Q. A newly formed city wants to construct a drinking water treatment plant and distribution system. Design capacity of the treatment plant is 1.75 m3/s, although initial demand is expected to be only 1.25 m3/s. The first customer is expected to receive water after 20 hr of disinfection and the customer at the end of the distribution system is expected to receive the water at a maximum of 3 days after disinfection. Chlorine will be used as the disinfectant and there is a requirement for a 3 log reduction in giardia cysts.
a) Design coagulation, flocculation, sedimentation, rapid sand filtration, and disinfection facilities from the following assumptions.
b) What effect does removing 70-100 sieve fraction have on the head-loss characteristics and on the depth of expanded bed?
c) If customers are neither expected to receive more than 0.60 mg/L nor less than 0.20 mg/L of chlorine (aesthetic requirement), how do you think you would meet this condition?
d) explain the meaning of Ct
e) explain why Ct values are different for different disinfectants (e.g. chlorine vs ozone vs chloramine vs chlorine dioxide)
f) compare the result obtained using the formula for Ct with Ct values from tables
g) comment on the differences and likely reasons for the differences
Make reasonable assumptions as done in the examples if you need. While the complete filter bed should cater for future demand (1.75 m3/s), it should have the capability to handle the current demand (1.25 m3/s) by having a few filters offline. Mainly carry out calculation of head loss of the clean bed, expanded bed, number of beds, width, length etc.
Chlorine decay characteristics usually do not follow first order. However, for the purpose of this exercise assume that chlorine decay can be described by first order decay characteristics. Assume that there is an initial drop of 0.50 mg/L within 15 min followed by slower first order decay; the decay rate is a function of DOC as given by Slow first order decay rate = 0.02 + 0.065*DOC d-1
THM and HAA formation characteristics are given as 40 and 25 µg/mg chlorine reacted respectively. The last customer is located about 3 days from the initial chlorine dosing.
The final chlorine concentration has to be at least 0.2 mg/L. Aesthetic consideration demands the chlorine levels to be maintained below 0.60 mg/L.