Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 164

Figure7.4 Algalandbacterialequilibrium
Aerator paddle
Effluent
Plan view
Algal-rich water
Side view
Figure7.5 Highratealgalpond
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 and 0.6m 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. These
ponds are not sensitive to fluctuations in daily feed, either in terms of quantity or
quality of effluent, providing that it is fundamentally of a kind suitable for this
type of treatment. Consequently, they may be fed on a continuous or intermittent
basis. The main influences which affect the system's performance are the compo-
sition of the effluent, the efficiency of mixing, the retention time, the availability
and intensity of light, pond depth and temperature. The latter two factors are
particularly interesting since they form logical constraints on the two groups of
organisms responsible for the system's function, by affecting the autotrophe's