Activated Carbon and P-Rich Fertilizer Production from Industrial Sludge by Application of an Integrated Thermo-Chemical Treatment

: The cost and environmental impact of sludge disposal methods highlight the necessity of new solutions for resource recovery. This study aims at concurrently producing activated carbon while recovering phosphorous by applying an integrated thermo-chemical treatment to a sludge of industrial origin. The sludge was ﬁrst subjected to slow pyrolysis on a laboratory scale at different temperatures, and the produced chars were processed by leaching to obtain biocoal. Leaching tests enabled us to deﬁne the optimal slow pyrolysis temperatures to maximize leaching performances. Then, sludge was processed in a slow pyrolysis pilot-scale plant, and the produced char was subjected to acid leaching and ﬁnally to physical activation. Chemical precipitation was then applied to the liquid leachate to recover phosphorous as a salt. Laboratory-scale slow pyrolysis and leaching tests showed that a higher pyrolysis temperature leads to a lower degree of demineralization by leaching. Leaching enabled us to reduce the char ash content by almost 88%, extracting 100% P, Mg, Ca, and Fe and almost 90% Al. Physical activation of biocoal with CO 2 at 700 and 800 ◦ C produced materials with a surface area of 353 and 417 m 2 g − 1 , respectively, that make them potentially applicable as adsorbents in wastewater treatment or in industrial emissions processes. Moreover, the activated carbons showed the atomic H/C and O/C ratios of anthracite, which opens a wide range of alternative market applications to fossil coal, such as metallurgy and the advanced material sector. In addition, the high P and K concentrations in the salt obtained by precipitation make it a promising fertilizing product in line with the current regulations.


Introduction
Europe's water is becoming a critical resource to be preserved.Economic activities, urbanization, and global warming affect the quality and availability of European freshwaters.For that, the collection and treatment of waste waters is one key element in the development of a sustainable water cycle [1].In the European Union (EU) context, industry is considered a key stakeholder in water consumption, from both a quantitative and qualitative point of view.The uptake of water by industry in Europe represents about 54% of the total uptake for human activities, equal to about 96 billion cubic meters per year [2].
According to the Urban Waste Water Treatment Directive, the wastewater treatment processes in the EU have strongly improved from 1990 to 2014 [3].and the number of wastewater treatment plants (WWTPs) has increased.Nowadays, 26523 urban wastewater treatment plants are operating in the EU [4].In addition, at least 30,000 large industrial facilities with related wastewater treatment systems should be considered [5].
The HTC process works at temperatures in the range of 180-250 • C and pressures in the range of 10-50 bar and operates in a reaction environment characterized by the presence of liquid water [13].The two main products of HTC are the solid, named "hydrochar", which is a carbonaceous, highly volatile solid, namely >50% dry basis (d.b.), and the water phase, rich in organic carbon [26].For this reason, HTC is also seen as a pre-treatment preceding the application of other recovery processes, such as anaerobic digestion of the water phase and pyrolysis of the hydrochar [27,28].
Slow pyrolysis needs sludge to be previously dewatered by drying or other pretreatment systems.The process works at higher temperatures, in a range between 400 and 700 • C, with a residence time in the order of hours and heating rates between 1 and 20 • C/min [29,30].The process always generates three products: a solid carbonaceous matrix (char), a mixture of condensable vapors (water and organic compounds), and a mixture of permanent gases (CO, CO 2 , CH 4 , H 2 , etc.) [31].After pyrolysis, most of the sludge's ashes are concentrated in the char, which usually represents more than 40% d.b. of the processed material, rising to over 50% d.b. in the case of ash-rich municipal sludges [32].Consequently, an effect of treating sludge by slow pyrolysis is the increase of the ash content in the produced char and often, by consequence, a reduction of the carbon content in comparison with the processed feedstock.
Char is a carbonaceous product, rich in inorganics when produced by sludge, hydrophobic, and with a low volatile content.Phosphorous, as well as calcium, iron, and silicon, are more concentrated in char compared to the processed feedstock.Due to its high ash content and its low heating value (9-14 MJ kg −1 d.b.), char from sludge slow pyrolysis is neither suitable as a solid biofuel nor applicable in the steel industry.However, the high concentration of both inorganic compounds and carbon makes sludge-derived char an interesting raw material that is usable as a source of nutrients and renewable carbon.
The scientific challenge of sludge management consists then in the valuation of resources from this waste, including carbon and inorganic compounds (with phosphorous in the first place).To this end, a chemical char upgrading step to extract phosphorus and other inorganic compounds could represent not only a promising solution to ensure a full valorization of these raw materials but also an opportunity to improve char quality and unlock its potential application in the cement, steel, and other industry sectors.
Chemical leaching is a process that allows for the separation of the soluble components of a solid material by dissolving them into a liquid phase.A common application of chemical leaching is low-grade coal cleaning, aiming to reduce the amount of mineral matter [15], which comprises ash and sulfur [16].The application of the acid leaching process for phosphorus extraction from pyrolyzed sludge has been tested by the authors in previous studies with promising results.Acid leaching performed on sludge-derived char enabled a high extraction rate of P, Ca (>90%), and other metals; at the same time, the process extracted only 50% of Al and was ineffective on SiO 2 extraction [32].The products obtained by the acid leaching process applied to sludge-derived char consist of an acid liquid containing the recovered inorganic elements, such as P, Ca, and Fe, and a biocoal (LBC), with a higher C content compared to the raw char and a reduced ash concentration.The inorganic elements retained in the liquid can be recovered by chemical precipitation, producing an inorganic compound with a high N, P, and K content.Biocoal, with its reduced ash content and, thus, increased carbon content, can be considered an alternative as a precursor for the production of adsorbents and biomaterials.
To the best of the authors' knowledge, many studies are available concerning the extraction of phosphorus from sludge ash [33][34][35] or the production of biochar by slow pyrolysis [36,37].However, limited literature was found addressing the simultaneous valorization of both carbon and phosphorus retained in sewage sludge, mainly involving hydrothermal carbonization as a thermochemical treatment process [38,39], characterized by high volatility and low market value.The aim of the proposed work is to test the integrated slow pyrolysis and acid leaching processes on an ash-rich industrial sludge to obtain a low-ash biocoal and extract most of the valuable inorganic elements.This integrated process was previously tested by the authors on an ash-rich municipal sewage sludge [25], but never before on a sludge of industrial origin.Moreover, this study investigated the extraction of inorganics from the leachate by chemical precipitation and the upgrading of the biocoal by means of physical activation to produce two high-value products: an inorganic N-P-K fertilizer in compliance with the EU FPR regulation and an activated carbon with high micro-and meso-porosity, reusable for wastewater treatment or as an alternative to fossil coal in different sectors.

Feedstock: Industrial Sludge
The feedstock consists of an industrial sludge generated by anaerobic digestion, centrifuging, and drying of the suspended solids from the WWTP of an Italian meat producer factory.The WWTP producing the sludge receives the waste streams of the agri-food production, the meat manufacturing process, and the wastewater of the whole industrial complex, including equipment washing, toilets, and other facilities.

General Description of the Experimental Activity
The experimental campaign started with slow pyrolysis testing at laboratory scale, performed at different operating temperatures.Then, the leaching of the produced chars was tested, aiming to assess the performance of the leaching process on char produced at different pyrolysis temperatures.Based on the results of the leaching and pyrolysis tests, the most promising operating conditions for the slow pyrolysis test were selected.In particular, the temperature and the residence time chosen for the pilot-scale pyrolysis tests are those that enabled the highest degree of demineralization in the laboratory.Subsequently, the char from the slow pyrolysis pilot scale test was leached, adopting more severe conditions compared to those tested in the laboratory in order to maximize the ash extraction efficiency.The dissolved inorganic compounds were recovered from the leachate by precipitation.Finally, the activation process for the biocoal was tested.The activities performed during the experimental campaign are described in the paragraphs below.

Laboratory-Scale Slow Pyrolysis Tests
Sludge slow pyrolysis tests in laboratory (PT 400 -PT 650 ) were performed on a thermogravimetric analyzer (LECO TGA701).Six pyrolysis tests were carried out at constant residence time (1 h) and heating rate (20 • C min −1 ), increasing the temperature by 50 • C, from 400 • C to 650 • C, among the tests.The following nomenclature is adopted to identify the laboratory-scale slow pyrolysis tests:

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PT 400 : test performed at the pyrolysis temperature of 400 The mass yield was determined for each trial, and the char from each test was then analyzed in laboratory.The carbon recovery rate was calculated for each test, after char characterization, as follows: where C BC (% d.b.) and mass BC (g) are the carbon content and the mass of the char (BC) from the test, respectively, and C sludge (% d.b.) and mass sludge (g) are the carbon content and the mass of the processed dry industrial sludge, respectively.

Pilot-Scale Slow Pyrolysis Test
A pilot-scale slow pyrolysis unit (Figure 1) was operated for the test in a real-world environment.The pyrolysis unit, called SPYRO, is an auger-type reactor of approximately 2 m in length and an inner diameter of 0.15 m.The unit can convert up to 3 kg h −1 of feedstock and can be operated up to 600 • C. The pyrolysis reactor is coupled with a condensation unit for the recovery of volatiles in the form of pyrolysis liquid.Details of the pilot unit are reported elsewhere [32].where C BC (% d.b.) and mass BC (g) are the carbon content and the mass of the char (BC) from the test, respectively, and C sludge (% d.b.) and mass sludge (g) are the carbon content and the mass of the processed dry industrial sludge, respectively.

Pilot-Scale Slow Pyrolysis Test
A pilot-scale slow pyrolysis unit (Figure 1) was operated for the test in a real-world environment.The pyrolysis unit, called SPYRO, is an auger-type reactor of approximately 2 m in length and an inner diameter of 0.15 m.The unit can convert up to 3 kg h −1 of feedstock and can be operated up to 600 °C.The pyrolysis reactor is coupled with a condensation unit for the recovery of volatiles in the form of pyrolysis liquid.Details of the pilot unit are reported elsewhere [32].The operational parameters of the pilot-scale slow pyrolysis test (PTP) are shown in Table 1.The rotating speed of the reactor screw was set at 0.4 rpm to achieve a solid residence time of almost 1 h.
where BCP mass (g) is the mass of char and sludge mass (g) is the mass of dry sludge.Then the char was analyzed.The pyrolysis oil obtained by the pilot plant condensation system was first collected and weighted, and then separated gravimetrically, obtaining three phases: a light oil phase (LOP), an aqueous phase (AP), and a heavy oil phase (HOP).After collection, the LOP and the HOP were analyzed.

Experimental Procedures for Chemical Leaching Tests
The char samples obtained from slow pyrolysis tests on the lab scale (BC) and on the pilot scale (BCP) were processed by chemical leaching.The experimental campaign was conducted at laboratory scale to extract and separate the desired inorganic elements from The operational parameters of the pilot-scale slow pyrolysis test (PT P ) are shown in Table 1.The rotating speed of the reactor screw was set at 0.4 rpm to achieve a solid residence time of almost 1 h.The char produced by the trial was collected and weighted to calculate char mass yield (PMY) as follows: PMY = BC P mass sludge mass •100 where BC P mass (g) is the mass of char and sludge mass (g) is the mass of dry sludge.Then the char was analyzed.The pyrolysis oil obtained by the pilot plant condensation system was first collected and weighted, and then separated gravimetrically, obtaining three phases: a light oil phase (LOP), an aqueous phase (AP), and a heavy oil phase (HOP).After collection, the LOP and the HOP were analyzed.

Experimental Procedures for Chemical Leaching Tests
The char samples obtained from slow pyrolysis tests on the lab scale (BC) and on the pilot scale (BC P ) were processed by chemical leaching.The experimental campaign was conducted at laboratory scale to extract and separate the desired inorganic elements from the char.First, chemical leaching tests were performed to identify the optimal leaching operating conditions.The following operating conditions were varied during the experiments:

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Mass ratio between leaching solution and processed char (liquid:char);

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Pure reagent concentration (HNO 3 ) in the leaching solution (molarity, mol L −1 ); All the leaching tests were performed using the same procedure.Char was initially oven dried at 105 • C to a constant weight.The leaching solution was prepared by mixing, in a beaker, demineralized water and nitric acid (dosed as a ≥65% solution by Honeywell, ACS grade).The char was ground manually at a size below 500 µm in the case of BC processing and below 250 µm in the case of BC P , and then added to the solution.Each test was performed using a heating plate equipped with a thermocouple and a magnetic stirrer.At the end of the contact time, the contents of the beaker were vacuum filtered to separate the liquid from the solid phase.The separated solid material was then washed with demineralized water for 30 min to remove the residual acid reagent.The separation of washing water was again performed through vacuum filtration.The solid material was finally oven-dried at 105 • C to a constant weight.

Performance Indicators of Chemical Leaching Tests
Leaching test performances were evaluated by two indicators: degree of demineralization (DD) and element extraction efficiency (EE).
The first, calculated for all leaching tests, expresses the percentage of extracted ash against the initial content in the processed char: where ash BC (% d.b.) is the ash in the processed char, and ash LBC (% d.b.) is the ash in the biocoal.
The second indicator was calculated for the leaching test performed on BC P (LT P ) only and expresses the percentage of the extracted element against the initial content of that element in the processed char: where element LL (mg) is the element mass in the leachate (LL), and element BC,i (mg) is the element mass in the processed char.Both were calculated given the mass of the LL and processed char, and the element concentration in the materials was determined analytically.

Chemical Leaching Tests on BC from Laboratory-Scale Slow Pyrolysis Tests
To select the process conditions to be performed in the slow pyrolysis pilot plant, the study included an assessment of the impact of slow pyrolysis temperatures on the efficacy of the leaching process in terms of inorganic compound extraction.Six chemical leaching tests at the same conditions were performed, processing the chars (BC) produced in the laboratory at different temperatures.In each test, 11.2 g of char were processed.The tests were performed using nitric acid (HNO 3 ) as a reagent and under the following operating conditions: liquid:char 10:1, molarity 0.5 mol L −1 , temperature 30 • C, and contact time 1 h.The following nomenclature is adopted to identify the chemical leaching tests:

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LT 400 : test performed on the BC from PT 400 ; The samples of biocoal (LBCs) were collected, and their ash content was determined.

Chemical Leaching Tests on BC from PT P
Finally, a chemical leaching test was performed on the char produced by the pilot-scale pyrolysis test (BC P ), aiming to extract the inorganics from the char and maximize the ash reduction in the final solid.In this test, 20 g of char in a 0.8 M HNO 3 acid solution were processed with a contact time of 2 h and a temperature set at 70 • C. The solid mass yield was calculated for the leaching test performed on BC P (LT P ), assuming that no organic matter is lost during the leaching process (neither in liquid nor in gas form).The equation adopted is the following: where: Mass BC P -Mass of processed char (g);
The outcome of the laboratory trials was used to define the process conditions for the pilot-scale slow pyrolysis test and for the chemical leaching test on the processed char.

Chemical Precipitation Test
The leachate from the LT P was adopted as the starting material for the precipitation test, aiming at recovering the inorganic compounds dissolved in the liquid in solid form by increasing the liquid pH.To this end, known volumes of a KOH solution were dosed in the glass beaker containing the acid liquid.During the process, the system was stirred, and the pH was monitored by a pH meter (Metrohm 827 pH).The 15% KOH solution adopted for the test was prepared by KOH pellets (≥97% purity, ACS grade by Merck).The dosage was stopped when a pH of around 9 was achieved in the liquid phase.The precipitated solid was then separated from the liquid phase by centrifugation and finally dried in an oven at 105 • C.

Activation Tests
The LBC from the LT P was then activated.The activation tests (AT) were performed using a batch tubular quartz reactor placed in a ceramic furnace.The furnace is equipped with k-type thermocouples both inside and outside the reactor, controlled by a programmable logic controller (PLC).First, a known quantity of LBC P was charged in the tubular reactor, which was then placed in the furnace.Subsequently, the desired process temperature and the heating rate were set.As a first step, nitrogen was fluxed to avoid oxygen entering and feedstock oxidation.Afterwards, CO 2 was fluxed at a flow rate of 3.5 l min −1 for the set residence time.During the first activation test (AT1), 9 g of LBC P were processed, while 8 g of LBC P were processed during AT2.The activation test operating conditions are reported in Table 2.The activated carbon (AC) mass yield obtained by the activation test (AMY) was calculated for both trials as follows: where AC mass (g) is the mass of activated carbon and LBC P mass (g) is the mass of biocoal.The total AC mass yield (ACY tot ) obtained by slow pyrolysis (PT P ), acid leaching (LTP), and activation (AT1, AT2) is calculated by the following formula:

Feedstock and Process Product Characterization
The characterization of the sludge used as pyrolysis feedstock and of the products obtained from the tested processes consisted of different physical-chemical analyses.
The industrial sludge was first air dried to determine its moisture content prior to its characterization.The first analysis consisted of the determination of the content of residual moisture, ash, determined at 550 • C (ash 550) and 710 • C (ash 710), volatiles, and fixed carbon (fixed C, calculated as the difference between 100 and the sum of moisture, volatiles, and ash 550, according to UNI EN 1860-2: 2005), in the sludge (proximate analysis).Then, sludge C, H, N, and S content (ultimate analysis) were determined.In addition, the higher heating value (HHV) was analyzed, calculating the lower heating value (LHV) by means of the HHV, H, and moisture content (following UNI EN ISO 18125: 2018 and UNI EN ISO 16948: 2015).Finally, the concentration of metals P and Si was determined by microwave plasma atomic emission spectroscopy (MP-AES).
Char from slow pyrolysis in the pilot unit was characterized by the same analysis and instruments as the feedstock, while for the chars from laboratory-scale pyrolysis, ash 550 and 710, volatiles, and C, H, N, and S content were determined.The biocoal was characterized by its ash (550 and 710), volatiles (C, H, and N), and its composition in metals (P and Si).Activated carbons were characterized by their ash 550 and 710, C, H, and N contents, surface area, and pore volume.In addition, Fourier-transform infrared spectroscopy (FT-IR) analysis of the activated carbons was performed.The surface area of the pilot unit char, of the biocoal, and of the activated carbons was determined via the Brunauer-Emmett-Teller (BET) method, while the plots of the pore volume and of the surface area of the activated carbons were determined by the density functional theory (DFT) method.The micropore volume of the activated carbons was also derived by the alpha-S method.
The HOP and the LOP of the condensed pyrolysis oil were analyzed, determining their water content, C, H, and N content, HHV, and LHV.
The characterization of the leachate and of the precipitated compound consisted of the analysis of metal, P, and Si concentrations.The precipitated compound composition was also expressed as oxides, starting from the analytical elemental composition and considering the different molar masses.
Moisture was determined by drying the feedstock sample at 105 • C until a constant weight was reached, according to UNI EN ISO 18134-2: 2017.Ash and volatiles were determined by a thermogravimetric analyzer (LECO TGA701), following UNI EN ISO 18122: 2016 and UNI EN 1860-2: 2005 (for ash 550 and ash 710, respectively, by heating around 1 g sample up to 550 • C or 710 • C under constant air flow until constant weight was reached) and UNI EN ISO 18123: 2016 (for volatiles, by heating around 1 g sample under nitrogen flow up to 900 • C).
C, H, N, and S were determined by a CHN-S analyzer (LECO TruSpec CHN-S).C, H, and N analysis was performed according to ASTM D5291-10 for liquid samples (HOP and LOP) and to UNI EN ISO 16948: 2015 for solid materials.ASTM D 4239 was followed for S content determination.For C, H, N, and S analysis, about 60-80 mg of sample material were combusted at high temperatures (950 • C for C, H, and N and 1350 • C for S) and converted by catalysts to carbon dioxide, water vapor, elemental nitrogen, and sulfur dioxide.Lower S content values (<0.1% w/w) were instead determined by an ion chromatography system (Metrohm 883 Basic IC Plus) after combustion by a bomb calorimeter (LECO AC500).A higher heating value (HHV) was obtained by a bomb calorimeter (LECO AC500), according to UNI EN ISO 18125:2018 for solid materials and DIN 51900-1:2000 and DIN 51900-3:2005 for liquid samples (HOP and LOP).The sample (around 0.30-1.0g) was weighted and placed in the combustion bomb, then flushed with oxygen reaching 30 bar.Before ignition, the temperature was stabilized for 3 min; then the ignition was started by an electrical ignition device, and the temperature was monitored for 5 min.HOP and LOP water content determination was performed by Karl Fischer titration (Metrohm 848 Titrino Plus, Herisau, Switzerland), following ASTM E203-08, titrating about 0.1-0.5 g with iodine-based Karl Fischer reagents.The concentration of metals, P, and Si (inorganic elements) was determined using microwave plasma atomic emission spectroscopy (MP-AES, by Agilent 4200 MP-AES), which uses nitrogen plasma, in compliance with UNI EN ISO 16967: 2015 and UNI EN ISO 16968: 2015.For liquid samples, around 500 mg were analyzed.For the determination of inorganic elements in the organic solids (sludge, chars, and biocoal), the samples (around 30 mg each) have been previously digested with 3 mL of hydrogen peroxide and 5 mL of nitric acid in a Milestone Start D microwave digestion system to be completely solubilized in a liquid sample, then analyzed by MP-AES.Pore volume and surface area were analyzed via a surface area and pore size analyzer (Quantachrome NOVA 2200E), following ASTM D6556-10.This instrument analyzes the pore size of the samples in the range of 2-52 nm.
Before the analysis, samples were dried at 200 • C for 48 h in oven.Then they were degassed in the analyzer at 200 • C for 24 h under vacuum to remove moisture and volatile compounds.Degassed samples were then weighed in a bulb cell and analyzed after immersion in liquid nitrogen to determine the correspondent adsorption isotherm, and then the sample surface area (by BET and DFT methods) and the pore volume (by DFT method) were derived.The cumulated surface area, cumulated pore volume, and alpha-S plot of the activated carbons were generated by the software NovaWin TM by Quantachrome (version 11.02).
FT-IR analysis was performed through a FT-IR Shimadzu IR Tracer 100 in attenuated total reflectance (ATR) mode with an optical resolution of 4 cm −1 and a spectral range from 600 to 4000 cm −1 averaged on 45 scans.

Feedstock Characterization
The characterization of the processed industrial sludge is reported in Tables 3 and 4.

Laboratory-Scale Slow Pyrolysis Tests
The char mass yield, the ash content, the volatiles content, and the C, H, N, and S content of the char obtained by the six laboratory-scale slow pyrolysis tests are reported in Table 5.The carbon recovery rate in the solid product is shown in Figure 2. As reported in the graph, over 45% of carbon was recovered as solid material produced at 400 °C, around 35% at 500 °C, and less than 30% at 650 °C.As reported in the graph, over 45% of carbon was recovered as solid material produced at 400 • C, around 35% at 500 • C, and less than 30% at 650 • C.

Effect of Pyrolysis Temperature on Leaching Process Performances
The efficacy of the leaching process on the char obtained at different pyrolysis temperatures (400, 450, 500, 550, 600, and 650 • C) under the same chemical leaching conditions was evaluated by calculating the DD of each test.The trend of the DD against pyrolysis temperature is reported in Figure 3.As reported in the graph, over 45% of carbon was recovered as solid material produced at 400 °C, around 35% at 500 °C, and less than 30% at 650 °C.

Effect of Pyrolysis Temperature on Leaching Process Performances
The efficacy of the leaching process on the char obtained at different pyrolysis temperatures (400, 450, 500, 550, 600, and 650 °C) under the same chemical leaching conditions was evaluated by calculating the DD of each test.The trend of the DD against pyrolysis temperature is reported in Figure 3.The comprehensive mass balance of the pilot-scale slow pyrolysis test is reported in Table 6.During the slow pyrolysis trial performed in the pilot plant, 3.71 kg of dry industrial sludge were processed, producing 1.60 kg of char, 1.27 kg of condensate, and about 0.84 kg of permanent gases, calculated by difference (Table 6).By gravimetric separation, the following pyrolysis oil fractions have been collected:

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Light oil phase (LOP): a brown color, homogeneous oil phase lighter than water, containing organic compounds, which represented about one-thirds of the condensed fraction; • Aqueous phase (AP): a gray liquid containing mainly water and representing about two-thirds of the condensable fraction collected; • Heavy oil phase (HOP): a black oil phase heavier than oil, rich in aromatic compounds, such as TAR, amounting to less than 2% of the total condensates.
The recovered products, characterized according to the methodology explained above, are described in the paragraphs below.
The mass yields of the pilot-scale test are also represented graphically in Figure 4.
The carbon balance resulting from the pyrolysis trial showed that 39% of the feedstock carbon remained in the solid, while 61% devolatilized as process gas.This result was similar to that achieved in the laboratory under the same process conditions, in which 37% carbon was recovered in the char.In addition, 38% of the feedstock nitrogen was recovered in the solid pyrolysis product (BC P ) and 33% in the two oil phases.• Heavy oil phase (HOP): a black oil phase heavier than oil, rich in aromatic compounds, such as TAR, amounting to less than 2% of the total condensates.The recovered products, characterized according to the methodology explained above, are described in the paragraphs below.
The mass yields of the pilot-scale test are also represented graphically in Figure 4.The carbon balance resulting from the pyrolysis trial showed that 39% of the feedstock carbon remained in the solid, while 61% devolatilized as process gas.This result was similar to that achieved in the laboratory under the same process conditions, in which 37%

Energy Balance
The theoretical chemical energy input to the pyrolysis plant in the form of dry sludge was about 62.5 MJ, calculated by multiplying the processed feedstock mass by its calorific value.The energy recovered as pyrolysis products, calculated using the same methodology, is distributed as follows: 22.4 MJ in the char, 13.4 MJ in the condensates, and 26.7 MJ in the gas (calculated as a difference).The extracted char contained about 36% of the feedstock chemical energy, while 22% was recovered in the condensed pyrolysis oil and 42% in the permanent gases, as shown in Figure 5. carbon was recovered in the char.In addition, 38% of the feedstock nitrogen was recovered in the solid pyrolysis product (BCP) and 33% in the two oil phases.

Energy Balance
The theoretical chemical energy input to the pyrolysis plant in the form of dry sludge was about 62.5 MJ, calculated by multiplying the processed feedstock mass by its calorific value.The energy recovered as pyrolysis products, calculated using the same methodology, is distributed as follows: 22.4 MJ in the char, 13.4 MJ in the condensates, and 26.7 MJ in the gas (calculated as a difference).The extracted char contained about 36% of the feedstock chemical energy, while 22% was recovered in the condensed pyrolysis oil and 42% in the permanent gases, as shown in Figure 5.

Pyrolysis Oil Characterization
The results of the two oil fraction characterizations (LOP and HOP) are shown in Table 7.

Pyrolysis Oil Characterization
The results of the two oil fraction characterizations (LOP and HOP) are shown in Table 7.Both LOP and HOP showed a carbon content higher than 60% and a relevant nitrogen content of 7.5% and 6.7%, respectively.The low water content of the oil phases results in an oil of high calorific value, which could be of interest for potential energy valorization.However, the of nitrogen could contribute to the formation of fuel NOx during combustion.

Char from Pilot-Scale Test (BC P ) Characterization
The results of BC P characterization are reported in Tables 8 and 9.   BC P shows a higher ash content compared to the starting industrial sludge due to organic matter volatilization and consequent ash concentration, as mineral matter does not take part in the pyrolysis process.Consequently, the concentration of volatiles is lower compared to the starting sludge, while the fixed carbon content is higher.The concentration of metals, phosphorous, and silicon is higher.

Performances of Chemical Leaching Tests on BC P
The extraction efficiency of key inorganic elements is reported in Figure 6.The chemical leaching process applied to BC P enabled the extraction of high percentages of P, Ca, and other inorganic elements contained in the char.A DD of 87.5% was achieved, reducing the ash content in the char from an initial value of 56.1% d.b. to a final value in the biocoal of 9.2% d.b.The high DD led to a high extraction efficiency for most of the inorganic elements.In particular, an EE of 100% was achieved for valuable elements like P, Mg, Ca, and Fe, and an extraction close to 90% for Al and K.These values are confirmed by the elemental analysis of the LBC P reported in Section 3.5.2.

Biocoal (LBCP) Characterization
The characterization of the biocoal (LBCP) focused on the total remaining ash content, the resulting concentration of C, H, and N, and the surface area (Table 10).

Biocoal (LBC P ) Characterization
The characterization of the biocoal (LBC P ) focused on the total remaining ash content, the resulting concentration of C, H, and N, and the surface area (Table 10).
In total, due to ash extraction, the content of carbon and nitrogen in the biocoal increased by 67% and 77%, respectively.The porosity increased as well, from 7 to 17 m 2 g −1 .LBC P mass yield by the leaching process, calculated according to Equation (3), resulted in 48.2% d.b.The elemental analysis of LBC p is reported in Table 11.

Inorganics Recovery by Precipitation
The inorganic compounds extracted by chemical leaching were recovered by precipitation.The of the acid liquid (LL P ) resulting from LBC P leaching is reported in Table 12.The mass balance of the precipitation process is reported in Table 13.As the table shows, a precipitated salt (PS) mass yield of approximately 5.1% was obtained from the LL P .Both the PS and the precipitation liquid (PL) obtained by the process were characterized to determine the performance of the process.The composition of both fractions is reported in Table 14.The recovery efficiency of the precipitation process was calculated considering the remaining fraction of each single element in the PL.The analysis shows that the PL contained only 2942 mg kg −1 of Ca, 10,609 mg kg −1 of K, 14 mg kg −1 of Fe, and 302 mg g −1 of Na.High recovery of all elements was achieved, with a 100% recovery for both P and Al.The PL could be reused in the precipitation process for the KOH solution preparation.

Activated Carbon Production
The activation process, performed at 700 • C and 800 • C in a tubular furnace, produced two activated carbons (AC1 and AC2) with a mass yield of 75.5% and 66.8%, respectively.The composition of the two products is reported in Table 15.AC1 showed a higher mass yield, but also a lower porosity and a higher nitrogen content.AC2, produced at 800 • C, resulted in a higher surface area and a lower nitrogen content.The lower concentration of nitrogen in AC2 can be attributed to the higher activation temperature, which led to the devolatilization of the nitrogen compounds.However, the mass yield obtained at 800 • C is about 11.7% less than at 700 • C.
In addition to the BET surface area (reported in Table 15), the DFT surface area and average pore volume were analyzed, and their trend is shown by Figures 7 and 8. AC1 resulted in 0.192 cm 3 g −1 as the total pore volume for a DFT surface area of 325 m 2 g −1 .AC2 resulted in 0.239 cm 3 g −1 as the total pore volume for a DFT surface area of 510 m 2 g −1 .
Sustainability 2023, 15, x FOR PEER REVIEW 18 of 25 Figure 8 reports AC1 and AC2 cumulative pore volume (y axis) compared to pore width (x axis).The graph shows that the total pore volume in AC1 was mainly generated by micropores (pore size < 2 nm), with an average pore width of 1.1 nm, contributing to about 61.6% of the total pore volume.Similarly, AC2 showed an average pore width of 1.4 nm, contributing about 67.5% of the total pore volume.From the mentioned analysis, the two activated carbons result in prevalently microporous materials.The alpha-S method was adopted to confirm the DFT method results for AC1 and AC2.In general, the alpha-S plot consists of the trend of the adsorbed N2 volume (in stand- Figure 8 reports AC1 and AC2 cumulative pore volume (y axis) compared to pore width (x axis).The graph shows that the total pore volume in AC1 was mainly generated by micropores (pore size < 2 nm), with an average pore width of 1.1 nm, contributing to about 61.6% of the total pore volume.Similarly, AC2 showed an average pore width of 1.4 nm, contributing about 67.5% of the total pore volume.From the mentioned analysis, the two activated carbons result in prevalently microporous materials.The alpha-S method was adopted to confirm the DFT method results for AC1 and AC2.In general, the alpha-S plot consists of the trend of the adsorbed N2 volume (in standard temperature and pressure conditions) against alpha-s (the ratio between the amount Figure 8 reports AC1 and AC2 cumulative pore volume (y axis) compared to pore width (x axis).The graph shows that the total pore volume in AC1 was mainly generated by micropores (pore size < 2 nm), with an average pore width of 1.1 nm, contributing to about 61.6% of the total pore volume.Similarly, AC2 showed an average pore width of 1.4 nm, contributing about 67.5% of the total pore volume.From the mentioned analysis, the two activated carbons result in prevalently microporous materials.
The alpha-S method was adopted to confirm the DFT method results for AC1 and AC2.In general, the alpha-S plot consists of the trend of the adsorbed N 2 volume (in standard temperature and pressure conditions) against alpha-s (the ratio between the amount adsorbed by a reference non-porous sample and the amount adsorbed at a relative pressure of 0.4) [40].For AC1, the alpha-S plot had a slope of 23.9 and an intercept of 74.0 cm 3 g −1 ; for AC2, the alpha-S plot had a slope of 23.7 and an intercept of 101.8 cm 3 g −1 .From this method, the micro-pore volume (<2 nm) of AC1 resulted in 0.114 cm 3 g −1 , which is consistent with that produced by DFT (Figure 8), equal to 0.118 cm 3 g −1 .The volume of AC2 micropores by alpha-S plot (0.157 cm 3 g −1 ) is consistent with that from DFT (0.162 cm 3 g −1 ) as well.
The total mass yield (ACY) achieved considering the three process steps (pyrolysis, leaching, and activation) resulted in 15.6% d.b. for AC1 and 13.8% d.b. for AC2.The porosity achieved at 800 • C resulted in up to double that obtained by physical or chemical activation of sewage sludge char in other studies [41,42].
The FT-IR spectra of AC1 and AC2 are reported in Figure 9, compared to those of a commercial activated carbon.adsorbed by a reference non-porous sample and the amount adsorbed at a relative pressure of 0.4) [40].For AC1, the alpha-S plot had a slope of 23.9 and an intercept of 74.0 cm 3 g −1 ; for AC2, the alpha-S plot had a slope of 23.7 and an intercept of 101.8 cm 3 g −1 .From this method, the micro-pore volume (<2 nm) of AC1 resulted in 0.114 cm 3 g −1 , which is consistent with that produced by DFT (Figure 8), equal to 0.118 cm 3 g −1 .The volume of AC2 micropores by alpha-S plot (0.157 cm 3 g −1 ) is consistent with that from DFT (0.162 cm 3 g −1 ) as well.
The total mass yield (ACY) achieved considering the three process steps (pyrolysis, leaching, and activation) resulted in 15.6% d.b. for AC1 and 13.8% d.b. for AC2.The porosity achieved at 800 °C resulted in up to double that obtained by physical or chemical activation of sewage sludge char in other studies [41,42].
The FT-IR spectra of AC1 and AC2 are reported in Figure 9, compared to those of a commercial activated carbon.

Discussion
Carbon, hydrogen, and nitrogen decreased with the increase in slow pyrolysis temperature (Table 5), as temperature affects the devolatilization of organic compounds.About 6.1% nitrogen was found in the solid produced at 400 °C, while less than 4% was found in that produced at 650 °C (Table 5).Nitrogen is estimated to be present in the sludge, mainly in the form of protein N (P-N).In fact, according to [43], despite the fact that pyridine N (N-6), pyrrole N (N-5), and nitrogen oxides have been found in the sludge, P-N and N-6 often represent more than 80% of the total N. On the contrary, sulfur increases (Table 5), probably due to the high stability of sulfur compounds and the presence of inorganic sulfur [44].
As shown in Figure 3, the DD achieved by the chemical leaching tests on the char produced on a laboratory scale slightly increases for the char produced at 450 °C in

Discussion
Carbon, hydrogen, and nitrogen decreased with the increase in slow pyrolysis temperature (Table 5), as temperature affects the devolatilization of organic compounds.About 6.1% nitrogen was found in the solid produced at 400 • C, while less than 4% was found in that produced at 650 • C (Table 5).Nitrogen is estimated to be present in the sludge, mainly in the form of protein N (P-N).In fact, according to [43], despite the fact that pyridine N (N-6), pyrrole N (N-5), and nitrogen oxides have been found in the sludge, P-N and N-6 often represent more than 80% of the total N. On the contrary, sulfur increases (Table 5), probably due to the high stability of sulfur compounds and the presence of inorganic sulfur [44].
As shown in Figure 3, the DD achieved by the chemical leaching tests on the char produced on a laboratory scale slightly increases for the char produced at 450 • C in com-parison with that obtained at 400 • C.This can be due to the solubility of the char organic matter produced at 400 • C, which reacts under leaching conditions, reducing the efficacy of leaching on inorganic compounds.At temperatures higher than 450 • C, the DD gradually decreases with increasing pyrolysis temperatures.This result is probably due to the reduction of the solubility of P, Ca, and other heavy metal compounds with the increase in temperature.The reduction of both heavy metals and P-Ca compounds solubility at higher pyrolysis temperatures has been widely investigated in several studies.A higher Ca-P crystallinity was found in the chars produced at high pyrolysis temperatures, correlated with low water-extractable P [45], and a reduced availability of phosphorus was identified above 600 • C pyrolysis [46].A reduced solubility was also found for other heavy metals when the pyrolysis temperature increased from 500 to 800 • C [47].The pyrolysis trials performed in the pilot plant showed a lower char mass yield compared to that obtained in the laboratory at the same temperature.However, the composition of the char obtained at the pilot scale was similar to that produced in the laboratory.Therefore, the different mass balance is probably related to the losses or partial combustion of char powder, which could take place during the test in the pilot plant.
Despite the mass yield of the activated carbon compared to the initial processed feedstock resulted from 13.8 to 15.6%, the amount of stored carbon in the produced activated carbons was 28.0% for AC2 and 31.4% for AC1.Thus, it can be estimated that about twothirds of the carbon retained in the feedstock was devolatilized during the process.Both the obtained activated carbons showed a high surface area, more than 20 times higher than that of the LBC P .Moreover, carbon concentration increased from 59.4% in LBC P to 72-73%, while hydrogen was reduced to less than 1 %.These results enabled us to consider both AC1 and AC2 as anthracitic carbons, according to the Van Krevelen coal classification (Figure 10), and hence comparable to those used in the steelmaking sector.comparison with that obtained at 400 °C.This can be due to the solubility of the char organic matter produced at 400 °C, which reacts under leaching conditions, reducing the efficacy of leaching on inorganic compounds.At temperatures higher than 450 °C, the DD gradually decreases with increasing pyrolysis temperatures.This result is probably due to the reduction of the solubility of P, Ca, and other heavy metal compounds with the increase in temperature.The reduction of both heavy metals and P-Ca compounds solubility at higher pyrolysis temperatures has been widely investigated in several studies.A higher Ca-P crystallinity was found in the chars produced at high pyrolysis temperatures, correlated with low water-extractable P [45], and a reduced availability of phosphorus was identified above 600 °C pyrolysis [46].A reduced solubility was also found for other heavy metals when the pyrolysis temperature increased from 500 to 800 °C [47].The pyrolysis trials performed in the pilot plant showed a lower char mass yield compared to that obtained in the laboratory at the same temperature.However, the composition of the char obtained at the pilot scale was similar to that produced in the laboratory.Therefore, the different mass balance is probably related to the losses or partial combustion of char powder, which could take place during the test in the pilot plant.Despite the mass yield of the activated carbon compared to the initial processed feedstock resulted from 13.8 to 15.6%, the amount of stored carbon in the produced activated carbons was 28.0% for AC2 and 31.4% for AC1.Thus, it can be estimated that about twothirds of the carbon retained in the feedstock was devolatilized during the process.Both the obtained activated carbons showed a high surface area, more than 20 times higher than that of the LBCP.Moreover, carbon concentration increased from 59.4% in LBCP to 72-73%, while hydrogen was reduced to less than 1 %.These results enabled us to consider both AC1 and AC2 as anthracitic carbons, according to the Van Krevelen coal classification (Figure 10), and hence comparable to those used in the steelmaking sector.Typical properties of the activated carbons are high carbon content, high specific surface areas, and a high level of porosity.The distribution of pores, in combination with the specific surface area, is dependent on the starting materials of activation and on the adopted process parameters [49].Therefore, specific absorption trials should be performed to better determine the quality of the produced activated carbons.However, the specific surface areas obtained by the tests were comparable to physically activated biomass [49,50].Typical properties of the activated carbons are high carbon content, high specific surface areas, and a high level of porosity.The distribution of pores, in combination with the specific surface area, is dependent on the starting materials of activation and on the adopted process parameters [49].Therefore, specific absorption trials should be performed to better determine the quality of the produced activated carbons.However, the specific surface areas obtained by the tests were comparable to physically activated biomass [49,50].
Since activated carbon has a large specific surface area and has developed micropores, it has strong absorptivity and a large adsorption capacity.Since the physical adsorption properties of activated carbons are mainly related to the specific surface area, it is expected that both AC1 and AC2 could be used as adsorbents in wastewater treatment or in industrial emissions processes.Activated carbon can be used either alone or in combination with other water treatment technologies [51].
The FT-IR spectra of both samples (Figure 9) show a lack of functionalization typical of graphitic material [52].The spectra appear to be smooth in the region 4000-1700 cm −1 , while low signals are detected around 1600 cm −1 , from aromatic C=C stretching, and around 1100 cm −1 , probably related to inorganic oxides left from the feedstock.The spectra were also compared to a commercial steam-activated charcoal, NORIT B SUPRA EUR, which shows a similar smooth spectrum as the thermally activated samples, with even fewer functional groups detected.The FT-IR analysis enabled us to determine the high graphitization rate of the activated carbons (AC1 and AC2) and their similarity with commercial adsorbents available on the market [53].The precipitated salt showed a high concentration of P, Ca, and Fe, including a relevant content of K derived from the KOH dosed for the precipitation test.In particular, converting the P and K concentrations resulting from PS analysis into equivalent P 2 O 5 and K 2 O, the resulting concentrations are 16% P 2 O 5 and 23% K 2 O.The composition of the obtained precipitated salt was compared to that reported in the current Italian regulation on fertilizers [54] (and following modifications).The obtained precipitate could be potentially framed as a couple of fertilizing products included by the decree, that is, mixed phosphate salts that shall have a minimum 10% P 2 O 5 concentration, or PK fertilizers, named "concimi PK", whose requirements include minimum P 2 O 5 and K 2 O concentrations of 5% each and a minimum overall concentration (P 2 O 5 + K 2 O) of 18% [55].The concentration of P, as P 2 O 5 , and K, as K 2 O, resulted largely above the minimum values required by the decree.
Moreover, the concentration of P 2 O 5 was in line with that required by the EU FPR for precipitated phosphate salts (recognized as Component Material Category 12).The high concentration of iron, above the 10% d.b.EU FPR limit for precipitated phosphate salts, could represent a limit for the marketability of the product in the fertilizer industry, despite the fact that iron is not considered a toxic element.However, due to the low precipitation pH of iron ions, a two-stage selective precipitation could be performed to separately collect iron and increase the quality of the inorganic fertilizer.

Conclusions
The study demonstrated that the sludge produced by industrial WWTPs could be a valuable source of carbon and inorganic nutrients to be recycled as end-of-waste products with high added value.The industrial sludge processed in this study showed a similar composition to civil sludge but with a much lower silica content.The lack of silicates in the sludge and, thus, in the char enabled a high degree of demineralization by a single-step acid leaching.The biocoal obtained after low-temperature slow pyrolysis and chemical leaching presented a high nitrogen and volatile content and a low surface area (17.2 m 2 g −1 ), which reduced its quality as a substitute for fossil coal.On the contrary, activation enabled a strong reduction of nitrogen, and increased carbon content to up to 73.2% d.b., and produced a material with a high surface area (470 m 2 g −1 ) that was potentially usable as an adsorbent for liquid or gas effluents.The recovery of the inorganics from the leachate by precipitation in a single step led to the formation of a precipitated salt with a high concentration of P and K but also containing the full amount of iron contained in the initial sludge.The separated collection and, thus, the reduction of iron concentration in the salt could lead to an increase in the value of the obtained product.

Figure 3 .Figure 3 .
Figure 3. Degree of demineralization (DD) of the chemical leaching process on char produced at different pyrolysis temperatures (LT400-650: leaching test on char produced from a laboratory-scale slow pyrolysis test at 400-650 °C).

Figure 4 .
Figure 4. Pyrolysis products distribution after the pilot-scale slow pyrolysis trial (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).

Figure 4 .
Figure 4. Pyrolysis products distribution after the pilot-scale slow pyrolysis trial (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).

Figure 5 .
Figure 5.Chemical energy distribution among the different pyrolysis products after the pilot-scale slow pyrolysis trial (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).

Figure 5 .
Figure 5.Chemical energy distribution among the different pyrolysis products after the pilot-scale slow pyrolysis trial (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).
Sustainability 2023, 15, x FOR PEER REVIEW 15 of 25 the ash content in the char from an initial value of 56.1% d.b. to a final value in the biocoal of 9.2% d.b.The high DD led to a high extraction efficiency for most of the inorganic elements.In particular, an EE of 100% was achieved for valuable elements like P, Mg, Ca, and Fe, and an extraction close to 90% for Al and K.These values are confirmed by the elemental analysis of the LBCP reported in Section 3.5.2.

Figure 6 .
Figure 6.Extraction efficiency (EE) of BCP (char from a slow pyrolysis test at pilot scale) inorganic elements.

Figure 7 .
Figure 7. Trend of the cumulative surface area compared to AC1 (activated carbon produced at 700 °C) and AC2 (activated carbon produced at 800 °C) pore width.

Figure 8 .
Figure8.Trend of the cumulative pore volume compared to AC1 (activated carbon produced at 700 °C) and AC2 (activated carbon produced at 800 °C) pore width.

Figure 8 .
Figure 8. Trend of the cumulative pore volume compared to AC1 (activated carbon produced at 700 °C) and AC2 (activated carbon produced at 800 °C) pore width.

Figure 8 .
Figure 8. Trend of the cumulative pore volume compared to AC1 (activated carbon produced at 700 • C) and AC2 (activated carbon produced at 800 • C) pore width.

Figure 10 .
Figure 10.Classification of produced coals by Van Krevelen Diagram [48] (LBC P : biocoal from leaching test on char from slow pyrolysis test at pilot scale, AC1: activated carbon produced at 700 • C, AC2: activated carbon produced at 800 • C).
PT 450 : test performed at the pyrolysis temperature of 450 • C; • PT 500 : test performed at the pyrolysis temperature of 500 • C; • PT 550 : test performed at the pyrolysis temperature of 550 • C; • PT 600 : test performed at the pyrolysis temperature of 600 • C; • PT 650 : test performed at the pyrolysis temperature of 650 • C.

Table 1 .
Operational parameters of the pilot-scale slow pyrolysis test (PTP).

Table 1 .
Operational parameters of the pilot-scale slow pyrolysis test (PT P ).
• LT 450 : test performed on the BC from PT 450 ; • LT 500 : test performed on the BC from PT 500 ; • LT 550 : test performed on the BC from PT 550 ; • LT 600 : test performed on the BC from PT 600 ; • LT 650 : test performed on the BC from PT 650.

Table 2 .
Activation tests performed on the biocoal (LBC P ).

Table 3 .
Proximate analysis and ultimate analysis of the processed industrial sludge.

Table 4 .
Inorganic elements concentration in the processed industrial sludge.

Table 5 .
Mass yield and composition of the chars from the laboratory scale slow pyrolysis trials (PT 400 -PT 650 : slow pyrolysis test at 400-650 • C).

Table 6 .
Mass balance of the pilot-scale slow pyrolysis test (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).
* Calculated by difference.

6 .
Mass balance of the pilot-scale slow pyrolysis test (AP: Aqueous Phase, LOP: Light Oil Phase, HOP: Heavy Oil Phase).
* Calculated by difference.

Table 7 .
Composition of LOP (Light Oil Phase) and HOP (Heavy Oil Phase).

Table 8 .
Proximate and ultimate analysis and surface area of BC P (char from the slow pyrolysis test at pilot scale).

Table 9 .
Inorganic elements concentration in BC P (char from slow pyrolysis test at pilot scale).

Table 10 .
Proximate and ultimate analysis, and surface area of the biocoal (LBCP).

Table 10 .
Proximate and ultimate analysis, and surface area of the biocoal (LBC P ).

Table 11 .
Inorganic elements concentration in LBC P .

Table 13 .
Mass balance of the precipitation test.

Table 14 .
Inorganic elements concentration in the PS (precipitated salt) and in the PL (precipitation liquid) recovered after precipitation.