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Article

Enhancing Wastewater Treatment Sustainability Through Integrated Anaerobic Digestion and Hydrothermal Carbonization: A Life-Cycle Perspective

by
Kayode J. Taiwo
1,†,
Andrada V. Oancea
2,†,
Nithya Sree Kotha
1 and
Joseph G. Usack
1,3,4,*
1
Department of Food Science and Technology, University of Georgia, 100 Cedar Street, Athens, GA 30602, USA
2
Environmental Biotechnology Group, Department of Geosciences, University of Tübingen, 94–96 Schnarrenbergstr, 72076 Tübingen, Germany
3
New Materials Institute, University of Georgia, 220 Riverbend Rd, Athens, GA 30602, USA
4
Institute for Integrative Agriculture, Office of Research, University of Georgia, 130 Coverdell Center, 500 D.W. Brooks Dr., Athens, GA 30602, USA
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2025, 17(16), 7545; https://doi.org/10.3390/su17167545
Submission received: 22 July 2025 / Revised: 13 August 2025 / Accepted: 15 August 2025 / Published: 21 August 2025
(This article belongs to the Section Sustainable Water Management)
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Abstract

Wastewater treatment plants (WWTPs) are critical infrastructure that lessen the environmental impacts of human activity by stabilizing wastewaters laden with organics, chemicals, and nutrients. WWTPs face an increasing global population, greater wastewater volumes, stricter environmental regulations, and additional societal pressures to implement more sustainable and energy-efficient waste management strategies. WWTPs are energy-intensive facilities that generate significant GHG emissions and involve high operational costs. Therefore, improving the process efficiency can lead to widespread environmental and economic benefits. One promising approach is to integrate anaerobic digestion (AD) with hydrothermal carbonization (HTC) to enhance sludge treatment, optimize energy recovery, create valuable bio-based materials, and minimize sludge disposal. This study employs an LCA to evaluate the environmental impact of coupling HTC with AD compared to conventional AD treatment. HTC degrades wastewater sludge in an aqueous medium, producing carbon-dense hydrochar while reducing sludge volumes. HTC also generates an aqueous byproduct containing >30% of the original carbon as simple organics. In this system model, the aqueous byproduct is returned to AD to generate additional biogas, which then provides heat and power for the WWTP and HTC process. The results indicate that the integrated AD + HTC system significantly reduces environmental emissions and sludge volumes, increases net energy recovery, and improves wastewater sludge valorization compared to conventional AD. This research highlights the potential of AD + HTC as a key circular bioeconomy strategy, offering an innovative and efficient solution for advancing the sustainability of WWTPs.

1. Introduction

With the global population projected to reach 8.5 billion by 2030 and 9.7 billion by 2050, the challenges associated with waste and sewage sludge management are estimated to increase exponentially [1]. This surge in population directly correlates with increased wastewater generation, placing an immense burden on wastewater treatment plants (WWTPs). Over the past decade, the volume of sewage sludge has steadily risen, leading to growing environmental and economic concerns regarding its disposal [1]. Managing the increasing volume of sludge has become a critical sustainability challenge in recent years, as disposal remains costly and environmentally detrimental. Among all the components removed during wastewater treatment, residual sludge accounts for the largest volume, and its improper disposal poses severe environmental hazards due to its high concentrations of organic matter, toxic compounds, and heavy metals [2]. Conventional sludge management methods, such as landfilling and incineration, are no longer sustainable due to stringent environmental regulations, health risks, high disposal costs, and depletion of landfill space [3,4,5]. As a result, there is a pressing need for innovative and sustainable sludge treatment technologies that not only reduce environmental impacts but also maximize energy recovery and resource utilization.
Anaerobic digestion (AD) has been widely used in wastewater treatment, providing a biological pathway for breaking down organic matter in sludge. This process produces (1) biogas, a versatile energy carrier, and (2) digestate, a nutrient-rich byproduct [6,7]. The open-culture microbiome that underpins the AD process serially degrades organic matter in the absence of oxygen through four trophic stages: (i) hydrolysis (depolymerization), wherein complex macromolecules, such as proteins, lipids, and polysaccharides, are broken down into their simpler monomeric forms (i.e., amino acids, fatty acids, and sugars, respectively), (ii) acidogenesis, wherein the monomers are further degraded into fermentation intermediates such as volatile fatty acids, alcohols, and gases such as CO2 and H2, (iii) acetogenesis, wherein the intermediates are converted into acetate, CO2, and H2, and finally, (iv) methanogenesis, in which acetoclastic and hydrogenotrophic methanogens convert acetate and H2/CO2 into CH4 and H2O. This stepwise trophic cascade underpins all bioenergy recovery in AD systems and is influenced by numerous operational parameters, including the substrate composition, pH, temperature, hydraulic retention time (HRT), and organic loading rate (OLR). A variety of reactor configurations can be applied for AD depending on the waste characteristics and treatment goals. The common AD reactor designs include continuously stirred tank reactors (CSTRs), upflow anaerobic sludge blanket reactors (UASBs), plug-flow reactors, and two-stage reactors that separate hydrolytic/acidogenic and methanogenic phases. AD reactors are typically operated under either a mesophilic (30–40 °C) or thermophilic (55–65 °C) temperature regime. AD systems vary widely in terms of their microbiome composition, resilience to inhibitory wastewater contaminants, biogas production, waste stabilization, and digestate quality, which have implications for downstream energy recovery/utilization and digestate applications [8].
However, despite its advantages, AD has inherent limitations, such as the large volume of residual sludge left behind after digestion, which still requires costly disposal [9]. Furthermore, the economic viability of AD is challenged by the low market price of biogas, reducing its profitability as a stand-alone solution [10]. To improve its efficiency and economic feasibility, the AD process can be integrated with other biological, electrochemical, physical, and thermochemical processes to generate higher-value products and maximize resource recovery [11,12]. One of the thermochemical processes includes hydrothermal carbonization (HTC), which occurs in a subcritical water environment (180–250 °C; 2–6 MPa) and facilitates the conversion of organic matter into hydrochar, process water, and gas [5,13]. Originally investigated by Friedrich Bergius in 1913 [14], HTC has recently gained attention as a viable sludge treatment option due to its ability to process high-moisture-content feedstocks without pre-drying, significantly reducing the energy consumption compared to dry pyrolysis or gasification [13]. Over the past few decades, it has been increasingly studied for its potential to convert residual sludge into valuable products [13]. Its application in WWTPs, particularly as a post-AD treatment before dewatering, reduces the sludge volume, thereby lowering the energy consumption and sludge transport costs [15]. Moreover, the carbon-dense hydrochar has a high calorific value, making it suitable for use as a heating fuel. Hydrochar also possesses similar physical and chemical properties to biochar, making it a suitable soil amendment [16]. The process water, on the other hand, has traditionally been considered a waste. Still, because a significant fraction of the biomass carbon is retained in the process water as dissolved organic compounds (i.e., up to 55%), current research efforts are focusing on its recovery and valorization through various means [6,16]. The AD treatment of process water, wherein dissolved organics are converted into biogas, has emerged as a potentially viable strategy. By integrating HTC and AD, it is also possible to establish a more energy-independent system, as the recovered biogas can be used to generate process heat for HTC, thereby offsetting the virgin energy inputs.
By moving toward a more circular and self-sustaining system, the integration of HTC with AD represents a paradigm shift in wastewater treatment. Several HTC projects have already demonstrated the feasibility of applying HTC technology at industrial scales, which would be necessary for implementing HTC in WWTPs. Companies such as SunCoal Industries (Ludwigsfelde, Germany), TerraNova Energy (Düsseldorf, Germany), AVA-CO2 (Zug, Switzerland), and Ingelia (Valencia, Spain) have implemented HTC for biomass and sludge treatment, proving its potential for large-scale application [17,18]. Despite these promising benefits, concerns exist regarding the fate of heavy metals and toxic contaminants in hydrochar and process water [19], as well as the potential leaching of hazardous elements into the environment, which must be carefully assessed before large-scale implementation can be realized [20]. Additionally, from a life-cycle assessment (LCA) perspective, integrating HTC in a WWTP may lead to increased greenhouse gas (GHG) emissions due to the energy-intensive nature of HTC, particularly when non-renewable energy sources are used. Indirect environmental burdens, such as emissions from process-derived contaminants and the transportation of hydrochar, must also be taken into account. These considerations underscore the necessity of comprehensive environmental impact assessments to fully understand the trade-offs associated with adopting HTC technology in wastewater treatment [21]. However, while studies have investigated AD + HTC for various biomass conversion applications, none have evaluated the life-cycle environmental impacts of applying HTC to treat post-AD solids, while recovering bioenergy via process water recycling and utilizing hydrochar as a soil amendment, in the context of a WWTP.
Therefore, this study aims to evaluate the environmental sustainability of integrating AD and HTC technology for wastewater treatment. The specific objectives are to (1) devise an optimal AD + HTC implementation strategy within the wastewater treatment process, (2) determine the net energy balance of a combined AD + HTC approach relative to an AD-only approach, and (3) compare the environmental impacts of the two approaches. This study employs LCA, a standardized methodology for assessing the environmental impacts of a product, process, or system across its entire life cycle. LCA has been widely used in wastewater treatment research since the late 1990s to quantify the resource efficiency, energy consumption, and environmental impacts [22,23]. By comparing conventional sludge management practices with an integrated AD + HTC system, this study will determine whether the combined process offers tangible environmental benefits over conventional AD treatment.

2. Materials and Methods

The LCA in this study was conducted in accordance with the ISO 14040 standards [24]. The study evaluated the environmental impacts of wastewater sludge treatment for two scenarios: (a) conventional wastewater treatment (WWT) using AD to treat mixed sludge (reference scenario) and (b) enhanced WWT integrating HTC with AD (proposed scenario). The LCA proceeded in four phases: (1) goal and scope definition, (2) life-cycle inventory (LCI), (3) life-cycle impact assessment, and (4) interpretation. The functional unit was defined as the treatment of 1 kg mixed sludge produced by a WWTP serving a capacity of 10,129 population equivalents (PEs). The geographic context for technology implementation was Stuttgart, Germany. The reference year for the analysis was 2024. The system boundary of the reference scenario included AD of mixed sludge, energy recovery from biogas via combined heat and power (CHP) production, sludge dewatering, final sludge transport, and disposal by landfilling (Figure 1). The proposed scenario encompassed all the unit processes in the reference scenario, as well as those required to integrate HTC. The additional unit processes included hydrochar production, heat recovery, HTC process water recycling, hydrochar drying, hydrochar transport, and hydrochar application as a soil-conditioning agent. The system boundary focused on the core sludge treatment processes that directly affect the comparison of the two technology scenarios. Upstream unit operations such as sludge screening and grit removal, primary and secondary clarification, and aerobic treatment are not directly affected by AD and HTC operation; therefore, they were not included in the system boundary. The decision was made to isolate and assess the environmental and energy performance of the AD + HTC integration itself. Including upstream processes would introduce shared infrastructure and operational steps that are common to both the reference and proposed scenarios, and therefore, will not affect the relative comparisons.
In the reference scenario (Figure 1A), the thickened mixed sludge, derived from the primary and secondary clarifiers, was delivered to a continuously stirred anaerobic digester (1st AD) operated at 37 °C with a hydraulic retention time of 26 days. The produced biogas entered a combined heat and power (CHP) system, generating electricity and thermal energy, assuming an electrical and thermal efficiency of 35% and 50%, respectively. The biogas composition consisted of 66% methane (CH4), 33% carbon dioxide (CO2), and 1% (v/v) of other constituents (e.g., NH3, H2S, VOCs). The heat captured by the CHP was used to heat the anaerobic digester, and the residual heat was assumed to be used for auxiliary WWTP operations [25]. Similarly, the produced electric power was allocated to meet the power demands of the 1st AD system, dewatering system, pumps, and other WWTP unit processes, with any surplus electricity fed into the grid. The displaced grid electricity was based on Germany’s national average energy mix. The remaining undigested sludge from the 1st AD system was directed to an anaerobic stabilization tank (2nd AD) to allow residual biogas release and sludge settling, as described by Uman et al. [26]. The supernatant fraction from the anaerobic stabilization tank was returned to the headworks, while the settled sludge fraction was sent to dewatering via centrifugation [27]. The resulting centrate was returned to the headworks, and the dewatered sludge was trucked 150 km to a sanitary landfill [28].
In the proposed AD + HTC system (Figure 1B), an HTC unit, operating at 200 °C and 40 bar pressure, and with a retention time of 3 h, was integrated to treat the settled sludge from the anaerobic stabilization tank [6]. The HTC process was powered using heat recovered from the CHP exhaust gas, and the heated fluid exiting the HTC unit was passed through a heat exchanger with the mixed sludge influent to sustain mesophilic temperatures in the 1st-stage AD. The hydrothermally treated process water, rich in organic compounds, was recycled into the 1st-stage AD to enhance biogas production. The HTC process was assumed to yield 50% (wt) of hydrochar from the sludge solid fraction [29,30]. The resulting carbon-densified hydrochar was subsequently separated and trucked 150 km for land application. It was assumed that hydrochar, when applied to land as a soil amendment, reduces the demand for commercial nitrogen fertilizers (i.e., ammonia nitrate) by 20% due to its nutrient-retaining properties [31]. The 20% reduction in commercial nitrogen fertilizer application was based on previous studies demonstrating hydrochar’s potential to enhance soil nitrogen availability and use efficiency. Hydrochar acts as a delayed-release nitrogen source, gradually mineralizing and providing sustained nitrogen to crops [32,33]. Additionally, hydrochar improves the nitrogen retention in soils by adsorbing ammonium and nitrate, reducing leaching losses, and stimulating microbial processes that promote nitrogen cycling [33]. These combined effects support a reasonable reduction in the synthetic fertilizer demand without compromising the crop yield.
The majority of the LCI data were derived from primary data from the Mühlhausen WWTP in Stuttgart, Germany. The remaining LCI data were obtained from the Ecoinvent v3.6 database, literature sources, and technical reports. The LCI data were compiled in Microsoft Excel with indexed referencing. The LCI accounted for the material and energy flows within the system boundary and key process assumptions. Assumptions were made where specific data were unavailable. A list of the key LCI data and major model assumptions is provided in the Supplementary Information [6,17,25,27,29,31,34,35,36,37,38,39,40,41]
The life-cycle impact assessment was conducted with SimaPro v9.0 software (PRé Sustainability, Amersfoort, the Netherlands) using the IMPACT2002+ v2.15 methodology for environmental impact characterization. The assessed midpoint impact categories included the global warming potential, aquatic and terrestrial eutrophication potentials, aquatic and terrestrial acidification potentials, ozone layer depletion, and human and ecosystem toxicity. The endpoint categories included human health, ecosystem quality, climate change, and resource depletion. Finally, a sensitivity analysis was conducted to evaluate the influence of key parameters of the AD + HTC system on climate change impacts, including the biogas CH4 content, CHP electrical efficiency, and CHP thermal efficiency. Each parameter was adjusted within a +/−10% range to evaluate the parameter sensitivity.

3. Results and Discussion

3.1. HTC Integration Reduced Dewatered Sludge Volume and Transport Burden with Minimal Change in Parasitic Energy Consumption

The integration of HTC with AD drastically reduced the mass of solids requiring disposal by effectively densifying half of the solids into hydrochar, while solubilizing ~45% into the process water, and gasifying the remaining 5% of solids into the off-gas. In the AD-only scenario, mechanical dewatering of settled sludge yielded 0.45 kg of dewatered sludge solids per kg of incoming mixed sludge solids, with approximately 89% of this fraction sent for landfilling (Figure 2A). In contrast, the post-AD settled sludge treated with HTC generated only 0.21 kg of hydrochar per kg mixed sludge solids, representing less than half the mass for export compared to dewatered sludge (Figure 2B). These mass fractionation results are consistent with previous studies, which indicate that hydrochar yields from digested sludge range from 40% to 73%, depending on the feedstock properties and process conditions [40,42,43]. The practical implications of these reductions are significant. HTC reduces the environmental and economic burdens associated with sludge dewatering, transportation, and landfill disposal, which not only lowers the environmental emissions and costs but also helps extend the lifespan of landfill facilities. Landfill space is a finite resource and should be reserved for materials that are fully unrecyclable.
AD + HTC also confers considerable energy efficiency and recovery benefits. By avoiding the enthalpic penalty of water vaporization (i.e., 0.627 kWh·kg−1), HTC processing requires less heat than other thermal processes, such as gasification or incineration. This feature is a key advantage. Many WWTPs are compelled to incinerate dewatered sludge rather than landfill it due to landfilling prohibitions (e.g., in certain EU countries) or limited land space (e.g., in urban areas or island localities). However, incineration is energy-intensive, costly, and requires strict pollution control measures to prevent the release of contaminants (e.g., SOx, NOx, heavy metals) into the atmosphere through the exhaust gas. HTC of dewatered sludge, therefore, presents a more sustainable option. Indeed, the energy analysis shows HTC integration results in only a slightly higher parasitic heat demand compared to the AD-only scenario (without incineration) because most of the HTC process heat required to raise the process water temperature from 37 °C to 200 °C (~10% of total) is subsequently recovered for AD substrate preheating. The majority of unrecovered heat (~3% of the total) was used to dry the hydrochar before transport (Figure 3A). Moreover, while this study assumed a mesophilic AD process (37 °C), the overall heat balance would be similar had the study assumed a thermophilic AD process (55–65 °C), except for slightly higher heat losses at the digester. Indeed, as discussed below, the AD treatment of HTC process water resulted in a significant surplus of bioenergy production, which could be applied to other WWTP operations. For example, the surplus heat from AD + HTC could be used to pasteurize AD co-substrates that require pathogen removal (e.g., slaughterhouse waste, food waste) in WWTPs that employ anaerobic co-digestion [44]. The surplus heat could also be used for district heating if the WWTP and district were reasonably co-located.
The AD + HTC scenario also reduced the overall parasitic electricity demand by replacing energy-intensive dewatering operations (44% of the parasitic electricity in the AD-only scenario) with less demanding HTC pumping and mixing operations (31% of the parasitic electricity in the AD + HTC scenario) (Figure 3A). At the same time, the additional biogas generated from the AD treatment of HTC process water led to surplus electricity production exceeding the parasitic losses (Figure 3C). This surplus electricity could be fed into the utility grid or used on-site for other WWTP operations that promote circularity. For example, the surplus electricity could be used to separate nutrients or heavy metals via electrodialysis from process effluents [45]. HTC process water, for instance, contains high levels of ammonia and orthophosphate, as well as various metal ions (e.g., Cr, Ni, Cu, Zn, Cd, Pb) [46], which would increase in concentration due to the recycling of HTC process water. Ammonia and metal ions can inhibit methanogenesis at elevated concentrations during the AD process [6,47]. However, the recycling ratio of process water (Rm) was calculated as 0.248, which suggests the accumulation of these components in the AD system would be marginal and not affect the water treatment effectiveness appreciably. Ammonia is also an essential nitrogen source for microbial growth. Therefore, additional ammonia would be needed to metabolize the organic carbon introduced by the process water. Moreover, most of these problematic components are cationic and could be separated (e.g., electrodialysis, precipitation) from the process water before AD, while leaving the non-ionic and anionic compounds, such as the reduced sugars, organic acids, and amino acids, intact for biomethanation. Sulphate supplementation has also been proposed to precipitate heavy metals during AD, thereby mitigating their inhibitory effects [47]. Sulphate is converted into H2S by sulphate-reducing bacteria, which then combines with the heavy metal cations. However, a drawback of this approach is further biogas H2S contamination.
The surplus electricity could also be used for water electrolysis (2H2O → 2H2 + 2O2, E0 = −1.23 V), creating pure streams of H2 and O2 gas. The H2 gas could be used for in situ or ex situ biogas upgrading to biomethane via hydrogenotrophic methanogenesis (4H2 + CO2 → CH4 + 2H2O, ΔG° = −32.7 kJ/mol) [11], while the O2 gas could be used in the aerobic treatment process to improve the oxygen transfer and uptake efficiency. Alternatively, H2O can be electro-synthesized into ozone (O3) for use in tertiary wastewater treatment applications [48]. Ozone is commonly used in WWTPs for disinfection, color and odor removal, and oxidation of recalcitrant organic pollutants. Fittingly, Yang et al. [49] demonstrated that ozone can be used to treat the recalcitrant compounds in hydrothermal process water. Ozone dosing up to 4.64 mg O3/mL fully degraded the phenols (100%) and partially degraded the N-heterocyclic compounds (21.7%) in the hydrothermal liquefaction process water. These examples demonstrate that AD + HTC makes WWTPs more versatile by recovering more energy from wastewater, which can be used subsequently for various applications.

3.2. AD + HTC Integration Promoted Energy Self-Sufficiency and Resource Recovery for Sludge Management in the WWTP

In conventional WWTPs, AD alone is typically insufficient to meet the total plant energy demand, supplying only 30–50% of the total energy requirements, depending on the plant scale and substrate characteristics [50]. This limitation is particularly important given that energy use accounts for 25–40% of the total WWTP operating costs [51,52]. Sludge dewatering alone represents approximately 7% of the plant’s energy load [53]. Indeed, sludge management contributes the largest share of both operational expenditures and GHG emissions across the treatment chain [54,55]. Here, implementing AD + HTC increased the CHP output by 69%, resulting in a net energy yield of 2.05 kWh·kgsludge−1 (3.58 kWh·mbiogas−3) in the AD + HTC scenario compared to 0.76 kWh·kgsludge−1 (2.46 kWh·mbiogas−3) in the AD-only scenario (Figure 3C), while maintaining similar parasitic energy consumption levels. While the net energy results do not include the energy demands of the unit operations upstream of the AD process (out-of-boundary) and may not meet the energy demands of the entire WWTP when included, they demonstrate that WWTPs employing AD + HTC are better positioned to achieve energy self-sufficiency compared to those that do not. Indeed, Ipiales et al. [56] reported that anywhere from 50 to 90% of the original feedstock energy can be recovered from the biogas and hydrochar in an integrated AD + HTC system, underscoring the energy efficiency of the process. AD alone is incapable of accessing the total amount of chemical energy contained in mixed sludge. A large fraction of the total volatile solids in the mixed sludge (i.e., 50–70%) is microbial biomass originating from the waste activated sludge process. The complex composition and structure of the microbial cell walls resist hydrolysis, which limits the biogas conversion rates and yields. The HTC process, on the other hand, is a brute force approach that readily hydrolyses this biomass, effectively serving as a pre-treatment for AD. The newly bioavailable chemical energy in the form of solubilized organics can then be converted into biogas.
In fact, most of the net energy benefit observed in the AD + HTC scenario was accrued through the biomethanation of the residual organic material in the HTC process water. The amount of energy released exceeded the energy demand of the HTC process itself, underscoring the intrinsic value of this byproduct stream. Hence, HTC process water should not be considered a waste, but rather, a potential feedstock for biological applications, not limited to AD. For example, process water could be used as a carbon or nutrient source in bio-hydrogen production or algae cultivation [57]. The review article prepared by Zhou et al. [6] indicates that HTC process water is rich in soluble organics such as reduced sugars, short-chain carboxylic acids, and amino acids, all of which could be used to supplement biological growth media. However, it is essential to note that process water also contains a variety of biologically recalcitrant compounds and heavy metals, which persist during biological treatment [6]. Therefore, additional protocols for testing, treating, and mitigating hazardous compounds are necessary to implement HTC safely for these purposes. For instance, inoculum from micro-aeration-assisted AD, which some WWTPs already practice for H2S removal, was found to improve the biodegradability and methane yields during the treatment of food-waste-derived process water by 38% compared to standard AD treatment [58].
Beyond energy considerations, the integration of HTC supports a broader shift from waste disposal to value-added resource recovery, including carbon recovery [59]. Unlike dewatered sludge, which is generally landfilled or subjected to further drying for incineration [60], hydrochar is a carbon-rich, highly porous solid that can be used as a solid fuel, adsorbent material, or catalyst [61]. Indeed, the AD + HTC process recovered ~64.7% (w/w) of the total feedstock carbon as useful end-products (i.e., CH4, hydrochar), compared to ~27.7% (w/w) in the AD scenario. Yet, recent studies show that heavy metals (e.g., Zn, Cu, Ni, Cr) tend to accumulate in hydrochar after HTC, with over 92–100% retained in the solid phase, where they are often transformed into more stable, less bioavailable forms [62,63]. Polycyclic aromatic hydrocarbons (PAHs) can also be present and tend to accumulate in the hydrochar rather than the process water, with concentrations increasing in the hydrochar at higher HTC temperatures. For instance, PAH concentrations of 3435.82 µg·kg−1 at 160 °C and 6221.98 µg·kg−1 at 240 °C have been reported [64]. Additives such as CaO have been shown to reduce the PAH and acidity levels in hydrochar. These findings suggest that contaminant stabilization is possible, but further research is needed to assess the long-term risks under field conditions.
Hydrochar can also serve as a soil amendment for carbon storage and crop cultivation. Multiple studies have reported the agronomic potential of hydrochar, particularly its ability to enhance the soil structure, improve nutrient retention, and stimulate microbial activity [16,65,66]. The present work considered using hydrochar as a soil amendment to enhance nutrient retention, thereby reducing the need for synthetic fertilizer. The transport-related energy consumption (i.e., diesel fuel) of hydrochar was considerably less due to its smaller quantity and improved handling properties compared to dewatered sludge in the AD-only scenario (Figure 3C). However, the overall amount of transport-related energy was low (<3%) compared to the total amount of energy produced and consumed in the WWTP. Nevertheless, the high value and reduced volume of the hydrochar production pathway offer a more sustainable approach to conventional sludge disposal [60]. These findings demonstrate that integrating AD and HTC not only enhances energy recovery and self-sufficiency in WWTPs but also facilitates a transition toward resource-oriented sludge management.
However, while hydrochar application can reduce the fertilizer demand and contribute to soil carbon enrichment, long-term ecological risks such as heavy metal accumulation and soil acidification must be considered. These risks are influenced by both the hydrochar properties and the receiving soil type. Site-specific guidelines, including application limits based on the soil buffering capacity and regular monitoring, are important to mitigate potential adverse effects. Co-application with alkaline amendments (e.g., lime) may also help counteract acidification. Future research should include long-term field trials across diverse soil conditions to assess the ecological safety and performance over time [65]. Finally, hydrochar handling and transport may pose health and safety risks that were not accounted for in this study. For example, hydrochar can generate dust that may irritate the eyes, skin, and respiratory tract. Therefore, WWTP operators handling hydrochar should wear personal protective equipment such as gloves, eye shields, long sleeves, and dust respirators. Moreover, hydrochar is combustible; therefore, it should be stored safely away from heat sources, sparks, or open flames due to its combustibility and possible dust explosion hazard. These additional safety measures are not overly burdensome, and WWTP personnel are well accustomed to handling hazardous substances and following safe-handling protocols.

3.3. Environmental and Human Health Impact Mitigation Through AD + HTC Coupling

This study considered the endpoint indicators human health, ecosystem quality, climate change, and resource depletion to quantify the environmental impacts of the proposed AD + HTC scenario and the conventional AD-only scenario. The IMPACT 2002+ v2.15 methodology was selected for its comprehensive midpoint-to-endpoint modeling and compatibility with wastewater treatment LCA studies. This comprehensive modeling framework enables a more integrated understanding of how sludge treatment processes affect broader environmental and public health outcomes. For example, Halleux et al. [67] compared Eco-Indicator 99, CML, and IMPACT 2002+ in evaluating a wastewater treatment plant. They reported that IMPACT 2002+ provided a balanced approach to assessing the midpoint impacts, without requiring model adaptation, unlike Eco-Indicator 99. Similarly, Patel and Singh [68] conducted a comparative LCA of multiple wastewater treatment and sludge pathways, such as AD and lime stabilization, in India. They reported that IMPACT 2002+ is effective for hotspot identification across both wastewater and sludge treatment processes.
The results consistently showed better environmental performance in the AD + HTC scenario compared to the AD-only scenario across all the indicators. For instance, the human health impacts were significantly lower in the AD + HTC scenario, showing a 295% improvement relative to the AD-only scenario (Figure 4A). The increase in surplus heat and electricity contributed the most to reducing the human health impacts, followed by the avoidance of landfilling and the recovery of fertilizer. The displacement of grid electricity resulted in more significant reductions in the human health impacts than the displacement of grid heat. Grid electricity remains heavily reliant on coal and oil, which, when combusted, emit relatively high levels of particulate matter, heavy metals, NOx + SOx, and carcinogens. These emissions are strongly linked to respiratory illnesses, cardiovascular disease, and cancer; therefore, they are weighted more heavily in IMPACT 2002+. The natural gas used for heat, on the other hand, is a much cleaner-burning fuel, emitting far fewer of these pollutants, which is why it has a lower human health impact score. Landfilling also contributes significant human health impacts due to the release of GHGs and contaminated leachates.
Finally, while the IMPACT 2002+ methodology accounted for many of the human health impacts, other impacts were not captured in the life-cycle impact assessment. For example, the hydrochar generated by HTC is effectively sterile, which would minimize the spread of pathogens that could otherwise pose a public health risk [5]. In contrast, dewatered solids retain a significant biological load, raising concerns for both workers and those exposed downstream. Moreover, the reduced need for sludge transport and handling further minimizes the occupational exposure risks and emissions from vehicle exhaust. Therefore, AD + HTC shows a clear advantage in reducing the human health impacts compared to standard practice.
Similar trends were also observed in the impacts on ecosystem quality (Figure 4B)—electricity production resulted in higher ecological impact reductions than heat production from natural gas. Electricity production from coal and oil emits heavy metals, such as mercury, lead, and cadmium, which are toxic to both aquatic and terrestrial organisms. Additionally, electricity production emits NOx and SOx, leading to the acidification of water bodies and soil through indirect atmospheric deposition. These impacts directly affect ecosystem quality by causing a loss of biodiversity, reduced plant and aquatic productivity, and soil degradation. Grid electricity also involves more upstream ecosystem impacts due to mining and oil extraction. The only difference in the trends observed here relative to the human health impacts was the greater influence of displaced fertilizer compared to landfilling. While landfilling contributes to ecological damage through GHG emissions, leachate emissions, and land use, these impacts are less significant than the environmental damage caused by synthetic fertilizer production.
Synthetic fertilizer production via the Haber–Bosch process is an energy-intensive industrial process that involves the production of ammonia through the splitting of molecular nitrogen and combining it with H2 derived from steam methane reforming. The ammonia is further converted into nitric acid (HNO3) via the Ostwald process, which is later neutralized to form ammonium nitrate. While the energy consumption alone contributes significantly to ecosystem damage, synthetic fertilizer production also emits considerable amounts of GHGs, ammonia, and NOx, which cause eutrophication and acidification. Here, the HTC hydrochar was applied as a soil amendment, effectively displacing synthetic fertilizers due to its nutrient-retaining properties.
During the HTC process, the organic nitrogen in sludge is converted into ammonia, some of which is entrained in the hydrochar [69]. Hydrochar also acts as a soil conditioner, promoting greater plant nutrient absorption and utilization by enhancing soil nutrient retention and soil porosity [70]. Additionally, while not accounted for in this life-cycle impact assessment, hydrochar can also bind pesticides, possibly reducing the risk of groundwater contamination [71]. Finally, due to its significantly reduced mass and volume relative to dewatered sludge [72], the transportation of hydrochar to agricultural sites results in a 50% reduction in the transport-related ecosystem impacts compared to transporting dewatered sludge. However, HTC also liberates additional nutrients (e.g., ammonium and phosphate) and heavy metals into the aqueous phase, which may necessitate further effluent polishing steps to meet discharge standards. Alternatively, these nutrients present a valuable opportunity for nutrient recovery, particularly in WWTPs aiming for resource circularity. As mentioned previously, the additional energy created via AD + HTC could be used for these recovery purposes. Similarly, solubilized forms of heavy metals may present an easier target for safe removal compared to heavy metals bound to the dewatered sludge. Future research should investigate the fractionation and fate of these nutrients and trace metals in the context of a fully integrated and continuous AD + HTC process, as well as the relevant downstream processes, to identify potential environmental trade-offs.
Overall, the AD + HTC scenario led to a 199% greater reduction in the ecosystem quality impacts compared to the AD-only scenario. However, the presence of toxic compounds and heavy metals in hydrochar remains a significant concern, which can only be partly ameliorated through HTC process optimization [73]. Hydrochar is also acidic and has a relatively high ash content. Thus, further research is needed to assess the long-term impact of hydrochar on the soil structure and biota. Finally, it is important to consider the logistical concern of matching the supply of hydrochar with the loading capacity of agricultural land. WWTPs may have to transport hydrochar long distances to distribute it across a sufficiently large land area. Despite these reservations, however, the LCA results indicate that AD + HTC provides significant environmental benefits in terms of ecosystem quality.
The impact categories climate change and primary resource depletion are both heavily influenced by fossil energy use. Climate change impacts are primarily driven by GHG emissions, which typically derive from burning fossil fuels, including the combustion of coal and oil (electricity), natural gas (electricity/heat), and diesel (transport). Primary resource depletion quantifies the extraction of non-renewable resources, which includes minerals alongside fossil fuels. WWTPs are energy-intensive, but they consume relatively few minerals; therefore, the impacts of climate change and primary resource depletion in this LCA were broadly similar, being primarily dictated by the WWTP’s net energy balance and capture efficiency. Indeed, both the AD-only and AD + HTC scenarios achieved a positive net energy balance through biogas-powered CHP; therefore, both resulted in significant net reductions in the climate change and resource depletion impacts (Figure 4C,D).
However, the AD + HTC scenario exhibited a substantially higher net energy balance compared to the AD-only scenario, resulting in a 16-fold reduction in the climate change impacts and a 3.9-fold reduction in the resource depletion impacts. The primary driver of this improvement was the approximately ten-fold increase in surplus heat generated by the AD + HTC system compared to the AD-only system, enabling the significant displacement of grid-supplied natural gas. It is important to note, however, that this ten-fold increase pertains specifically to surplus heat production; the gross heat output in the AD + HTC scenario was only 35% higher than that of the AD-only configuration. In the AD-only scenario, the majority of heat generated via CHP was consumed internally, primarily for influent preheating and maintaining digester temperature, leaving only a minimal heat surplus. By contrast, in the AD + HTC scenario, the additional biogas produced through the anaerobic digestion of HTC process water contributed substantially to the net heat gains, as the parasitic thermal demands of the system remained essentially unchanged from the AD-only baseline.
This improved heat recovery efficiency highlights the energetic advantage of integrating HTC with AD, particularly when process optimization enables surplus energy to be directed toward grid displacement or other on-site applications. While the observed impact reductions in the AD + HTC scenario were mainly attributable to enhanced biogas production from the recovered HTC process water, the avoidance of landfilling, which is a notable methane emitter, and the substitution of synthetic fertilizer, which causes indirect GHG emissions, led to substantial climate change impact reductions (Figure 4C). Finally, while the GHG emissions generated from the transportation of hydrochar to agricultural lands were relatively insignificant, they were also lower than the GHG emissions emitted during the transport of dewatered sludge to landfills. Overall, these results align with the findings of Mannarino et al. [21] who reported that applying HTC for sludge management could reduce the GHG emissions by over 98% relative to conventional sludge composting practices outside the water resource recovery facility. In conclusion, these results show the environmental and human health co-benefits that could be realized by integrating HTC with AD in sludge treatment systems. By minimizing the emissions across key impact categories, the proposed approach demonstrates strong potential for supporting low-carbon and environmentally conscious wastewater management frameworks.
These results clearly demonstrate the environmental benefits of AD + HTC integration; however, several practical challenges need to be addressed before real-world implementation can be realized. Two significant barriers to HTC implementation are its high capital cost and the various fundamental physical constraints that technically limit HTC scaling, which may render HTC infeasible for large-scale WWTPs. Indeed, HTC requires specialized containment vessels capable of tolerating high temperatures and pressures. As the capacity of the HTC unit increases, more structural material is required, which quickly increases the size and cost of these containment vessels [74]. Moreover, higher-volume vessels face heat transfer issues, such as slower heat-up and cool-down rates. Larger vessels also tend to distribute heat less uniformly, which can reduce the hydrochar consistency and yields [75]. Integrating AD and HTC would also require careful process coordination, particularly in balancing the CHP heat supply with the HTC heat demand. Finally, there is considerable regulatory uncertainty regarding the land application of hydrochar, particularly in terms of the dosing rates, contaminant monitoring and thresholds, and product classification. Given the variability of hydrochar properties (i.e., due to differences in the sludge composition), Dang et al. [76] presented the additional challenge of ensuring consistent performance as a soil amendment. Future research should, therefore, focus on process intensification, long-term performance monitoring, and techno-economic analysis to help support the sustainable deployment of AD + HTC systems in various WWTP contexts

3.4. Sensitivity Analysis Demonstrates the Dependence of Climate Change Impacts on Net Bioenergy Production During Wastewater Treatment

To test the robustness of the environmental performance outcomes of the proposed AD + HTC sludge treatment process, a sensitivity analysis was performed using three key energy-related parameters: the methane content of the biogas, the electrical efficiency of the CHP unit, and the thermal efficiency of the CHP unit, with the climate change impacts serving as the sensitivity indicator. Each parameter was varied by +10% from the base case values. Specifically, the electrical efficiency increased from 35% to 38.5%, the thermal efficiency increased from 50% to 55%, and the methane content in biogas increased from 66% to 72.6%, and the LCA model was rerun. The other life-cycle inventory items (e.g., transport distance, fertilizer efficiency, landfilling) played a relatively minor role in the overall LCA outcomes; therefore, they were not included in the sensitivity analysis.
Among the variables examined in the sensitivity analysis, the methane concentration in biogas emerged as the most influential parameter affecting the climate change impacts, followed by the thermal efficiency and electrical efficiency of the CHP unit (Figure 5). Specifically, a 10% increase in the methane content resulted in a 17.5% greater reduction in the climate change impacts under the AD + HTC scenario, compared to a 12.9% reduction for thermal efficiency and a 4.7% reduction for electrical efficiency. These results are consistent with expectations and reflect the interplay of multiple process-level factors. Electrical efficiency was the least sensitive of the three parameters due to the inherently lower conversion efficiency of electricity generation in CHP systems compared to thermal conversion efficiency. Moreover, the relative contribution of the parasitic electricity demand was modest, comprising only 5.3% of the gross electricity yield. In contrast, the parasitic heat demands consumed a significantly larger share (65.6% of the gross heat yield). As a result, marginal improvements in thermal efficiency yielded more substantial net energy gains, which in turn led to greater reductions in the climate-related impacts compared to the equivalent gains in electrical efficiency. Crucially, increases in the methane content of biogas simultaneously enhanced both the electrical and thermal outputs of the CHP unit, amplifying the system’s energy recovery potential and, consequently, its capacity to displace fossil-derived grid energy. This dual benefit explains why the methane content had the highest sensitivity among the variables tested. From a broader perspective, these findings underscore the importance of optimizing biogas quality (not just quantity) as a critical lever for enhancing the environmental performance of AD systems, particularly when integrated with energy-intensive processes such as HTC.
Zhu et al. [77] and Saboohi and Hosseini [78] similarly concluded that optimizing the methane yield is among the most effective strategies for enhancing the environmental performance of biogas-based energy systems. The sensitivity analysis applied a presupposed nominal improvement of 10% to allow a direct comparison between parameters. A variety of operational interventions during AD could be employed to increase the methane content and yield of biogas. These include co-digesting high-lipid, highly biodegradable substrates [79], allowing headspace pressurization for methane enrichment [80], injecting H2 gas for biomethanation [80], supplementing with trace metals [81], and optimizing the organic loading rate (OLR) and hydraulic retention time (HRT) [82]. Considering the scale and infrastructural constraints of many WWTPs, as well as the costs, co-digestion stands out as the most practical option because it does not require significant infrastructural changes (if any) and typically generates revenue via tipping fees. Headspace pressurization and biomethanation with H2 can be implemented at these scales [11] but would require an upfront cost to retrofit the AD system with pressure-rated and safety-compliant infrastructure. Trace metal supplementation would be easy to employ, but given the variability of the sludge composition, would need to be evaluated on a case-by-case basis. Trace metal supplementation may not be economically feasible given the cost of certain trace metals and the amount required at scale. Finally, while the OLR and HRT can be manipulated to increase methane yields, the margin for their adjustment would be constrained by the working capacity of the AD system and the availability of sludge staging tanks.
Improving methane yields and the overall energy recovery efficiency has direct implications for reducing the environmental footprint of WWTPs, primarily by lowering their reliance on external fossil-based energy sources. However, given the relatively mature state of AD and CHP technologies, the opportunities for further performance improvements through conventional optimization are becoming increasingly limited. Therefore, future improvements are more likely to come from the integration of complementary or novel technologies that overcome the inherent limitations of existing systems. In this context, the results of the present LCA demonstrate that the integration of HTC with AD represents a transformative opportunity to enhance the environmental performance of wastewater treatment. By unlocking additional energy recovery pathways and reducing residual sludge volumes, AD + HTC addresses key sustainability and circularity challenges that conventional AD systems have struggled to overcome.
However, as with any LCA study, several methodological limitations could affect these results, including the definitions of the system boundaries and process assumptions. Also, while the functional unit (i.e., 1 kg of sludge) aligns with industry-standard LCA practice, alternative functional units (e.g., 1 MJ of energy recovered; kg of solids removed) could influence the absolute values. However, the choice of functional unit would not change the outcomes of the comparative analysis. In addition, the IMPACT 2002+ methodology, which LCA practitioners frequently use for environmental impact assessment, introduces uncertainty through its use of characterization models, weighting factors, and assumptions that may vary across regions or databases. Finally, while primary data were used where available, especially for AD operations, HTC-specific data were partly obtained from the literature sources and may not fully capture the process variability across facilities or scales.

4. Conclusions and Future Perspectives

Integrating AD with HTC presents a compelling pathway to enhance the sustainability and circularity of WWTPs. This synergistic combination of technologies enables more effective sludge management by significantly reducing residual sludge volumes, improving energy recovery, and generating hydrochar—a multifunctional co-product that can be utilized for soil conditioning, carbon sequestration, or energy production, among other applications. In addition to improving solids reduction, HTC contributes to better environmental performance by minimizing emissions, particularly GHGs, and reducing transport-related impacts due to the lower mass and superior handling characteristics of hydrochar. The LCA conducted in this study revealed that AD + HTC integration produced substantially higher net energy yields per kilogram of treated sludge compared to conventional AD alone within the system boundary. These enhanced energy yields were primarily driven by the conversion of the substantial organic carbon content in the HTC process water into biogas.
Beyond improving energy self-sufficiency, these energy surpluses make WWTPs more operationally versatile, enabling the integration of advanced waste treatment and resource recovery technologies. Such technologies include in situ biomethanation, electrochemical separation of nutrients and heavy metals, and tertiary treatment processes such as disinfection, odor and color removal, and the oxidation of recalcitrant organic pollutants. Moreover, many WWTPs are evolving into regional co-digestion facilities, accepting externally sourced organic waste, including food waste, fats, oils, and greases (FOGs), as well as beverage industry effluents. These high-energy co-substrates can further enhance biogas yields and may enable WWTPs to transition from net energy consumers to net energy producers, thereby extending their role as essential providers of environmental services. Given their existing infrastructure and regulatory framework, WWTPs are uniquely well positioned to lead this transition compared to many other industries. Finally, while this study focused primarily on the unit operations involved in AD + HTC integration, future research should broaden the system boundary to consider the environmental fate of hydrochar in land applications. The key considerations include its impacts on the soil biota, heavy metal mobility, and nutrient cycling. Additionally, evaluating economic feasibility, exploring alternative process configurations, and identifying new valorization pathways, such as the use of hydrochar in catalysis or materials production, will be critical for assessing the full potential of AD + HTC systems. In conclusion, the integration of AD and HTC, particularly when combined with innovative process optimization strategies, holds significant promise as a platform technology for enabling energy-positive, resource-efficient, and environmentally sustainable wastewater treatment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17167545/s1, Table S1: A list summarizing all the major process and life-cycle modeling values and assumptions.

Author Contributions

Conceptualization, K.J.T., A.V.O. and J.G.U.; methodology, K.J.T. and A.V.O.; formal analysis, K.J.T., A.V.O. and J.G.U.; investigation, K.J.T. and A.V.O.; resources, J.G.U.; data curation, A.V.O. and N.S.K.; writing—original draft preparation, A.V.O.; writing—reviewing and editing, K.J.T., N.S.K. and J.G.U.; visualization, K.J.T. and A.V.O.; supervision, J.G.U.; project administration, J.G.U.; funding acquisition, J.G.U. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported, in part, by (1) the Agricultural Experiment Station at the University of Georgia, (2) the Office of Global Engagement at the University of Georgia through the Global Research Collaboration Grant program, and (3) the Institute of Integrative Precision Agriculture at the University of Georgia through the Seed Grant program.

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors would like to thank Roy Posmanik from the Technion–Israel Institute of Technology, Israel, and the staff of the Mühlhausen WWTP in Stuttgart, Germany, for providing technical advice and data related to the hydrothermal carbonization (HTC) and wastewater treatment processes.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

ADAnaerobic digestion
CHPCombined heat and power
GHGGreenhouse gas
HTCHydrothermal carbonation
LCALife-cycle assessment
WWTWastewater treatment
WWTPWastewater treatment plant

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Figure 1. Schematic overview of the two evaluated scenarios, showing the major unit processes, material and energy flows, and system boundary. The unit processes shaded in gray are not included within the system boundary. Scenario (A) represents the reference scenario involving conventional anaerobic digestion (AD); scenario (B) represents the proposed scenario integrating anaerobic digestion and hydrothermal carbonization (HTC).
Figure 1. Schematic overview of the two evaluated scenarios, showing the major unit processes, material and energy flows, and system boundary. The unit processes shaded in gray are not included within the system boundary. Scenario (A) represents the reference scenario involving conventional anaerobic digestion (AD); scenario (B) represents the proposed scenario integrating anaerobic digestion and hydrothermal carbonization (HTC).
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Figure 2. Mass balance Sankey diagrams illustrating the mass flow through the primary unit operations of the (A) reference scenario: conventional anaerobic digestion (AD) (top), and (B) the proposed scenario: anaerobic digestion combined with hydrothermal carbonization (AD + HTC). The mass balance values were calculated based on 1 kg of total solids contained in the mixed sludge (i.e., dry basis).
Figure 2. Mass balance Sankey diagrams illustrating the mass flow through the primary unit operations of the (A) reference scenario: conventional anaerobic digestion (AD) (top), and (B) the proposed scenario: anaerobic digestion combined with hydrothermal carbonization (AD + HTC). The mass balance values were calculated based on 1 kg of total solids contained in the mixed sludge (i.e., dry basis).
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Figure 3. Energy analysis of the anaerobic digestion scenario (AD) and the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC), showing (A) the parasitic electricity consumption, (B) the parasitic heat consumption, and (C) the total energy balance. The energy values, in kWh, were normalized to 1 kg of mixed sludge (i.e., the functional unit).
Figure 3. Energy analysis of the anaerobic digestion scenario (AD) and the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC), showing (A) the parasitic electricity consumption, (B) the parasitic heat consumption, and (C) the total energy balance. The energy values, in kWh, were normalized to 1 kg of mixed sludge (i.e., the functional unit).
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Figure 4. Environmental impact assessment for the damage categories quantified using the IMPACT 2002+ methodology for the anaerobic digestion scenario (AD) and the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC). The environmental damage categories include (A) human health, (B) ecosystem quality, (C) climate change, and (D) resources. DALY = daily adjusted life years; PDF = potentially disappeared fraction of species; eq. = equivalents.
Figure 4. Environmental impact assessment for the damage categories quantified using the IMPACT 2002+ methodology for the anaerobic digestion scenario (AD) and the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC). The environmental damage categories include (A) human health, (B) ecosystem quality, (C) climate change, and (D) resources. DALY = daily adjusted life years; PDF = potentially disappeared fraction of species; eq. = equivalents.
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Figure 5. Sensitivity analysis of the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC) for the following parameters: (1) electrical efficiency of the CHP unit, (2) thermal efficiency of the CHP unit, and (3) the methane content of the biogas. The sensitivity results were evaluated using the climate change impacts as the indicator. The % reduction values were calculated relative to the net climate change impacts in the base-case AD + HTC scenario.
Figure 5. Sensitivity analysis of the anaerobic digestion combined with hydrothermal carbonization scenario (AD + HTC) for the following parameters: (1) electrical efficiency of the CHP unit, (2) thermal efficiency of the CHP unit, and (3) the methane content of the biogas. The sensitivity results were evaluated using the climate change impacts as the indicator. The % reduction values were calculated relative to the net climate change impacts in the base-case AD + HTC scenario.
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MDPI and ACS Style

Taiwo, K.J.; Oancea, A.V.; Kotha, N.S.; Usack, J.G. Enhancing Wastewater Treatment Sustainability Through Integrated Anaerobic Digestion and Hydrothermal Carbonization: A Life-Cycle Perspective. Sustainability 2025, 17, 7545. https://doi.org/10.3390/su17167545

AMA Style

Taiwo KJ, Oancea AV, Kotha NS, Usack JG. Enhancing Wastewater Treatment Sustainability Through Integrated Anaerobic Digestion and Hydrothermal Carbonization: A Life-Cycle Perspective. Sustainability. 2025; 17(16):7545. https://doi.org/10.3390/su17167545

Chicago/Turabian Style

Taiwo, Kayode J., Andrada V. Oancea, Nithya Sree Kotha, and Joseph G. Usack. 2025. "Enhancing Wastewater Treatment Sustainability Through Integrated Anaerobic Digestion and Hydrothermal Carbonization: A Life-Cycle Perspective" Sustainability 17, no. 16: 7545. https://doi.org/10.3390/su17167545

APA Style

Taiwo, K. J., Oancea, A. V., Kotha, N. S., & Usack, J. G. (2025). Enhancing Wastewater Treatment Sustainability Through Integrated Anaerobic Digestion and Hydrothermal Carbonization: A Life-Cycle Perspective. Sustainability, 17(16), 7545. https://doi.org/10.3390/su17167545

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