Q. 1 The wastewater from a city, containing a high concentration of biodegradable organics, is being steadily discharged into a flowing river at a location S. If the rate of aeration of the river water is lower than the rate of degradation of the organics, then the dissolved oxygen of the river water. [2017 : 1 Mark, Set-I]
(a) is lowest at the location S.
(b) is lowest at a point upstream of the location S.
(c) remains constant all along the length of the river.
(d) is lowest at a point downstream of the location S.
Hence, DO is minimum at some point downstream of S.
Q. 2 A waste water stream (flow = 2 m3/s, ultimate BOD = 90 mg/l) is joining a small river (flow = 12m3/s, ultimate BOD = 5 mg/I). Both water streams get mixed up instantaneously. Cross- sectional area of the river is 50 m2. Assuming the de-oxygenation rate constant, K = 0.25/day, the BOD (in mg/l) of the river water, 10 km downstream of the mixing point is [2014 : 2 Marks, Set-Il]
Flow of waste water stream, Qw= 2 m3/sec
Yw = 90 mg/l = 90 gm/m3
Flow of river, Qr = 12 m3/sec
Ultimate BOD of river,
Yr = 0.5 mg/l = 5 gm/m3
BOD of mixture,
Area of river = 50 m2
Flow of river =12 m3/sec