CIVE 450 Homework Problems, Fall 2015

CIVE 450 HOMEWORK PROBLEMS SET 2 COAGULATION / FLOCCULATION FILTRATION DISINFECTION 1. A flocculator paddle of the design and dimensions shown below is rotated through water at 20oC. with an angular speed of 4.0 r/min. (a) How much power is dissipated into the water? (b) If the tank in which this paddle is rotating has the dimensions of 4 x 4 x 4 m and the flow through the tank is 5000 m3/d, determine the Gt value for the flocculator. Assume density of water= 998.2 and the viscosity = 1.002 x 10-3N.s/m2 at 20oC. 2. An average flow in a treatment plant is 15,000 cubic meters / day. It is desired to design a flocculation unit with 30 min hydraulic residence time and a mean velocity gradient of 45/sec. Motor drive efficiency of 60% is expected. Estimate the required motor power in watts. Assume a vicosity of 0.001 kg/m.sec. 3. A filter plant is to be constructed to process 75,700 m3/d. Pilot-plant analysis on mixed media indicated that a maximum filtration rate of 15 m/h will be acceptable. Assuming a surface configuration of approximately 5 x 8 m, how many filter units will be required? Allow one unit out of service for backwashing. The backwash velocity required to expand the filter is 36 m/h. Each backwash period requires 30 min during which the back wash flow is for 10 min, and the water is wasted for the first 10 min of each filter run. Determine the net production of each filter if it is backwashed once per day. 4. Assuming that anthracite (Sp.Gr.1.5) is to be added at a later stage to the sand medium to produce more effective treatment, what should be the size of the anthracite particle in order to get good stratification in the bed. (choose size to have a weight less by 5% of that of sand). Use the following data for calculation: - Sand has the following characteristics: S.G. 2.6 - effective size 0.5 mm, uniformity coefficient smallest size 0.3 mm. - Depth of filter medium is 600 mm - Viscosity of the water is 1.038 centipoise - Density of water is 1.00 - Porosity of sand is 0.45 5. The required dosage to disinfect a water supply system at break point chlorination was found to be 8.6 mg/l leaving a residual of 0.4 mg/l . If it is desired to disinfect 32,000 cu.m./day of water that is supplied in 18 hrs. of pumping per day and maintain a residual free chlorine of 2 mg/l; a. What should be the dosage of chlorine to be applied. b. Draw an approximate curve for the chlorine break point. c. What should be the chlorine feed rate , Kgs/day. d. What is the chlorine quantity needed for two weeks operation. e. What is the difference between plain chlorination and post chlorination. 6. The results of a chlorine demand test on a raw water at 20oC are given in the following table: Sample No. Chlorine Dosage Residual Chlorine mg/l mg/l 1 0.2 0.19 2 0.4 0.37 3 0.6 0.51 4 0.8 0.50 5 1.0 0.20 6 1.2 0.40 7 1.4 0.60 8 1.6 0.80 a. Draw the chlorine demand curve. b. Determine the breakpoint chlorine dosage. c. Determine the chlorine demand at a dosage of 1.4 mg/l. d. If you need to chlorinate 100,000 m3/day and have a residual free chlorine of 0.2 mg/l, what should be the dosage required and how many kgs of chlorine do you need to maintain an operation for two weeks. What would be the combined residual chlorine at this dosage. e. If the supply of the total quantity of water is to be performed in 16 hours, at what daily rate would you set the chlorinator dial.