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By Thomas Gail Haws, P.E.
SANITARY SEWER COLLECTION SYSTEM DESIGN FLOW PEAKING FACTORS
VERY SMALL FLOWS AND THE UNIFORM PLUMBING CODE
PEAKING FACTORS FOR VERY SMALL FLOWS
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:
|Upstream Population||Peaking Factor|
|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
|Collection Main Diameter||Peaking Factor|
|10 inches and smaller||4.0|
|12 to 15 inches||3.5|
where P= the design contributing population in thousands
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)
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.
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:
|Fixture Units||Flow Range
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
|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|
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:
|Population||Avg daily flow (gpd)||From UPC||From Alberta Environmental||From Harmon's Peaking Factor||From Arizona A.A.S Title 18|
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.
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