Preliminary and Primary Treatment
Preliminary treatment
Preliminary treatment protects downstream equipment. Bar screens (coarse, roughly 1 inch and larger openings; fine screens smaller) capture rags, plastics, and debris; screenings are dewatered and landfilled. Grit removal uses velocity control or aerated/ vortex chambers to settle dense inorganic material - sand, eggshells, coffee grounds - that would abrade pumps and clog digesters. Trap: confusing screenings (large, buoyant, fibrous debris) with grit (dense, inorganic, fast-settling). Comminutors and grinders shred rather than remove, and simply move the problem downstream.
Primary clarification
Primary clarifiers are settling tanks, usually rectangular or circular, with scrapers moving sludge to a hopper and skimmers removing scum. Three design numbers govern performance: surface overflow rate (flow ÷ surface area, gpd/ft²), hydraulic detention time (hours), and weir loading (flow ÷ weir length, gpd/ft). Typical values are 800–1,200 gpd/ft² at average flow, 1.5–2.5 hours detention, and weir loading under 10,000–20,000 gpd/ft. Well-run primary clarification removes about 50–65% of TSS and 25–40% of BOD5.
Traps and troubleshooting
Questions often mix up overflow rate (units of velocity) with weir loading (units of flow per length), or assume primary treatment removes most BOD. Short-circuiting, density currents, and sludge blanket buildup all reduce actual versus theoretical performance. Rising flows hurt twice: overflow rate rises and detention time falls.
Rule of thumb
Double the flow and the overflow rate doubles while detention time halves - expect poorer removal. If effluent TSS rises with normal sludge blanket depth, suspect short-circuiting or a hydraulic overload rather than biology.