Bioremediation is a process that uses soil’s naturally occurring micro-organisms to decompose contaminants such as toxic or hazardous substances. Bioremediation works because most of the organic compounds that comprise hazardous wastes can be used as food by micro-organisms (ENSR, 1992, p24). The biological treatment of contaminated soils is primarily based on the actions of microbes to oxidise (metabolise) organic compounds and reduce them into their constituent parts, producing by-products such as cell matter, carbon dioxide, water and other inert materials. This may be caused by the action of a single micro-organism but “more often involves the interaction of two or more microbial species” (Armishaw et al, 1992, pl26). Biodegradation can occur in a number of different ways but the success, or otherwise, of the treatment process will depend upon factors such as the chemical composition of the substance to be treated and the micro-organisms involved, as well as the chemical and physical, e.g. aerobic or anaerobic, environment within which they are located.
Not all contaminants are amenable to treatment by biological processes, “although they are effective against a wide range of common contaminants under the correct conditions” (Armishaw etal, 1992, p i27), with even some man made (xenobiotic) compounds being amenable to treatment. It is therefore necessary to have a good
understanding of the nature of the contaminants, their locations and concentrations before the appropriate biological treatment can be selected.
If “the natural microbial community does not display the desired ability to remove the site contaminants, provision can be made to investigate the feasibility of treating the site with non-indigenous/commercially available bacterial innoculants if a biological treatment is still the desired option” (Armishaw et al,
1992, p. 130), nutrients may also be added and the physical structure of the soil can be modified so as “to enhance mass transfer of oxygen to the site of microbial activity” (Bewley, 1992, p272).
Biological treatments can be undertaken in situ but for difficult soils ex situ
methods may be preferred. The in-situ treatment of contaminated soil does not require excavation but may involve the addition of surfactants, or other agents, to water lying within or infiltrating the contaminant so as to increase its mobility.
Ex-situ biological treatments, such as composting, require the excavation of the contaminated soil and its placement in a purpose designed treatment bed, where it may be mixed with a suitable bulking agent, such as wood chips or sand, to aerate the material and inoculation with water, nutrients and, if necessary, additional microbes.
5.4.2 Chemical systems
Chemical treatment processes alter hazardous constituents in waste streams to reduce their toxicity or mobility, or produce inert compounds from the original material (ENSR, 1992, p36). A wide range of chemical treatments may, in theory, be applied to the remediation of contaminated soil and these may be
categorised according to the chemical processes involved, for example, oxidation, reduction, neutralisation, mobilisation, hydrolysis and polymerisation. The majority of the chemical treatment processes require soil to be in a slurry form or for the contaminants to be mobilised in a liquid medium such as groundwater (Armishaw et al, 1992, p9).
As with bioremediation, chemical treatments can be applied both in-situ and ex- si tu, although “relatively few chemical processes have been applied directly to contaminated soils” but “have been used more widely for cleaning of a wide range of other contaminated materials” (Armishaw et al, 1992, p96). The ENSR Report (1992) describes two chemical methods which have been used at hydrocarbon contaminated sites:
• in-situ (chemical) oxidation and, • ultraviolet-enhanced oxidation.
In certain case, chemical oxidants may be used to decompose or oxidise hydrocarbons in the subsurface (ENSR, 1992, pp36-37). The process is similar to chemical burning and the oxidant is usually a dilute hydrogen peroxide solution, which is injected into the contamination through injection wells at carefully controlled rates. This method is beneficial where hydrocarbons are too highly concentrated, or are too toxic, for successful bioremediation.
Ultraviolet oxidation uses the injection of oxidants, usually hydrogen peroxide or ozone either alone or together, to chemically decompose the hydrocarbons, with the injected stream of material being passed through a bank of ultraviolet lamps to ‘activate’ the oxidisers. “This very active solution then rapidly attacks the hydrocarbons to produce carbon dioxide, water and chloride ions (when
chlorinated hydrocarbons are present)” (ENSR, 1992, p37). Ultraviolet oxidation is only effective on clear aqueous streams but it is capable of destroying some chlorinated hydrocarbons. Pre-treatment may be needed in order to remove suspended solids, colloidal or other material which may plate the ultraviolet lamps but a benefit of the system is that it does not produce any secondary waste streams. Other forms of chemical treatment include those listed in Box 5.4.
BOX 5 .4
_____________TYPES OF CHEMICAL TREATMENT___________________ Reduction - the addition of chemical reducing agents, such as aluminium, sodium
and zinc metals, alkaline polyethylene and glycol, which are then oxidised. Chemical dechlorination - uses reduction reagents to cleave chlorine atoms from
hazardous chlorinated molecules to leave less hazardous compounds, the process can be applied to liquid wastes, sludges and soils.
Extraction - includes techniques such as the use of organic solvents, supercritical extraction and metal extraction with acids.
Supercritical Fluid Extraction - a form of solvent extraction which uses highly compressed gases as the solvent, this requires the temperature and pressure of the solvent to be maintained close to its critical point so that the gas is in its liquid phase and able to dissolve the contaminant.
Electrochemical - has been used for the destruction of PCBs in contaminated fluids and involves mixing the contaminated liquid with a conducting solution in an electrochemical operating at low temperature and low voltage, in the presence of a reagent.
Neutralisation - refers to the adjustment of soil or groundwater pH to an acceptable level (usually in the range pH 6 to 9) using dilute or weak acids or bases (Armishaw
etal, 1992).
Precipitation - used to render contaminants insoluble and thus facilitate their removal from liquids, such as groundwater, by physical processes, for example flocculation, sedimentation or filtration.
5.4.3 Physical systems
“Physical processes do not destroy contaminants and can therefore be considered as first-stage treatment techniques in a multi-stage process; the final step being the destruction or stabilisation of the contaminant” (Armishaw et al, 1992, p45). The objective of physical processing is to separate or isolate the contaminants from the uncontaminated host material, or to concentrate the contaminants, thereby reducing their bulk.
Perhaps the best known physical process is soil washing, which “provides one of the few rapid, relatively cheap, contaminated soil treatment systems The principle of a soil washing system is to:
• separate from the soil those particles containing the contaminants and so produce a concentrate, or
• transfer the contamination into an aqueous medium that can subsequently be treated using a sorbent or by precipitation.”
(Pearl and Wood, 1994, pi) Much of the equipment used for soil washing originated in the minerals processing and metals extraction industries, is therefore widely available and has been well tested, albeit for other purposes. “Soils washing, therefore, does not fall into the category of developing or unproven technologies” (Boyle, 1993, p i57). Whether or not soils washing will be suitable to treat a particular contaminated soil will depend upon the extent to which it can reduce the bulk of the contaminated residues, leaving a smaller volume of material, with a more concentrated level of contamination, for further treatment or disposal.
“As a generalization, if soils washing is to prove cost effective on a given site, it should be possible to recover 70-90% of the mass of the feed material as cleaned, leaving 10-30% as contaminated residue” (Boyle, 1993, pl58). The cleaned material can then be disposed of as uncontaminated, or can be returned to the development site for re-use. As a general rule, coarse, sandy, soils or fill materials with a high proportion of gravel, ash or clinker are best suited to soils washing, as the contaminants tend to adhere to the finer particles of the soil. Therefore the process of washing the finer particles out of the coarser medium should result in maximisation of the recovery of cleaned material. “If the
recovery of cleaned product drops much below 70%, it is unlikely that the application of soils washing will be justified” (Boyle, 1993, p i58). Other physical processes include those listed in Box 5.5.
BOX 5 .5
_____________PHYSICAL TREATMENT METHODS___________________ Ex-situ Steam Stripping - the compounds are evaporated by the steam and the vapours
produced are treated by a number of downstream processes which separate the volatile contaminants from water, such as, steam condensation, water-immiscible oil separation and activated carbon adsorption.
Soil Vapour Extraction or Air Venting Techniques - may be used for the in situ
treatment of volatile or semi-volatile organic compounds (VOCs or SVCs). A series of pipes or wells is sunk into the contaminated ground. These are either connected to vacuum pumps, in which case the negative pressure gradients induce sub-surface air flows which volatise the contaminants, or hot air and steam are injected into the ground so as to volatise not only the VOCs but also many SVCs.
Electroremediation can be applied to the clean-up of soils with a relatively high moisture content, a direct current (DC) is passed through an array of electrodes embedded in the soil and this induces contaminant flow in the pore water to the electrodes by a number of processes;
______ (a) electrolysis, (b) electro-osmosis, and (c) electrophoresis._______________ 5.4.4 Solidification systems
In the context of waste management “the term ‘solidification’ means the conversion of a liquid or a sludge into a solid with good physical characteristics (such as high compressive strength, low permeability etc.) so that the physical handling involved in disposal is made easy. However, solidification does not guarantee stabilization of the hazardous waste”. (Soundararajan, 1992, pi 60) In order to effect treatment through the use of solidification it may be necessary to either accept that the material will still retain its hazardous properties, or to undertake some form of treatment or stabilisation as part of the solidification process.
According to Soundararajan (1992, pl61) the entire phenomenon of organic stabilization may be explained in the following two phases:
Phase 1: a binder is used with an organic compound or compounds in its matrix. The compound(s) would have a similar polarity to the contaminant, so that the organic waste is selectively dissolved in itself. Since the organic compound is part of the binder this may be called the stationary phase.
Phase 2: once the organic molecule has been retained in the stationary phase several chemical reactions, producing different kinds of chemical bonds, can be created between the binder and the waste molecule. A strong interaction between the binder and the waste molecule, reduces the availability of the contaminant to the environment by leach processes.
Solidification systems are generally classified according to the binder system as either inorganic or organic (Armishaw et al, 1992, p i98) and the most important of each includes the following techniques:
• Inorganic - cement based, pozzolan2 based, lime based, liquid silicate and vitrification
• Organic - thermoplastic microencapsulation, thermosetting and macroencapsulation.
The most commonly used binder systems are those which are cement based, often with cement being used in conjunction with pulverised fly ash (PFA) and sodium silicate solution. According to Armishaw et al (1992) lime based systems must always be used in conjunction with a pozzolan material and most contaminated soils could be expected to contain a significant proportion of pozzolan.
Vitrification involves heating the contaminated soil to temperatures exceeding 1,000°C, at which point the inorganic toxic components are incorporated into a hard glass or ceramic-like substance, with any organic pollutants being incinerated. An effective emission control system is required to remove volatile toxic metals and any organic products formed during the process. At the time of
A substance that contains silicates or aluminosilicates that can react with lime and water to form stable, insoluble, compounds which possess cementing properties.
the Armishaw report (1992) both in-situ and ex-situ vitrification systems were being investigated. The same report also stated that “organic binder systems have not been used for the remediation of contaminated soil as the process is very expensive, although a bench-scale test has been described” (Armishaw et al,
1992, pl99). 5.4.5 Thermal systems
There are two principal ways of using heat treatment to remove contaminants: (i) removal of the contaminant by evaporation - either by direct heat transfer
(convection or radiation) from heated air (or other gases), or an open flame, or by direct heat transfer, and
(ii) destruction of the contaminants by direct or indirect heating of the soil to
an appropriate temperature. (Smith, 1987, p i30)
Thermal treatment methods are based upon the fact that all organic and inorganic contaminants have a definite vapour point (Bohm, 1992, p i99). At this point the compound transfers from the solid to the gaseous state and, depending upon the energy input, chemical reactions take place. Oxidation will occur if oxygen is present but if it is not then vaporisation of the compounds will result. The residual compounds can be collected and either condensed out or incinerated.
Most thermal systems are applicable to as wide a range of soil types as their associated handling systems will permit (Armishaw et al, 1992, p i67) and every installation for the thermal treatment of soil basically contains three process stages (Bohm, 1992, p i99), as follows:
preliminary treatment - to sort the soil so as to remove unsuitable material, such as metal parts, following which the soil is pulverised and any unsuitable soil portions removed;
thermal treatment - when the soil is dried, the contaminants driven off and partially destroyed, following which the soil is cooled, with care being taken to recover heat for recycling;
exhaust air treatment and cleaning - which guarantees that the pollutants are fully destroyed or removed and that emission control regulations are complied with.
5 5 EXTENSIVE TREATMENTS
For the most part the treatment methods described in the previous section are intended to remediate the contaminated land in a relatively short period of time, so as to prepare the land for redevelopment, to prevent harm to potential targets or to mitigate harm which is actually occurring. Some of the treatments are still at the experimental stage, whilst others may involve such high levels of cost as to render them commercially unacceptable. This is especially the case when immediate redevelopment is not contemplated, only a restricted type of development is to be permitted, or when the ‘do-nothing’ option has been selected. In such circumstances it may be appropriate to select a long term or extensive form of treatment, in order to ameliorate the contaminative state of the site over a period of several years.
Bardos and van Veen (1995) identified the opportunities for extensive technologies as including:
• the polishing of partially remediated sites under development though still subject to restrictions on use, to ensure continued remediation of the site during the lifetime of the development and hence increase its value by the end of the lifetime of the proposed development;
• remediation of active industrial sites over the remainder of their operating lifetime so that they are remediated by the time disposal of the site is envisaged;
• remediation of sites which have been dealt with by isolation, monitoring and containment, so that the site is remediated within the design lifetime of the containment measures;
• treatment of sites that are too large to be cleaned economically using “intensive” technologies; managing downstream contamination from persistent and inaccessible sources;
• treatment of excavated material removed from sites.
(Bardos and van Veen, 1995, pi) Research is being undertaken into the use of hyperaccumulator plants, active as opposed to passive containment barriers, the stimulation of natural on-site processes of attenuation and decay, and the promotion of biological activity through the use of plant roots. Extensive methods under consideration for use include composting or digestion for soil and waste co-treatment, in-situ
precipitation of metal sulphides under anaerobic conditions and in-situ treatments contained in emplacements across aquifers or other drainage pathways. Technologies with the potential for use in extensive treatments include bioventing and various containment methods coupled with in-situ treatments.
Risk management is a key issue in the selection of an extensive treatment approach (Bardos and van Veen, 1995, p3) and the fact that the lifetime of the contaminant will be extended must be taken into account in the risk assessment exercise. It may therefore be necessary to contain the contaminated soil within a secure area, which may for example require its excavation and re-deposition within a bunded area formed with an impervious material such as clay or a geomembrane. A treatment technology which is gradual and exploits the natural processes occurring within the ground can then be employed to break down the
contaminants. Monitoring of the containment area will be required, in order to ensure its security and to assess the effectiveness of the process.
Although extensive treatments are not relevant in situations where immediate, or short term, redevelopment is desired, there may be a place for them within the development process, especially when sufficient land is available within the overall site for a containment area to be established. For example if it is intended that only part of a site will be redeveloped, with the remainder being allocated to public open space, it may be appropriate to remove contaminated soil from within the development site and establish a secure containment in the public open space area. Whether or not it will be able to put the open space area to its intended use within the short to medium term will depend upon the nature of the contaminants and the type of treatment selected, but deferring the availability of the public open space may be preferable to disposing of the contaminated soil into landfill.
5.6 COST AND EFFECTIVENESS OF TREATMENTS
The cost and effectiveness of available treatment options will have a direct impact on most, if not all, of the nine categories of factors which influence the choice of remedial strategy and design, listed in section 5.1. Certainly effectiveness of the treatment will be important when considering the legal implications, as the authorities responsible for ensuring compliance with the legislation will need to be convinced that satisfactory standards of remediation are achieved. Cost would, at first sight, seem to be less important from the legal viewpoint, however the general principle laid down in the Environmental Protection Act 1990 is one of Best Available Techniques Not Entailing Excessive Cost (BATNEEC),
therefore it would seem that cost is a relevant issue when considering treatment options. Similarly cost and effectiveness are important issues in the political context, as government and even international policies may rule out certain types of treatment.
Cost is perhaps the most important commercial issue but the effectiveness of the treatment should be of equal concern to the intending developer, because unless the effectiveness of the selected option can be satisfactorily proven the likelihood