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August 2017
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City of Morgan Hill
Sewer System Master Plan
3.2.1
Unit Flow Factors Methodology
Sewer unit factors are developed by using water consumption records and applying a return to
sewer ratio for each land use to estimate sewer flow coefficients. There are several methods for
developing the unit factors. This analysis relied on the use of the
City’s
water consumption billing
records, and the Public Water System Statistics (PWSS) report, which lists the monthly water
consumption per customer account, by land use type, to estimate the unit factors within the
service area.
3.2.2
Average Daily Wastewater Unit Flow Factors
Wastewater flow factors were based on water demands as
extracted from the City’s
water
consumption billing records. A return to sewer ratio was applied to each unadjusted water demand
factor for individual land uses, and sewer flows were balanced to wastewater treatment plant
flows. Generally, non-residential land uses return the majority of the water demand to the sewer
system. These unit factors were estimated at ranging from 45 percent to 75 percent return to
sewer ratio. The same concept can be applied to multi-family residential lots, which were
estimated at ranging from 25 percent to 80 percent return to sewer ratio. Single family residential
lots often have the lowest return to sewer ratio. This is largely due to water lost for landscape
irrigation. Single family lots were estimated ranged from 25 percent to 40 percent return to sewer
ratio. Lastly, unit factors were adjusted to 100 percent occupancy, and rounded.
This analysis generally indicates that existing residential land uses have higher flow generation
factors than that of non-residential land uses. The existing unit factor analysis is shown on
Table
3.2
.
3.2.3
Peaking Factors
The sanitary sewer system is evaluated based on its ability to convey peak sewer flows. Peaking
factors represent the increase in sewer flows experienced above the average dry weather flows
(ADWF). The various peaking conditions are numerical values obtained from a review of historical
data and, at times, tempered by engineering judgment.
The peaking conditions that are significant to hydraulic analysis of the sewer system include:
x
peak dry weather flows (PDWF)
x
peak wet weather flows (PWWF)
Typical values for peaking factors of 2.0 or less are generally used to estimate peak flows at
treatment facilities where flow fluctuations are smoothed out during the time of travel in the sewer,
while peaking factors between 3.0 and 4.0 are used to estimate peak flows in the smaller
upstream areas of the system where low flow conditions are prone to greater fluctuations.