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Do Laboratory Scale Experiments Improve Constructed Wetland Treatment Technology? Jan Vymazal* Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 169, 165 21 Praha 6, Czech Republic
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need to be mentioned. The laboratory experiments are usually carried out with artificial wastewater, which is not comparable with real wastewater. In artificial wastewaters, only one organic compound is generally used, and this compound is usually easily degradable (e.g., glucose or acetic acid). Under these conditions, the bacterial assemblage developing in the system is entirely different as compared to those developing in fullscale CWs treating real wastewaters. The laboratory experiments are very short (several months or even several weeks), and plants are not given time enough to fully develop; therefore, the role of plants during the short wastewater treatment experiments is distorted. On one side, short-term experiments overestimate uptake of nutrients, as plants are only in the early growth stage when uptake is much higher as compared to later growth stage; on the other hand, it does not allow for full development of roots, which play an important role as substrate for bacterial attachment and sites for oxygen release to the rhizosphere. In addition, in small area, the plants’ growth pattern is different as compared to that in large areas. Some experiments are carried out in microcosms with surface area less than 0.05 m2, so it is more than obvious that the results from such systems cannot be transferred to full-scale constructed wetlands with surface area of several hectares. The short duration of the experiments does not allow for long-term performance evaluation. It is also obvious that laboratory experiments cannot be used as a basis for guidelines for constructed wetlands design, construction, operation, or maintenance. It is no surprise that such guidelines have been produced in countries where the research has been carried out mostly in full-scale CWs or pilotscale CWs such as Denmark,1 Austria,2 Germany,3 and New Zealand.4 Laboratory experiments are necessary to obtain basic information on the treatment processes and removal mechanisms for certain compounds; however, unless the results are confirmed in the pilot-scale systems and then in fullscale systems, the laboratory results “vanish in the haze” and do not contribute to the improvement of CWs treatment technology.
uring the 1990s, most papers dealing with constructed wetlands (CWs) for wastewater treatment were based on results from the full-scale systems. However, since then, the constructed wetland research has shifted very much to laboratories and greenhouses using artificial wastewater. As indicated in Figure 1, while in 1995, 88% of the papers
Figure 1. Distribution of research papers (reviews not included) dealing with constructed wetlands for wastewater treatment recorded in Web of Science in 1995, 2005, 2015, and 2017.
recorded in the Web of Science database were based on fullscale CWs, in 2015 and 2017, it was only 39 and 26%, respectively. On the other hand, in 1995, only one paper dealing with CWs was based on laboratory study; in 2015 and 2017, it was 30 and 49%, respectively. The question arises of why such shift has occurred. The possible answers are summarized briefly: Researchers and especially doctoral students need to publish quickly and as much as possible. The laboratory experiments allow for detailed monitoring (laboratory is usually “next door”, field systems are usually not near the university or other research institutions) and the conditions are steady (e.g., flow, wastewater inflow composition, temperature) and, therefore, much easier for statistical analyses as compared to wide variation in these parameters in the field. The number of samples is much higher as compared to field studies, creating the image of very intense evaluation. The experiments are usually very short, sometimes only several weeks, because the results “must” be published as soon as possible. In general, the results of the laboratory experiments are seldom used in the design of full-scale CWs. The question is whether the results of laboratory experiments can actually be used for field full-scale systems. There are several points which © XXXX American Chemical Society
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jan Vymazal: 0000-0002-8113-4670 Received: October 10, 2018
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DOI: 10.1021/acs.est.8b05709 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Notes
The author declares no competing financial interest.
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REFERENCES
(1) Brix, H.; Johansen, N. H. Retningslinier for etablering af beplantede filteranlæg op til 30 PE (Guidelines for vertical flow constructed wetland systems up to 30 PE). Økologisk byfornyelse og spildevandsrensning Nr. 52; Miljøstyrelsen, Miljøministeriet (in Danish), 2004. (2) DWA-A-262E. Principles for Dimensioning, Construction and Operation of Wastewater Treatment Plants with Planted and Unplanted Filters for Treatment of Domestic and Municipal Wastewater. German Association for water, Wastewater and Waste: Hennet, Germany, 2017. (3) Ö NORM B 2505. Wastewater treatment plants − Intermittently loaded effluent filtration systems (Constructed Wetlands) − Application, dimensioning, installation, operation, service and inspection. Austrian Standards Institute: Vienna, Austria, 2009. (4) Tanner, C. C.; Sukias, J. P. S.; Yates, C. R. New Zealand guidelines: Constructed Wetland Treatment of Tile Drainage. NIWA Information Series No. 75; National Institute of Water and Atmospheric Research Ltd., 2010.
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DOI: 10.1021/acs.est.8b05709 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
