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Biological treatment of contaminated land
Biological treatment of contaminated land
All industrialised countries have a legacy of contaminated land. In the UK, with its high population density, relatively small land area and its concern for countryside and the environment, the decontamination of previously developed and often polluted land, the so called "brownfield sites", to make them suitable for redevelopment is rapidly becoming a social necessity as well as a target of government policy. A fairly recent example of this process is the removal of contaminated ground from the Greenwich Gas Works site to a registered land fill site before the construction of the Dome.
Not only is the re-use of previously developed sites an important government policy for the protection and conservation of the countryside but their decontamination is a social necessity because of the harm that ground pollution can cause to the health and wellbeing of the local and of the wider community. Owners of contaminated land now risk prosecution if there is a statutory breach of legislation covering the prevention of harm to people or to the environment. In addition, land owners face the possibility of enforced remediation where harmful contamination is proved.
Planning policy guidance
Department of the Environment, Transport and the Regions Guidance Note No.3: Housing, identifies that the Government's focus for additional housing should be existing towns and cities with the objective of making better use of previously developed land. It requires Regional and Local Planning Authorities to give priority to re-using previously developed land within urban areas in preference to the development of greenfield sites. The national target is that, by 2008, 60 per cent of additional housing should be provided on previously developed land and through the conversion of existing buildings. To help to achieve this target, planning authorities will have to seek to ensure that housing requirements are apportioned between local authorities in a way that maximises the use of previously developed land and buildings and minimises greenfield land use. Their search sequence should start with the re-use of previously developed land and buildings within urban areas. Land for development should be allocated on the presumption that previously developed sites should be developed before greenfield sites, making allowance for "windfalls", previously developed sites such as sites resulting from factory closures that unexpectedly become available. It is thus clear that the treatment of contaminated land will play an important part in the realisation of Government planning policy.
The investigation of brownfield sites
Before brownfield sites can be redeveloped, a thorough investigation is needed to assess the suitability of the site for development, to identify any obstructions and contaminants that need to be dealt with before construction works commence and to cost these advance works for budget purposes.
If harmful contaminants are identified either by pre-development site investigation or, as sometimes happens, during the course of construction, they can be treated in a number of ways.
The migration of pollutants can be prevented by containment within barriers or impermeable membranes and polluted ground can be given a protective cover.
Contaminated soil can be excavated and disposed of in suitable, registered landfill sites.
Pollutants can be chemically stabilised, solidified or vitrified before they are safely disposed of.
The separation of contaminants can be achieved by hydraulic treatment, including soil washing and steam stripping and the subsequent treatment or safe disposal of the contaminated effluent, or by solvent or by vacuum extraction.
Contaminants can be destroyed by either incineration or by chemical or biological treatment.
Biological or bio-remedial treatment involves the degradation of organic contaminants, either aerobically or anaerobically, by micro- organisms. Anaerobic degradation will produce methane, which itself requires controlled handling and the release of which into the atmosphere has been identified as a source of global warming. Bio-remedial treatment cannot be used to degrade inorganic contaminants, though it may reduce them to a less harmful or to a more easily treated state.
Bio-remedial treatments can be either in situ or ex situ (that is, after excavation or de-watering) and can be more speedy and more cost effective than other treatments. Bio-remedials can be used to treat petrols, oils, tars, pesticides, dyes, solvents and some explosives and PCBs. Their use may mean that after treatment little or none of the contaminated soil need go to a landfill site, with resulting cost savings. As a result, they may be cost competitive with containment or landfill solutions. In addition, in situ bio-remedial treatment may enable ground to be decontaminated below existing structures so that pollution problems in built up areas can be tackled. Because bio-remedial treatment destroys contamination, the use of ex situ treatments on site may enable the treated soil to be reused as fill reducing the need to bring fresh material to site as well as avoiding disposal costs.
Before biological treatments can be specified a toxicity map will be prepared using information obtained from biosensors to identify the contaminated areas and the degree of contamination in the ground throughout the site. Using this information a remedial strategy can be tailored to suit the individual site conditions. This can usually be done relatively quickly compared with traditional survey methods. The biosensors typically contain both a biological component, which reacts to some property of the soil with a reduced reaction in the presence of pollutants, and an electrical component which measures the reaction. For example, the biosensor may contain a strain of bacteria which emits luminescence when in contact with soil nutrients or non-toxic effluent but with reduced luminescence when toxins are present. Biosensors are already used to monitor alcohol levels in beer production and to check the quality of water and sewage effluents.
Ex situ treatment
Of the two alternative bioremedial treatments, ex situ treatment is more commonly used. It is more easily monitored and controlled with the possibility of varying the external conditions under which it is effected. Alternative methods of ex situ treatment include:
Landfarming. Contaminated soil is screened for large debris before it is spread within a protected band, tilled and treated with suitable nutrients.
Composting. Large debris is removed and the contaminated soil is laid in long rows. It is then periodically turned to increase the oxygen supply to the microbes that are breaking down the pollutants.
Biopiles. The contaminated soil is actively aerated and the lechate is recirculated through it.
Bioreactors. The contaminated soil is physically contained for treatment. This enables less readily degradable contaminants to be treated.
These processes may also be combined for more effective treatment. For example, soil washing or ground water extraction may be followed by biological treatment of the contaminated effluent.
In situ treatment
In situ treatments are used for less accessible contaminants or where contamination is at a low to moderate level and may be widespread across a site. Alternative in situ treatments include:
Bio-venting. Air is circulated through the ground above the water table to increase oxygen levels in order to stimulate the biological degradation of the pollutants.
Air sparging. Treatment is similar to bio-venting but it enables treatment to be effected both above and below ground water level.
Pump and treat. This treatment creates a "bioreactor" in the ground by pumping contaminated water to the surface, where oxygen and nutrients are added and the treated water is reinjected into polluted soil.
Natural attenuation. The natural physical, chemical and biological processes present in the ground are used to limit the spread of contamination when the source of contamination has been removed. Before natural attenuation is sanctioned the movement of the contaminants needs careful modelling to show that it will not have a significant environmental impact.
The need to redevelop old industrial sites as industries change or decay and commercial and housing demands for land increase, together with the current awareness of the damage to health and the environment posed by industrial contamination, has forced developers to clean these sites of pollutants to standards not previously attempted. Biological treatment of contaminated soils, either after excavation or in the ground, can offer a socially acceptable and cost-effective way of dealing with pollution from many toxic organic substances. It avoids most of the problems of long-distance transportation or long-term containment resulting from some of the solutions conventionally adopted, in the face of increasing pressure from a public aware as never before of the hazards of industrial pollution.