Environmental Biotechnology - Chapter 5 potx

The importance of land remediation in cleaning up theresidual effects of previous human activities on a site lies in two spheres.. In broad terms it is possible to view the driving force

5 Contaminated Land

and Bioremediation

Contaminated land is another example of a widely appreciated, yet often poorlyunderstood, 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 themanifestation of the other The importance of land remediation in cleaning up theresidual 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 areal drive for compliance and a much greater awareness of liability issues withinindustry 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 itsmany individual forms, is only one Though it will, of course, provide the mainfocus of this discussion, it is important to realise that the arguments presentedelsewhere 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, expresslynonbiological methods of remediation may be indicated as the best practicableenvironmental option (BPEO) It is impossible to disassociate contextual factorsfrom wider issues entirely Accordingly, and to establish the relevancy of thewider 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 ofsites may give rise to possible contamination concerns, such as asbestos works,chemical works, garages and service stations, gas works, incinerators, iron andsteel works, metal fabrication shops, paper mills, tanneries, textile plants, timbertreatment 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

of 360 000 hectares (900 000 acres) of land may be affected by contamination inone form or another (BioWise 2001) Much of this will, of course, be in primeurban 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 UKEnvironment 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 importancebecause of pressures on industry and developers The motive force is, then, alargely commercial one and, consequently, this imposes its own set of conditionsand 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 arenormally 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 asale or transfer, comply with legislative, licensing or planning requirements, or

to bolster corporate image or public relations Generally, one or more of thesehave to be present before remediation happens

Having established the need for treatment, the actual remedies to be employedwill 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 toremediate is essentially commercial, only land for which remediation is eithernecessary or worthwhile will tend to be treated and then to a level which eithermakes it suitable for its intended use or brings it to a condition which no longerposes 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 wholecontaminated land issue Hence, in purely economic terms, remediation will onlytake place when one or more of the driving forces becomes sufficiently com-pelling to make it unavoidable It will also tend towards the minimum acceptablestandard necessary to achieve the required clean-up This is not an example ofindustrial self-interest at its worst, but rather the exercise of responsible manage-ment, since resources for remediation are typically limited and so their effectiveuse is of great importance To ‘over’ remediate any one given site could seriously

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 sothat it no longer poses unacceptable risk and, once the relevant aim has beenachieved, 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, whichmaximises the potential reuse of former industrial land thereby protecting urbanopen 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 pose In this respect, discussions of absolute quality become less relevant than aconsideration of minimum acceptable standards

pur-The choice of method and the determination of the final remediation standardwill always be chiefly governed by site-specific factors including intended use,local conditions and sensitivities, potential risk and available timeframe For thisreason, it is appropriate to take a brief overview of the available technologies atthis point, to set the backdrop for the discussions of the specifically biotechno-logical methods to come

Biological methods involve the transformation or mineralisation of contaminants

to less toxic, more mobile, or more toxic but less mobile, forms This can includefixation or accumulation in harvestable biomass crops, though this approach isdiscussed 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 andtheir generally nontoxic, ‘green’ image On the other hand, the process end-pointcan be uncertain and difficult to gauge, the treatment itself may be slow and notall contaminants are conducive to treatment by biological means

Chemical

Toxic compounds are destroyed, fixed or neutralised by chemical reaction Theprincipal advantages are that under this approach, the destruction of biologically

recalcitrant chemicals is possible and toxic substances can be chemicallyconverted to either more or less biologically available ones, whichever is required.

On the downside, it is possible for contaminants to be incompletely treated, thereagents necessary may themselves cause damage to the soil and often there is aneed for some form of additional secondary treatment

Physical

This involves the physical removal of contaminated materials, often by tration and excavation, for further treatment or disposal As such, it is not trulyremediation, though the net result is still effectively a clean-up of the affectedsite Landfill tax and escalating costs of special waste disposal have made remedi-ation an increasingly cost-effective option, reversing earlier trends which tended

concen-to favour this method The fact that it is purely physical with no reagent additionmay be viewed as an advantage for some applications and the concentration ofcontaminants significantly reduces the risk of secondary contamination However,the contaminants are not destroyed, the concentration achieved inevitably requirescontainment measures and further treatment of some kind is typically required

Solidification/vitrification

Solidification is the encapsulation of contaminants within a monolithic solid ofhigh structural integrity, with or without associated chemical fixation, when it isthen termed ‘stabilisation’ Vitrification uses high temperatures to fuse contami-nated materials

One major advantage is that toxic elements and/or compounds which cannot bedestroyed, are rendered unavailable to the environment As a secondary benefit,solidified soils can stabilise sites for future construction work Nevertheless, thecontaminants are not actually destroyed and the soil structure is irrevocably dam-aged Moreover, significant amounts of reagents are required and it is generallynot suitable for organic contaminants

Thermal

Contaminants are destroyed by a heat treatment, using incineration, tion, pyrolysis or volatisation processes Clearly, the principal advantage of thisapproach is that the contaminants are most effectively destroyed On the nega-tive side, however, this is achieved at typically very high energy cost, and theapproach is unsuitable for most toxic elements, not least because of the strongpotential for the generation of new pollutants In addition, soil organic matter,and, thus, at least some of the soil structure itself, is destroyed

gasifica-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

on no more than where the treatment takes place – on the site or off it – butsince 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 thepresent 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 ings and features to remain undisturbed, in many cases They also avoid many

build-of the potential delays with methods requiring excavation and removal, whileadditionally 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 somedepth within, a site, and of low to medium concentration Additionally, sincethey are relatively slow to act, they are of most use when the available time fortreatment is not restricted

These methods are not, however, without their disadvantages and chief amongstthem is the stringent requirement for thorough site investigation and survey,almost invariably demanding a high level of resources by way of both desktop andintrusive methods In addition, since reaction conditions are not readily controlled,the supposed process ‘optimisation’ may, in practice, be less than optimum andthe true end-point may be difficult to determine Finally, it is inescapable that allsite 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 whetherthe 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 readilyoptimised, process control is easier to maintain and monitoring is more accurateand simpler to achieve In addition, the introduction of specialist organisms, onthose 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 theinevitably increased likelihood of spillage, or potential secondary pollution, rep-resented by such movement Obviously these approaches require a supplementaryarea 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

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 onindividual 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 hasemerged, 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, highintervention strategies, with a heavy demand for resources and high initiation,running and support costs Their key factors are a fast response and low treatmenttime, which makes them excellent for heavy contamination conditions, since theycan make an immediate lessening in pollutant impact Soil washing and thermaltreatments 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 resourcerequirement and lower initiation, running and support costs These technolo-gies have a slower response and a higher treatment time, but their lower costsmake wider application possible, particularly since extensive land remediationtreatments do less damage to soil quality Accordingly, they are well suited

to large-scale treatment where speed is not of the essence Examples include

composting, the promotion of biological activity in situ within the root-zone,

precipitation of metal sulphides under anaerobic conditions and the cropping ofheavy metal accumulator plants

All these systems of classification are at best generalisations, and each can beuseful at different times, dependent on the purpose of the consideration They aremerely 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 ispossible 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 tion is that it is defined on the basis of representing the fate of the pollutant,

systemisa-Figure 5.2 Technology classification

rather than the geographical location of the work or the level of complexity ofthe technology used, as in the previous cases In addition, it can also be relativelyeasily extended to take account of any given technology.

Process Integration

However they are classified, the fact remains that all the individual gies available each have their limitations As a result, one potential means ofenhancing remediation effectiveness which has received increasing attention isthe use of a combination approach, integrating different processes to provide anoverall treatment The widespread application of this originated in the USA andthe related terms used to describe it, ‘bundled technologies’ or ‘treatment trains’have quickly become commonly used elsewhere The goal of process integrationcan be achieved by combining both different fundamental technologies (e.g bio-

technolo-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 makesthe whole approach to cleaning up land far more flexible The enhanced abil-ity this confers for individually responsive interventions stands as one of the keyfactors 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 whereappropriate, switching over to less engineered or resource-hungry technologiesfor the long-haul to achieve full and final treatment

As has been mentioned before, commercial applicability lies at the centre ofbiotechnology, and process integration has clear economic implications beyondits ability simply to increase the range of achievable remediation One of the mostsignificant of these is that complex contamination scenarios can be treated morecheaply, by the integrated combination of lower cost techniques This opens

up the way for higher cost individual methods to be used only where lutely necessary, for example in the case of major contamination events or acutepollution incidents With limited resources typically available for remediationwork, treatment trains offer the possibility of maximising their utilisation byenabling responsible management decisions to be made on the basis of meaning-ful cost/benefit analysis

abso-This is an important area for the future, particularly since increased experience

of land remediation successes has removed many of the negative perceptionswhich were previously commonplace over efficiency, speed of treatment andgeneral acceptability For many years remediation techniques, and bioremedia-tion especially, were seen in a number of countries as just too costly comparedwith landfill As changes in waste legislation in several of these regions havedriven up the cost of tipping and begun to restrict the amount of biodegradablematerial entering landfills, the balance has swung the other way, making remedi-ation the cheaper option There is a certain irony that the very alternative whichfor so long held back the development of remediation should now provide such a

strong reason for its use In the future, wider usage of extensive technologies mayincrease the trend, since they offer the optimum cost/benefit balance, with inten-sive processes becoming specialised for fast-response or heavy contaminationapplications In addition, the ‘treatment train’ approach, by combining technolo-gies to their maximum efficiency, offers major potential advantages, possiblyeven 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 residualeffects of previous human activities on a site, typically relies on the inherentabilities and characteristics of indigenous bacteria, fungi or plant species In thepresent discussion, the emphasis will concentrate on the contribution made bythe first two types of organism The use of plants, including bioaccumulation,phytoextraction, phytostabilisation and rhizofiltration, all of which are sometimescollectively 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, liganddegradation or oxidation Microbes capable of utilising a variety of carbon sourcesand 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/orefficiently This is the basis of the majority of bioremediation and proceeds bymeans 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 generationand possible accumulation of intermediate byproducts, which may themselves befurther 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 orplant species

Unsurprisingly, given the expressly biological systems involved, ation is most suited to organic chemicals, but it can also be effective in thetreatment of certain inorganic substances and some unexpected ones at that Met-als and radionuclides are good examples of this Though, obviously, not directlybiodegradable themselves, under certain circumstances their speciation can bechanged which may ultimately lead to their becoming either more mobile and

bioremedi-Table 5.1 Potential for bioremediation of selected contaminants

Readily possible Possible under certain

circumstances

Currently impossible

Chlorophenols Pesticides, herbicides

Crude oil and petroleum and fungicides

Developments in bioprocessing continually redefine the definitive catalogue ofwhat 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 ofthe 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 existfor which the application of remediating biotechnologies could have potentialrelevance 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 thecharacter of the contamination itself and those which depend on environmentalconditions The former encompass both the chemical nature of the pollutantsand the physical state in which they are found in a given incident Thus, inorder for a given substance to be open to bioremediation, clearly it must be bothsusceptible to, and readily available for, biological decomposition Generally itmust 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 ofsignificance 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 erated However, for greatest efficiency, the ideal range is around 20–30◦C, as

tol-this tends to optimise enzyme activity In much the same way, a pH of 6.5–7.5would be seen as optimum, though ranges of 5.0–9.0 may be acceptable, depen-dent on the individual species involved Generally speaking, sands and gravelsare the most suitable soil types for bioremediation, while heavy clays and thosewith a high organic content, like peaty soils, are less well indicated However,this is not an absolute restriction, particularly since developments in bioremedi-ation techniques have removed the one-time industry maxim that clay soils wereimpossible to treat biologically.

It should be apparent that these are by no means the only aspects whichinfluence the use of remediation biotechnologies Dependent on the circum-stances; nutrient availability, oxygenation and the presence of other inhibitorycontaminants can all play an important role in determining the suitability ofbioremediation, but these are more specific to the individual application A num-ber of general questions are relevant for judging the suitability of biologicaltreatment The areas of relevance are the likes of the site character, whether it iscontained or if the groundwater runs off, what contaminants are present, wherethey are, in what concentrations and whether they are biodegradable Other typi-cal considerations would be the required remediation targets and how much time

is available to achieve them, how much soil requires treatment, what alternativetreatment methods are available and at what cost

Clearly then, there are benefits to the biological approach in terms of tainability, contaminant removal or destruction and the fact that it is possible totreat large areas with low impact or disturbance However, it is not without itslimitations For one thing, compared with other technologies, bioremediation is

sus-often relatively slower, especially in situ, and as has been discussed, it is not

equally suitable for all soils Indeed, soil properties may often be the largest gle influence, in practical terms, on the overall functional character of pollution,since they are major factors in modifying the empirical contamination effect.The whole issue may be viewed as hierarchical The primary influence consists

sin-of the contaminants themselves and actual origin sin-of the contamination, whichclearly have a major bearing on the overall picture However, edaphic factorssuch as the soil type, depth, porosity, texture, moisture content, water-holdingcapacity, humus content and biological activity may all interact with the primaryinfluences, and/or with each other, to modify the contamination effect, for better

or worse Figure 5.3 is a simplified illustration of this relationship

Hence, it is not enough simply to consider these elements in isolation; thefunctional outcome of the same contaminant may vary markedly, dependent onsuch site-specific differences

After consideration of the generalised issues of suitability, the decision remains

as to which technique is the most appropriate This is a site-specific issue, for all

of the reasons discussed, and must be made on the basis of the edaphic mattersmentioned previously, together with proper risk assessment and site surveys Atthe end of all these studies and assessments, the site has been investigated by

Figure 5.3 Modification of effect by edaphic factors

desk top and practical means, empirical data has been obtained, the resident tamination has been characterised and quantified, its extent determined, relevantrisk factors identified and risk assessment has been performed The next stage

con-is the formulation of a remediation action plan, making use of the data obtained

to design a response to the contamination which is appropriate, responsible andsafe At this point, having obtained the clearest possible overview, technologyselection forms a major part of this process

When this has been done, and approval has been gained from the relevant tory, regulatory or licensing bodies, as applicable, the last phase is to implementthe remediation work itself

statu-Biotechnology Selection

Although the primary focus of remediation methods commonly falls on gies dependent on a relatively high engineering component, there is one purelybiological treatment option which can be a very effective means of clean-up.Known variously as ‘natural attenuation’, ‘passive remediation’, ‘bioattenuation’

technolo-or ‘intrinsic remediation’, it is appropriate ftechnolo-or sites where the contamination doesnot currently represent a clear danger to human health or the environment Though

it is not an engineered solution, neither is it a ‘do nothing’ approach as is times stated, since it is not an exercise in ignoring the problem, but a reasoneddecision on the basis of the necessary site investigations, to allow nature to takeits course The approach works with natural cycles and the pre-existing indige-nous microbial community to bring about the required treatment The need for agood initial survey and risk assessment is clear, and typically a comprehensivemonitoring programme is established to keep a check on progress