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Insitu subsurface active biofilm barriers MJ Brough and Dr RJ Martin of University of Birmingham and Dr A Al-Tabbaa of the University of Cambridge.

The current trend in contaminated land remediation is a move away from passive containment systems to either passive containment technology combined with some form of remediation or to active containment. Both systems aim to provide a degree of contaminated ground treatment in the longer term. One treatment method which has been receiving considerable attention is the process of bioremediation which, in its various forms, includes natural biodegradation and bioaugmentation. In addition, for a completely different purpose, micro-organisms have been used in the petroleum industry where they were injected in oil wells to reduce the permeability of hydrocarbon bearing strata. A recent and novel concept that is currently under investigation and forms the basis of the research work presented here, is to combine the microbial activities of biodegradation and permeability reduction to develop a system of active containment.

Some micro-organisms have the ability to reduce the permeability of porous media by the development of a biofilm, consisting of micro-organisms immobilised at a surface and embedded in an organic polymer matrix (exopolysaccharides) produced by the adhered micro-organisms. The current research investigates the development of biofilm barriers as a form of active containment in an attempt to exploit the potential of certain microbial populations to develop in porous media in such a way as to facilitate the dual role of containment and remediation.

Initial work has recently been completed on biofilm system in soil columns where the biofilm barrier has been optimised in terms of permeability reduction and biodegradation efficiency. This investigation identified the overriding parameters involved in the promotion of either permeability reduction or biodegradation of a synthetic 'sewage' feed that was permeated through porous media inoculated with activated sludge. Biofilm barrier formation within medium to coarse sands and slightly clayey to clayey sands was examined with varied methods of application and feeding. Soil was pre-mixed or injected with activated sludge and fed on a batch or continuous basis in order to reproduce the possible options for using a biofilm barrier and formation in the sub- surface. Results showed that biofilm accumulation was improved in pre-mixed systems that were continuously permeated with the synthetic sewage resulting in the continuous removal of the permeant chemical oxygen demand. The role of biodegradation appeared to be dependent upon biofilm viability within porous media, while the process of clogging was due to a combination of many factors and not just a result of an increased number of viable micro-organisms. This improvement in viability enhanced the bioclogging of porous media; however other factors which reduced hydraulic permeability were identified and involved the liberation of exopolysaccharide and gas production. Initial work has demonstrated that a reduction in permeability of one order of magnitude can be achieved with this system.

Further column work is currently being carried out to establish the effects of permanent flow rate and composition on the relative processes of waste containment and bioremediation within porous media. These studies will also involve examination of biofilm barrier performance in columns using a consortium of bacteria with specific degradative capabilities (covering a wide range of petroleum derivatives) inoculated into sands, slightly clayey sands and clayey sands. These biologically active columns will permeated with a diesel/kerosene/gasoline based feed supplemented with nitrogen and phosphorous. Permeability, biofilm viability and biodegradation within columns will be monitored to examine biofilm barrier performance with the introduction of contaminant of environmental significance. This work will also examine the degree and extent of mobility of the micro- organisms following inoculation and the biofilm. The required reduction in permeability to allow different degrees of biodegradation to take place will also be studied. Part of this work will use commercially available micro-organisms tailored towards the biodegradation of the contaminants selected for this work in collaboration with International Biotechnology Services.

The next stage of this investigation will apply the results of the column work to simulate insitu application of the technology. The insitu process can be broken down into two stages. The first is the introduction of the micro-organisms into the soil. The technology adopted is one of the soil mixing simulated using a laboratory-scale model auger (60mm in diameter and 300mm high) as shown schematically in Figure 1. The auger breaks up and mixes the soil insitu, the micro-organisms, supplement nutrients and oxygen are then introduced and mixed with the soil which is then compacted to the required degree on withdrawal of the auger. By augering several of these biologically active columns together, a biologically active wall should be produced. With the imposition of a contaminated groundwater flow across the wall, a biofilm barrier should form. This barrier would in practice form a part of a larger three-dimensional containment system which will ensure that the contaminated groundwater is directed towards the barriers and cannot escape from the treatment system.

Maintenance of the subsurface biofilm barrier then takes place through the repeated appli-

cation, whenever required, of supplement nutrients and oxygen. Two methods will be employed to study this process: the first is by re-mixing the biofilm barrier in place, as in Figure 1, adding the nutrients and then re-examining the establishment of the biofilm barrier conditions; the second is by supplementing the contam- inated groundwater upstream of the biofilm barrier, Figure 2, again by soil mixing.

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