Micro in Concentrations, Mega in Concerns
They may be the mathematical equivalent of needles in a haystack, but in the scientific literature and in public perception microconstituents oftentimes appear to loom larger than the haystack itself. This month’s library is an update on these compounds. It is meant to give you both information and perspective. We now have the tools to detect these compounds at very low concentrations with some level of accuracy. By low here I mean low parts per billion. A part per million is $0.01 per $10,000.00. To turn that into a part per billion, take that $10k and up it to $10,000,000, still using that same penny as a basis for comparison.
Granted, unlike other compounds, at least certain of these compounds can induce very subtle changes over time- at least in aquatic species. There are even concerns about the impact of these compounds on human species, although scientific consensus and common sense pretty much suggest that biosolids can’t be a significant exposure route.
While this library and most of its readers focus on the solids end of the process, the main hazard potentials associated with these emerging compounds have been defined for aquatic systems. As a result of these studies, additional work has been done to see how existing treatment processes can be altered to maximize decomposition of these compounds to reduce any potential risk. But does the risk merit the expense and effort to remove or decrease?
The first article is a general survey paper on the fate of a very broad range of these compounds in WWTPs. There are some nice diagrams to show how adsorption works, how biodegradation can happen etc. For me, the most useful part of this paper is the extensive lists of compounds with concentrations in WWTPs and some description of what happens to them within plants. They include the name of the compound, what it does, the % removal within a plant, mechanism of removal, effluent concentration, and relevant environmental concentrations. These tables are complete with references if you want to get more information. So if someone asks you about a compound that you have never heard of, this paper may just have it included in the list. Unfortunately this paper is all about the water with nothing about the solids.
For some information on the solids you can look at paper #2. This study, out of Canada, looks at the concentrations of a subset of these compounds in the solids coming out of aerobic and anaerobic treatment systems. In other words, do they decay or decrease or transform more readily in the presence of oxygen or in an anaerobic environment, kept at a comfortable temperature. For most compounds, degradation is more pronounced in the presence of oxygen. The authors note that while aerobic digestion leads to more complete destruction of the compounds, that process comes at greater expense and without the potential for energy generationboth characteristics of anaerobic digestion. Is it worth the expense and are the benefits associated with reducing the concentrations of these compounds greater or less than the benefits associated with anaerobic digestion?
For two opinions on whether extra treatment is worth the expense, we turn to papers #3 and 4. Both are short opinion pieces, and, even better, they disagree with each other. Both address new regulations in Switzerland that will require additional treatment to remove compounds such as estrogens from WWTP effluent. The justifications for these regulations are the negative impact of these compounds on fish downstream from the treatment plants.
The first paper says: ‘Wait a minute here. Are you sure that this is a good idea?’ The authors point out that aquatic life in these rivers has gotten more diverse and healthier over time, not less. In rivers in the U.K., for example, species richness of invertebrates in rivers has improved from 1991- 2008, not declined. Although sex changes have been observed, these guys say ask the people who fish in these rivers if there is a problem. And the answer is ‘no’. They note that the carbon emissions associated with the extra efforts to increase removal will likely have a greater negative consequence than any benefits associated with removal. They also point out that it could be that these river systems are adapted to the characteristics of the effluent, and changing those characteristics might have an unanticipated detrimental impact. I am not sure I concur with that last point, but my money is generally on these guys.
The 4th piece is the response where they say ‘not so fast.’ I think that here the primary argument is similar to the philosophy that change in any way is not a good thing and that these compounds constitute change. We may be causing harm that we can’t detect, but what right do we have to cause that harm? I understand that in principle, but I am not sure I agree in practice. Humans, in general, with or without the TCC in the toothpaste, are the main cause of so much disturbance. Focusing on the toothpaste is like missing the forest for the leaf.
The last paper provides a unique and fresh perspective on this whole discussion. The authors’ argue that we should consider the environmental impacts of compounds before we allow them to be mass marketed. Talk about taking the discussion out of the sewer. The authors propose ten recommendations for consideration before a pharmaceutical is approved for general use. While I am not convinced that this approach would ever be used to stop the distribution of any highly effective medication, it would be a very effective tool to stop the distribution and use of compounds shown to have no effect, such as the range of antimicrobials.
This question of risk and benefit is one that has not been adequately addressed in the literature on these compounds. It certainly seems to be absent from public perception. This review focuses on ecosystem risk. For a broad discussion of human risk, I invite you to make the trip to Lake Chelan for NBMA’s Biofest this September. Results from a human health risk assessment that I participated in will be presented.
Sally Brown, University of Washington