The Disposal of Sewage Sludge

The sludge from both the primary and secondary treatment stages of sewage is principally water and organic matter. It can be digested anaerobically, in a process that takes several weeks to complete. Bacteria levels in the sludge are not thereby completely eliminated, but the levels are reduced about a thousandfold. The sludge that remains after this further organic decomposition has occurred and after the supernatant water is removed is sometimes then incinerated or simply dumped into a landfill or into a water body such as the ocean. However, sludge is high in plant nutrients, so about half the sewage sludge in North America and Europe is spread on farm fields, golf courses, and even residential lawns as low-grade fertilizer sometimes called biosolid.

Unfortunately, sewage sludge may contain toxic substances, which potentially could be incorporated into food grown on the land or could contaminate groundwater under the fields. In particular, heavy metal concentrations often are higher in sewage sludge than in soil, principally because industrial wastes are sometimes released directly into sewage lines shared by households. For example, the lead level in municipal sludge can range from several hundred to several thousand parts per million, compared to an average of about 10 ppm in the Earth's crust. In a few communities, an attempt is made to eliminate these toxic materials before final disposal occurs. Some scientists have worried that food crops grown in soil fertilized by sewage sludge may incorporate some of the increased amounts of heavy metals. Control experiments indicate that vegetables vary greatly in the extent to which they will absorb increased amounts of the metals; e.g., the uptake of lead by lettuce is particularly large, but that by cucumbers is negligible. The concentration of arsenic in agricultural soils is greatly increased if arsenic pesticides are applied to them; crops planted on these soils subsequently absorb some of the adsorbed arsenic. Other substances of concern in using sewage sludge as fertilizer for food are alkylphenols from detergents, brominated fire retardants, and pharmaceuticals— especially antibiotics given to farm animals.

Continue reading here: The Destruction of Volatile Organic Compounds

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