Water System Planning - Public Water Systems - page 129

Commercial and industrial demands will vary significantly, depending upon the type
of use.
Residential Equivalents Some planners describe average connection demands in
terms of equivalent residential units (ERUs). An ERU is generally defined as the total
single-family residential demand divided by the number of single-family residential
connections. Other demand classifications can then also be defined in ERUs by divid-
ing the total demand for that classification by the calculated ERU unit value. For
example, demand for a typical multifamily unit connection may be determined to be
0.8 ERU, while that for an average commercial connection may be 2.0 ERU. Although
convenient to use this shorthand technique, it does not translate well to maximum-day
or peak-hour demand since water use variation for different user classes is not the
same.
Per Capita Demands The residential and total system demands may be divided by
the population to determine per capita demands for both categories. Typical units of
measurement are gallons per capita per day (gpcd) or liters per day per capita (l / d per
capita). If the information is available, the population may also be divided into single-
and multifamily categories to determine the average number of people for each con-
nection category. Typical single-family residential connections average between 20 and
30 people per connection; typical multifamily connections average between 1.0 and
2.0. Depending upon location, climate, cost, and mix of residential versus commercial/
industrial connections, typical average-day demands may vary between 80 and
150 gpcd (300 and 570 L / d per capita) for residential demand and between 100 and
200 gpcd (380 and 760 L / d per capita) when all system demands are included. Of
course, unusual mixes of connections and large industrial and irrigation demands can
cause significant variations in these typical values.
Peaking Factors Although multipliers can be used to estimate demand variations,
good planning requires measurement of maximum-day and peak-hour demands. Flow
demand variation depends on climate, size of system, and the characteristics of the
connections. For example, a system dominated by residential connections will exhibit
significantly different weekly and diurnal flow variations than a system dominated by
industrial / commercial users.
For determining source-of-supply, storage, and conveyance needs, the following
three demand periods need to be determined: average daily demand (ADD), maximum
daily demand (MDD), and peak-hour demand (PHD). For typical water systems serv-
ing a mix of both residential and commercial connections, MDD ranges from 1.5
ADD to 3.0 ADD. PHD ranges from 1.25 MDD to 2.5 MDD, or 2.5 ADD
to 5 ADD. (Note that these values do not apply to all systems in all areas.)
For most typical systems, peak-hour demands (other than emergency / fire flows)
generally occur during weekdays in the late-afternoon or early-evening hours. This
corresponds to peak residential use after the range of working and school hours. In
most climates, peak demands also occur during summer months as a result of increased
domestic use and irrigation demands. Figure 7-5 illustrates diurnal flow variation in
a typical mixed-use water system.
Fire Fire demands are determined based upon land use densities and the critical
structure classification (i.e., highest fire flow requirement) for each pressure or service