Sanitary Sewer Peaking Factors for Very Low Flows--A Study

By Thomas Gail Haws, P.E.


CONTENTS

SANITARY SEWER COLLECTION SYSTEM DESIGN FLOW PEAKING FACTORS

VERY SMALL FLOWS AND THE UNIFORM PLUMBING CODE

PEAKING FACTORS FOR VERY SMALL FLOWS

SUMMARY

SANITARY SEWER COLLECTION SYSTEM DESIGN FLOW PEAKING FACTORS

INTRODUCTION

Various published sources provide flow peaking factors for sanitary sewer collection system design. These peaking factors are generally multiplied by average daily dry weather sewer flow (often 100 gallons per capita per day) to obtain design (dry weather) peak flows for sewer velocity and capacity design. Many governing agencies establish minimum velocity requirements (often 2.0 ft/s) based on these peak flows. The flow peaking factors given by these sources are generally in the form of tables or equations such as the following:

TABLE 1: PEAKING FACTORS FROM 2000 ARIZONA A.A.S. Title 18

Upstream Population Peaking Factor
100 3.62
200 3.14
300 2.90
400 2.74
500 2.64
600 2.56
700 2.50
800 2.46
900 2.42
1000 2.38
1001 to 10,000 PF = (6.330 x p -0.231) + 1.094
10,001 to 100,000 PF = (6.177 x p -0.233) + 1.128
More than 100,000 PF = (4.500 x p -0.174) + 0.945

PF = Peaking Factor

P = Upstream Population

TABLE 2: PEAKING FACTORS FROM 2000 CITY OF BOULDER COLORADO DESIGN AND CONSTRUCTION STANDARDS, CHAPTER 6

Collection Main Diameter Peaking Factor
10 inches and smaller 4.0
12 to 15 inches 3.5

EQUATION 1: HARMON'S PEAKING FACTOR

PF=1+14/(4+P1/2)

where P= the design contributing population in thousands

EQUATION 2: PEAKING FACTOR FROM 1997 ALBERTA ENVIRONMENTAL PROTECTION STANDARDS AND GUIDELINES SECTION 7.1.1.2

PF=35.803(QAVG-0.168) to a maximum of PF=5.0

where QAVG = average flow rate

in gallons per day (converted from SI units assuming 3.875 liters/gallon)

SUMMARY

The above 4 methods are typical in that they produce peaking factors no higher than 5.0. I have encountered no method that yields peaking factors any higher than 6.0. With its maximum constraint removed, the Alberta equation yields a peaking factor of 11 for 960 gpd, the flow from, say, three homes with 3.2 residents each. It yields a peaking factor of 6.5 for a daily flow of 25,000 gpd.

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VERY SMALL FLOWS AND THE UNIFORM PLUMBING CODE

UNIFORM PLUMBING CODE SEWER DESIGN PROCEDURE

Building designers and building mechanical engineers use a simplified design procedure from the Uniform Plumbing Code to design sewers (drainage piping) within buildings. The procedure assigns Fixture Units to various waste generating building fixtures (such as sinks and showers), then establishes authoritatively the permissible pipe sizes and slopes for a given number of total Fixture Units contributing to a point in a sewer (drainage) system. The following information is extracted from that procedure:

TABLE 3: FIXTURE UNIT FLOWS FROM 1997 UNIFORM PLUMBING CODE TABLE 7-4

Fixture Units Flow Range
(gpm)
Flow Range
(l/s)
1 0-7.5 0-0.47
2 8-15 0.50-0.95
4 16-30 1.00-1.89
6 31-50 1.95-3.15

TABLE 4: FIXTURE UNIT LOADING ON DRAINAGE PIPES FROM 1997 UNIFORM PLUMBING CODE TABLE 7-5

PIPE
SIZE (INCHES)
PIPE
SLOPE
MAX.
FIXTURE
UNITS
MIN.
FIXTURE
UNITS
1 1/4 0.0208 1  
1 1/2 0.0208 1  
2 0.0208 8  
2 1/2 0.0208 14  
3 0.0208 35  
4 0.0208 216  
5 0.0208 428  
6 0.0208 720  
8 0.0208 2640  
10 0.0208 4680  
12 0.0208 8200  
8 0.0052   1500
10 0.0052   1600
12 0.0052   1700

EXTRACTING PEAKING FACTORS FROM THE UNIFORM PLUMBING CODE

Peaking factors for very small flows/areas can be extracted from the Uniform Plumbing Code by calculating the maximum flow capacity or minimum flow requirement of the pipes presented in the Table 4 above and estimating the represented population from the fixture units assuming 21 fixture units per home and 3.2 people per home

TABLE 5: PEAK FACTORS EXTRACTED FROM 1997 UNIFORM PLUMBING CODE TABLE 7-5 WITH CALCULATED FLOWS BY TOM HAWS

PIPE
SIZE (INCHES)
PIPE
SLOPE
MAX.
FIXTURE
UNITS
MIN.
FIXTURE
UNITS
APPROXIMATE
H-W FLOW
W/ C=100
(GPM)
APPROX.
POP.
APPROX.
AVG.
DAILY
FLOW
APPROX.
PEAKING
FACTOR
1 1/2 0.0208 1   10 0.2 15 709
2 0.0208 8   21 1.2 122 260
2 1/2 0.0208 14   39 2.1 213 261
3 0.0208 35   62 5.3 533 167
4 0.0208 216   130 33 3291 57
5 0.0208 428   240 65 6522 53
6 0.0208 720   390 110 10971 51
8 0.0208 2640   830 402 40229 30
10 0.0208 4680   1500 713 71314 30
12 0.0208 8200   2400 1250 124952 28
8 0.0052   1500 130 (2 fps) 228 22800 8.2
10 0.0052   1600 140 (2 fps) 244 24400 8.3
12 0.0052   1700 150 (2 fps) 259 25900 8.3
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PEAKING FACTORS FOR VERY SMALL FLOWS

While governing agencies have commonly established minimum pipe sizes (6" or 8"), and required that they simply be laid to provide minimum velocities (2.0 or 2.5 ft/s) at full (or half) flow irrespective of actual projected flows in collection system upper reaches, some have attempted to verify the achievement of minimum velocities with appropriate slopes in these upper reaches of sewer collection systems, such as commercial sites and residential cul-de-sacs. Typical peaking factors fail to adequately estimate peak flows for these locations, where a single fixture might produce a peak flow 100 times the daily average.

A clothes washer located in a single family residence sustains a pumped drainage rate of 12 (measured at home by author) to 20 (reported by Maytag) gallons per minute for 1 1/2 to 3 minutes (35 to 38 gallons for 12 to 18 lb. capacity washer). A low-flow water closet uses 3.5 gallons per flush, and flushes in 5 to 10 seconds. A typical shower produces a flow of 5 gpm (7200 gpd) for 5 to 10 minutes. Based on these flows, one would expect to see a daily peak flow of at least 20 gpm (30,000 gpd) for a 3-home cul-de-sac (QAVG=960 gpd) based only on clothes washer drainage. Without carefully addressing probabilities of simultaneous washer drains, shower drains, and water closet flushes, plus other fixtures, or flow attenuation, it seems reasonable to say that daily peak flow for a 3-home cul-de-sac coming from extended simultaneous drainage might be as high as 40 gpm (60,000 gpd) and that weekly peak flow might be as high as 60 or 80 gpm (90,000 or 120,000 gpd). These drainage rates indicate that a peak factor between 62 and 125 would be appropriate for a 3-home cul-de-sac (Population=9.6, QAVG=960 gpd).

Based on the evidence presented above, it seems very appropriate to extract sanitary sewer collection system design peaking factors from the Uniform Plumbing Code for very small flows. For a population of 9.6 individuals, the method just cited yielded a peaking factor of 60 to 125. For the same population, the UPC extraction method yields a peaking factor of about 150.

Based on the minimum UPC fixture requirements for 8", 10", and 12" pipes presented above, it seems appropriate to assign a peaking factor of 8 to flows near 25,000 gpd. This is tantalizingly close to the peaking factor of 6.5 yielded by the Alberta Equation (Equation 2) above. We can therefore construct a continuous table and graph of peak flow factors that includes very low flows:

TABLE 6: SANITARY SEWER PEAKING FACTORS FOR VERY SMALL FLOWS

Population Avg daily flow (gpd) From UPC From Alberta Environmental From Harmon's Peaking Factor From Arizona A.A.S Title 18
1 100 260      
3 300 230      
10 1,000 150      
30 3,000 55      
100 10,000        
300 30,000 8      
1,000 100,000   5.2 3.8 2.4
3,000 300,000   4.3 3.4 2.1
10,000 1,000,000   3.5 3.0 1.8
30,000 3,000,000   2.9 2.5 1.7
100,000 10,000,000   2.4 2.0 1.6
  30,000,000   2.0 1.7 1.4
1,000,000 100,000,000   1.6 1.4 1.4

The table above is graphed below. Note that for populations between 10 and 300, with daily flows between 1,000 and 30,000 gpd, the resultant peak flow varies only from 104 to 167 gpm. This indicates that the minimum peak flow to be expected for a public residential sewer would be in that range (100 to 170 gpm) and would be relatively insensitive to population changes in the uppermost reaches of a residential system.

FIGURE 1: SANITARY SEWER PEAKING FACTORS FOR VERY SMALL FLOWS

Graph of Sewer Peaking Factors

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SUMMARY

The graph above shows peaking factors for very low sanitary sewer flows. For populations between 10 and 300, the resulting minimum peak flow for a public residential sewer would be around 100 gpm to 170 gpm, which is relatively insensitive to population changes for the uppermost reaches of a residential system


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