Question

Problem #3: Biochemical Oxygen Demand (BOD) and Stream Quality

A wastewater treatment plant designed for a community of 45,000 people has a flow of 100 gal/person/day and a BOD₅ loading of 0.2 lb/person/day. The upstream characteristics of the receiving waters are:

stream flow rate = 20 cfs and BOD₅ = 2 mg/L

The water quality standards require the in-stream BOD₅ to be less than 7 mg/L in this stretch of the river.

The minimum BOD removal efficiency that the treatment plant should achieve is most nearly:

(a) 96          (b) 87          (c) 84          (d) 91

Problem #4: Wastewater Treatment

A wastewater plant receives a flow of 6 MGD with an influent BOD of 200 mg/L. The plant consists of aerated grit chamber, primary clarifier, activated sludge tank, final clarifier and disinfection through UV.

The primary clarifier removes 60% solids and 35% BOD. The parameters for the activated sludge process are volume is 175,000 ft³, mixed liquor suspended solids concentration is 2500 mg/L, biological yield is 0.6 and decay coefficient is 0.06 /day. Final plant effluent needs to have a BOD of 10 mg/L. the peaking factor is assumed to be 3.

1. What are the dimensions (ft) of the grit chamber if the detention time is 1 min at peak flow and the l:w:d is 1.5:1.5:1 ?

(a) 10.5 x 10.5x7     (b) 13.5 x 13.5 x 9 (c) 12x12x8          (d) 15x15x10

2. What is the depth (closest in ft) of the circular primary clarifier if the overflow rate is 1000 gpd/ft² and the detention time is 2 hours for the average flow ?

(a) 11             (b) 15          (c) 9            (d) 13

Answer 1