TO-SYN-FUEL Project to Convert Sewage Sludge in Value-Added Products: A Comparative Life Cycle Assessment ^†

Abstract

Second-, third-, and fourth-generation biofuels represent an important response to the challenges of clean energy supply and climate change. In this context, the Horizon 2020 “TO-SYN-FUEL” project aimed to produce advanced biofuels together with phosphorus from municipal wastewater sludge through a combination of technologies including a Thermo-Catalytic Reforming system, Pressure Swing Adsorption for hydrogen separation, Hydrodeoxygenation, and biochar gasification for phosphorous recovery. This article presents the environmental performance results of the demonstrator installed in Hohenberg (Germany), with a capacity of 500 kg per hour of dried sewage sludge. In addition, four alternative scenarios are assessed, differing in the source of additional thermal energy used for sludge drying: natural gas, biogas, heat pump, and a hybrid solar greenhouse. The environmental performance of these scenarios is then compared with that of conventional fuel. The comparative study of these scenarios demonstrates that the biofuel obtained through wood gasification complies with the Renewable Energy Directive, while natural gas remains the least sustainable option. Heat pumps, biogas, and greenhouse drying emerge as promising alternatives to align biofuel production with EU sustainability targets. Phosphorus recovery from sewage sludge ash proves essential for compliance, offering clear environmental benefits. Although sewage sludge is challenging due to its high water content, it represents a valuable feedstock whose sustainable management can enhance both energy recovery and nutrient recycling.

1. Introduction

The ongoing climate crisis requires the entire economy to seek alternative solutions to drastically reduce greenhouse gas (GHG) emissions across all sectors. The European Green Deal has established ambitious targets to achieve climate neutrality in Europe by 2050. Within this framework, the transport sector is one of the most critical and challenging to decarbonize, due to the rigidity of its infrastructure and the complexities involved in producing economically viable, sustainable, and advanced renewable fuels [1]. To address this issue, the Renewable Energy Directive (RED III) serves as the central regulatory framework, established by the European Commission for the fuel sector [2]. In quantitative terms, the average share of renewable energy sources (RES) in the European transport sector was approximately 10.8% in 2023, and is expected to increase to 29% by 2030, as mandated by RED III [3,4]. This target includes both renewable fuels and renewable electricity. Moreover, the directive sets a binding sub-target of 5.5% for advanced biofuels and renewable fuels of non-biological origin, to be achieved by 2030. RED III also reinforces sustainability criteria for the use of biomass for energy, aiming to minimize the risk of unsustainable bioenergy practices. In particular, the directive focuses on “advanced biofuels”, defined as fuels produced from non-food feedstocks that are considered sustainable and at low risk of indirect land use change. Complementing RED III, Regulation (EU) 2019/631 on CO₂ emission performance standards for passenger cars and vans, along with the proposed regulation for heavy-duty vehicles, imposes stringent emissions reduction targets [5]. By 2030, CO₂ emissions must be reduced by 55% for passenger cars [6] and 45% for heavy-duty vehicles [7], respectively. In this context, biofuels can play a significant and non-negligible role.

Beyond the European Union (EU), biofuel production is also experiencing continuous growth. Numerous non-EU countries are implementing policies and strategies to promote the use of biofuels, adopting diverse approaches tailored to their available resources, climate goals, and economic priorities. Leading this global effort are the United States and Brazil, the world’s top biofuel producers [8]. As early as 2005, the U.S. Congress established the Renewable Fuel Standard program to reduce greenhouse gas emissions and expand the national renewable fuels sector [9]. This act mandated that increasing volumes of renewable fuel be blended into transportation fuel each year, reaching over 130 billion liters by 2022 [10]. In Brazil, the large-scale use of biofuels began during the oil crisis of the 1970s, with the partial replacement of gasoline by ethanol. The sector experienced a strong revival in the 2000s, driven by the development of flex-fuel technologies. In 2022, ethanol accounted for approximately 22% of the energy used in the transport sector, and production exceeded 27 billion liters [11]. Other major global producers include Indonesia, Thailand, China, India, Argentina, and Canada [12].

The growing interest in biofuels has been accompanied by a heightened awareness of the various environmental concerns linked to the use of dedicated crops for their production, highlighting the need to identify alternative feedstocks [13,14]. This shift has driven the development of second-, third- and fourth-generation biofuels, commonly referred to as advanced biofuels, as mentioned above [15,16]. This has been embodied in numerous regulations. For example, in the European Union, advanced biofuels must be produced from a positive list of feedstocks (mostly wastes and residues) set out in Part A of Annex IX of RED III [2]. This list includes sewage sludge.

Over the past few decades, a wide range of feedstocks has been investigated for the production of advanced biofuels, including agricultural residues, forestry waste, and non-food dedicated crops (second-generation biofuels); microalgae (third-generation); as well as sewage sludge, organic waste, CO₂, and genetically modified plants (fourth-generation) [17,18,19]. This paper focuses on sewage sludge (also known as biosolids in North America) as a valuable feedstock for the production of advanced biofuels. It presents several advantages that make its utilization particularly attractive: it is generated in large quantities, it is homogeneously distributed, it is not affected by seasonality, and it contains significant amounts of bioavailable nitrogen and phosphorus [20,21]. Additionally, since sludge disposal involves significant costs, its use as a feedstock can offer economic benefits [21]. Circular economy approaches need to be applied when dealing with sewage sludge from wastewater treatment plants, as the recovery of energy and valuable materials can improve both the economic and the environmental impacts of the processes. Despite regulatory efforts to change the status of sewage sludge from waste to a resource (e.g., in Europe through the Water Framework Directive [22], the Waste Framework Directive [23]), only one-third to half of sludge is recycled in agriculture or composting, where incineration and landfilling remain the modus operandi in many countries [24]. Currently, in many countries, the loss of organic matter, nitrogen, and phosphorus means that the management of sewage sludge misses an opportunity to contribute to a more circular economy, in which secondary raw materials could replace primary ones, especially concerning the recovery of phosphorus.

However, there are also some drawbacks to consider. The organic content of sewage sludge can be highly variable and sometimes quite low; it may also contain pollutants such as heavy metals and hydrocarbons [24]. Furthermore, its high-water content poses challenges for certain innovative treatment technologies [24].

This study evaluates the environmental performance of a system for the conversion of sewage sludge into advanced biofuels and added value products within the TO-SYN-FUEL project [25], using the life cycle assessment (LCA) methodology. TO-SYN-FUEL (The Demonstration of Waste Biomass to Synthetic Fuels and Green Hydrogen) was a Horizon 2020 research and innovation project that ran from 2017 to 2022. Its main objective was to demonstrate the technology through the construction of a plant at near commercial scale, designed with a nominal dry feedstock capacity of 500 kg/h [25]. The project implemented a new integrated process combining Thermo-Catalytic Reforming (TCR), hydrogen separation through Pressure Swing Adsorption (PSA), hydro deoxygenation (HDO), and charcoal gasification to produce a fully equivalent gasoline and diesel substitute (compliant with EN228 [26] and EN590 [27] European Standards), and phosphorous. LCA is acknowledged by the European Commission as the most comprehensive framework currently available for assessing the environmental impacts of products and processes, as it effectively prevents burden shifting and facilitates the identification of environmental hotspots [28,29]. This study aims at studying environmental advantages and disadvantages of such technology by comparing it with alternative solutions for drying sewage sludge.

2. Materials and Methods

This section provides a detailed description of the production process under investigation, together with the methodological framework adopted for the LCA. The procedures applied to define system boundaries, functional unit, and impact categories are reported. Finally, the different scenarios considered for the analysis are outlined.

2.1. Production Process Description

This process enables production of fully equivalent substitutes for gasoline and diesel—compliant with European standards EN228 [26] and EN590 [27]—as well as green hydrogen suitable for internal use. The plant was designed to operate with a capacity of 500 kg per hour of dried sewage sludge (water content of 5–15%), enabling the annual processing of approximately 2100 tons of feedstock into around 210,000 L of HDO bio-oil, which were further distilled into transport fuels. The demonstration facility, located in Hohenberg, Germany, became operational in the autumn of 2021. By the end of the project, the demonstrated TCR technology had been successfully validated at Technology Readiness Level (TRL) 7 [25].

The TCR system is a reactor that combines intermediate pyrolysis with a post-reforming stage (see Figure 1). It is externally heated by a flue gas heating system. The pyrolysis intermediates are directed into the reformer, where interactions between char and vapors occur. This process enhances the quality of all resulting product streams. As a result of the catalytic properties of the char itself, reforming can take place at temperatures below 700 °C, eliminating the need for an external catalyst. Furthermore, biochar is gasified to produce thermal energy and electricity. The bio-oil produced is upgraded using hydrogen derived from the TCR gas fraction. Before use, this hydrogen must be separated from other gas components, such as carbon monoxide, carbon dioxide, and methane, via PSA, a process that exploits the varying adsorption capacities of materials under different pressures. The purified hydrogen is then compressed and directed to the HDO unit, where heteroatoms, such as sulfur, nitrogen, and oxygen, are removed via catalytic reactions from biooil, thereby improving its quality. The process operates with an excess of hydrogen, which, in line with economic efficiency principles, is recovered and recycled through the PSA system [30]. Off-gases from PSA and HDO are sent to a combined heat-power (CHP) system.

2.2. Application of the Environmental Life Cycle Assessment

The goal of the environmental life cycle assessment is to evaluate the performances of the biofuel produced from sewage sludge using the TCR/PSA/HDO integrated system through a 15-set of longitudinal environmental impact indicators. This will be compared with the impacts of conventional fossil fuel.

A “Well-To-Wheel (WTW)” approach is adopted, as recommended by the JRC–EUCAR–CONCAWE consortium (JEC), whose overarching aim is to support the sustainability of the European automotive and oil industries. The WTW approach focuses on the fuel production and vehicle use phases, excluding the energy and emissions associated with facilities construction, vehicle manufacturing, or end-of-life aspects [31].

The system boundaries of this study (see Figure 1) encompass the following processes: (1) sludge thickening and dewatering (reducing water content from 99% to 75%); (2) sludge transport (assuming a distance of 100 km); (3) sludge drying (water content reduced from 75% to 10%); (4) TCR; (5) PSA; (6) HDO; (7) CHP generation; (8) biochar gasification; (9) wastewater treatment plant (WWTP) of wastewater and process water generated in the different steps of the system; (10) phosphorus recovery; (11) HDO bio-oil distillation; (12) fuel distribution; (13) fuel combustion. Additional thermal energy is required for sludge drying, since the thermal output of the system alone is insufficient to satisfy the process heat requirements. As for the feedstock, municipal sewage sludge entering the system from wastewater treatment plants is considered to have a zero-burden boundary, meaning that it is treated as a waste material with no upstream environmental impact allocated with its generation. The functional unit for the analysis is defined as 1 MJ of fuel, expressed in terms of Higher Heating Value (HHV).

2.2.1. Scenarios Description

Sewage sludge moisture content—typically exceeding 95%—necessitates water removal prior to any thermal processing. For pyrolysis to be feasible, the sludge must be dried to reduce its moisture content to below 10%. The drying process is extremely energy-intensive and can therefore significantly affect the overall environmental performance of the entire biofuel production system.

This study analyzes five scenarios that differ in the type of drying system employed. Each scenario is defined based on the source of the additional thermal energy required for the drying process: (1) “Natural Gas”—additional heat is provided by natural gas combustion; (2) “Biogas”—additional heat is supplied using biogas produced from the anaerobic digestion of dedicated energy crops; (3) “Wood Gasification”—represents the baseline scenario, as the demonstrator TCR/PSA/HDO plant was installed at a sewage sludge drying facility powered by forest wood chips gasification in Hohenburg (Amberg-Sulzbach district); (4) “Heat Pump”—drying is achieved through a heat pump powered by the European electricity mix; (5) “Greenhouse”—drying is carried out in a hybrid solar greenhouse [32]. A sixth scenario was implemented as benchmark, which represents conventional fuel (gasoline or diesel). This scenario aims to assess the environmental performance of average European gasoline and diesel production and use. It should be noted that both biogasoline and biodiesel are obtained during the HDO bio-oil distillation process; however, for the sake of brevity, this article compares only the results related to (bio-)gasoline. However, the trends are fully comparable for biodiesel production.

2.2.2. Life Cycle Inventory

This study is based on operational data from the TCR/PSA/HDO integrated system in its final configuration at demonstration scale, as developed within the framework of the TO-SYN-FUEL project. Primary data were used for the processes occurring within the demonstration plant, while engineering estimates, design specifications, and literature sources were employed for the processes outside the TO-SYN-FUEL plant but included within the system boundaries. LCA was conducted using GaBi 10.6 software (now “LCA for Experts” by Sphera) [33]. Background data were obtained from the GaBi Professional Database [34] and the Ecoinvent Database [35]. Sources of primary data used in this study are listed in

Table 1. Primary and secondary data used in this study.

Process	Data Source
Sludge thickening and dewatering	Bianchini et al., 2015 [37]
Sludge transport	Marazza et al., 2019 [38]
Sludge drying	GaBi Professional Database [34]
TCR	Engineering and design data
PSA	Operating data
HDO	Operating data
CHP generation	Operating data
Biochar gasification	Engineering and experimental data
WWTP	Engineering and experimental data
Phosphorus recovery	GaBi Professional Database [34]
HDO bio-oil distillation	Remy, 2015 [36]
Fuel distribution	Iribarren et al., 2012 [39]
Fuel combustion	GaBi Professional Database [34]

. Recovery of phosphorus from the ash resulting from biochar gasification has not been implemented in the Hohenberg demonstration plant. Rather, it was hypothesized and included within the system boundaries to enhance sustainability from a circularity and waste valorization perspective. Data on phosphorus recovery were obtained from the European project P-REX [36]. The selected process involves the digestion of ash in a large excess of phosphoric acid (H₃PO₄). Following digestion, insoluble residues are removed through filtration. The resulting liquid solution, rich in H₃PO₄ and containing various dissolved impurities from the ash, undergoes purification via a multi-stage ion exchange process. This step effectively removes divalent salts, metals, and other contaminants, including heavy metals. After purification, part of the recovered H₃PO₄ is recycled back into the ash digestion step, while the remaining portion is recovered as a product and further concentrated by steam evaporation.

GaBi Professional Database was used for modeling drying through natural gas, biogas, heat pump and wood gasification, and the conventional fuel scenarios. Inventory data used to model the “Greenhouse scenario” are from a pilot plant located in Ostia, Italy [32].

To address the multifunctionality of the integrated system, an energy-based allocation was applied to the main products: biogasoline and biodiesel. Namely, it was assumed that 0.110 kg of biogasoline (HHV 47.1 MJ/kg) and 0.174 kg of biodiesel (HHV 46 MJ/kg) are produced from every 500 kg of sewage sludge. In addition, system expansion was used to account for co-products such as thermal energy, power and phosphorus (in the form of phosphoric acid).

The following inclusions and assumptions have been adopted: (1) sludge thickening and dewatering includes electricity and flocculants consumption; (2) sludge transport includes fuel production and direct emissions of the transport; (3) sludge drying includes thermal and power consumption; (4) TCR includes thermal energy and power consumption, water, nitrogen and lubricating oil consumption, wastewater production, and thermal energy recovery; (5) PSA includes power consumption, nitrogen consumption, compressed air consumption, and zeolite (activated carbon) production and disposal; (6) HDO includes power consumption, nitrogen consumption, compressed air consumption, steam consumption, water consumption, wastewater production, and catalyst production and disposal; (7) CHP generation includes power and thermal energy production, lubricating oil consumption, direct emission, NaOH and water consumption, and wastewater production (8) biochar gasification includes power and thermal energy production, steam consumption, lubricating oil consumption, and direct emission; (9) WWTP is based on a specific dataset on wastewater treatment; (10) phosphorus recovery includes energy consumption, chemical production, resin production and its disposal, steam consumption, wastewater treatment, residual ash disposal, and credits from phosphoric acid production; (11) HDO bio-oil distillation includes thermal energy and power consumption; (12) fuel distribution includes fuel production and direct emissions of the transport; (13) fuel combustion includes the directed emission of the bio-fuels combustion in car engines where the emissions of CO₂ and CH₄ are biogenic; (14) only the operational phase is considered, excluding facility construction and equipment assembly and dismantling; and (15) maintenance is not included. Studies have shown that for long-lived technical systems, the environmental impact of the construction phase is often negligible compared to that of the operation phase [40,41,42].

A detailed Life Cycle Inventory is presented in

Table 2. Input and output flows of the analyzed foreground system.

Categories	Flow	Amount	Unit
Feedstock	Sludge (water content 99% w/w)	4.05 × 101	kg
Chemicals	Polyelectrolyte consumption	6.61 × 10−5	kg
	Sodium hydroxide	3.25 × 10−3	kg
	Hydrochloric acid	1.87 × 10−2	kg
Materials	Silicon carbide	3.52 × 10−3	kg
	Catalyst 1: TK-455	2.32 × 10−4	kg
	Catalyst 2: TK-341	1.85 × 10−4	kg
	Activated coal	5.66 × 10−6	kg
	Cationic resin	5.07 × 10−6	kg
	Lubricating oil	7.62 × 10−5	kg
Utilities	Tap water	8.17 × 10−2	kg
	Water (desalinated; deionized)	1.42 × 10−1	kg
	Nitrogen gaseous	1.23 × 10−2	kg
	Compressed air	1.31 × 10−2	Nm3
	Steam	7.06 × 10−1	kg
Energy	Power	7.19 × 10−1	MJ
	Thermal energy for TCR/PSA/HDO	4.24 × 100	MJ
	Thermal energy from natural gas for other processes	2.48 × 10−2	MJ
Waste	Wastewater	2.17 × 10−1	kg
	Silicon carbide	3.52 × 10−3	kg
	Catalyst 1: TK-455	2.32 × 10−4	kg
	Catalyst 2: TK-341	1.85 × 10−4	kg
	Activated coal	5.66 × 10−6	kg
	Cationic resin	5.07 × 10−6	kg
	Residual ash	1.56 × 10−1	kg
Products	Gasoline	1.00 × 100	MJ
	Electricity	1.03 × 100	MJ
	Thermal energy	1.26 × 100	MJ
	H3PO4	5.52 × 10−2	kg

. Note that the five scenarios differ only in terms of the source used to supply the additional thermal energy for the TCR/PSA/HDO process.

2.2.3. Life Cycle Impact Assessment

With regard to life cycle impact assessment (LCIA), the methods recommended in the ILCD Handbook were applied [43,44]. The following 15 impact categories were included in a first comparison among all scenarios: Global Warming Potential excluding biogenic carbon (GWPebc), Acidification Potential (ACP), Freshwater Aquatic Ecotoxicity Potential (FAETP), Freshwater Eutrophication Potential (FETP), Marine Eutrophication Potential (METP), Terrestrial Eutrophication Potential (ETP), Human Toxicity Potential with cancer effects (HTPc), Human Toxicity Potential with non-cancer effects (HTPnc), Ionizing Radiation Potential with human health impacts (IRPhh), Land Use Change Potential (LUCP), Ozone Layer Depletion Potential (ODP), Respiratory Inorganics Impact Potential with particulate matter (RIPpm), Photochemical Ozone Formation Potential (POFP), Water Resource Depletion Potential (WRDP), and Abiotic Resources Depletion Potential (ADP). The GWP was calculated excluding biogenic carbon, on the basis that biogenic CO₂ was not accounted for either as stored in the feedstock or as released during the oxidation of biofuels and biochar. It was therefore treated as carbon-neutral. In this regard, it is useful to clarify the interpretation of the emissions balance underlying the LCA indicators: (i) a negative impact score reflects avoided environmental impacts resulting from the substitution of other products; (ii) a positive impact score indicates a direct contribution to environmental impacts.

3. Results

The results are presented in three sections: climate change potential, comprehensive environmental profile analysis, and evaluation of the relative contributions of the individual processes comprising the system. The objective is to assess their capacity to reduce greenhouse gas emissions in the transport sector without generating impacts in other categories, and to trace the origin of these impacts to support system design and engineering optimization.

3.1. Climate Change Potential

Figure 2 illustrates the Global Warming Potential (expressed in kg CO₂ eq.) associated with the production of 1 MJ of bio-gasoline for the five TCR/PSA/HDO scenarios. The sixth column represents the impact score of conventional gasoline. Results are compared against the RED III fossil fuel comparator (94 g CO₂ eq./MJ, black dashed line) and the benchmark for maximum allowable emissions (32.9 g CO₂ eq./MJ, red dashed line). From Figure 2, it can be observed that only three TCR/PSA/HDO scenarios fully meet the benchmark. Specifically, the “Heat Pump” scenario shows a positive impact score, indicating a contribution to global warming, whereas the “Wood Gasification” and “Greenhouse” scenarios result in avoids GHG emissions. The “Biogas” scenario falls just short of meeting this criterion, while the “Natural Gas” scenario shows the least favorable performance, even compared with conventional gasoline. A detailed analysis of the contributions behind these results is provided in Section 3.3.

3.2. Comprehensive Environmental Profile

Table 3. Results of LCIA for the five scenarios and conventional gasoline.

Impact Cat.	U.M.	Natural Gas	Biogas	Wood Gasification	Heat Pump	Greenhouse	Conventional Gasoline
GWPebc	kg CO2 eq.	1.35 × 10−1	3.55 × 10−2	−2.22 × 10−2	1.03 × 10−2	−6.79 × 10−2	8.59 × 10−2
ACP	Mole of H+ eq.	−2.21 × 10−3	−4.61 × 10−4	−2.14 × 10−3	−2.21 × 10−3	−2.38 × 10−3	1.46 × 10−4
FAETP	CTUe	1.51 × 100	1.64 × 100	1.74 × 100	1.51 × 100	1.50 × 100	1.21 × 10−2
FETP	kg P eq.	−4.25 × 10−5	−3.45 × 10−5	−3.70 × 10−5	−4.23 × 10−5	−4.26 × 10−5	3.37 × 10−7
METP	kg N eq.	1.61 × 10−4	7.78 × 10−4	1.73 × 10−4	1.40 × 10−4	9.88 × 10−5	3.44 × 10−5
TETP	Mole of N eq.	1.61 × 10−3	7.84 × 10−3	1.71 × 10−3	1.34 × 10−3	9.36 × 10−4	5.32 × 10−4
HTPc	CTUh	−3.79 × 10−8	−3.70 × 10−8	−3.64 × 10−8	−3.80 × 10−8	−3.80 × 10−8	5.31 × 10−10
HTPnc	CTUh	1.60 × 10−7	1.86 × 10−6	1.64 × 10−7	1.57 × 10−7	1.54 × 10−7	4.88 × 10−9
IRPhh	kBq U235 eq.	−7.83 × 10−3	−6.79 × 10−3	5.36 × 10−3	1.63 × 10−2	−2.16 × 10−2	4.33 × 10−4
LUCP	kg C def. eq.	−4.19 × 10−1	2.61 × 100	4.99 × 10−2	−3.91 × 10−1	−4.35 × 10−1	3.26 × 10−2
ODP	kg CFC-11 eq.	−5.60 × 10−9	−5.60 × 10−9	−3.76 × 10−9	−5.59 × 10−9	−5.60 × 10−9	1.19 × 10−14
RIPpm	kg PM2.5 eq.	−1.50 × 10−4	−8.45 × 10−5	−1.43 × 10−4	−1.50 × 10−4	−1.58 × 10−4	5.47 × 10−6
POFP	kg NMVOC eq.	1.75 × 10−4	7.64 × 10−4	2.32 × 10−4	1.00 × 10−4	−9.31 × 10−7	1.05 × 10−4
WRDP	m3 eq.	−9.23 × 10−4	5.69 × 10−3	−8.84 × 10−4	2.28 × 10−3	−3.13 × 10−3	1.73 × 10−4
ADP	kg Sb eq.	−6.50 × 10−7	−5.99 × 10−7	−2.27 × 10−7	−5.22 × 10−7	−8.09 × 10−7	3.48 × 10−8

presents the impact scores of the five scenarios across the 15 impact categories recommended in the ILCD Handbook. Figure 3 was generated to easily catch the general trends; it provides the percentage values of the scenarios, with “Natural gas” chosen as the reference scenario. Note that “Natural gas” scenario is set to 100% or −100% depending on whether it generates an impact or an avoided impact. As it is possible to observe from Figure 3, the results are very heterogeneous. To facilitate their discussion, three groups of impact categories have been identified: (1) the ones where all five scenarios perform much better than the conventional fuel, such as ACP, FETP, HTPc, ODP, RIPpm, and ADP; (2) the ones where all five scenarios perform much worse than the conventional fuel, such as FAETP, METP, TETP, and HTPnc; (3) the ones where some scenarios perform much better and other scenarios perform much worse than the conventional fuel, such as IRPhh, LUCP, WRDP, GWPebc, and POFP.

Note that to quantify the performance of the TCR/PSA/HDO scenarios relative to conventional gasoline, it is sufficient to normalize the impact scores of the various scenarios to that of gasoline (Table 3).

The impact categories of the first group for the TCR/PSA/HDO scenarios all show negative impact scores, indicating avoided environmental impacts, whereas the conventional gasoline scenario exhibits positive scores. In contrast, the conventional gasoline scenario shows values ranging from approximately <1% to 30% of those observed in the TCR/PSA/HDO scenarios of the second group. All of these scenarios result in emissions or material consumption, with none leading to avoided environmental impacts. In Group 3, the situation varies considerably across scenarios, with some leading to negative impacts and others to positive ones.

The results indicate that none of the TCR/PSA/HDO scenarios outperform conventional fuel in all 15 impact categories. The presence of multiple trade-offs underscores the importance of evaluating the environmental impact of technologies and products through a comprehensive life cycle perspective that encompasses the full spectrum of environmental interactions between human activities and ecosystems. This approach is preferable to focusing solely on a few impact indicators, such as greenhouse gas emissions and abiotic resource depletion.

The impact score values further indicate that the “Greenhouse” scenario consistently exhibits the highest performance among the five TCR/PSA/HDO scenarios, both in terms of generated and avoided impacts. Conversely, no single scenario can be identified as the overall worst performer. Each of the remaining four scenarios emerges as the least favorable in at least one impact category. Among them, the “Biogas” scenario shows the lowest performance across nine categories, whereas “Natural gas” and “Heat pump” rank lowest in only one (Global Warming Potential).

A further consideration is that while in impact category Groups 1 and 2 the orientation (positive or negative) remains the same across scenarios despite differences in scores, in other categories—specifically Group 3—some scenarios exhibit opposite behaviors: while certain scenarios generate impacts, others contribute to their avoidance.

3.3. Contribution Analysis

Figure 4, Figure 5 and Figure 6 show the percentage contribution of each of the 13 processes into which the system is divided (see Section 2.2). The impact categories are grouped according to the same three groups described earlier. As previously noted, the impact categories in Group 1 show significantly better performance for the TCR/PSA/HDO scenarios compared to conventional gasoline. As shown in Figure 4, this improvement is entirely attributable to phosphorus recovery (orange component), which allows for the avoidance of the conventional treatment of phosphate rock to obtain phosphoric acid. Without the resource and emission savings associated with phosphorus recovery from the gasification ash, all impact categories would perform worse than those of conventional gasoline. The avoidance of phosphoric acid production via the conventional process results in the preservation of phosphate mineral resources and the prevention of emissions of acidifying substances (e.g., SO₂, SO₃, NO_x), eutrophying compounds (e.g., phosphates), organic pollutants (e.g., aromatic compounds, phthalates, carboxylic acids) [45,46], and heavy metals (e.g., Cu, Cd, Fe, Zn, Pb) [47,48], as well as particulate matter.

Group 2 comprises the impact categories where the performances of the TCR/PSA/HDO scenarios are noticeably inferior to that of conventional gasoline (Figure 5). Here as well, phosphorus recovery (orange component) plays a significant role, substantially affecting the total impact score. However, its behavior varies considerably across the four impact categories. In the case of human toxicity and aquatic ecotoxicity, all processes involved—including phosphorus recovery—contribute to increasing the scores. Conversely, phosphorus recovery in marine and terrestrial eutrophication has a negative score, thanks to avoiding conventional phosphorus production, which partially reduces the total score. The avoided impacts on eutrophication are primarily linked to the reduction of nitrogen emissions, credited to the substitution of mineral phosphoric acid. Conversely, although the avoided production of phosphoric acid results in negative impact scores, these are offset by the toxic and ecotoxic emissions associated with the production of hydrochloric acid required for phosphorus extraction from biochar ash. In Group 2, processes other than phosphorus recovery also play a significant role. In the ecotoxicity and human toxicity impact categories, HDO and wastewater treatment contribute approximately 30–40% of the total score. On the other hand, in the categories of marine and terrestrial eutrophication, important contributions come from fuel use (light green component), biochar gasification (dark gray component), combined heat and power production (CHP, yellow component), HDO (blue component), and wastewater treatment (green component). A special case is the biogas scenario: here, the drying process is by far the most impactful, making the contributions from the other processes comparatively minor. Indeed, the impact of the drying process varies considerably depending on the TCR/PSA/HDO scenario; on the contrary, the contribution from fuel combustion (“use phase”) is very similar between the TCR/PSA/HDO scenarios and conventional gasoline.

Group 3 includes the impact categories in which conventional gasoline exhibits intermediate performance compared to the TCR/PSA/HDO scenarios. Once again, in these categories, phosphorus recovery (orange component) plays an important role; however, in this case, other processes also contribute significantly to the final score—namely, sludge drying (light gray component), combined heat and power (CHP, yellow component) production, biochar gasification (dark gray component), and fuel use (light green component). Since the scores associated with these processes vary significantly across scenarios, the final results show a highly uneven trend from one scenario to another. Clearly, the process that most strongly influences this variability is sludge drying, or more precisely, the energy source used for drying. With regard to the impact associated with ionizing radiation, drying contributes very significantly when the energy source is either directly (heat pump) or indirectly (wood gasification) electricity. Indeed, in the European electricity mix, a substantial share of power is generated from nuclear energy. Other impact categories are also influenced—though to varying degrees—by the drying process, depending on the energy source employed. Land use impact is particularly high in the biogas scenario, which relies on dedicated energy crops. For water depletion, the biogas and heat pump scenarios stand out due to the significant water demand associated with dedicated crop production and electricity generation, respectively. The Photochemical Ozone Formation Potential is mainly driven by direct or indirect emissions of NO_x and VOCs linked to energy use and is particularly important in the biogas scenario. Finally, the highest greenhouse gas emissions are associated with natural gas, followed by biogas and electricity. By contrast, energy recovery from CHP and biochar gasification in the greenhouse scenario results in avoided emissions of ionizing radiation and greenhouse gases, as well as water savings, thereby reinforcing the negative score associated with phosphorus recovery. Finally, it should be noted that the use process of conventional gasoline (light green component) leads to high greenhouse gas emissions, in contrast to what occurs in all the TCR/PSA/HDO scenarios.

4. Discussion

The LCA results demonstrate that the scenario representing the configuration of the TCR/PSA/HDO integrated system, as implemented in the Hohenberg demonstrator—specifically, the “Wood gasification” scenario—complies fully with the greenhouse gas emission limits established by the RED III Directive for biofuels. However, such a scenario is not representative of the industrial thermal energy demand within the European Union—and in most other regions worldwide. Indeed, according to a recent Position Paper by the EHPA [49], 80% of the heat used in EU industrial sectors is still derived from fossil fuels, with approximately 60% coming from natural gas [50]. Therefore, the “Natural gas” scenario best represents the current European context but is the least aligned with the sustainability criteria defined by the RED III Directive. Nonetheless, it is important to note that this share is gradually decreasing in favor of renewable sources [51]. In 2021, bioenergy was the leading source of non-fossil industrial process heat, particularly within the forest-based industries [52], accounting for approximately 10% of total industrial heat consumption. Consequently, the “Wood gasification” scenario—beyond reflecting the configuration of the demonstrator—can be considered both plausible and partially representative of existing practices within the European context. Currently, a small additional share of the heat used in the industrial sector comes from electricity. Another recent position paper by the EHPA [53] states that industrial heat pumps could already meet around 10% of the industry’s total final energy demand, 60% of which is heat. According to the EHPA [49], 62% of the heat currently used in the industrial sector could be electrified using existing technologies. Given these figures, the “Heat Pump” scenario appears both feasible and relevant. This scenario meets the requirements of the RED III Directive for biofuels, although, unlike the “Wood Gasification” scenario, it does not result in avoided emissions at the considered energy mix level. Higher levels of RES penetration in the energy mix are expected to deliver higher performance. The “Biogas” scenario appears less attractive, as currently less than 0.1% of total industrial heat comes from biogas [54] and LCA results barely meet the target set by the directive. However, the share of biogases in total gaseous fuel demand could grow from 1% in 2023 to around 5–10% by 2050 [55]. The fifth and final scenario analyzed is the “Greenhouse” scenario. It includes all cases in which sludge drying occurs without the use of additional thermal energy, relying only on a minimal amount of electricity. As shown in Figure 6, this scenario results in the lowest GHG emissions and is the only one that includes avoided emissions not solely attributable to phosphorus recovery (represented by the orange component in Figure 6). The growing focus on technologies that enable sludge drying through the use of solar energy is making the “Greenhouse” scenario an increasingly concrete and viable option, rather than a futuristic concept. Currently, numerous initiatives are underway to develop and test solar-powered sludge drying systems [56,57,58]. In Europe, several industrial-scale facilities are already operational or under construction. A large-scale solar drying plant has been operating in Bayreuth, Bavaria since 2016, while a smaller facility is active in Łomianki, Poland [59]. Another large solar–thermal plant is under construction in Bottrop, Germany [60], and additional greenhouse-based sludge drying systems are being developed in Apulia, Italy [61]. This growing body of evidence confirms that the Greenhouse scenario is no longer theoretical—it is becoming a realistic and scalable solution.

Another highly relevant aspect emerging from the results concerns the process of phosphorus recovery from the ashes obtained via gasification of char. As shown in Figure 4, Figure 5 and Figure 6, none of the five scenarios analyzed would meet the targets set by the RED III Directive without the contribution of avoided emissions resulting from phosphorus recovery (orange portion of the bars). Currently, phosphorus recovery is mandated only by a few national regulations in Europe—Germany [62], Switzerland [63], and Austria [64]—while in some other countries, legislative proposals (Sweden [65]), management plans (France [66]), or national platforms (Italy [67]) are under development. Industrial-scale facilities are still limited, not only due to technological constraints but also due to economic and regulatory barriers [68]. Nonetheless, it is evident that the future direction will be to exploit the phosphorus content in sewage sludge, as wastewater represents a significant secondary source of this element [69], it is a non-renewable resource included by the European Union in the list of critical raw materials [70], and recovery efforts are fully aligned with the circular economy policies promoted by the EU [71,72]. It is noteworthy that among the several technologies studied to extract phosphorus from municipal sewage sludge, the recovery from sludge ash is one of the most promising [73]. In 2024, the Altenstadt mono-incineration plant (Germany) was upgraded with a thermochemical process under the R-Rhenania project, achieving a production capacity of 15,000 t/year of phosphate fertilizers [74]. What has just been illustrated confirms the hypothesis that, within a few years, phosphorus recovery from sewage sludge ash will become a concrete reality across much of Europe, making the assumption formulated in this study both plausible and well-founded. The data used in this study are literature-based and derived from a research project [36]; therefore, both inventory data and impact scores carry a high degree of uncertainty. However, phosphorus recovery will undoubtedly lead to significant reductions in environmental impacts compared to the mining of phosphate rock [75,76].

A third topic to explore is the implications of using sewage sludge from municipal wastewater treatment as feedstock for the TCR/PSA/HDO system. As previously explained, sewage sludge typically contains more than 90% water, making it a challenging feedstock for pyrolysis and gasification processes, as it requires drying before being subjected to thermochemical conversion—an operation that entails high energy demand [77,78,79]. On the other hand, sewage sludge represents a valuable resource from a bioeconomy perspective and fits well within the principles of the circular economy. First of all, it is an abundant and ubiquitously available material across Europe: in 2019, sewage sludge production in Europe reached 8.1 million tons per year (Mt/y) of dry matter [80]. A slight increase is expected in the coming years, driven by the expansion of sewer networks, population growth, and improvements in wastewater treatment efficiency [24]. These huge quantities need to be managed, and their disposal as waste is both costly and complex [81]. Directive 1999/31/EC prohibits the landfilling of liquid or non-stabilized organic waste and requires pre-treatment before disposal [82]. In general, European countries prioritize agricultural and/or energy recovery [24]. Indeed, municipal sewage sludge is rich in organic carbon, making it attractive for bioeconomic and bioenergetic applications, and it also contains nutrients such as nitrogen, potassium, and phosphorus, a particularly relevant element for agriculture. One of the main challenges to be addressed for the proper valorization of sewage sludge is the possible presence of organic and inorganic contaminants [82,83,84,85]. This issue becomes even more critical in the ashes generated by the thermal treatment of sludge due to the accumulation of specific pollutants [79,86]. To address these issues, one option would be to apply the biochar from the TCR/PSA/HDO process directly in agriculture rather than gasifying it. This approach would enable the destruction of organic pollutants [87,88], facilitate long-term carbon storage in soils [89,90,91,92], and return nutrients (mainly phosphorous) to the cycle. It should be noted, however, that limit values for heavy metals must be respected, and that in some EU countries this type of use is not permitted under current regulations [20,24].

5. Conclusions

This study shows that the TCR/PSA/HDO configuration with wood gasification, as in the Hohenberg demonstrator, complies with RED III and is partly representative of current practices. The natural gas scenario best reflects today’s context but is the least sustainable, while heat pumps and bioenergy appear as promising alternatives, and the greenhouse scenario stands out as the most advantageous. Overall, renewable-based heat supply and innovative drying technologies seem the most effective strategies to align biofuel production with European sustainability targets.

Phosphorus recovery from sewage sludge ash is also crucial for the TCR/PSA/HDO system to meet RED III requirements, as none of the scenarios would comply without the avoided emissions it provides. Despite uncertainties in the available inventory data, recovery offers clear environmental benefits, and recent initiatives and pilot projects indicate that large-scale implementation is becoming increasingly feasible.

Finally, sewage sludge is a challenging yet valuable feedstock. Its high water content demands energy-intensive drying, but its abundance and nutrient content make it a key resource in the circular bioeconomy. Sustainable management strategies, including phosphorus recovery and potential biochar applications, are essential to reduce disposal burdens while enhancing energy recovery and nutrient recycling.