FIGURE 14. Compound-loop chlorination system flow diagram.
Calculation Procedure:
- Compute the sanitary sewage flow rate
Table 4 shows the typical population per acre for various residential areas and the flow
rate used in sewer design. Using the typical population of 500 persons per acre (4046 m
2
)
given in Table 4, we see the total population of the area served is (30 acres)(500 persons
per acres) = 15,000 persons.
Since this is a midwestern city, the sewer design basis, per capita, used for Des
Moines, Iowa, 200 gal/day (8.76 mL/s), Table 4, appears to be an appropriate value.
Checking with the minimum flow recommended in Table 4, 100 gal/day (4.38 mL/s), we
see the value of 200 gal/day (8.76 mL/s) seems to be well justified. Hence, the sanitary
sewage flow rate that the main sewer must handle is (15,000 persons)(200 gal/day) =
3,000,000 gal/day.
- Convert the flow rate to cfs
Use the relation cfs = 1.55 (gpd/10
6
), where cfs = flow rate, fWs; gpd = flow rate, gal/24
h. So cfs = 1.55(3,000,000/1,000,000) = 465 fWs (0.13 m
3
/s).
- Compute the required size of the main sewer
Size the main sewer on the basis of its flowing full. This is the usual design procedure fol-
lowed by experienced sanitary engineers.
Two methods can be used to size the sewer pipe, (a) Use the chart in Fig. 4, Section
14, for the Manning formula, entering with the flow rate of 4.65 fWs (0.13 m
3
/s) and pro-
jecting to the slope ratio or hydraulic gradient of 0.0035. Read the required pipe diameter
as 18 in (457 mm).
(b) Use the Manning formula and the appropriate conveyance factor from Table 5.
When the conveyance factor Cfis used, the Manning formula becomes Q = CfS
m
, where
Q = flow rate through the pipe, ft^3 /s; Cf= conveyance factor corresponding to a specific n
value listed in Table 5; S = pipe slope or hydraulic gradient, ft/ft. Since Q and S are
known, substitute and solve for C 7 , or Cf= Q/Sm = 4.65/(0.0035)1/2 = 78.5. Enter Table 5
Receiving Water
or Reuse
Chlorine Residual
Analyzer
Control
Signal
Compressed
Gas Storage
Chlorinator
Chlorine Gas
Control Signal
Flow meter Injector Rotometer
Chlorine Solution
Waterwater
Effluent
Chlorine
Contact
Channel
