CIV235
DEPARTMENT OF CIVIL AND STRUCTURAL ENGINEERING
Water and Wastewater Treatment
Autumn Semester 2016 – 2017
Q1
a) Describe the key components of the urban water cycle and use a sketch to show its linkage with the natural aquatic environment. Explain how the urban water cycle can impact on the natural aquatic environment. (6 marks)
b) Explain why significantly different amounts of drinking water are used per person, per day in the USA, the Netherlands and the UK. (6 marks)
c) The BOD of a wastewater water sample was measured 30 days after collection and found to be 520mg/L. If the de-oxygenation rate constant was determined to be 0.170 day-1 estimate the value of BOD5 of this sample? (6 marks)
d) A new wastewater treatment plant is to be constructed. It will discharge 0.3m3/s of wastewater with an ultimate BOD of 320 mg/L and a dissolved oxygen (DO) level of 1.2 mg/L. During the winter the receiving river has an average flow rate of 1.8 m3/s, an average cross-sectional flow velocity of 0.3 m/s and a water temperature of 11°C. In the river, the background ultimate BOD is 5.0 mg/L, the DO level is 6.5 mg/L, and the saturated DO level can be assumed to be 10.2 mg/L. Determine the critical DO level and the distance downstream of the wastewater treatment plant that the critical DO level occurs, if the rate co-efficients for re-aeration and degradation are 0.2 day-1 and 0.15 day-1 , respectively. (12 marks)
e) In the summer the river water temperature rises to 15 °C, the average flow rate reduces by 0.25 m3/s and the average flow velocity to 0.27 m/s. Describe what will happen at the location of the river with the critical DO level. Will the value of the critical DO rise or fall in comparison to the winter conditions? Explain your answers. (4 marks)
Q2
a) Describe the sequence of processes found within a water treatment plant and the goals of each of them. Use a sketch to explain the inter-relationship between the processes. (10 marks)
b) A sample of water has the following concentrations of ions: Ca2+ 35 mg/L, Mg2+ 10 mg/L, Cl- 11 mg/L, SO42- 72 mg/L. Using this information calculate the equivalent CaCO3 concentration in mg/L, and the total hardness of this water sample. Assume the following values for the molecular weight in mg/mmol for the cations: Ca2+ 40.1, Mg2+ 24.3, Cl- 35.5 and SO42- 96.1. (10 marks)
c) The raw water that is used in the Headingley Water Treatment Works is slightly coloured and exhibits high levels of turbidity. Explain how turbidity is measured and what treatment processes are used to reduce this level of turbidity to an acceptable level. (13 marks)
Q3
a) Describe the processes found within a wastewater treatment plant. Use a sketch to explain the relationship between all the processes. In particular, identify the processes that remove soluble BOD and describe how they remove soluble BOD. (12 marks)
b) An aeration tank for a completely mixed aeration process is being sized for a design wastewater flow of 4800 m3/day. The influent BOD5 is 180 mg/L. The desired design effluent BOD5 is 5 mg/L.
Given the data in Table Q3 and the following equations, all symbols have their usual meanings, calculate:
(i) The volume of the aeration tank marks (6 marks)
(ii) The food/micro-organism ratio (5 marks)
(iii) The sludge production rate (5 marks)
(iv) The oxygen requirement (5 marks)
Table Q3. Parameters adopted in the aeration tank design.
|
Design Parameter
|
Wastewater
|
|
Sludge Age θc , days
|
8
|
|
MLVSS, mg/L
|
1500
|
|
Sludge index volume, mg/L
|
90
|
|
Yield coefficient Y mg VSS/mg BOD5
|
0.6
|
|
Decay rate of microbes kd, day-1
|
0.05
|
|
Conversion factor for converting BOD5 to ultimate BODu, f
|
0.68
|