City of Morgan Hill Sewer System Master Plan 2017

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.

August 2017

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City of Morgan Hill Sewer System Master Plan