Agricultural Use Of Sewage Sludge

Now here's where a thoughtful person may ask, "Why not put sewage sludge back into the soil for agricultural purposes?"

One reason: government regulation. When I asked the supervisor of my local wastewater treatment plant if the one million gallons of sludge the plant produces each year, from a population of 8,000 people, was being applied to agricultural land, he said, "It takes six months and five thousand dollars to get a permit for a land application. Another problem is that due to regulations, the sludge can't lie on the surface after it's applied, so it has to be plowed under shortly after application. When farmers get the right conditions to plow their fields, they plow them. They can't wait around for us, and we can't have sludge ready to go at plowing time." It may be just as well.

Problems associated with the agricultural use of sewage sludge include groundwater, soil and crop contamination with pathogens, heavy metals, nitrates, and toxic and carcinogenic organic compounds.34 Sewage sludge is a lot more than organic agricultural material. It can contain DDT, PCBs, mercury and other heavy met-als.35 One scientist alleges that more than 20 million gallons of used motor oil are dumped into sewers every year in the United States.36

America's largest industrial facilities released over 550 million pounds of toxic pollutants into U.S. sewers in 1989 alone, according to the U.S. Public Interest Research Group. Between 1990 and 1994, an additional 450 million pounds of toxic chemicals were dumped into sewage treatment systems, although the actual levels of toxic discharges are said to be much higher than these.37

Of the top ten states responsible for toxic discharges to public sewers in 1991, Michigan took first prize with nearly 80 million pounds, followed in order by New Jersey, Illinois, California, Texas, Virginia, Ohio, Tennessee, Wisconsin and Pennsylvania (around 20 million pounds from PA).38

An interesting study on the agricultural use of sludge was done by a Mr. Purves in Scotland. He began applying sewage sludge at the rate of 60 tons per acre to a plot of land in 1971. After fifteen years of treating the soil with the sludge, he tested the vegetation grown on the plot for heavy metals. On finding that the heavy metals (lead, copper, nickel, zinc and cadmium) had been taken up by the plants, he concluded, "Contamination of soils with a wide range of potentially toxic metals following application of sewage sludge is therefore virtually irreversible." 39 In other words, the heavy metals don't wash out of the soil, they enter the food chain, and may contaminate not only crops, but also grazing animals.40

Other studies have shown that heavy metals accumulate in the vegetable tissue of the plant to a much greater extent than in the fruits, roots, or tubers. Therefore, if one must grow food crops on soil fertilized with sewage sludge contaminated with heavy metals, one might be wise to produce carrots or potatoes instead of lettuce.41 Guinea pigs experimentally fed with swiss chard grown on soil fertilized with sewage sludge showed no observable tox-icological effects. However, their adrenals showed elevated levels of antimony, their kidneys had elevated levels of cadmium, there was elevated manganese in the liver and elevated tin in several other tis-


Estimated to contain 10 billion microorganisms per gram, sludge may contain many human pathogens.43 "The fact that sewage sludge contains a large population of fecal coliforms renders it suspect as a potential vector of bacterial pathogens and a possible contaminant of soil, water and air, not to mention crops. Numerous investigations in different parts of the world have confirmed the presence of intestinal pathogenic bacteria and animal parasites in sewage, sludge, and fecal materials." 44

Because of their size and density, parasitic worm eggs settle

Table 5.1

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