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

Thermal activation of peroxymonosulfate for enhanced volatile fatty acids production and phosphorus release during anaerobic fermentation of iron-rich sludge

Heat-peroxymonosulfate (PMS) pre-treatment was simultaneously used for phosphorus (P) release and volatile fatty acids (VFAs) production in this study. Maximum P concentrations increased from 10.3 ± 0.4 mg/L in PMS-0 to 246.1 ± 1.6 mg/L in PMS-0.8, with 41.4 % of total P released. VFAs production on day 5 increased from 2409.1 ± 30.8 mg chemical oxygen demand (COD)/L in PMS-0 to 2995.4 ± 86.5 mg COD/L in PMS-0.8. Metagenomic analysis showed that an increase in PMS dosage was detrimental to P release during polyphosphate hydrolysis by polyphosphate-accumulating organisms; functional genes involved in S cycling increased, suggesting that sulfate reduction was a critical cause of P release from iron-rich sludge during anaerobic fermentation (AF). These results provide important insights for the improvement of P release efficiency and acid production during AF, enhancing the potential for resource recovery from iron-rich sludge.

A chemically enhanced primary treatment and anammox-based process for sustainable municipal wastewater treatment: The advantage and application prospects

Low-carbon nitrogen removal, bioenergy production, and phosphorus recovery are key goals for sustainable municipal wastewater treatment. Traditional activated sludge processes face an energy demand conflict. Anaerobic ammonium oxidation (Anammox) offers a solution to this issue, with the A-B process providing a sustainable approach. Stable and cost-effective nitrite supply for mainstream anammox has gained attention, while the interactions between A-B stage processes are also crucial. This paper reviews the benefits and challenges of mainstream anammox, bioenergy, and phosphorus recovery. A combined process of chemically enhanced primary treatment, partial denitrification and anammox is identified as effective for sustainable treatment. Additionally, the stable nitrite supply from the sidestream partial nitrification provides a 54% nitrogen removal contribution to the mainstream anammox. Anaerobic digestion with sulfate reduction is proposed as an efficient method for simultaneous bioenergy and phosphorus recovery from iron-enhanced primary sludge. The recycling of iron and sulfate reduces excess sludge and cuts costs. A novel wastewater treatment scheme, supported by a mass balance analysis, is presented; the proposed process is capable of recovering >50% of the carbon and phosphorus, while reduced 40% dosing of Fe and S chemicals, reducing the cost of chemical dosing and treatment of the digestate while meeting the high-quality effluent. The paper also explores the potential for transitioning from conventional activated sludge processes and suggests directions for future research.

Insight into the evolution of phosphorous conversion, microbial community and functional gene expression during anaerobic co-digestion of food waste and excess sludge with spicy substances exposure

Garlic and chili are widely used as food flavoring agents in food cooking, therefore might be accumulated in large amounts in food waste (FW). The effects of garlic and chili on the dissolution, hydrolysis, acidification and methanation in an anaerobic co-digestion system were investigated during the combined co-digestion of FW and excess sludge (ES). Additionally, the transformation of phosphorus form and microbial metabolism changes during the process were analyzed. The results showed the addition of garlic and chili promoted the release of protein in the soluble chemical oxygen demand. Secondly, the addition of garlic and chili up-regulated the relative abundances of key coding genes pstS, pstA, pstB and pstC. The relative abundances of the pstS and pstC genes increased by 0.0113% and 0.0021%, respectively, when 10 g garlic was added compared with no garlic. Furthermore, with respect to phosphorus conversion, the addition of garlic inhibited the conversion of solid phosphorus to gaseous phosphorus, whereas the addition of chili had the opposite effect. Meanwhile, garlic and chili inhibited the expression of key coding genes in phosphofructokinase. This work provides new insights into the phosphorus conversion and microbial metabolism in the process of anaerobic co-digestion of FW and ES under the influence of spicy substances.

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.

Inhibition of phosphorus removal performance in activated sludge by Fe(III) exposure: transitions in dominant metabolic pathways

Introduction

Simultaneous chemical phosphorus removal process using iron salts (Fe(III)) has been widely utilized in wastewater treatment to meet increasingly stringent discharge standards. However, the inhibitory effect of Fe(III) on the biological phosphorus removal system remains a topic of debate, with its precise mechanism yet to be fully understood.

Methods

Batch and long-term exposure experiments were conducted in six sequencing batch reactors (SBRs) operating for 155 days. Synthetic wastewater containing various Fe/P ratios (i.e., Fe/P = 1, 1.2, 1.5, 1.8, and 2) was slowly poured into the SBRs during the experimental period to assess the effects of acute and chronic Fe(III) exposure on polyphosphate-accumulating organism (PAO) growth and phosphorus metabolism.

Results

Experimental results revealed that prolonged Fe(III) exposure induced a transition in the dominant phosphorus removal mechanism within activated sludge, resulting in a diminished availability of phosphorus for bio-metabolism. In Fe(III)-treated groups, intracellular phosphorus storage ranged from 3.11 to 7.67 mg/g VSS, representing only 26.01 to 64.13% of the control. Although the abundance of widely reported PAOs (Candidatus Accumulibacter) was 30.15% in the experimental group, phosphorus release and uptake were strongly inhibited by high dosage of Fe(III). Furthermore, the abundance of functional genes associated with key enzymes in the glycogen metabolism pathway increased while those related to the polyphosphate metabolism pathway decreased under chronic Fe(III) stress.

Discussion

These findings collectively suggest that the energy generated from polyhydroxyalkanoates oxidation in PAOs primarily facilitated glycogen metabolism rather than promoting phosphorus uptake. Consequently, the dominant metabolic pathway of communities shifted from polyphosphate-accumulating metabolism to glycogen-accumulating metabolism as the major contributor to the decreased biological phosphorus removal performance.

Effects of sodium sulfide concentration on the solid and solution chemistry of a biosolids slurry for phosphorus recovery and reuse

Municipal biosolids contain organic and inorganic phosphorus (P) that could be recovered for reuse as P fertilizer. Inorganic P compounds include iron phosphates that precipitate and/or adsorbed phosphate ions as a consequence of soluble iron addition in order not to exceed total phosphorus (TP) emission limits. The inorganic orthophosphate (o-Pi) minerals within biosolids can have low solubilities. One P recovery strategy is to maximize the dissolution of o-Pi from biosolids for reuse. Dissolving iron phosphates in biosolids by adding sodium sulfide was assessed as an o-Pi dissolution strategy. 10 % w/w biosolids slurries with a total phosphorus (TP) of 0.97 ± 0.03 mmol P/dry g were mixed with sulfide/TP (S2-/TP) molar ratios from 0 to 4 for up to 96 h. The maximum o-Pi concentration (48 ± 7 mM, or 42 ± 6 % of TP) was obtained for 4 S2-/TP after 24 h at room temperature (RT). Iron concentrations measured by colorimetry (ferrozine) reduced from 0.6 ± 0.1 mM to less than 0.01 mM (S2-/TP > 1). X-ray diffraction and FTIR suggest that sulfide treatment preferentially dissolved amorphous o-Pi-containing solids, vivianite, and minerals with iron, aluminum, phosphate, sulfate, and other subsitutions. Poorly crystalline erdite (NaFeS2 ·2H2O) was detected in products after S2-/TP treatment ratios ≥ 2. Incubation at RT or 37 °C did not affect the o-Pi concentrations for 0 or 4 S2-/TP over 47 h. Sulfide addition could also increase the risk of construction material corrosion, and reduce the efficiency of P recovery by precipitation. There are disadvantages to using sulfide to dissolve o-Pi from biosolids as a potential P recovery process.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Elemental sulfur redox bioconversion for selective recovery of phosphorus from Fe/Al-bound phosphate-rich anaerobically digested sludge: Sulfur oxidation or sulfur reduction?

The biological oxidation of elemental sulfur (S0) to sulfate and the reduction of S0 to sulfide provide a potential route for extracting and reclaiming phosphorus (P) from anaerobically digested sludge (ADS). However, the treatment performance, stability, and cost-effectiveness of the two opposing bioprocesses based on S° for selective P recovery from ADS remain unclear. This study aimed to compare the roles of S0-oxidizing bacteria (S0OB) and S0-reducing bacteria (S0RB) in liberating insoluble P from ADS through single-batch and consecutive multibatch experiments. Changes in P speciation in the sludge during the biological extraction processes were analyzed by using complementary sequential extraction and P X-ray absorption near-edge spectroscopy. Results showed that S0OB treatment extracted more phosphate from the sludge compared with S0RB treatment, but it also released a considerable amount of metal cations (e.g., heavy metals, Mg2+, Al3+, Ca2+) and negatively affected sludge dewaterability due to intense sludge acidification and cell lysis. At pH 1.2, the S0OB treatment released 92.9% of P from the sludge, with the dissolution of HAP, Fe-PO4, Mg3(PO4)2, and P-fehrrihy contributing 26.8%, 22.1%, 12.8%, and 10.5%, respectively. The S0RB treatment released 63.6% of P from the sludge at pH 7.0, with negligible dissolution of metal cations, thereby avoiding costly purification of the extract and alkali neutralization for pH adjustment. This treatment involved the replacement of phosphates bounded with Fe-PO4 (FePO4 and P-fehrrihy) and Al-PO4 (P-Alumina and AlPO4) with biogenic sulfides, with contributions of 72.7%, and 20.9%, respectively. Consecutive bioprocesses for P extraction were achieved by recirculating the treated sludge. Both S0OB and S0RB treatments did not affect the extent of sludge dewatering but considerably weakened the dewatering rate. The S0OB-treated sludge exhibited prolonged filtration time (from 3010 s to 9150 s) and expressing time (from 795 s to 4690 s) during compression dewatering. After removing metal cations using cation exchange resin (CER) and neutralizing using NaOH, a vivianite product Fe3(PO4)2·8H2O (purity: 84%) was harvested from the S0OB-treated extract through precipitation with FeSO4·7H2O. By contrast, a vivianite product Fe3(PO4)2·8H2O (purity: 81%) was directly obtained from the S0RB-treated extract through precipitation with FeSO4·7H2O. Ultimately, 79.8 and 57.9wt% of P were recovered from ADS through S0OB extraction-CER purification-alkali neutralization-vivianite crystallization, and S0RB extraction-vivianite crystallization, respectively. Collectively, biological S0 reduction is more applicable than biological S0 oxidation for selectively reclaiming P from Fe/Al-associated phosphate-rich ADS due to better cost-effectiveness and process simplicity. These findings are of significance for developing sludge management strategies to improve P reclamation with minimal process inputs.

Co-digestion of sulfur-rich vegetable waste with waste activated sludge enhanced phosphorus release and hydrogenotrophic methanogenesis

Anaerobic co-digestion of sulfur-rich vegetable waste (SVW) with waste activated sludge (WAS) and the underlying mechanisms associated with methane production and phosphorus (P) release were investigated. Four types of SVW (Chinese cabbage, cabbage, rapeseed cake, and garlic) were utilized for co-digestion with WAS, and the methane yield increased by 7.3%-35.3%; in the meantime, the P release amount from WAS was enhanced by 9.8%-24.9%. The organic carbon in SVW promoted methane production, while organic sulfur and the formation of FeS facilitated P release. Among the four types of SVW, rapeseed cake was identified as the most suitable co-digestion substrate for enhancing both methane production and P release due to its balanced nutrients and relatively high sulfur content. Syntrophic bacteria working with hydrogenotrophic methanogens, iron-reducing bacteria, sulfate-reducing bacteria, and hydrogenotrophic methanogens were enriched. Metabolic pathways related to sulfate reduction and methanogenesis were facilitated, especially hydrogenotrophic methanogenesis. Enzymes involved in hydrogenotrophic methanogenesis were promoted by 76.05%-407.98% with the addition of Chinese cabbage, cabbage, or rapeseed cake. This study provides an eco-friendly technology for promoting P resource and energy recovery from WAS and an in-depth understanding of the corresponding microbial mechanisms.

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

Energy-neutral municipal wastewater treatment based on partial denitrification-anammox driven by side-stream sulphide

"Low-carbon" has become an important evaluation index of modernisation construction. In the area of wastewater treatment has also caused considerable concern. Anaerobic ammonium oxidation (anammox) is a novel autotrophic nitrogen removal process that provides an opportunity for low-carbon remodelling of municipal wastewater treatment plants (MWTPs). The stable supply of nitrite is of great significance for the application of anammox. As a process with stable nitrite supply, partial denitrification (PD) is of great significance in the coupling nitrogen removal with anammox in municipal wastewater. Furthermore, innovation of the low-carbon nitrogen removal process can enable the recovery of abundant bioenergy resource from MWTPs. The low-carbon nitrogen removal via PD-anammox process and the bioenergy recovery for municipal wastewater in the previous studies has been summarised. On this basis, a novel energy-neutralisation municipal wastewater treatment process based on partial denitrification-anammox driven by sulphide produced in the side-stream has been proposed. The long-term retention of mainstream anammox and improvement of energy recovery efficiency under the requirement of ensuring nitrogen removal require additional detailed investigation.

A review on sulfur transformation during anaerobic digestion of organic solid waste: Mechanisms, influencing factors and resource recovery

Anaerobic digestion (AD) is an economical and environment-friendly technology for treating organic solid wastes (OSWs). OSWs with high sulfur can lead to the accumulation of toxic and harmful hydrogen sulfide (H₂S) during AD, so a considerable amount of studies have focused on removing H₂S emissions. However, current studies have found that sulfide induces phosphate release from the sludge containing iron‑phosphorus compounds (FePs) and the feasibility of recovering elemental sulfur (S⁰) during AD. To tap the full potential of sulfur in OSWs resource recovery, deciphering the sulfur transformation pathway and its influencing factors is required. Therefore, in this review, the sulfur species and distributions in OSWs and the pathway of sulfur transformation during AD were systematically summarized. Then, the relationship between iron (ferric compounds and zero-valent iron), phosphorus (FePs) and sulfur were analyzed. It was found that the reaction of iron with sulfide during AD drove the conversion of sulfide to S⁰ and iron sulfide compounds (FeS_x), and consequently iron was applied in sulfide abatement. In particular, ferric (hydr)oxide granules offer possibilities to improve the economic viability of hydrogen sulfide control by recovering S⁰. Sulfide is an interesting strategy to release phosphate from the sludge containing FePs for phosphorus recovery. Critical factors affecting sulfur transformation, including the carbon source, free ammonia and pretreatment methods, were summarized and discussed. Carbon source and free ammonia affected sulfur-related microbial diversity and enzyme activity and different sulfur transformation pathways in response to varying pretreatment methods. The study on S⁰ recovery, organic sulfur conversion, and phosphate release mechanism triggered by sulfur deserves further investigation. This review is expected to enrich our knowledge of the role of sulfur during AD and inspire new ideas for recovering phosphorus and sulfur resources from OSWs.

Formation of vivianite in digested sludge and its controlling factors in municipal wastewater treatment

Engineering solutions to recover phosphorus from municipal wastewater are required to close the anthropogenic phosphorus cycle. After chemical phosphorus elimination by iron, the ferrous iron‑phosphorus mineral vivianite forms in digested sludge, and its separation is being researched at the pilot scale. In this study, sludge samples from 16 wastewater treatment plants (WWTPs) demonstrated that phosphorus bound to biomass and redox-sensitive iron in activated sludge was transformed into other phosphorus binding forms, including vivianite, during digestion. Vivianite quantity was approximated using X-ray diffraction and two sequential extractions. These three independent methods of approximating vivianite quantity were closely related confirming their relationship to the vivianite content in the samples. The digested sludge from three WWTPs exhibited comparatively high levels of vivianite-bound phosphorus approximated between 31 % and 51 % of total phosphorus. The controlling factors of vivianite formation were investigated in order to enhance its formation in digested sludge and increase the amount of phosphorus recoverable as vivianite. They were identified using single and multivariate correlation (MLR), considering the sludge properties, sludge composition, and process parameters within the operating range of the 16 WWTPs. Increasing iron content was verified as the primary predictor of significantly increased vivianite formation (MLR: p < 0.001). In addition, increasing sulphur content was found to be an additional significant factor that decreased vivianite formation (MLR: p < 0.05). Furthermore, a comparison of plants using sulphur-free (FeCl2 and FeCl3) and sulphur-containing (FeSO4 and FeClSO4) precipitants indicated that the latter could increase the sulphur content in digested sludge (one-tailed Welch two-sample t-test: t(14.6) = 2.3, p = 0.02). Thus, by increasing the sulphur content, the use of sulphur-comprising precipitants may counteract vivianite formation, whereas sulphur-free precipitants may facilitate it and, hence, promote vivianite recovery.

Simultaneous phosphate recovery and sodium removal from brackish aquaculture effluent via diafiltration-nanofiltration process

Expansion of the aquaculture industry has been accompanied by environmental impact as the discharged effluent contains excess nutrients such as phosphorus compounds. Recovery of such nutrients is not economically feasible as it presents in trace amounts. Furthermore, brackish aquaculture effluent which contains high sodium chloride (NaCl) content makes the treated solution inappropriate for fertilizer production. Herein, this study proposed a diafiltration-nanofiltration route to perform a simultaneous phosphate concentrating and osmotion (sodium) removal from brackish aquaculture effluent. Effects of operating pressure, phosphate, and sodium content on membrane performance were first determined using Desal-5 DK membrane with three types of solutions namely (i) freshwater without NaCl, (ii) dilute brackish water with 1,500 mg/L NaCl, and (iii) brackish water with 10,000 mg/L NaCl. It was found that at 4 bar operating pressure, it could achieve higher phosphate rejection and sodium permeance. The presence of NaCl negatively influenced both phosphate rejection and concentrating factor (CF) due to the salt screening effect. It was noteworthy that negative sodium rejection (up to -16%, CF <1) could be attained, indicating the concentrating effect for sodium was negligible. The concentrating process was effective to concentrate phosphate by 2-fold but less effective in removing sodium. Diafiltration was then introduced and resulted in about 76% of sodium removal. Diafiltration-nanofiltration (DF-NF) mode was shown to be a more efficient method than nanofiltration-diafiltration (NF-DF) mode as phosphate could be concentrated up to 2 factors with 99 wt% of sodium being removed from the real brackish aquaculture effluent. These findings showed that DF-NF is a feasible approach for concentrating phosphate while removing sodium ions from aquaculture effluent and the recovered nutrient solution has huge potential to be applied as liquid fertilizer for hydroponic plants.

Simultaneous enhancing phosphorus recovery and volatile fatty acids production during anaerobic fermentation of sewage sludge with peroxydisulfate pre-oxidation

Phosphorus release and sludge hydrolysis are the keys for phosphorus and carbon recovery from sewage sludge via anaerobic process. In this study, iron-rich sludge (a common phosphorus-rich sewage sludge) was pre-oxidized by heat-activated peroxydisulfate (PDS) to enhance volatile fatty acids (VFAs) production and iron-bound phosphorus (Fe-P) release during anaerobic fermentation (AF). With low-dosage PDS pre-oxidation (33.75 mg/g total solids), the concentration of recoverable phosphorus increased by 49.3% than that noted in control along with enhanced VFAs production after 4 days. This is mainly because PDS oxidation not only effectively disintegrated sludge, but also generated sulfate simultaneously. Sludge disintegration enhanced organic matter hydrolysis, promoting VFAs yield, while sulfate was reduced to sulfide during AF and precipitated with iron, leading to Fe-P release. The application of PDS pre-oxidation on iron-rich sludge could not only improve the resourcefulness of sludge but also reduce secondary pollution (sulfate or hydrogen sulfide).

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.

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.

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.

Short reaction times coupled with alkalization improves the release of phosphorus from Al-waste activated sludge

In this study, the performance and mechanism of P release from Al-waste activated sludge (WAS) via wet-chemical treatment at different reaction times were investigated. The maximum P release (46% of TP) was achieved at 20 min when the pH was maintained at 2 during acidic treatment. During alkali treatment, the maximum P concentration (363.96 mg/L, 46.07%) was achieved at 10 min when pH was initially adjusted to 12. Acidic treatment took twice as long to achieve the same efficiency of released P as the alkali treatment. Furthermore, P release mainly originated from Al-P and Ca-P during acidic treatment and Al-P dissolution during alkali treatment. The cost of chemical consumption was 483.96 USD/ton TS sludge with acidic treatment, which was 8.49 times higher than that of alkali treatment without pH control. Thus, short reaction times (ca. 10 min) coupled with alkalization provide an effective approach for improving P release from Al-WAS.

Novel CaO2 beads used in the anaerobic fermentation of iron-rich sludge for simultaneous short-chain fatty acids and phosphorus recovery under ambient conditions

A novel composite CaO₂ bead was prepared to improve total short-chain fatty acids (TSCFAs) production and phosphorus (P) recovery from iron-rich waste activated sludge (WAS) during ambient anaerobic fermentation. Results showed that CaO₂ mass percentage of 5% and CaCl₂:nylon66 = 1:1 (mass ratio) were the optimal prescription for the preparation of CaO₂ beads with porous structure, loose morphology, and sustained-release of CaO₂. The highest TSCFAs production (356 mg/g VSS) was observed and about 9% of P in sludge could be recovered on beads. The decrease of Fe-phosphate and Fe-oxides in the sludge were due to different mechanisms. Microbial community analyses showed that CaO₂ beads effectively enriched dissimilatory iron-reducing bacteria (DIRB) and promoted iron-reduction related genes. After fermentation, the P-rich beads are easy to separate from sludge for further P recovery, and the supernatant carrying abundant acetate and Fe²⁺ can be returned to the wastewater treatment line to improve nutrient removal.

Short-chain fatty acids recovery from sewage sludge via acidogenic fermentation as a carbon source for denitrification: A review

Wastewater treatment plants face the problem of a shortage of carbon source for denitrification. Acidogenic fermentation is an effective method for recovering short-chain fatty acids (SCFAs) as a carbon source from sewage sludge. Herein, the most recent advances in SCFAs production from primary sludge and waste activated sludge are systematically summarised and discussed. New technologies and problems pertaining to the improvement in SCFAs availability in fermentation liquids, including removal of ammoniacal nitrogen and phosphate and extraction of SCFAs from fermentation liquids, are analysed and evaluated. Furthermore, studies on the use of recovered SCFAs as a carbon source for denitrification are reviewed. Based on the above summarisation and discussion, some conclusions as well as perspectives on future studies and practical applications are presented. In particular, the recovery of carbon source/bioenergy from sewage sludge must be optimised considering nutrient removal/recovery simultaneously.

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.

Enhanced production of volatile fatty acids by adding a kind of sulfate reducing bacteria under alkaline pH

Anaerobic digestion could make sludge stable and harmless, and the volatile fatty acids (VFAs) produced from it. The objective of this study was to reduced sludge production and realize the resource utilization of VFAs through enhance anaerobic sludge fermentation by adding sulfate reducing bacteria (SRB) under alkaline pH. Under the neutral and alkaline pH, SRB was added into the sludge fermentation liquid with sole stock solution and sterilization treatment respectively, while the liquid without any additives was used as control. The results indicated that obvious increase of the production of VFAs was observed after adding SRB under alkaline pH. And, more protein and polysaccharide were obtained which were the main substrates for the production of VFAs. The concentration of ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) were also increased with the addition of SRB. So, a high yield production of VFAs could be achieved through the addition of SRB + alkaline pH.

In-situ ammonia stripping with alkaline fermentation of waste activated sludge to improve short-chain fatty acids production and carbon source availability

Alkaline fermentation of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs) has been proved to be promising to develop internal carbon source for denitrifying processes in municipal wastewater treatment plants. However, a large amount of ammoniacal nitrogen also releases during fermentation, resulting in inhibition of acidogenic process and reduction of carbon source availability. Alkaline fermentation of WAS combined with in-situ ammonia stripping was proposed to improve SCFAs production and carbon source availability simultaneously. The results showed that a maximal SCFAs production of 308.7 ± 4.8 mg chemical oxygen demand/g volatile suspended solid was achieved under the pH = 10 + In-situ ammonia stripping, which was 21.7% and 141.5% higher than that of the pH = 10 and the control, respectively. Most of the ammoniacal nitrogen was stripped out, and the WAS-derived SCFAs availability as carbon source increased by 103.2%. These findings indicate in-situ ammonia stripping could make alkaline fermentation of WAS more practical.

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.

Effect of Fe on inorganic polyphosphate level in autotrophic and heterotrophic cells of Rhodospirillum rubrum

Inorganic polyphosphate is involved in metal homeostasis in microorganisms. The aim of the study was to reveal differences in polyphosphate metabolism of Rhodospirillum rubrum under autotrophic and heterotrophic cultivation in the presence of Fe (2.3 mg Fe³⁺ L^-1) and without Fe (traces). Heterotrophic conditions without Fe resulted in cell lysis and low biomass yield. High polyphosphate content and low exopolyphosphatase activity were observed in the cells cultivated autotrophically in the presence of Fe. The cells grown heterotrophically in the presence of Fe contained more phosphate and low-molecular polyphosphate; on the contrary, the content of the high molecular polyphosphate decreased in parallel with the increase in exopolyphosphatase activity. The possible involvement of Pi and polyphosphate to the formation of Fe-containing inclusions is discussed.

Phosphorus release and recovery from Fe-enhanced primary sedimentation sludge via alkaline fermentation

Phosphorus release and recovery from Fe-based chemically enhanced primary sedimentation (CEPS) sludge via alkaline fermentation was investigated. The coagulation results showed that 78% of organic matter and 95% of phosphorus were concentrated from sewage into sludge with the optimum dosages of 25 mg/L FeCl₃. The batch fermentation results revealed that 69.35% of the phosphorus in the Fe-sludge can be released and the maximum phosphorus concentration was 20.57 mg/L at pH 11. In the recovery stage, 90% of the P released in the fermented sludge supernatant was precipitated at a 2:1 ratio of magnesium to phosphorus and pH 11. The result of X-ray diffraction indicated that magnesium ammonium phosphate (MAP) was the major component of the precipitated solids. Thus, the present study provides an alternative option for phosphorus release and recovery as MAP from CEPS sludge via alkaline fermentation.