164 Environmental Biotechnology
Aerator paddlePlan viewSide viewAlgal-rich waterEffluentEffluentFigure 7.5 High rate algal pond
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 composition 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 ability to photosynthesise and the heterotrophe’s to respire. While
a deeper cell permits greater resident biomass, thus elevating the numbers of
micro-organisms available to work on the effluent, beyond a certain limit, the
law of diminishing returns applies in respect of light available to algae in the
lower reaches. Warmer temperatures increase metabolic activity, at least within
reason, and the rate of straightforward chemical reactions doubles per 10◦C
rise, but at the same time, elevated water temperatures have a reduced oxygen-
carrying capacity which affects the bacterial side of the equilibrium mentioned
earlier. As with so much of environmental biotechnology, a delicate balancing
act is required.
After a suitable retention period, which again depends on the character of the
effluent, the design and efficacy of the treatment pond and the level of clean-up
required, the water is discharged for use or returned to watercourses. Obviously,
after a number of cycles, algal and bacterial growth in a functionally eutrophic
environment would, as discussed earlier in the section, begin to inhibit, and then
eventually arrest, the biotreatment process. By harvesting the algal biomass, not
only are the contaminants, which to this point have been merely biologically iso-
lated, physically removed from the system, but also a local population depression
is created, triggering renewed growth and thus optimised pollutant uptake. The
biomass recovered in this way has a variety of possible uses, of which compost-
ing for ultimate nutrient reclamation is without doubt the most popular, though