Reduced in bulk and which can be safely returned to the environment. It is, in effect, a low-moisture, solid-substrate fermentation process, as previously discussed (chapter 4). To be totally effective, it should only use as substrates readily decomposable solid organic wastes, the final product is mostly used for soil improvement, but in more specialised operations using specific organic raw substrates(straw, animal manures, etc.), the final product can become the substrate for the worldwide commercial production of the mushroom Agaricus bisporus.
Composting has long been recognized not only as a means of safely treating solid organic wastes but particularly as a form of recycling of organic matter. Composting will increasingly play a significant role in future waste management schemes since it offers the means of re-use of organic material derived from domestic, agriculture and food industry wastes. The increased interest in composting derives from the growing awareness of the many environmental problems associated with some of the main ways now practiced for treating solid organic wastes, e.g. incineration and landfilling. The overwhelming majority of municipalities and individuals are opposed to having incinerators and landfill sites established within their communities.
Composting has only recently become a serious waste-management technology, and both theoretical and practical development of the technology is still in its infancy. The primary aim of a composting operation is to obtain, in a limited time within a limited compost, a finl compost with a desired product quality. A composting plant must function under environmentally safe conditions.
Composting is carried out in packed bed of solid organic particles in which the indigenous microbes will grow and reproduce. Free access to air is an essential requirement. The starring materials are arranged in static piles (windrows) , aerated piles or covered tunnels or in rotating bioreactors (drums or cylinders). Some form of pre-treatment of the waste may be required, such as particle-size reduction by shredding or grinding. The basic biological substrates with oxygen to produce carbon dioxide, water and other organic by-products (fig. 9.6). After the composting process is completed, the final product most often needs to be left for variable time periods to stabilize
Successful composting requires optimization of the growth conditions for the microorganisms. It is a mixed-culture fermentation and an outstanding example of microbial ecology in action. The large bulk of most operations acts as insulation and, as a result of the biological heat generated by the microbial reactions, there can be rapid internal heat build-up. Over-heating can seriously impair microbial activity. Compost processes should be regulated to prevent temperature rising above 55 c. The moisture level of the organic substrates should normally be 45-60%, free moisture will accumulate, filling the interparticle spaces and restricting aeration, while below 40%, conditions become too dry for successful microbial colonization.
Solid organic materials are only slowly solubilized by exo-enzyme secretion by the fermenting microbes. This reaction step is generally considered as ratelimitingng. Cellulose and lignin are abundantly present in most solid wastes. A high lignin content, for example, in straw and wood materials, hampers the overall process of degradation. Lignin is especially resistant to degradation and is only slowly degraded. In many instances it can shield other substances which are otherwise more easily degraded. Ready access to air is an essential ingredient for a successful, balanced bioreactor.
For large-scale commercial composting, the aerated pile system is carried out in closed buildings to facilitate the control of odour emissions. In these systems forced aeration with regular turning used to create good composting conditions. There are now several plants in Europe with a capacity of over 60000 tonners per year.
Tunnel composting is performed in closed plastic tunnels 30-50 m long and 4-6 m in width and height. Such tunnel systems have been in operation for many years for the composting of sewage sludge and domestic wastes and for specialized substrate preparation for mushroom production. Some plants can operate at up 10000 tonnes per year.
Rotating drum systems in various sizes have been used for composting domestic wastes worldwide. The large Dano process is especially useful for wettish organic waste. Small drum systems have been widely accepted for small quantities of garden waste which can readily be used for recycling.
In some composting processes the gas outlets can odour problems owing to the presence
Of sulphur and nittogen compounds. Special attention is now given to reducing or removing these odours by gas scrubbers or filttation since environmental regulations can lead to the closure of offending plants. The most widely used form of biofilrration involves a fixed bed or mass of organic material, e.g. mature compost or microbially embeddedwood chips. The gases pass throough the mixture and the resulting biochemical activity can greatly reduce the offensive chemical smells.
Composting is undoubtedly one of the principal strategies for solid organic waste tteatment and recycling back into the environment.
For future expansion of composting and recycling four criteria will need to be achieved.
(1) a suitable infrastructure must be in place;
(2) suitable quality and quantity of substrates must be available;
(3) there must be markets for the end-products;
(4) processes must be environmentally sound and demonstrate econmicviability viability.
In 1992, 7% of all European municipal solid waste was composted and, by 2000, this had grown to almost 18%. Germany, as a renowned ’green’ nation, already has a much higher rate. During 1992, no less than 120 composting plants were being built, enlarged or planned in Germany. This would add a compost production capacity of nearly I million tonnes annually.
A major reason for this expansion has been the separation of domestic waste at source. This is the expansion has been the separation of domestic waste at source. This is the three-bin approach widely practiced in Europe, namely one for recyclables (glass, metals, plastics), one for fully degradables (vegetable wastes, papers – the bio-bin), and one for other materials and hazardous wastes.
In Germany alone it has been calculated that an annual demand for 20 million tonnes (MT) of compost can be identified, thus:
10.8 MT – agriculture
1.2 MT – viniculture
1 MT – forestry
3.6 MT – substrates and soils
3.4 MT – land reclamation
The process of composting has been with us in many forms for centuries, recycling vegetable wastes into useful products. It is simple, natural and invariably costs less than landfill and incineration. But above all, all, it is safe, free from toxic emissions, and needs minimum financial resources.
9.6 Bioremdiation
Large areas of the earth’s and the oceans and other waterways have already been contaminated with oil-derived compounds and toxic chemicals. More than two mailion tonnes of oil are estimated to enter the sea each yeas. Approximately half will be derived from industrial effluents, sewage and seepage from below the seafloor. It is considered that only about 18% of the total is coming from refineries, off-shore operation and tanker activities. Unlike most other pollutants, oil spillages can readily be seen and have become an emotive subject on the TV screen. Most oils do have a relatively low toxicity to the environment in general but can have catastrophic and immediate effects on bird and animal lif associated with water. Some of the major oil spills of recent years are shown in Fig. 9.7.
The contamination of soil normally results from a range of activities related to our industrialised society. Contaminated land is viewed as land that contains substances which, when present in sufficient quantities or concentrations, can probably cause harm to man – directly or indirectly – and to the environment in general. Many xenobiotic (industrially derived) compounds can show high levels of recalcitrance and, while in many cases only small concentrations get into the environment, they can be subject to biomagnification. Biomagnification, in essence, implies an increase in the concentration of a chemical substance, e.g. DDT , as the substance is passed through the food chain.
Hazardous wastes and chemicals have become one of the major problems of modern society worldwide. In the USA, in excess of 50 000 contaminated sites and sevetal hundred thousand leaking underground storage tanks have been identified. The estimated cost of treating these polluted sites is $ 1.7 ×10 12 and, to date, the Us Environmental Pollution Agency (EPA) has given $15.2 ×109 to clean up hazardous waste sites. Hazardous wastes and toxic chemicals pose complex environmental problems by directly affecting the air, water, soil and sedimeants while indirectly and unpredictably affecting living organisms that use these resources.
The commitment of biotechnology for the environmental management of such hazardous wastes or contaminants can be seen as the development of systems that involve biological catalysts to degrade, detoxify or accumulate contaminating chemicals. The application of biological
agents – mostly microorganisms – for the treatment of environmental chemicals has mostly been directed towards remedial activities.
There are three main approaches to be assessed in dealing with contaminated sites, namely: (1) identification; (2) assessment of nature and degree of the hazard, and degree of the hazard, (3) the choice of remedial action.
When dealing with contaminated soil, clean-up operations can involve onsite processing, in situ treatment or off-site processing. Up to the pres