Past Year Questions: Disposal of Sewage Effluents | Environmental Engineering - Civil Engineering (CE) PDF Download

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.
Ans: (d)

• Before point T, rate of aeration < rate of degradation, so DO continuously decrease from S till T.
• At point T, rate of aeration = Rate of degradation. Flence DO is minimum at T, 
• After T, rate of aeration > Rate of degradation, so DO starts increasing.

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]
 (a) 1.68
(b) 12.63
(c) 15.46
(d) 1.37
Ans: (c)

Flow of waste water stream, Q_w= 2 m³/sec
Ultimate BOD,
Y_w = 90 mg/l = 90 gm/m³ 
Flow of river, Q_r = 12 m³/sec
Ultimate BOD of river,
Y_r = 0.5 mg/l = 5 gm/m³

BOD of mixture,

Area of river = 50 m² 
Flow of river =12 m³/sec