The accumulated, semiliquid material consisting of suspended, colloidal, and dissolved organic and inorganic matter separated from wastewater during treatment. Sludges are developed as contained pollutants, and contaminants are separated by mechanical, hydraulic, biological, or chemical processes. The various classes of solids that are removed and collected must be disposed of in a safe, nuisance-free manner without hazard to health or the environment. Collection, handling, transporting, and disposal of removed solids are difficult and costly, since they are offensive and putrescible, with 92–99.5% water content. Sewage solids must be treated by thickening, chemical conditioning, mechanical dewatering, thermal action, biological stabilization, or digestion to convert putrescible organic matter to relatively inert end products, remove water, and reduce weight and volume.
Sewage solids are classified as screenings, scum, grit, septage, or sewage sludges. Screenings are large solids, carried by incoming wastewater, that are captured mechanically on screens or racks with openings of various sizes. These protective units remove floating debris, including wood, clothing, cans, rags, paper, rubber and plastic goods, and stringy material that could damage equipment or create problems in plant maintenance and operation.
Scum is defined as the floating fraction of sewage solids, with specific gravity under 1.0, that, under quiescent conditions, rises to the surface of the wastewater. Primary tank skimmings contain oils, fats, soaps, rubber and plastic hygienic products, cigarette filter tips, paper, and similar materials.
Heavy suspended solids consisting of sand, cinders, coffee grounds, seeds, small metal objects, and other generally inorganic particles carried in wastewater inflow are collectively known as grit. The amount of grit varies with type of sewer, season, weather, intensity of runoff, condition of streets and sewers, and use of household garbage disposal units.
Septage consists of partially digested material pumped from on-site sanitary waste-water disposal systems. It contains a mixture of grit, scum, and suspended solids, adding to treatment plant sludge. See also Leaching; Septic tank.
Sludge derives its name from the unit process from which it settles out. Primary sludge, or raw sludge, develops as solids in incoming wastewater settle hydraulically. Raw sludge, containing up to 5% solids by weight, is gray, greasy, viscous, unsightly, contains visible fecal solids and scraps of household wastes, and has a disagreeable odor. Sludge thickening is a process that is used to remove water, increase the concentration of solids, reduce weight and volume, and prepare sludges for further treatment and handling. See also Precipitation (chemistry); Water treatment.
Solids are generally disposed of in landfills, buried, composted, or recycled as soil amendments. See also Air pollution; Hazardous waste; Sewage; Sewage disposal; Sewage treatment.
