TUTORIAL CHAPTER 1 - author.uthm.edu.my
Transcript of TUTORIAL CHAPTER 1 - author.uthm.edu.my
Streeter-Phelps oxygen sag equation
DO drops rapidly
dissolved oxygen
reaches its minimum value
and river conditions are at
their worst
Beyond the critical
point, the remaining organic
matter in the river has diminished
to the point where oxygen is
being added to the river by
reaeration faster than it is being
withdrawn by decomposition, and
the river begins to recover.
A city discharges 20000 m3/day of sewage into a river whose
rate of flow is 0.7 m3/sec. Determine critical DO & time and
DO deficit profile for 100 km from the following data:
Given: Velocity of mix = 0.25 m/s, kr=0.4/day, kd = 0.23/day
Solution
Q River = 0.7 m3/sec
Q Sewage discharge = 20000/(24x 3600) = 0.231 m3/sec
MASS BALANCE
𝐶 =𝑄𝑢𝐶𝑢 + 𝑄𝑑𝐶𝑑𝑄𝑢 + 𝑄𝑑
BOD of mix
𝐵𝑂𝐷 =(0.7)(3.4) + (0.231)(45)
0.7 + 0.231= 13.71 mg/l
DO of mix
𝐷𝑂 =(0.7)(8.2) + (0.231)(0.2)
0.7 + 0.231= 6.66 mg/l
Temperature of mix
𝑇 =(0.7)(23) + (0.231)(26)
0.7 + 0.231= 23.74 oC
Saturation value of D.O. at 23.74 oC is 8.57 mg/l
Ultimate BOD 𝐵𝑂𝐷𝑡 = 𝐵𝑂𝐷𝑢 (1 − 𝑒−𝑘𝑡)
13.71 = 𝐵𝑂𝐷𝑢 (1 − 𝑒−0.23(5))
𝐵𝑂𝐷𝑢 = 𝐿𝑜 = 20.88 𝑚𝑔/𝐿
Initial DO deficit (D0)
𝐷𝑜 = 8.57 − 6.66 = 1.91 𝑚𝑔/𝐿
Adjustments to
Deoxygenation (kd) and reoxygenation (kr) coefficients at 23.74 oC temperature
𝑘𝑑 = 0.23 × 1.056(23.74−20) = 0.282 𝑘𝑟 = 0.4 × 1.024(23.74−20) = 0.437