SFPUC Wholesale Customer Water Demand Projections ... - BAWSCA

SECTIONTHREE
Establishing Base - Year Conditions
Fixture Model Water Use Computation
In the fixture model the amount of water used by an end use is the product of the
amount of water used by the fixture per use multiplied by the number of users in
the customer-billing account (i.e., persons per household or employees per account)
multiplied by the number of uses per day. An example of toilet use computed by a
fixture model is shown below:
• Single-Family Residential Toilet Use (sample values in a fixture model)
– Average users per Single-Family account (household size): 3.0 people per
household
– Average toilet flush volume in service area 12 : 3.5 gallons per flush
– Average flushes per person per day: 5.0 times per day
– Volume used to flush toilets: 3.0 x 3.5 x 5.0 = 52.5 gal/day/household
Comparing Fixture Model Computations with Indoor and Outdoor Use
The fixture model computations for each end use by customer-billing category are
compared to the percentage of indoor and outdoor water use and by end use for a
customer-billing account provided by AWWARF, presented in Table 3-3.
Consistent with the fixture model computation provided above, if the single-family
indoor use for a particular account is 200 gal/day and the percentage allocated to
toilets is 26.7% (Mayer et al. 1999; see Table 3-3), then 53.4 gal/day of the 200
gal/day are used for toilet flushing in the house. Comparing the 53.4 gal/day with
the computed value in the fixture model for toilet flushing in that house there is a
difference of 1.4 gal/day. Therefore, adjustments need to be performed to either the
fixture model or the percentage of water use by end use so that these two numbers
match.
Matching Fixture Model Computations with Initial End-Use Percentages
The fixture model computations and the end-use percentages can be matched in two
ways: either adjust the fixture model components or adjust the initial percentages of
end use for total water. In this example, small adjustments can be made to the
fixture model to increase the volume used to flush toilets up to 53.4 gal/day or
small adjustments can be made to the percentage of water use associated with
toilets to reduce its value to 52.5 gal/day. For example, adjusting the fixture model
may involve increasing the number of flushes per person per day from 5.0 to 5.086
or decreasing the percentage of indoor use water use allocated to toilets to 26%.
12 Average toilet flush volume is derived by taking the initial proportions of each type of fixture (Table 3-10)
combined with the volume-per-use estimates (Table 3-12) computing a service-area-specific volume-per-use
weighted average.
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