Environmental Problems, Their Causes, and Sustainability 1

PrimarySecondaryGritBar screen chamber Settling tank Aeration tank Settling tankChlorinedisinfection tankTo river, lake,or oceanRaw sewagefrom sewers Sludge (kills bacteria)Activated sludgeAir pumpSludge digesterFigure 22-16 Solutions: primary andsecondary sewage treatment.Sludge drying bedDisposed of in landfill orocean or applied to cropland,pasture, or rangelandwater coloration and disinfection to kill disease-carryingbacteria and some but not all viruses. The usualmethod for doing this is chlorination. But chlorine canreact with organic materials in water to form smallamounts of chlorinated hydrocarbons. Some of thesechemicals can cause cancers in test animals and maydamage the human nervous, immune, and endocrinesystems (Case Study, p. 416).Use of other disinfectants, such as ozone and ultravioletlight, is increasing. But they cost more and theireffects do not last as long as chlorination.What Should We Do with Sewage Sludge?An Unsettled ProblemSewage sludge can be used as a soil conditioner,but this can cause health problems if it containsinfectious bacteria and toxic chemicals.Sewage treatment produces a gooey sludge containinga slimy mixture of bacteria-laden solids and oftentoxicchemicals and metals when sewer systems mixindustrial and household waste. In the United States,about 9% by weight of this sludge is placed in largecircular digesters and kept warm for several weeks toallow anaerobic bacteria to decompose organic materialsand produce compost for use as a soil conditioner.About 36% of the sludge, also known as biosolids, isused to fertilize farmland, forests, golf courses, cemeteries,parkland, highway medians, and degradedland. The remaining 55% is dumped in conventionallandfills where it can contaminate groundwater, or isincinerated. Such burning of waste can pollute the airwith toxic chemicals, and it produces a toxic ash usuallyburied in landfills that the EPA says will eventuallyleak.From an ecological standpoint, it is desirable to recycleplant nutrients in sewage sludge to the soil onland. But there are problems with using sewage sludgeto fertilize crops (Figure 22-17, p. 512). As long as harmfulbacteria and other pathogens and toxic chemicalsare not present, sludge can fertilize land used for foodcrops or livestock. But removing bacteria (usually byheating), toxic metals, and organic chemicals is expensiveand rarely done in the United States. According toa 2002 report by the National Academy of Sciences, theEPA is using outdated science to set standards for usingsewage sludge as a fertilizer in the United States.Agrowing number of alleged health problems andlawsuits have resulted from use of sludge to fertilizecrops in the United States. To protect consumers andavoid lawsuits, some food packers such as Del Monteand Heinz have banned produce grown on farms usingsludge as a fertilizer.How Can We Improve Sewage Treatment?Eliminate ToxicsPreventing toxic chemicals from reaching sewagetreatment plants would eliminate such chemicals fromthe sludge and water discharged from such plants.Environmental scientist Peter Montague calls for redesigningsewage treatment systems to prevent toxichttp://biology.brookscole.com/miller14511