Environmental Engineering Reference
In-Depth Information
Systems can be complex or simple and active or passive. Complex active systems require
pipes, pumps, pressure tanks, and filtration where the simple systems may have all of the
same components, but on a much smaller level. The passive systems generally refer to
gravity landscape systems. If an active system is selected any type of water storage sys-
tems can be designed to fit a site's intended use and supply demands.
With nonpotable rainwater harvesting and stormwater reuse systems, a level of use for
the captured water needs to be determined. There are typically four levels of use or levels
of commitment to provide a water source for the intended water demand. The four levels
of commitment include occasional, intermittent, partial, or full. A full commitment is
typically a large storage capacity system that provides all of the water needed by the user
(or demand) for an entire year.
21.3 Typical System Components
A nonpotable rainwater harvesting or stormwater reuse system has five basic components:
1. Catchment area: the surface upon which the rain falls. It may be a roof, other
impervious surfaces and may include landscape areas.
2. Conveyance: transport channels or pipes from catchment area to storage.
3. Roof washing: the systems that filter and remove contaminants and debris. This
includes first-flush devices.
4. Storage: cisterns or tanks where collected rainwater is stored.
5. Distribution: the system that delivers the rainwater, either by gravity or pump.
One additional component would be needed to provide a potable rainwater harvesting or
stormwater reuse systems; purification. This may include filtering equipment, ultraviolet,
chlorination or other methods of disinfection, and additives to settle, help filter, and
disinfect the collected rainwater. Depending on catchment surface material and rainfall
intensity, a loss of potentially collected rainwater can range from 20% to 70%. This loss
is due to runoff material absorption or infiltration, evaporation, and inefficiencies in the
collection process.
21.3.1 Catchment Area
A catchment area is the defined surface area upon which rainwater falls and is eventually
collected. Rainwater harvesting for nonpotable use can be accomplished with any roofing
material. Although rooftops are the typical catchment area, patio surfaces, driveways,
parking lots, or channeled swales can also serve as catchment areas. Rainwater is slightly
acidic, which means it will dissolve and carry minerals into the storage system from any
catchment surface.
The total amount of water that is received in the form of rainfall over an area is referred
to as the rainwater endowment of that area. The actual amount of rainwater that can be
effectively harvested from the rainwater endowment is called the rainwater harvesting
potential. Rainwater yields vary with the size and texture of the catchment area. A maxi-
mum of 90% of a rainfall can be effectively captured through rooftop rainwater harvesting.
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