Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 163

Algal Treatment Systems (ATS)
Algae have principally been employed to remove nitrogen and phosphorus from
wastewaters, though some organic chemicals can also be treated and a relatively
new application has emerged which makes use of their efficient carbon seques-
tration potential.
Effluent treatment
Algal effluent treatment systems work on the basis of functional eutrophication
and rely on a dynamic equilibrium between the autotrophic algae themselves
and the resident heterotophic bacteria, which establishes a two-stage biodegrada-
tion/assimilation process, as shown in Figure 7.4. In effect this is an ecological
microcosm in which organic contaminants present in the wastewater are biolog-
ically decomposed by the aerobic bacteria, which make use of oxygen provided
by algal photosynthesis, while the algae grow using the nutrients produced by
this bacterial breakdown, and photosynthesise producing more oxygen.
Though the process is self-sustaining, it is also self-limiting and left to pro-
ceed unchecked, will result in the well-appreciated characteristic eutrophic stages
leading to the eventual death of all component organisms, since true climactic
balance is never achieved in the presence of continuously high additional nutri-
ent inputs. The removal of excess algal and bacterial biomass is, therefore, an
essential feature, vital to maintaining the system's efficiency.
Of all the engineered algal systems for effluent, the high rate algal pond
(HRAP) is one of the most efficient and represents a good illustration of this
use of phytotechnology. Figure 7.5 shows a typical example.
The system consists of a bioreactor cell in the form of a relatively shallow
reservoir, typically between 0.2-0.6metres deep, with a length to width ratio of
2:1 or more, the idea being to produce a large surface area to volume ratio. The
void is divided with internal baffles forming walls, to create a channel through
which the effluent flows. A mechanically driven paddle at the end nearest to
the effluent input both aerates and drives the wastewater around the system.
Figure7.4 Algalandbacterialequilibrium