Evolution of phosphorus complexation and mineralogy during (hydro)thermal treatments of activated and anaerobically digested sludge: Insights from sequential extraction and P K-edge XANES

Migration of phosphorus in pig manure during pyrolysis process and slow-release mechanism of biochar in hydroponic application

Pyrolysis is an effective method for treating of livestock and poultry manure developed in recent years. It can completely decompose pathogens and antibiotics, stabilize heavy metals, and enrich phosphorus (P) in biochar. To elucidate the P migration mechanism under different pig manure pyrolysis temperatures, sequential fractionation, solution 31P nuclear magnetic resonance, X-ray photoelectron spectroscopy, X-ray diffraction, and K-edge X-ray absorption near-edge structure techniques were used to analyze the P species in pig manure biochar (PMB). The results indicated that most of the organic P in the pig manure was converted to inorganic P during pyrolysis. Moreover, the transformation to different P groups pathways was clarified. The phase transition from amorphous to crystalline calcium phosphate was promoted when the temperature was above 600 °C. The content of P extracted by hydrochloric acid, which was the long-term available P for plant uptake, increased significantly. PMB pyrolyzed at 600 °C can be used as a highly effective substitute for P source. It provides the necessary P species (e.g. water-soluble P.) and metal elements for the growth of water spinach plants, and which are slow-release comparing with the Hogland nutrient solution.

Enhanced heavy metal stabilization and phosphorus retention during the hydrothermal carbonization of swine manure by in-situ formation of MgFe2O4

Hydrothermal carbonization is an efficient technique for the disposal of livestock manure, enabling its harmless treatment, quantity reduction, and resourceful utilization. Co-hydrothermal of modified materials facilitates the production of more valuable carbonaceous materials. However, further exploration is needed to understand their potential impact on the environmental risks associated with livestock manure disposal and the application of products derived from it. Therefore, the carbonization degree, heavy metals stabilization, and phosphorus retention during the hydrothermal treatment of swine manure were systematically investigated in this study under the influence of in-situ formed MgFe2O4. The results revealed that the in-situ formation of MgFe2O4 improved the dehydration and decarboxylation of organic components in swine manure, thereby improving its carbonization degree. Furthermore, both hydrothermal carbonization and MgFe2O4 modified hydrothermal carbonization resulted in an enhanced stabilization of heavy metals, leading to a significant reduction in their soluble/exchangeable fraction and reducible fraction. Phosphorus was predominantly retained in the hydrochars, with the highest retention rate reaching 88%, attributed to the significant decrease in soluble and exchangeable phosphorus fractions facilitated by the in-situ formation of MgFe2O4. Moreover, MgFe2O4 modified hydrochars exhibited remarkable adsorption capacity for Pb(II) and Cu(II) without any leaching of heavy metals. Overall, the findings indicated that the in-situ formation of MgFe2O4 positively influenced the hydrothermal of swine manure, improving certain economic benefits in its practical application.

Phosphorus Footprint in the Whole Biowaste-Biochar-Soil-Plant System: Reservation, Replenishment, and Reception

Two phosphorus (P)-rich biowastes, sewage sludge (SS) and bone dreg (BD), were selected to clarify P footprints among biowaste, biochar, soil, and plants by introducing a novel "3R" concept model. Results showed that pyrolysis resulted in P transformation from an unstable-organic amorphous phase to a stable-inorganic crystalline phase with a P retention rate of 70-90% in biochar (P reservation). In soil, SSBC released more P in acid red soil and alkaline yellow soil than BDBC, while the opposite result appeared in neutral paddy soil. The P released from SSBC formed AlPO4 by combining with Al in soil, whereas P from BDBC transformed into Ca5(PO4)3F(or Cl) in conjunction with Ca in the soil (P replenishment). Various plants exhibited an uptake of approximately 2-6 times more P from biochar-amended soil than from the original soil (P reception). This study can guide the application of biochar in various soil-plant systems for effective nutrient reclamation.

Characterization of biochar produced from sewage sludge and its potential use as a substrate and plant growth improver

Biochar is a product rich in carbon produced by pyrolysis of different kinds of biomass and it modifies the physical, chemical, and biological properties of soil. In this study, biochar, produced at different pyrolysis temperatures (590 °C, 665 °C, and 765 °C), was physico-chemically characterized. It was explored whether biochar made from sewage sludge can become an alternative solution for future water and phosphorus management in agricultural production. A pot experiment was conducted using Chinese cabbage (Brassica rapa subsp. pekinensis) to investigate the effect of applying different biochars to the substrate, taking into account different growth parameters and the biochemical composition of the plants, as well as the physico-chemical properties of the substrate. According to the results, pyrolysis temperature influences the content of elements in biochar and their availability to plants, with total phosphorus contents in biochar ranging from 4.6% to 4.9%. In addition, applying biochar to the substrate significantly increases the volumetric water content up to 4.5 fold more compared to the control, which indicates a promising application in drought stress conditions and, at the same time, is a source of nutrients and can help to reduce the amount of mineral fertilizer application.

Hydrothermal carbonization of milk/dairy processing sludge: Fate of plant nutrients

Dairy processing sludge (DPS) is a byproduct generated in wastewater treatment plants located in dairy (milk) processing companies (waste activated sludge). DPS presents challenges in terms of its management (as biosolids) due to its high moisture content, prolonged storage required, uncontrolled nutrient loss and accumulation of certain substances in soil in the proximity of dairy companies. This study investigates the potential of hydrothermal carbonization (HTC) for recovery of nutrients in the form of solid hydrochar (biochar) produced from DPS originating from four different dairy processing companies. The HTC tests were carried out at 160 °C, 180 °C, 200 °C and 220 °C, and a residence time of 1h. The elemental properties of hydrochars (biochars), the content of primary and secondary nutrients, as well as contaminants were examined. The transformation of phosphorus in DPS during HTC was investigated. The fraction of plant available phosphorus was determined. The properties of hydrochar (biochar) were compared against the European Union Fertilizing Products Regulation. The findings of this study demonstrate that the content of nutrient in hydrochars (biochars) meet the requirements for organo-mineral fertilizer with nitrogen and phosphorus as the declared nutrients (13.9-26.7%). Further research on plant growth and field tests are needed to fully assess the agronomic potential of HTC hydrochar (biochar).

Two birds with one stone: The multiple roles of hydrothermal treatment in dewatering municipal sludge and producing value-added products

Sludge dewatering and resource recovery are key steps in the sustainable treatment of municipal sludge (MS) owing to the high levels of moisture and nutrients. Among the treatment options available, hydrothermal treatment (HT) is promising to efficiently improve dewaterability and recover biofuels, nutrients, and materials from MS. However, hydrothermal conversion at different HT conditions generates multiple products. Integrating the characteristics of dewaterability and value-added products under different HT conditions facilitates the application of HT for the sustainable management of MS. Therefore, a comprehensive review of HT for its multiple roles in MS dewatering and value-added resource recovery is conducted. First, the impact of HT temperature on sludge dewaterability and key mechanisms are summarized. Then, this study elucidates the characteristics of biofuels produced (combustible gases, hydrochars, biocrudes, and H2-rich gases), nutrient recovery (proteins and phosphorus), and value-added materials under a wide range of HT conditions. Importantly, along with the integration and evaluation of HT product characteristics under different HT temperatures, this work proposes a conceptual sludge treatment system that integrates the different value-added products in different HT stages. Furthermore, a critical evaluation of the knowledge gaps in the HT for sludge deep dewatering, biofuels, nutrients, and materials recovery is provided along with recommendations for further research.

Optimizing directional recovery of high-bioavailable phosphorus from human manure: Molecular-level understanding and assessment of application potential

Phosphorus (P) recovery from human manure (HM) is critical for food production security. For the first time, a one-step hydrothermal carbonation (HTC) treatment of HM was proposed in this study for the targeted high-bioavailable P recovery from P-rich hydrochars (PHCs) for direct soil application. Furthermore, the mechanism for the transformation of P speciation in the derived PHCs was also studied at the molecular level. A high portion of P (80.1∼89.3%) was retained in the solid phase after HTC treatment (120∼240°C) due to high metal contents. The decomposition of organophosphorus (OP) into high-bioavailable orthophosphate (Ortho-P) was accelerated when the HTC temperature was increased, reaching ∼97.1% at 210°C. In addition, due to the high content of Ca (40.45±2.37 g/kg) in HM, the HTC process promoted the conversion of low-bioavailable non-apatite inorganic (NAIP) into high-bioavailable apatite inorganic P (AP). In pot experiments with pea seedling growth, the application of newly obtained PHCs significantly promoted plant growth, including average wet/dry weight and plant height. Producing 1 ton of PHCs (210°C) with the same effective P content as agricultural-type calcium superphosphate could result in a net return of $58.69. More importantly, this pathway for P recovery is predicted to meet ∼38% of the current agricultural demand.

Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO 3 - and NH 4 + in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH 4 + concentration was observed. Conclusions: HTC allows for the production of hydrochar as a potential fertilizer material that requires further processing. Adjusting HTC conditions enhanced P-recovery in the hydrochar, while retrieving higher nitrate concentrations in the liquid product. Optimizing HTC for the production of qualified hydrochars requires further treatment of Cr content, studying the availability of P in the products and enhancing the hydrochar yield for economic feasibility.

P K-Edge XANES Calculations of Mineral Standards: Exploring the Potential of Theoretical Methods in the Analysis of Phosphorus Speciation

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy is a technique routinely employed in the qualitative and quantitative analysis of phosphorus speciation in many scientific fields. The data analysis is, however, often performed in a qualitative manner, relying on linear combination fitting protocols or simple comparisons between the experimental data and the spectra of standards, and little quantitative structural and electronic information is thus retrieved. Herein, we report a thorough theoretical investigation of P K-edge XANES spectra of NaH₂PO₄·H₂O, AlPO₄, α-Ti(HPO₄)₂·H₂O, and FePO₄·2H₂O showing excellent agreement with the experimental data. We find that different coordination shells of phosphorus, up to a distance of 5-6 Å from the photoabsorber, contribute to distinct features in the XANES spectra. This high structural sensitivity enables P K-edge XANES spectroscopy to even distinguish between nearly isostructural crystal phases of the same compound. Additionally, we provide a rationalization of the pre-edge transitions observed in the spectra of α-Ti(HPO₄)₂·H₂O and FePO₄·2H₂O through density of states calculations. These pre-edge transitions are found to be enabled by the covalent mixing of phosphorus s and p orbitals and titanium or iron d orbitals, which happens even though neither metal ion is directly bound to phosphorus in the two systems.

Calcium enhances phosphorus reclamation during biochar formation: Mechanisms and potential application as a phosphorus fertilizer in a paddy soil

Transformation of phosphorus (P) species during pyrolytic production of biochar from P-rich biowastes with a subsequent soil amendment is important to P reclamation. Aiming at increasing the content of plant-available P and restraining the formation of easily mobile P in pyrolysis product, this study used exogenous calcium ions (20 wt% CaCl₂) addition prior to pyrolysis to regulate the pyrolytic transformation of P chemical fractions from sewage sludge and bone dreg. Results showed that active Ca catalyzed the decomposition of organic P to transform into inorganic orthophosphate. Based on Hedley's sequential extraction method, this study found that addition of Ca ions remarkably reduced the content of soluble P, exchange P, Fe/Al bound P, and occluded P in biochar, while increased Ca bound P from 78 to 85% to 85-96%. Liquid ³¹P NMR indicated that exogenous Ca induced the crack of the P-O-P bond in pyrophosphate to become orthophosphates. It also explained why new orthophosphates including chlorapatite (Ca₅(PO₄)₃Cl) and calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) appeared in the Ca-composite biochar compared to pristine biochar. Combined with rapid P-release test in paddy soil (pH 6.27) and 30-days rice seedling growth test under flooded condition (10 wt% biochar addition ratio), it was confirmed that compared to pristine biochar, Ca-composite biochar released more P in paddy soil, but also promoted more P to be taken in by rice root and stalk. These results suggested that pretreating biowaste with Ca ion was a friendly approach to enhance P reclamation during biochar formation, making it a promising P fertilizer.

Effects of persulfate-assisted hydrothermal treatment of municipal sludge on aqueous phase characteristics and phytotoxicity

Hydrothermal technology (HT) has received much attention in recent years as a process to convert wet organic waste into hydrochar. The aqueous phase (HTAP) produced by this process is still a burden and has become a bottleneck issue for HT process development. In this study, we provide the first investigation of the HTAP characteristics, phytotoxicity, and their correlation with persulfate (PS) (PS, 2.0 mmol/g TS)-assisted municipal sludge HT. The results showed that PS accelerated the hydrolysis of protein substances and increased the concentration of NH₄⁺ by 13.4% to 190.5% and that of PO₄^3- by 24.2% to 1103.7% in HTAP at hydrothermal temperatures of 120 to 240 °C. PS can reduce the phytotoxicity of HTAP by reducing aldehydes, ketones, N heterocyclic compounds, and particle size and by increasing its humification index. The maximum values of the root length and biomass of pakchoi (Brassica chinensis L.) seedlings occurred when electrical conductivity was 0.2 mS/cm of HTAP. This work provided a new strategy for the selection and design of HTAP management strategies.

Improvement of fungal extraction of phosphorus from sewage sludge ash by Aspergillus niger using sludge filtrate as nutrient substrate

Fungal extraction is a promising approach for reclaiming phosphorus (P) from sewage sludge ash (SSA). However, this approach faces notable technical and economic challenges, including an unknown P speciation evolution and the addition of expensive chemical organic carbon. In this study, the use of an organic-rich effluent produced in sludge dewatering as nutrient source is proposed to initiate the fungal extraction of SSA-borne P with Aspergillus niger. The changes in P speciation in the ash during fungal treatment was analyzed by combined sequential extraction, solid-state ³¹P nuclear magnetic resonance, and P X-ray absorption near edge spectroscopy. Results showed that after 5 days of fungal treatment using sludge-derived organics, 85 % of P was leached from SSA. Dominantly, this considerable release of P resulted from the dissolution of Ca₃(PO₄)₂, AlPO₄, FePO₄, and Mg₃(PO₄)₂ in the ash, and their individual contribution rates to P released accounted for 28.0 %, 24.3 %, 20.6 %, and 18.8 %, respectively. After removal of metal cations (e.g., Mg²⁺, Al³⁺, Fe³⁺, and heavy metals) by cation exchange resin (CER), a hydroxyapatite (HAP) product with a purity of > 85 % was harvested from the extract by precipitation with CaCl₂. By contrast, without CER purification, a crude product of Ca/Mg-carbonates and phosphates mixture were obtained from this extract. A total of 73.2 wt% of P was ultimately recovered from SSA through integrated fungal extraction, CER purification, and HAP crystallization. These findings provide a mechanistic basis for the development of waste management strategies for improved P reclamation with minimal chemical organics consumption.

Co-hydrothermal carbonization of agricultural waste and sewage sludge for product quality improvement: Fuel properties of hydrochar and fertilizer quality of aqueous phase

Hydrothermal carbonization (HTC) is a promising carbon-neutral technology for converting sewage sludge (SS) and agricultural waste into energy. However, HTC-generated aqueous phase (AP) impedes the development of the former. This study investigated the potential of SS with rice husk (RH) and wheat straw (WS) co-HTC to form hydrochar and AP as substitutes for fuel and chemical fertilizer, respectively. Compared with single SS hydrochar, the yield of co-HTC-based hydrochar and higher heating value significantly increased by 10.9%-21.6% and 4.2%-182.7%, reaching a maximum of 72.6% and 14.7 MJ/kg, respectively. Co-HTC improves the safe handling, storage and transportation, and combustion performance of hydrochar. The total nitrogen concentration in AP-SS was 2575 mg/L, accounting for 67.7% of that found in SS. Co-HTC decreased and increased the amine and phenolic components of AP, respectively. AP-SS-RH and AP-SS-WS significantly increased pakchoi dry weight by 45.5% and 49.4%, respectively, compared with AP-SS. The results of the hydroponic experiments with AP instead of chemical fertilizers revealed that AP-SS did not reduce pakchoi dry weight by replacing <20% chemical fertilizers. However, AP-SS-RH or AP-SS-WS replaced 60% chemical fertilizers. Therefore, the co-HTC of SS and agricultural waste increased the AP substitution of chemical fertilizer from 20% to 60%. These findings suggest that the co-HTC of agricultural waste with SS is a promising technology for converting SS into renewable resource products for fuels and N-rich liquid fertilizer while significantly improving fuel and fertilizer quality.

Dispose of Chinese cabbage waste via hydrothermal carbonization: hydrochar characterization and its potential as a soil amendment

Landfill of waste biomass not only poses a threat to environmental protection but also leads to a great waste of biomass resources. Hydrothermal carbonization (HTC) has been considered a promising method to convert the wet biomass into hydrochar, a high-value-added product with multiple application potentials. The cabbage waste, typical wet waste biomass with a huge production per year, was hydrothermally carbonized under 190 °C and 260 °C, respectively. The results indicated that the majority of nutrients from feedstock were dissolved in spent liquor during HTC, with only a few amounts retained on hydrochar. Temperature showed a more significant impact on hydrochar properties than retention time, which enables hydrochar to be potentially used as a soil conditioner. Particularly, the hydrochar produced at 190 °C could improve plant nutrition in the short term, while that produced at 260 °C may benefit in C sequestration. Moreover, the hydrochar dominated by meso/macropores (> 90%) would be conducive to the storage of plant-available water. But both BTX and VOCs may release during hydrochar application; thus, further field experiments are needed to test the environmental risks of hydrochar when applied as a soil amendment.

Energy and nutrient recovery by spent mushroom substrate-assisted hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) has been recognized as a promising sewage sludge (SS) treatment technology for effective pathogen elimination, bioenergy recovery, organic contaminant destruction and volume reduction. However, the solid product (hydrochar) of SS after HTC as fuel has the problems of high ash content, high nitrogen content and low calorific value. The aqueous phase (AP) produced is still considered a burden and has become a bottleneck in the development of HTC. In this study, co-HTC of SS with spent mushroom substrate (SMS) is conducted, and the fuel properties of hydrochar and the quality of the AP as a liquid fertilizer are investigated. In comparison with hydrochar of single SS, the energy yield and higher heating value of the hydrochar from co-HTC were significantly increased by 12.1-44.8 % and 33.2-137.8 %, respectively, reaching their maximum of 72.75 % and14.98 MJ/kg, respectively. Co-HTC can improve safe handling, storage and transportation, and combustion performance of hydrochar. Furthermore, the AP of co-HTC could significantly increase the biomass of pakchoi, which was 140.9 % and 90.7 % of AP from single SS and Hoagland nutrition solution (represents commercial fertilizer), respectively. The AP of co-HTC as fertilizer can recover 62.03-64.65 % nitrogen from SS and SMS. These findings suggest that co-HTC of SMS with SS is a promising technology for the conversion SS into renewable resource products for fuels and N-rich liquid fertilizer while also significantly improving fuel and fertilizer quality.

Effects of lignocellulosic biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment

Sludge is a nutrient-rich organic waste generated from wastewater treatment plants. However, the application of sludge as a nutrient source is limited by its high contents of water and pollutants. In this study, the effects of biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment (HTT) were investigated. Blending biomass with digested sludge for HTT at 180-240 °C increased the recovery of nitrogen in the treated solids. At the HTT temperature of 240 °C, HTT with hardwood sawdust led to the highest nitrogen recovery of 70.6%, compared to the lowest nitrogen recovery of 36.5% without biomass. Blending biomass slightly decreased the recovery of phosphorus compared to those without biomass. Nevertheless, the lowest phosphorus recovery of 91.3% with the use of hardwood sawdust at the HTT temperature of 240 °C was only ∼7.0% less than that without biomass. Blending biomass reduced the contents of macro-metals such as Ca, Fe, Mg and Al in treated solids but the metal contents varied with different biomasses. Regarding the heavy metals, the use of rice husk did not decrease the contents of Ni and Co while blending bagasse did not decrease the content of Cr at HTT temperatures of 210 °C and 240 °C compared to the use of other biomasses. The different effects of biomass type on nutrient recovery and heavy metals were likely related to the types and abundances of organic acids such as acetic acid, oxygen-containing functional groups such as C-OH and COOH, oxide minerals such as silica from biomasses and the overall effects of these factors. This study provides very useful information in selection of lignocellulosic biomass for HTT of sludge for nutrient recovery and heavy metal removal.

Changes of phosphorus species during (hydro) thermal treatments of iron-rich sludge and their solubilization mediated by a phosphate solubilizing microorganism

This study systematically evaluated phosphorus (P) solubilization from pyrochar and hydrochar derived from both raw sludge and iron-rich sludge. The data indicated, that an increase in thermal treatment temperature and the presence of iron promoted the accumulation of P in both pyrochar (derived at 300, 500, and 800 °C) and hydrochar (derived at 100, 200, and 280 °C). After incubating pyrochar and hydrochar with a phosphate solubilizing microorganism (PSM) (Pseudomonas aeruginosa) for 30 days, PSM significantly promoted the solubilization of P in pyrochar and hydrochar synthesized at low temperatures rather than those at high temperatures, with a 59 % increase for the pyrolysis of raw sludge at 300 °C than that pyrolyzed at 800 °C and a 62 % increase for the hydrothermal treatment of raw sludge at 100 °C than that treated at 280 °C. And the phenomena were more obvious on the char samples derived from iron-rich sludge. The mass balance of different P species in the solid and liquid phases indicated that after incubating with PSM for 30 days, NaOH-P was the main P solubilized from the solid phase of pyrochar and HCl-P was the main P solubilized from the solid phase of hydrochar. Considering P availability to plants, the preliminary economic analysis indicated that the hydrothermal treatment of iron-rich sludge at 100 °C showed the highest economic benefits for P recovery, with the net cost of 28.79 USD/ton wet sludge. This study was useful in giving novel insights into the reuse of char samples as P fertilizer, and also suggested the importance of Pseudomonas aeruginosa and other bacteria in sludge application, particularly in terms of P solubilization.

A review on the production of P-enriched hydro/bio-char from solid waste: Transformation of P and applications of hydro/bio-char

Phosphorus (P) is a necessary element for plant growth and animal health. Most P utilized by anthropogenic activities is released within the generation of various solid wastes such as sewage sludge, animal manure, and wetland plant, which increase the risk of water contamination. (Hydro)thermal treatment could be employed for solid waste treatment with the production of value-added hydro/bio-char, and the behavior of P during the thermochemical treatment process is critical for the further utilization of hydro/bio-char. This study provides a systematic review of the migration and transformation mechanisms of P during thermochemical treatment of various solid wastes, and special emphasis is given to the potential applications of P-enriched hydro/bio-char. Future challenges and perspectives in the thermal treatment of P-enriched solid waste are presented as well. The distribution and speciation of P were affected by feedstock properties, thermal technique, and reaction conditions, correspondingly affecting hydro/bio-char applications. The derived P-enriched hydro/bio-char was mainly applied as an agricultural soil amendment, P recovery source, and heavy metal sorbent, which could be adjusted by varying treatment process parameters. Additionally, potentially toxic substances, such as heavy metals in the solid waste, should be addressed during the production and application of hydro/bio-char. Overall, the production of P-enriched hydro/bio-char from solid waste is a promising route to simultaneously achieve P reclamation and solid waste treatment.

Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects

Phosphorus (P) and sulfur (S) are usually involved simultaneously in the immobilization of heavy metals in sewage sludge during pyrolysis, and thus their speciation in sewage sludge-derived biochar (SSB) profoundly affects the recycling of the nutrients and the environmental risks of sewage sludge. Here, we investigated the speciation evolution of P and S in SSB induced by ageing processes in soil using X-ray absorption near edge structure spectroscopy. Results showed that Ca-bound compounds like hydroxyapatite dominated the P forms, while over 60% of S existed as reduced inorganic sulfides in the SSB. The stable Ca-associated P species in SSB tended to be transformed gradually into relatively soluble species during ageing in soil. The speciation composition of S in SSB remained almost unaffected when aged in pot soils, whereas about 33.6% of reduced sulfides were transformed into oxidized species after 1-year ageing in field soils. SSB significantly increased the proportion of sulfides and the contents of available P and S in the amended soil but showed relatively weak effects on the speciation distribution of P in the soil because of their similar compositions. These findings provide insights into biogeochemistry of nutrients and behaviors of heavy metals in SSB after its application to the soil environments.

Targeted clean extraction of phosphorus from waste activated sludge: From a new perspective of phosphorus occurrence states to an innovative approach through acidic cation exchange resin

Waste activated sludge (WAS) is an important source of non-renewable phosphorus (P) recovery. Given the factor that the occurrence states of phosphorus in WAS determines its recovery efficiency, the spatial distribution and chemical speciation of phosphorus were comprehensively and simultaneously analyzed by in-situ and step-by-step extraction methods for the first time. It was confirmed that the phosphorus in solid phase of WAS could be mainly divided into three parts: polyphosphate in cells, extracellular polymeric substances (EPS)-bound P, and phosphate precipitated with metals (P-precipitates) in extracellular inorganic minerals. Among these forms, EPS-bound P (mainly orthophosphate, Ortho-P) and P-precipitates (mainly Ca-P, Fe-P, Al-P, and Mg-P) were the major forms of phosphorus in WAS, accounting for 65%-82% of total phosphorus (TP). Owing to the acid solubility of P-precipitates, acid extraction could be a potentially effective means for phosphorus recovery. However, the co-solution of metals may hinder the phosphorus recovery and the EPS-bound P cannot be recovered by acid extraction. To enhance phosphorus release from EPS and reduce metal interference, a targeted clean extraction technology using acidic cation exchange resin (ACER) was also developed. The results showed that a low dosage ACER could effectively extract EPS-bound P and P-precipitates, and the content of phosphorus in the extract exceeded 50% of TP. Compared with acid extraction, the release efficiency of TP increased by 13%-23%, and the dissolved metal content decreased by more than 90% in the extract by ACER. This was attributed to the acidification and metal capture by ACER. Finally, more than 90% of Ortho-P in the extract was recovered as calcium phosphate, which alleviated the depletion of phosphorus resources.

Influence of persulfate on transformation of phosphorus and heavy metals for improving sewage sludge dewaterability by hydrothermal treatment

Activated persulfate oxidation has been proven to be an efficient advanced sludge treatment technique to improve sludge dewaterability. This study investigates the influence of persulfate on the transformation of phosphorus (P) and heavy metals (HMs) during the hydrothermal treatment of sewage sludge. The hydrothermal temperature, time, and persulfate concentration are optimized by a Box-Behnken design to obtain the best sludge dewaterability, which is expressed by capillary suction time (CST). The highest CST reduction efficiency is 90.5% at the optimal hydrothermal temperature, time, and concentration of persulfate, which are 145 °C, 2 h, and 150 mg/g dry sludge (DS), respectively. The distribution and transformation of P and HMs with different persulfate concentrations (100-200 mg/g DS) during the hydrothermal process are investigated. Results show that more than 90% of the P and HMs in the sludge are retained in sludge cakes after the hydrothermal treatment. The addition of SPS can make the P in the sludge cakes transform into more stable P species according to the extraction capacity of sequential extracts. It can be found from the ecological risk indexes of the HMs that the addition of SPS during the hydrothermal treatment of sludge can reduce the environmental risk of HMs. This study provides insights into the P and HM distribution and transformation during hydrothermal treatment with persulfate, providing a reference for sludge recovery strategies.

Crop fertilisation potential of phosphorus in hydrochars produced from sewage sludge

Sewage sludges are a rich underused source of phosphorus (P) which contributes to environmental degradation, yet if recaptured, could return significant amounts of P to agricultural systems. Hydrothermal carbonisation (HTC) can efficiently recover P, with the added ability to transform P species into potentially more desirable forms for direct application to crops. P dynamics in hydrochars have primarily examined P speciation and chemical extractability as indicators of P bioavailability, but few studies directly evaluate the agronomic effectiveness of hydrochars as P fertilisers. As such, there is a clear need to assess the suitability of hydrochar as a source of bioavailable P in plant systems and the influence of HTC synthesis conditions. Response Surface Modelling of HTC synthesis conditions (pH, temperature and time), revealed initial pH significantly influence P distribution. Mild conditions of 180 °C for 30 min at pH 8.0 maximised P recovery (99%) along with carbon (62%) and nitrogen (43%) in hydrochars. Systematic characterisation of hydrochar P by chemical extraction and P L_2,3-edge X-ray absorption near edge spectroscopy revealed H₂O, NaHCO₃ and NaOH- P fractions were significantly (p < 0.05) reduced in all hydrochars, while HCl-P fraction increased with HTC temperatures at pH 7. In contrast, P L_2,3-edge XANES spectra were remarkably similar in raw sludges and corresponding hydrochars, regardless of HTC temperature or pH, revealing P was predominantly present as ferric phosphate with some hydroxyapatite. Multiple linear regression modelling suggested a significant relationship between chemical extractability and P bioavailability to wheat present in the raw sludges and hydrochars. This research provides further insight into the potential to use hydrothermal treatment for recovery and agricultural reuse of P, the importance of operational conditions on P transformation and the relationship between P speciation and bioavailability. The value of sewage sludge in a more sustainable global P cycle is also highlighted.

Co-Pyrolysis of Sewage Sludge and Wetland Biomass Waste for Biochar Production: Behaviors of Phosphorus and Heavy Metals

Large amounts of sewage sludge (SS) and wetland plant wastes are generated in the wastewater treatment system worldwide. The conversion of these solid wastes into biochar through co-pyrolysis could be a promising resource utilization scheme. In this study, biochar was prepared by co-pyrolysis of SS and reed (Phragmites australis, RD) using a modified muffle furnace device under different temperatures (300, 500, and 700 °C) and with different mixing ratios (25, 50, and 75 wt.% RD). The physicochemical properties of biochar and the transformation behaviors of phosphorus (P) and heavy metals during the co-pyrolysis process were studied. Compared with single SS pyrolysis, the biochar derived from SS-RD co-pyrolysis had lower yield and ash content, higher pH, C content, and aromatic structure. The addition of RD could reduce the total P content of biochar and promote the transformation from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). In addition, co-pyrolysis also reduced the content and toxicity of heavy metals in biochar. Therefore, co-pyrolysis could be a promising strategy to achieve the simultaneous treatment of SS and RD, as well as the production of value-added biochar.

Correlating phosphorus transformation with process water during hydrothermal carbonization of sewage sludge via experimental study and mathematical modelling

Recently, hydrothermal carbonization (HTC) based phosphorus (P) recovery from sewage sludge (SS) has attracted considerable interests worldwide. However, they concentrated on P transformation in the hydrochars, while ignored that the variations of process water (PW) might influence P transformation, since it exposed to water thoroughly during HTC. In this study, correlation of P transformation with PW were examined via experimental study and mathematical modelling. The results showed that statistical significance (p < 0.05) of HTC temperature and feedwater pH on NH₄⁺-N concentration in the PW was observed due to deamination and ring opening reactions of amino acids, confirming by their excellent correlation with R² = 0.988. NH₄⁺-N concentration dominated increasing PW pH, which stimulated the transformation of NAIP to AP. Associated model was developed with satisfactory R² = 0.938. Although P transformation during HTC was significantly influenced by HTC temperature and feedwater pH, supporting by their strong correlation with R² = 0.956, its transformation was PW pH dependent. Ultimately, detailed P transformation pathways during HTC was proposed with incorporation into the impact of PW. This work can provide new insights into HTC-based P transformation in the pristine SS.

Physico-chemical and biological treatment strategies for converting municipal wastewater and its residue to resources

An increase in urbanization and industrialization has not only contributed to an improvement in the lifestyle of people, but it has also contributed to a surge in the generation of wastewater. To date, conventional physico-chemical and biological treatment methods are widely used for the treatment of wastewater. However, the efficient operation of these systems require substantial operation and maintenance costs, and the application of novel technologies for the treatment and disposal of sludge/residues. This review paper focuses on the application of different treatment options such as chemical, catalyst-based, thermochemical and biological processes for wastewater or sludge treatment and membrane-based technologies (i.e. pressure-driven and non-pressure driven) for the separation of the recovered products from wastewater and its residues. As evident from the literature, a wide variety of treatment and resource recovery options are possible, both from wastewater and its residues; however, the lack of planning and selecting the most appropriate design (treatment train) to scale up from pilot to the field scale has limited its practical application. The economic feasibility of the selected technologies was critically analyzed and the future research prospects of resource recovery from wastewater have been outlined in this review.

Influence of rice husk addition on phosphorus fractions and heavy metals risk of biochar derived from sewage sludge

This study investigated the effects of rice husk dose and pyrolysis temperature on the phosphorus (P) fractions and environmental risk of heavy metals in biochar co-pyrolyzed from sewage sludge and rice husk. Biochar properties were analyzed, and the transformation of P and heavy metals speciation during co-pyrolysis were also discussed. Co-pyrolysis of raw sludge and rice husk (10-50 wt%) could increase the carbonization degree and stability of biochar at 500 °C. The organic P (OP) in raw sludge (68 wt%) was transformed to inorganic P (IP) during co-pyrolysis, indicating that the addition of rice husk could improve biochar-P bioavailability by promoting the transformation of IP. The IP content increased from 71.5 wt% of sludge biochar to 92 wt% of blended biochar (50 wt% sludge and 50 wt% rice husk) at a pyrolysis temperature of 500 °C. With the mass ratio of sludge to rice husk of 5:5, the OP content decreased from 3 mg g^-1 to 0.75 mg g^-1 as the pyrolysis temperature increased from 300 °C to 700 °C. The ³¹P nuclear magnetic resonance spectra and X-ray photoelectron spectroscopy results showed that P species in biochar mainly existed as orthophosphate, which can be directly taken up by plants. After co-pyrolysis, the toxicity and mobility of heavy metals gradually decreased with increasing rice husk dose and pyrolysis temperature. The study indicates that co-pyrolysis of sewage sludge and rice husk could be a promising P reuse strategy.

Species, fractions, and characterization of phosphorus in sewage sludge: A critical review from the perspective of recovery

Phosphorus recovery from municipal sewage sludge is a promising way to alleviate the shortage of phosphorus resources. However, the recovery efficiency and cost depend greatly on phosphorus species and fractions in different sewage sludges, i.e., waste activated sludge and chemically enhanced primary sludge. In this review, the phosphorous (sub-)species and fractions in waste activated sludge and chemically enhanced primary sludge are systematically overviewed and compared. The factors affecting phosphorus fractions, including wastewater treatment process, as well as sludge treatment methods and conditions are summarized and discussed; it is found that phosphorus removal method and sludge treatment process are the dominant factors. The characterization methods of phosphorus species and fractions in sewage sludge are reviewed; non-destructive extraction of poly-P and microscopic IP characterization need more attention. Anaerobic fermentation is the preferable solution to achieve advanced phosphorus release both from waste activated sludge and chemically enhanced primary sludge, because it can make phosphorus species and fractions more suitable for recovery. A post low strength acid extraction after anaerobic fermentation is recommended to facilitate phosphorous release and improve the total recovery rate.

Speciation of Main Nutrients (N/P/K) in Hydrochars Produced from the Hydrothermal Carbonization of Swine Manure under Different Reaction Temperatures

Hydrothermal carbonization (HTC) has been proved to be a promising technology for swine manure (SM) treatment. Currently, there is a lack of systematic understanding of the transformation characteristics of nutrient speciation in the HTC of SM. In this study, the speciation of the main nutrients (N/P/K) in SM-derived hydrochar produced at different reaction temperatures (200-280 °C) was investigated. The recovery of P (61.0-67.1%) in hydrochars was significantly higher than that of N (23.0-39.8%) and K (25.5-30.0%), and the increase in reaction temperature promoted the recovery of P and reduced the recovery of N. After the HTC treatment, the percentage of soluble/available P was reduced from 61.6% in raw SM to 4.0-23.9% in hydrochars, while that of moderately labile/slow-release P was improved from 29.2% in raw SM feedstock to 65.5-82.7%. An obvious reduction was also found in the amounts of available N (from 51.3% in raw SM feedstock to 33.0-40.5% in hydrochars). The percentages of slow-release N and residual N in hydrochars produced at 240 °C reached the maximum and minimum values (46.4% and 18.9%), respectively. A total of 49.5-58.3% of K retained in hydrochars was residual (invalid) potassium. From the perspective of the mobility and availability of N, P and K only, it was suggested that the HTC of SM should be carried out at 220-240 °C. Compared with the original SM, it is safer and more effective to use the SM-derived hydrochar as an organic fertilizer.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

Biochar and hydrochar derived from freshwater sludge: Characterization and possible applications

Freshwater sludge (FS) is generated in large quantities during the production of drinking water every day. It is largely underutilized, and has long been filter pressed to sludge cake and then disposed of in landfills. The search for more economical and sustainable disposal or reuse options is urgently needed. Biochar and hydrochar are increasingly popular wastes derived materials with huge potential for soil improvement, environmental remediation, and mitigation of climate change, but there is a lack of research on the production of FS derived biochar and hydrochar. In this study, biochar was produced by pyrolysis at 300, 500 or 700 °C for 1 h, and hydrochar was produced by hydrothermal carbonization (HTC) at 140, 160, 180 or 200 °C for 4 h. Proximate analyses show that the biochar has a higher carbon stability and is possibly suitable for carbon sequestration, while the hydrochar contains more labile carbon structures. The ultimate analysis indicates that the surface hydrophobicity is in the order of: biochar > hydrochar > FS. The phytotoxicity tests indicate their positive effects on germination of wheat seeds. This study provides a new treatment to reuse numerous FS and put forward the possible applications of its carbonaceous products, which is expected to facilitate a circular economy and realize the zero-waste target.

Thermochemistry of sulfur during pyrolysis and hydrothermal carbonization of sewage sludges

Sulfur (S) is an abundant and redox-active element in urban wastewater systems and plays a critical role in both the wastewater and sludge treatment processes. This study comparatively characterized the transformation of S and several closely associated metals (Cu, Zn, and Fe) during pyrolysis (250 to 750 °C) and hydrothermal carbonization (HTC, 150 to 275 °C) treatments of sewage sludge. S, Fe, Zn, and Cu K-edge X-ray absorption spectroscopy was applied to quantitatively evaluate the fate of S and contribution of different S species in regulating metal speciation. During pyrolysis, aliphatic-S and sulfonate were preferentially degraded at low temperature (below 350 °C) and sulfate was thermochemically reduced at temperature above 450 °C, while metal sulfides (up to 27%) and thiophenes (up to 70%) were increasingly formed. Similar to the pyrolysis process, metal sulfides (up to 40% at temperature above 200 °C) and thiophenes were formed during HTC. The degradation of thiols and organic sulfide, as well as sulfate reduction, released sulfide and strongly affected metal speciation. For example, almost all Cu and half of Zn were transformed into Cu-Fe- or Zn-Fe-sulfides during HTC, whereas they were partially desulfidized during pyrolysis. High abundance of reduced S species (S^-1 and S^-2) in hydrochars may contribute to their strong reductive adsorption of Cr(VI). Results from this work reveal the thermochemical reactions driving the transformations of S and its associated metals during pyrolysis and HTC. The results provide fundamental knowledge for selecting thermochemical sludge treatment techniques for value-added applications of the products.

Migration and transformation mechanism of phosphorus in waste activated sludge during anaerobic fermentation and hydrothermal conversion

This study investigated migration and transformation mechanism of P in waste activated sludge (WAS) during anaerobic fermentation (AF) process and the subsequent hydrothermal conversion (HTC) process. Control of pH during the AF processes was found to be significant, whereby the use of acidic (pH = 5.5) or alkaline conditions (pH = 9.5) facilitated the release of either apatite phosphorus (AP) or non-apatite inorganic phosphorus (NAIP) and organic phosphorus, respectively. At the same pH of 9.5, NaOH promoted the transfer of P into liquid phase, and P in the solid phase was mainly in the form of NAIP. In contrast, Ca(OH)₂ enhanced the incorporation of P into the solid products, with the P mainly in the form of AP. The subsequent HTC process promoted the NAIP transferred to AP, and the bioavailability of P in the HTC solid products was decreased. The P K-edge X-ray absorption near edge structure analysis provided detailed information about the phosphates. It demonstrated that the conversion of Ca₈H₂PO₄·6.5H₂O to Ca₅(PO₄)₃·OH was facilitated by HTC under the alkaline condition. This study sheds lights on transformation mechanism of P speciations during AF and HTC processes, which would provide fundamental information for effective utilization of P in bio-wastes.

Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive

Phosphorus (P) recovery from sewage sludge is one of the promising substitutes to the phosphate ores to alleviate the shortage of P supply in the future. In this study, how hydrothermal carbonization (HTC) temperature functioned in the migration and transformation of P from sewage sludge during HTC process was clarified. The enhancement effect of Ca-based additive on the transformation of P to hydroxyapatite via HTC was systematically studied. Additionally, leaching characteristics of P in the hydrochars with Ca-based additive were analyzed to assess their P-bioavailability. Results indicated that favoring by increased pH in the process water, HTC temperature played a significant role in the migration and transformation of P during HTC. Therefore, higher HTC temperature was beneficial for P enrichment and transformation to apatite phosphorus in the hydrochars. Both alkaline environment and existence of sufficient calcium ions were essential for enhanced formation of apatite phosphorus during HTC. Adding CaO could stimulate almost complete transformation of non-apatite inorganic phosphorus to apatite phosphorus such as hydroxyapatite, causing a maximum increase of apatite phosphorus by 252%. Concurrently, P-bioavailability by 2% (w/w) formic acid extraction of the hydrochars increased by 233% at CaO = 4%. These findings were confirmed by XRD analyses. Ca-associated apatite phosphorus such as Ca₅(PO₄)₃OH and Ca₂P₂O₇ ∙ 2H₂O, was detected in the hydrochars. With the assistance of thermochemical calculation, transformation pathways of P after CaO addition have been proposed, which coincided with our experimental results well. The stable P-containing minerals of hydroxyapatite (Ca₅(PO₄)₃OH) was calculated at equilibrium state.

The influence of inorganic components and carbon-oxygen surface functionalities in activated hydrothermally carbonized waste materials for water treatment

In this study, we have examined how the activation of hydrothermally carbonized sewage sludge and horse manure influences the inorganic component of these materials and surface chemistry. This was examined through statistical correlations between kinetic tests using trimethoprim, fluconazole, perfluorooctanoic acid, and copper, zinc, and arsenic and physicochemical properties. Yield and inorganic content varied considerably, with potassium hydroxide-activated materials producing lower yields with higher inorganic content. Phosphoric acid activation incorporated inorganically bound phosphorus into the material, although this showed no statistically relevant benefit. A maximum surface area of 1363 m²g^-1 and 343 m²g^-1 was achieved for the horse manure and sewage sludge. Statistical analysis found positive correlations between carbon-oxygen functionalities and trimethoprim, fluconazole, perfluorooctanoic acid, and copper removal, while inorganic content was negatively correlated. Conversely, arsenic removal was positively correlated with inorganic content. This research provides insight into the interactions with the organic/inorganic fraction of activated waste materials for water treatment.

Process Waters from Hydrothermal Carbonization of Sludge: Characteristics and Possible Valorization Pathways

Hydrothermal carbonization (HTC) is an innovative process capable of converting wet biodegradable residues into value-added materials, such as hydrochar. HTC has been studied for decades, however, a lack of detailed information on the production and composition of the process water has been highlighted by several authors. In this paper the state of the art of the knowledge on this by-product is analyzed, with attention to HTC applied to municipal and agro-industrial anaerobic digestion digestate. The chemical and physical characteristics of the process water obtained at different HTC conditions are compared along with pH, color, organic matter, nutrients, heavy metals and toxic compounds. The possibility of recovering nutrients and other valorization pathways is analyzed and technical feasibility constraints are reported. Finally, the paper describes the main companies which are investing actively in proposing HTC technology towards improving an effective process water valorization.

The transformation of phosphorus fractions in high-solid sludge by anaerobic digestion combined with the high temperature thermal hydrolysis process

Transforming inactive phosphorus (P) to active P to recover it from waste activated sludge is important. The transformation of P fractions from high-solid sludge by the anaerobic digestion (AD) and acidification phase of AD (AAD) combined with a high temperature thermal hydrolysis process (HTTHP) was investigated. The results showed that the sequence of P release effects by three processes was HTTHP + AAD > AD + HTTHP > HTTHP + AD. The PO₄^3--P release from high-solid sludge was directly affected by the temperature of HTTHP. At 140 °C, each process had more PO₄^3--P release than that at 160 °C. The total amount of PO₄^3--P release in AD + HTTHP was approximately 6 times that of HTTHP + AD. Based on the experimental results, a new process of mesophilic AD - post HTTHP was recommended, in which, enhancement of P release by sulfide ions was also proposed.

Coevolution of Iron, Phosphorus, and Sulfur Speciation during Anaerobic Digestion with Hydrothermal Pretreatment of Sewage Sludge

Anaerobic digestion (AD) with hydrothermal (HT) pretreatment is an emerging technology for enhanced resource recovery from sewage sludge. This study investigates the speciation of Fe, P, and S during sequential HT-AD treatment of sewage sludge using sequential chemical extraction, X-ray diffraction, and X-ray absorption spectroscopy. Results suggest strong correlations between Fe and P species as well as Fe and S species, affecting the solubility and bioavailability of each other. For instance, much vivianite formed in the hydrochars after HT treatment at low temperature, while more strengite precipitated at higher HT temperature. During the subsequent AD process, microbial reduction of strengite and other Fe(III) species led to the formation of more vivianite, with concurrent P release into the solution and adsorption onto other minerals. HT pretreatment of sewage sludge had a weak effect on the sulfidation of Fe during the AD process. This work has important implications for understanding the nutrient speciation and availability in sludge-derived hydrochars and AD solids. It also provides fundamental knowledge for the selection and optimization of HT pretreatment conditions for enhanced resource recovery through sequential HT-AD process.

The impact of hydrothermal carbonization on the surface functionalities of wet waste materials for water treatment applications

Hydrothermal carbonization (HTC) is an energy-efficient thermochemical process for converting wet waste products into value added materials for water treatment. Understanding how HTC influences the physicochemical properties of the resultant materials is critical in optimizing the process for water treatment, where surface functionality and surface area play a major role. In this study, we have examined the HTC of four wet waste streams, sewage sludge, biosludge, fiber sludge, and horse manure at three different temperatures (180 °C, 220 °C, and 260 °C). The physicochemical properties of these materials were examined via FTIR, SEM and BET with their adsorption capacity were assessed using methylene blue. The yield of solid material after hydrothermal carbonization (hydrochar) decreased with increasing temperature for all samples, with the largest impact on horse manure and fiber sludge. These materials also lost the highest degree of oxygen, while HTC had minimal impact on biosludge and sewage sludge. The differences here were due to the varying compositions of each waste material, FTIR identified resonances related to cellulose in horse manure and fiber sludge, which were not detected in biosludge and sewage sludge. Adsorption capacities varied between 9.0 and 68 mg g^-1 with biosludge HTC at 220 °C adsorbing the highest amount. Adsorption also dropped drastically at the highest temperature (260 °C), indicating a correlation between adsorption capacity and HTC conditions. This was attributed to the loss of oxygen functional groups, which can contribute to adsorption. These results suggest that adsorption properties can be tailored both by selection of HTC temperature and feedstock.

A perspective on hydrothermal processing of sewage sludge

The US annually produces 79 million dry tons of liquid organic waste including sewage sludge. Anaerobic digestion can only reduce the sludge volume by 50% in mass, leaving the other half as a growing waste management and hygienic problem. Hydrothermal processing (HTP), a set of several chemical digestion processes, could be used to convert sewage sludge into valuable products and minimize potential environmental pollution risks. Specifically, hydrothermal carbonization and hydrothermal liquefaction have been extensively studied to sustainably manage sludge. Two of the main reasons for this are the high upscalability of HTP for public waste management and that it is estimated that HTP can recover eleven times more energy from waste products than landfilling. An integration of HTP with anaerobic digestion or recycling the soluble organics (in the HTP aqueous products) into the HTP process could lead to a higher overall rate of energy recovery for municipal sewage sludge.

Energy and Nutrient Recovery from Sewage Sludge and Manure via Anaerobic Digestion with Hydrothermal Pretreatment

Global expectation for sustainability has prompted the transition of practices in wastewater treatment plants toward not only waste management but also energy and nutrient recovery. It has been shown that low-temperature hydrotherm (HT) treatment can enhance downstream biogas production via anaerobic digestion (AD). Yet, because the application of combined HT and AD is still at an early stage, a systematic understanding of the dynamic speciation evolution of important elements is still lacking. This study investigates energy and nutrient recovery from sewage sludge and swine manure via combined HT-AD treatment. Bench-scale investigation was conducted to evaluate biogas production and understand the dynamic evolution of organic carbon (C) and phosphorus (P) speciation. C and P speciations were characterized using complementary chemical and spectroscopic techniques, including ¹³C nuclear magnetic resonance (NMR) spectroscopy, P X-ray absorption near edge structure (XANES) spectroscopy, and sequential chemical extraction. Results from this study suggest that low-temperature HT pretreatment can achieve enhanced biogas production for sludge compared to the minimal effect on the biogas production from manure. It also provides guidance for P recovery from liquid digestate and solid residue after the AD process.

Experimental and thermodynamic studies of phosphate behavior during the hydrothermal carbonization of sewage sludge

Recovered phosphate from sewage sludge is becoming a key product in the fertilizer market. This study investigates the fate of phosphate during the hydrothermal carbonization of digested sewage sludge to support the development of an economic and sustainable solution for dealing with sewage sludge for phosphate recovery. The solid products from the hydrothermal carbonization of digested sewage sludge in a batch reactor (180, 220, and 260 °C; 1, 2, and 4 h; digested sewage sludge-to-water ratios of 0.2 and 0.1 w/w) were analyzed using a sequential chemical extraction procedure to understand and predict the formation of phosphate species and the related extraction behavior of phosphate. The obtained results were compared with the thermochemical equilibrium composition of hydrothermal carbonization products, calculated using the software FactSage 7.2. The majority of phosphate was retained as Al, Ca, and Fe salts in hydrochar. The decomposition of organic phosphates was observed by processing at lower temperatures. Hydrothermal carbonization at temperatures higher than 180 °C resulted in the transformation of the Al-bound phosphate into Ca-bound phosphate. Hydroxyapatite (Ca₅(PO₄)₃(OH)) and Fe₇(PO₄)₆ were calculated as stable phosphate-containing minerals at equilibrium. This study suggests that kinetic constraints inhibit the formation of these minerals in the batch reactor and presents a mechanism of phosphate transformation using the obtained data. The results allow for targeted optimization of phosphate recovery strategy.

Phosphorus complexation of sewage sludge during thermal hydrolysis with different reaction temperature and reaction time by P K-edge XANES and 31P NMR

Thermal hydrolysis pretreatment (THP) can improve sludge disintegration and biogas production. Phosphorus (P) is immobilized on hydrochars of sewage sludge (SS). It is critical to understand changes in P speciation in SS hydrochars under different reaction temperatures and reaction times during THP for reclamation and (re)cycling of P. This study combined sequential extraction, ³¹P liquid and solid-state nuclear magnetic resonance (NMR), and P K-edge X-ray absorption near edge structure (XANES) spectroscopy to systematically analyze variation in P speciation and related metals in hydrochars. The temperature of the THP has more influence on P extraction content than reaction time; the bioavailability of P from hydrochars declined with increasing temperature. A ³¹P liquid and solid-state NMR analysis demonstrated that orthophosphate was the most abundant phosphate present small deviations in the chemical shift were observed in different samples under different conditions. The results of XANES demonstrated that CaP was more stable than FeP and AlP. LCF results suggested OcataCa proportion raised under stable CaP ratio in samples when the temperature was above 155 °C and duration was 60 min. This study provides a theoretical basis that can meet the practical application of THP for sludge disposal and P reclamation on subsequent anaerobic digestion.

Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus

The recovery of phosphorus from sewage sludge was critical due to the depletion of phosphate ore. The present research aims to identify the phosphorus speciation and reveal the phosphorus transformation mechanism of dewatered sewage sludge during hydrothermal conversion (HTC) process, as well as to achieve the high efficiency recovery of phosphorus. Multiple analysis of SMT method, VK diagram, XANES and NMR showed that most phosphorus (>80%) was transferred to the hydrochar and presented as inorganic phosphorus (IP) after the HTC process. A dehydration trend was observed of the HTC process with the increase of sub-critical temperature. Ca-associated phosphorus increased significantly as the temperature increased. The Pyro-P gradually transformed to Ortho-P with the increase of HTC temperature and disappeared at 320 °C. The addition of HCl (6.13 and 12.3 mmol/g) in the HTC process resulted in a high percentage (>80%) of phosphorus transferred to the aqueous phase, and the bioavailability of the residual phosphorus increased significantly. The recovery rate of phosphorus could achieve 98.37% at the pH of 7.52, with the struvite purity of 90.41%. The results of this study provide new insights into the selective transfer of phosphorus in dewatered sludge by HTC process, in addition to some efficient ways for the utilisation of the HTC products.

Hydrothermal carbonization of sewage sludge: A critical analysis of process severity, hydrochar properties and environmental implications

Hydrothermal carbonization (HTC) of sewage sludge reduces the waste volume and can be source of energy and valuable products. Furthermore, HTC offers several advantages over conventional dry-thermal pre-treatments, as no prior drying is requested, and the high quality of the char produced promotes applications as energy production and storage, wastewater remediation, and soil amendment. Relationships between char yields, physicochemical properties and process parameters are here analysed, with the aim to provide insight into the choice of the process severity required to fit the desired application. Moreover, presence and fate of heavy metals and organic contaminants are discussed. The highest reaction temperature is the main parameter affecting the physicochemical characteristics of the char produced, while the heating rate governs the heat mass transfer and the rate of intermediates formation. Depolymerization of the biomass results in a reduction of the oxygen to carbon ratio and, therefore, in augmented high heating values, further increased by deposition of 5-(hydroxymethyl)furfural. Recirculation of process water may enhance dehydration reactions and the deposition of degraded polymers, increasing dewaterability and yield, but field trials are recommended to assess the feasibility of this option. An overuse of chars for energy generation purposes would be deleterious for the environmental life cycle. Further research is encouraged to assess the pollutants abatement and their degradation pathways when incorporated in the carbonaceous product, to promote the application of hydrochars as soil amendment, as well as for environmental remediation purposes.

Management of biosolids-derived hydrochar (Sewchar): Effect on plant germination, and farmers' acceptance

Hydrothermal carbonization is a promising approach of biosolids management and its utilization as a soil amendment. This study evaluated the physical and chemical properties of hydrothermally converted biosolids (Sewchar) and its effect as a potential soil amendment on the growth of rice, beans, and radish. The germination experiment was conducted in a greenhouse in a randomized design using five Sewchar doses (0, 10, 20, 40 and 60 Mg ha^-1). The results showed that hydrothermal carbonization influences the physicochemical properties of the biosolids, such as promoting pore structure and trace elements below the threshold values for use in agriculture. The spectroscopic techniques demonstrated higher presence of oxygen-containing functional groups (e.g., CO/OH) on surfaces of Sewchar than that of biosolids. The Sewchar doses of 10 Mg ha^-1 and 60 Mg ha^-1 yielded the highest dry biomass for beans and rice respectively. Increasing Sewchar doses negatively correlated with radish dry biomass, as indicated by linear regression equation fitting (p < 0.05). Thus, biomass responses to Sewchar application into the soil varied with Sewchar dose and type of plant. For a proper environmental management, a survey was conducted to assess farmers' perception and acceptance of Sewchar as a soil amendment. The survey revealed that younger farmers who had higher education qualifications were more prone to use Sewchar as soil amendment. Additionally, farmers who would not use Sewchar as soil amendment attributed the highest level of importance to economic criteria, such as fertilizer and freight prices. In the future, studies on a longer term under field conditions should be performed to elucidate the interactions between Sewchar and soil properties on plant growth and to ensure the safe use of Sewchar as a soil amendment.

Amino-functionalized sewage sludge-derived biochar as sustainable efficient adsorbent for Cu(II) removal

Sludge biochar as promising low-cost adsorbent has increasingly gained interests, but its poor surface functionality severely undermines its adsorption capacity and selectivity. Here a facile amino functionalization strategy was first proposed to enhance the surface functionality of sewage sludge derived biochar (SSDB) via the combination of sol-gel process for mesoporous silica coating and silylation for highly selective removal of Cu(II). The prepared amino-functionalized SSDB showed excellent adsorption capacity of 74.51 mg/g at room temperature, increasing by nearly 118% with regard to the unfunctionalized SSDB, and prominent selectivity (minute separation factor SF_{Co, Ni, Zn/Cu}) toward Cu(II) uptake. Characterizations demonstrated amine groups on the SSDB, the surface density of which reached 1.34 mg/m². The adsorption kinetics of Cu(II) on amino-functionalized SSDB was well described by a pseudo-second order kinetic model while the adsorption isotherm data was well fitted by Sips model. The pH range in which the adsorption preferentially occurred was 3-5. The occurrence of amine group protonation undermined adsorption performance at a pH < 3. These effects of amino-functionalized SSDB toward Cu(II) uptake were a result of the grafted amine groups specifically complexing with Cu(II) in the tetrahedron. Hence, prominent adsorption performance and low-cost feedstock make amino-functionalized SSDB a sustainable adsorbent for Cu(II) removal in water resulting in a cleaner utilization of sewage sludge.