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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 biodegradation/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 – 0.6 metres 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 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 various attempts have also been made to turn the algal crop into a number of different products, including animal feed and insulating material.
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