Environmental Engineering Reference
In-Depth Information
the highest concentration of debris and soluble pollutants. First-flush devices collect
and dispose of this initial rain before it contaminates previously harvested and stored
rainwater.
Capacities of first-flush devices may vary depending on the catchment size and ultimate
use of the rainwater. Rainwater collected from a rooftop will typically be cleaner than
rainwater collected from a ground level surface or pavement area. Thus, the storage capacity
of the rooftop first-flush device does not need to be as large as a ground level surface
catchment area first-flush. Rainwater collected from surface or pavement areas where dirt
and debris are more prevalent may require longer settling periods for suspended solids
and an absorbent material to remove oil and grease. Therefore, a more sophisticated and
larger capacity first-flush device is typically required.
Use of a first-flush device is especially important when rain events follow a long dry
period; during dry spells, debris and other pollutants build up on catchment surfaces.
In this case, a large volume of water may be required to remove the catchment surface
contaminants surpassing the volume allowed in a specified first-flush device. This means
some contaminants will not be diverted and will enter the rainwater storage system.
When a second rain event closely follows one that was strong enough to sufficiently
“wash” the catchment area, use of the first-flush device during the second rain event
may not be required. However, if the first rain event was not strong enough to move
the catchment area contaminants, diversion of the second rain event's initial rainwater
runoff to the first-flush device may be warranted. Multiple first-flush devices may be
required on large surfaces as the time needed for the dirty water starting farthest from
the first-flush is greater than dirty water closer to the device. In this case, the dirty water
located some distance away will mix with cleaner water close by the devise. This can be
avoided by having several first-flush devices evenly spaced apart in the catchment area.
21.3.4 Storage
Most of the components of a rainwater harvesting system are assumed costs in a building
project. For example, all buildings have a roof and some have gutters and downspouts.
Most homes and businesses also have irrigation systems and landscape materials placed
around the structures. The cisterns or storage tanks represent the largest investment in a
rainwater harvesting system because most homes and businesses are not initially fitted
with a storage system.
Most cisterns and tanks have three distinct components all of which need to be
waterproofed: base, sides, and a cover. They also contain several minor components
including water inlet, water outlet, access hatch, overflow pipe, and means of draining.
A typical storage cistern is covered and made of stone, steel, concrete, ferro-cement, plastic,
or fiberglass. A storage system should be durable, attractive, able to withstand the forces of
standing water, watertight, clean, smooth inside, sealed with a nontoxic joint sealant, and
easy to operate. A tight cover is essential to prevent evaporation and mosquito breeding,
and to keep insects, birds, lizards, frogs, and rodents from entering the tank. Cisterns and
tanks should not allow sunlight to enter or algae will grow inside the container and the
water will not age correctly.
Some storage tanks contain settling compartments to encourage any roof or pavement
runoff contaminants to settle rather than remain suspended. Storage tanks can have an
inlet from a sand filter or directly from the gutters through a leaf and debris filter. They
must also have an overflow equal in size to the inlet flow rate, and an outlet or drain.
The overflow should daylight to a landscape basin or an adjacent drainage system.
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