Biological Treatment, Softening & Disinfection

• Lime softening removes carbonate hardness; soda ash is added for noncarbonate hardness.
• Disinfection is credited by CT: residual concentration (mg/L) × contact time (min).

Worked: an aeration basin

A 4 MGD plant sends primary effluent at 180 mg/L BOD to a 1.2 MG aeration basin holding 2,500 mg/L MLVSS. The BOD load is 4 × 180 × 8.34 = 6,000 lb/day. F/M = (4 × 180)/(1.2 × 2,500) = 0.24 per day, within the conventional range. θ = 1.2/4 = 0.3 day = 7.2 hours.