Preliminary and Primary Treatment
What you must know
Preliminary treatment protects downstream equipment: bar screens and comminutors remove large debris; grit chambers remove dense inorganics (sand, grit) to prevent abrasion; equalization tanks dampen flow and strength variations, critical in industrial plants with batch discharges. Flow measurement (Parshall flume, magnetic flowmeter) and pH adjustment often occur here.
Primary treatment removes settleable solids via gravity. Primary clarifiers (sedimentation tanks) achieve roughly 50–70% TSS removal and 25–40% BOD removal depending on detention time, surface overflow rate (SOR), and solids loading. Design parameters: surface overflow rate (m³/m²·d), weir loading rate, and hydraulic detention time. Scum removal and sludge collection (scrapers) matter. Chemical addition (coagulants, flocculants) can enhance removal for industrial streams with colloidal or emulsified contaminants. Dissolved air flotation (DAF) is preferred for low-density solids, oils, and greases.
Traps in questions
- Confusing SOR (flow/area) with solids loading rate (mass/area·time).
- Assuming primary treatment removes dissolved organics - it does not.
- Forgetting that equalization improves both flow and load dampening.
- Mixing up grit removal (dense inorganics) with screenings (large debris).
Worked example / rule of thumb
A clarifier with 500 m² area treating 10,000 m³/d has SOR = 20 m³/m²·d. If typical design SOR is 30–40 m³/m²·d for primary clarification, the tank is conservatively sized. Rule of thumb: primary clarifiers remove about 60% TSS and 35% BOD.