Lokesh P. Padhye
Erick R. Bandala
Buddhi Wijesiri
Ashantha Goonetilleke
Nanthi Bolan- 1Department of Civil and Environmental Engineering, The University of Auckland, Auckland, New Zealand
- 2Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, NV, United States
- 3School of Civil and Environmental Engineering, Queensland University of Technology (QUT), Brisbane, QLD, Australia
- 4School of Agriculture and Environment, The University of Western Australia, Perth, WA, Australia
- 5The UWA Institute of Agriculture, The University of Western Australia, Perth, WA, Australia
The United Nations 17 Sustainable Development Goals (SDGs) are a universal call to action to end poverty, protect the environment, and improve the lives and prospects of everyone on this planet. However, progress on SDGs is currently lagging behind its 2030 target. The availability of water of adequate quality and quantity is considered as one of the most significant challenges in reaching that target. The concept of the ‘Circular Economy’ has been termed as a potential solution to fasten the rate of progress in achieving SDGs. One of the promising engineering solutions with applications in water treatment and promoting the concept of the circular economy is hydrochar. Compared to biochar, hydrochar research is still in its infancy in terms of optimization of production processes, custom design for specific applications, and knowledge of its water treatment potential. In this context, this paper critically reviews the role of hydrochar in contributing to achieving the SDGs and promoting a circular economy through water treatment and incorporating a waste-to-value approach. Additionally, key knowledge gaps in the production and utilization of engineered hydrochar are identified, and possible strategies are suggested to further enhance its water remediation potential and circular economy in the context of better natural resource management using hydrochar. Research on converting different waste biomass to valuable hydrochar based products need further development and optimization of parameters to fulfil its potential. Critical knowledge gaps also exist in the area of utilizing hydrochar for large-scale drinking water treatment to address SDG-6.
Introduction
The United Nations 17 Sustainable Development Goals (SDGs) are intended to ensure peace and prosperity for people and safeguard the natural environment from human-induced pollution and climate change (UNDESA, 2018). SDGs progress, however, is currently lagging behind its stipulated 2030 target because of continued environmental degradation, increasing social problems such as poverty, inequality and inequity, and financial and economic instability (Jackson, 2009; Banerjee et al., 2011; Sachs, 2015). While all the proposed SDGs are equally important and of high priority, the first and central SDG of “No Poverty” is strongly linked to water (Merrey, 2015). Water permeates all aspects of our daily lives as the availability of water of adequate quality and quantity is not only essential for the survival of humans but also for maintaining healthy ecosystems, including agroecosystems, which in turn translates to community wellbeing. Good water quality can promote healthy lives, while degraded water quality will adversely impact human wellbeing and economic development. In addition, multiple interdependencies involving water as a key natural resource, such as water-food-energy nexus, highlights its role in the sustainable development framework. Hence, availability of clean water has been termed as a “critical entry point” to achieve sustainable development and nearly all of the proposed SDGs (Merrey, 2015).
In recent years, the concept of the “Circular Economy” has gained increasing attention with the hope of overcoming some of the intractable sustainability challenges (Ghisellini et al., 2016). The classical linear economic system utilizes natural resources and non-renewable energy to produce goods and services, which eventually generate mostly non-biodegradable waste. On the other hand, the circular economy is based on the concepts of conservation, reuse, and recycling to produce goods and services, as illustrated in Figure 1. More formally, “the circular economy” is defined by Geissdoerfer et al. (2017) as “a regenerative system in which resource input and waste, emission, and energy leakage are minimized by slowing, closing, and narrowing material and energy loops”. In this context, a circular economy is relevant to all sectors of the economy, including energy, materials, industry, health care, financial and information technology, and can be a driver for accelerating progress in achieving SDGs (UNGA, 2019).
FIGURE 1. Illustration of the concepts of linear and circular economies.
One of the more promising solutions that have emerged in recent decades to close the materials and energy loops mentioned above is the conversion of waste biomass to biofuels. It can be achieved either through thermochemical or biochemical conversion processes. Out of these two alternatives, thermochemical processes are being widely explored due to inefficiencies and practical difficulties associated with biochemical processes (Tripathi et al., 2016). Pyrolysis is the most commonly used thermochemical process and is carried out at temperatures higher than 300°C and in the absence of oxygen. The solid output of this process is called ‘biochar’, which has been extensively discussed in the literature for its environmental, agricultural, and material applications (Bolan et al., 2021). Due to the higher energy input required for pyrolysis and expensive pretreatment needs, hydrothermal carbonization (HTC) has replaced pyrolysis as the preferred thermochemical process for some waste biomass, especially those with high moisture content. HTC is carried out at relatively lower temperatures, typically from 80 to 240°C, under subcritical water pressure (Nadarajah et al., 2021). The solid output of this process is called “hydrochar”.
Hydrochar is gaining popularity as an environmentally and economically sustainable material with multiple applications, including as an effective adsorbent to remediate polluted water. Hydrochar can be produced using a range of widely available biomass, including agricultural waste, sewage sludge, manure, and microalgae through HTC, which ensures sustainability in raw materials sourcing, energy consumption, and waste management (Nadarajah et al., 2021). However, compared to biochar, hydrochar is still in the early stages in terms of optimization of production processes, custom design for specific applications, and knowledge of pollutant removal processes. In this context, this paper examines the role of hydrochar in contributing to achieving the relevant SDGs and promoting a circular economy through water quality remediation and incorporating a waste-to-value approach. Additionally, some of the knowledge gaps in the production and utilization of engineered hydrochar are identified, and possible strategies are suggested to further enhance the circular economy in the context of better natural resource management using hydrochar.
Hydrochar and Water Quality
Only one out of the 17 UN SDGs focuses explicitly on water, “SDG 6—clean water and sanitation for all”. However, water is also an enabler to achieving the other 16 SDGs. There is also growing recognition of the multiple interdependencies or “nexus” among the various SDGs, including water and energy, water and food, water and environment, or water and economy. Hence, addressing the growing water scarcity and quality challenges is critical to achieving UN SDGs by 2030 (Merrey, 2015). Unfortunately, the latest United Nations data on SDG 6 (UNDESA, 2020; UN_Water, 2020) indicate very concerning trends in the provision of safely managed drinking water resources and sanitation services across the world. The data show that the proportion of the urban population using safely managed drinking water services exhibits a slightly decreasing trend over time (86.2% in 2000; 85.9% in 2010; 85.2% in 2017). Essentially, safe drinking water provision is not meeting the growing demand of the ever-increasing urban population. Similarly, in rural areas, the availability of contaminant-free safe water has shown only a slight improvement. It remains behind target, with nearly half of the total population not having access to contaminant-free freshwater sources. These findings need to be viewed in the context that the urban population is increasing whilst the rural population is decreasing in all countries around the world. More strikingly, the proportion of the global population using drinking water free of contamination shows an increase of only four percentage points over the five years (2015-2020) since the adoption of SDGs. To meet the target set out for SDG 6 by 2030, the rate of progress in achieving quality clean water resources needs to be more than triple for the remaining years (Ritchie and Roser, 2021). Hence, developing sustainable and inexpensive water treatment solutions is the need of the hour.
It is where hydrochar, as an inexpensive material because of the abundance of waste biomass (e.g., agriculture, forestry, food, and wastewater treatment) and less energy-intensive HTC process, can play a key role. As a result of hydrolysis and biomass monomer reactions during HTC, hydrochar surface hosts various chemically reactive functional groups, including acidic surface functional groups, that can drive the adsorption of contaminants through electrostatic interactions (Sumaraj et al., 2020). However, HTC alone produces hydrochar with low surface area, and hence, it may need to be combined with additional activation steps to increase its porosity and surface area desired for certain environmental remediation purposes (Tasca et al., 2019). The physicochemical properties of contaminants and the surface chemistry of adsorbents control the chemisorption of water contaminants (Sumaraj and Padhye, 2017). Typical functional groups on hydrochar can be optimized for increasing their adsorptive capacity or removing selective contaminants. It can be effectively achieved by altering physicochemical properties with additional activation steps (Peng et al., 2019; Teng et al., 2020; Çatlıoğlu et al., 2020; Mittapalli et al., 2021).
Although most studies have reported specific techniques for optimizing surface functionality of hydrochar for efficient adsorption of contaminants, a significant knowledge gap exists in relation to their real-world application. For example, recent studies by Zhang et al. (2022) and Lv et al. (2022) have reported methods to make hydrochar surfaces more positively- and negatively charged, respectively, for higher adsorption of anionic and cationic contaminants in wastewater. Both studies claim that their modified hydrochars could be used for wastewater treatment, but neither of these studies used actual wastewater in their batch experiments. The lack of consideration given to various parameters/factors, including complex aqueous matrices, continuous flow reactors, large-scale setups, competition by solutes for active sites in the presence of organic matter, and data related to regeneration efficiencies for real-world applications, make such claims questionable. Even if the surface modifications show promising results in the batch-scale studies, more in situ or field measurements should be conducted on a longer-term to examine the changes in surface chemistry and adsorption performance of chemically modified hydrochars in complex environments.
For the removal of uncharged or nonpolar contaminants, hydrochars need morphological and/or chemical modifications to achieve efficient adsorption. Although hydrochars have typically low surface area, with the help of chemical activation, surface areas higher than 1,000 m2/g, similar to activated carbons, have been reported (Zhu et al., 2015; Zhang et al., 2019). The activation of hydrochar by KOH, NaOH, K2CO3, H3PO4, ZnCl2 and other acids, bases, salts, and oxidizing agents have been reported in the literature to increase their surface areas significantly (Zhu et al., 2015). Lower peak temperatures and lower residence times of the HTC process have been reported to enhance surface area; thus, effective activation achieved at these conditions can result in hydrochars with higher surface areas and surface reactivity (Fang et al., 2015; Zhu et al., 2015). However, most of the reported research on activation of hydrochar has solely focussed on surface area enhancement, while little or no information is available regarding the cost and energy input for such activation at large scale. Weight ratios of activating agents:hydrochar have been reported as high as 1:1 (Zhu et al., 2015; Zhang et al., 2019), which can make their large scale applications economically unfeasible. Hence, there exists a significant research gap on cost-benefit analysis and scaling-up of activation processes.
The presence of contaminants, including potentially toxic elements in the feedstock, is another concern in relation to hydrochar application for water remediation. For example, wastewater sludge may contain toxicants and, when used as a feedstock, can lead to the leaching of these contaminants from sludge-derived hydrochars during their applications to treat water. In such cases, specific inexpensive pre- or post- HTC treatments can significantly lower the concentrations of residual contaminants (Hoang et al., 2022). However, ecotoxicological risks posed by the use of such hydrochars are rarely investigated. Consequently, in-depth studies are needed to understand the toxicological risks of using sludge-derived hydrochars for extended periods of water treatment.
As shown in Table 1, most of the research on hydrochar for water remediation has focused on removing dyes, heavy metals, and pharmaceuticals. There is a need for more studies on understanding the interactions of hydrochars with pollutants commonly found in urban and/or agricultural runoffs, wastewaters, and surface waters such as inorganic nitrogen contaminants, phosphorus, and emerging contaminants. Especially with the increasing number of studies reporting the presence of poly- and perfluoroalkyl substances (Lenka et al., 2022), personal care products (Kumar et al., 2019a), illicit drugs (Kumar et al., 2019b), and disinfection by-products, such as N-nitrosodimethylamine (Jasemizad et al., 2021), in urban waters, it is critical that the hydrochar’s applicability for water treatment should consider its performance against modern-day contaminants.
TABLE 1. Selected studies on aqueous contaminant removal using hydrochar.
Hydrochar and Circular Economy
Agricultural residues are increasingly being used as a sustainable source of energy in rural areas (Mau and Gross, 2018; Huang et al., 2019). As such, hydrochar production using agricultural residues in rural areas would provide additional economic benefits by integrating renewable energy, water treatment, and waste management. Similarly, sewage sludge and food waste are increasingly generated in urban areas due to the increasing population, where their use for hydrochar production can yield multiple benefits, as described above (Tasca et al., 2019). In fact, urban-produced wastes are often found to cause air, water, and soil pollution while, at the same time, cities account for 75% of global greenhouse gas emissions (Van Hullebusch et al., 2021). Hence, using these wastes for generating value-added products for various sustainable applications is a prime example of a circular economy in action. Hydrochar production is a promising pathway to support the transition to a circular economy by adding innovation to most basic services, such as water and wastewater services, waste materials management, energy production, construction materials etc., in big cities and small towns (Zvimba et al., 2021). Table 2 lists examples of different hydrochar applications to achieve a circular economy and contribute towards achieving SDGs.
TABLE 2. Hydrochar production and its applications in line with principles of circular economy.
As shown in Table 2, despite the numerous applications of hydrochar, the available information on its use to reduce adverse environmental impacts of cities or generate sustainable construction practices using local materials is very limited.
For example, selected studies on converting sewage sludge and municipal solid waste to hydrochar are included in Table 2, where the use of different additives was tested to increase the heating value of produced hydrochar or machine learning tools were explored to predict hydrochar properties. However, no information is available for scale-up operation of hydrochar production and its evaluation under field conditions. The production of hydrochar from either municipal solid waste or sewage sludge is a topic receiving significant attention in recent years (Sharma and Dubey, 2020; Huezo et al., 2021; Liu et al., 2021). However, it is worth noting that the technology readiness level is very low for all cases. This is a significant knowledge gap as there is a need for technologies that incorporate circular economy principles to establish sustainable communities. The lack of information on further technology demonstration, system development and launch and operation may delay implementation or even hinder knowledge of their technologic and economic feasibility, which in turn could result in failure of the timely achievement of SDGs goals.
Using hydrochar from solid waste for the improvement of asphalt surfaces for road construction has attracted attention recently because the application of solid wastes to road infrastructure through its conversion to hydrochar provides a practical and economical solution to the construction and maintenance of roads, especially in rural areas with a ready supply of feedstocks available for large-scale hydrochar synthesis. Augmented rheological properties of hydrochar-modified asphalt have been reported, which has also been suggested as an environmentally friendly asphalt modifier (Qin et al., 2021). Other studies, however, have suggested that hydrochar has adverse impacts on road surfaces as it increases bitumen viscosity, thus creating difficulties with workability and poor storage stability (Wu and Hu, 2021). These conflicting findings present an interesting avenue for further research. Assessing whether hydrochar performance as an asphalt modifier depends on its chemical and/or physical characteristics is fundamental to better understanding its potential use. Finding engineering solutions to overcome drawbacks on the use of hydrochar from solid waste as an asphalt modifier can open the door for circular economy solutions for sustainable solid waste management.
The application of hydrochar for energy-related applications such as electricity generation or energy storage is also gaining traction. For example, the use of hydrochar made of kitchen waste has been reported with a high potential for energy storage (Zhou et al., 2021). Researchers have found that hydrochar, as a supercapacitor, surpassed the performance of commercial activated carbon, reaching specific energy values 6.66 and 8.52 Wh/kg in acidic and neutral electrolytes, respectively (Lang et al., 2021). The effect of HTC variables on the performance of the hydrochar produced, though, remains poorly understood and has been identified as an important knowledge gap that merits further exploration to optimize the process and produce the best materials for energy storage.
Similarly, generating hydrochar from spent algal materials after biofuel extraction is another promising application of circular economy. For example, using such hydrochar (5 wt%) has been reported to have more than double co-hydrothermal gasification yield (Sztancs et al., 2020). Several challenges, however, remain to be answered, including problems associated with processing aqueous phase discharges with a diluted load of by-products (e.g., microalgal slurry) (Khoo et al., 2020). The impact of the solid content of spent algal waste on hydrochar production yield is a significant knowledge gap worthy of further exploration. Addressing the problem of using dilute microalgal slurry (e.g., 1 wt% biomass) for hydrothermal treatment will allow recovery of valuable materials in addition to biofuels production from biorefinery processes.
Conclusion
As evident from the above discussion, hydrochar production and application is a promising pathway towards a circular economy and achieving UN SDGs through water treatment and waste management. However, significant research gaps exist and will need to be addressed as a matter of priority due to the SDGs timeframe. It is important to note that the current research tends to focus on the application of hydrochar for treating non-potable water such as stormwater and wastewater output from other treatment technologies (e.g., industrial treatment plants, bioretention systems and wetlands) (Cui et al., 2020; Merzari et al., 2020; Wang et al., 2021). Therefore, critical research gaps exist in the area of utilizing hydrochar for large-scale drinking water treatment to address SDG-6 directly. Similarly, although several laboratory-scale studies (Table 2) have shown the promise of converting different waste biomass to valuable hydrochar based products, such technologies are in their infancy and need further development and optimization of parameters to enable making a significant contribution to improve human wellbeing and contribute to achieving the UN SDGs. Therefore, although promising in principle, hydrochar research needs the immediate attention of the research community, industry, and policymakers to make a lasting and positive impact on the environment in the current SDG cycle and beyond.
Author Contributions
LP—Conceptualisation, Literature Review, Coordination, Writing the Original Draft, Revising. EB—Conceptualisation, Literature Review, Writing the Original Draft, Revising. BW—Literature Review, Writing the Original Draft AG—Conceptualisation, Coordination, Review. NB—Review and Proofread.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s Note
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Keywords: hydrochar, water quality, sustainability, sustainable development goals, circular economy
Citation: Padhye LP, Bandala ER, Wijesiri B, Goonetilleke A and Bolan N (2022) Hydrochar: A Promising Step Towards Achieving a Circular Economy and Sustainable Development Goals. Front. Chem. Eng. 4:867228. doi: 10.3389/fceng.2022.867228
Received: 31 January 2022; Accepted: 07 March 2022;
Published: 07 April 2022.
Edited by:
Kevin M. Van Geem, Ghent University, BelgiumReviewed by:
Chao He, Tampere University, FinlandCopyright © 2022 Padhye, Bandala, Wijesiri, Goonetilleke and Bolan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Lokesh P. Padhye, bC5wYWRoeWVAYXVja2xhbmQuYWMubno=