The reuse of waste water (e.g., for irrigation in agriculture) after appropriate treatment is a very useful process, especially in areas which are densely populated and arid at the same time. According to a recent report, however, the treatments necessary to make the waters reusable cause high emissions of nitrous oxide (N2O), a powerful greenhouse gas. Here are the details of the study performed by researchers at the University of Cincinnati and the University of California, Irvine.
Waste water treatment and water reclamation
Waste waters can have different origins, for instance domestic households or industrial plants. These normally contain pollutants of a different nature. It is necessary to treat these waters before releasing them into the environment.
In recent years, there has been an increasing tendency to reuse such waste waters; this process is called water reclamation. This practice is common in areas such as California, due to the scarcity of fresh water supplies. The reclaimed water can be used, for instance, in agriculture, for irrigation, or for industrial processes where water is needed, or to recharge ground water supplies. Water reclamation can avoid (or at least minimize) long-distance water importation.
More extensive treatments
For the waters to be reclaimed, however, they have to be cleaned in a more complete way, with several treatments. This is to ensure that all the pollutants and/or toxic compounds are removed from them, and that their use poses no health threat.
In particular, the waste waters have to undergo to processes to remove nutrients, such as phosphorus and nitrogen. In fact, according to the United States Environmental Protection Agency, very high water concentrations of these elements can be dangerous for the ecosystem; it can, for instance, cause an excessive growth of plants and algae (eutrophication). Furthermore, as reported by Bódalo et al., nitrogen in the form of the ammonium ion (NH4+) is harmful and can cause disease.
Nitrogen removal process and nitrous oxide formation
The removal of nitrogen from waste waters is a two-stage process. In the first stage, the ammonium ion (NH4+) is converted to the nitrate ion (NO3–); this part is called nitrification. In the second stage, the nitrate ion is converted into molecular nitrogen (N2); this is named denitrification.
During both these processes, there can be a release of nitrous oxide (N2O). N2O is a gas with very high greenhouse gas properties; its Global Warming Potential (GWP) is 298 times higher than that of carbon dioxide.
If the nitrogen removal process is applied at industrial scale for waste waters treatment, relatively high quantities of N2O could be formed and, consequently, released into the atmosphere.
Monitoring the nitrous oxide emissions
Recently a systematic analysis of the emissions of nitrous oxide for different waste water treatments was reported by professor Amy Townsend-Small and her coworkers. This work was a joint collaboration between the University of Cincinnati and the University of California, Irvine.
They monitored the nitrous oxide emissions from plants with or without the nutrient removal. In the first case (with the removal), the effluent could be used as reclaimed waters; in the second case, on the contrary, waters could not be reclaimed but were discharged into the open sea.
Their results showed that, when the nitrogen removal was performed, the atmospheric nitrous oxide emissions were much higher – more than three times higher than without nitrogen removal. This is the downside of reclaiming the waste waters.
Reclaimed waters may still be good for the environment
Professor Townsend-Small tells Decoded Science: “Despite our findings, I still think that we should continue to reuse wastewaters as reclaimed waters. In a place like southern California, drinking waters are imported over a very long distance; this results in large energy consumption, and therefore a high emission of carbon dioxide. If drinking water resources could be supplemented with recycled water, the process of water recycling can still help reducing the overall emissions of greenhouse gases.”
There may be other cases where this is not true, where the high emissions of N2O are not “balanced” by the non-emissions of other greenhouse gases. This study shows how water reclamation is a complex topic, where many issues have to be considered, and every situation has to be evaluated separately.
These results of Professor Townsend-Small and her coworkers were published in the Journal of Environmental Quality.
The Environmental Protection Agency. The problems with water nutrients. Accessed September 20, 2011.
Bódalo A., Gómez J.L., Gómez G., León G., Tejera M.: “Ammonium removal from aqueous solutions by reverse osmosis using cellulose acetate membranes.” Accessed September 20, 2011.
Desalinization, 184, 149 (2005).
Townsend-Small A., Pataki D.E., Tseng L.Y., Tsai C.Y, Rosso D.: “Nitrous Oxide Emissions from Wastewater Treatment and Water Reclamation Plants in Southern California.” Journal of Environmental Quality, 40, 1542 (2011).
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