Co-pyrolysis technology for enhancing the functionality of sewage sludge biochar and immobilizing heavy metals
Graphical abstract
Introduction
Urban sewage treatment is becoming more and more necessary due to the acceleration of global urbanization and the ongoing enhancement of human needs for environmental preservation. Production of sewage sludge (SS) is also increased by the quick expansion of sewage treatment facilities. Up to now, the SS produced by urban sewage treatment has exceeded 60 million m3 year−1 in China (calculated by the water content of 80%) (Liu et al., 2021d). SS disposal appears to be receiving insufficient attention even if sewage disposal technology has advanced greatly in recent years. Nearly 50% of the world's SS is not treated harmlessly (Gao et al., 2020). The environment and human life will be significantly threatened if the SS is not properly disposed of. The high production scale, high hazard and potential resource properties of SS have prompted people to seek new and alternative treatment methods (Gao et al., 2020).
The traits of SS define its risk, and correspondingly those traits also determine the treatment ideas and approaches. SS is composed of inorganic particles, organic matter, and water. It has a complex composition with a high water content (Gao et al., 2020; Liew et al., 2021). The inorganic components of SS mainly include inorganic minerals and inorganic nutrients such as N, P, and K (Liew et al., 2021). About 60% of the dry weight is made up of organic ingredients, which include lipids, proteins, polysaccharides, humic compounds, nucleic acids, etc. (Chen et al., 2018; Jiang et al., 2021; Liew et al., 2021). SS also contains a variety of poisonous and dangerous pollutants, such as pathogenic bacteria, Escherichia coli, and refractory organic pollutants. Numerous types of heavy metals (HMs) are detected in SS (Gao et al., 2020). They include Zn, Cu, As, Cr, Cd, Ni, Hg and Pb. SS thus possesses the dual characteristics of pollution source and resource. To prevent secondary pollution caused by the pollutants mentioned above, it is vital to find a sustainable SS treatment strategy. Based on the characteristics of the SS, the three primary goals of the current SS disposal procedure are the detoxification, reduction and recycling (Fig. 1) (Yuan and Dai, 2017; Liang et al., 2020; Fang et al., 2021; Liu et al., 2021b; Wang et al., 2021d; Yang et al., 2021b). Currently, the main treatment techniques of SS worldwide include sanitary landfills, land application, building materials utilization, and thermochemical treatment (Fig. 1) (Gao et al., 2020). The typical thermochemical technologies include pyrolysis, gasification and combustion (Jiang et al., 2021). The advantages, drawbacks and cost of each technology are detailed in Table 1.
The current immature technology makes the SS treatment costly and low efficient. The entire cost of various SS disposal technology is made up of construction investment cost and operating cost. The overall cost of SS thermal drying and incineration accounts for the majority, while the total cost of aerobic fermentation and landfill is relatively low. However, aerobic fermentation or landfill disposal techniques is not a long-term solution when time costs, site constraints and environmental benefits are taken into account (Gao et al., 2020; Shahbeig and Nosrati, 2020; Han et al., 2021). A policy constraint on landfill technology, for instance, results from a lack of available land resources; further risk assessments must be carried out to ensure low environmental hazards before the treated SS is ready for agricultural or land uses. Labor, manufacturing, and management expenses will all rise as a result of this. Therefore, people are still highly interested in developing novel alternative routes for SS treatment due to socioeconomic and environmental reasons. The precondition for adopting thermochemical technology for resource utilization in SS is the abundance of organic matter. Pyrolysis has been an extensively studied technology for energy or materials recovery from SS (Gao et al., 2020; Jiang et al., 2021). In particular, thermodynamics in an oxygen-free environment can modify the original molecular structure of SS, allowing it to be converted into various phase hydrocarbons or biochar (Fonts et al., 2012; Singh et al., 2020). Pathogenic microorganisms and organic pollutants can be destroyed and converted at high temperatures or under high pressure, whereas HMs can be stabilized (Liew et al., 2021). The recycling of SS resources can also be accomplished concurrently in the form of synthetic bio-oil, syngas, and biochar. Pyrolysis requires complex equipment or processes, yet it may outperform other treatment methods in terms of efficiency, economic performance, and volume reduction (Bora et al., 2020; Shahbeig and Nosrati, 2020). In general, pyrolysis technology may achieve the objectives of resource use, reduction, and detoxification all at once. It has significant practical implications and a wide range of development opportunities in energy storage, new material development and emergency treatment.
However, SS is not entirely suited for pyrolysis due to high moisture content, high ash concentration, and abundance of HMs (Gao et al., 2020; Haghighat et al., 2020). The drying process must be included because the moisture level in SS must be less than 15% prior to pyrolysis (Zhang et al., 2018; Wu et al., 2021). Synthetic SS-based biochar (SSB) usually contains a high ash content, resulting in a low carbon content and calorific value. (Wang et al., 2021b, 2021d). Besides, the pore structure of SSB produced by SS pyrolysis alone is not ideal, and the types and numbers of surface functional groups are few (Han et al., 2022). All of these limit the functionality and utilization range of SSB. Co-pyrolysis with additives is a process that is interesting and useful for lowering environmental risks while also enhancing the quality and quantity of SS products (Djandja et al., 2020; Gao et al., 2020; Jiang et al., 2021). The reported types of additives for co-pyrolysis include biomass (Urych and Smoliński, 2021; Zhao et al., 2021a), organic chemicals (Hu et al., 2019; Zaker et al., 2021), inorganic minerals (Liu et al., 2020; Xia et al., 2020) and wastes (Fan et al., 2021a; Wang et al., 2021c) (Fig. 2). The thermodynamics/kinetics and the quality of products are typically affected by the additives (Fan et al., 2022). For instance, co-pyrolysis can boost bio-oil's calorific value and reduce the release of oxygenated molecules, water, and other contaminants from the bio-oil (Fonts et al., 2012; Qiu et al., 2020). Significantly, co-pyrolysis can alter the SSB's chemical and physical characteristics (Gao et al., 2020; Tu et al., 2021). According to various additives, SSB's carbon, HMs, and nutritional contents can be modified (Fig. 2). Biomass additives (mainly agricultural and forestry biomass) have advantages in increasing carbon content, enriching pore structure, and reducing HMs concentration and bioavailability (Urych and Smoliński, 2021; Zhao et al., 2021a). Chemical additives help form specific functional groups that improve particular functionalities (Fan et al., 2021a; Wang et al., 2021c). Alkaline inorganic salts show excellent ability to enrich pore and increase specific surface area (Wang et al., 2021e). The effect of waste depends on its complex composition. Organic solid waste exhibits similar effects to biomass, while inorganic solid waste resembles inorganic minerals. In addition, additives tend to dilute the concentration of nutrients provided by SS in SSB (Wang et al., 2021d; Xiong et al., 2021).
Co-pyrolysis SSB has shown greater application potential in soil improvement (Yang et al., 2021a; Yin et al., 2021), pollutant adsorption (Liu et al., 2020; Mian et al., 2020), catalytic reactions (Yuan and Dai, 2016; Wilk et al., 2021) and other fields (Fig. 2). Through improved adsorption, complexation, and the formation of eutectic compounds, co-pyrolysis can also accomplish immobilization and stabilization of HMs (Gao et al., 2020; Tong et al., 2021; Li et al., 2022a) (Fig. 2). Since 2018, more than 50 excellent reviews have summarized the SS thermochemical conversion process and highlighted the benefits of SS pyrolysis (Fonts et al., 2012; Liu et al., 2019; Gao et al., 2020; Schnell et al., 2020; Jiang et al., 2021). For instance, Gao et al. (2020) give a thorough overview of SS thermochemical conversion from the standpoint of energy and resource recovery. Discussion topics include SS composition and typical disposal technology, thermochemical routes, drying technology, co-feed and catalytic processes, reaction kinetics, reactor technology, etc. This paper emphasizes the importance of co-processing and catalyst design to improve the selectivity of pyrolysis or gasification products. The pre-treatment, activation, and modification technologies of SSB are introduced in certain reviews (Xiao et al., 2022; Liu et al., 2021a). Some reviews concentrate on the kinetic mechanisms and fuel production (Liu et al., 2021d; Shahbeig and Nosrati, 2020) as well as the HMs migratory characteristics (Li et al., 2022a; Liew et al., 2021; Chanaka Udayanga et al., 2018).
Unfortunately, there is no specific review yet to analyze the catalysis and synergy effects of additives on the biochar functionality and HMs immobilization from the perspective of co-pyrolysis. Through straightforward raw material blending, the co-pyrolysis can enhance the quality of pyrolysis products and reinforce the immobilization of HMs. Co-pyrolysis may therefore be the simplest and most practical technology for widespread industrial applications. However, the mechanism of co-pyrolysis is complicated. Complex synergistic interactions between various components and the catalytic effects of additives all contribute to the pyrolysis process. The clarification of the mechanism is also the crucial basis for industrialization. Therefore, it is crucial and urgent to summarize the state of SS co-pyrolysis research at this point. We examined the benefits and drawbacks of several SS disposal techniques in this review, and we focused on the advantages of the SS co-pyrolysis technology in terms of SS detoxification, reduction, and resource utilization. For the first time, the research progress of co-pyrolysis in enhancing the functionality of SS biochar (in the fields of land use, pollutant adsorption and catalysis) and immobilizing HMs is reviewed in detail. The catalytic or synergistic effects of typical additives during co-pyrolysis and their impact on biochar properties and HM behavior are summarized. Limitation and challenges in SS co-pyrolysis are also discussed. Finally, future research prospects are proposed in detail from seven perspectives as pyrolysis process optimization, co-pyrolysis additive selection, catalytic mechanism research of process and product, biochar performance improvement and application field expansion, cooperative immobilization of HMs, and life cycle assessment. Hopefully, this review can help researchers rapidly understand the research status and provide insightful recommendations and direction for further research.
Section snippets
Co-pyrolysis of SS to produce high-value biochar
Biochar, one of the primary products of SS co-pyrolysis, has developed pore structure, sizable specific surface area, high N and P contents, and rich surface functional groups (Rangabhashiyam et al., 2022; Xiao et al., 2022). The combination of SS and additives also exhibits the catalytic and synergistic effect (Hakeem et al., 2022). Co-pyrolysis SSB has more significant application potential than SSB in terms of enhancing soil nutrients, adsorbing multiple pollutants, and catalyzing diverse
HMs behavior in the co-pyrolysis process
HMs in sewage mainly come from industrial wastewater. Consumables such as food, medicine, and cosmetics, as well as surface runoff, also introduce a small amount of HMs into sewage. The vast majority of HMs are enriched in SS through the treatment process of sewage treatment plants. SS contains a variety of HMs, the most common including Zn, Cu, Cr, Ni, Pb and Cd, which tend to have high concentrations and huge potential environmental risks (Wang et al., 2021c). The HMs in the treated SS are
Challenges of SS thermochemical disposal
Although SS pyrolysis technology can stabilize HMs, destroy harmful pathogens, and reduce SS volume, it still faces challenges.
- (1)
SS has high moisture content and must be dehydrated before pyrolysis. The dehydration process consumes much energy and takes a long time. The cost disadvantage of dehydration conditioner is significant;
- (2)
Only a few numbers of catalysts and reactors are utilized for SS pyrolysis because of the complicated compositional features of SS. High sludge ash content may also lead
Future research prospects of SS co-pyrolysis
To cope with the above challenges, we summarize the previous research results combined with our own research experience and put forward the following prospects.
- (1)
The pretreatment process of SS, the design of the pyrolysis process and the optimal design of pollutant emission control system are equally important in future research. Firstly, it is necessary to innovate and develop new pyrolysis reactors, continuously optimize reaction parameters, and strengthen heat and mass transfer efficiency. The
Conclusions
This review introduces the advantages of SS pyrolysis technology in terms of detoxification, reduction and resource utilization. The co-pyrolysis technology, which can significantly improve the quality of pyrolysis products and reinforce the immobilization of HMs, is now the most straightforward and practical technology available. It is also the most promising technology to be industrialized first. The review then introduces the research progress of co-pyrolysis in enhancing the functionality
Author contributions statement
Zeyu Fan: Conceptualization, Data curation, Formal analysis, Investigation, Writing–original draft, Supervision, Funding acquisition. Xian Zhou: Data curation, Formal analysis, Investigation, Funding acquisition. Ziling Peng: Data curation, Formal analysis. Sha Wan: Data curation, Formal analysis, Investigation. Zhuo Fan Gao: Methodology, Formal analysis, Investigation. Shanshan Deng: Methodology, Formal analysis, Investigation. Luling Tong: Writing–original draft, Conceptualization, Resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the support from the National Natural Science Foundation of China (No. 52200143, 51979011), the Central Non-Profit Scientific Research Fund for Institutes (No. CKSF2021483/CL, CKSF2021437/CL and CKSF2021430/CL) and the Jiangxi Academy of Water Science and Engineering Open Project Fund (No. 2021SKSG04).
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