Bioremediation is the use of living organisms (primarily micro-organisms) to degrade pollutants previously introduced into the environment or to prevent pollution through treatment of waste streams before they enter the environment.
Bioremediation is emerging as one of several alternate technologies for removing pollutants from the environment, restoring contaminated sites, and preventing further pollution.
ADVERTISEMENTS:
Biodegradation of waste is the conversion of waste materials by biological processes to simple inorganic molecules and to a certain extent, to biological materials. The complete bioconversion of a substance to inorganic species such as carbon dioxide, ammonia, and phosphate is called mineralisation.
Biological waste treatment is a generic term applied to processes that use micro-organisms to decompose organic wastes either into water, carbon dioxide, and simple inorganic substances or into simpler organic substances, such as aldehydes and acids. The purpose of a biological treatment system is to control the environment for micro-organisms so that their growth and activity are enhanced, and to provide a means for maintaining high concentrations of the micro-organisms in contact with the wastes.
Since biological treatment systems do not alter or destroy inorganic substances, and high concentrations of such materials can severely inhibit decomposition activity, chemical or physical treatment may be required to extract inorganic materials from a waste stream prior to biological treatment.
Detoxification refers to the biological conversion of a toxic substance to a less toxic species, which may still be relatively complex or biological conversion to an even more complex material. An example of detoxification is the enzymatic conversion of paraoxon (a highly toxic organophosphate insecticide) to p-nitrophenol, which has only about 1/200 the toxicity of the parent compound.
Biodegradation is usually carried out by the action of micro-organisms, particularly bacteria. However, micro-organisms such as bacteria are not panaceas for chemical wastes. Their function can vary depending upon the specific bacterium and the metabolic processes of the bacteria.
Biotransformation is the conversion of a substance through metabolisation thereby causing an alteration to the substance by biochemical processes in an organism. Metabolism is divided into the two general categories of catabolism, which is the breaking down of more complex molecules, and anabolism, which is the building up of life molecules from simpler materials. The substances subjected to biotransformation may be naturally occurring or anthropogenic (made by human activities). They may consist of xenobiotic molecules that are foreign to living systems.
ADVERTISEMENTS:
An important biochemical process that occurs in the biodegradation of many chemical waste materials is co-metabolism. This does not serve a useful purpose to an organism in terms of providing energy or raw material to build biomass, but occurs concurrently with normal metabolic processes.
An example of co-metabolism of chemical waste is provided by the white rot fungus which degrades a number of kinds of organic chlorine compounds (including polychlorobiphenyls, cyanides, and chlorodioxins) under the appropriate conditions. The enzyme system responsible for this degradation is one that the fungus uses to break down lignin in plant material under normal conditions.
Biodegradation of chemical waste that can be metabolised takes place whenever the wastes are subjected to conditions conducive to biological processes. The most common type of biodegradation is that of organic compounds in the presence of air, that is, aerobic processes. However, in the absence of air, anaerobic biodegradation may also take place. Furthermore, inorganic species are subject to both aerobic and anaerobic biological processes.
Although biological treatment of chemical waste is normally regarded as degradation to simple chemical species such as carbon dioxide, water, sulphates, and phosphates, the possibility must always be considered of forming more complex (in some cases hazardous) chemical species. An example of the latter is the production of volatile, soluble, toxic methylated forms of arsenic and mercury from inorganic species of these elements by bacteria under anaerobic conditions.
ADVERTISEMENTS:
Physical, chemical and biological treatment processes are employed for waste-water treatment. In addition, chemicals are introduced for precipitation of nutrients, followed by coagulation and filtration for removing solids remaining after biological treatment. In some cases, granular activated carbon or membrane filtration or a combination of membrane-assisted solvent extraction are used for additional purification of the groundwater streams and waste streams. This higher level of treatment is advisable because of the damage that any visual traces of chemical waste can do to the appearance of the water. In addition, the treatment may combat the potential eutrophic effect that the nutrients phosphorus and nitrogen may have on the receiving water.
For the most part, anthropogenic compounds resist biodegradation much more strongly than do naturally occurring compounds. This is generally due to the absence of enzymes that can bring about an initial attack on the compound. A number of physical and chemical characteristics of a compound are involved in its amenability to biodegradation.
Such characteristics include hydrophobicity, solubility, volatility and affinity for lipids. Some organic structural groups impart particular resistance to biodegradation. These structural groups include branched carbon chains ether linkages, meta-substituted benzene rings, chlorine, amines, methoxy groups, sulphonates, and nitro groups.
Acintomycetes are micro-organisms that are morphologically similar to both bacteria and fungi. They are involved in the degradation of a variety of organic compounds, including degradation-resistant alkanes, and lignocellulose. Other compounds attacked include pyridines, non-chlorinated aromatics, and chlorinated aromatics. Fungi are particularly noted for their ability to attack long-chain and complex hydrocarbons and are more successful than bacteria in the initial attack on polychlorobiphenyls.
ADVERTISEMENTS:
Phototrophic micro-organisms, which include algae, photosynthetic bacteria, and cyanobacteria (blue- green algae) tend to concentrate organophilic compounds in their lipid stores and induce photochemical degradation of the stored compounds.
Usually the products of biodegradation are molecular forms that tend to occur in nature and that are in greater thermodynamic equilibrium with their surroundings than the starting materials. Detoxification refers to the biological conversion of a toxic substance to a less toxic species.
Microbial bacteria and fungi possessing enzyme systems required for biodegradation of wastes are usually best obtained from populations of indigenous micro-organisms at a chemical waste site where they have developed the ability to degrade particular kinds of molecule. Although it has some shortcomings in the degradation of complex chemical mixtures, biological treatment offers a number of significant advantages and has considerable potential for the degradation of chemical wastes, even in situ.
The biodegradability of a compound is influenced by its physical characteristics, such as solubility in water and vapour pressure, and by its chemical properties including molecular mass, molecular structure, and presence of various kinds of functional groups, some of which provide a ‘biochemical handle’ for the initiation of biodegradation. With the appropriate organisms and under the right conditions, even substances such as phenol that are considered to be biocidal to most micro-organisms can undergo biodegradation.
Properties of chemical wastes and their media can be changed to increase biodegradability. This can be accomplished by adjustment of conditions to optimum temperature, pH (usually in the range of 6-9), stirring, oxygen level, and the amount of the material. Biodegradation can be aided by removal of toxic organic and inorganic substances, such as heavy metal ions.
Biodegradability of polymers has become a focal point in both environmental protection and landfill space management. The impact of plastic materials in municipal solid waste (MSW) is even greater when it is considered that while plastics make up only about 9 per cent, by mass, they make up 28 per cent of the volume of MSW.
The biodegradability issue of plastic articles has been brought to the public attention via – (i) startling discovery of tragic deaths of baby dolphins due to their mouths getting stuck by discarded plastic loop carriers of 6-pack cans; (ii) finding of undecomposed newspapers, still completely readable, that had been buried over 70 years in landfills; and (iii) an increasing number of deaths of wild birds at national or municipal park sites due to injuries from carelessly discarded snack wrappers.
Most commodity polymers of molecular weight of about 1000 or higher are essentially not biodegradable, in their present forms. Biodegradation is often accompanied or preceded by photodegradation by which polymers go through reduction in molecular weights. Efforts have been made to make polymers and polymeric articles biodegradable after serving for intended uses. Some of the successful and noteworthy products include polyhydroxybutyratevalerate (PHBV), ethylene-carbon monoxide (E/CO), copolymer, aliphatic polyester, polycaprolactone and vinyl ketone photodegradable polymers.
Aerobic processes for the treatment of waste utilise aerobic bacteria and fungi that require molecular oxygen. These processes are often favoured by micro-organisms, in part because of the high energy yield obtained when molecular oxygen reacts with organic matter. Aerobic and anaerobic digestion processes are well adapted to the use of an activated sludge process. These treatments can be applied to wastes such as chemical process wastes and landfill leachates. Some systems use powdered activated carbon as an additive to absorb non-biodegradable organic wastes.
Contaminated soils can be treated in variety of ways to eliminate contaminates. It is possible in principle to treat contaminated soils biologically in place by pumping oxygenated, nutrient-enriched water through the soil in a recirculating system.
Anaerobic processes for the treatment of waste are those processes in which micro-organisms degrade wastes in the absence of oxygen and can be practiced on a variety of organic wastes. Compared to the aerated activated sludge process, anaerobic digestion requires less energy yields, less sludge by-product, generates hydrogen sulphide which precipitates toxic heavy metal ions, and produces methane which can be used as an energy source.
Activated sludge is the biologically active sediment produced by the repeated aeration and settling of sewage and/or organic wastes. The dissolved organic matter acts as food for the growth of aerobic flora. These species produce a biologically active sludge which is usually brown in colour and which destroys the polluting organic matter in the sewage and waste.
The process is known as the activated sludge process. Briefly, the activated sludge process (Fig. 12.10) is a versatile and effective waste treatment process. Micro-organisms in the aeration tank convert organic material in waste-water to microbial biomass and carbon dioxide. Organic nitrogen is converted to ammonium ion or nitrate and organic phosphorus is converted to orthophosphate.
The microbial cell matter formed as part of the waste degradation processes is normally kept in the aeration tank until the micro-organisms are past the log phase of growth, at which point the cells flocculate and separate from the liquid. These solids settle out in a settler and a fraction of them is discarded. Part of the solid, the return sludge, is recycled to the head of the aeration tank and comes into contact with fresh sewage. The combination of a high concentration of micro-organisms in the return sludge and a rich food source in the in-flowing sewage provides optimum conditions for the rapid degradation of organic matter.
However, in terms of pollution by humans, sewage sludge is not the beneficial organic fertiliser that many believe it to be. Sewage sludge spread on land may contaminate water by release of a variety of organic and inorganic contaminants. The spread of disease in ancient and medieval times due to contamination of water supplies will attest to this.
Similarly, materials leached from landfills by (acid) rain can also cause serious contamination of land and water systems. In addition, leachates from unlined pits and lagoons containing chemical liquids/sludge may cause a specific pollution (drinking water) problem or a more general pollution problem. Recent studies show that concentrated municipal sludges can be decisively decomposed to final mineralised compounds using supercritical water oxidation process.
Soil is a natural medium for a number of living organisms that may have an effect upon biodegradation of chemical wastes. Of these, the most important are bacteria. Wastes that are amenable to land treatment are biodegradable organic substances. However, in soil contaminated with chemical wastes, bacterial cultures may develop that are effective in degrading normally recalcitrant compounds through acclimation over a long period of time. Land treatment is mostly used for petroleum refining wastes and is applicable to the treatment of fuels and wastes from leaking underground storage tanks.
On the note of storage tanks, especially aboveground tanks, the regulations are sufficiently strict that precautions to counteract spillage must be taken to avoid fines.
Land treatment can also be applied to biodegradable organic chemical wastes, including some organic halogen compounds. Land treatment is not suitable for the treatment of wastes containing acids, bases, toxic inorganic compounds, salts, heavy metals, and organic compounds that are excessively soluble, volatile, or flammable. Composting of chemical wastes is the biodegradation of solid or solidified materials in a medium other than soil.
Bulking material such as plant residue, paper, municipal refuse, or sawdust may be added to retain water and enable air to penetrate to the waste material. Successful composting of chemical waste depends upon a number of factors, such as the selection of the appropriate micro-organism or inoculum. Once a successful composting operation is underway; a good inoculum is maintained by recirculating spent compost to each new batch.
Other parameters that must be controlled include oxygen supply, moisture content (which should be maintained at a minimum of about 40 per cent), pH (usually around neutral), and temperature. The composting process generates heat so, if the mass of the compost pile is sufficiently high, it can be self- heating under most conditions. Some wastes are deficient in nutrients, such as nitrogen, which must be supplied from commercial sources or from other wastes.
Bioreactors have been used for waste-water treatment processes for decades. The reactors may be either fixed film or slurry phase. Fixed film reactors are similar to the traditional trickling filters or rotating biological contactors (RBCs) of the waste-water industry. In either case the micro-organisms are supported on the medium of the filter.
The wastes are passed over the filter (or in the case of rotating biological contactors the filter is passed over the waste) allowing the micro-organisms to come into contact with the wastes and break down the organic material. Slurry phase reactors are tanks into which the wastes, nutrients, and micro-organisms are placed. The tank is mixed and may be aerated, many instances, contaminated groundwater is used to create the waste slurry. Both fixed film and slurry phase treatments are either batch or continuous mode.
Solid phase bioremediation, often referred to as land farming, treats wastes using conventional soil management practices to enhance the microbial degradation of the wastes. The wastes are placed directly on the ground or in shallow tanks, if required by RCRA restrictions. Nutrients and micro-organisms are normally added to the wastes which are routinely tilled during the treatment process. This tilling improves aeration and the contact of the organisms with the wastes. While treatment may occur throughout the upper three to five feet of the soil, mostly occurs within the top foot, called the zone of incorporation.
Soil heaping is piling wastes in heaps of several feet high on an asphalt or concrete pad. Nutrients, micro-organisms, and air are provided through perforated piping placed throughout the pile. The pile is covered to contain volatile organic compounds, to stabilise the micro-organism’s environment, and to control soil erosion. The volatile organic compounds can be further controlled by applying a vacuum to the pile and treating the exhaust.
Composting is another application of bioremediation. In this process, the wastes are normally mixed with a structurally firm bulking material such as chopped hay and wood chips. As with the other bioremediation technologies, nutrients, air, and micro-organisms must be added. The three major types of composting are open windrow, static windrow, and reactor systems.
The differences among the three relate to how aeration is accomplished. In the open windrow system, the compost piles are open to the air whereas in the static windrow system the air is mechanically forced into the compost piles. When reactors are used, the compost is mechanically mixed to ensure aeration.
One of the advantages of bioremediation is that it can be effectively applied to treat wastes in place.
The process usually entails introduction of nutrients, micro-organisms and air to the soil/waste through a series of injection wells or infiltration trenches. The term bioventing has also been applied to this technology, although the term could just as easily be applied to composting or to soil heaping. Nevertheless, whatever the name applied to the technology, it has shown some success for hydrocarbon degradation.
If the soil does not have sufficient moisture content, water may also have to be added. In situ bioremediation is often applied in conjunction with groundwater pump and treat systems and soil flushing activities.
There is also the concept of gene manipulation as a means degrading polynuclear aromatic hydrocarbons. The concept offers promise for many sites (such as town gas sites where wastes containing polynuclear aromatic hydrocarbons are evident). However, the degradation products from such interactions may require cleanup. But, it is quite possible that the degradation products are easier to clean than the original polynuclear aromatic hydrocarbons. There is also the concept of using biodegradation on such wastes where the waste has been reduced to residual saturation by flushing technologies. A final flushing to remove the biodegraded material will be necessary.