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Incorporating Biochar into Wastewater Eco-treatment Systems: Popularity, Reality, and Complexity Shubiao Wu*,† and Haiming Wu*,‡ †
Aarhus Institute of Advanced Studies, Aarhus University, Høegh-Guldbergs Gade 6B, DK-8000 Aarhus C, Denmark College of Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, PR China that can be utilized for wastewater treatment. The results of recent studies have illustrated various benefits of biochar addition to wastewater eco-treatment systems, from enhancing the removal of diverse pollutants (e.g., nutrients, heavy metals, emerging organic pollutants, and fecal bacteria) to facilitating the enrichment of functional microorganisms and promoting microbial activity. However, most of the results are only tested under short-term laboratory scale experimental conditions and are not yet applied in full-scale systems. Biochar may not be as universally beneficial as often assumed. The outcomes of biochar application still vary widely depending on experimental conditions. Therefore, rather than thinking of biochar as an omnipotent enhancer in eco-treatment systems, it is imperative to focus attention to this intensification strategy and urge a rethink on its future research development.
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‡
1. ENHANCE ADSORPTIVE REMOVAL OF CONTAMINANTS Incorporating biochar into wastewater eco-treatment systems, for example, CWs, is a direct and easy-operative mean to improve contaminant removal via adsorption. However, this adsorptive characteristic may hardly differentiate biochar from other effective adsorbents. The effectiveness of biochar as an adsorbent is highly dependent on parameters affecting its manufacture, for example, feedstock type and pyrolysis conditions. Moreover, the use of negatively charged biochars mainly benefits adsorptive removal of more cations than anions. Thus, to meet different requirements (e.g., biochar lifespan lengthening and anion control improvement), the possibility of using various surface modifications of raw biochar has been attracting research attention.3 Although extensive research has been done on modified biochars, the cost of these modifications is still the key to applying and promoting these engineered biochars in eco-treatment systems, which are particularly characterized by low-cost construction and operation. Besides, the effect of biochar addition on limiting fecal bacteria growth has also been documented to associate with its adsorptive characteristics, however, the underlying mechanism is still not clear.
Biochar has grown in popularity in recent years because of its ability to significantly contribute to climate change mitigation over the long-term.1 Besides, due to its excellent structural properties, biochar has been employed to improve nutrient and water retention in soil, and thus increase fertilization efficiency and crop yield. These overwhelming statements may lead readers to believe that the initiative to use biochar is solely directed toward agribusiness applications. However, this is not the case. In 2009, biochar was applied to wastewater treatment for the first time as a potentially low-cost alternative adsorbent. Since then, the application of biochar has expanded rapidly to various wastewater treatment systems and attracted increasing attention in this field. As wastewater treatment plants are undergoing a transition from being centralized and energyintensive to becoming decentralized facilities, eco-treatment systems such as constructed wetlands (CWs) and soil infiltration systems are gaining popularity.2 However, the vast areas of land occupation of these eco-treatment systems have been recognized as a critical factor limiting their widespread application. Thus, exploring intensification strategies to upgrade the pollutant-removal performance of these systems to a higher level comparable to those of centralized activated sludge systems, while also maintaining their eco-characteristics and economic advantages, has driven the new focus on the integration of biochar into these treatment systems. Owing to its highly porous structure, large surface area, and high cation exchange capacity, biochar exhibits a high adsorptive capacity © XXXX American Chemical Society
2. STIMULATE BIOGEOCHEMICAL CYCLE OF NITROGEN A growing number of studies are demonstrating higher total nitrogen removal in wastewater eco-treatment systems when biochar is used as an amendment for denitrification. The documented mechanisms underlying these observations Received: February 21, 2019
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DOI: 10.1021/acs.est.9b01101 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
Figure 1. Concept of integrating biochar in wastewater treatment eco-systems on promoting circular economy and sustainability.
sustainability of these ecosystems, further scrutiny considering underlying mechanisms and long-term economic analyses of mature cases are still necessary. While bringing multiple benefits, biochar applications may not solve all problems and substantial uncertainties still exist. Therefore, we need to focus more on exploring biochar interaction mechanisms and tailoring its applications to different field circumstances.
include enhanced denitrifier abundance and carbon source provision.4 Generally, biochar structure is dominated by a core of aromatic carbon rings that are highly stable and resistant to microbial decomposition. Even though some recent studies have found that the employed biochar releases humic acid-like substances, which can act as a bioavailable carbon source and associate with denitrifying bacteria. These humic acid-like substances are more derived from incompletely pyrolyzed biochar at low temperatures. Under such conditions, the porous structure and adsorptive capacity of biochar decreases significantly compared to that of biochar produced via hightemperature pyrolysis. Therefore, the questions on how to balance the demand between organic substance release and effective adsorption still exist. Besides, the high electron exchange capacity and electrical conductivity of biochar play important roles in the electron transfer process. The integration of biochar in CWs may stimulate denitrification and reduce nitrous oxide (N2O) accumulation simultaneously by enabling the effective transfer of electrons between nitrate and organic contaminants.5 Biochar-induced changes can affect soil biological and physiochemical processes driving nutrient cycling and N2O emission. However, the mechanism of N2O flux suppression related to different biochar characteristics is still not sufficiently understood.
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AUTHOR INFORMATION
Corresponding Authors
*(S.W.) E-mail:
[email protected]. *(H.W.) E-mail:
[email protected]. ORCID
Shubiao Wu: 0000-0003-1203-0680 Haiming Wu: 0000-0002-6618-3175 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Woolf, D.; Amonette, J. E.; Street-Perrott, F. A.; Lehmann, J.; Joseph, S. Sustainable biochar to mitigate global climate change. Nat. Commun. 2010, 1, 56. (2) Wu, S.; Lyu, T.; Zhao, Y.; Vymazal, J.; Arias, C. A.; Brix, H. Rethinking Intensification of Constructed Wetlands as a Green EcoTechnology for Wastewater Treatment. Environ. Sci. Technol. 2018, 52 (4), 1693−1694. (3) Wang, B.; Gao, B.; Fang, J. Recent advances in engineered biochar productions and applications. Crit. Rev. Environ. Sci. Technol. 2017, 47 (22), 2158−2207. (4) Zhou, X.; Wang, R.; Liu, H.; Wu, S.; Wu, H. Nitrogen removal responses to biochar addition in intermittent-aerated subsurface flow constructed wetland microcosms: Enhancing role and mechanism. Ecol. Eng. 2019, 128, 57−65. (5) Liu, X.; Qu, J.; Li, L.; Zhang, A.; Zheng, J.; Zheng, J.; Pan, G. Can biochar amendment be an ecological engineering technology to depress N2O emission in rice paddies?A cross site field experiment from South China. Ecol. Eng. 2012, 42, 168−173.
3. PROMOTE CIRCULAR ECONOMY AND OPERATIONAL SUSTAINABILITY The development of wastewater eco-treatment systems, for example, CWs, is always facing some historical challenges including that of low purification performance, and those with respect to the post-treatment of saturated media, economic benefit driven operation, and so on. Incorporating biochar into wastewater eco-treatment systems can help to handle these challenges and promote their sustainability. Wetland plants are valuable feedstocks for pyrolysis, producing not only biochar but also bio-oil and syngas to offset wetland operating costs (Figure 1). The integration of biochar into CWs would enhance pollutant removal and reduce greenhouse gas emission. In addition, adsorption-saturated biochar can be recycled as nutrient loaded soil amendment. However, in closing this nutrient loop, the possibility of metals and antibiotics accumulation in the exhausted biochar should be examined. Finally, although the concept of integrating biochar into wastewater eco-treatment systems appears, at first glance, to be a “green” solution to promoting circular economy and the B
DOI: 10.1021/acs.est.9b01101 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
