Environmental Engineering Reference
In-Depth Information
bioreactor through diffuser pipes located at the bottom of the tank or at another location
depending on the configuration. The tank may be divided into sedimentation and clarification
zones. Sludge that settles to the bottom of the tank needs periodical removal. The main func-
tion of the anaerobic bioreactor is to reduce the biological oxygen demand and suspended
solids before sending the wastewater to the succeeding steps (EPA, 2002).
The second step, the anoxic reactor, is a tank that operates in a complete mix aeration
mode, which is promoted by coarse air bubbling at the bottom of the tank. The purpose of the
anoxic tank is further removal of biological oxygen demand and starting the nitrification of
ammonia, which is converted to nitrate (NO
3
−
). Settled biosolids from following steps are
frequently fed back into the anoxic tank to act as a source of carbon for the nitrifying bacteria
without the need of supplementation (EPA, 2002).
The closed aerobic tank serves the purpose of reducing biological oxygen demands to
lower levels, continue the nitrification process, and remove odors. Aeration is promoted by
injecting air at the bottom of the tank through a diffuser that creates fine bubbles. A biofilter
that contains plants is used on the surface of the tank to capture odors by bacteria that develop
in the biofilters.
In the open aerobic tanks, floating plants, such as hyacinths and pennywort, or plants sup-
ported by a rack are placed on the surface of these tanks, so their roots act as a support for the
growth of nitrifying organisms (EPA, 1997). The plants also absorb nutrients and serve as
habitats for insects and other organisms. Other organisms such as algae, snails, shrimp, and
fish can be also included at this stage. Open aerobic tanks are installed in series, so the nitrifi-
cation process can be completed at the last tank (EPA, 2002).
The clarifier is a settling tank to remove solids in suspension. The surface of the clarifier is
generally covered with duckweed to prevent the development of algae (EPA, 2002).
In the last step, ecological fluidized beds (EFB) polish the effluent to reduce the biological
oxygen demand even further and remove dissolved solids and nutrients. Instead of the EFB,
some systems use a constructed wetland to finish polishing the water before reusing or
discharging it to surface water bodies (EPA, 1997; Barista, 2001).
Constructed wetlands
A constructed wetland is similar to a natural wetland, which is an area of land with permanent
or seasonal saturation of moisture. Natural wetlands can be transition zones between land and
bodies of water and as a result saturated with fresh, salty, or brackish water, or they can be
saturated as a result of the ground water table reaching the surface. Natural wetlands have
several functions such as storing and filtrating water, retaining sediments and pollutants, and
sustaining highly biodiverse ecosystems.
In an effort to replicate natural wetlands, constructed wetlands, also known as artificial
marshlands, are wetlands built with the purpose of wastewater treatment in areas where wet-
lands previously were not present. Constructed wetlands are not the same as “created or
restored wetlands,” which are imitations of natural wetlands with the purpose of supporting
wildlife habitat. These created or restored wetlands contain open water of variable depth,
irregular shorelines, and aquatic vegetation as well as shrubs and trees (EPA, 2000).
Constructed wetlands are shallow ponds or channels (called cells) frequently less than 1
meter deep. Cells are impermeabilized with a liner or impervious clay and engineered structures
are placed to control water level and flow circulation. Wetlands can be constructed by excavat-
ing basins, by constructing earth embankments, or a combination of the two (EPA, 1995).
There are three types of constructed wetlands: free water surface (FWS), vegetated
submerged bed (VSB), and the hybrid system. FWS wetlands resemble natural wetlands
