Biotechnology in waste management

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Sewage farms - a public service we all need but prefer not to think about - are a classic example of traditional biotechnology. So are the compost heaps in many suburban gardens. The voracious appetites of bacteria are used to break down the huge quantities of human wastes discharged into the world's sewers each day. They and other microbes also help turn leaves, twigs and vegetable scraps into fertile humus to improve garden soil.

However, modern biotechnology can do more. Recent developments in biotechnology are providing new ways to clean up industrial wastes and yielding efficient new production methods that are less polluting than traditional processes. Biotechnology can even help convert industrial and other wastes into useful products.

What is biotechnology doing for industrial wastes?

Treating Waste Water

Biotechnology has always played a key role in removing organic solids like human waste from the millions of litres of waste water generated every day in Australia. Other contaminants, though, like phosphorus and nitrogen, have often been discharged into rivers and other waterbodies where they can disrupt the delicate ecological balance.

Being nutrients, phosphorus and nitrogen can cause excessive plant and algal growth. Overgrowth of aquatic plants can choke rivers and dams, and algae can produce toxins that poison fish and livestock. To remove nutrients from waste water, sewage farms have been using costly chemicals. Biotechnology, however, offers a cheaper and cleaner alternative. An extra anaerobic stage (one without oxygen) is added to the beginning of the sewage treatment process to help break down the organic materials. In the next aerobic phase, where oxygen is available, bacteria responsible for phosphorus removal can proliferate and consume the readily available organic food source.

During the sewage treatment process, bacteria also help to convert nitrogenous compounds into gaseous forms of nitrogen which are allowed to escape into the atmosphere.

Cleaning up Chemicals

Biotechnology is providing environmentally acceptable methods of modifying or destroying chemical wastes so they are no longer toxic to the environment. This usually involves finding bacteria or other microbes that can digest the target pollutants. If necessary, these organisms can be genetically engineered to provide strains with better contaminant-degrading potential than their natural counterparts. An example is the research being carried out at old military dumps where TNT (2,4,6 - trinitrotoluene) explosive is being made safe by using white rot fungi to degrade the dangerous explosives to harmless products.

Decontaminating Soil

Biotechnology has uncovered some strange coincidences that can work to our advantage. White rot in timber, for example, is caused by a harmless fungus that can digest the tough lignin component of wood. It happens that white rot fungi also enjoy eating organochlorine compounds such as DDT, dieldrin, aldrin and polychlorinated biphenyls, all of which have some structural similarities to lignin. This feature offers a relatively cheap and environmentally sound way of disposing of noxious compounds which, in the past, were valued for their stability and used extensively in refrigerants, fire retardants, paints and varnishes, solvents, herbicides and pesticides.

Normally, disposing of these chemicals involves high temperature incineration or quarantining of contaminated land. However, Australian scientists are conducting field trials using one strain of white rot fungi (of which there are six to ten thousand known species) to detoxify a PCB-contaminated site in the United States. The scientists also aim to develop a process for degrading bulk toxic wastes presently stored in drums on industrial sites throughout the world.

Removing Organic Pollutants from Industrial Effluents

White rot fungi also promise to provide a cheap and practical method of treating effluent from other industries.

For example, pulp and paper mills have a particular problem with toxic effluent from bleached paper production, with chlorine bound to the lignin component being the main pollutant. Biological treatment of the effluent using the lignin-digesting white rot fungus could offer both an economically and environmentally acceptable solution.

Treatment of waste products from other industries such as food processing, chemical manufacturing, textiles, brewing and distilling can also benefit from biotechnology, which can help devise biological effluent treatment processes suited to individual waste streams. Biotechnology is also used in more direct ways in many of these industrial processes, using, for example, fermentation and enzyme technologies.

Treating Petroleum Sludge and Oil Spills

Oil sludge, normally discharged into the sea from petroleum refineries, contains toxic compounds that are a major threat to the marine ecology. All forms of aquatic life are adversely affected, and contaminated fish, when eaten by humans, present a serious health hazard.

Biotechnology, however, has shown that particular species of bacteria and fungi, normally found in soil, can protect the marine environment by breaking down various types of hydrocarbons, the main component of petroleum. To be effective in cleaning up marine oil spills, however, micro-organisms must be able to withstand the marine environment _ for example they need to survive in high salt concentrations and to grow at low temperatures.

It may be necessary to use some of the techniques of modern biotechnology to introduce these characteristics into the appropriate oil-eating micro-organisms.

Making Fuels from Waste

Biotechnology is already benefitting developing countries by providing a cheap, clean and renewable alternative to fossil fuels, but the costs of biomass fuels such as ethane are still high relative to fossil fuel equivalents. Biomass fuels are greenhouse gas neutral (i.e., carbon dioxide is consumed by photosynthesis during the growth of the plant, and equal amounts are released when the biomass fuel is burned).

Human, animal or vegetable wastes are fed into a sealed unit, called a digester, where bacteria working without oxygen ferment the wastes to produce methane gas. Methane can be used for purposes including small-scale electricity production, cooking, lighting and heating. The slurry left at the end of the process is a useful crop fertilizer. Western nations are also looking at methane as a renewable energy source. Fruit and vegetable processing factories produce millions of tonnes of solid wastes each year. These are usually dumped, burnt or fed to animals. Pilot plants, however, have shown these wastes to be a good source of methane. Some local councils in Australia and elsewhere are also running trial programs to tap the methane produced during natural decay of rubbish at municipal garbage tips.

BIOTECHNOLOGY CAN HELP

• treat human and domestic wastes to maintain a healthy environment
• treat industrial wastes to reduce pollution
• convert some wastes to safe fertilizer to improve crop production
• remove or detoxify harmful chemical residues and accidental spills
• turn industrial and human wastes into useful fuel.

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Compost

Interest in composting as a waste management technique has increased enormously as people turn their attention to recycling technologies. The awareness that yard waste makes up 18 percent of the waste stream and food waste adds another 8 percent makes composting a high potential area for waste reduction.

In composting, a biochemical process occurs in which complex organic matter decomposes, through the action of microorganisms, into more stable organic matter. The results of this process are dark, humus-like materials that can be used for landscaping and certain agricultural purposes.

Because most of the plants start with mixed garbage , they must overcome the hurdle that much of the garbage is inorganic and does not break down in a composting process. Therefore, mixed municipal solid waste composting plants require before and after the composting process, sorting operations to remove inappropriate materials such as glass, metal, plastics and textiles.

After sorting out the inorganic materials, the composting process can begin. To get a sense of what good large-scale composting requires, the most important factors affecting the process are:

• Temperature. The temperature must be between 132 and 160 degrees Fahrenheit for a minimum of three days for a fast ans succesfull composting process.
• Moisture. At the beginning of the composting process, digesting material should have a moisture content of 50-60 percent.
• Controlled availability of oxygen. The micro-organisms in the waste need the oxygen in air to oxidize carbon matter and thus cause decomposition.
• Appropriate sizing of the waste particles. A ton of waste divided into many small pieces has more surface area than a ton of waste divided into fewer, larger pieces. Increased surface area allows micro-organisms more decomposing surface, resulting in faster composting.
• Chemical balance. The pH of the waste, which is a measure of acidity or alkalinity, should be maintained in the 7-8 range. The ratio of carbon to nitrogen is critical to the rate of composting.

Roughly, two different processes are used in the compost recycling. The first handles only yard waste. The second handles mixed solid waste.

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Recent advances

All the composting-processes which we have been describing above are so-called aerobic processes. This means that the material is composted under influence of air. However, there have been advances in composting with anaerobic processes. In these processes, the material is being composted by bacteries without influence from air. Although these anaerobic processes are currently more expensive than the traditional aerobic processes, the anaerobic processes have some advantages. They need less energy, in fact they even produce energy. Furthermore, these processes attribute less to the greenhouse- effect

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Processing mixed solid waste

There are several production steps common to MSW composting systems:

• Preprocessing. In this step large bulky items are removed and waste is shredded.
• Digestion/Composting. For this step there are two main technologies available:
  • Windrow systems. Here the waste is laid out in large, long piles known as windrows. Air is forced through these windrows or material is mechanically turned upside down to speed up the composting process.
  • Invessel systems. These systems involve continuous aeration and mixing, tumbling or turning of waste in various types of containment structures such as silosm agitated beds or tunnel reactors. In these systems the composting process needs 5-7 days to finish.
• Curing. Curing is an important phase of the composting process. It gives the material time to stabilize after the active phase of digestion. Usually this is done by laying the material in large windrows. This process takes approximately 4 weeks.
• Postprocessing. Postprocessing is performed before or after curing to remove remaining contaminants such as glass, ceramics or plastic from the compost before it is marketed. Postprocessing also can include other steps to prepare compost for end users, such as pelletizing.

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Processing yard waste

The yard waste composting process can be done in different ways depending on the available land and capital. One can distinguish a low technology system, in which composting can take 2 to 3 years. There is a medium technology system, in which composting will take 2 years. Furthermore, there is a high technology system which takes a year. The low technology approach requires much land but doesn't doesn't need much capital or technology. The high technology approach does require a lot of capital and technology but it doesn't take many land.

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