Replacement of existing chemical methods of production with those based on microbial or enzyme action is an important potential area of primary pollution prevention and is one role in which the use of genetically modified organisms could give rise to significant environmental benefit. Biological synthesis, either by whole organisms or by isolated enzymes, tends to operate at lower temperatures and, as a result of high enzymatic specificity, gives a much purer yield with fewer byproducts, thus saving the additional cost of further purification. There are many examples of this kind of industrial usage of biotechnology. In the cosmetics sector, there is a high demand for isopropyl myristate which is used in moisturising creams. The conventional method for its manufacture has a large energy requirement, since the process runs at high temperature and pressure to give a product which needs further refinement before it is suitable for use. An alternative approach, using enzyme-based esterification offers a way to reduce the overall environmental impact by deriving a cleaner, odour-free product, and at higher yields, with lower energy requirements and less waste for disposal.
There is a long tradition of the use of biological treatment methods in the clothing and textile industry, dating back to the first use of amylase enzymes from malt extract, at the end of the nineteenth century, to degrade starch-based sizes for cheap and effective reduction of fabric stiffness and improvement to its drape. Currently, novel enyzmatic methods provide a fast and inexpensive alternative to traditional flax extraction by breaking down the woody material in flax straw, reducing the process time from seven to ten days, down to a matter of hours. The enzyme-based retting processes available for use on hemp and flax produce finer, cleaner fibres, and, consequently, novel processing techniques are being developed to take advantage of this. Interest is growing in the production of new, biodegradable polymeric fibres which can be synthesised using modified soil bacteria, avoiding the current persistence of these materials in landfills, long after garments made from them are worn out.
In natural fibre production enzymes are useful to remove the lubricants which are introduced to prevent snagging and reduce thread breakage during spinning, and to clean the natural sticky secretions present on silk. The process of bioscour-ing for wool and cotton, uses enzymes to remove dirt rather than traditional chemical treatments and bio-bleaching uses them to fade materials, avoiding both the use of caustic agents and the concomitant effluent treatment problems such conventional methods entail. Biological catalysts have also proved effective in shrink-proofing wool, improving quality while ameliorating the wastewater produced, and reducing its treatment costs, compared with chemical means.
A process which has come to be called biopolishing involves enzymes in shearing off cotton microfibres to improve the material’s softness and the drape and resistance to pilling of the eventual garments produced.
Biostoning has been widely adopted to produce ‘stone-washed’ denim, with enzymes being used to fade the fabric rather than the original pumice stone method, which had a higher water consumption and caused abrasion to the denim.
However, perhaps the most fitting example of environmental biotechnology in the textile industry, though not really in a ‘clean technology’ role, is the incorpo-ration of adsorbers and microbes within a geotextile produced for use in land man-agement around railways. Soaking up and subsequently biodegrading diesel and grease, the textile directly reduces ground pollution, while also providing safer working conditions for track maintenance gangs and reducing the risk of fire.
The leather industry has a lengthy history of using enzymes. In the bating process, residual hair and epidermis, together with nonstructural proteins and carbohy-drates, are removed from the skins, leaving the hide clean, smooth and soft. Traditionally, pancreatic enzymes were employed. Moreover, something in the region of 60% of the input raw materials in leather manufacturing ultimately ends up being discarded and enzyme additions have long been used to help manage this waste. Recent advances in biotechnology have seen the upsurge in the use of microbially-derived biological catalysts, which are cheaper and easier to produce, for the former applications, and the possibility of converting waste products into saleable commodities, in the latter.
As well as these improvements on existing uses of biotechnology, new areas of clean application are emerging for tanners. Chemical methods for unhairing hides dissolve the hairs, making for efficient removal, but adding to the treat-ment cost, and the environmental implications, of the effluents produced, which are of high levels of COD and suspended solids. Combining chemical agents and biological catalysts significantly lessens the process time while reducing the quantities of water and chemicals used. The enzymes also help make intact hair recovery a possibility, opening up the prospect of additional income from a cur-rent waste. It has been estimated that, in the UK, for a yearly throughput of 400 000 hides, enzymatic unhairing offers a reduction of around 2% of the total annual running costs (BioWise 2001). While this may not seem an enormous contribution, two extra factors must be borne in mind. Firstly, the leather indus-try is very competitive and, secondly, as effluent treatment becomes increasingly more regulated and expensive, the use of clean manufacturing biotechnology will inevitably make that margin greater.
Degreasing procedures are another area where biotechnological advances can benefit both production and the environment, since conventional treatments pro-duce both airborne volatile organic compounds (VOCs) and surfactants. The use of enzymes in this role not only gives better results, with a more consistent qual-ity, better final colour and superior dye uptake, but also considerably reduces VOC and surfactant levels. The leather industry is also one of the places where biosensors may have a role to play. With the ability to give almost instanta-neous detection of specific contaminants, they may prove of value in giving early warning of potential pollution problems by monitoring production processes as they occur.
Microbial desulphurisation of coal and oil represents a further potential example of pollution control by the use of clean technology. The sulphur content of these fossil fuels is of environmental concern principally as a result of its having been implicated in the production of acid rain, since it produces sulphur dioxide (SO2) on combustion. Most of the work done to date has tended to focus on coal, largely as a result of its widespread use in power stations, though similar worries equally surround the use of high-sulphur oils, particularly as the reserves of low-sulphur fuels dwindle. The sulphurous component of coal typically constitutes between 1 – 5%; the content for oil is much more variable, dependent on its type and original source.
There are two main ways to reduce SO2 emissions. The first is to lessen the sulphur content of the fuel in the first place, while the second involves removing it from the flue gas. There are a number of conventional methods for achieving the latter, the most commonly encountered being wet scrubbing, though a dry absorbent injection process is under development. At present, the alternative approach of reducing the sulphur present in the initial fuel, works out around five times more expensive than removing the pollutant from the flue gas, though as stock depletion forces higher sulphur coals and oils to be burnt, the economics of this will start to swing the other way. Methods for achieving a sulphur content reduction include washing pulverised coal and the use of fluidised bed technology in the actual combustion itself, to maximise clean burn efficiency.
Sulphur is present in coal in a variety of different forms, both organic and inor-ganic and biological methods for its removal have been suggested as alternatives to the physical means mentioned above. Aerobic, acidophilic chemolithotrophes like certain of the Thiobacillusspecies, have been studied in relation to the desulphurisation of the inorganic sulphur in coal (Rai 1985). Microbes of this genus have long been known to oxidise sulphur during the leaching of metals like copper, nickel, zinc and uranium from low grade sulphide ores. Accord-ingly, one possible application which has been suggested would be the use of a heap-leaching approach to microbial desulphurisation at the mine itself, which is a technique commonly employed for metals. However, though this is, clearly, a cheap and simple solution, in practice it is difficult to maintain optimum condi-tions for the process. The micro-organisms which have most commonly been used to investigate this possible approach are mesophiles and the rapid temperature increases experienced coupled with the lengthy period of contact time required, at around 4 – 5 days, form major limiting factors. The use of extreme thermophile microbes, like Sulfolobus sp. may offer the way ahead, giving a faster rate of reaction, though demanding the more sophisticated and engineered environment of a bioreactor if they are to achieve their full process efficiency.
The removal of organic sulphur from coal has been investigated by using model organic substrates, most commonly dibenzothiophene (DBT). In labora-tory experiments, a number of organisms have been shown to be able to remove organic sulphur, including heterotrophes (Rai and Reyniers 1988) like Pseu-domonas, Rhizobium and the fungi Paecilomyces and chemolithotrophes like Sulfolobus, mentioned earlier. These all act aerobically, but there is evidence tosuggest that some microbes, likeDesulfovibrio can employ an anaerobic route (Holland et al. 1986). While the use of such model substrates has some validity, since thiophenes are the major organic sulphur components in coal, how well their breakdown accurately reflects the situation for the real material remains much less well known.
A range of putative bioreactor designs for desulphurisation have been put for-ward, involving treatment systems of varying complexity, which may ultimately provide an economic and efficient method for removing sulphur from these fuels prior to burning. However, the state of the art is little advanced beyond the lab-oratory bench and so the benefits of large-scale commercial applications remain to be seen.
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