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The commercial success of hydrothermal carbonization (HTC) is contingent on seeking solutions for the downstream wastewater (process water) generated during the process which is still regarded largely as a nuisance. In the present study, the reusability and valorization strategy of process wastewater generated during co-HTC of organic fraction of municipal solid waste (food and yard waste) at 220 °C for 1 h was established. The process wastewater was anaerobically digested in the first part to determine its methane-generating capacity; and in the second part, it was recirculated up to five times to understand the evolution of physicochemical and fuel characteristics of hydrochar. The process water was characterized by the presence of high total organic carbon (up to 40 g/L) and chemical oxygen demand (up to 96 g/L). The decreasing trend of heavy metals with increasing recirculation suggested possible adsorption/immobilization mechanism taking place to the hydrochar surface. The process water generated from co-HTC condition has anaerobic biodegradability of 72% while experimental and theoretical methane yield observed were 224 mL/g COD and 308 mL/g COD, respectively. The presence of high organic and ionic species in recirculated process water promoted the overall carbonization process which was evident from the increased energy yield (86 to 92%), carbon content (68 to 71%) and calorific value (20 to 27 MJ/kg). The recirculation also enhanced overall combustion characteristics of hydrochar as analyzed by thermogravimetric analysis. The recirculation strategy enhanced fuel properties of hydrochar while making sure upstream and downstream water related burden is reduced (as illustrated by life cycle analysis) to create a cleaner production system for renewable solid biofuels production.

Yard waste is either dumped or is being openly burned to get rid of it, instead of using it as a valuable renewable energy source. In this study, hydrothermal carbonization of yard waste was conducted to valorize it as a solid bio fuel, using a batch reactor. The effect of process parameter on yield, energy and physicochemical properties of the valorized solid bio fuel (hydrochar) was examined in this study by varying reaction temperature (160–200 °C for 2 h) and reaction time (2–24 h at 200 °C). The calorific value of hydrochar was within a range of 17.72–24.59 MJ/kg as compared to 15.37 MJ/kg for untreated yard waste. Hydrochar mass yield decreased from 78.6% at operating temperature – time of 160 °C –2 h to 45.6% at 200 °C –24 h. The plot of atomic ratios (H/C and O/C) demonstrates improvement in the coalification process which was mainly governed by decarboxylation and dehydration reactions. The grindability of the prepared hydrochar was comparable to that of coal. Hydrochar produced at lower reaction condition (160–200 °C at 2 h) have better flowability as compared to that produced at higher reaction condition (4–24 h at 200 °C). The reaction time longer than 12 h has a minimal effect on the yield, energy and physicochemical properties of hydrochar. Increasing reaction time and temperature improved the ignition and burnt temperature of hydrochar. All reaction condition has an energy ratio (energy output to energy input) of more than one making HTC process a net energy producer.

In recent times, the mutation of complex municipal solid waste (MSW) biodegradable portion to an energy recovery resource became an extraordinary innovation as it conserves our precious natural resources and maintains the ecological balance. The various thermo-chemical and biological treatment approaches also succeeded in synthesizing efficient energy recovery resources like solid biofuel (char) and biogas. Despite, the process instability due to precursor heterogeneity, environmentally non-benign, and non-viable processes have shown their inefficiency in on-site executions (Liu and Balasubramanian 2012). Nevertheless, on the other side intercontinental MSW generation is almost 1.3 × 103 million tons and reaches 2.2 × 103 million tons by 2025 (Xu 2013). Therefore, a promising futuristic viable and sustainable treatment technology for producing active energy recovery resource is the need of the hour. Likewise, hydrothermal carbonization (HTC) has recently gained well-built recognition as key enabling mechanization for converting the MSW biodegradable portion to desirable commodities (Wang et al. 2018; Zhao et al. 2014). The HTC process operates under lower temperatures, making use of distilled/de-ionized water as a catalyst or medium within a residence time of minutes to several hours (Ramke et al. 2009). The process finally yields into an energy-dense product named “hydrochar”, can be the replacement by lignite/bituminous coal. The usage of HTC as a waste conversion technology can notably minimize the greenhouse gas emissions (GHG’s) via compost centres and municipal solid waste (MSW) landfill sites (Berge et al. 2013). Also, the synthesized energy-dense hydrochar can be practiced as an economical replacement to different environmental applications to deal with the imminent universal energy shortage and environmental challenges (Wang et al. 2020). However, HTC is manifesting as a robust technology, along with the process advantages and hydrochars wide variety applications in environmental and energy sectors, the process disadvantages are also discussed in this chapter for its better practical scale execution.

Hydrothermal carbonization (HTC) of yard waste (YW) and food waste (FW) was performed in landfill leachate (LL) to overcome the unnecessary exploitation of our limited natural resources. The physicochemical properties and combustion behavior of the resulting hydrochars were compared with those obtained using distilled water (DW) as reaction medium. Although performing HTC in LL led to lower hydrochar mass yields (43% YWH and 36% FWH) than DW (47.1% YWH and 41.5% FWH), it had minimal impact on the fuel characteristics of the hydrochars. Notably, the higher heating value of the hydrochars prepared in LL (22.8 MJ kg−1 for YWH and 30.2 MJ kg−1 for FWH) is comparable to that of conventional solid fuels, and may, therefore, be considered as inexpensive alternatives to fossil fuels. Overall, the results of this study conclusively suggest that the use of LL as an alternative moisture source can significantly improve the sustainability of HTC technology.

In this study, Co-HTC of food waste with yard waste was conducted for biofuel pellets production, and also to understand any possible synergy between two feedstock types. The calorific value of blended raw feedstock was 13.5 MJ/kg which increased to 27.6 MJ/kg after Co-HTC at 220 °C for 1 h. Energy yield and fuel ratio calculated was 45% and 0.65 respectively. Hydrochar produced demonstrated a stable combustion profile as compared to reactive combustion profile for raw samples. The blend of food and yard waste hydrochar was easily pelletized, and its pellets showed improvement in mechanical properties as compared to pellets made from mono-substrate((food waste) hydrochar. Pellets produced from the blend of food and yard waste hydrochar showed higher energy (46.4 MJ/m3) and mass density (1679 kg/m3) as compare to the pellet produced from food waste hydrochar alone. Tensile strength obtained for the blended hydrochar pellet was 2.64 MPa while same for the pellets produced from food waste hydrochar alone was 1.30 MPa. In addition to improving hydrophobicity, soften lignin from yard waste also helped in binding the food waste hydrochar particles together within the pellets matrix during heated pelletization. The results presented in the study indicated that in the presence of all favorable conditions, there is a potential that approximately 11% of the global coal consumption could be replaced by the combustion of hydrochar produced from food and yard waste globally.

The major challenges associated with the anaerobic digestion of urban lignocellulosic biomass are its recalcitrance nature and the disposal of resulted digestate. In this study yard waste was thermally pretreated to overcome inherent recalcitrant nature during anaerobic digestion. The resulted digestate after anaerobic digestion, which poses a serious land disposal problem was hydrothermally treated to produce energy-rich solid biofuel knows as hydrochar. After pretreatment the biogas production was improved from 328 to 364 mL/g VS. Then hydrochar prepared from the digestate at a temperature of 180 and 200 °C for a treatment duration of 6 h. The produced hydrochar had a calorific value in the range of 20–23.5 MJ/kg as compared to 15 MJ/kg for raw digestate. The present research on integration of anaerobic digestion and hydrothermal carbonization not only improved the bioenergy production but also minimized the waste production.

The aim of this study was to investigate the anaerobic granular sludge (AGS) as feedstock for hydrochar production using the hydrothermal carbonization (HTC) process. HTC experiments were conducted at 250 °C for 2 h. The hydrochar produced was then co-pelletized with yard waste to produce densified biofuel pellets. The hydrochar yield was around 80%, while the hydrochar energy yield was around 93%. The calorific value of hydrochar increased to approximately 19 MJ/kg as compared to 16 MJ/kg for the raw AGS. HTC improved the combustion behavior of hydrochar by improving ignition and burnout temperatures. Hydrochar pellets were able to withstand the compressive force of 3.10 MPa as compared to 2.44 MPa by raw AGS pellets thus proving its enhanced robustness. This study demonstrated that combined HTC and pelletization could be integrated with the wastewater treatment plant for solid biofuel production from AGS, providing a valuable renewable energy source.

Developing a treatment technology which minimizes the production of by-product (waste) is need of an hour. In this study, municipal yard waste (primary raw material) was microwave-pretreated before anaerobic digestion (AD) to improve biogas production. The anaerobically digested, Pretreated Yard Waste (PTY) and the Untreated Yard Waste (UTY) (waste/secondary raw material) was Hydrothermally Carbonized as a downstream treatment technique to produce energy rich hydrochar. The Hydrothermal carbonization (HTC) was conducted at a temperature of 180 °C and 200 °C for 6 h to produce carbon-rich hydrochar. Physicochemical, structural and combustion properties of PTY and UTY hydrochar were characterized and compared using a range of techniques to gain detailed insight into individual hydrochar samples. Microwave pretreatment of yard waste enhanced the biogas production from 264 ± 11 mL/g VS to 370 ± 14 mL/g VS. The carbon content and higher heating value of digestate increased considerably from 44 and 44.35% to 53–56% and15–16 MJ/kg to 21–23 MJ/kg, respectively after HTC. Thermal gravimetric analysis of the prepared hydrochar showed that the high-temperature carbonization increased the combustion properties of hydrochar. The hydrochar prepared from PTY showed enhanced physicochemical, structural and combustion properties as compared to hydrochar prepared from UTY. The finding asserted that the pretreatment of yard waste before AD not only improved biogas production but also improved yield with better quality hydrochar when its resulting digestate was hydrothermally carbonized. AD of yard waste yields biogas and HTC of the resulting digestate yields hydrochar; both are biofuel, hence, augmenting HTC as a downstream treatment process along with AD would result in the creation of near-zero loss process.

The energy demand of the world is expected to reach 739 quadrillions BTU in 2040, which therefore demand for exploring more alternative source of renewable energy. Waste biomass though vast in reserve for generating renewable energy has its own downside. High moisture, fibrous nature, high bulk volume, hydrophilic nature and low calorific value are some of the inferior quality of waste biomass which creates bottleneck for easy renewable energy generation. Pre-treatment of biomass to overcome these challenges has created a new research interest. Among the treatment options available, the hydrothermal carbonization (HTC) method, which can process wet waste has become the most preferred choice among researchers recently. The HTC eliminates energy-intensive pre-drying process needed for other treatment methods such as pyrolysis, dry torrefaction and incineration. Through this article, we attempt to provide a detailed review of how renewable biomass can be effectively used to produce renewable energy by improving their inherent inferior characteristics. The review also highlights bottlenecks that constrain the deployment of renewable energy using HTC methods. The scope of further research direction is well identified in this review. The paper also present recent advancements which are filling the knowledge gap of HTC technology that were there earlier. Critical analysis of microwave assisted HTC and conventional heated HTC is also presented in this review. The analysis in this paper reveals that biomass is a valuable resource, and should be explored to take advantage of its renewable energy generation potential. The HTC method of biomass upgradation improves transport, storage and fuel characteristics by improving grindability, pellets durability, hydrophobicity, energy density, combustion behaviour and calorific value, and also helps in improving the environmental performance of solid fuel produced. Despite the fact that the technology is in the early stage of development and there still exist knowledge gap and shortcomings, the vast literature reviewed suggests that it has a potential of being future technology. Therefore, it needs further investigation which should fill existing shortcoming of the technology.