Page 95 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

76 Cha pte r T w o

The volume criteria for treating runoff for quality control for this credit are based
on runoff from 90 percent of the average annual rainfall. For the purposes of this credit,
90 percent of the average annual rainfall may be estimated in a specific precipitation
event as up to 1 in of rainfall for humid areas, 0.75 in of rainfall for semiarid areas, and
0.5 in of rainfall for arid zones. For all events less than the maximum volume criteria as
so specified, all the associated runoff would be treated prior to discharge from the site;
and for all storm events greater than the maximum specified, the runoff associated with
this maximum criterion would need to be treated.
As far as the treatment requirement goes, it may be assumed according to this credit
that all rain infiltrated on-site is 100 percent treated and any runoff that is discharged
from the site from the specified maximum rainfall amount must be treated to attain an
80 percent TSS removal. The removal method must be one that is either designed by
acceptable state or local BMP performance standards or supported by appropriate in-
field monitoring data. It is also assumed that any rainwater appropriately reused on-
site does not contribute to the TSS load. However, it should be ascertained that the
BMPs used adhere to any local and state health and environmental codes. There are
many devices, such as rainwater harvesting systems, that have strict health code
requirements for items such as length of time in the storage container, minimum
treatment required, or differentiation from potable water systems and the “appearance”
of potable water systems.
Treating for total phosphorus is no longer a requirement for this credit as it was
in earlier versions of LEED. This seems reasonable to the author for a combination of
reasons. First, much of the phosphorus is absorbed in, or part of, the suspended
solids and will be removed by the TSS process. Second, there are usually two main
nutrients that may promote eutrophication or other environmental problems in
receiving waters: nitrogen and phosphorus. In general, phosphorus is the limiting
nutrient for freshwater systems, and nitrogen is the limiting nutrient for saltwater
bodies. Therefore, phosphorus is not always considered an important pollutant in
many receiving waters. Many watersheds where phosphorus is the limiting nutrient
already have discharge criteria limiting the load of phosphorus and are thus already
protected.
A summary of the steps for complying with this credit is as follows:

1. Determine whether your project is in a humid, semiarid, or arid region.
2. Determine the minimum volume of water that the system must be designed to
handle with a combination of on-site infiltration, appropriate on-site use, or
treatment for 80 percent TSS removal prior to discharge.
a. If in a humid region, multiply the site area by 1 in to determine this minimum
volume of runoff that must be infiltrated, used appropriately on-site, or
treated prior to discharge.
b. If in a semiarid region, multiply the site area by 0.75 in to determine this
minimum volume of runoff that must be infiltrated, used appropriately on-
site, or treated prior to discharge.
c. If in an arid region, multiply the site area by 0.5 in to determine this minimum
volume of runoff that must be infiltrated, used appropriately on-site, or
treated prior to discharge.
3. If WQ , WQ , and WQ are defined as the water quality volumes for humid,
VH VS VA
semiarid, and arid climates, respectively, then the total rainfall which must be