Physical, Chemical, and Biological Treatment Units
Unit processes and design parameters
Physical, chemical, and biological units are selected by the contaminant and the required effluent quality. Preliminary treatment removes grit and debris to protect pumps. Primary clarification removes settleable solids at surface overflow rates (SOR) of roughly 600-1200 gpd/ft2 and weir loadings under 10,000-20,000 gpd/ft. Secondary clarification is usually the hydraulic bottleneck; SOR of 400-800 gpd/ft2 and solids loading rate (SLR, lb/day/ft2) govern design, not detention time alone.
Biological units fall into suspended growth (activated sludge, MBBR, lagoons) and attached growth (trickling filters, RBCs). Nitrification consumes about 7.14 mg/L alkalinity as CaCO3 per mg/L of ammonia nitrogen oxidized and 4.57 mg/L O2 per mg/L NH3-N; denitrification recovers roughly 3.57 mg/L alkalinity per mg/L nitrate reduced. Anaerobic versus anoxic is a common trap: anaerobic means no oxygen and no nitrate; anoxic means no oxygen but nitrate present.
Chemical units: coagulation, flocculation, precipitation (ferric, alum, lime), and disinfection. Chlorine CT is residual concentration times contact time, corrected for baffling and pH-dependent species; UV is dose (mJ/cm2) and depends on transmittance.
Traps: confusing SOR with SLR; using average flow when peak governs; assuming detention time alone ensures settling; ignoring short-circuiting; and forgetting alkalinity consumption in nitrification.
Worked example: a 70-ft-diameter clarifier has area = pi x 35 squared = 3,848 ft2. At 0.9 MGD, SOR = 900,000 / 3,848 = about 234 gpd/ft2. The limiting check is solids loading: 0.9 MGD x 3000 mg/L x 8.34 / 3,848 = about 5.9 lb/day/ft2, within typical limits.