Preliminary and Primary Treatment
Purpose of the unit processes
Preliminary processes protect equipment; primary sedimentation removes settleable and floatable solids and reduces the organic load on the biological system.
Screening. Coarse bar screens (openings roughly ¼ in to 2 in) catch rags, wipes, plastics, and wood that foul pumps and plug diffusers. Fine screens (about 6 mm or smaller) remove more material and are common ahead of membranes and fixed-film processes. Comminutors shred solids in the flow instead of removing them; shredded rags still ball up and grit still passes, so removal is generally preferred.
Grit removal. Grit is dense inorganic material (sand, gravel, eggshells, coffee grounds) that abrades pumps and accumulates in channels and digesters. Velocity-controlled channels, aerated chambers, and vortex units use short detention (about 1–3 minutes at peak flow) and a controlled velocity to settle grit while keeping lighter organics in suspension.
Primary sedimentation. Clarifiers are sized by surface overflow rate (roughly 600–1,200 gpd/ft² at average flow), detention time (about 1.5–2.5 h), weir loading, and solids loading rate. Expect roughly 50–65% TSS and 25–40% BOD removal from a well-operated tank.
Traps
Check units and basis: SOR is gpd per square foot and is usually evaluated at average flow, while peak flow governs hydraulics. Do not confuse surface overflow rate with solids loading rate (lb/day/ft²). Short-circuiting from a missing baffle wrecks performance, and sludge left too long goes septic and floats.
Worked example
A clarifier with 2,000 ft² of surface area receives 1.5 MGD. SOR = 1,500,000 gpd ÷ 2,000 ft² = 750 gpd/ft², a normal value. Double the flow and the rate doubles to 1,500 gpd/ft², which is why peak flows are checked separately.