Environmental Bioremediation Technologies

The current body of knowledge concerning metal effects on biodegradation is still in its infancy, yet the timely and cost-effective remediation of metal and organic co-contaminated sites will require a lucid understanding of factors important in determining the extent to which toxic metals inhibit organic biodegradation. Past attempts to measure impacts of metals on biodegradation are difficult to interpret, because they have generally been based on total metal rather than solution phase or bioavailable metal concentrations. This has resulted in reported inhibitory concentrations of metals that vary by as many as 5 orders of magnitude. A critical first step will be to consistently report solution phase or bioavailable metal concentrations so that legitimate comparisons of biodegradation behaviors in co-contaminated sites can be made. Currently, a useful approximation is to measure and use solution phase metal data; however, new methods of defining and determining bioavailable metal are rapidly being developed. Despite the enormous variance among reported inhibitory concentrations of metals, it remains clear that metals have the potential to inhibit organic biodegradation in both aerobic and anaerobic systems. The mechanisms and patterns by which metals inhibit biodegradation vary with the composition and complexity of each system and include both physiological and ecological components. A more thorough understanding of these systems, taking into account various levels of complexity, is needed to develop new approaches to bioremediate co-contaminated sites. Nevertheless, there already exist several approaches including addition of metal resistant microorganisms and additives that reduce metal bioavailability. Field trials are needed to validate these approaches.

Bioremediation has developed from the laboratory to a fully commercialised technology over the last 30 years in many industrialised countries. However, the rate and the extent of development has varied from country to country. A successful bioremediation scheme relies on the management of soil microbial populations capable of catabolising the contaminants. The role of soil microbiota in the biochemical conversion of organic and inorganic contaminants has been realised, priority research needs have been identified and effort has been made to understand the ecological, biochemical and genetic basis of microbial contaminant degradation, with a view to enhancing microbial capabilities and thus designing more effective bioremediation processes.

Phytoremediation is an emerging technology for contaminated sites and is attractive due to its low cost, high public acceptance and environmental friendliness nature. It is not a panacea for all waste problems, but a supplement to the existing technologies. The technology has been demonstrated, but not yet commercially exploited. More research background for development of plant tailored for remediation needs use of genetic engineering. The concept of manipulating plant genes for toxic metal uptake is today a cutting edge research area. The likelihood of public acceptance of genetically engineered plants for phytoremediation will be welcomed, since it will clean up the environment of toxic metals. No doubt phytoremediation technology has attracted a great deal of attention in recent years and it is expected that phytoremediation will capture a significant share of the environmental market in the coming years.

In future, modification and adaptation of nanotechnology will extend the quality and length of life. The breath of anticipated opportunities, cross-disciplinary nature, potential for innovation, historical track records and the impact of the potential gains of nanotechnology research have led to the recognization of this area with special emphasis. The social benefits are significant from nanomaterials and the new products are applicable to information technology, medicine, energy, and environment. An important challenge in nanotechnology is to tailor optical, electric and electronic properties of nanoparticles by controlling the size and shape. Utilization of microbe for intracellular/extracellular synthesis of nanoparticles with different chemical composition, size/shapes and controlled monodispersity can be a novel, economically viable and eco-friendly strategy that can reduce toxic chemicals in the conventional protocol.

Phytoremediation is an eco-friendly cost-effective technology, as compared to classical physical, chemical and even to the microorganisms-based bioremediation techniques. It is useful for the remediation of sites contaminated with non-biodegradable toxic heavy metals, hazardous air pollutants like oxides of nitrogen and sulfur, and photoxidants like ozone, recalcitrant organic pollutants, like chlorinated pesticides, organophosphate, insecticides, petroleum hydrocarbons, polynuclear aromatic hydrocarbons (PAHs), sulphonated biphenyl (PCBs) and chlororinated solvents (TCE, PCE) etc.

Surface flow constructed wetlands are being designed for the treatment of municipal waste waters in developed nations. However, use of constructed wetlands is not gaining momentum in tropical nations due to water scarcity and high surface evapotranspitration. But, in there countries for the bioremediation mine drainage, agricultural waste waters and flood water there is considerable scope as they have rich plant diversity.

Thus, there are several plant, edaphic and environmental factors which regulate plant resistance to air pollution. Suitability of plants for the pollution abatement depends on how fast they are able to absorb pollutants from the atmosphere and metabolise or detoxify them at cellular levels. However, the plants with pollutant avoidance mechanism may not be recommended for mitigating air pollution level in urban or industrial areas. This makes crystal clear that effectiveness of avenue trees in urban areas, and greenbelts in and around industrial units largely depends on the selection of suitable plant species and its number.

Phytoremediation by VGT is a low cost technology as compared to conventional (engineering) methods for site remediation. It is also virtually maintenance free, the grasses regrow very quickly and its efficiency improves with age (Truong 1999). Social acceptance of a particular technology in remediation of contaminated lands and water bodies has also become an important issue, as it directly affects the life of community. Biological technologies based on the use of plants are more acceptable to people, as it creates a green and aesthetic view and also provides some useful materials. Several plants are being identified and trialed to be used in the phytoremediation task. Important among them are other grasses like the Bermuda grass (Cynodon dactylon), Bahia grass (Paspalum notatum), Rhodes grass (Chloris guyana), the tall wheat grass (Thynopyron elongatum), common reed grass (Phragmites australis), the munj grass (Sachharum munja) and Imperata cylindrica. Other plants are the marine couch (Sporobolus virginicus), cumbungi (Typha domingensis) and Sarcocrina spp. They are highly salt and toxicity tolerant and have extensive root binding system. They were tried in the rehabilitation works, but none succeeded so well as vetiver. There is need to educate the society, the general people and the planner about the ecological and economic value of this ‘wonder grass’.

Humans have had a major impact on the earth’s water reservoirs: rivers, lakes, oceans as well as groundwater. Nitrate is listed as second most common pollutant of groundwater next to pesticides. Whether it is by deforestation of riparian zones, inundating agricultural fields with fertilizer, faulty septic systems or poorly designed waste water overflow systems, the detrimental effects of human activities have started to become apparent. With the growing awareness of the increasing nitrate problem and its impact on ecosystems as well as human health, the question remains: what alternatives do we have? Are our only choices to reduce the human population, dig millions upon millions of miles of tunnels underneath towns and cities, prohibit the use of fertilizers, or fund tertiary waste water treatment? Some of these suggestions are more farfetched than others. In the past fifty years or so, strides have been made using processes which incorporate physico-chemical or biological means to help restore an area or remove this pollutant from soil and water. The fact is that most of the above actions are either extremely expensive or completely unethical, a much less expensive, and more environment friendly alternative could be phytoremediation. Taking into consideration the world’s growing population and the adverse effect humans have had on the nitrate concentrations of water bodies, more measures both effective and eco-friendly are needed to remedy the menace of growing NO3⁻ pollution in groundwater.

Several strategies have been attempted for bioremediation of hydrocarbonpolluted sites. Bioaugmentation with designed bacterial consortium, followed by the addition of rhamnolipid biosurfactant and NPK solution to soils contaminated with up to 10% tank bottom sludge, enhanced the rate of biodegradation over a period of 56 days. Pre-treatment of hydrocarbon contaminated soil with biosurfactants enhanced bioavailability of the hydrocarbons to microbial population. Furthermore, supplementation with inorganic nutrients like NPK solution enhanced the secondary successions of crude petroleum utilizers. For bioremediation, a single inoculation with the biosufactant-producing hydrocarbon degrading bacterial consortium at the beginning of the process would reduce the cost of inoculum preparation considerably. Hence we suggest a combined treatment as a possible bioremediation technology for the reclamation of oil sludge polluted soils.

Wastewater treatment plants, such as activated sludge and methanogenic reactors, are not the natural habitat of WRF, since these organisms prefer solid substrates and well-aerated environments. The fact, that constructed wetlands (e.g. sub-surface flow systems with rooted emergent macrophytes), are transitional environments, i.e. are intermediate between terrestrial and aquatic ecosystems, can be an advantage in the treatment of polluted effluents. The wetlands system treats wastewater by physical, chemical and biotic processes, in a close association of appropriated plants, microorganisms, macro-organisms and substrates. Macrophytes enhance physical filtration, prevent clogging in vertical flow systems, mediate oxygen transfer to the rhizosphere and favour microorganism colonisation (Brix et al. 1996; Brix 1997). In sub-surface systems, there is an oxygen gradient, with high partial pressures near the plant roots, to be replaced progressively by anaerobic and anoxic environments. The mixture of aerobic, anoxic and anaerobic zones stimulates different microbial communities that can degrade complex organic substances (such as azo dyes) almost to mineralisation. The extent of dyes biodegradation must be evaluated, since the formation of intermediate compounds can enhance toxicity (Sweeney et al. 1994). The use of constructed wetlands is a low cost technique, with low maintenance needs (Schwitzguébel et al. 2002; Susarla et al. 2002). It is able to tolerate high fluctuations in flow, temperature (Winthrop et al. 2002) and the composition and/or concentration of pollutants in wastewater. Finally, it is likely to find widespread acceptance with the public for its obvious technological and aesthetic qualities.