended solids.

In terms of regulatory standards, suspended solids (SS) are listed as a basic control item for water pollutant discharge from municipal wastewater treatment plants. According to the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002), basic control items comprise 19 conventional pollutants that affect the water environment and can be removed by general treatment processes, with suspended solids being one of them and thus a mandatory control item. 

The standard classifies conventional pollutant values into Grade 1 (Standard A and Standard B), Grade 2, and Grade 3. Specifically, Grade 1 Standard A sets the suspended solids discharge limit at 10 mg/L, Grade 1 Standard B at 20 mg/L, Grade 2 at 30 mg/L, and Grade 3 at 50 mg/L. The applicable grade depends on the environmental function of the receiving surface water, protection objectives, and the treatment processes employed.

Beyond effluent suspended solids, the sludge generated during wastewater treatment is also subject to specific regulations. GB 18918-2002 requires that sludge from municipal wastewater treatment plants undergo dewatering treatment, with the moisture content of dewatered sludge required to be less than 80 percent. 

When sludge is used for agricultural purposes, its pollutant content must comply with the Control Standards of Pollutants in Sludge for Agricultural Use (GB 4284-2018), which specifies pollutant control indicators for sludge from municipal wastewater treatment plants when applied to cultivated land, garden land, and pasture land.