Environmental Engineering Reference
In-Depth Information
oxygen for their metabolism. The system consists of a biological reactor and a secondary
sedimentation tank to separate the biomass from the final effluent. Biological reactors
could be subdivided into two major categories - attached growth reactors and suspended
growth reactors.
In attached growth systems (Fig. 8.1) the biological culture, known as “biofilm”, is
attached to a fixed media, which could consist of natural material - stone or gravel, or
plastic blocks with a high specific surface area. The reactor, where the process takes
place is known as trickling filter (biofilter). The wastewater is spread over the filter
media and on its way through it, gets in contact with the biofilm. The air current, formed
due to the temperature difference between the reactor and the surrounding environment,
supplies oxygen for the process. Figure 8.1 shows a treatment scheme commonly used in
the region.
In suspended growth systems, also called activated sludge systems, the biological
culture (biomass), is mixed with the wastewater and held in suspension within open tanks
(aeration tanks). The oxygen needed for the process is supplied by means of mechanical
aerators or through the provision of compressed air, dispersed by diffusers in the form of
bubbles. A large variety of different types of aeration systems exist, which have the
purpose not only to supply oxygen for the process but also to keep the mixture of biomass
Figure 8.1. Sewage treatment scheme.
and wastewater in suspension. The required concentration of the biomass in the reactor is
maintained by the recirculation of a portion of the sludge, retained in the secondary
sedimentation tank, back to the reactor. The treatment effect with respect to the removal
of the organic material, expressed as BOD
5
, would depend on the design of the system
and the configuration of the reactor in terms of hydraulic conditions, retention times of
the wastewater and the biomass, and organic loading rates.
Attached growth systems require less energy consumption and are simpler to operate
and maintain. For this reason, they have been a preferred secondary treatment alternative
in many conventional wastewater treatment plants in the region. The first plants of such
