Cleaning wastewater by phytoremediation

30 May 2006




Introduction Phytoremediation combines the Greek word 'phyton' (plant), with the Latin word 'remediare' (to remedy) to describe a system whereby certain plants, working together with soil organisms, can transform contaminants into harmless and, sometimes, valuable forms. This practice is increasingly used to remediate sites contaminated with heavy metals and toxic organic compounds. Whilst the technology has historically been applied to soil clean-up, it can also be applied to the treatment of tannery waste. This article provides an overview of the technique, together with information concerning the specific application to the leather industry (ie the use of reed beds). Phytoremediation uses green plants to rid soils and wastewater of toxic heavy metals and metalloids. It can be defined as the clean-up of pollutants primarily mediated by photosynthetic plants. These plants and their microbially-active rhizosphere (root zone of influence) can transform pollutants and the nutrient nitrogen into valuable biomass, with the remaining water removed via evaporation and transpiration. The range of biological treatments for environmental problems, as described by the term phytoremediation, actually consists of several specific processes: * Phytoextraction - Uptake of substances from the environment, with storage in the plant (phytoaccumulation) * Phytostabilisation - Reducing the movement or transfer of substances in the environment. For example, limiting the leaching of substances contaminating soil * Phytostimulation - Enhancement of microbial activity for the degradation of contaminants, typically around plant roots * Phytotransformation - Uptake of substances from the environment, with degradation occurring within the plant (phytodegradation) * Phytovolatilisation - Removal of substances from the soil or water with release into the air, possibly after degradation * Rhizofiltration - The removal of toxic metals from groundwater Phytoremediation takes advantage of the nutrient utilisation processes of the plant to take in water and nutrients through roots, transpire water through leaves, and act as a transformation system to metabolise organic compounds, such as oil and pesticides. Alternatively they may absorb and bio-accumulate toxic trace elements, including heavy metals such as lead, cadmium and selenium. Heavy metals are closely related to the elements plants use for growth. Phytoremediation is an affordable technology that is most useful when contaminants are within the root zone of the plants (top three to six feet of the soil). For sites with contamination spread over a large area, phytoremediation may be the only economically feasible technology. Wetlands phytoremediation Wetlands are defined as land where the water level is near the ground surface long enough each year to maintain saturated soil conditions. Marshes, bogs and swamps are all examples of naturally occurring wetlands. A 'constructed' wetland is defined as a wetland specifically constructed for the purpose of pollution control and waste management. There are two types of constructed wetlands: the free surface wetland and the sub-surface flow wetland. Both types utilise aquatic vegetation and are similar in appearance to a marsh. For the purposes of phytoremediation, wetlands are shallow waters with at least 50% aerial cover of submerged of emergent macrophytes or attached algae. Natural wetlands have long been used for the disposal of waste. Any treatment occurring in early waste disposal wetland was incidental and confined to some reduction in the biological oxygen demand (BOD). Thus natural or constructed wetlands are best reserved for two purposes: * Polishing of already partially oxidised industrial or domestic waste * Removal of specific pollutants, such as nitrogen, phosphorus, copper, lead, organic compounds and pesticides from all wastes including agricultural or urban storm run-off These treatment wetlands utilise plant-based enzymatic biochemical processes, which work in conjunction with indigenous microbial activity to optimise rhizospheric biodegradation and plant tissue phytodegradation. Bioremediation Soil micro-organisms can degrade organic contaminants. This is called bioremediation and has been used for many years both as an in-situ process and in land farming operations with soil removed from sites. It has been demonstrated, for example, that certain varieties of mustard plant can remove metals such as chromium, lead, cadmium and zinc from contaminated soil. Also hydroponic plant cultures have been used to remove toxic metals from aqueous waste streams. Plants can accelerate bioremediation in surface soils by their ability to stimulate soil micro-organisms through the release of nutrients from and the transport of oxygen to their roots. The rhizosphere is a zone of increased microbial activity and biomass at the root-soil interface that is under the influence of the plant roots. This zone of soil, being closely associated with the plant root, has much higher numbers of metabolically active micro-organisms than unplanted soil. It is this symbiotic relationship between soil microbes that is responsible for the accelerated degradation of soil contaminants. One of the more important roles of soil micro-organisms is the decomposition of organic residues with the release of plant nutrient elements such as carbon, nitrogen, potassium, phosphate and sulphur. A significant amount of the CO2 in the atmosphere is utilised for organic matter synthesis, primarily through photosynthesis. This transformation of carbon dioxide and the subsequent sequestering of the carbon as root biomass could reduce potential problems associated with global warming. Absorption of large amounts of nutrients by plants (and only a small amount of plant toxins that might be harmful to them) is the key factor for this technique to succeed. Plants generally absorb large amounts of elements they need for growth and only small amounts of toxic elements that could harm them. Economical aspects Phytoremediation is considered to be a cost-effective alternative to conventional remediation methods. Cleaning the top 15cm of contaminated soil by applying phytoremediation costs an estimated £1,500-8,000 per hectare, compared to £5,000-13,000 per hectare for on-site microbial remediation. If the soil is moved, the costs escalate, but phytoremediation costs are still far below those of traditional remediation methods, such as stripping the contaminants from the soil using physical, chemical or thermal processes. Plants are effective at remediation of soils contaminated with organic chemical wastes such as solvents, petrochemicals, wood preservatives, explosives and pesticides. The conventional technology for soil clean-up is to remove the soil and isolate it in a hazardous waste landfill or to incinerate it. Organic contaminants are in many cases completely destroyed (converted to CO2 and H2O), rather than simply immobilised or stored. Reed beds What is a reed bed? Reed beds are self-contained, artificially engineered, wetland ecosystems. In simple terms, a reed bed is a hole in the ground, lined with an impermeable liner, filled with one or more solid media (eg soil) and planted with a sufficiently robust reed species. The system is then fed effluent and drained by gravity. Reed beds can be built in a number of variants but mainly they are of the horizontal flow or down flow configuration. The two types can also be used in combination where necessary. Horizontal flow reed beds can be of the surface flow or sub-surface flow type. By far the most common type is the sub-surface flow which is often used for final polishing or tertiary treatment applications. Surface flow reed beds are often used for metals removal and settlement applications. Reed beds are a cost-effective method of sewage treatment. The systems are robust and well-proven, requiring only a fraction of the maintenance of traditional methods of treatment. Chemicals are not required and, provided that just over one metre of hydraulic head is available between the process and final discharge point for the treated effluent, no power is required. Sludge disposal has become a major waste treatment issue for producers. In the past, sludge, both in liquid and solid form, could be disposed of to landfill at reasonable cost. Recently, however, legislative measures have been instigated which make landfill an increasingly expensive and time-limited disposal option. Sludge treatment reed beds will fulfil the requirements of the 'safe sludge matrix', producing an 'enhanced treated product'. Furthermore, the bio solids produced are highly mineralised and dewatered to a dry solids content of some 40%. Reed beds are becoming increasingly popular for the treatment of both industrial and domestic effluents, offering a simple, robust and cost-effective means of wastewater treatment. Reed beds have been applied to the treatment of domestic effluents in rural communities, where the relatively small volumes of effluent may mean that conventional systems are not cost-effective. BLC is developing reed bed technology for leather effluent treatment in co-operation with the company ARM Ltd. Reed beds are a promising technology for the developing world, where high tech treatment is too expensive or impractical, but still aims to fulfil local discharge compliance. BLC can offer testing, implementation and start-up of large scale reed beds for industrial wastewater treatment. For further details contact Stuart Booth on [email protected] or +44 1604 679956.



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