Please be neat, state assumptions and reference any sources of information (e.g.
ID: 479688 • Letter: P
Question
Please be neat, state assumptions and reference any sources of information (e.g., Table 6.14 in text). 1. The Local Water utility serves a town of 50,000 residents. The utility blends water from two reservoirs and treats the water with lime-soda softening to remove both hardness and total suspended solids. The process flow diagram is show below. The blended water is split equally among 4 identical treatment trains. Ume-soda softening &sedimentation; Car 110 mg Tofiltration and disinfection Alkalinity 6SmeqL TSS-12.55 mg Reservoir CO2-0.6 meq L Ca 45 mg Li Mga a 25 mg Li Alkalinity 1.8 meq Li TSS a 7.9 m (a) Calculate the values of all of the water quality parameters of the blended water before it enters the treatment processes. (b) Calculate the total daily mass of lime and soda ash required to treat the blended water. (c) Assuming that the lime soda softening process has a TSS removal efficiency of 85%, calculate the total daily mass of solid residuals produced in the softening process. (d) Calculate the additional fractional removal of TSS that must be achieved in the filters for the finished water to meet the Safe Drinking Water Act requirement of TSS s 1 mg L (e) Calculate the size of each softening/sedimentation tank if the residence time of water in the lime- soda softening process must be 1.75 h.Explanation / Answer
a)
Quality parameter in the blend, Q = (Q1 * F1 + Q2 * F2) / (F1 +F2) ... (1)
F1 = 15 ML/day
F2 = 9 ML/day
All water quality parameter are tabulated below using equation (1)
Units
Reservoir A
Reservoir B
Blend
CO2
meq/L
0
0.6
0.225
Ca2+
mg/L
110
45
85.625
Mg2+
mg/L
60
25
46.875
Alkalinity
meq/L
6.5
1.8
4.738
TSS
mg/L
12.5
7.9
10.775
Flow, F
ML/day
15
9
24
b)
Mass of lime (CaO, MW = 56 g/mol) required to treat
= 0.225 * 56 + 85.625 / 40 * 56 + 46.875 / 24 * 56 + 4.738 * 56 mg/L
= 12.6 + 119.875 + 109.375 + 265.3 mg/L
= 507.15 mg/L or kg/ML
Daily Mass of lime required to treat
= 507.15 kg/ML * 24 ML/day
= 12171.6 kg/day
c)
Total TSS = 10.775 mg/L * 24 ML/day
= 258.6 kg/day
Total daily mass of solid residuals produced
= 258.6 * (100 - 85) / 100
= 38.79 kg/day
d)
Safe drinking TSS limit
= 1 mg/L * 24 ML/day
= 24 kg/day
Additional removal required = 38.79 - 24 = 14.79 kg/day
Additional fractional removal = 14.79 / 38.79 = 0.38
e)
Flow rate = Total Volume / Mean residence time
24 ML/day = 4 * Volume of each tank / 1.75 h
1 ML/hr = 4 * Volume of each tank / 1.75 h
Volume of each tank = 0.4375 ML
Units
Reservoir A
Reservoir B
Blend
CO2
meq/L
0
0.6
0.225
Ca2+
mg/L
110
45
85.625
Mg2+
mg/L
60
25
46.875
Alkalinity
meq/L
6.5
1.8
4.738
TSS
mg/L
12.5
7.9
10.775
Flow, F
ML/day
15
9
24