Q. 1 The ultimate BOD (L_{0}) a wastewater sample is estimated as 87% of COD. The COD of this wastewater is 300 mg/L. Considering first order BOD reaction rate constant k (use natural log) = 0.23 per day and temperature coefficient θ = 1.047, the BOD value (in mg/L, up to one decimal place) after three days of incubation at 2 7 °C for the w astew ater will b e _______ [2018 : 2 Marks, SetI]
Ans: 160.226 mg/l
Ultimate BOD = 0.87
COD = 0.87 x 300 = 261 mg/l
For municipal sewage, at standard temperature, value of k((base e) = 0.23 per day. Thus, value 0.23 per day given is w.r.t. the standard temperature of 20°C.
= 0.23 (1,047)^{2720}
= 0.317 days^{1}
BOD_{3} = 261 (1  e^{0.317 x 3})
= 160.226 mg/l
Q. 2 For a given water sample, the ratio between BOD_{5day}, 2o°c and the ultimate BOD is 0.68. The value of the reaction rate constant k (on base e) (in day^{1}, up to two decimal places ) is [2017 : 2 Marks, SetII]
Ans: 0.23 day^{1}
Q. 3: Two wastewater streams A and B, having an indentical ultimate BOD are getting mixed to form the stream C. The temperature of the stream /A is 20°C and the temperature of the stream C is 10°C. It is given that
The 5day BOD (in mg/l, up to one decimal place) of the stream C, calculated at 10°C , is [2017 : 2 Marks, SetI]
Ans: 21.21mg/l
For stream A,
For stream C,
Q. 4 For a wastewater sample, the threeday biochemical oxygen demand at incubation temperature of is estimated as 200 mg/l. Taking the value of the first order BOD reaction rate constant as 0.22 day^{}^{1}, the fiveday BOD (expressed in mg/l) of the wastewater at incubation temperature of would be______ . [2016 : 2 Marks, SetIl]
Ans: 276.19 mg/l
From equation (i) and (ii), we get,
Q. 5 : The 2day and 4day BOD values of a sewage sample are 100 mg/l and 155 mg/l, respectively. The value of BOD rate constant (expressed in per day) is________ . [2016 : 2 Marks, SetI]
Ans: 0.299
1.55  1,55x = 1  x^{2}
x^{2}  1.55x + 0.55= 0
Q. 6: Ultimate BOD of a river water sample is 20 mg/L. BOD rate constant (natural log) is 0.15 day^{1}. The respective values of BOD (in %) exerted and remaining after 7 days are: [2015 : 2 Marks, SetII]
(a) 45 and 55
(b) 55 and 45
(c) 65 and 35
(d) 75 and 25
Ans: (c)
BOD remaining after 7 days
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