|
Edited on Fri Jan-30-04 12:43 PM by NNadir
It is of course now illegal to discharge heavy metals into sewage systems in most cities.
Before I address the title of the post, I have an interesting aside. In some places, the ban on heavy metal contamination has a curious effect, inasmuch as it has lead to increased corrosion of septic systems. This is because heavy metals form insoluble sulfides. One of the chief compounds found in sewage, accounting for some of the noxious odor is hydrogen sulfide formed from the biodegradation of the amino acids methionine and cysteine in proteins. In the past, this hydrogen sulfide would react with heavy metal contaminants in the sewage to form heavy metal sulfides, such a mercuric sulfide, lead sulfide and iron sulfide. This had the happy effect of rendering the heavy metals insoluble and removing the hydrogen sulfide. When the dumping of heavy metals into sewage lines was forbidden, the quantity of hydrogen sulfide went up. In the presence of oxygen, hydrogen sulfide is oxidized to sulfuric acid. Sulfuric acid in turn decomposes concrete (as in acid rain). Concrete is used to make sewage pipes. The overall result is to cause increased corrosion of sewer lines.
Common heavy metals that remain in sewage today are copper (from oxidation of water pipes that bring water into homes, silver (from sodder) and unfortunately lead (also from sodder, especially in older homes). All of these elements are found in trace quantities in homes with soddered copper plumbing in the drinking water. Additional quantities of lead occur in sewage from waste lines, especially toilet bowl seals, which are also made of lead. It is fortunate that lead forms a very insoluble sulfide, PbS, the chief constituent of the widely distributed ore known as galena. (The city of Galena, Illinois, I believe is named for this compound.)
Silver forms an insoluble chloride and it hardly ever finds its way into the food chain. Copper sulfide is also insoluble (it is the form of many copper ores.) This is unfortunate in a way, since copper, although it can be toxic at high levels, is an essential element: It is a constituent of important proteins involved in oxidative phosphorylation, a key reaction on which all eucaryotic life, including human beings, depend. (A high concentrations however copper can compete with iron in certain heme containing proteins, of which hemaglobin is just one.)
In any case the concentrations of lead are extremely small in most sewage, and what is there is at best of very low solubility owing to the presence of hydrogen sulfide. I don't really believe that much of this lead will find its way into the foodchain, something that is easily verifiable by chemical analysis.
If it can be shown, however that the quantities of lead (or any other element) found in sewage sludge do in fact find their way into the food chain, especially via a mechanism by which it is concentrated, it is a realitively simple affair to set specifications for heavy metal contamination. One can then ban lots exceeding this specification. This is exactly what is done for all industrial materials, be they chemicals, fertilizers, or foodstuffs.
This of course begs the question. If you ban using all sewage sludge from use, what exactly do you do with it? I think a law against the use of toilets would attract many violators.
|