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4.7 Variations in Wastewater Flows 145
for which shape as well as size are governing factors. Low flows edge upward and high
flows move downward. Indeed, wastewater and stormwater flows are quite like stream and
flood flows and hence subject also to analysis by flood-routing procedures.
H. M. Gifft (1945) has evaluated damping effects by the following observational
relationships:

Q max day >Q avg day 5.0P 1>6 (4.23)
Q min day >Q avg day 0.2P 1>6 (4.24)
Q max day >Q min day 25.0P 1>3 (4.25)

Here Q max day , Q avg day , and Q min day are, respectively, the maximum, average, and mini-
mum daily rates of flow of spent water and P is the population in thousands.
The most commonly used ratio is Q max day /Q avg day , which is known as the peaking
factor, PF. In this way the maximum flow is calculated by multiplying the average flow
by the peaking factor:
(4.26)
Q max day PF Q avg day
Other common variable peaking factor (PF) calculation methods include the following:

5.0
Babbitt : PF = 0.2 (4.27)
P
14.0
Harmon: PF = 1.0 + 0.5 (4.28)
4.0 + P
18 + 2P
Ten-States Standards: PF = (4.29)
4 + 2P
2.69
Federov: PF = 0.121 (4.30)
Q avg day
where P is the population, in thousands; and Q avg day is the average daily flow, in L/s.
For population-based peaking factor methods, the peaking factor decreases as the pop-
ulation increases. A larger population means that peak loads from different sources are
likely to occur in a more staggered manner. Thus, the peak loads are likely to be less pro-
nounced compared to the average loading rate. In systems servicing smaller populations,
peak loads from different sources are more likely to coincide, causing more pronounced
differences in average loading rates. The graph in Fig. 4.7 shows how the extreme flow
factor decreases with increasing population when applying the Ten-States Standards.

Figure 4.7 PF Versus 4.0
Contributing Population from 3.5
the Ten-States Standards. 3.0
Peaking factor, PF 2.5

2.0
1.5
1.0
0.5
0.0 1.0 2.0 3.0 4.0 5.0
Contributing population in millions