Arias, CA

Brix, H

Lyu, T

Vymazal, J

Wu, S

Zhao, Y

Shubiao Wu

Tao Lyu

Yaqian Zhao

Jan Vymazal

Carlos A. Arias

Hans Brix

Crossref

Environmental Science & Technology

Rethinking Intensification of Constructed Wetlands as a Green Eco-Technology for Wastewater Treatment

Nottingham Trent Institutional Repository (IRep)

1  Rethinking Intensification of Constructed Wetlands as a Green 1 Eco-technology for Wastewater Treatment 2 Shubiao Wu*†‡, Tao Lyu§, Yaqian Zhao#, Jan Vymazal⊥, Carlos A. Arias∥, Hans Brix∥ 3 †College of Engineering, China Agricultural University, Beijing 100083, China 4 ‡Aarhus Institute of Advanced Studies, Aarhus University, Høegh-Guldbergs Gade 6B, DK-8000 Aarhus C, 5 Denmark 6 §School of Animal, Rural and Environmental Sciences, Nottingham Trent University, Nottinghamshire 7 NG25 0QF, UK 8 ⊥Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kymýcká 129, 165 21 Praha 9 6, Czech Republic 10 # Centre for Water Resources Research, School of Civil Engineering, University College Dublin, Belfield, 11 Dublin, 4, Ireland 12 ∥Department of Bioscience, Aarhus University, 8000 Aarhus C, Denmark 13 *E-mail: wushubiao@gmail.com 14 Constructed wetlands (CWs) comprise a suite of recognized eco-technologies 15 that are designed and constructed to mimic and manipulate the simultaneous physical, 16 chemical, and biological processes occurring in natural wetlands for wastewater 17 treatment purposes. Besides the water quality improvement function, CWs also 18 provide a multitude of other functions, such as biodiversity, habitat, climatic, 19 hydrological, and public use functions. Since the 1960s, the annual number of 20 publications in the Web of Science Core Collection in this field has increased 21 exponentially to >350 in 2017 (Figure 1). The most attractive benefit of this technology 22 is its “green” and ecofriendly way of treating wastewater, with low operational costs 23 and easy maintenance. Because of these eco-characteristics and economic advantages, 24 CWs have been well documented in guidelines as an alternative to conventional 25 centralized wastewater treatment systems.  26 Along with the growing attention to CW technology paid by both scientists and 27 engineers in the last five decades, knowledge gained from diverse fields of research has 28 gradually turned understanding on CW from a “black box” model to a “grey box” model. 29 However, the effort devoted to understanding the cycling of basic elements, such as 30 carbon, phosphorus, nitrogen, and sulphur, and their complex interactions in these 31 systems, seems to lag far behind the pace of engineering applications. This 32 understanding is needed particularly where CWs are trialed for the treatment of 33 various industrial effluents that have not been attempted before (1). In recent years 34 however, several intensifying strategies involving new system configurations, 35 sometimes even without plants, technical amendments, and operations with high 36 external energy use, are emerging in literature. Currently, the number of publications 37 reporting on various types of intensified CWs constitutes nearly a quarter of those 38 published in this field (Figure 1). Although it is questionable to define many of the 39 2  “intensified CWs”, these new types of CWs are driving the CW technology away from 40 its eco-characteristics and economic benefits. The CW intensification is responding to 41 new challenges where CWs passive systems present limitations and cannot produce 42 the demanded effluent quality. Some of the new intensifying techniques are, however, 43 only tested under laboratory scale conditions and are not as yet applied in full-scale 44 systems. 45 In our view, it is time to focus attention to this intensification strategy and urge a 46 rethink on the future developments of CW technology. Should we intensify these 47 systems using technical means to increase their pollutant removal performance and 48 capacity, but risk losing their ancillary benefits? This in turn raises the question to 49 rethink about how it would be wise to further intensify the performance of CW systems 50 but maintain their ancillary functions.  51 (1) Intensification reduces the footprint of CWs 52 A main benefit of CW intensification lies in the improved treatment capacity and 53 the concurrent lower footprint of the systems (2). However, in low-populated rural 54 areas and in many developing countries, land availability may not be such an issue. 55 Even though the intensification of CWs can improve treatment efficiency, it would 56 certainly increase the maintenance complexity and operational costs of the systems, 57 which could be a challenge in some regions with limited financial resources. Some 58 slightly intensified CWs, without too much process intensification and energy-demand, 59 might be feasible. Besides, the strong seasonal nature of pollutant loading and the need 60 to have a system with an established microbial community in a fixed bed may warrant 61 such ‘intensification’. However, the balance of its basic eco-characteristics, low 62 operational costs, and easy maintenance, along with treatment capacity, must remain 63 an important consideration. 64 (2) Intensification widens the application of CWs 65 The application of CWs has expanded quickly to treat various types of wastewater, 66 which may contain a range of toxicants, including high concentrations of salt, high acid 67 or alkaline compounds, and a diverse array of toxic chemical compounds. The long-68 term effects of these toxicants on wetland vegetation and associated microorganisms 69 have not been well studied. But these toxic compounds may be the reason for the 70 observed withering of plants and disruption of treatment performance in CWs handling 71 different types of industrial wastewater. Even though intensification may accelerate 72 the pollutant transformation processes by increasing the functional microbial activity, 73 and concurrently relieve the potential negative impact from specific toxic contaminants, 74 integration of other wastewater treatment technologies is recommended in the 75 treatment train, which cannot be replaced by intensified CWs (3).  76 (3) Intensification may make wetland vegetation redundant  77 Wetland vegetation is an indispensable component on CWs, since the plants 78 provide many significant functions in relation to the treatment processes as well as 79 3  ancillary functions, such as biodiversity and food chain support. However, 80 intensification strategies, such as forced pressurized aeration may make the role of 81 plants obviously become less or non-existent. Also, the forced aeration may impede 82 the growth of the plants (4). In such systems, from a treatment perspective, it might be 83 an advantage not to have vegetation growing in the beds, as the systems can then be 84 covered or even buried under the ground. The reason to maintain a vegetative cover in 85 these intensified systems seems to be that the green image of the CW-technology can 86 be used as a sales argument. 87 (4) Intensification may reduce microbial community diversity  88 Due to strict discharge limits for specific water quality parameters, intensification 89 of CWs often results in the stimulation of microbial communities processing the 90 targeted pollutants, e.g., organic matter or ammonium. The role of other microbial 91 communities may be weakened or lost under this treatment intensification. These 92 microbial communities may be important for degradation of emerging pollutants, such 93 as pharmaceuticals and personal care products. High microbial diversity is a critical 94 characteristic of an eco-treatment system, while this aspect may be undermined in 95 intensified CWs.  96 Therefore, it is necessary to urge engineers and scientists to keep in mind one of the 97 principles of ecological engineering in relation to constructed wetlands: “do not over-98 engineer the system, design it with nature, not against it” (5). Considering pollutant 99 nature, concentration, and seasonality, some intensifications are suitable; however, 100 operation and maintenance issues and the natural character of CWs must be the 101 priority. 102 Declaration  103 The authors declare no competing financial interest. 104 References 105 (1) Vymazal, J. Constructed wetlands for wastewater treatment: Five decades of experience. 106 Environmental Science & Technology. 2011, 45(1), 61-69. 107 (2) Wu, S. B., P. Kuschk, H. Brix, J. Vymazal and R. J. Dong. Development of constructed wetlands in 108 performance intensifications for wastewater treatment: A nitrogen and organic matter targeted 109 review. Water Research. 2014, 57, 40-55. 110 (3) Liu, R. B., Y. Q. Zhao, L. Doherty, Y. S. Hu and X. D. Hao. A review of incorporation of constructed 111 wetland with other treatment processes. Chemical Engineering Journal. 2015, 279: 220-230. 112 (4) Butterworth, E., A. Richards, M. Jones, H. Brix, G. Dotro and B. Jefferson. Impact of aeration on 113 macrophyte establishment in sub-surface constructed wetlands used for tertiary treatment of 114 sewage. Ecological Engineering. 2016, 91: 65-73. 115 (5) Mitsch, W. J. Landscape design and the role of created, restored, and natural riparian wetlands in 116 controlling nonpoint source pollution. Ecological Engineering. 1992, 1(1–2): 27-47. 117  118  119 

Rethinking intensification of constructed wetlands as a green eco-technology for wastewater treatment

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Rethinking intensification of constructed wetlands as a green eco-technology for wastewater treatment

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