Sewage Systems
"Out of sight, out of mind" has been the traditional philosophy of waste disposal. Technologically, it may well be the last thing scientists want to be associated with, so it has received relatively little attention until recent decades, when the negative results of lax policies have shown themselves. Improper sewage disposal results in pollution-related problems--besides being foul and unpleasant, pollution can damage plant and animal communities, contaminate food and water supplies, and spread serious human diseases.
In the 1870s, Louis Pasteur (1822-1895) discovered the existence of microbes, and made the association between pollution and diseases such as cholera and typhoid. Yet it took the better part of a century for practice to come into line with knowledge, and many parts of the world still do not have adequate sanitation.
Sewer systems have been part of the urban scene since ancient times. Latrines and drains were used in the Indus Valley circa 2500 B.C., and Rome boasted sewers as early as 500 B.C.. Water closets and toilets began to appear in homes during the 1700s. In cities, the effluent had to be carried in buckets to gutters and cesspools, while it was buried or used as fertilizer in rural areas. In both cases, water and food supplies were easily contaminated, either directly or by bacteria carried by rats, mosquitoes, and flies. The first sewer system to resemble modern underground sewer plans was built in Hamburg in 1842. In 1854, London rebuilt its sewer system after a cholera outbreak. Though Pasteur had yet to make his discovery, suspicions about the link between disease and improper disposal of human waste had surfaced.
The new London sewer system made extensive use of Portland cement, a highly impervious material invented by Joseph Aspden in 1824. Yet nothing was done to treat the effluent before it was sent into the Thames River. Waste treatment began about 1890 when filters and chemicals started to be used in some cities. In 1916 two United States innovators, Lockett and Ardern, devised the activated sludge method, relying on biological organisms to purify waste, but modern waste treatment evolved slowly during the twentieth century.
About 80% of waste has industrial origins. The special nature of each factory and its pollutants requires individualized attention at the site before the waste is sent into the public system. Incineration, radiation treatment, and composting are among those treatments. Modern sewage treatment is comprised of three stages. The primary stage separates out larger solids, or sludge. The second stage includes further filtering and allows bacteria to break down the particles in aeration tanks. The third stage introduces lime to aggregate the remaining suspended particles and allow it to settle in ponds. In rural areas, where homes and businesses are widely scattered, sewage systems are expensive to build. Therefore, septic tanks are located near each building. Invented in 1896, they collect the effluent, break it down with bacteria, and gradually introduce it to the surrounding soil.
One method for disposal of sewage wastes, the spreading of sludge, is used by many countries, and has had renewed, and controversial, interest in the United States in recent years. Sludge, sometimes referred to as biosolids, is processed sewage waste, left over after attempts to remove debris (e.g. gum wrappers), heavy metals (e.g. lead from old city pipes), and pathogens ( viruses, molds and bacteria). Some sludge, deemed to be cleaner, is heat-treated and sold as fertilizer. The rest, if not considered toxic, is used to feed the soil, and provide a cheap method of sewage disposal. The waste industry, and some state and federal government agencies, advocate the spreading of sludge as beneficial to growing and grazing land, and as the best way to handle the nation's sewage disposal problems. But citizen and environmental groups have protested vigorously that the sludge frequently contains toxic heavy metals, fecal disease-causing fecal organisms, or radioactive compounds. Scientists have discovered that the practice of spreading only thin layers of sludge increases the amount of mercury--an environmental contaminant--that evaporates into the atmosphere and which, ultimately, condenses out in cold Artic air. There is also concern that processed sludge sold as fertilizer may contain disease-causing fecal organisms such as Salmonella. The real problem, some scientists point out, is that the vast array of pollutants in sludge cannot all be treated by one, single disposal method. As population increases, the spreading of sludge as a means of sewage disposal is sure to remain contentious.
Promising future directions in sewage handling include the use of reed beds to remove toxins, reliance on peat or lignite to absorb harmful bacteria, and the immobilization of bacteria in a solid medium like carbon powder.
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