Primary Treatment (Sedimentation, Clarifiers)
Design parameters
Primary clarifiers remove settleable solids by gravity. Performance is described by surface overflow rate (SOR, gpd/ft²), detention time (hours), and weir loading (gpd per foot of weir). Typical SOR for primary clarification is roughly 600–1,200 gpd/ft² at average flow, with detention of about 1.5–2.5 hours. Short-circuiting, wind, and density currents from temperature differences reduce actual performance below theoretical values.
What to watch on the unit
Sludge blanket depth, collector speed, scum box condition, influent distribution, and weir levelness. A rising blanket means sludge is being pulled over the weirs; a blanket left too long turns septic and floats. Waste sludge often enough to prevent septicity but thick enough to remain pumpable - roughly 4–6% solids for primary sludge.
Expected performance
Primary treatment typically removes 50–65% of suspended solids and 25–35% of BOD. Results far from that range point to high flows, short detention, short-circuiting, or an industrial discharge upstream.
Traps
Mixing SOR (flow per area) with hydraulic loading on a trickling filter; using diameter instead of area for a circular unit; forgetting that weir length for a circular clarifier is the circumference, not the diameter.
Worked example
A rectangular clarifier is 60 ft long × 20 ft wide at 2.0 MGD. Area = 1,200 ft², so SOR = 2,000,000/1,200 = 1,667 gpd/ft² - high for primary clarification, so expect solids carryover. With 10 ft side-water depth, volume = 12,000 ft³ = 89,760 gal, and detention = 89,760/2,000,000 × 24 = 1.08 hours, also short.