Clarification of phosphorus fractions and phosphorus release enhancement mechanism related to pH during waste activated sludge treatment

Study on the mechanisms of efficient phosphorus recovery by a pilot-scale biofilm sequencing batch reactor under low carbon demand

To study the mechanism of a novel pilot-scale biofilm sequencing batch reactor (PS-BSBR) for efficient phosphorus recovery under low carbon demand. The phosphate uptake/release performance and carbon source utilization efficiency of PS-BSBR and a typical enhanced biological phosphate removal (EBPR) -A2O process were compared, and the detection methods of different phosphorus forms were improved. The results showed that phosphate uptake/release content of PS-BSBR were 3.07 times and 4.47 times of that of A2O process under high carbon source utilization efficiency, respectively. The PS-BSBR mainly used inorganic phosphorus (IP) in the form of non-apatite inorganic phosphorus (NAIP) in EPS (85-90%), which was dependent on the adsorption of biologically induced extracellular polymers (EPS). The A2O process was mainly based on the IP in the form of NAIP (60-70%) in the cell for phosphate uptake and release, that was, relying on the biological phosphorus metabolism in the cell of polyphosphate-accumulating organisms (PAOs). Macroomics sequencing revealed that PS-BSBR had a variety of PAOs and a high-abundance glycogen-accumulating organisms (GAOs). By up-regulating the expression of key genes related to cellular phosphorus metabolism and EPS secretion, PS-BSBR promoted the phosphorus metabolism of PAOs cells and the biologically induced phosphate adsorption and desorption, which were dominated by the synthesis and decomposition of EPS. Therefore, the phosphorus absorption and release performance of PS-BSBR process was significantly better than that of A2O process. This study could provide theoretical support and regulatory guidance for the application of PS-BSBR process in sewage phosphorus recovery under the consumption of low carbon sources.

Unraveling the effects of polyhydroxyalkanoates accumulation in Pseudomonas extremaustralis growth and survival under different pH conditions

Polyhydroxyalkanoates (PHAs) are intracellular polymers that enhance bacterial fitness against various environmental stressors. Pseudomonas extremaustralis 14-3b is an Antarctic bacterium capable of accumulating, short-chain-length PHAs (sclPHAs), composed of C3-C5 monomers, as well as medium-chain-length PHAs (mclPHAs) containing ≥ C6 monomers. Since pH changes are pivotal in bacterial physiology, influencing microbial growth and metabolic processes, we propose that accumulated PHA increases P. extremaustralis fitness to cope with pH changes. To test this, we analyzed the production of sclPHA and mclPHA at different pH levels and its effect on bacterial survival against pH stress. P. extremaustralis was able to grow and accumulate PHA when the culture media pH ranged from 6.0 to 9.5, showing a marked loss of viability outside this range. Additionally, based on the analysis of different PHA-deficient mutants, we found that when exposed to both acidic and alkaline conditions, sclPHA and mclPHA conferred different protection against pH stress, with sclPHA making the main contribution. These results highlight the importance of PHA in supporting survival in pH-stressful environments.

The Pb capture mechanism of soil prophylactic agents prepared from phosphorus tailings and the influence of phosphorus speciation on its slow-release mechanism

This research activated phosphorus tailings to prepare a high‑phosphorus core (HPC) for multi-species composite slow-release heavy metal soil prophylactic agents (MCP), aiming to extend the slow-release period of MCP and enhance the efficiency of Pb stabilization. During the preparation of HPC, the proportion of non-apatitic inorganic phosphorus (NAIP) and apatite phosphorus (AP) continuously decreased with increasing polymerization temperature. At 400 °C, polyphosphates (PP) began to form, reaching 74.26 % at 600 °C. Initially, the rapidly soluble NAIP remained the major component of HPC, but the proportion of AP increased with higher polymerization temperatures, reaching 40.8 % at 600 °C. After 120 days of cultivation with four MCPs (MCP 300-21, MCP 400-12, MCP 500-14, MCP 600-14), the total soil phosphorus (TSP), soil organic matter (SOM), and Pb stabilization capacity of the cultivated soil showed significant improvements, reaching maximum values of 2.39 mg/g, 38.16 mg/g, and 45.4 mg/g, respectively, which are 9.9, 4.4, and 5.9 times higher than those of the CK soil. KEGG (Kyoto Encyclopedia of Genes and Genomes) functional prediction analysis indicated that MCPs contribute directly or indirectly to the forms and chemical stability of Pb by stimulating soil physiological and biochemical processes. This research proposes a novel approach for using phosphates in soil heavy metal management strategies and provides new insights into the mechanisms of heavy metal stabilization in soil using environmental functional materials.

Substituting phosphorus and nitrogen in hydroponic fertilizers with a waste derived nutrients solution: pH control strategies to increase substitution ratios

Hydroponics, despite its potential for efficient crop production, relies heavily on chemical fertilizers derived from nonrenewable resources and thus contributes to environmental burdens and unsustainable use of phosphorus. Integrating hydroponics into a circular phosphorus economy is crucial for mitigating these impacts. This study quantitatively assessed the capacity of filtrates from nitrified biogas digestate (f-NBD), a nutrient solution derived from organic waste, to replace phosphorus and nitrogen in hydroponic chemical nutrient solutions. Additionally, the influence of pH control methods on phosphorus recovery and substitution was investigated using a novel pH-rebound approach involving acidification followed by alkalinization to pH 6. The experimental results demonstrated that the pH-rebound method effectively dissolved apatite phosphorus, the predominant form of precipitated phosphorus in NBD, without inducing significant reprecipitation upon alkalinization. This pH adjustment enhanced the phosphorus solubility and optimized the nitrogen-to-phosphorus (N/P) ratio in f-NBD, enabling it to substitute up to 77% of the phosphorus and 100% of the nitrogen in standard hydroponic nutrient solutions. The study also revealed that, under certain conditions, f-NBD is as a more viable option for phosphorus recovery than struvite, a widely recognized recovered phosphorus product. The identified substitution ratios and pH optimization strategies provide valuable insights for mitigating the environmental burdens of hydroponic fertilizers. By partially replacing chemical nutrient solutions with f-NBD, hydroponics can be integrated more effectively into a circular phosphorus economy, reducing the reliance on nonrenewable resources and curtailing the environmental impacts associated with the production and use of conventional fertilizers. This research provides a basis for future initiatives aimed at developing sustainable hydroponic systems and offering new utilization methods for biogas digestate.

Integrating enhanced biological phosphorus removal in adsorption-stage to treat real domestic sewage

Wastewater treatment innovation toward resource recovery facilities raises concerns about the adsorption and bio-degradation (A-B) process. This study integrated enhanced biological phosphorus removal (EBPR) into the A-stage for real domestic sewage treatment using the short sludge retention time (S-SRT) approach. The S-SRT approach resulted in outstanding phosphorus (over 90 %) and COD removal (approximately 88 %), increased sludge yield and organic matter content, and a 1.68-fold increase in energy recovery efficiency by sludge anaerobic digestion. The inhibition of nitrification relieved competition for carbon sources between denitrification and phosphorus removal, allowing for the enrichment of phosphorus-accumulating organisms (PAOs) such as Tetrasphaera and Halomonas, leading to enhanced phosphorus removal activities. Biological adsorption also plays a significant role in achieving steady phosphorus removal performance. This study demonstrates the potential of the S-SRT approach as an effective strategy for simultaneous carbon and phosphorus capture in the A-stage, contributing to energy and nutrient recovery from sewage.

Efficient phosphorus recovery from waste activated sludge: Pretreatment with natural deep eutectic solvent and recovery as vivianite

Recycling phosphorus from waste activated sludge (WAS) is an effective method to address the nonrenewable nature of phosphorus and mitigate environmental pollution. To overcome the challenge of low phosphorus recovery from WAS due to insufficient disintegration, a method using a citric acid-based natural deep eutectic solvent (CA-NADES) assisted with low-temperature pretreatment was proposed to efficiently release and recover phosphorus. The results of 31P nuclear magnetic resonance (NMR) confirmed that low-temperature pretreatment promoted the conversion of organic phosphorus (OP) to inorganic phosphorus (IP) and enhanced the effect of CA-NADES. Changes in the three-dimensional excitation-emission matrix (3D-EEM) and flow cytometry (FCM) indicated that the method of CA-NADES with low-temperature thermal simultaneously release IP and OP by disintegrating sludge flocs, dissolving extracellular polymeric substances (EPS) structure, and cracking cells. When 5 % (v/v) of CA-NADES was added and thermally treated at 60 °C for 30 min, 43 % of total phosphorus (TP) was released from the sludge. The concentrations of proteins and polysaccharides reached 826 and 331 mg/L, respectively, which were 6.30 and 14.43 times higher than those of raw sludge. The dewatering and settling of the sludge were also improved. Metals were either enriched in the solid phase or released into the liquid phase in small quantities (most efficiencies of less than 10 %) for subsequent clean recovery. The released phosphorus was successfully recovered as vivianite with a rate of 90 %. This study develops an efficient, green, and sustainable method for phosphorus recovery from sludge using NADES and provides new insights into the high-value conversion of sludge.

Comprehensive insights into phosphorus solubility and organic matter's impact on black phosphate leaching

The current study introduces groundbreaking insights into how organic matter (OM) of the black phosphate (RB-Ph) uniquely influences phosphorus (P) solubility during acetic acid (AA) leaching, expanding our understanding in this crucial area. To highlight such role, the OM of the RB-Ph was treated separately by different procedures including calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph). The mineralogical, chemical and morphological characteristics of phosphatic and non-phosphatic components of these phosphatic materials were carefully examined pre- and post-treatment via different techniques. The P dissolution of the precursor RB-Ph and its modified derivatives all over the applied experimental parameters traced the following trend: NB-Ph > RB-Ph > CB-Ph > HB-Ph. Intensive grinding to nanoscale resulted in amorphous components with conspicuous OM content (TOC, 0.410%), significantly enhanced P dissolution rate of NB-Ph (730-980 ppm), despite the noticeable reduction in its P2O5 content to 22.34 wt.%. The precursor RB-Ph, thanks to its high OM content (TOC, 0.543%), also displayed a sufficient P dissolution rate (470-750 ppm) compared to the two other modified derivatives, CB-Ph (410-700 ppm) and HB-Ph (130-610 ppm). Such deep and conspicuous impact of OM on P solubility can be tied to their decomposition, releasing not only organic acids but also the adsorbed P by the OM's surficial binding sites to the solution. Finally, the optimum conditions of P leaching were attained at 2:1 acid/solid (w/w) ratio and 2 h of retention time of all investigated samples.

Efficient phosphate and hydrogen recovery from sludge fermentation liquid by sacrificial iron anode in electro-fermentation system

Electro-fermentation (EF) has been extensively studied for recovering hydrogen and phosphorus from waste activated sludge (WAS), while was limited for the further application due to the low hydrogen yield and phosphorus recovery efficiency. This study proposed an efficient strategy for hydrogen and vivianite recovery from the simulated sludge fermentation liquid by sacrificial iron anode in EF. The optimum hydrogen productivity and the utilization efficiency of short chain fatty acids (SCFAs) reached 45.2 mmol/g COD and 77.6% at 5 d in pH 8. Phosphate removal efficiency achieved at 90.8% at 2 d and the high crystallinity and weight percentage of vivianite (84.8%) was obtained. The functional microbes, i.e., anaerobic fermentative bacteria, electrochemical active bacteria, homo-acetogens and iron-reducing bacteria were highly enriched and the inherent interaction between the microbial consortia and environmental variables was thoroughly explored. This work may provide a theoretical basis for energy/resource recovery from WAS in the further implementation.

Enzymatic phosphatization of fish scales-a pathway for fish fossilization

Phosphatized fish fossils occur in various locations worldwide. Although these fossils have been intensively studied over the past decades they remain a matter of ongoing research. The mechanism of the permineralization reaction itself remains still debated in the community. The mineralization in apatite of a whole fish requires a substantial amount of phosphate which is scarce in seawater, so the origin of the excess is unknown. Previous research has shown that alkaline phosphatase, a ubiquitous enzyme, can increase the phosphate content in vitro in a medium to the degree of saturation concerning apatite. We applied this principle to an experimental setup where fish scales were exposed to commercial bovine alkaline phosphatase. We analyzed the samples with SEM and TEM and found that apatite crystals had formed on the remaining soft tissue. A comparison of these newly formed apatite crystals with fish fossils from the Solnhofen and Santana fossil deposits showed striking similarities. Both are made up of almost identically sized and shaped nano-apatites. This suggests a common formation process: the spontaneous precipitation from an oversaturated solution. The excess activity of alkaline phosphatase could explain that effect. Therefore, our findings could provide insight into the formation of well-preserved fossils.

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.

Fundamental properties and phosphorus transformation mechanism of soybean straw during microwave hydrothermal conversion process

Microwave hydrothermal (MHT) conversion is emerging as a promising technology for the disposal and reutilization of biowastes. This study investigated the fundamental properties and phosphorus transformation mechanism of soybean straw during the MHT conversion process. The oxygen-containing functional groups in soybean straw were stripped, and a trend of dehydration was observed as the temperature increased during the MHT process. Cellulose was identified as the major component of the MHT solid products at high temperature. Glucose and glucuronic acid in the MHT liquid products were gradually converted to formic acid and acetic acid with increasing temperature and holding time. The characteristics of the MHT products directly affected the changes in P speciation and transformation. Most of the P was distributed in liquid products and the impact of holding time was not significant on P distribution at low MHT temperature. With the increase in temperature and holding time, P gradually transferred into the solid products. The proportion of organic phosphorus and soluble inorganic phosphorus in soybean straw was high, and it decreased noticeably after the MHT process. The increase in MHT temperature promoted the conversion of OP and AP into IP and NAIP respectively. P K-edge X-ray absorption near edge structure analysis reveals that Ca5(PO4)3(OH) was the major component of soybean straw and more Ca5(PO4)3(OH) was formed at lower MHT temperature. This study provides fundamental knowledge on the property changes of soybean straw and the transformation of phosphorus during MHT conversion process, which is essential for its disposal and further utilization.

Clarification of the phosphorus release mechanism for recovering phosphorus from biofilm sludge in alternating aerobic/anaerobic biofilm system

A novel wastewater treatment plant process was constructed to overcome the challenge of simultaneous nitrate removal and phosphorus (P) recovery. The results revealed that the P and nitrate removal efficiency rose from 39.0 % and 48.4 % to 92.8 % and 93.6 % after 136 days of operation, and the total P content in the biofilm (TPbiofilm) rose from 15.8 mg/g SS to 57.8 mg/g SS. Moreover, the increase of TPbiofilm changed the metabolic mode of denitrifying polyphosphate accumulating organisms (DPAOs), increasing the P concentration of the enriched stream to 172.5 mg/L. Furthermore, the acid/alkaline fermentation led to the rupture of the cell membrane, which released poly-phosphate and ortho-phosphate of cell/EPS in DPAOs and released metal‑phosphorus (CaP and MgP). In addition, high-throughput sequencing analysis demonstrated that the relative abundance of DPAOs involved in P storage increased, wherein the abundance of Acinetobacter and Saprospiraceae rose from 8.0 % and 4.1 % to 16.1 % and 14.0 %. What's more, the highest P recovery efficiency (98.3 ± 1.1 %) could be obtained at optimal conditions for struvite precipitation (pH = 7.56 and P: N: Mg = 1.87:3.66:1) through the response surface method (RSM) simulation, and the precipitates test analysis indicated that P recovery from biofilm sludge was potentially operable. This research was of great essentiality for exploring the recovery of P from biofilm sludge.

Prediction of biological nutrients removal in full-scale wastewater treatment plants using H2O automated machine learning and back propagation artificial neural network model: Optimization and comparison

The effective control of total nitrogen (ETN) and total phosphorus (ETP) in effluent is challenging for wastewater treatment plants (WWTPs). In this work, automated machine learning (AutoML) (mean square error = 0.4200 ∼ 3.8245, R2 = 0.5699 ∼ 0.6219) and back propagation artificial neural network (BPANN) model (mean square error = 0.0012 ∼ 6.9067, R2 = 0.4326 ∼ 0.8908) were used to predict and analyze biological nutrients removal in full-scale WWTPs. Interestingly, BPANN model presented high prediction performance and general applicability for WWTPs with different biological treatment units. However, the AutoML candidate models were more interpretable, and the results showed that electricity carbon emission dominated the prediction. Meanwhile, increasing data volume and types of WWTP hardly affected the interpretable results, demonstrating its wide applicability. This study demonstrated the validity and the specific advantages of predicting ETN and ETP using H2O AutoML and BPANN model, which provided guidance on the prediction and improvement of biological nutrients removal in WWTPs.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

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).

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.

Sludge disintegration and phosphorus migration in anaerobic fermentation of waste activated sludge by the addition of EDTA-2Na

The effects of the addition of EDTA-2Na on sludge disintegration and phosphorus (P) migration during anaerobic fermentation (AF) of waste activated sludge (WAS) are investigated. The efficiency of sludge disintegration was positively correlated with the dose of EDTA-2Na from 0.5-2.0 g/g SS, and an enormous quantity of P was liberated into the aqueous phase, accompanied by sludge disintegration. The proper dose of EDTA-2Na for P release from WAS was 1.5 g/g SS, with an orthophosphate concentration of 394.72 mg/L. P release was more consistent with the pseudo second-order kinetic model. The migration of P species during AF with EDTA-2Na addition was also studied. Orthophosphate was the main species in both of the liquid phase and the loosely bound extracellular polymeric substances (EPS), but organic P (OP) was much more abundant in tightly bound EPS. Inorganic P (IP) was the dominant P speciation in the solid and was mainly distributed in the fraction of non-apatite IP, which accounted for more than 62.8% of IP in the presence of EDTA-2Na. In addition, both IP and OP in the solid contributed to the accumulation of P and the former was outperformed. Furthermore, the increased total dissolved P mainly came from cells. However, the fermented sludge tended to be smaller and to have low compressibility, which is detrimental to its further treatment.

Response of anaerobic digestion of waste activated sludge to types of alkalis: Contribution identification of metal ions

Alkaline pretreatment is one promising strategy for promoting anaerobic digestion of waste activated sludge (WAS). This study selected three types of alkalis with monovalent (NaOH and KOH), divalent (Ca(OH)₂ and Mg(OH)₂), and trivalent (Fe(OH)₃ and Al(OH)₃) cations to reveal the roles of metal ions on short chain fatty acids (SCFAs) production. The enhanced production potentials of SCFAs were reduced by order of alkalis with monovalent, divalent, and trivalent cations. Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no contributions on SCFAs production, while Fe³⁺ and Al³⁺ performed better than control, especially the latter. The mechanism analysis proved that Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no significant effects on solubilization, hydrolysis, acidification and methanogenesis stages, while the first three stages were improved by Fe³⁺ and Al³⁺ and the methanogenesis stage was inhibited. The findings may provide some new insights when using alkalis or residual metal ions to improve anaerobic digestion of WAS.

Acid/alkali pretreatment enhances the formation of vivianite during anaerobic fermentation of waste activated sludge

Phosphorus (P) recovery from waste activated sludge (WAS) of wastewater treatment plants is significant in the world suffering from P shortage. Recently, vivianite crystallization has been regarded as an essential method of recovering P from anaerobic fermentation (AF) of WAS. This study performed acid/alkali pretreatment (pH 3/pH 10) on AF of WAS to improve iron reduction and vivianite formation. The results showed that the maximum iron reduction rate (R_max) in the pH 3 and pH 10 groups was increased by 1.9 and 1.7 times compared with that in the Control-Fe group, and the iron reduction efficiency (E_Fe) was increased by 17.5% and 12.0% respectively. The Fe bound P (Fe-P) proportion in the sludge in the pH 3 and pH 10 groups increased by 50.0% and 33.7%, respectively. Furthermore, the relative abundance of the iron-reducing bacteria Clostridium_sensusensu in the pH 3 group was higher; and the Fe-P proportion in the sludge and the size of vivianite crystal after AF were larger. With these results, pH 3 pretreatment was preferred for promoting Fe²⁺ release and vivianite formation during AF.

Comparison on anaerobic phosphorus release and recovery from waste activated sludge by different chemical pretreatment methods: Focus on struvite quality and benefit analysis

Phosphorus recovery from waste activated sludge (WAS) is expected to alleviate the shortage of phosphate rock and reduce eutrophication. In this study, acid, alkali and sodium polyacrylate (PAAS) were compared to enhance phosphorus release and recovery from WAS. During anaerobic fermentation (AF) stage, the optimal pretreated conditions for ortho-phosphate release were the pH of 4 (AF 12 h), 13 (AF 12 h) and 22.4 g PAAS/L (AF 24 h) with the phosphorus release efficiencies of 40.9%, 62.6% and 31.7%, respectively. Acid, alkali and PAAS addition were beneficial for apatite phosphorus (AP), non-apatite inorganic phosphorus (NAIP) and organic phosphorus (OP) release from WAS, respectively. Strong acidic (pH = 4) and alkaline (pH = 12 and 13) conditions inhibited the release of soluble ammonia, while PAAS would not have a negative impact on the release of soluble ammonia. By means of precipitation crystallization, the ortho-phosphate could be almost recovered after acid/alkali pretreatment compared with PAAS (88.9%) at optimal Mg/P molar ratio of 1.5:1. The XRD, FT-IR and SEM-EDX analyses confirmed the main component in the product was struvite. The purity of the struvite in the product recovered from acid (named Pre_AC, 78.9%) and alkali (named Pre_AL, 89.6%) pretreated sludge were higher than that of the PAAS (named Pre_PA, 72.3%) by elemental analysis. The mercury and chromium content existed in Pre_AC were above the Control Standards of Pollutants in Sludge for Agricultural Use, whereas detected heavy metal elements level of the Pre_AL and Pre_PA were below the standard. By means of cost analysis, acid/alkali pretreatment could obtain economic benefits compared with PAAS. Thus, those discoveries would broaden the phosphorus recovery way to serve in practice.

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.

Speciation evolution and transformation mechanism of P during microwave hydrothermal process of sewage sludge

Due to the global shortage of phosphate ore, sewage sludge is an important resource for P recovery. This study aims to investigate how P was migrated and transformed during the microwave hydrothermal (MHT) process of sewage sludge. The effects of MHT and hydrothermal (HT) conversion were compared. The results reveals that there were no significant differences on the P distribution and speciation changes between the HT and MHT products, especially under high hydrothermal temperature. Ortho-P/Pyro-P was the dominant P form in the hydrothermal solid products, and high temperature promoted the transformation of C-O-P to Ortho-P/Pyro-P. The analysis of X-ray absorption near edge structure (XANES) shows that Ca₅(PO₄)₃OH was formed after the hydrothermal processes. The relative abundance of Ca-P decreased first and then increased with increasing hydrothermal temperature. Moderate MHT temperature (170 °C) and holding time (30-60 min) promoted the transformation of P to the liquid products. Generally, the effect of MHT temperature was more significant than that of heating type and holding time on the variations of P distribution and speciations.

Freezing-low temperature treatment facilitates short-chain fatty acids production from waste activated sludge with short-term fermentation

This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides, Romboutsia and Paraclostridium, were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS.

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.

Biological recovery of phosphorus from waste activated sludge via alkaline fermentation and struvite biomineralization by Brevibacterium antiquum

Struvite biomineralization is a promising method for phosphorus recovery from wastewater treatment plant streams, and the growth of responsible microorganisms in mixed cultures is one of the most critical points for applying this process in pilot and full-scale. This study aimed to investigate the growth and bio-struvite production of Brevibacterium antiquum in mixed sludge culture. Alkaline fermentation was applied at different pH conditions to enhance the phosphorus content of sludge for an efficient recovery, and pH 8 was determined as the most feasible considering the phosphorus release and sludge characteristics. Growth optimization studies showed that NaCl's presence decreases the growth rate of Brevibacterium antiquum and bio-struvite production. At the same time, pH in the range of 6.8-8.2 did not alter the growth significantly. In addition, studies showed the ability of Brevibacterium antiquum in unsterilized fermented sludge centrate to grow and recover the phosphorus as struvite. Thus, our results indicated the potential of struvite biomineralization in full-scale wastewater treatment plants.

Transformation of phosphorus by MgCl2 and CaCl2 during sewage sludge incineration

Phosphorus (P) recovery from sewage sludge (SS) have been regarded as an effective method of P recycling. The effects of incineration temperature, incineration time, and chlorine additives on the distribution of P speciation during sludge incineration were studied. Moreover, the reactions between model compounds AlPO4 and additives (MgCl2 and CaCl2) were investigated by thermogravimetric differential thermal analysis and X-ray diffraction measurements. The results demonstrated that the increase in temperature and time stimulated the volatilization of non-apatite inorganic phosphorus (NAIP) instead of apatite phosphorus (AP). MgCl2 and CaCl2 can greatly promote the conversion of NAIP to AP. Additionally, AlPO4 reacted with MgCl2 are incinerated at 500-600 °C to form Mg3(PO4)2, which is mainly due to the reaction of the intermediate product MgO and AlPO4. Reactions between AlPO4 and CaCl2 occurred at 700-750 °C and produced Ca2PO4Cl, which can be directly used with high bioavailability. These findings suggested that chlorine additives in the SS incineration process can obtain phosphorus-containing minerals with higher bioavailability to realize the resource utilization of P in sludge.

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.

Enhancement of phosphorus release from waste activated sludge by electrochemical treatment

The enhancement of phosphorus (P) released from waste activated sludge (WAS) by electrochemical treatment was investigated in this study. Results showed that the concentration of orthophosphate (ortho-P) and organic phosphorus (OP) in liquid both increased after electrochemical treatment. The ortho-P and OP concentration reached a maximum of 5.020 and 1.888 mg/L under the optimal condition respectively (voltage of 4.5 V and time of 60 min), which were 2.86 and 4.93 times higher than that in raw sludge. Meanwhile, the role of extracellular polymeric substances (EPS) in this process was also studied. The variation trends of P-release in tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS) were different. In TB-EPS, the concentration of total phosphorus (TP) and ortho-P decreased when the voltage increased. In contrast, the concentration of TP and ortho-P in LB-EPS increased and reached the maximum under the optimal condition. Released metal ions (Ca, Mg, Fe, and Al) had some effects on P-release both in liquid and EPS, which indicated that EPS played an important role. SCOD and TSS revealed that the disintegration of sludge was also enhanced by electrochemical treatment. Additionally, the P fractions in sludge phase suggested that OP was more likely to be released in liquid phase.

Waste activated sludge lysate treatment: Resource recovery and refractory organics degradation

Sludge lysate is an unavoidable and refractory liquid produced from the waste activated sludge hydrothermal pyrolysis, which contains plenty of hazardous refractory organic compounds and value-added organic resources. Here, the proof of concept for an integrated strategy that couples technically compatible pretreatment to microbial electrolysis assisted AD (ME-AD) system is investigated for sludge lysate treatment and resource recovery. The pretreatment process shows a positive effectiveness on the ME-AD by reducing the organic load and inhibitory matters, which promote the residual refractory organic compounds (Maillard reaction products and humic acid-like substances) and carbon sources further biodegradation and bio-transformation. Combining membrane separation with ME-AD increased not only both the yield and purity of methane to 268.76 mL CH₄/g COD and 98%, respectively, but also the recovery of 70.0~82.4% crude proteins (9.1 ± 0.5 g/L) from sludge lysate. Alternatively, the alkaline precipitation combined with ME-AD enhanced the recovery efficiency of short-chain fatty acids (SCFAs). The visible decreasing in the unpleasant color of the effluents was observed, implying that the degradation of harmful refractory organic was almost eliminated in sludge lysate. This strategy is worthy to be developed in WWTP for sludge lysate treatment with considerable bio-resources recovery and refractory organics removal.

Occurrence, effect, and fate of residual microplastics in anaerobic digestion of waste activated sludge: A state-of-the-art review

The plastic products have large consumption over last decades, resulting in a serious microplastics (MPs) pollution. Specially, the main removal way of MPs from wastewater is to transfer MPs from liquid to solid phase, leading to its enrichment in waste activated sludge (WAS). Anaerobic digestion has been served as the most potential technique to achieve both resource recovery and sludge reduction, herein this review provides current information on occurrence, effect, and fate of MPs in anaerobic digestion of WAS. The effects of MPs on WAS anaerobic digestion are greatly related to forms, particles sizes, contents, compositions and leachates of MPs. Also, the presence of MPs not only can change the effects of other pollutants on anaerobic digestion of WAS, but also can affect the fates of them. Besides, the future perspectives focused on the fate, effect and final removal of MPs during WAS anaerobic digestion process are outlined.

Improvement of phosphorus release from sludge by combined electrochemical-EDTA treatment

In this paper, combined with the addition of ethylenediaminetetraacetic acid (EDTA), the electrochemical treatment of waste activated sludge (WAS) was investigated to explore its effect on the release of phosphorus (P) from WAS. The results showed that during the electrochemical treatment, the addition of EDTA could significantly promote the release of P from the WAS to the supernatant, the optimal amount of EDTA was 0.4 g/g total suspended solids (TSS), when the release of total dissolved phosphorus (TDP), organic phosphorus (OP) and molybdate reactive phosphorus (PO₄^3--P) were 187.30, 173.84 and 13.46 mg/L, respectively. OP was the most likely form of P to be released during this process. Moreover, combined electrochemical-EDTA treatment could promote the release of P and metal ions from extracellular polymeric substances (EPSs) to the supernatant, and increase the solubility and disintegration of sludge. EDTA chelated the metal ions of sludge flocs and phosphate precipitates to cause sludge floc decomposition, thereby promoting the release of P from WAS.

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.

Response of anaerobic digestion of waste activated sludge to residual ferric ions

This study was conducted to investigate the effects of residual ferric ions (FI), released from iron or its oxides for wastewater or waste activated sludge (WAS) treatment, on anaerobic digestion of WAS. Herein it was found that the anaerobic digestion process was greatly affected by FI dosages as well as FI distributions. The responses of performance and microorganism suggested that a low FI (e.g., 0.125 mmol/g volatile suspended solid (VSS)) enhanced methane production by 29.3%, and a medium FI (e.g., 0.3 mmol/g VSS) promoted short chain fatty acids accumulation to reach the maximum of 247 mg chemical oxygen demand /g VSS, conversely, a high FI (e.g., 0.9 mmol/g VSS) led to severe inhibition on acidogenesis and methanogenesis. The findings may provide some new insights for mechanism understanding on anaerobic digestion process influenced by iron or its oxides, as well as the disposal of WAS contained FI.

Phosphorus concentration and availability in raw organic waste and post fermentation products

The aim of the study was to determine the mobility of phosphorus forms in raw organic waste and from the solid and liquid fractions of digestate. To achieve the purpose of this study, the components (including livestock manure, agricultural waste, food waste, sewage sludge) and their post fermentation products were considered. Subsequently, the effect of the fermentation process on the mobility of phosphorus forms in post-fermentation fractions (solid and liquid) was investigated. Then, the evaluation of the fertilising potential of digestate fractions was assessed. The available organic and inorganic phosphorus forms were determined according to the Standards in Measurements and Testing (SMT) Programme extraction protocol and according to the acid molybdate spectrophotometric method. It has been shown that phosphorus in digestates occurred mainly in inorganic forms with Fe, Al, Mn, Mg and Ca ions. Its proportion in relation to total phosphorus ranged from 80 to 90%. The lowest phosphorus content was found in digestate from the fermentation of agricultural and food waste (fruit and vegetables), while digestate from livestock manure and sewage sludge fermentation was rich in phosphorus. It was shown that the solid fractions of digestate represented from 30 to 70% of highly labile phosphorus (i.e. phosphorus with organic matter and in bonds with Al, Fe, Mg and Mn oxides and hydroxides) in relation to total phosphorus. However, the share of labile phosphorus forms in the liquid fraction of digestates was much higher and accounted for 80-90% of the total phosphorus.

The influence of a stepwise pH increase on volatile fatty acids production and phosphorus release during Al-waste activated sludge fermentation

In this study, the performance of volatile fatty acids (VFAs) production and phosphorus (P) release during Al-waste activated sludge (Al-WAS) anaerobic fermentation with stepwise pH increases from 8 to 11 was investigated via a long-term acclimation strategy. As results, VFAs concentration increased with increasing pH and the maximum yield of VFAs was 358.03 mg-COD/g VS at pH 11, which was much higher than at pH 8. P was also released during the process, and the P concentration increased gradually from 26 mg/L at pH 8 to 186 mg/L at pH 11, accounting for 35.8% of the total P in the Al-WAS. The P distribution results demonstrated the dissolution of non-apatite inorganic phosphorus (NAIP) and organic P in the sludge contributed to release P into the liquid at pH 8, 9, and 10, while inorganic P release originated from the dissolution of NAIP at pH 11.

Integration of electrochemical and calcium hypochlorite oxidation for simultaneous sludge deep dewatering, stabilization and phosphorus fixation

A hybrid electrochemical process with Ca(ClO)₂ addition for simultaneous sludge dewaterability, stabilization and phosphorus fixation was proposed. Under optimal conditions (150 mg/g VS Ca(ClO)₂, 15 V), the capillary suction time (CST) and specific resistance to filtration (SRF) were decreased by 88% and 92%, respectively. Efficient sludge stabilization with E. coli colonies of less than 1000 MPN/g TS was achieved. Phosphorus of 99% was removed from the filtrate and successfully fixed in the sludge cake and on the electrode surface. The integration of electrochemical and hypochlorite oxidation could effectively degrade the tightly bound extracellular polymeric substances (TB-EPS) structure with a total organic carbon (TOC) reduction of 52%. Besides, the disintegration of microbial cell envelopes was also achieved, with a reduction of living cell fraction of 98%. Furthermore, system pH could be maintained at near neutral (7.45) and the conversion of Fe(II) to Fe(III) was also facilitated with the addition of Ca(ClO)₂, resulting in improved electrocoagulation process for enhanced sludge dewatering and phosphorus fixation. The multifunctional effects were achieved with the cooperated extracellular electrooxidation for EPS destruction and the active chlorine for intracellular microbial cell disintegration. This research provides a promising strategy for integrated sludge treatment and recycling for possible land utilization.

Insight into efficient phosphorus removal/recovery from enhanced methane production of waste activated sludge with chitosan-Fe supplementation

Fe(III)-loaded chitosan (CTS-Fe) composite was used for the first time to remove and recover phosphorus (P) from waste activated sludge (WAS) via anaerobic digestion (AD). The P transformation pathway and the effect of CTS-Fe addition on the AD process were investigated using batch experiments. The P fractionation results indicate that non-apatite inorganic phosphorus (NAIP) reduction in the solid phase of sludge at 20 g/L of CTS-Fe addition (6.72 mg/g-SS) was 2.4 times higher than that in the control (2.77 mg/g-SS, no CTS-Fe addition). This is probably brought about by the added CTS-Fe enhanced the reduction of Fe(III)-P compounds in the sludge with phosphate released into the liquid phase. CTS-Fe can efficiently recover 95% of P from the liquid digestate of WAS. Notably, partial Fe(III) on the CTS-Fe was reduced and effectively combined with P to form vivianite crystals on the CTS-Fe surface during the AD process. Characterization analysis demonstrated that ligand exchange and chemical precipitation were the dominant mechanisms for P removal/recovery. Furthermore, the addition of CTS-Fe increased methane production by 11.9 - 32.2% under the tested conditions, likely attributable to the enhanced hydrolysis of WAS under CTS-Fe supplementation. As the P-loaded CTS-Fe particles can be easily separated and recovered from the AD system and further reutilized in agriculture, this study could provide a new approach for simultaneous P removal/recovery and enhanced methane production from AD of WAS.

Impacts of electrokinetic isolation of phosphorus through pore water drainage on sediment phosphorus storage dynamics

Pore water is a crucial storage medium and a key source of sediment phosphorus. A novel equipment based on electrokinetic geosynthetics (EKGs) was used for isolating phosphorus from eutrophic lake sediments through pore water drainage. Three mutually independent indoor group experiments (A, B, and C) were conducted to investigate the effects of voltage gradient (0.00, 0.25, and 0.50 V/cm) on pore water drainage capacity, phosphorus removal performance, sediment physicochemical properties, and phosphorus storage dynamics. The average reduction in the sediment moisture and total phosphorus content was 2.5%, 4.3%, and 4.6% and 28.15, 75.95, and 112.65 mg/kg after 6 days of treatment for A, B and C, respectively. Efficient pore water drainage through gravity and electroosmotic flow and electromigration of phosphate were the main drivers of sediment-dissolved and mobilized phosphorus separation. A high voltage gradient facilitated the migration of pore water and the phosphorus in it. The maximal effluent total phosphorous (TP) concentration was up to 27.9 times that in the initial pore water samples, and negligible effluent TP was detected when the pore water pH was less than 2.5. The TP concentration was exponentially and linearly related to the pH and electronic conductivity of the electroosmotic flow, respectively. The migration of H⁺ within the sediment matrix promoted the liberation of metals bounded to phosphorus, particularly of Ca-P and Fe-P. Pore water drainage through an EKG resulted in Ex-P separation of up to 87.50% and a 13.84 mg/kg decrease in Ca-P and 125.35 mg/kg accumulation of low mobile Fe-P in the weak acid anode zone.

Migration and transformation of phosphorus in waste activated sludge during ozonation

For phosphorus (P) recovery from waste activated sludge (WAS), the most important step is to release P into the solution. This study aimed to explore the migration and transformation of P in WAS during ozonation based on the Standards Measurements and Testing Program analysis. The results showed that WAS contained 7.10% P element and could be selected as potential substitution of phosphate rock. Inorganic phosphorus (IP) was the major P fraction in raw WAS (68.10%), and non-apatite inorganic phosphorus (NAIP) occupied 62.40% of IP. Ozonation facilitated the P application in agriculture as the bio-available P in the solid phase increased by 23.63% at ozone dosage 0.20 gO₃/gSS. The highest concentration of total phosphorus in liquid (TP_(L)) (40.68 mg/L) was achieved at ozone dosage 0.20 gO₃/gSS, and 89.62% of TP_(L) was PO₄^3--P, which was easy to be recovered by struvite precipitation. The contributions of different P fractions in solid phase to TP_(L) were related to ozone dosage. The analysis of P mass balance suggested that the optimum ozone dosage for P recovery was 0.15 O₃/gSS.

Transformation and migration of phosphorus in excess sludge reduction pretreatment by alkaline ferrate oxidation combined with anaerobic digestion

Recently, more and more attention has been paid to the strong oxidation ability of newly prepared potassium ferrate (NAPF) in sludge reduction process, but less attention has been paid to the change of phosphorus in this process. The feasibility of phosphorus migration and transformation during excess sludge reduction pretreatment using NAPF pre-oxidation combined with anaerobic digestion was investigated. After 70 mg/g suspended solids NAPF pretreatment and 16 days anaerobic digestion, the solid-phase volatile suspended solids decreased by 44.2%, and much organic matter had been released into the liquid-phase and then degraded during digestion by indigenous microorganisms. As the sludge pre-oxidation process was performed, solid-phase organic phosphorus and chemically combined phosphorus also released into the liquid-phase as PO₄^3-, peaking at 100 mg/L. During anaerobic digestion, the Fe³⁺ in the liquid-phase was gradually reduced to Fe²⁺, and then formed Fe²⁺-PO₄^3- compound crystals and re-migrated to the solid-phase. The concentration of PO₄^3- decreased to 17.08±1.1 mg/L in the liquid-phase after anaerobic digestion. Finally, the phosphorus in the Fe²⁺-PO₄^3- compound accounts for 80% of the total phosphorus in the solid-phase. A large number of vivianite crystals in sludge were observed. Therefore, this technology not only effectively reduces sludge, but also increases the proportion of PO₄^3- in the sludge in the form of Vivianite.

Carbon source and phosphorus recovery from iron-enhanced primary sludge via anaerobic fermentation and sulfate reduction: Performance and future application

Anaerobic fermentation and sulfate reduction (AF-SR) was firstly used for recovery of carbon sources and phosphorus from Fe-enhanced primary sludge (Fe-sludge). With FeCl₃ dosage of 30 mg Fe/L, 63.0% of the chemical oxygen demand (COD) and 97.3% of the phosphorus were concentrated from sewage into Fe-sludge. Batch anaerobic fermentation tests of Fe-sludge with and without sulfate addition (AF-SR and control) were performed. The results showed that volatile fatty acid concentrations of the control and AF-SR were 211.0 and 270.2 mg COD/g volatile suspended solids, respectively. Furthermore, 33.2% (control) and 56.2% (AF-SR) of the total phosphorus in Fe-sludge was released. The recovery performances of carbon source and phosphorus were calculated based on struvite precipitation. The available carbon source of the AF-SR system was 44.5% higher than that of the control. A novel integrated wastewater and sludge treatment process based on chemically enhanced primary sedimentation and AF-SR is proposed for future application.

Phosphorus release during alkaline treatment of waste activated sludge from wastewater treatment plants with Al salt enhanced phosphorus removal: Speciation and mechanism clarification

Chemical phosphorus removal (CPR) is being increasingly adopted in wastewater treatment plants (WWTPs) to enhance P elimination to comply with stringent discharge limits. However, strategies to recover P enriched in the produced waste activated sludge (WAS) are not well developed. In this study, we investigated the release of P in WAS from three WWTPs employing Al salt enhanced CPR by alkaline treatment. We also monitored P mobilization by tracking the dynamics of P fractions and species, the dissolution of major metals, and sludge cell integrities as pH was altered. The level of aqueous total phosphorus (TP_aq) in the sludge increased significantly to >200 mg/L (from <11 mg/L in the raw sludge) as the pH was increased to 12, with the majority being PO₄-P_aq especially at high pHs. The dominance of non-apatite inorganic phosphorus (NAIP) in the sludge-P, a good correlation observed between aqueous PO₄-P and aqueous Al, and the reversibility of P mobilization all suggest that the dissolution of Al-bound P was largely responsible for the sludge-P release. Sludge cell integrity, on the other hand, was not closely correlated with TP_aq concentrations. Although the level of TP released in this study is among the highest, a more efficient strategy still needs to be developed to further enhance sludge-P release when TP content in the sludge mixture (TP_mx) is considered (TP_mx was >800 mg/L in this work).

Enhanced short-chain fatty acids production from waste activated sludge with alkaline followed by potassium ferrate treatment

This study reported an efficient approach, i.e., alkaline followed by potassium ferrate (PF) pretreatment, to enhance short chain fatty acids (SCFAs) production from waste activated sludge anaerobic fermentation process. The optimum condition was initial pH of 10.0 and PF dosage of 28 mg Fe(VI)/g total suspended solid, with the highest SCFAs production of 382 mg chemical oxygen demand/g volatile suspended solid, which was 2.03 and 2.06 times higher than that of corresponding sole treatments. It was found that the alkaline + PF treatment could provide more soluble substrates for subsequent acidification process by accelerating disruption of both microbial cells and extracellular polymeric substances. And the alkaline + PF treatment also benefited to the activity promotion of specific hydrolases and inhibition of methanogens. Besides, the abundances of microorganisms related to SCFAs production, such as Proteiniclasticum and Macellibacteroides, were increased greatly, whereas the main SCFAs consumer, Proteobacteria, was decreased from 29.1% to 14.4%.

Fractionation and identification of iron-phosphorus compounds in sewage sludge

Iron (Fe) salts are widely used to remove phosphorus (P) in wastewater treatment plants (WWTPs). Understanding the existing species of iron-phosphorus compounds (FePs) in sludge is conducive to P recovery. In this study, the chemical equilibrium modeling software, sequential chemical extraction methods and instrumental analytical techniques were used to establish a reliable method for fractionation and identification of FePs in sewage sludge. Five FePs-containing sludge samples obtained in the laboratory or from WWTPs were investigated. Modified chemical extraction methods for P and Fe fractionations combined with Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis are preferable for qualitative and quantitative identification of FePs in sewage sludge. The analytical results revealed that more than half of P was bound to Fe in all the sludge samples. Approximately 83% and 14% of Fe was present as iron phosphates in the samples prepared in the laboratory with Fe(III) dosed to a phosphate solution and lab-scale secondary effluent, respectively. Ferrihydrite and hematite were the dominant iron oxides in these two samples, respectively. In the sludge samples collected from WWTPs, Fe bound to iron oxides (71%), mainly hematite, was the dominant Fe fraction in the returned sludge. Mature iron hydroxyphosphate together with some lepidocrocite was the dominant FePs in both the thickened sludge and mixed sludge before digestion.

Temperature effect on extracellular polymeric substances (EPS) and phosphorus accumulating organisms (PAOs) for phosphorus release of anaerobic sludge

Phosphorus (P) is an essential element for living organisms and anaerobic sludge is an attractive source for P recovery. Anaerobic P release depends on both phosphorus-accumulating organisms (PAOs) and extracellular polymeric substances (EPS). However, the P release contributed by the microbial cells and EPS was not addressed completely and the effect of temperature on the mechanism of P release and transformation was rarely considered. This study, therefore, investigated the effects of temperature on the P fraction and the relationship between PAOs metabolic pathway and EPS reaction using the Standards in Measurements and Testing (SMT) protocol and the 31P nuclear magnetic resonance (31P-NMR) experiments. Experimental results showed that the temperature not only affected the metabolism of PAOs, but also significantly influenced the EPS components and the hydrolysis of EPS-associated polyphosphate (poly-P). And the P release mainly occurred due to biological mechanisms with a conversion from non-reactive P (NRP) in both intracellular and extracellular substances to reactive P (RP) fractions. The highest concentration of total P in the supernatant (TPL) occurred at 15 °C, and the TPL release from the solid to liquid phase was better fitted with pseudo-second-order kinetic model. More organic P in the sludge (OPs) released from the sludge phase at 35 °C would convert into inorganic P (IPs) and non-apatite inorganic phosphorus (NAIPs) was the most labile P fraction for P release. The hydrolysis of EPS-associated poly-P was enhanced by higher temperatures with the degradation of the long-chain poly-P by PAOs. Meanwhile, a lower temperature could obviously improve the P release because the dominance of PAOs would potentially shift to GAOs with the increase of temperature. But the very-low temperature (5 °C) was not beneficial for the P release and suppressed the microbial activities.

Simultaneous release of polyphosphate and iron-phosphate from waste activated sludge by anaerobic fermentation combined with sulfate reduction

Iron is widely used in sewage treatment systems and enriched into waste activated sludge (WAS), which is difficult and challenging to phosphorus (P) release and recovery. This study investigated simultaneous release performance of polyphosphate and iron-phosphate from iron-rich sludge via anaerobic fermentation combined with sulfate reduction (AF-SR) system. Batch tests were performed, with results showing that AF-SR system conducted a positive effect due to the relatively low solubility of ferrous sulfide in comparison with ferric phosphate precipitates. Simulation study was performed to investigate the total P release potential from actual waste activated sludge, finding that about 70% of the total P could release with the optimized pH of 7.0-8.0 and the theoretical S^2-/Fe²⁺ molar ratio of 1.0. A potential new blueprint of a wastewater treatment plant based on AF-SR system, towards P, N recovery and Fe, S, C recycle, was finally proposed.

Acidification and recovery of phosphorus from digested and non-digested sludge

Acidification was used to dissolve phosphorus from digested and non-digested sludge from five wastewater treatment plants in order to make phosphorus accessible for subsequent recovery. More phosphorus was dissolved from digested sludge (up to 80%), with respect to non-digested sludge (∼25%) and the highest release was observed at pH 2. The acid consumption for digested sludge was higher than for non-digested sludge due to the presence of the bicarbonate buffer system, thus CO₂ stripping increased the acid consumption. In all the experiments, the sludge was exposed to acid for 1 h. For the five tested sludge types, 60-100 mmol o-P was released per added mol H₂SO₄. It was mainly iron and calcium compounds that accounts for the phosphorus release at low pH. The release of heavy metals was in general low (<30%) for all the wastewater treatment plant, as Zn, Cd and Ni showed the most critical release after acidification of non-digested sludge.

Speciation, mass loadings, and fate of phosphorus in the sewage sludge of China

Phosphorus (P) in sewage sludge (SS) has been a concern for decades. Sludge-borne P could exacerbate eutrophication problems once released to aquatic environments. Meanwhile, sludge-borne P would be a valuable alternative source to P minerals. A comprehensive understanding of the occurrence forms and fate of P in SS is required prior to the assessment of environmental implications of sludge-borne P. In the present study, we conducted a nationwide survey on speciation of phosphorus in the SS of China. The average concentration of total phosphorus (TP) in SS was 17.3 ± 5.1 g· kg^-1, of which 67.1 ± 8.9% occurred as inorganic forms. Non-apatite inorganic phosphorus constituted the major component (77.6 ± 11.4%) of inorganic phosphorus, with the rest minor portion occurring as apatite phosphorus. Organic P accounted for 26.8 ± 7.9% of TP in SS, showing an average concentration of 4.5 ± 4.2 g· kg^-1. Majority of phosphorus in the SS of China occurs as labile forms, suggesting that the environmental implications of SS disposal approaches should be assessed seriously. Sewage sludge produced by China contained 1.1 × 10⁵ t of TP in 2016, and the amount accumulated to 1.0 × 10⁶ t since 2001. Landfill sites are the single most important fate, receiving 8.1 × 10⁵ t of sludge-borne TP since 2001. Land application and building materials are the other two fates of sludge-borne phosphorus. Spatial-temporal variations of phosphorus in the SS of China were also discussed. Graphical abstract.

Free ammonia-based pretreatment enhances phosphorus release and recovery from waste activated sludge

The recovery of phosphorus from waste activated sludge (WAS) was usually at low levels due to low phosphorus release. This study presents a novel, cost-effective and eco-friendly pretreatment method, e.g., using free ammonia (FA) to pretreat WAS, to enhance the phosphorus release from WAS. Experimental results showed that the phosphorus release from WAS was significantly increased after FA pretreatment at up to 189.4 mg NH₃-N L^-1 for 24 h, under which the released PO₄^3--P (i.e. 101.6 ± 6.7 mg L^-1) was higher than that pH 9 (i.e. 62.6 ± 4.54 mg L^-1) and control (without pH and FA pretreatment) (i.e. 15.1 ± 1.86 mg L^-1). More analysis revealed that the FA induced improvement in phosphorus release could be attributed to the disintegration of extracellular polymeric substances (EPS) and cell envelope of sludge cells. Moreover, the released phosphorus recovered as magnesium ammonium phosphate (MAP) was confirmed. The findings reported may guide engineers to develop an economic and practical strategy to enhance resources and energy recovery from WAS.