Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Highlights

• First comprehensive review about municipal sludge-derived hydrochar.

• Formation pathways from municipal sludge to hydrochar are summarized.

• Unique properties of sludge-derived hydrochar are systematically encapsulated.

• The current state of hydrochar valorization is comprehensively evaluated.

• Challenges and prospects for the valorizing approaches of hydrochar are identified.

Abstract

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

Introduction

Due to increasing population and rapid urbanization, a significant amount of wastewater solids, often called municipal sludge (MS), are generated at municipal wastewater treatment plants (WWTPs) worldwide (Kor-Bicakci and Eskicioglu, 2019). In China, the annual MS production doubled within five years and reached 5.7 million dry tonnes in 2013 (Yang et al., 2015). In the United States (US), the MS generation rate is nearly 12.7 million dry tonnes per year (2018) (Marrone et al., 2018). The world total production rate of MS was recorded at 45 million dry tonnes per year in 2017 (Gao et al., 2020). Originating from households, food-processing, agricultural, and industrial wastewater and associated biological treatment, MS including primary sludge (PS), secondary sludge (SS), and digested sludge (DS) is a reservoir of organic materials, nitrogen (N), phosphorus (P), and other inorganic nutrients (Zhai et al., 2014a). MS has been identified as a complementary P sink in regions with limited phosphate rock resources (Shi et al., 2019). However, MS, especially non-stabilized sludge, may also contain various hazardous materials, including pathogens (Lopes et al., 2020), organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) (Brookman et al., 2018), inorganic pollutants (e.g., heavy metals) (Chen et al., 2020d), and emerging contaminants or micropollutants (e.g., antibiotics, hormones, pharmaceuticals, and personal care products) (Taboada-Santos et al., 2019). Considering the magnitude of sludge production and its potential nutrients and hazards, the search for alternative treatment options has been stimulated for decades.

Proper management of MS through conventional disposal methods, such as landfilling, composting, land application, and incineration, requires significant expenditures (Xu et al., 2018a). It has been reported that managing MS could cost as much as 57% of the total operation cost in a WWTP (Ma et al., 2018). Even though anaerobic digestion (AD) treatment has been applied to break down the organic matters in MS and generate biogas (mainly methane), a considerable amount of sludge (approximately 40–50% of the input) remains and requires appropriate disposal (Zhen et al., 2017). The long solids retention time (SRT) (typically 15 – 20 days) requirement further limits the efficiency of conventional AD treatment. Given the increasing sludge amount, conventional treatment methods for MS may not be sustainable in the future.

The high moisture content in sludge (almost 98 wt%) causes the biggest handling challenges: The massive volume and consequent high cost of treatment. An emerging technique, hydrothermal processing (HTP), also called hydrothermal conversion, is promising to address these challenges efficiently and economically (Huang et al., 2019). HTP can treat waste with high moisture content through a thermochemical process. This ability is a significant advantage compared to other techniques that require dry feedstocks (e.g., incineration and pyrolysis). It has been reported that the drying cost of MS is the majority energy input of pyrolysis treatment, occupying 65–75% of the total inputs (Kim and Parker, 2008). Conversely, without drying requirement, HTP could substantially reduce the energy input. HTP utilizes hot pressurized water as a reaction medium to break down large complex organic matters or macromolecules into smaller and simpler units at elevated temperature and pressure (Mathimani and Mallick, 2019). The reaction rate especially raises when the treatment conditions reach the critical point of water (374.3°C and 22.1 MPa) (He et al., 2014a). Thus, HTP can efficiently decompose organic matters and reduce the volume of residual solids. The dewaterability of MS is also significantly enhanced after HTP treatment, even at low temperatures (e.g., 180°C). Wang et al. (2014) reported that the moisture content of excess sludge was reduced to 27% after HTP at 180°C for 1 h followed by mechanical dewatering, while over 65% of moisture was retained when only mechanical dewatering technologies were used. Ahmed et al. (2021) found that after HTP of DS at 190°C for 1 h, the capillary suction time decreased by 91%. Moreover, the greatest benefit of HTP is its use to produce renewable biofuels (e.g., coal-like char, biocrude, and syngas) from sludge (Moreno and Espada, 2019). In summary, HTP is used to simultaneously recover energy, promote organic pollutants decomposition, enhance dewaterability and eliminate pathogens (via high-temperature sterilization).

In recent decades, many researchers have focused on sludge valorization using HTP treatment (Merzari et al., 2019). HTP for sludge-to-energy conversion is classified into three main categories based on the treatment temperature, pressure, and featured fuel products: Hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG) (as shown in Table 1). Other hydrothermal technologies, such as thermal hydrolysis, wet oxidation, and supercritical water oxidation, do not aim for energy recovery and thus are not considered in this review. In this review, the carbonaceous char produced in the form of solid residue from HTP processes is defined as hydrochar. As can be seen in Table 1, different fuel products or coproducts (i.e., hydrochar, biocrude, syngas, and aqueous phase) are generated in all hydrothermal conditions; however, their yields vary.

Fig. 1 presents the normal distribution of product yields based on data gathered from numerous HTP studies. As the treatment severity intensifies in the order of HTC, HTL, and HTG, there is a noticeable trend of decreasing yield of solid and aqueous phase products, while the gas yield increases. The biocrude yield tends to be maximized through the HTL of MS. It is noted that a considerable amount of hydrochar remains as a product or coproduct despite the hydrothermal conditions. As the figure shows, average yields of hydrochar are 60.2%, 44.7%, and 20.5%, dry basis (db), from HTC, HTL, and HTG treatment of MS, respectively (Fig. 1). However, it should be noted that the yield of hydrochar varies substantially depending on the reaction severity and processing conditions. For example, in a continuous plug-flow HTL system (276–358°C for 18–30 min), the hydrochar yield from PS, SS, and DS were 9.5, 20.5, and 36.4%, db, respectively (Marrone et al., 2018).

The reduced mass percent of hydrochar causes a high concentration of nutrients (particularly P) and contaminants, such as heavy metals, PAHs, and PCBs (Chanaka Udayanga et al., 2018). Yu et al. (2019) reported that almost all P (> 90%) remained in the hydrochar after HTC treatment of PS. However, due to the accumulation of toxic contaminants (e.g., Cr and Ni), direct recycling of hydrochar as P fertilizer is restricted by many jurisdictions (Chanaka Udayanga et al., 2018). Direct burning of HTC hydrochar for heat generation would cause a wide distribution of P-rich ash and potential secondary pollution (Oliver-Tomas et al., 2019). Therefore, enrichment of the nutrients and pollutants in hydrochar seems to be a key bottleneck for HTP application to MS. Several studies for P recovery from MS-derived hydrochar and risk assessment of contaminants have been conducted in the past ten years (Li et al., 2012; Liu et al., 2018b; Ovsyannikova et al., 2019; vom Eyser et al., 2015). To date, few reviews have detailed the characteristics of MS-derived hydrochar produced under various HTP conditions or mentioned its application opportunities. Therefore, it is necessary to critically and comprehensively investigate current information and expose knowledge gaps of MS-derived hydrochar for its sustainable management.

This review aims to show current knowledge of the properties of MS and its corresponding hydrochar from HTP and to present and evaluate sustainable application processes for MS-derived hydrochar. Recent studies of HTP technique are application-oriented and often issued as technical reports. Consequently, this review involves an extensive search of all related peer-reviewed journal articles, conference proceedings, theses, books, as well as technical reports from bibliometric databases (e.g., Google Scholar, Science Direct, Scopus, and Web of Science). The following keywords have been searched in different combinations: Hydrochar, sludge, municipal sludge, sewage sludge, hydrothermal, hydrothermal conversion, HTC, HTL, HTG, HTP, and supercritical water gasification. The search period is concentrated in the last twenty years because more attention has been paid to HTP treatment of MS since 2000. After rejecting articles referring to industrial sludge or irrelevant hydrothermal processes (e.g., thermal hydrolysis and oxidation) based on title, abstract, and scanning, the remaining articles (in total 319 references) were analyzed thoroughly. Based on collected information from all selected papers, this review is divided into the following sections. Firstly, the formation of hydrochar from MS is comprehensively presented from the perspectives of mechanisms and hydrothermal conditions. Secondly, unique characteristics of MS-derived hydrochar and the associated contaminants of concern are summarized to guide the application of hydrochar. Thirdly, this review evaluates current utilizations of MS-derived hydrochar for sustainable management. Lastly, special attention is given to the feasibilities and challenges of each technology for hydrochar valorization.