While many ancient civilisations had an appreciation of the need to protect the quality of water to be used for human consumption, it was not until 1855 that it was demonstrated that cholera was transmitted by water contam-inated with faeces. A similar route for typhoid fever was shortly to be demon-strated. By the end of the nineteenth century, the microbial ecology of many human diseases had been shown to have an anal–oral route of transmission, which finally confirmed the health hazards associated with water contaminated with faeces. The introduction of sewage systems in developed societies during the nineteenth century allowed, for the first time, the possibility of treat-ment of municipal and industrial wastes before discharge into natural water systems.
Growth in human populations has generally been matched by a concomi-tant formation of a wider range of waste products, many of which cause serious environmental pollution if they are allowed to accumulate in the ecosystem.
In rural communities, recycling of human, animal and vegetable wastes has been practised for centuries, providing in many cases valuable fertilisers or fuel. However, it was also a source of disease to humans and animals by residual pathogenicity of enteric (intestinal) bacteria. In urban communities, where most of the deleterious wastes accumulate, efficient waste collection and specific treatment processes have been developed, since it is impractical to discharge high volumes of waste into natural land and waters. The intro-duction of these practices in the last century was one of the main reasons for the spectacular improvement in health and well-being in the developed countries.
Mainly by empirical means, a variety of biological treatment systems have been developed, ranging from cesspits, septic tanks and sewage farms to gravel beds, percolating filters and activated sludge processes coupled with anaerobic digestion. The primary aims of all of these systems or bioreactors is to alleviate health hazards and to reduce the amount of biologically oxidisable organic compounds, producing a final effluent or outflow that can be discharged into the natural environment without any adverse effects.
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Fig. 9.2 Aerial view of bioreactors at the sewage treatment plant for the city of Glasgow, Scotland.
Such bioreactor assemblies rely on the metabolic versatility of mixed micro-bial populations (micromicro-bial ecology) for their efficiency. The systems in which they perform their biological functions can be likened to other industrial bioreactors (e.g. as in antibiotic production); large-scale plants, e.g. municipal forced-aeration tanks (Fig. 9.2), can be extremely complex, requiring the skills of the engineer and the microbiologist for successful operation. The funda-mental feature of these bioreactors is that they contain a range of microorgan-isms with the overall metabolic capacity to degrade most organic compounds entering the system.
The development of these systems was an early example of biotechnology.
Indeed, in volumetric terms, biological treatment of domestic waste-waters and sewerage in the industrialised nations is by far the largest biotechnological industry and the least recognised by lay people. Controlled use of microorgan-isms has led to the virtual elimination of such waterborne diseases as typhoid, cholera and dysentery in these communities. Yet, if water and sewage treat-ments are seriously interrupted, major epidemics may quickly develop, as wit-nessed in 1968 in Zermatt, Switzerland, where typhoid developed following the breakdown of the water treatment plant.
CH4
Fig. 9.3 Stages of sewage treatment in a complex incorporating anaerobic digestion.
Thus, biotechnology not only generates a whole new range of useful prod-ucts, it also plays an indispensable part – through water- and sewage-treatment processes – in the reduction of infectious diseases of humans and animals.
The biological disposal of organic wastes is achieved in many ways through-out the world. A widely used practice for sewage treatment is shown in Fig. 9.3.
This complex but highly successful system involves a series of three stages of primary and secondary processing followed by microbial digestion. An optional tertiary stage involving chemical precipitation may be included. The primary activity is to remove coarse particles and solubles, leaving the dis-solved organic materials to be degraded or oxidised by microorganisms in a highly aerated, open bioreactor. This secondary process requires consid-erable energy input to drive the mechanical aerators that actively mix the whole system, ensuring regular contact of the microorganisms with the sub-strates and air. The microorganisms multiply and form a biomass or sludge, which can either be removed and dumped, or passed to an anaerobic digester (bioreactor) which will reduce the volume of solids, the odour and the num-ber of pathogenic microorganisms. A further useful feature is the generation of methane or biogas, which can be used as a fuel. However, the value of biogas is marginal because of its content of carbon dioxide and hydrogen sulphide.
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Another important means of degrading dilute organic liquid wastes is the percolating or trickling filter bioreactor. In this system the liquid flows over a series of surfaces, which may be stones, gravel, plastic sheets, etc., on which attached microbes remove organic matter for essential growth. Excessive microbial growth can be a problem, creating blockages and loss of biological activity. Such techniques are widely used in water purification systems.
Abundant availability of water is vital for modern urban and industrial development. Water makes up more than 70% of the human body and about 2 litres per day is usually sufficient to keep an adult healthy. Water acts as a transport medium for essential nutrients within the body, helps to remove toxins and waste materials, stabilises the body temperature and performs a crucial part in the structure and function of the circulatory system. In essence, water is the elixir of life. In the natural world the ecosystem regenerates and recycles water. Increasingly, man’s intrusion into nature by industrialisation, extensive farming practices, deforestation, etc., has severely unbalanced this process. It is now accepted that two-thirds of the world’s nations are water-stressed – using clean water faster than it is replenished in aquifers or rivers.
Biotechnology will play an important role in reclamation and purification of waste waters for re-use. Water must be recycled in the sustainable use of resources. The most important threat mankind faces in the coming decades is not global warming or energy deficiency but an increasing shortage of high-quality water.
What are the future areas of importance? Microbiological effluent treatment will be a major field of biotechnological interest in the future. Integrated sys-tems will be developed for treating complex wastes. The role of the biocatalyst or microbe will be constantly reassessed.
In countries with high annual hours of sunlight, there has been consid-erable development of combined algal/bacterial systems for waste and water treatments. Such processes can lead to the formation of relatively pure water and algal/bacterial biomass, which may be used for animal feeding, for biogas formation or, perhaps more ambitiously, for bulk organic chemical formation.
A novel biotechnological innovation in waste-water treatment is the deep-shaft fermentation system developed by ICI. The deep deep-shaft is, in fact, a hole in the ground (up to 150 metres in depth), divided to allow the cycling and mixing of waste water, air and microorganisms (Fig. 9.4). It is most economical in land use and power, and produces much less sludge than conventional systems.
A comparison of several widely used treatment processes for liquid wastes is shown in Table 9.1.
Compressor
Start-up air
Process air
Downcomer
Shaft lining Riser Sludge recycle
OUTLET Air
INLET
Fig. 9.4 The principle of the deep-shaft fermentation system used in waste-water treatment.