Environmental Engineering Reference
In-Depth Information
remove this portion of the organic pollution, nutrients or heavy metals, which is
associated with small-size suspended particles and colloidal material retained in the filter
bed. In many cases, filters are incorporated as part of combined treatment units, which
have compartments for sedimentation and/or other treatment methods.
Filter media might include different materials other than sand, such as anthracite,
activated carbon or synthetic granules. The filter bed might be designed to contain layers
of different materials with different size fractions. An important point to be remembered
when filters are applied, is the fact that they have limited capacity to retain pollutants and
need regular cleaning or replacement.
3.3.5
Infiltration structures
These structures could be classified more as facilitating structures, rather than treatment
facilities. As in the case of detention reservoirs, infiltration structures allow for partial
retention and infiltration of the runoff into the soil. They could be used at a small scale
(on site structures), or at a large scale to allow for the runoff infiltration of a whole
suburb or region.
Small-scale structures are usually designed as infiltration trenches, which have a
similar construction as the well known “soak ways”. These are trenches, filled with
variable size inert porous material (stone, rubble, sand, etc), which are arranged to permit
a smooth passage of the rainwater from the conveying drainage perforated pipes, through
the drainage material, surrounding the pipe, into the soil. It is common practice to
incorporate infiltration trenches within the drainage system, parallel to roads, streets or
parking slots, in order to minimize the runoff volume, which needs to be transported
downstream.
Large-scale structures include infiltration basins, which allow for the infiltration of the
accumulated water through the bottom and walls of the reservoir. They might be designed
in the shape of an embankment or an excavated structure as a shallow pond, which
reaches the permeable soil layer. During most of the time these are dry structures, which
could be incorporated in the landscape and form part of parks, recreational areas, etc.
Care should be taken to clean periodically the debris, sludge and slime left after heavy
storms, in order to prevent clogging of the surface. It is recommended that such types of
basins be covered with grass, in order to help the biological regeneration of deposited
organic material.
Infiltration structures usually have large land requirements, as the main design
parameter is the infiltration capacity of the soil per unit area. The lower this parameter is,
the larger infiltration areas will be needed. One important consideration for the design of
infiltration basins is the retention time of the runoff volume, before its infiltration, which
should not be excessively long in order to prevent the organic decomposition of debris,
unpleasant odors and mosquito breeding.
3.3.6
Storm water reuse
Storm water is one of the basic sources of supply and replenishment of natural water
resources. In many countries in arid and semi-arid regions, where natural water resources
are limited, the practice of rainwater harvesting is applied. It consists in the collection and
