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

Combined effect of thermal hydrolysis process and low-temperature pyrolysis on the classification and bioavailability of phosphorus in sewage sludge

Existing phosphorus (P) resources are becoming increasingly scarce, so it is necessary to recover P from potential sources. This paper is based on thermal hydrolysis process (THP) at 140-180 °C, coupled with low-temperature pyrolysis at 300 °C, to study its effect on the recovery and conversion of P from sewage sludge. Most significant change was observed in apatite P, which increased from 3.43 ± 0.48 mg/g in raw sludge to 30.17 ± 1.17 mg/g in biochar (BTHP-180-4-300) during optimal process (THP condition: 180 °C, 4 h; pyrolysis condition: 300 °C). Reactions between phosphates and metal ions became more complete during this combined process. Unstable forms of P were converted into more stable forms, with transformations from Al-P and Fe-P toward Ca-P compounds like Ca3(PO4)2, Ca3Mg3(PO4)4, Ca2P2O7, and Ca(H2PO4)2, making P less degradable and more suitable as slow-release fertilizers. Additionally, P characteristics of actual THP in a sewage treatment plant were similar to those of laboratory THP.

Simultaneously enhancement of methane production and active phosphorus transformation by sludge-based biochar during high solids anaerobic co-digestion of dewatered sludge and food waste: Performance and mechanism

Biochar has been proved to improve methane production in high solids anaerobic co-digestion (HS-AcoD) of dewatered sludge (DS) and food waste (FW), but its potential mechanism for simultaneous methane production and phosphorus (P) transformation has not been sufficiently revealed. Results showed that the optimal preparation temperature and dosage of sludge-based biochar were selected as 300 °C and 0.075 g·g-1, respectively. Under this optimized condition, the methane production of the semi-continuous reactor increased by 54%, and the active phosphorus increased by 18%. The functional microorganisms, such as Methanosarcina, hydrogen-producing, sulfate-reducing, and iron-reducing bacteria, were increased. Metabolic pathways associated with sulfate reduction and methanogenesis, especially hydrogenotrophic methanogenesis, were enhanced, which in turn promoted methanogenesis and phosphorus transformation and release. This study provides theoretical support for simultaneously recovery of carbon and phosphorus resources from DS and FW using biochar.

Enhancing Phosphorus Release from Sewage Sludge in Anaerobic Digestion via Thermal Hydrolysis Pretreatment: Insights from Phosphorus Speciation and Molecular Biological Pathways

This study explores the mechanisms enhancing phosphorus (P) release from sludge in anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) using sequential chemical extraction, X-ray absorption near-edge structure spectroscopy (XANES), 31P NMR, and multiomics. THP-treated sludge notably increased liquid-phase P by 53.8% over 3 days compared to sewage sludge (SS), identifying solid-phase Fe-P as the primary P source. The THP+AD also provided a higher abundance of bacteria that contributed to P release through multiple pathways (MPRPB), whereas SS+AD enriched some microbial species with single P release pathway. Moreover, species co-occurrence network analysis underlined the pivotal role of P-releasing bacteria in THP+AD, with 8 out of 16 keystones being P-releasers. Among the 63 screened genes that were related to P transformations and release, the poly beta-hydroxybutyrate (PHB) synthesis genes associated with polyphosphate bacteria-mediated P release were more abundant in THP+AD than in SS+AD. Furthermore, the upregulation of genes involved in methyl phosphonate metabolism in the THP-treated sludge enhanced the methane production potential of the AD process. These findings suggested that MPRPB were indeed the main contributors to P release, and enrichment in the THP+AD process enhanced their capability for P liberation.

Release of phosphorus through pretreatment of waste activated sludge differs essentially from that of carbon and nitrogen resources: Comparative analysis across four wastewater treatment facilities

The accumulation of phosphorus in activated sludge in wastewater treatment plants (WWTPs) provides potential for phosphorus recovery from sewage. This study delves into the potential for releasing phosphorus from waste activated sludge through two distinct treatment methods-thermal hydrolysis and pH adjustment. The investigation was conducted with activated sludge sourced from four WWTPs, each employing distinct phosphorus removal strategies. The findings underscore the notably superior efficacy of pH adjustment in solubilizing sludge phosphorus compared to the prevailing practice of thermal hydrolysis, widely adopted to enhance sludge digestion. The reversibility of phosphorus release within pH fluctuations spanning 2 to 12 implies that the release of sludge phosphorus can be attributed to the dissolution of phosphate precipitates. Alkaline sludge treatment induced the concurrent liberation of COD, nitrogen, and phosphorus through alkaline hydrolysis of sludge biomass and the dissolution of iron or aluminium phosphates, offering potential gains in resource recovery and energy efficiency.

Separation of nutrients and acetate from sewage sludge fermentation liquid in flow-electrode capacitive deionization system: Competitive mechanisms of ions and influence of activated carbon

Effective separation of volatile fatty acids (VFAs), ammonia (NH4+-N) and reactive phosphorous (RP) generated from anaerobic fermentation liquid is critically important for efficient resource recovery. Flow-electrode capacitive deionization (FCDI) is proven to be capable of efficient removal of ions, environmentally friendly and cost-effective in operation. The performances of FCDI system in the separation of NH4+-N, RP, and acetate and mechanism of pHs and activated carbon on their performances were investigated. Results showed that a pH of 5.0 promoted the removal of NH4+-N (53.1 %) and RP (39.5 %), and 72.0 % of acetate was retained in the solution, which revealed that removal of NH4+-N and RP, and retention of acetate were evidently affected by speciation of ions. Furthermore, the recovery of NH4+-N and RP was undermined by the adsorption of ions on activated carbon. This study provides a novel insight of ion selective mechanism during the operation of the FCDI system.

Anaerobic digestion of hydrothermally pretreated dewatered sewage sludge: effects of process conditions on methane production and the fate of phosphorus

The hydrothermal pretreatment (HTP) characteristics and the fate of phosphorus (P) and anaerobic digestion (AD) performance of dewatered sewage sludge (DSS) were investigated at different hydrothermal conditions. The maximum methane yield reached 241 mL CH₄/g COD when the hydrothermal conditions were 200 °C-2 h-10% (A4), and the yield was 78.28% higher than that without pretreatment (A0) and 29.62% higher than that of the initial hydrothermal conditions (A1, 140 °C-1 h-5%). Proteins, polysaccharides, and volatile fatty acids (VFAs) were the main hydrothermal products of DSS. 3D-EEM analysis revealed that tyrosine, tryptophan proteins, and fulvic acids decreased after HTP, but the content of humic acid-like substances increased, and this phenomenon was more noticeable after AD. Solid-organic P was converted into liquid-P during the hydrothermal process, and nonapatite inorganic P was converted into organic P during AD. All samples achieved positive energy balance, and the energy balance of A4 was 10.50 kJ/g VS. Microbial analysis showed that the composition of the anaerobic microbial degradation community changed as the sludge organic composition was altered. Results showed that the HTP improved the anaerobic digestion of DSS.

Role of humic substances and alkaline in phosphorus release from sludge pre-treated by (alkali-) hydrothermal

The scarcity of phosphorus (P) resources makes the recovery of P urgent. Sludge is a secondary resource rich in P, and the release of P from it is a key step for recovery. Hydrothermal (HT) is currently a popular method for sludge pretreatment, and its combination with alkaline (alkali-hydrothermal, AHT) could reduce the energy consumption in treatment. This study tried to compare their P release profiles in treating activated sludge in which organic P (OP) and non-apatite inorganic P (NAIP) were co-existence. Apart from the OP release in cell lysis, P release from NAIP brought by the joint effect of OH^- and humic substances (HS) formed in treatment was focused. The results showed that, compared to HT treatment, more P was released when OH^- participated (AHT), and the peak P release was observed at 160 °C. Variation of P distribution in the treated sludge revealed that more P was released from NAIP in AHT than in HT. HS formed in treatments was extracted and characterized. The amount and the structure of the HS varied significantly with the treatment conditions, and there was a linear correlation ship between PO₄^3--P release and the humic acid (HA) amount in HS. Mechanism study indicated there was a synergism between HS and OH^- in promoting PO₄^3--P release from NAIP. This study linked HS produced by sludge with P release, which provided a new perspective for subsequent P recovery from sludge.

Enhanced phosphorus release from waste activated sludge using ascorbic acid reduction and acid dissolution

Due to the widespread application of various iron (Fe)-derived substances used in phosphorus (P) removal during wastewater treatment, Fe-P species generated in this process constitute an important part of P speciation in non-digested sludge. SEM-EDS and sequential extraction methods were utilized to analyze the speciation, distribution, and spatial variation of P contained in the sludge. Inorganic P accounted for 91.3% of the total P, and Fe(III)-P represented the greatest percentage (68.5%) in the inorganic P fraction. Ascorbic acid, also known as vitamin C (VC), performed well in releasing P from sludge, especially in combination with subsequent pH adjustment to 3.0 using HCl. Fe(III)-P in sludge was first reduced to Fe(II)-P by VC, then dissolved in acidic conditions to release Fe²⁺ and PO₄^3-. Other metal-P compounds were also partially dissolved and released. VC disrupted the sludge floc structure, releasing organic P via organic efflux. There was a positive correlation (R²>0.97, p<0.05) between the amount of released P and the amount of reductant (VC). There was a synergistic effect between 120 mmol/L VC and acidity, producing the greatest P release of 67.1% of total sludge P. The P release efficiency achieved in this study was higher than other reported methods. Additionally, VC provides a more sustainable option due to its natural biodegradability. Released P and Fe²⁺ can be recovered as vivianite with recovery rates of 88% and 99%, respectively. This finding provides a new direction for effective, sustainable sludge P recovery and utilization.

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.

Phosphorus Recovery by Adsorption from the Membrane Permeate of an Anaerobic Membrane Bioreactor Digesting Waste-Activated Sludge

The recovery of phosphorus (P) from waste activated sludge (WAS) is a promising approach for sustainable resource management. During the anaerobic digestion of WAS, orthophosphate is released, and this P species is favorable for adsorption recovery. In the present study, an anerobic membrane bioreactor (AnMBR) with a P-adsorption column was developed to generate biogas from WAS and to recover P from membrane permeate simultaneously. The effects of the hydraulic retention time (HRT) and solid retention time (SRT) of the AnMBR on P solubilization were investigated. As a result, the maximum P solubilization was 21% when the HRT and SRT were 45 days and 100 days, respectively. Orthophosphate in the membrane permeate was adsorbed and recovered using a mesoporous material called zirconium sulfate–surfactant micelle mesostructure (ZS) in the column. The adsorbed P could be desorbed from the ZS with a NaOH solution, and P was recovered as a concentrated solution by a factor of 25. When the HRT was 19 days, the biogas yield and biogas production rate were 0.26 L/g-VSinput and 0.123 L/L/d, respectively. The average methane content in the biogas was 80%. The developed membrane-based process may be effective for resource recovery from WAS.

Enhancing phosphorus recovery from sewage sludge using anaerobic-based processes: Current status and perspectives

Anaerobic-based processes are green and sustainable technologies for phosphorus (P) recovery from sewage sludges economically and are promising in practical application. However, the P release efficiency is always not satisfied. In this paper, the P release mechanisms (regarding to different P species) from sewage sludge using anaerobic-based processes are systematically summarized. The obstacles of P release and the updated achievements of enhancing P release from sewage sludges are analyzed and discussed. It can be concluded that different P species can release from sewage sludge via different anaerobic-based processes. Extracellular polymeric substances and excessive metal ions are the two main limiting factors to P release. Acid fermentation and anaerobic fermentation with sulfate reduction could be two promising ways, with P release efficiencies of up to 64% and 63%. Based on the summarization and discussion, perspectives on practical application of P recovery from sewage sludge using anaerobic-based processes are proposed.