1+0.23Qf = 36.84
Q= 155.8 mg-min/LFor a residence time of 15 minutes, the required residual is
Ct = (155.8 mg-min/L)/15 min = 10.38 mg/LThe chlorination system should be designed to provide chlorine residuals over a range
of operating conditions and should include an adequate margin of safety. Therefore, the
dosage required at peak flow will be set at 15 mg/L.
- Determine the required capacity of the chlorinator
The capacity of the chlorinator at peak flow with a dosage of 15 mg/L is calculated using:
Cl 2 (Ib/d) = (Dosage, mg/L)(Avg flow, Mgd)(P.F.)(8.34)where
Cl 2 = Pounds of chlorine required per day (kg/d)
Dosage = Dosage used to obtain coliform reduction
Avg. Flow = Average flow
P.F. = Peaking factor for average flow
8.34 = 8.34 lb-L/Mgal-mg
Using values from above:
Cl 2 (Ib/d) = (15 mg/L)(0.5 Mgd)(2.8)(8.34) = 175 Ib/d (79.5 kg/d)The next largest standard chlorinator size is 200 Ib/d (90.8 kg/d). Therefore, two 200
Ib/d (90.8 kg/d) chlorinators will be used with one serving as a spare. Although the peak
capacity will not be required during most of the day, it must be available to meet chlorine
requirements at peak flow.
- Compute the daily consumption of chlorine
The average daily consumption of chlorine assuming an average dosage of 15 mg/L is
Cl 2 (Ib/d) - (15 mg/L)(0.5 Mgd)(8.34) = 62.5 Ib/d (28.4 kg/d)A typical chlorination flow diagram is shown in Fig. 14. This is a compound loop sys-
tem which means the chlorine dosage is controlled through signals received from both ef-
fluent flow rate and chlorine residual.
SANITARY SEWER SYSTEM DESIGN
What size main sanitary sewer is required for a midwestern city 30-acre (1.21 * 10
5
m
2
)
residential area containing six-story apartment houses if the hydraulic gradient is 0.0035
and the pipe roughness factor « = 0.013? One-third of the area is served by a branch sew-
er. What should the size of this sewer be? If the branch and main sewers must also handle
groundwater infiltration, determine the required sewer size. The sewer is below the nor-
mal groundwater level.