Contrarian friend Cliff White doesn’t share
Lindsay Brown’s impatience with HRM Council’s decision to spend $50,000 studying the safety of fertilizer derived from the municipality’s sewage treatment plants.
Among other things, she mentions studies that go back eighty years. I'd suggest that studies going back even half that time wouldn't be testing even half the chemicals, toxins, and metal compounds likely to be found in today's sewage. Since any cursory search of the literature will show that not all of these products are removed at the treatment plant, three questions arise:
- First, how effective are our local sewage plants are in extracting heavy metals, toxins and other chemicals before it becomes sludge and then fertilizer?
- Second, what are the national and provincial standards for levels of these products in fertilizers?
- Finally, are these standards adequate to protect both the environment and human health?
A quick search of the literature will show that different countries have widely different standards in this regard, suggesting that this is a legitimate area for concern. Given the reasonable scientific concern regarding sewage sludge I don't think a study of the local stuff is unwarranted.
In an earlier letter, Cliff forwarded information he extracted from
a 2009 US EPA Report.
The sampling effort collected sewage sludge from 74 randomly selected publicly owned treatment works in 35 states. Samples were collected in 2006 and 2007. The TNSSS Technical Report provides results for 145 analytes, including:
- four anions (nitrite/nitrate, fluoride, water-extractable phosphorus),
- 28 metals,
- four polycyclic aromatic hydrocarbons,
- two semi-volatiles,
- 11 flame retardants,
- 72 pharmaceuticals, and
- 25 steroids and hormones.
Some analytes were found in all 84 samples, while others were found in none or only a few of the sewage sludge samples.
After the jump, more extracts from the report, detailing the number of samples in which various chemicals were found. That list will probably scare some readers. Certain environmentalists like to cite such lists precisely because they sound scary, and because they lend a false aura of scientific credibility to their arguments. Such lists are all but meaningless without two essential pieces of information:
- In what concentrations were the chemicals found? (For many chemicals, minuscule amounts are both routine and harmless.)
- What level of exposure to people, plants, or animals would result if the sludge were used for its intended purpose? (How much actually gets to people is the real worry, and Cliff's list tells you nothing about that.)
To answer these questions, scientific risk assessors use a model known as
source, pathway, receptor. In the case of a person who eats carrots grown in soil treated with fertilizer derived from composted sewage sludge, the sludge is the source, eating a carrot is the pathway, and the person is the receptor.
For each chemical, the risk assessor will determine the amount present in the sludge, and the amount that might make its way into a carrot and then into a person who eats the carrot. The risk depends on the actual exposure a person might experience. These calculations typically use ultra-conservative assumptions: the receptor is a developing child; the child eats only vegetables grown in soil treated with the fertilizer; Large amounts of fertilizer are used.
This is exactly the kind of analysis used to set allowable levels of Cliff's scary sounding chemicals. Find the Canadian Council of Ministers of Environment (CCME) report on this process
here [pdf]. Regular sampling confirms that composted Halifax sewage sludge meets these standards. Dozens of municipalities have safely used sewage sludge for decades, with less advanced equipment that that used in HRM. And let us not forget, Halifax's sewage treatment plants solved a real environmental menace--the dumping of raw sewage into Halifax Harbour.
For all these reasons, real environmentalists should be delighted, and HRM Council should not waste public money pandering to anti-science zealots who will never be persuaded on this issue.
29 May, 2011
