Environmental Engineering Reference
In-Depth Information
Closed
aerobic
reactor
Open aerobic tank
Anaerobic
digester
Raw
wastewater
Constructed wetland
Clarifier
Treated
wastewater
Figure 9.8
Advanced ecologically engineered system shown with a constructed wetland instead of the
ecological fluidized bed.
Adapted from Barista, 2001; Environmental Protection Agency (EPA), 1997, 2002.
When suspended solids are an issue, a clarifier can precipitate suspended particles and the
sludge accumulated at the bottom returned to anaerobic reactor. A second alternative is the use
of microfiltration units with membrane pore sizes between 0.1 and 0.4
m. The permeate of
the membrane separation unit goes to the next treatment step while the retenate containing
particles is directed back to the reactor.
Conventionally, the next step is to polish the effluent even further with an advance waste-
water treatment system. However, as was stated previously, these systems consume significant
amounts of energy; therefore, if the objective is to reduce energy consumption and emissions
alternative methods need to be considered, such as engineered natural systems.
μ
Engineered natural systems
Wastewater from food-processing plants is high in biological oxygen demands, but it is not
considered toxic, which makes this wastewater a good candidate for treatment in engineered
natural systems. These systems contain similar processes to conventional biological systems,
but they use microorganisms, protozoa, snails, and plants to carry out each stage of the process
(Environmental Protection Agency [EPA], 2002).
The best well-known system is the “Advanced Ecologically Engineered System” also
known by the trademark Living Machine
®
(EPA, 2002). After the effluent is screened, the
system contains six steps that start with (1) an anaerobic bioreactor followed by (2) an anoxic
reactor, (3) a closed aerobic reactor, (4) an open aerobic reactors, (5) a clarifier, and (6) eco-
logical fluidized beds or a constructed wetland (Fig. 9.8). These are all the components that
may be present but do not have to be included in a system and their addition is based on the
characteristics of the effluents. In warm climates, the whole system can be installed outdoors;
however, in cold climates, with exception of the anaerobic reactor, the rest of the components
has to be housed in a greenhouse.
The anaerobic bioreactor is a horizontal tank, totally or partially buried below grade, which
operates similar to a septic system. After screening, the untreated wastewater is fed into the
