Environmental Biotechnology - Chapter 5 potx

5 Contaminated Land and Bioremediation Contaminated land is another example of a widely appreciated, yet often poorly understood, environmental problem, in much the same way as discussed for pol- lution in the last chapter. That this should be the case is, of course, unsurprising, since the two things are intimately linked, the one being, in essence, simply the manifestation of the other. The importance of land remediation in cleaning up the residual effects of previous human activities on a site lies in two spheres. Firstly, throughout the world, environmental legislation is becoming increasingly strin- gent and the tightening up of the entire regulatory framework has led to both a real drive for compliance and a much greater awareness of liability issues within industry. Secondly, as the pressure grows to redevelop old, unused or derelict so- called ‘brown-field’ sites, rather than develop previously untouched ‘green-field’, the need to remove any legacy of previous occupation is clear. A number of tech- nologies are available to achieve such a clean-up, of which bioremediation, in its many individual forms, is only one. Though it will, of course, provide the main focus of this discussion, it is important to realise that the arguments presented elsewhere in this book regarding the high degree of specificity which governs technology selection within biotechnological applications also applies between alternative solutions. In this way, for some instances of contamination, expressly nonbiological methods of remediation may be indicated as the best practicable environmental option (BPEO). It is impossible to disassociate contextual factors from wider issues entirely. Accordingly, and to establish the relevancy of the wider setting, alternative remediation techniques will be referred to a little later in this chapter. The idea of ‘contaminated land’ is something which is readily understood, yet, like pollution, somewhat more difficult to define absolutely. Implicit is the pres- ence of substances which, when present in sufficient quantity or concentration, are likely to cause harm to the environment or human health. Many kinds of sites may give rise to possible contamination concerns, such as asbestos works, chemical works, garages and service stations, gas works, incinerators, iron and steel works, metal fabrication shops, paper mills, tanneries, textile plants, timber treatment plants, railway yards and waste disposal sites. This list is not, of course, exhaustive and it has been estimated that in the UK alone something in the region 90 Environmental Biotechnology of 360 000 hectares (900 000 acres) of land may be affected by contamination in one form or another (BioWise 2001). Much of this will, of course, be in prime urban locations, and therefore has the potential to command a high market price, once cleaned up. Since the whole question of contaminated land increasingly forms the basis of law and various professional codes of practice, there is an obvious need for a more codified, legal definition. The version offered in Section 57 of the UK Environment Act 1995 is a typical example: any land which appears to be in a condition that significant harm is being caused or there is a significant possibility of significant harm (or) pollution of controlled waters. In this, harm is expressly defined as to human health, environment, property . As was mentioned earlier, land remediation continues to grow in importance because of pressures on industry and developers. The motive force is, then, a largely commercial one and, consequently, this imposes its own set of conditions and constraints. Much of environmental biotechnology centres on the ‘unwanted’ aspects of human activity and the clean-up of contaminated land is no exception to this general trend. As such, it is motivated by necessity and remedies are normally sought only when and where there is unacceptable risk to human health, the environment and occasionally to other vulnerable targets. In broad terms it is possible to view the driving forces on remediation as characterised by a need to limit present or future liability, increase a site’s value, ease the way for a sale or transfer, comply with legislative, licensing or planning requirements, or to bolster corporate image or public relations. Generally, one or more of these have to be present before remediation happens. Having established the need for treatment, the actual remedies to be employed will be based on a realistic set of priorities and will be related to the risk posed. This, of course, will require adequate investigation and risk assessment to deter- mine. It is also important to remember in this context that, since the move to remediate is essentially commercial, only land for which remediation is either necessary or worthwhile will tend to be treated and then to a level which either makes it suitable for its intended use or brings it to a condition which no longer poses an unacceptable risk. It should be apparent, then, from the preceding discussion that the economics of remediation and the effective use of resources are key factors in the whole contaminated land issue. Hence, in purely economic terms, remediation will only take place when one or more of the driving forces becomes sufficiently com- pelling to make it unavoidable. It will also tend towards the minimum acceptable standard necessary to achieve the required clean-up. This is not an example of industrial self-interest at its worst, but rather the exercise of responsible manage- ment, since resources for remediation are typically limited and so their effective use is of great importance. To ‘over’ remediate any one given site could seriously Contaminated Land and Bioremediation 91 compromise a company’s ability to channel sufficient funds to deal with others. The goal of treating land is to make it suitable for a particular purpose or so that it no longer poses unacceptable risk and, once the relevant aim has been achieved, further treatment is typically not a good use of these resources. Gen- erally it would be judged better to devote them to cleaning up other sites, which maximises the potential reuse of former industrial land thereby protecting urban open spaces and the countryside from development pressure. In the long term, the sustainable use of land largely depends on making sure that it is maintained at a level which enables its continued best use for its current or intended pur- pose. In this respect, discussions of absolute quality become less relevant than a consideration of minimum acceptable standards. The choice of method and the determination of the final remediation standard will always be chiefly governed by site-specific factors including intended use, local conditions and sensitivities, potential risk and available timeframe. For this reason, it is appropriate to take a brief overview of the available technologies at this point, to set the backdrop for the discussions of the specifically biotechno- logical methods to come. Remediation Methods The currently available processes for soil remediation can be divided into five generalised categories: • biological; • chemical; • physical; • solidification/vitrification; • thermal. Biological Biological methods involve the transformation or mineralisation of contaminants to less toxic, more mobile, or more toxic but less mobile, forms. This can include fixation or accumulation in harvestable biomass crops, though this approach is discussed more fully later in Chapter 7. The main advantages of these methods are their ability to destroy a wide range of organic compounds, their potential benefit to soil structure and fertility and their generally nontoxic, ‘green’ image. On the other hand, the process end-point can be uncertain and difficult to gauge, the treatment itself may be slow and not all contaminants are conducive to treatment by biological means. Chemical Toxic compounds are destroyed, fixed or neutralised by chemical reaction. The principal advantages are that under this approach, the destruction of biologically 92 Environmental Biotechnology recalcitrant chemicals is possible and toxic substances can be chemically converted to either more or less biologically available ones, whichever is required. On the downside, it is possible for contaminants to be incompletely treated, the reagents necessary may themselves cause damage to the soil and often there is a need for some form of additional secondary treatment. Physical This involves the physical removal of contaminated materials, often by concen- tration and excavation, for further treatment or disposal. As such, it is not truly remediation, though the net result is still effectively a clean-up of the affected site. Landfill tax and escalating costs of special waste disposal have made remedi- ation an increasingly cost-effective option, reversing earlier trends which tended to favour this method. The fact that it is purely physical with no reagent addition may be viewed as an advantage for some applications and the concentration of contaminants significantly reduces the risk of secondary contamination. However, the contaminants are not destroyed, the concentration achieved inevitably requires containment measures and further treatment of some kind is typically required. Solidification/vitrification Solidification is the encapsulation of contaminants within a monolithic solid of high structural integrity, with or without associated chemical fixation, when it is then termed ‘stabilisation’. Vitrification uses high temperatures to fuse contami- nated materials. One major advantage is that toxic elements and/or compounds which cannot be destroyed, are rendered unavailable to the environment. As a secondary benefit, solidified soils can stabilise sites for future construction work. Nevertheless, the contaminants are not actually destroyed and the soil structure is irrevocably dam- aged. Moreover, significant amounts of reagents are required and it is generally not suitable for organic contaminants. Thermal Contaminants are destroyed by a heat treatment, using incineration, gasifica- tion, pyrolysis or volatisation processes. Clearly, the principal advantage of this approach is that the contaminants are most effectively destroyed. On the nega- tive side, however, this is achieved at typically very high energy cost, and the approach is unsuitable for most toxic elements, not least because of the strong potential for the generation of new pollutants. In addition, soil organic matter, and, thus, at least some of the soil structure itself, is destroyed. In Situ and Ex Situ Techniques A common way in which all forms of remediation are often characterised is as in situ or ex situ approaches. These represent largely artificial classes, based Contaminated Land and Bioremediation 93 on no more than where the treatment takes place – on the site or off it – but since the techniques within each do share certain fundamental operational sim- ilarities, the classification has some merit. Accordingly, and since the division is widely understood within the industry, these terms will be used within the present discussion. In situ The major benefit of approaches which leave the soil where it is for treatment, is the low site disturbance that this represents, which enables existing build- ings and features to remain undisturbed, in many cases. They also avoid many of the potential delays with methods requiring excavation and removal, while additionally reducing the risk of spreading contamination and the likelihood of exposing workers to volatiles. Generally speaking, in situ methods are suited to instances where the contamination is widespread throughout, and often at some depth within, a site, and of low to medium concentration. Additionally, since they are relatively slow to act, they are of most use when the available time for treatment is not restricted. These methods are not, however, without their disadvantages and chief amongst them is the stringent requirement for thorough site investigation and survey, almost invariably demanding a high level of resources by way of both desktop and intrusive methods. In addition, since reaction conditions are not readily controlled, the supposed process ‘optimisation’ may, in practice, be less than optimum and the true end-point may be difficult to determine. Finally, it is inescapable that all site monitoring has an in-built time lag and is heavily protocol dependent. Ex situ The main characteristic of ex situ methods is that the soil is removed from where it originally lay, for treatment. Strictly speaking this description applies whether the material is taken to another venue for clean-up, or simply to another part of the same site. The main benefits are that the conditions are more readily optimised, process control is easier to maintain and monitoring is more accurate and simpler to achieve. In addition, the introduction of specialist organisms, on those occasions when they may be required, is easier and/or safer and generally these approaches tend to be faster than corresponding in situ techniques. They are best suited to instances of relatively localised pollution within a site, typically in ‘hot-spots’ of medium to relatively high concentration which are fairly near to the surface. Amongst the main disadvantages are the additional transport costs and the inevitably increased likelihood of spillage, or potential secondary pollution, rep- resented by such movement. Obviously these approaches require a supplementary area of land for treatment and hence they are typically more expensive options. As Figure 5.1 illustrates, the decision to use in situ or ex situ techniques is a comparatively straightforward ‘black-or-white’ issue at the extremes for either 94 Environmental Biotechnology Figure 5.1 Factors affecting technology suitability option. However, the middle ground between them comprises many more shades of grey, and the ultimate resolution in these cases is, again, largely dependent on individual circumstance. Intensive and Extensive Technologies Though the in situ/ex situ classification has established historic precedence, of recent times an alternative approach to categorise remediation activities has emerged, which has not yet achieved the same widespread recognition or accep- tance, but does, nevertheless offer certain advantages over the earlier approach. Perhaps the most significant of these is that it is a more natural division, based on genuine similarities between technologies in each class. Thus the descriptions ‘intensive’ and ‘extensive’ have been suggested. Intensive technologies can be characterised as sophisticated, fast-acting, high intervention strategies, with a heavy demand for resources and high initiation, running and support costs. Their key factors are a fast response and low treatment time, which makes them excellent for heavy contamination conditions, since they can make an immediate lessening in pollutant impact. Soil washing and thermal treatments are good examples of ‘intensive’ approaches. Extensive methods are lower-level interventions, typically slower acting, based on simpler technology and less sophisticated engineering, with a smaller resource requirement and lower initiation, running and support costs. These technolo- gies have a slower response and a higher treatment time, but their lower costs make wider application possible, particularly since extensive land remediation treatments do less damage to soil quality. Accordingly, they are well suited to large-scale treatment where speed is not of the essence. Examples include Contaminated Land and Bioremediation 95 composting, the promotion of biological activity in situ within the root-zone, precipitation of metal sulphides under anaerobic conditions and the cropping of heavy metal accumulator plants. All these systems of classification are at best generalisations, and each can be useful at different times, dependent on the purpose of the consideration. They are merely a convenient way of looking at the available techniques and should not be regarded as anything more than a helpful guide. As a final aspect of this, it is possible to examine the various forms of land remediation technologies in terms of their overall functional principle. Hence, the approaches may be categorised as ‘destructive’, ‘separating’ or ‘containing’, dependent on their fundamental mode of operation, as Figure 5.2 illustrates. The principal attraction of this systemisa- tion is that it is defined on the basis of representing the fate of the pollutant, Figure 5.2 Technology classification 96 Environmental Biotechnology rather than the geographical location of the work or the level of complexity of the technology used, as in the previous cases. In addition, it can also be relatively easily extended to take account of any given technology. Process Integration However they are classified, the fact remains that all the individual technolo- gies available each have their limitations. As a result, one potential means of enhancing remediation effectiveness which has received increasing attention is the use of a combination approach, integrating different processes to provide an overall treatment. The widespread application of this originated in the USA and the related terms used to describe it, ‘bundled technologies’ or ‘treatment trains’ have quickly become commonly used elsewhere. The goal of process integration can be achieved by combining both different fundamental technologies (e.g. bio- logical and chemical) and sequences of in situ or ex situ, intensive or extensive regimes of processing. In many respects, such a ‘pick-and-mix’ attitude makes the whole approach to cleaning up land far more flexible. The enhanced abil- ity this confers for individually responsive interventions stands as one of the key factors in its wider potential uptake. In this way, for example, fast-response appli- cations can be targeted to bring about a swift initial remediation impact where appropriate, switching over to less engineered or resource-hungry technologies for the long-haul to achieve full and final treatment. As has been mentioned before, commercial applicability lies at the centre of biotechnology, and process integration has clear economic implications beyond its ability simply to increase the range of achievable remediation. One of the most significant of these is that complex contamination scenarios can be treated more cheaply, by the integrated combination of lower cost techniques. This opens up the way for higher cost individual methods to be used only where abso- lutely necessary, for example in the case of major contamination events or acute pollution incidents. With limited resources typically available for remediation work, treatment trains offer the possibility of maximising their utilisation by enabling responsible management decisions to be made on the basis of meaning- ful cost/benefit analysis. This is an important area for the future, particularly since increased experience of land remediation successes has removed many of the negative perceptions which were previously commonplace over efficiency, speed of treatment and general acceptability. For many years remediation techniques, and bioremedia- tion especially, were seen in a number of countries as just too costly compared with landfill. As changes in waste legislation in several of these regions have driven up the cost of tipping and begun to restrict the amount of biodegradable material entering landfills, the balance has swung the other way, making remedi- ation the cheaper option. There is a certain irony that the very alternative which for so long held back the development of remediation should now provide such a Contaminated Land and Bioremediation 97 strong reason for its use. In the future, wider usage of extensive technologies may increase the trend, since they offer the optimum cost/benefit balance, with inten- sive processes becoming specialised for fast-response or heavy contamination applications. In addition, the ‘treatment train’ approach, by combining technolo- gies to their maximum efficiency, offers major potential advantages, possibly even permitting applications once thought unfeasible, like diffuse pollution over a large area. The Suitability of Bioremediation Bioremediation as a biotechnological intervention for cleaning up the residual effects of previous human activities on a site, typically relies on the inherent abilities and characteristics of indigenous bacteria, fungi or plant species. In the present discussion, the emphasis will concentrate on the contribution made by the first two types of organism. The use of plants, including bioaccumulation, phytoextraction, phytostabilisation and rhizofiltration, all of which are sometimes collectively known as phytoremediation, is examined as part of a separate chapter. Thus, the biological mechanisms underlying the relevant processes are biosorp- tion, demethylation, methylation, metal-organic complexation or chelation, ligand degradation or oxidation. Microbes capable of utilising a variety of carbon sources and degrading a number of typical contaminants, to a greater or lesser extent, are commonly found in soils. By enhancing and optimising conditions for them, they can be encouraged to do what they do naturally, but more swiftly and/or efficiently. This is the basis of the majority of bioremediation and proceeds by means of one of the three following general routes. Mineralisation, in which the contaminant is taken up by microbe species, used as a food source and metabolised, thereby being removed and destroyed. Incomplete, or staged, decomposition is also possible, resulting in the generation and possible accumulation of intermediate byproducts, which may themselves be further treated by other micro-organisms. Cometabolism, in which the contaminant is again taken up by microbes but this time is not used as food, being metabolised alongside the organism’s food into a less hazardous chemical. Subsequently, this may in turn be mineralised by other microbial species. Immobilisation, which refers to the removal of contaminants, typically met- als, by means of adsorption or bioaccumulation by various micro-organism or plant species. Unsurprisingly, given the expressly biological systems involved, bioremedi- ation is most suited to organic chemicals, but it can also be effective in the treatment of certain inorganic substances and some unexpected ones at that. Met- als and radionuclides are good examples of this. Though, obviously, not directly biodegradable themselves, under certain circumstances their speciation can be changed which may ultimately lead to their becoming either more mobile and 98 Environmental Biotechnology Table 5.1 Potential for bioremediation of selected contaminants Readily possible Possible under certain circumstances Currently impossible Acids Chlorinated solvents Asbestos Alcohols Cyanides Asphalt Aldehydes and ketones Explosives Bitumen Ammonia PCBs Inorganic acids Creosote PAHs Chlorophenols Pesticides, herbicides Crude oil and petroleum and fungicides hydrocarbons Tars Glycols Timber treatments Phenols Surfactants accessible or less so. The net result produced in either case can, under the right conditions, be a very effective functional remediation. A list of typical contam- inants suitable for bioremediation would include the likes of crude oil and its derivatives, some varieties of fungicides and herbicides, hydrocarbons, glycols, phenols, surfactants and even explosives. Developments in bioprocessing continually redefine the definitive catalogue of what may, and may not, be treated and many chemicals once thought ‘impossible’ are now routinely dealt with biologically. Table 5.1 reflects the current state of the art, though this is clearly subject to change as new approaches are refined. As a result, it should be obvious that a large number of opportunities exist for which the application of remediating biotechnologies could have potential relevance. Even so, there are a number of factors which affect their use, which will be considered before moving on to discuss practical treatment issues themselves. Factors Affecting the use of Bioremediation It is possible to divide these into two broad groups; those which relate to the character of the contamination itself and those which depend on environmental conditions. The former encompass both the chemical nature of the pollutants and the physical state in which they are found in a given incident. Thus, in order for a given substance to be open to bioremediation, clearly it must be both susceptible to, and readily available for, biological decomposition. Generally it must also be dissolved, or at the very least, in contact with soil water and typ- ically of a low–medium toxicity range. The principal environmental factors of significance are temperature, pH and soil type. As was stated previously, biore- mediation tends to rely on the natural abilities of indigenous soil organisms and so treatment can occur between 0–50 ◦ C, since these temperatures will be tol- erated. However, for greatest efficiency, the ideal range is around 20–30 ◦ C, as [...]... information be required 106 Environmental Biotechnology Figure 5. 7 Illustrative long-term monitoring scheme However, it is worth noting that for some sites it may be necessary to continue monitoring into the future Under these circumstances, a comprehensive environmental management and audit scheme can be put in place to monitor environmental effects of such operations and Figure 5. 7 shows a suitable illustrative... the costs for remediation were as shown in Table 5. 2 Figure 5. 10 Pie chart of remediation technologies use in the UK (1997) Table 5. 2 A cost comparison of selected technologies Technology Bioremediation Chemical Encapsulation Excavation/Disposal Incineration Soil washing Typical cost (£/m3 ) 10–80 10–100 20–180 30– 75 100–400 15 40 Source: Biotechnology Means Business 1996/7 figures Contaminated Land... treated, down into the region of contamination The extra oxygen availability thus achieved, as in the previous approach described, stimulates the resident microbes, which then treat 104 Environmental Biotechnology Figure 5. 5 Bioventing the polluting substances The air flow through the soil is further driven by vacuum extractors peripheral to the treatment zone, which increases the dissolved oxygen levels... established within the soil matrix, with the actual clean-up activity taking place both within the groundwater and also externally to it The major characteristic of this technique is the two-well system sunk into the ground, the ‘injection well’ and the ‘recovery well’, the former being located Contaminated Land and Bioremediation 1 05 Figure 5. 6 Injection recovery ‘upstream’ of the latter Nutrients... years or more of research in the USA, which gave rise to the ‘Part 50 3 Rule’ Issued in February 1993, the Clean Water Act, specifically the part of it called Title 40 of the Code of Federal Regulations, Part 50 3 – The Standards for the Use or Disposal of Sewage Sludge, which is commonly referred to as the ‘Part 50 3 Rule’ or even simply ‘Part 50 3’, sets out benchmark limits for the USA Typical European regulations... contaminated soil requires it to improve the overall texture, ease of aeration, water-holding capacity or organic matter content This technique is sometimes termed ‘soil composting’ 108 Environmental Biotechnology because of the similarity it has with the windrow method of treatment for biowaste material, which is described in Chapter 8 It is not a true example of the compost process, though there are many... Figure 5. 10 Though this may be of limited relevance in universal terms, since, as has been pointed out throughout, the situation in one country does not necessarily bear any resemblance to that in another, in many ways, it does serve as a useful illustration of the link between economics and the uptake of environmental biotechnology Over the same period, the costs for remediation were as shown in Table 5. 2... bioaugmentation, or occasionally a mixture of both 102 Environmental Biotechnology Bioenhancement concentrates solely on the existing microfauna, stimulating their activity by the manipulation of local environmental conditions Bioaugmentation, by contrast, requires the deliberate introduction of selected microbes to bring about the required clean-up These additions may be unmodified ‘wildtype’ organisms,... isolated from the surroundings by an impermeable clay or high density polyethylene (HDPE) liner, as shown diagrammatically in Figure 5. 8, and typically relies on the activities Contaminated Land and Bioremediation 107 Figure 5. 8 Schematic diagram of land farming of indigenous micro-organisms to bring about the remediation, though specialist bacteria or fungi can be added if required The soil to be treated...Contaminated Land and Bioremediation 99 this tends to optimise enzyme activity In much the same way, a pH of 6 .5 7 .5 would be seen as optimum, though ranges of 5. 0–9.0 may be acceptable, dependent on the individual species involved Generally speaking, sands and gravels are the most suitable soil types for bioremediation, while heavy clays . situ techniques is a comparatively straightforward ‘black-or-white’ issue at the extremes for either 94 Environmental Biotechnology Figure 5. 1 Factors affecting technology suitability option. However,. availability thus achieved, as in the pre- vious approach described, stimulates the resident microbes, which then treat 104 Environmental Biotechnology Figure 5. 5 Bioventing the polluting substances the reader is recom- mended to examine such publications at first hand should detailed information be required. 106 Environmental Biotechnology Figure 5. 7 Illustrative long-term monitoring scheme However,