Environmental Trade-Offs in Phosphorus Recovery: A Comparative LCA of Pyrolysis and Hydrothermal Carbonization of Poultry Manure

Abstract

Phosphorus is a non-renewable resource critical for global food security, yet its natural reserves are rapidly depleting. Meanwhile, the poultry industry generates vast amounts of nutrient-rich waste that pose serious environmental risks if not managed properly. While valorizing these wastes offers a sustainable raw material alternative, investigating the environmental impacts of recovering them as a phosphorus source is crucial. This study evaluates phosphorus recovery from poultry litter via acid leaching following Hydrothermal Carbonization (HTC) and pyrolysis processes holistically. By conducting a Life Cycle Assessment (LCA) using this specific substrate and method combination, this work aims to provide comprehensive environmental insights. The impact assessment reveals that the total Global Warming Potential (GWP) is 6.00 kg CO₂ eq for the pyrolysis scenario and 4.18 kg CO₂ eq for the HTC scenario. Methodologically, a ‘system expansion’ approach was applied to integrate the avoided burdens from poultry manure management into the system boundaries. Furthermore, the inventory analysis revealed that chemical consumption (specifically NaOH and H₂SO₄) in the production process is the dominant factor not only for Global Warming Potential (GWP) but also across other environmental impact categories evaluated. The findings clearly indicate that chemical intensity predominantly determines the environmental performance across carbon footprint, acidification and other environmental impact categories.

1. Introduction

Phosphorus is an essential and non-substitutable nutrient for plant metabolism, playing a fundamental role in energy transfer, photosynthesis, and biomass formation; therefore, it is indispensable for agricultural productivity, crop quality, and ultimately human food security [1]. The steadily increasing global demand for phosphorus, coupled with the geographical concentration of phosphate rock reserves and market volatility, has raised significant concerns regarding long-term supply security. In response to these risks, the European Commission designated phosphorus as a critical raw material in 2014, highlighting its economic importance and vulnerability to supply disruption, and thereby catalyzing intensified research efforts into phosphorus recovery technologies as well as the reinforcement of circular waste management policies. Additionally, the limited availability of phosphate rock resources and forecasts indicating the imminent depletion of economically viable reserves have expedited the investigation of secondary and alternative phosphorus sources, especially those originating from agricultural residues, animal manure, and wastewater streams [2]. Currently, the agricultural fertilizer and animal feed industries consume more than 80% of the approximately 150 million tons of phosphate rock produced annually [3,4,5].

At the same time, the rapid growth of poultry and livestock production in recent decades has resulted in a significant increase in animal manure generation, causing serious environmental challenges. The traditional land application of poultry manure has become increasingly problematic due to excessive nutrient loading, which emphasizes the need to implement sustainable management and disposal strategies [6]. In this context, phosphorus recovery from phosphorus-rich waste streams—such as poultry manure, meat and bone meal, sewage sludge, and their incineration ashes—has emerged as one of the most promising approaches to enhance phosphorus resource security while simultaneously mitigating environmental impacts [7,8]. Hydrothermal carbonization (HTC) and pyrolysis stand out as two fundamental thermochemical methods for the valorization of animal wastes like poultry manure, offering significant potential for both energy conversion and nutrient recovery [9]. Acid leaching of biochar or hydrochar produced from pyrolysis or HTC releases phosphorus, which makes it possible to acquire more of it back. Pyrolysis is better for making biochar or activated carbon with a lot of surface area, but it uses more energy than HTC because it needs to be done at high temperatures and the feedstock needs to be dried first [10]. Conversely, HTC can operate with lower energy requirements as it does not require pre-drying for wet feedstocks; however, it introduces different environmental burdens due to additional steps such as process water treatment and chemical usage, which can lead to increased greenhouse gas emissions and potential water pollution [10,11].

Evaluating phosphorus recovery technologies through Life Cycle Assessment (LCA) is a critical point for revealing all these environmental burdens. LCA studies examining the environmental impacts of phosphorus recovery processes have focused on technological development, raw material diversity, and methodological approaches over the last decade. These studies have primarily concentrated on comparing recovery technologies with conventional mineral fertilizers and identifying environmental “hotspots” [12]. In the reviewed literature, the preferred waste types for phosphorus recovery are urban wastewater and sewage sludge, while the most applied and compared recovery methods are identified as chemical precipitation (e.g., struvite) and wet chemical extraction (acid leaching) [13,14,15]. On the other hand, Linderholm et al. [16] examined phosphorus recovery from ash obtained by sludge incineration (Ash Dec process) in Swedish scenarios. Similarly, Behjat et al. [17] specifically investigated “dairy wastewater” and compared it with urban wastewater. Previous studies have demonstrated that phosphorus content varies significantly across different substrates, which directly influences the quantity of chemical inputs and, consequently, the environmental impacts of the recovery process. For instance, Behjat et al. [17] determined that the phosphorus content of municipal wastewater is generally lower than that of dairy industry wastewater, highlighting the importance of feedstock selection. Furthermore, Amann et al. [13] emphasized that in low-concentration streams, the increased chemical demand is the dominant factor driving the cumulative energy demand and global warming potential of the process. Consequently, poultry manure was chosen as the feedstock for this study because it has a much higher phosphorus content than sewage sludge and wastewater. This confers on it a stoichiometric advantage in terms of reducing chemical use per unit of recovered phosphorus.

Furthermore, comprehensive studies exist in the literature regarding the disposal of poultry litter via thermochemical methods. For instance, Bora et al. [18] compared six different technologies, including pyrolysis, gasification, and hydrothermal carbonization (HTC), from both environmental (LCA) and techno-economic (TEA) perspectives, demonstrating the superiority of these methods over direct land application. Additionally, Preuss and You [19] focused on the significance of system boundaries, evaluating the environmental performance of technologies such as pyrolysis, HTC, and hydrothermal liquefaction (HTL) by comparing attributional and consequential LCA methods. Similarly, Zhao et al. [20] concentrated on the economic and environmental optimization of pyrolysis-based systems. However, the majority of these studies have modeled the direct utilization of the resulting solid products (biochar or hydrochar) as soil amendments. The integration of an acid leaching process to recover phosphorus with higher efficiency and purity from the solid products obtained after thermochemical treatment, along with the environmental implications of this specific integration, has not yet been addressed in detail in the literature. To address this critical research gap, this study evaluates phosphorus recovery from poultry litter via acid leaching following HTC and pyrolysis processes holistically. By conducting a life cycle assessment using this specific substrate and method combination, this work aims to provide comprehensive environmental insights, thereby filling the missing link in the current body of knowledge.

2. Materials and Methods

2.1. Goal and Scope Definition

The Life Cycle Assessment (LCA) study was conducted in accordance with the ISO 14040 and 14044 standards [21]. The primary goal of this study is to evaluate and compare the environmental performance of two thermochemical valorization pathways—pyrolysis and hydrothermal carbonization (HTC)—for phosphorus (P) recovery from poultry manure. The study specifically aims to quantify the trade-offs between the energy-intensive pre-drying step of pyrolysis and the chemical-intensive downstream processing of HTC.

• Functional Unit

To ensure a consistent basis for comparison, the functional unit (FU) was defined as ‘1 kg of recovered phosphorus (P)’ in the form of amorphous calcium phosphate precipitate. Consequently, all mass and energy flows in the inventory analysis were normalized to this FU. This approach allows for a direct and standardized comparison of the environmental burden per unit of recovered nutrient, independent of the total mass of the precipitate.

• System Boundaries

The system boundaries were defined as “gate-to-gate” (Figure 1).

Included processes: Raw material acquisition, pre-processing (thermal drying for pyrolysis), thermal conversion (pyrolysis at 300 °C and HTC at 220 °C), and downstream phosphorus extraction (acid leaching and precipitation).

Excluded processes: Construction and decommissioning of the infrastructure (capital goods), transportation of feedstock to the facility, and the final application of the recovered phosphorus product to soil (use phase), as the P-bioavailability is assumed to be comparable for both scenarios.

2.2. Life Cycle Inventory (LCI) Analysis

The Life Cycle Inventory (LCI) phase involved the collection and calculation of data to quantify inputs and outputs for the defined system.

2.2.1. Data Collection and Assumptions

Data regarding reaction temperatures, residence times, acid concentrations, and product yields were obtained from the laboratory experiments [9].

Pyrolysis Scenario: It was assumed that wet chicken manure (25% moisture) undergoes thermal drying to <10% moisture prior to pyrolysis. The drying energy was calculated based on the latent heat of water vaporization, assuming an industrial dryer efficiency of 70%.

HTC Scenario: The process was modeled to treat wet biomass directly. The reactor heating energy was calculated using the specific heat capacity of the biomass–water mixture.

Leaching & Precipitation: For both scenarios, an optimized counter-current leaching system was simulated to reduce water and acid consumption compared to lab-scale batch experiments. The chemical requirement for neutralization was calculated stoichiometrically using Sodium Hydroxide (NaOH).

The rationale for selecting 300 °C (pyrolysis) and 220 °C (HTC) is grounded in the experimental optimization data provided by Topçu et al. [9]. These specific temperatures were identified as the optimal operating conditions, representing the most efficient balance between ‘Maximum Product Yield’ and ‘Maximum Phosphorus Recovery Potential.’

2.2.2. Scale-Up Framework

Since the experimental data were obtained at a laboratory scale, a rigorous scale-up framework was applied to model a hypothetical industrial-scale plant (1 ton dry matter/day capacity) (

Table 1. Key design parameters and assumptions used for the industrial-scale modelling.

Parameter	Value/Assumption	Reference
Scale-up Methodology	Bottom-up Approach	[22]
Initial Moisture Content	25% (wet basis)	Experimental/Field Data [9]
Drying Thermal Efficiency	70%	[23]
Latent Heat of Water	2260 kJ/kg	Thermodynamic Constant
Specific Heat Biomass	1.6 kJ/kg °C	[24]
Reactor Efficiency	85% (Electric heating)	[25]
Acid Consumption (Leaching)	Stoichiometric + 10% excess	Based on lab optimization (0.1 M) [9]
Neutralization Agent	NaOH (Stoichiometric)	Chemical calculation

). The scale-up procedure followed the methodology proposed by Piccinno et al. [22], adopting a two-step approach:

Mass Balance: Process yields (char production, P-extraction efficiency) were derived directly from the laboratory experiments.

Energy Balance: Energy consumption was modeled based on thermodynamic principles rather than direct lab-scale measurements, which often reflect heat losses typical of small equipment, thus providing a more accurate representation of energy efficiency in larger-scale operations.

Laboratory devices, such as small-scale tubular furnaces, do not accurately represent industrial realities when measuring electricity consumption due to significant heat losses and low efficiency. Therefore, energy consumption was modeled theoretically based on the following assumptions.

In the pyrolysis scenario, the energy required to reduce the moisture content of chicken manure from 25% to below 10% was calculated using the latent heat of vaporization of water (2260 kJ/kg) and the sensible heat needed to reach the boiling point. An industrial dryer thermal efficiency of 70% was assumed, consistent with reference (

Table 2. Life Cycle Inventory based on the Functional Unit.

Parameter/Variable	Unit	Pyrolysis (PYR)	HTC	Data Source/Calculation Method
A. Process performance assumptions
Process temperature	°C	300	220	[9]
Leaching (recovery) efficiency	%	90	95	[9]
P content	%	1.8	2.1	Assumption
Solid Yield	%	65%	55%	[9]
B. Functional unit calculation (1 kg P recovered)
Target output	kg P	1.0	1.0	Functional unit
Required solid product	kg	62.6	48.4	Formula: 1/P Content × Leaching Efficiency
Required feedstock (manure)	kg	94.9	88	Formula: Required Char Mass/Solid Yield
Added water for HTC process			50 L	Moisture of the feedstock (52%)
C. Energy modeling assumptions
Feedstock moisture content	%	25	25	Initial moisture content
Target moisture after drying	%	<10	Not applicable	Drying required for pyrolysis; unnecessary for HTC
Dryer thermal efficiency	%	70	–	[23]
Reactor/furnace efficiency	%	85	85	[25]
D. Life cycle inventory inputs (SimaPro)
Total electricity consumption	kWh	39.5 Drying Energy Reactor Energy	25.9	Thermodynamic estimation
Sulfuric acid (H2SO4)	kg	6.1	4.7	Stoichiometric consumption proportional to processed solid mass
Sodium hydroxide (NaOH)	kg	5.0	3.9	pH neutralization requirement
Process water (tap water)	m3	0.90	1.95	Liquid-to-solid ratio (L/S = 10) during leaching and washing
OUTPUTS
Gas Emission (Water Vapor)	kg	186.3	-

) [23].

For reactor heating, the energy needed to elevate the feedstock to reaction temperatures—300 °C for pyrolysis and 220 °C for hydrothermal carbonization (HTC)—was determined using the specific heat capacity (Cp) equation (Q = m. Cp. ΔT). The specific heat capacities of biomass and water were considered as 1.6 kJ/kg °C and 4.18 kJ/kg °C, respectively [25]. An energy efficiency of 85% for the reactor was applied in the industrial-scale scenario.

To address the multifunctionality of the system, the system expansion method was employed in accordance with ISO 14040. As noted by Topçu et al. [9], direct land application is the traditional management approach for poultry manure. However, this method is deemed unsustainable due to the uncontrolled greenhouse gas emissions and nutrient runoff into water bodies it may create. Consequently, the SimaPro inventory analysis modeled the raw material input as an “avoided process,” representing the environmental credit obtained from diverting waste from conventional disposal rather than treating it as an environmental burden through a system expansion approach. The “Laying hen, 17 weeks, wet manure management” dataset was selected as the baseline reference scenario.

It is important to emphasize that if manure were classified as a valuable by-product (for example, in a market with established manure trading), an allocation procedure would need to be implemented. In an economic allocation scenario, a portion of the impacts from feed production and coop operations would be assigned to the manure, thus increasing the input environmental load of the treatment processes. However, given the current lack of a structured manure market and the prevalence of open storage in the region, classifying manure as waste provides a more accurate representation of the environmental benefits, which raises the possibility of thermochemical conversion to mitigate existing disposal emissions.

2.3. Life Cycle Impact Assessment (LCIA)

The LCIA phase was performed using SimaPro 9.4.0.3 software. The ReCiPe 2016 Midpoint (H) method was selected for impact characterization due to its broad global applicability. The 18 impact categories in SimaPro software were evaluated as they are most relevant to waste management and nutrient recovery systems.

3. Results

The environmental impacts of the phosphorus recovery systems were evaluated based on the functional unit of 1 kg of recovered phosphorus. The results are presented separately for the Pyrolysis and Hydrothermal Carbonization (HTC) scenarios, followed by a comparative analysis.

3.1. Impact Assessment Results for Pyrolysis Scenario

The impact assessment for the pyrolysis scenario reveals that the total Global Warming Potential (GWP) is 6.00 kg CO₂ eq (

Table 3. Environmental impact results and process contribution analysis for 1 kg recovered phosphorus (Pyrolysis).

Impact Category	Unit	Total	Sulfuric Acid (RoW)	Tap Water (RoW)	Pyrolysis	Sodium Hydroxide (Chlor-Alkali)	Electricity (TR, Medium Voltage)
Global warming	kg CO2 eq.	6.00	1.00	0.526	−0.507	4.35	0.632
Stratospheric ozone depletion	kg CFC-11 eq.	6.5 × 10−7	6.71 × 10−7	4.91 × 10−7	−4.32 × 10−6	3.62 × 10−6	1.85 × 10−7
Ionizing radiation	kBq Co-60 eq.	0.366	0.0665	0.0292	−0.0255	0.291	0.0043
Ozone formation, human health	kg NOx eq.	0.0157	0.0062	0.00124	0.000409	0.00646	0.00144
Fine particulate matter formation	kg PM2.5 eq.	0.0262	0.0141	0.000997	0.00384	0.00243	0.00487
Ozone formation, terrestrial ecosystems	kg NOx eq.	0.0158	0.0063	0.00132	0.000236	0.00551	0.00145
Terrestrial acidification	kg SO2 eq.	0.0493	0.0451	0.00161	−0.00842	0.00825	0.00283
Freshwater eutrophication	kg P eq.	0.00355	0.00193	0.000254	0.000487	0.000221	0.000652
Marine eutrophication	kg N eq.	−0.000342	5.11 × 10−5	1.78 × 10−5	−0.00051	5.92 × 10−5	4.02 × 10−5
Terrestrial ecotoxicity	kg 1,4-DCB eq.	161	156	2.28	−0.893	2.88	0.425
Freshwater ecotoxicity	kg 1,4-DCB eq.	1.39	1.39	0.0317	−0.0525	0.00518	0.0231
Marine ecotoxicity	kg 1,4-DCB eq.	1.89	1.81	0.0415	0.00507	0.00774	0.0311
Human carcinogenic toxicity	kg 1,4-DCB eq.	0.462	0.273	0.0999	0.0374	0.0116	0.0400
Human non-carcinogenic toxicity	kg 1,4-DCB eq.	28.4	26.4	0.506	0.426	0.294	0.751
Land use	m2 a crop eq.	−0.63	0.0942	0.0904	−0.934	0.115	0.00463
Mineral resource scarcity	kg Cu eq.	0.10	0.0907	0.00927	−0.000595	0.000743	0.000276
Fossil resource scarcity	kg oil eq.	1.90	0.392	0.103	0.103	1.14	0.160
Water consumption	m3	1.49	0.0838	0.907	−0.0219	0.518	0.0046

). As shown in the process contribution analysis, the environmental hotspots are heavily concentrated in the chemical consumption stage rather than energy usage. The consumption of sodium hydroxide (NaOH) for neutralization is the single largest contributor, accounting for 4.35 kg CO₂ eq (approximately 72% of the total GWP). Additionally, sulfuric acid production contributes 1.00 kg CO₂ eq. This high chemical burden is attributed to the lower phosphorus extraction efficiency of biochar, which necessitates higher acid volumes and subsequent neutralization agents to achieve the functional unit. Electricity consumption contributes a relatively minor share of 0.632 kg CO₂ eq, reflecting the assumed auto-thermal operation of the industrial pyrolysis reactor. However, the scenario exhibits a high-water consumption impact of 1.49 m³, driven by the high liquid-to-solid ratio required during the leaching of biochar. The avoided product credit provides a reduction of −0.507 kg CO₂ eq, which partially offsets the process burdens but is insufficient to neutralize the high chemical impacts.

The contribution analysis indicates that the environmental performance of the phosphorus recovery system is primarily driven by the intensity of chemical reagent use (Figure 2). Sulfuric acid consumption emerges as the dominant contributor across multiple impact categories. Sulfuric acid production and use substantially influence freshwater eutrophication, terrestrial acidification, and both carcinogenic and non-carcinogenic human toxicity categories. Sulfur dioxide emissions, high energy demand, and trace metal releases associated with sulfuric acid manufacturing closely link these impacts. In addition, indirect effects related to water use during acid production and leaching stages contribute noticeably to the overall water consumption category, particularly through the depletion of local water resources and potential impacts on aquatic ecosystems.

Sodium hydroxide use plays a secondary yet non-negligible role in shaping the environmental profile of the process. Due to its production via the energy-intensive chlor-alkali process, NaOH contributes primarily to toxicity-related categories, including non-carcinogenic human toxicity as well as freshwater and marine ecotoxicity. However, the results clearly show that sodium hydroxide does not act as a primary environmental driver; instead, it serves as a complementary contributor that amplifies the impacts associated with the chemical recovery stages.

Electricity consumption mainly affects energy-related impact categories, particularly climate change and fossil resource scarcity. Emissions associated with electricity generation, including CO₂ (carbon dioxide) and NO_x (nitrogen oxides) from fossil-based energy sources, contribute to climate change and ozone formation, which can harm human health and terrestrial ecosystems. Nevertheless, the influence of electricity use on the overall environmental performance remains limited due to offsetting effects generated by energy recovery and avoided upstream processes, such as the use of renewable energy sources that reduce emissions and the implementation of energy efficiency measures that lower overall consumption.

The pyrolysis stage serves a crucial balancing role in the overall life cycle performance of the system. Pyrolysis negatively impacts mineral resource scarcity and fossil resource scarcity through energy recovery and fossil fuel substitution. Additionally, these substitution effects lead to net environmental benefits in terrestrial, freshwater, and marine ecotoxicity categories by decreasing dependence on virgin materials and reducing overall emissions linked to resource extraction and processing. This finding emphasizes that pyrolysis functions not only as a waste treatment technology but also as an effective means of mitigating life cycle environmental burdens via energy and resource substitution.

3.2. Impact Assessment Results for HTC Scenario

The Hydrothermal Carbonization (HTC) scenario demonstrates a lower total GWP of 4.18 kg CO₂ eq. The distribution of environmental burdens in this scenario differs significantly from the pyrolysis route, characterized by a trade-off between electrical energy and chemical efficiency. Electricity consumption is responsible for 1.17 kg CO₂ eq. This aligns with the theoretical expectations for HTC, where heating the aqueous feedstock slurry requires substantial energy input despite heat recovery assumptions. Sodium hydroxide contribution is the main emission source at 3.39 kg CO₂ eq, and sulfuric acid contributes 0.773 kg CO₂ eq. These values are notably lower than in the pyrolysis scenario, confirming that the higher phosphorus availability in hydrochar allows for more efficient extraction with reduced chemical inputs. A critical finding is the substantial environmental credit of −1.26 kg CO₂ eq generated through system expansion (

Table 4. Environmental impact results and process contribution analysis for 1 kg recovered phosphorus (HTC).

Impact Category	Unit	Total	Sulfuric Acid (RoW)	Tap Water (RoW)	HTC	Sodium Hydroxide (Chlor-Alkali)	Electricity (Medium Voltage, TR)
Global warming	kg CO2 eq.	4.18	0.773	0.103	−1.26	3.39	1.17
Stratospheric ozone depletion	kg CFC-11 eq.	−1.59 × 10−6	5.17 × 10−7	9.60 × 10−8	−5.37 × 10−6	2.83 × 10−6	3.42 × 10−7
Ionizing radiation	kBq Co-60 eq.	0.257	0.0512	0.00571	−0.0350	0.227	0.00796
Ozone formation, human health	kg NOx eq.	0.0117	0.00477	0.000242	−0.000992	0.00504	0.00267
Fine particulate matter formation	kg PM2.5 eq.	0.0216	0.0109	0.000195	−0.000393	0.00190	0.00901
Ozone formation, terrestrial ecosystems	kg NOx eq.	0.0117	0.00486	0.000258	−0.00121	0.00507	0.00269
Terrestrial acidification	kg SO2 eq.	0.0337	0.0347	0.000315	−0.0130	0.00644	0.00524
Freshwater eutrophication	kg P eq.	0.00283	0.00149	4.97 × 10−5	−8.38 × 10−5	0.000173	0.00121
Marine eutrophication	kg N eq.	−0.000489	3.93 × 10−5	3.48 × 10−6	−0.000653	4.62 × 10−5	7.43 × 10−5
Terrestrial ecotoxicity	kg 1,4-DCB eq.	122	120	0.445	−1.51	2.25	0.787
Freshwater ecotoxicity	kg 1,4-DCB eq.	1.03	1.07	0.00619	−0.0867	0.00404	0.0428
Marine ecotoxicity	kg 1,4-DCB eq.	1.44	1.39	0.00812	−0.0258	0.00603	0.0575
Human carcinogenic toxicity	kg 1,4-DCB eq.	0.317	0.211	0.0195	0.00385	0.00904	0.0740
Human non-carcinogenic toxicity	kg 1,4-DCB eq.	21.8	20.4	0.0989	−0.259	0.229	1.39
Land use	m2 a crop eq.	−0.936	0.0726	0.0177	−1.12	0.0897	0.00857
Mineral resource scarcity	kg Cu eq.	0.0718	0.0699	0.00181	−0.000997	0.000580	0.000511
Fossil resource scarcity	kg oil eq.	1.47	0.302	0.0202	−0.0396	0.889	0.295
Water consumption	m3	0.623	0.0646	0.177	−0.0310	0.404	0.00851

).

The contribution analysis results for the HTC scenario indicate that the environmental impact profile is largely determined by the intensity of chemical inputs used (Figure 3). Across the contribution chart, sulfuric acid consumption clearly emerges as the dominant positive contributor in nearly all impact categories. Sulfuric acid plays a decisive role in multiple environmental impact categories, particularly freshwater eutrophication, terrestrial acidification, human toxicity (both carcinogenic and non-carcinogenic), and ecotoxicity. This behavior is directly linked to the high energy demand associated with sulfuric acid production, SO₂ emissions, and the use of auxiliary chemical inputs (Figure 3).

Sodium hydroxide consumption represents a secondary but still significant contributor to the environmental performance of the HTC system. Due to its chlor-alkali-based production pathway, NaOH generates noticeable contributions, especially in the human toxicity, freshwater ecotoxicity, and marine ecotoxicity categories. Climate change, ozone formation (affecting human health and terrestrial ecosystems), and fossil resource scarcity are the primary categories where electricity consumption contributes. CO₂ and NO_x emissions associated with electricity generation shape the environmental burdens in these categories. However, within the HTC process, energy consumption exhibits a more limited influence on overall environmental performance when compared to chemical inputs.

Another noteworthy aspect highlighted in the contribution analysis is the presence of negative (environmental benefit) contributions associated with the HTC process. Negative values seen in some ecotoxicity and resource use categories show that HTC reduces environmental damage by stabilizing waste biomass (poultry manure). This finding suggests that HTC should be regarded not merely as a pretreatment or carbonization technology, but also as a process component that contributes to the reduction in environmental burdens from a life cycle perspective.

3.3. Uncertainty Analysis

An evaluation of the Monte Carlo uncertainty analysis results (95% confidence interval) for the HTC and pyrolysis scenarios reveals that uncertainty patterns are largely consistent across both pathways but differ in magnitude for specific impact categories. For energy- and emission-driven impact categories, such as global warming potential, fossil and mineral resource scarcity, photochemical ozone formation, and terrestrial acidification, both scenarios exhibit low coefficients of variation (generally below 10%) and relatively narrow confidence intervals. This indicates that the results in these categories are statistically robust. To evaluate the impact of operational uncertainties, a sensitivity analysis was conducted on the thermal efficiency of the drying and reaction steps. A 10% reduction in efficiency (simulating suboptimal industrial conditions) resulted in an increase in the Global Warming Potential (GWP) of the HTC and pyrolysis process, shifting the value from 4.18 to 4.31 kg CO₂-eq/kg P and 6.00 to 6.24 kg CO₂-eq/kg P. This improvement corresponds to a relative change of only 3.1% and 3.6%. This finding demonstrates that while energy consumption is a significant contributor to the carbon footprint, the overall environmental performance of the system is robust and not critically sensitive to minor fluctuations in dryer or reactor efficiencies. In contrast, impact categories related to eutrophication, toxicity, and certain resource use indicators show substantially higher uncertainty levels in both scenarios. Freshwater and marine eutrophication and toxicity categories are characterized by wide confidence intervals and high coefficients of variation, frequently exceeding 40%.

In the literature, uncertainties regarding phosphorus recovery processes have been addressed through sensitivity analysis, scenario-based assessments, and standard deviation. In this study, however, Monte Carlo uncertainty analysis was applied to represent the systematic and probabilistic propagation of uncertainties. Goel et al. [26] demonstrated via scenario-based sensitivity analysis that eutrophication potential is highly sensitive to variations in recovery efficiency and background parameters. Similarly, Amann et al. [13], through a qualitative data quality assessment, emphasized that high uncertainties in toxicity and eutrophication categories stem from inherent variations in background datasets related to chemical production and agricultural emissions rather than the recovery technology itself. Although different uncertainty management approaches (sensitivity analysis and qualitative matrices) were used in these studies, their conclusions regarding the structural uncertainty created by background datasets (especially fertilizer and chemical production) in these impact categories corroborate the findings of the probabilistic Monte Carlo simulation in this study.

Despite methodological differences, the findings obtained are consistent with the literature regarding the distribution of uncertainty. Specifically, while low uncertainties were observed in energy- and emission-based impact categories such as global warming potential and terrestrial acidification, high uncertainties emerged in categories such as eutrophication and human toxicity, consistent with previously reported studies. This indicates that different uncertainty analysis approaches point to similar environmental sensitivities and support the methodological robustness of the obtained results.

The primary uncertainty in this study arises from the use of laboratory-scale experimental data to project industrial-scale environmental impacts. To mitigate the inherent discrepancies in energy efficiency between lab-scale equipment and industrial machinery, a theoretical upscaling framework based on thermodynamic mass and heat balances was employed for the pyrolysis and leaching stages. Regarding the applicability of the method to varying sludge or manure compositions, the Life Cycle Inventory (LCI) was constructed using a stoichiometric modeling approach. In this model, chemical consumption is not fixed but is dynamically calculated as a function of the feedstock’s phosphorus content and leaching efficiency, ensuring robustness against feedstock heterogeneity. Furthermore, analytical errors were minimized by utilizing average inventory data derived from replicated experimental runs as reported by Topcu et al. [9].

Overall, the uncertainty analysis suggests that comparative conclusions between HTC and pyrolysis should primarily rely on low-uncertainty indicators such as global warming potential and terrestrial acidification. In contrast, impact categories related to eutrophication and toxicity should be evaluated as supportive rather than decisive indicators, consistent with the literature stating that trade-offs between energy-derived benefits and toxicity-derived uncertainties are prevalent in nutrient recovery systems.

In summary, the results of the uncertainty analysis demonstrated that the source of uncertainty varies depending on the impact category. While uncertainty levels for energy-based impact categories, such as global warming and acidification, remained within a reliable range, high levels of uncertainty were observed for categories like eutrophication and toxicity. The uncertainty associated with toxicity and eutrophication was primarily attributed to background databases, which may lack comprehensive data or contain inconsistencies that affect the reliability of the results.

4. Discussion

A quantitative comparison between the HTC and pyrolysis scenarios (HTC–PYR) reveals a consistently improved environmental performance for HTC across most impact categories. In terms of global warming potential, HTC results in a net impact of 4.18 kg CO₂ eq, compared to 6.00 kg CO₂ eq for pyrolysis, corresponding to a reduction of approximately 1.8 kg CO₂ eq (~30%), primarily driven by stronger negative contributions from the HTC process itself. Similar trends are observed in ionizing radiation, which decreases from 0.366 kBq Co-60 eq (pyrolysis) to 0.257 kBq Co-60 eq (HTC), and in ozone formation (human health), which is reduced from 0.0157 to 0.0117 kg NO_x eq. HTC also shows lower impacts in fine particulate matter formation (0.0216 vs. 0.0262 kg PM_2.5 eq) and terrestrial acidification (0.0337 vs. 0.0493 kg SO₂ eq), indicating a systematically lower burden in air-pollution-related categories.

More pronounced differences emerge in ecotoxicity-related categories, where HTC consistently outperforms pyrolysis. Terrestrial ecotoxicity decreases from 161 to 122 kg 1,4-DCB eq (≈24% reduction), while freshwater ecotoxicity declines from 1.39 to 1.03 kg 1,4-DCB eq, and marine ecotoxicity from 1.89 to 1.44 kg 1,4-DCB eq. These reductions are largely attributable to stronger negative contributions associated with the HTC stage, which more effectively offsets upstream chemical-related impacts, such as the use of less harmful chemicals and improved processing methods that reduce overall toxicity. In addition, human toxicity impacts are notably lower under the HTC scenario: carcinogenic toxicity decreases from 0.462 to 0.317 kg 1,4-DCB eq, and non-carcinogenic toxicity from 28.4 to 21.8 kg 1,4-DCB eq, reflecting a substantial mitigation of chemical-driven toxicity burdens.

Both systems yield net environmental benefits in marine eutrophication and land use, yet HTC exhibits more pronounced advantages. Marine eutrophication shifts from −3.42 × 10⁻⁴ kg N eq (pyrolysis) to −4.89 × 10⁻⁴ kg N eq (HTC), while land use benefits increase from −0.63 to −0.94 m² a crop eq, indicating a stronger avoidance of land occupation. HTC also results in lower impacts for mineral resource scarcity (0.0718 vs. 0.10 kg Cu eq) and fossil resource scarcity (1.47 vs. 1.90 kg oil eq). Finally, water consumption is substantially reduced under HTC (0.623 m³) compared to pyrolysis (1.49 m³), corresponding to a reduction of nearly 60%, driven by lower overall process water demand and more effective internal offsets.

Overall, the HTC–PYR comparison demonstrates that while both pathways benefit from avoided primary phosphorus production, HTC provides systematically lower net impacts across climate change, toxicity, resource scarcity, land use, and water consumption categories, highlighting its superior environmental performance under the assumptions applied in this study. Fonseca et al. [27] reported similar trade-offs in their comparative LCA of hydrothermal carbonization (HTC) and pyrolysis for sewage sludge treatment. Their findings demonstrated that while HTC offers benefits for mitigating climate change through carbon storage and fertilizer substitution, it presents challenges regarding toxicity impacts due to heavy metal mobility; conversely, pyrolysis was found to produce a char with better metal retention and lower toxicity risks.

Within the reviewed literature, the Global Warming Potential (GWP) values reported for phosphorus recovery processes and their equivalent mineral fertilizer production are distributed over a wide range, from −5.2 kg CO₂ eq to values exceeding 89 kg CO₂ eq [13,14]. Amann et al. [13] emphasized that the carbon footprint of sewage sludge-based phosphorus recovery processes varies between −5.2 and 8.7 kg CO₂ eq. Havukainen et al. [28], based on Europe-centered production data, reported an average carbon footprint of mineral phosphorus of 3.1 kg CO₂ eq per kg P. This value represents facilities with high energy efficiency and optimized production processes. On the other hand, Gong et al. [29], based on production conditions in China, reported a considerably higher value of approximately 16.8 kg CO₂ eq for the production phase alone. One possible explanation for this difference is that while one study relies on European emission factors, the other is based on Chinese emission sources, which are characterized by a higher share of coal-based energy. In addition, emissions increase substantially during the production of complex fertilizers such as DAP, as ammonia synthesis is also involved (approximately 16.8 kg CO₂ eq for the production phase). In this study, the HTC and pyrolysis processes exhibit emission profiles that are three to four times lower compared to China-based production, while being closer in magnitude to those reported for European production systems (

Table 5. Global Warming Potential (GWP) of recovered phosphorus (this study) against mineral phosphorus fertilizer production.

Production Route/Technology	GWP Impact kg CO2 Eq
HTC (This study)	4.18
Pyrolysis (This study)	6.00
Reference (Mineral Fertilizer)
Mineral P (EU Standard—BAT) [27]	3.10
Mineral P (Global/China Average) [28]	16.30

).

Amann et al. [13], who examined a broad spectrum of phosphorus recovery technologies (18 different technologies), demonstrated that the environmental performance of recovery processes is largely dependent on the type and amount of chemicals used, particularly sulfuric acid and sodium hydroxide. Pradel and Aissani [12] emphasized that high chemical consumption in recovery processes is the main factor increasing environmental burdens. One of the most recent LCA reviews on phosphorus recovery technologies, conducted by Abdolrezayi et al. [14], similarly showed that the environmental performance of recovery processes is strongly influenced by chemical consumption and the selected system boundaries (

Table 6. GWP Comparison of P recovery technologies.

Commercial Name/Method	Recovered Technology	Carbon Footprint (GWP) (kg CO2-Eq/kg P)		Reference
Mineral P Production (China)	Conventional (Mineral Fertilizer)	16.3 (Production Only)	High emissions due to coal-based energy and mining.	[29]
Sludge Acid Leaching	Wet Chemical (Sludge)	>89.0	Low yields of P recovery associated with a low P concentration of sludge and need for large amounts of energy and reactants.	[12]
Liquid phase	Crystallization (Liquid Phase)	−1.4 to −0.5	Recovery from liquid phase-low energy	[13,30]
Sewage sludge	MEPHREC, AquaReci	0.7 to 8.7	Acid usage	[13]
Sewage sludge ash	RecoPhos, EcoPhos	−5.2 to −2.4	High potential for P recovery	[13]
EU Standard Fertilizer	Conventional (Mineral)	3.1	Optimized plants in Europe (BAT)	[28]
Mineral P	Variation in data on TSP	−1 to 4.8	-	[16]
HTC	Thermochemical (Thermochemical- Acid Leaching)	4.18	Higher emission due to chemical consumption	This Study
Pyrolysis	Thermochemical (Thermochemical- Acid Leaching)	6.00	Higher emission due to chemical consumption	This Study

).

The findings from the pyrolysis scenario in this study, which indicate that increased GWP correlates with elevated NaOH consumption, align completely with the assertion by Amann et al. [13] that chemically intensive processes result in greater environmental burdens. However, the lower GWP value obtained for the HTC scenario (4.18 kg CO₂ eq/kg P) highlights the importance of feedstock characteristics. Compared to the municipal sewage sludges evaluated by Amann et al. [13], the poultry manure used in this study has a substantially higher phosphorus content, which optimizes the amount of chemicals consumed per unit of recovered phosphorus and improves the overall environmental efficiency of the process.

In addition, Amann et al. [13] reported that liquid-phase recovery technologies (e.g., struvite precipitation) exhibit low environmental impacts; however, their recovery potential remains limited (10–25%). The HTC-based method suggested in this study, on the other hand, works on the solid phase and has recovery rates of over 90%. This means that it is a better option because it has a high recovery rate and low environmental impact, which solves the problem of balancing environmental impact and recovery rate.

The results (4.18 kg CO₂ eq for HTC and 6.0 kg CO₂ eq for pyrolysis) are higher than the average values of 3–4 kg CO₂ eq reported in the literature for mineral phosphorus fertilizers [30]. This observation is consistent with the findings of Pradel and Aissani [12], who reported that chemically based phosphorus recovery routes may exhibit higher carbon footprints than conventional mining-based production. Nevertheless, the values obtained in the present study are significantly lower than the extremely high emission levels reported by Pradel and Aissani [12] for certain scenarios (>89 kg CO₂ eq). This difference primarily arises from fundamental differences in the applied LCA approaches between the two studies. The observed increase in GWP associated with high NaOH consumption in the present study further supports this conclusion. The HTC scenario, on the other hand, has a lower chemical consumption and a higher recovery efficiency, which lowers the overall impact to 4.18 kg CO₂ eq. This makes the recovery process competitive with mineral fertilizers.

While the calculated GWP impacts currently exceed the mineral phosphorus benchmark, the competitiveness of the proposed HTC-based recovery process could be significantly enhanced by adopting industrial symbiosis strategies relevant to European conditions. The chemical intensity of the leaching step can be mitigated by adopting a circular acid management approach, similar to the EcoPhos technology. In such systems, the acid is not merely consumed; instead, the process is designed to convert spent acid into marketable by-products (e.g., food-grade phosphates, gypsum, or salts), thereby distributing the environmental burden across multiple products. On the other hand, the current study represents a conservative scenario where the acid-leached hydrochar (solid residue) and its field application were not fully optimized. This residue retains a significant calorific value and stable carbon structure. If this side-stream is valorized as a solid fuel for energy recovery (offsetting fossil energy) or engineered as a soil conditioner for carbon sequestration, the net GWP of the system would decrease substantially. Therefore, the environmental competitiveness of HTC-P should be re-evaluated under these ‘optimized valorization’ scenarios, where the system credits from acid recovery and hydrochar utilization are fully accounted for.

Abdolrezayi et al. [14] highlighted methodological differences in LCA studies, particularly with respect to system boundaries and functional units, and emphasized how the treatment of “avoided products” (e.g., substitution of mineral fertilizers or avoidance of waste disposal) can substantially influence the results. From this perspective, Pradel and Aissani [12] considered sewage sludge not as a “waste” but as a “product” of the wastewater treatment plant and allocated the upstream operational burdens of the facility to the sludge. This allocation approach dramatically increased the environmental burden attributed to the recovered phosphorus.

In contrast, in the present study, poultry manure was treated as a “waste,” and the avoidance of its disposal (e.g., avoided landfill or land spreading) was accounted for as a system credit using the system expansion approach. Goel et al. [26], in an LCA study conducted for India—a country with a similar fertilizer import profile—reported that conventional DAP fertilizer supply exhibited the highest impacts in the global warming and fossil depletion categories. Their study further demonstrated that decentralized recovery from waste streams such as septic tank liquor can significantly reduce environmental burdens compared to imported DAP fertilizer. In the present study, treating poultry manure as an avoided process offsets the energy demands associated with the HTC and pyrolysis processes, resulting in negative net impact values for these two pathways. Therefore, the manner in which by-products or waste management options are defined and credited in LCA studies constitutes a critical determinant in the interpretation of results.

The system boundaries of this study were restricted to a gate-to-gate approach, thereby excluding emissions associated with the post-production use phase. Since downstream activities—such as transportation to the end-user and field application—do not provide inputs to the HTC or pyrolysis production processes, their exclusion does not affect the calculation of production-based emissions. Moreover, assuming similar transport distances and application methods for the solid products (hydrochar and biochar), the inclusion of the use phase would likely add a constant burden to both scenarios, leaving the relative comparison between HTC and pyrolysis unchanged. However, it must be acknowledged that these boundaries represent a limitation, and this exclusion should be carefully considered when comparing these results with cradle-to-grave studies in the broader literature.

In impact categories other than GWP, particularly in marine eutrophication, the results of this study show a positive deviation from the general trend reported in the literature. Mayer et al. [30] reported a positive environmental burden of approximately 3.0 × 10⁻² kg N-eq for thermal treatment processes, mainly due to nitrogen (N) losses to the gas phase or the formation of NO_x emissions. In contrast, the results obtained in this study indicate that both pyrolysis (−3.42 × 10⁻⁴ kg N-eq) and HTC (−4.89 × 10⁻⁴ kg N-eq) provide a net environmental benefit (negative values) in terms of marine eutrophication. The primary reason for this positive environmental performance is attributed to the conversion of poultry manure into value-added products via HTC and pyrolysis instead of open storage within the defined system boundaries.

With respect to the human toxicity category, comprehensive LCA studies reported in the literature [31,32,33] clearly demonstrate that the direct land application of sewage sludge exhibits the highest toxicity potential, ranging between 210 and 2260 kg 1,4-DCB-eq. Mayer et al. [31] showed that, across different scenarios, HTC and pyrolysis reduce human toxicity impacts from approximately 4 × 10² kg 1,4-DCB to 1.3 × 10² kg 1,4-DCB and 1.2 × 10² kg 1,4-DCB, respectively, compared to direct land application, although pyrolysis was reported to exhibit higher acidification values due to its energy requirements.

In terms of acidification potential (AP), the HTC (0.0337 kg SO₂-eq) and pyrolysis (0.0493 kg SO₂-eq) values obtained in this study display a profile consistent with thermal treatment scenarios reported in the literature. The results are markedly lower than the high acidification burdens (0.083–0.30 kg SO₂-eq) associated with direct land application, which has been identified as the “worst-case scenario” by Mayer et al. [31] and Ravi et al. [32]. Although the HTC scenario yields a somewhat higher AP than the optimized HTC scenario reported by Mayer et al. [31] (0.017 kg SO₂-eq), it nevertheless reduces the environmental burden by approximately half through the mitigation of emissions originating from poultry manure, similar to the trends observed for eutrophication. The close match between the pyrolysis result from this study (0.0493 kg SO₂-eq) and the value reported by Mayer et al. [31] (0.041 kg SO₂-eq) further shows how important energy use is during drying and processing stages in determining acidification effects.

Overall, the thermal treatment methods evaluated in this study represent a more sustainable alternative to direct land application, which is associated with high acidification risks. In their comprehensive assessment of 18 phosphorus recovery technologies from municipal wastewater, Amann et al. [13] demonstrated that environmental impacts are driven primarily by chemical consumption—particularly sulfuric acid and sodium hydroxide—rather than energy demand. According to their findings, the Gifhorn process, which requires intensive acid use, exhibits the highest acidification burden (95.7 g SO₂-eq), whereas technologies providing by-product credits, such as EcoPhos (1.1 g SO₂-eq) and low-energy liquid-phase struvite recovery (38 g SO₂-eq), emerge as considerably more sustainable alternatives. Although acid use is also required in the HTC and pyrolysis routes evaluated in this study, the resulting acidification impacts (HTC: 0.0337 kg SO₂-eq; pyrolysis: 0.0493 kg SO₂-eq) are comparable to those reported for struvite recovery in the study by Amann et al. [13]. This outcome is clearly attributable to the waste management trade-offs incorporated in the system, as reflected by the waste management credits reported in Table 6 and

Table 7. Other Environmental Impact Comparison of P Recovery Technologies.

Scenario/Technology	Acidification (AP)	Eutrophication (EP)	Human Toxicity	Notes	Reference
Direct Land App.	8.3 × 10−2 kg SO2-eq	3.1 × 10−2 kg N-eq (Marine) −5.7 × 10−2 kg P-eq (Freshwater)	4.0 × 102 kg 1,4-DCB	Land application has the highest toxicity and acidification	[30]
Incineration	2.0 × 10−2 kg SO2-eq	3.0 × 10−2 kg N-eq	1.3 × 102 kg 1,4-DCB	Incineration reduces toxicity by 60–70%
HTC + Incineration	1.7 × 10−2 kg SO2-eq (Best)	2.7 × 10−2 kg N-eq	1.3 × 102 kg 1,4-DCB	HTC is the most balanced method for acidification and eutrophication
Pyrolysis + Incineration	4.1 × 10−2 kg SO2-eq	3.0 × 10−2 kg N-eq	1.2 × 102 kg 1,4-DCB	Pyrolysis has the lowest toxicity, but energy load increases acidification
Cement Kiln (Current)	−0.10 kg SO2-eq	-	−22 kg 1,4-DCB	Positive effect due to coal substitution credit	[31]
Land Application	0.30 kg SO2-eq	-	210 kg 1,4-DCB	High toxicity due to zinc and copper
Mono-Incineration	0.12 kg SO2-eq	-	43 kg 1,4-DCB	Moderate impact level
Struvite (Crystal)	-	-	1.36 × 103 CTUh	Struvite crystallization significantly reduces toxicity	[32]
BioAcid (Acidification)	-	-	2.23 × 103 CTUh	Chemical use does not reduce toxicity
Incineration	42 g SO2-eq	-	-		[13]
Gifhorn (Sludge Acid)	95.7 g SO2-eq	-	-	Acid use increases acidification
EcoPhos (Ash Acid)	1.1 g SO2-eq	-	-	Ash processing credits eliminate acidification load
Struvite (Liquid Phase)	38 g SO2-eq	-	-	Low energy and chemical requirements
HTC	0.0337 kg SO2 eq	−0.000342 kg N-eq (Marine) 0.00283 kg P-eq (Freshwater)	0.317 kg 1,4-DCB eq	Low impact compared to direct application credits	This study
Pyrolysis	0.0493 kg SO2 eq	−0.000489 kg N-eq (Marine) 0.00355 P-eq (Freshwater)	0.462 kg 1,4-DCB eq	Low impact compared to direct application credits	This study

(HTC: −0.130 kg SO₂-eq; pyrolysis: −0.00842 kg SO₂-eq).

As a result, Life Cycle Assessment (LCA) demonstrates that HTC-based phosphorus recovery becomes environmentally competitive with mineral phosphorus (TSP/SSP) only under conditions where process yield is increased, thereby minimizing chemical consumption (particularly acid) in downstream steps. While Behjat et al. [17] highlight that acid usage in non-optimized HTC processes exacerbates Acidification and Global Warming Potential (GWP) burdens, it is evident that increased HTC yield reduces chemical requirements, leading to significant improvements across GWP, Acidification, and Eutrophication categories. In this context, the adoption of integrated leaching strategies—characterized by the substitution of sulfuric acid with phosphoric acid and the valorization of by-products, as exemplified by the ECOPHOS technology—represents a critical turning point. Data from Smol et al. [15] demonstrate that this approach can render the process carbon-negative (−2.1 kg CO₂ eq) compared to the reference system. Consequently, sustainability in HTC systems hinges on resolving the inevitable trade-off between the goal of ‘maximum P recovery’ and the ‘chemical-induced environmental burden’ through circular acid management and high process yield.

LCA-based circular strategies in food and resource management play a critical role in enhancing resource efficiency and mitigating indirect environmental burdens, such as virtual water consumption [34]. In this context, recovering phosphorus from poultry manure goes beyond simple waste management; it serves as a vital strategy for closing the nutrient loop in agriculture. regarding technological benchmarking.

5. Conclusions

In this study, phosphorus recovery from poultry waste was investigated through thermochemical treatment followed by acid leaching. The study quantitatively demonstrates the environmental impacts of chemical inputs—particularly NaOH and H₂SO₄—and clearly shows that chemical intensity is the dominant factor determining environmental performance in terms of carbon footprint, water consumption, and toxicity-related impact categories. The evidence indicates that the study is not only comparative in nature but also provides guidance for process optimization.

From a comparative perspective between HTC and pyrolysis, both technologies were found to be advantageous for the valorization of poultry waste; however, the high chemical consumption required during the phosphorus recovery stage was identified as a key driver increasing the overall environmental burden. Therefore, in acid-based recovery technologies, scenarios in which acids can be valorized or recovered as by-products represent a critical improvement pathway. It also should be noted that this study focuses on the production phase; therefore, the environmental burdens associated with the agricultural use phase and the uncertainties regarding toxicity impacts were excluded from the system boundaries.

The findings of this study demonstrate that the environmental sustainability of phosphorus recovery depends not solely on the selected technology but strongly on recovery efficiency, reduction in chemical consumption, and the substitution or mitigation of chemical use through alternative solutions. In this respect, the study provides a guiding LCA framework that clearly identifies improvement hotspots and offers direction for future pilot- and industrial-scale implementations. Consequently, the improvement recommendations derived from this study are proposed in order of priority as follows:

• Substitution of single-use mineral acids (H₂SO₄) with regenerable acids (H₃PO₄) or waste acids, as exemplified by the Ecophos model.
• Optimizing P-distribution in solid/liquid phases to reduce chemical consumption per functional unit.
• Utilizing the residual solid by-product as an energy source to increase the system’s carbon credit.