Waste water and sewage treatmentItruduction;
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 contaminated by faeces.
A similar route for typhoid fever was shortly to
be demonstrated. 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 treatment of municipal and industrial wastes before
discharging into natural water systems.
Growth in human populations has generally been matched by a concomitant 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 andvegetable 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 introduction 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
Such bioreactor assemblies rely on the metabolic versatility of mixed microbial populations (microbial ecology) for their efficiency.
The systems in which they perform their biological functions can be likened to other industrial bioreactors (e.g. antibiotic production); large-scale plants, for example municipal forced aeration tanks can be extremely complex,
requiring the skills of the engineer and the microbiologist for successful operation.
The fundamental feature of these bioreactors is that they
contain a range of microorganisms 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 largestbiotechnological industry, and the least recognised by lay people.
Controlled use of microorganisms has led to the virtual elimination of such waterborne diseases as typhoid, cholera and dysentery in these communities.
Yet, if water and sewage treatments are seriously interrupted, major epidemics may quickly develop as witnessed in 1968 in Zermatt, Switzerland, where typhoid developed following the breakdown of the water treatment plant.
Thus, biotechnology not only generates a whole new range of useful products, 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 throughout the world widely used practice for sewage treatment is shown in Fig.
three stages of primary and secondary processing followed by microbial digestion.
An optional tertiary stage involving chemical precipitation may
The primary activity is to remove coarse particles and solubles
leaving the dissolved organic materials to be degraded or oxidised
by microorganisms in a highly aerated, open bioreactor.
This secondary process requires considerable energy input to drive the mechanical aerators that actively mix the whole system, ensuring regular contact of the microorganisms with the substrates 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) that will reduce the volume of solids, the odour and the number 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.
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, 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 two litres a 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 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, human 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 aquivers or rivers.
Biotechnology will play an important role for 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 systems will be developed for treating complex wastes.
The role of the biocatalyst or microbe will be constantly reassessed.
Biotechnologists are now designing increasingly specific and efficient bioreactors to contain selected consortia of microorganisms best adapted to a range of different waste streams.
In countries with high annual hours of sunlight there has been considerable 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, biogas formation or, perhaps more ambitiously, for bulk organic chemical formation.
A comparison of several widely used treatment processes for liquid wastes is shown in Table 7.1, while Table 7.2 defines the various operating components.
Water is now being recognised as an increasingly expensive component of many industrial processes. Industries worldwide use vast quantities of quality water in their manufacturing procedures, e.g. steel, textiles, food, etc. For example, for each tonne of steel produced approximately 280 tonnes of water will be used.
In the past many of these industries simply
discharged the waste water into water courses often resulting in extensive down-river or estuarine pollution. Stringent anti-pollution laws together with greatly increased water charges have prompted such companies to develop new waste-water treatment systems that function in a closed-loop manner.
Almost two-thirds of water consumption worldwide is utilised for agricultural irrigation.
In many cases where water is in short supply raw
domestic sewage is used, which invariably leads to crop contamination
ADVANTAGES OF WASTE WATER TREATMENT
- High BOD removal efficiency.
- Low operating costs.
- Low operator skills required.
- Sensitive to cold weather.
- Activated sludge High BOD removal efficiency.
- Moderate ground requirements.
- Trickling filters Low operator costs.
- Moderate space requirements. Resistant to sudden high inputs.