An existing water treatment plant is producing an average flow of 0.5 MGD. Due to several limitations the plant only has to operate from 9:00 AM to 5:00 PM. As a consulting engineer you are hired to evaluate the operation and the unit processes within the plant and make recommendations for improvements. You are asked to consider the following units.
1. The existing flash mixer is about 20 years old and needs to be replaced. What is your recommended horsepower for a new mixer (calculate both winter and summer conditions). Is there adequate detention time in the flash mix basin?
2. The plant uses a two-stage flocculator with two equal compartments. The flocculator motors in the two compartments are 1 and 1/2 horsepower, respectively. Calculate the "G" values of the two stages and comment on their acceptability for the winter condition.
3. Calculate the detention time, horizontal velocity, overflow rate, and weir loading rate of the sedimentation basin. Assume a simple weir across each tank (i.e. weir length is the width of the tank). Comment on the value of each parameter calculated.
4. A filter unit is 15 ft by 30 ft. After filtering 2.5 million gallons in a 24 hour period, the filter is backwashed at a rate of 15 gpm/sq ft for 12 min. Compute the average filtration rate and the quantity and percentage of treated water used in backwashing. Explain the concepts of air scouring and filter to waste related to the filter backwash process.
5. A conventional surface-water plant with coagulation, flocculation, sedimentation, and filtration produces filtered water with turbidity less than 0.3 NTU, pH 7, and temperature of 10oC at design flow of 2.5 MGD. After filtration, the water is chlorinated in a baffled reservoir with t10 value of 56 min.
a) What is the significance of t10 and why is it used in disinfection?
b) What is the required disinfection of the filtered water using free chlorine?
c) If the plant were to use direct filtration without coagulation, flocculation and sedimentation, what would be the required disinfection of the filtered water?