Page 141 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED W ater Ef ficiency 121

Now both the baseline and the design blackwater annual generation rates can be
determined. The blackwater baseline (BWB) generation rate, in gallons per year, can be
calculated using the fixture flow rate numbers in Table 3.2.2 for the most typical
conventional toilet as follows:
BWB = (1.6 × TWCU) + (1.0 × TUU) gal/yr (3.2.4)

To calculate the blackwater design (BWD) generation rate, the factors in Eq. (3.2.4) are
replaced with those appropriate for the alternate fixture type i used such as WCR (the fixture
i
rate for water closet type i) and UR (the fixture rate for urinal type i) from Table 3.2.2:
i
BWD = (WCR × TWCU) + (UR × TUU) gal/yr (3.2.5)
i i
Note that for Option 1 the only differences between the base blackwater generation rate
and the design blackwater generation rate are the fixture flow rates. However, Option 1
of WE credit 3.2 is based on a reduction in the wastewater generated, a reduction in
potable water used for blackwater fixtures, or a combination of both. Therefore, the
annual usage of other water sources, such as rainwater or graywater, for these fixtures
can be subtracted from the total design blackwater generation rate. The designer should
provide calculations to estimate the rate of these alternative waters available for use on
an annual basis. For instance, rainwater availability is seasonal, so the totals available
during the rainy seasons may be the only opportunity for potable water reduction.
Note also that there are local and state health and other regulatory codes that cover
the use, storage, and possibly treatment of nonpotable waters in buildings. In addition,
there is a large variation in the definitions used from locality to locality and from state
to state on what constitutes graywater, similar to with what is included in blackwater.
The designers, owners, and managers should become familiar with the appropriate
local definitions and any special health-related requirements for the use of reuse waters
or rainwaters so that any special protective measures or treatments can be included in
the design and operations. The designer should include the appropriate local definitions
of graywaters and other reuse waters, as well as the associated costs and systems in the
design, and include a narrative of them in the submittal. Table 3.2.3 lists some of the
various state interpretations of what may be included in graywater as taken from
Appendix 3: US State Regulations compiled in 1999 on the www.weblife.org website,
humanure section.
Let GW be the annual graywater or other alternative nonpotable water source usage
in the wastewater fixtures; then WE credit 2 is obtained via Option 1 if the following is true:
BWD − GW
≤ 050 (3.2.6)
.
BWB
Option 1: Residential Uses The building designer should declare the design occupancy
(DO) for the buildings. Note that design occupancies are not the same as maximum
occupancies. LEED 2009 gives some recommended residential design occupancies as
two persons per one bedroom unit and one additional person per additional bedroom.
In addition, usually residential facilities do not have urinals and do have equal gender
distributions as noted in Table 3.2.1. Equation (3.2.2) can be used to estimate the total
water closet usage. In this case, the transient occupancies are zero, there are no urinals,
and there are no shifts. Using a water closet rate of 5 uses per day as given in Table 3.2.1,