Water and Wastewater Treatment:
problem 1 (Please put "X" at the correct answer):
A) In water softening, which of the precipitates will settle out?
a) Ca(OH)2
b) MgCO3
c) Mg(OH)2
B) For particle sizes larger than 1μ in size:
a) Mechanical agitation will enhance particle agglomeration to form large flocs.
b) Brownian motion will help in particle agglomeration.
c) All of the above.
d) None of the above.
C) In non-ideal type II settling:
a) The settling volume is reduced through consolidation.
b) The fluid flows through the interstices of the flocs, thereby controlling the settling rate.
c) The particle sizes increase during sedimentation.
D) During coagulation, the colloidal particles remain un-aggregated because:
a) The electrostatic repulsive forces are too weak at shorter distances between the particles.
b) The van der Waal’s attractive forces are too weak at shorter distances between the particles.
c) None of the above.
E) The aim during water softening is to precipitate calcium and magnesium as calcium carbonate and magnesium hydroxide by:
a) Raising the pH of water to greater than 10.3 for Mg precipitation.
b) Raising the pH of water to greater than 11.0 for Ca precipitation.
c) None of the above.
F) Carbon adsorption breakthrough increases with:
a) An increase in the influent pH.
b) An decrease in the effective particle size of the carbon.
c) A decrease in the concentration of the adsorbate.
G) The most commonly used method for rapid filter cleaning is:
a) Surface wash with fluidization backwash.
b) Up flow wash with complete fluidization.
c) Air scour assisted backwash.
problem 2:
a) Briefly answer the following:
i) Discuss the terms “short circuiting” and “turbulence” in settling tanks under the following headings - causes, effects and design improvements/remedial measures taken to avoid them.
ii) Discuss the mechanism of coagulation in terms of the electrostatic and van der Waal’s forces acting on the colloidal particles.
Draw diagrams to illustrate your answer.
iii) What is aeration used for in drinking water purification? Discuss some methods by which ventilation can be improved to increase gas transfer through the water.
b) A water treatment plant is to process 30,000 m3/d of raw water. A tapered flocculation basin having four flocculators with velocity gradients 40/sec, 30/sec, 20/sec and 10/sec is to be designed with cross-flow paddle units. The basin may be a maximum of 10 m wide and 4 m deep to connect to the settling basin. The optimum Gt value for this system was found to be 3.5 × 104 at 20 °C. Assume that each compartment is square in shape. Determine:
i) Basin dimensions.
ii) Draw the plan and cross sections of the flocculator with dimensions.
iii) Power requirements in each compartment.
problem 3:
a) describe why not all of the organic matter in wastewater is converted to carbon dioxide and water in biological treatment.
b) What is the significance of the BOD in the characterization of wastewater?
c) Why are samples incubated at 20°C in the dark for 5 days in the BOD test?
e) A town of 25,000 inhabitants is to discharge treated domestic sewage to the neighboring stream which has a minimum flow of 0.25 m3/s and a 5-day BOD of 3 mg/L. Water consumption in the town is 175 L/capita.day, and the BOD contribution is 0.06 kg/person.day. If the BOD in the stream below the sewage outfall is not to exceed 4 mg/L, determine the maximum effluent BOD permissible.
problem 4:
a) Describe the purposes of equalization basins, grit chambers, primary sedimentation tanks, and waste stabilization ponds in wastewater treatment.
b) Would you consider a biological or a physical-chemical treatment process to be better for treating the wastewater produced from a potato processing plant? Why?
c) If the sludge concentration in the return sludge line from the secondary clarifier were the same as the MLVSS concentration, would recycle of return sludge increase the sludge age? Why?
d) What is the SVI of mixed liquor with a TSS concentration of 3150 mg/L that settles to a volume of 160 mL in a 1L cylinder?