Environmental Biotechnology - Theory and Application - G. M. Evans & J. C. Furlong

164 Environmental Biotechnology

Aerator paddle

Effluent

Effluent

Side view

Figure 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 oxygencarrying 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 isolated, 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 composting for ultimate nutrient reclamation is without doubt the most popular, though

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

Carbon sequestration

Their use as a carbon sink is a simpler process, only requiring the algae themselves. However, even as a functional algal monoculture, just as with the joint algal/bacterial bioprocessing for effluents, without external intervention to limit the standing burden of biomass within the bioreactor, reduced efficiency and, ultimately, system collapse is inevitable.

In nature, huge amounts of many elements are held in global reservoirs, regulated by biogeochemical cycles, driven by various interrelated biological and chemical systems. For carbon, a considerable mass is held in organic and inorganic oceanic stores, with the seas themselves being dynamic and important component parts of the planetary carbon cycle. Marine phytoplankton utilise carbon dissolved in the water during photosynthesis, incorporating it into biomass and simultaneously increasing the inflow gradient from the atmosphere. When these organisms die, they sink, locking up this transient carbon and taking it out of the upper oceanic ‘fast’ cycle into the ‘slow’ cycle, which is bounded by long-term activities within the deep ocean sediments. In this respect, the system may be likened to a biological sequestration pump, effectively removing atmospheric CO2 from circulation within the biosphere on an extended basis. The number, mass and extent of phytoplankton throughout the world’s seas thus provide a carbon-buffering capacity on a truly enormous scale, the full size of which has only really become apparent within the last 10 – 15 years, with the benefit of satellite observation.

In the century since its effectiveness as a means of trapping heat in the atmosphere was first demonstrated by the Swedish scientist, Svante Arrhenius, the importance of reducing the global carbon dioxide emissions has come to be widely appreciated. The increasing quantities of coal, oil and gas that are burnt for energy has led to CO2 emissions worldwide becoming more than 10 times higher than they were in 1900 and there is over 30% more CO2 in the air, currently around 370 parts per million (ppm), than before the Industrial Revolution. Carbon dioxide is responsible for over 80% of global warming and according to analysis of samples of the Antarctic ice, the world today has higher levels of greenhouse gases than at any time in the past 400 000 years. The UN Intergovernmental Panel on Climate Change has warned that immediate action is required to prevent further atmospheric increases above today’s level. In the absence of swift and effective measures to control the situation, by 2100 they predict that carbon dioxide concentrations will rise to 550 ppm on the basis of their lowest emission model, or over 830 ppm in the highest.

In 1990, over 95% of the western industrialised nations’ emissions resulted from burning fossil fuels for energy, with the 25% of the world’s population who live in these countries consuming nearly 80% of the energy produced globally.

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Unsurprisingly, energy industries account for the greatest share (36%) of carbon dioxide emissions, a large 1000 Megawatt coal-fired power station releases something in the region of 5 12 million tonnes of CO2 annually. Clearly, the current focus on reducing fossil fuel usage, and on minimising the emissions of carbon dioxide to atmosphere, is important. In one sense, the most straightforward solution to the problem is simply to stop using fossil fuels altogether. However, this is a rather simplistic view and just too impractical. While great advances have been made in the field of renewable energy, a wholesale substitution for gas, coal and oil is not possible at this time if energy usage is to continue at an unabated rate. The potential role of existing nonfossil fuel technology to bridge the gap between the current status quo and a future time, when renewables meet the needs of mankind, is a vital one. However, it is ridiculous to pretend that this can be achieved overnight, unless the ‘global village’ really is to consist of just so many mud huts.

In many respects, here is another case where, if we cannot do the most good, then perhaps we must settle for doing the least harm and the application of phytotechnology stands as one very promising means by which to achieve this goal. The natural contribution of algal photosynthesis to carbon sequestration has already been alluded to and the use of these organisms in an engineered system to reduce CO2 releases, simply capitalises on this same inherent potential in an unaltered way.

There have been attempts to commercialise the benefits of algae as carbon sinks. In the early 1990s, two prototype systems were developed in the UK, aimed at the reduction of CO2 emissions from various forms of existing combustion processes. The BioCoil was a particularly interesting integrated approach, removing carbon dioxide from generator emissions and deriving an alternative fuel source in the process. The process centred on the use of unicellular algal species in a narrow, water-containing, spiral tube made of translucent polymer, through which the exhaust gases from the generator was passed. The carbon dioxide rich waters provided the resident algal with optimised conditions for photosynthesis which were further enhanced by the use of additional artificial light. The algal biomass recovered from the BioCoil reactor was dried, and being unicellular, the effective individual particle size tended to the dimensions of diesel injection droplets, which, coupled with an energy value roughly equivalent to medium grade bituminous coal at 25 MJ/kg, makes it ideal for use in a suitable engine without further modification. Despite early interest, the system does not appear to have been commercially adopted or developed further.

Around the same time, another method was also suggested by one of the authors. In this case, it was his intent specifically to deal with the carbon dioxide produced when biogas, made either at landfill sites or anaerobic digestion plants, was flared or used for electricity generation. Termed the algal cultivation system and carbon sink (ACSACS), it used filamentous algae, growing as attached biofilter elements on a polymeric lattice support. CO2 rich exhaust gas

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Figure 7.6 Schematic ACSACS

was passed into the bottom of a bioreactor vessel, containing the plastic filter elements in water, and allowed to bubble up to the surface through the algal strands as shown in Figure 7.6.

Again, this approach to carbon sequestration was based on enhanced intrareactor photosynthesis, the excess algal biomass being harvested to ensure the ongoing viability of the system, with the intention of linking it into a composting operation to achieve the long-term carbon lock-up desired. The ACSACS though performing well at both bench and small pilot scale, never attained industrial adoption though remaining an interesting possible adjunct to the increasing demand for methane flaring or utilisation at landfills.

A similar idea emerged again recently, with a system being developed by Ohio University, which, in a perfect example of selecting an organism from an extreme environment to match the demands of a particular manmade situation, utilises thermophilic algae from hot springs in Yellowstone National Park. In this process, which has received a $1 million grant from the US Department of Energy, smoke from power stations is diverted through water to permit some of the CO2 to be absorbed and the hot, carbonated water produced then flows through an algal filter formed on vertical nylon screens.

This design, which is essentially similar to the earlier ACSACS, enables the largest possible algal population per unit volume to be packed into the filter unit, though like the previously described HRAP, light is a limiting factor, since direct sunlight will only penetrate through a few feet of such an arrangement. However, it is claimed that these carbon biofilters could remove up to 20% of the carbon dioxide, which would, of course, otherwise be released to atmosphere. This makes solving the problem something of a priority. One solution involves the use of a centralised light collector, connected to a series of fibre-optic cables

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seems likely that the increasingly prioritised initiatives to find low-cost systems to bring about effective remediation, effluent control and carbon sequestration will begin to reverse this situation in the near future. However, it is unavoidably true that the wider uptake of all plant-utilising bioengineering applications will depend as much on local modalities as on the actual state of the biotechnologies themselves. In this respect, phytotechnology has much in its favour. For one thing, it has the enormous benefit of virtually assured universal public acceptance, which is rare for any biotechnology. Commercially, it is a relatively low-intervention, highly ‘green’ and thoroughly noncontentious approach to environmental management, which has a strong potentially positive contribution to corporate image, with a relatively low negative influence on the balance sheet. The great advantage that almost all plant-based systems bring to biological engineering is the tremendous energy saving represented by their solar-powered nature. This, combined with their essentially integrated and intrinsically complex array of metabolic mechanisms makes a variety of plant species extremely useful in an environmental context, and typically without any additional need for modification. Given their inherent flexibility, acceptability, efficiency and costeffectiveness, it is difficult to imagine that phytotechnological systems will not be further developed and more widely adopted in the future.

References

Brooks, R., Chambers, M., Nicks, L. and Robonson, B. (1998) Phytomining,

Trends in Plant Science, 3:359 – 62.

Burken, J. and Schnoor, J. (1998) Predictive relationships for uptake of organic contaminants by hybrid poplar trees, Environmental Science and Technology, 32: 3379 – 85.

Chaney, R., Malik, M., Li, Y., Brown, S., Brewer, E., Angel, J. and Baker, A. (1997) Phytoremediation of soil metals, Current Opinion in Biotechnology, 8: 279 – 84.

Foth, H. (1990) Fundamentals of Soil Science, 8th edition, Wiley, New York. Heathcote, I. (2000) Artificial Wetlands for Wastewater Treatment, Prentice Hall/

Pearson, New Jersey.

Hutchinson, G. (1975) A Treatise of Limnology: Volume III. Limnological Botany, Wiley, New York.

Murphy, M. and Butler, R. (2002) Eucalyptus could be worth weight in gold,

Chemistry and Industry, 1: 7.

Otte, M., Kearns, C. and Doyle, M. (1995) Accumulation of arsenic and zinc in the rhizosphere of wetland plants, Bulletin of Environmental Contamination Toxicology, 55: 154 – 61.

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Seven beds, amounting to a total of five hectares (12.5 acres) were constructed in 1990, with the design specification of ultimately being able to treat 3000 m3 of effluent daily.

The system has some noteworthy and unusual features. Reed beds are often constructed by excavation, but at the Billingham site, one-metre high clay bunds were created surrounding each of the seven individual beds. The resulting cells were lined with an impermeable polymer membrane, equipped with drainage pipes and filled in with soil before planting. The drainage arrangements are also an important part of the overall process. Unlike many macrophyte beds, where the main prevailing hydraulic conduction lies effectively in a horizontal direction, at Billingham much of the treatment takes place during the vertical flow, down through the soil, which is further enhanced by the provision of outlet pipes at intervals across the bed, rather than simply at one end. As a result of this somewhat atypical design, it has been possible to achieve the required level of effluent polishing with a significantly smaller land requirement than would otherwise have been necessary, which has important practical and economic implications.

Historically, the environmental record of the chemical industry globally has not always been a glorious one. However, the successes of commercial projects like this one, clearly show that the application of biotechnology in general, and phytotechnology in particular, can bring very real benefits, without compromising either the local ecology or economy.

8

Biotechnology and Waste

As mentioned in the first chapter, waste represents one of the three key intervention points for the potential use of environmental biotechnology. Moreover, in many ways this particular area of application epitomises much of the whole field, since the management of waste is fundamentally unglamorous, typically funded on a distinctly limited budget and has traditionally been viewed as a necessary inconvenience. However, as the price of customary disposal or treatment options has risen, and ever more stringent legislation been imposed, alternative technologies have become increasingly attractive in the light of their greater relative cost-effectiveness. Nowhere has this shift of emphasis been more apparent than in the sphere of biological waste treatment.

With all of environmental biotechnology it is a self-evident truism that whatever is to be treated must be susceptible to biological action and hence the word ‘biowaste’ has been coined to distinguish the generic forms of organic-origin refuse which meet this criterion, from waste in the wider sense, which does not. This approach also removes much of the confusion which has, historically, dogged the issue, since the material has been variously labelled putrescible, green, yard, food or even just organic waste, at certain times and by differing authors, over the years. By accepting the single term biowaste to cover all such refuse, the difficulties produced by regionally, or nationally, accepted criteria for waste categorisation are largely obviated and the material can be viewed purely in terms of its ease of biodegradability. Hence a more process-based perspective emerges, which is often of considerably greater relevance to the practical concerns of actually utilising biotechnology than a straightforward consideration of the particular origins of the waste itself.

The Nature of Biowaste

Biowaste arises from a number of human activities, including agriculture, horticulture and industry, broadly falling into one of the following three major categories: faeces/manures, raw plant matter or process waste. This fits neatly into the process-orientated approach mentioned above, since the general characteristics of each are such that biological breakdown proceeds in essentially the same manner within the group and, thus, the ease of their decomposition is closely similar. Although, at least chemically speaking, biowaste can be seen