| Sewerage Paradigm | |
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| Appropriateness of the Paradigm |
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There have been many changes since the paradigm was set at the beginning of the 19th Century: What is important in the setting of the sewerage engineering paradigm at this time is that firstly, the choice of methods was not based on technical superiority in terms of performance in achieving effluent purification. Nor was the choice made by the British Royal Commission which nevertheless played an important role in dismissing exaggerated claims for some treatment methods and setting standards. The choice was made by engineers on the basis of their search for `good enough' solutions at a minimum cost; solutions that the public would accept at the turn of the century. The economics of the various solutions depended only on capital and operating costs for the particular stage of treatment being considered. They did not include possible environmental costs. The economics of utilising the sewage was calculated on early twentieth century price structures which reflected the cheapness and attractiveness of artificial fertilisers, resource availability (including water), pumping costs and the abundance of water supplies at that time. Composition of the sewage - chemicals, grease It is not only economic values which have changed in the past seventy years. The actual composition of city sewage has also changed substantially with the growth of industry and the increased use of inorganic and artificial materials in industrial processes. Sewage treatment methods within the paradigm are aimed at removing suspended solids which will settle out of the effluent and decreasing the oxygen demand of the sewage by breaking down organic material with the use of naturally occurring microorganisms contained within the sewage and in the environment. (Oxygen demand is a particular problem in rivers because oxygen is required by other living organisms in the river and oxygen may not be replaced or regenerated quickly enough to ensure these organisms survive.) These methods do not remove or treat toxic chemicals, heavy metals, organochlorines or most of the grease and oil that is contained in the sewage. In fact some of these substances actually interfere with the microorganisms necessary for secondary and tertiary treatment, killing them off and turning whole batches of sewage `off'. Engineers have coped with this problem partly by restricting what can be put into the sewers but this cannot be successfully policed and enforced without a large and expensive force of inspectors. Moreover, the effects of these substances in waterways is uncertain and it is only when a disaster occurs such as happened in Minamata, Japan, where hundreds of fish-eating people got mercury poisoning, that the adverse health effects can be proven. It is notable in this regard that mercury is one of the few substances that is completely banned from Sydney's sewer systems. Other substances are restricted by concentration and an `over-careful' approach is rejected by industries who have an economic bonus in the use of the sewers for waste disposal. Another problem which arises from the industrial waste in the sewage and which is subject to much research and experimental work is the disposal of the sludge. Sludgwe is a by-product of sewage treatment and consists of the solids which have been settled out of the sewage together with a certain amount of liquid. This problem has been present since the nineteenth century but has been exacerbated by the tendency for viruses and heavy metals to concentrate in the sludge making incineration, burial and sea dumping of the sludge, even after treatment, environmentally hazardous procedures. Grease is seen, by engineers as a major problem for swimming beaches near sewage outfalls because the grease, which forms a floating slick on the surface of the sea, makes the sewage field highly visible and leaves obvious traces in the form of grease balls on the sand. Some grease is removed from the sewage during sedimentation treatment by skimming the floating grease from the surface of the sewage in the tank. This has caused engineers to note the inappropriateness of the treatment paradigm,
Theories of disease, discovery of viruses There has been much controversy, which has yet to be settled, as to the danger that swimming in sewage polluted water poses to people. Treatment methods were not designed to eliminate pathogenic bacteria from sewage, but rather to prevent the waterways becoming a nuisance after the treated effluent was discharged into them. The paradigm was set before viruses were known. As a result, although sewage may contain as many as 110 different types of virus, conventional sewage treatment processes cannot be counted on to remove them. Primary sedimentation does not remove viruses or pathogenic bacteria at all. A representative of the World Health Organisation has said:
Because the paradigm does not specifically deal with viruses or pathogenic bacteria, their presence is not monitored. Monitoring of sewage effluent is confined to measuring levels of faecal coliform which are not dangerous in themselves but merely indicate the presence of sewage. Authorities, who will not set standards that cannot be met by the available technology, set standards for bathing waters in terms of concentrations of these faecal coliforms which are generally agreed not to correlate statistically with viral counts because faecal coliforms have a more rapid die-off rate than many viruses and pathogens. The second problem is the fact that conventional sewage treatment does not remove the nutrients from the sewage and this has caused the choking up of many waterways with excessive plant growth. Research into solving this problem has been tackled in terms of a search for a further stage of treatment, which can be added to the paradigm, and will remove the nutrients from the effluent before discharge. Public values, eg towards recreation and environment Changing community expectations have also created problems for the paradigm on two levels. The public is far less tolerant of the degradation of recreational facilities and more willing to pay for higher degrees of treatment but many treatment plants built when sewage flows were smaller and public expectations lower do not have the space available nearby to expand and incorporate, for example, secondary treatment. This has lead to a solution for ocean outfalls of extending the outfalls under the sea for a few kilometres. Such an ad hoc solution aims at keeping the sewage from view by discharging it at greater depths where it will be more dispersed and may be kept beneath the surface when the temperature difference between the top and lower levels of water is great enough to produce a thermoclyne (Caldwell Connell, 1976). The other change in community expectations arises from the greater environmental awareness that has been manifest since the 1960s and 70s. This awareness has meant that the public is not only concerned with their own health but also with the preservation of river and marine environments and the species that live in them. Very little research has been done into the effects of sewage, especially industrial wastes, on such ecosystems and the phenomenon of bioaccumulation of certain substances up the food chain has only been discovered fairly recently. References: Caldwell Connell (1976), Sydney Submarine Outfall Studies, (Sydney: M.W.S.&D.B.). Melnick, Joseph (1976) ÔViruses in water: An IntroductionÕ, in Gerald Berg and et al (eds), Viruses in Water, American Public Health Association). Ryan, Paul, (undated) ÔSubmarine Ocean Outfall SewersÕ, (Sydney: SPCC).
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© 2003 Sharon Beder