Glycine-assisted phosphorus release and recovery from waste-activated sludge†

Sheqi Cen ^a, Yao Zou *^ad, Hang Chen ^a, Xuhan Deng ^a, Fu Huang ^a, Liping Chen ^a, Le Li ^a, Tenghui Jin ^a, Chaohai Wei ^abc, Lichao Nengzi *^a and Guanglei Qiu *^abc
aSchool of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. E-mail: qiugl@scut.edu.cn
^bKey Laboratory of Pollution Control and Ecological Restoration in Industrial Clusters, Ministry of Education, Guangzhou 510006, China
^cGuangdong Provincial Key Laboratory of Solid Wastes Pollution Control and Recycling, Guangzhou 510006, China
^dGuangdong Society of Environmental Sciences, Guangzhou, 510000, China

First published on 28th August 2024

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Water impact Phosphorus (P) recovery from wastewater treatment is crucial to protect the water eco-environment and to alleviate the pressure of P-reserve depletion. We report an innovative method for P extraction and recovery from waste activated sludge with the assistance of glycine as a recyclable P-extraction agent. This work provides a new strategy for green P extraction and is of great significance for the development of sustainable P-recovery approaches.

1. Introduction

Phosphorus (P) is an essential and non-renewable resource that is important for food security and the sustainable development of human beings. However, it is estimated that the global P reserves are under threat of depletion in the near future due to the increased consumption of primary P rocks.¹ On the other hand, approximately 1.3 Mt of P enters wastewater treatment plants (WWTPs) globally, around 90% of which ends up in waste activated sludge (WAS).^2–4 Given the concerns about the increased demand for P in industry and agriculture,^5,6 there has been a paradigm shift in wastewater treatment strategies and an increasing emphasis on the pursuit of resource-oriented solutions for wastewater treatment.^1,7–9 In this regard, P recovery from WAS has become a key strategy for resource recovery and promoting sustainable global development.²

To recover P from WAS, a variety of P release/extraction methods have been developed to maximize the transfer of P into the liquid phase and then to recover it as mineral precipitates.^10–18 P extraction from the solid phase may be accomplished by wet chemical (e.g., pH adjustment by acid, alkaline, or both), physical (e.g., hydrothermal, ultrasonic), or biological (e.g., anaerobic digestion) treatment methods to induce sludge disintegration, cell lysis, and P release.^19–21 Among them, P recovery from anaerobic digestion is a commonly adopted approach, where nitrogen may be recovered simultaneously in the form of struvite with or without magnesium addition (depending on the magnesium contents in the supernatant; typically, external magnesium addition is required). However, the cost of magnesium salts and those related to the separation of impurities in this method have limited its cost-effectiveness and application.²² Additionally, P recovery from WAS before anaerobic digestion is considered necessary to prevent undesirable P precipitation in the digester and possible resultant pipe and equipment clogging.^23,24 Acid- or alkali-assisted P extraction from WAS before anaerobic digestion via pH adjustment has been reported to show desirable performance.²⁴ Acid or alkali could damage cell membrane via protein denaturation and promote the transformation of P and polyphosphate in WAS into dissolved phosphate,²⁴ achieving greater soluble PO₄³⁻-P release than at natural pH. The percentages of total P released from the activated sludge reached 30% and 34% at pH 2 and pH 12 by adding hydrochloric acid or sodium hydroxide solutions, respectively.²⁵ Though extreme pH levels could result in total dissolved phosphates folds higher than that at natural pH, acidic conditions pose risks in terms of high heavy metal extraction and a resultant quality degradation of the recovered P products (e.g., for use as fertilizers).^15,24,26

Enhanced biological phosphorus removal (EBPR) has been proposed as an alternative engineering approach, where anaerobic carbon uptake induces P release. It has been suggested that this may be employed for the extraction of P from WAS to generate a P-enriched solution for P recovery.^27,28 Under anaerobic conditions, a diverse range of carbon sources (e.g., volatile fatty acids) could effectively induce P release by polyphosphate-accumulating organisms (PAOs), since the microorganisms take up these carbon sources and store them as polymers by using the energy generated via polyphosphate hydrolysis.^29,30 Notably, the supply of carbon sources in this method inevitably results in additional chemical costs.³¹ Consequently, it would be desirable to have alternative methods that could effectively extract P without excessive carbon consumption.³²

Recently, Tian et al.³³ reported that glycine has a universal adverse effect on PAOs in field activated sludge, where P release was observed without glycine uptake. While this effect is detrimental to EBPR, it could render glycine a promising agent for P extraction from WAS. Nonetheless, this potential is yet to be verified. Additionally, it is still unknown if other amino acids have the same effect as glycine and may enable more effective and efficient P extraction.

Consequently, the aim of the present study was to develop a targeted and sustainable P-extraction method using amino acids as green and recyclable agents to facilitate P recovery from WAS. We summarize and compare the approaches reported in the literature in recent years for P release and recovery (Table S1†). Wet chemical treatment seems to be more desirable due to its simple operation and high efficiency for P recovery, albeit problems of metal dissolution and pipeline corrosion and clogging still need to be addressed. Based on the results, we compared glycine-assisted P extraction with the conventional acid/alkali P-extraction methods. According to the pre-experiments (Fig. S1†), glycine showed an extraordinary ability to induce P release compared to acid/alkali treatments. It would be useful to systematically analyze the potential effectiveness and efficiency of other amino acids. Therefore, in this study, 19 amino acids (note, tyrosine was excluded due to its low solubility in water) were evaluated as potential agents for P extraction from WAS. Based on its P-release induction ability and non-self-consumption characteristics, glycine was selected as a P-extraction agent to develop the P-extraction and -recovery method. The effects of the mixed liquor suspended solids (MLSS) concentration and temperature on the P-extraction process were analyzed. Glycine was added to the activated sludge to obtained a P-enriched supernatant. After using calcium chloride (CaCl₂) to precipitate the extracted P in the supernatant, the solution was filtered to obtain calcium phosphate (CaP) precipitates. The glycine-containing filtrate was reused in the next P-extraction operation with the addition of fresh activated sludge at the same MLSS concentration. The feasibility of glycine as a recyclable P-extraction agent was verified by repetitive P-extraction tests with the recovery and reuse of the glycine solution. The CaP precipitates were analyzed to evaluate the composition of the P products. This study provides a green and cost-effective alternative for P extraction and recovery from WAS, benefiting sustainable P recycling.

2. Experimental

2.1 Sludge characterization and chemicals

Activated sludge samples were obtained from two full-scale wastewater treatment plants (WWTPs) in Guangzhou, China. The WWTPs employ the anaerobic/anoxic/oxic (AAO) process for simultaneous nitrogen and P removal. All the activated sludges were stored at 4 °C and used for the experiments within 48 h. The chemicals included sodium hydroxide (NaOH), hydrochloric acid (HCl), CaCl₂, sodium acetate (CH₃COONa), and 19 amino acids (glycine, asparagine, aspartic acid, threonine, serine, cysteine, valine, alanine, glutamic acid, arginine, glutamine, methionine, proline, tryptophan, isoleucine, histidine, phenylalanine, lysine, leucine; note, all of them were L-isomers except for glycine) were purchased from Guangzhou Wego Technology Co., Ltd (Guangzhou, China) and used without further purification.

2.2 Phosphorus extraction tests with the 19 amino acids

To evaluate the performance of the amino acids in P extraction from activated sludge, batch tests were conducted with the 19 amino acids using activated sludge from the WWTPs. The activated sludge was concentrated to a MLSS concentration of 8.0 g L⁻¹, and the solution pH was adjusted to 7.0 using 0.1 M HCl or 0.1 M NaOH. The sludge was allocated into 100 mL serum bottles. Nitrogen gas was sparged for 15 min to confer anaerobic conditions before the bottles were sealed with rubber stoppers. The 19 amino acids listed above were tested as P-extraction agents with acetate for comparison. Activated sludge without any chemical addition served as the control. The concentrations of amino acids and acetate were chosen based on previous studies.^33–35 Acetate and each amino acid were injected into individual serum bottles to a final TOC concentration of 100 mg L⁻¹. The sealed bottles were incubated in an incubator at 180 rpm and 30 °C for 7 h. Mixed liquid samples were taken every 2 h. Anaerobic conditions were maintained during sampling. The samples were filtered immediately through a 0.45 μm cellulose acetate filter membrane for PO₄³⁻-P and TOC analysis. The experiments were performed in duplicate.

2.3 Effects of the MLSS concentration and temperature

To improve the efficiency of the glycine-assisted P extraction, batch tests were performed to evaluate the effects of the MLSS concentration on the extraction efficiency. The experiment was conducted in a series of 100 mL serum bottles. Before transferring into serum bottles, the activated sludge was concentrated under gravity and diluted with deionized water to a series of MLSS concentrations of 0, 2, 4, 6, 8, and 10 g L⁻¹. The pH was adjusted to 7.0 by using 0.1 M HCl or 0.1 M NaOH. The bottles were sparged with nitrogen gas for 15 min to create anaerobic conditions and were then sealed. Glycine was added into each bottle to a final TOC concentration of 100 mg L⁻¹. The sealed bottles were incubated at 180 rpm and 30 °C. Mixed liquid samples were collected every 20 min and filtered immediately though 0.45 μm cellulose acetate filter membranes for PO₄³⁻-P and TOC analysis. The experiment was performed in duplicate.

To understand the effects of temperature on the P-extraction efficiency of glycine, the experiment was performed at different temperatures, i.e., 5 °C, 25 °C, 30 °C, and 35 °C, respectively, in a thermostat water bath. The activated sludge was concentrated to a MLSS concentration of 7.5 g L⁻¹ in 100 mL serum bottles. The pH was adjusted to 7.0 by using 0.1 M HCl or 0.1 M NaOH. Before the serum bottles were sealed, anaerobic conditions in the bottles were obtained by sparging nitrogen gas for 15 min. Glycine was added into each bottle to a final TOC concentration of 100 mg L⁻¹. The sealed bottles were incubated at 180 rpm. Mixed liquid samples were taken every 20 min and filtered immediately though 0.45 μm cellulose acetate filter membranes for PO₄³⁻-P and TOC analysis. The experiment was performed in duplicate.

2.4 Glycine as a recyclable P-extraction agent

To verify the feasibility of glycine as a recyclable P-extraction agent, repetitive P-extraction experiments were performed. The activated sludge was concentrated to a final MLSS concentration of 8.5 g L⁻¹ in 100 mL serum bottles. Glycine or acetate was added individually into the serum bottles to give a final TOC concentration of 100 mg C per L. The group without any chemical addition served as the control group. The sealed bottles were incubated (180 rpm, 30 °C) under anaerobic conditions for 3 h. After the first P-extraction test, the mixed liquid was centrifuged (at 8000 rpm for 2 min) before collecting the P-enriched supernatant. Next, 1 M CaCl₂ solution was added into the P-enriched supernatant to achieve a Ca/P molar ratio of 1.67:1. The supernatant pH was adjusted to 10.0 using 1 M NaOH for phosphate precipitation. The supernatant was shaken at 180 rpm and 30 °C for 30 min. After the reaction, the precipitates were collected via filtration using 0.45 μm membranes and were air-dried (103 °C, 24 h) for scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analyses. The filtrate was collected and reused by adding it into fresh WAS to a MLSS concentration of 8.5 g L⁻¹ to start the next P-extraction test. The above-mentioned glycine/acetate solution recovery and reuse experiment was repeated four times under identical conditions. Mixed liquid samples were collected at the beginning and end of each cycle, and filtered immediately though 0.45 μm cellulose acetate filter membranes for PO₄³⁻-P and TOC analysis. The experiment was performed in duplicate.

2.5 Analytical methods

The MLSS and PO₄³⁻-P concentrations were measured following the standard methods.³⁶ The pH was monitored with a pH meter (Sartorius, Germany). TOC was analyzed using a TOC analyzer (Shimadzu, Japan). The morphology and chemical composition of the recovered P precipitates were analyzed by using a SEM-EDS (Merlin, Germany) at 15 kV. XRD (Empyrean, The Netherlands) with Cu Kα radiation (λ = 1.54 Å) were used to analyzed the crystalline structure of the precipitates, with a 2θ range of 10–90°. The analysis of the XRD spectra was performed by using MDI Jade v6.0, comparing with the standard HAP card (ICDD 41-0490) to find corresponding peaks. The elemental composition of the precipitates was determined by inductively coupled plasma-mass spectrometry (ICP-MS, Thermo Fisher Scientific, US).

2.6 DNA extraction and 16S rRNA gene amplicon sequencing

The activated sludge samples used in this study were analyzed for determining their bacterial community compositions. The activated sludge samples were snap-frozen in liquid N₂ and stored at −80 °C until analysis. DNA was extracted using the E.Z.N.A.® Soil DNA kit (Omega Bio-tek, US), following the manufacturer's instructions. 16S rRNA gene amplicon sequencing was performed targeting the V4 region (primer set: 515F 5′-GTGCCAGCMGCCGCGG-3′ and 806RmodR 5′-GGACTACNVGGGTWTCTAAT-3′). PCR amplification was carried out with the following program: 3 min denaturation at 95 °C; 27 cycles of 30 s at 95 °C, 30 s annealing at 55 °C, and 45 s elongation at 72 °C, and a final extension at 72 °C for 10 min. Purified and indexed amplicons were pooled and paired-end sequenced (2 × 300) on an Illumina MiSeq platform (Illumina, US) according to the standard protocols at Majorbio Bio-Pharm Technology Co. Ltd. (Shanghai, China). The obtained data were deposited in the NCBI database under the BioProject No. PRJNA1076689.

3. Results and discussion

3.1 P-extraction performances of the 19 different amino acids

To analyze the feasibility and performance of the different amino acids as P-extraction agents, experiments were conducted on activated sludge obtained from a local full-scale WWTP. The TOC concentration of each amino acid was 100 mg L⁻¹. Acetate with the same concentration was used for contrast. The results showed that the supernatant P concentrations increased with the treatment time but varied across amino acids (Fig. 1). After 7 h reaction, glycine induced the highest P release (60.9 mg P per L), comparable to that with acetate (63.4 mg P per L), followed by asparagine (55.4 mg P per L), and aspartic acid (52.9 mg P per L). Leucine induced the lowest P release (13.3 mg P per L). The P-release rate (mg P per L per h) versus final TOC consumption (mg L⁻¹) for different amino acids are presented in Fig. 1(c). The 19 amino acids could be divided into 4 major groups: (1) the group represented by glycine, threonine, serine, cysteine, valine, and alanine, which resulted in the highest P-release rates (8.71 mg P per L per h for glycine, and 6.15, 5.86, 5.78, 5.72, 5.38 mg P per L per h for threonine, serine, cysteine, valine, and alanine, respectively) and minimal consumption (14.0 mg TOC per L for glycine, and 28.1, 13.5, 20.9, 2.7, 30.0 mg TOC per L for threonine, serine, cysteine, valine, and alanine, respectively), (2) the group represented by asparagine and aspartic acid, which resulted in very high P-release rates (7.93 and 7.56 mg P per L per h for asparagine and aspartic acid, respectively) but they were significantly consumed (71.1 and 71.5 mg TOC per L for asparagine and aspartic acid, respectively), (3) the group represented by glutamic acid, methionine, proline, isoleucine, histidine and leucine, which resulted in low P-release rates (4.91, 3.73, 3.72, 2.79, 2.55, and 1.90 mg P per L per h for glutamic acid, methionine, proline, isoleucine, histidine, and leucine, respectively) but they were largely consumed (54.1, 44.1, 52.7, 74.2, 30.7, and 31.6 mg TOC per L for glutamic acid, methionine, proline, isoleucine, histidine, and leucine, respectively), and (4) the group represented by arginine, glutamine, tryptophan, phenylalanine, and lysine, which resulted in low P-release rates (4.30, 3.86, 2.81, 2.29, and 2.16 mg P per L per h for arginine, glutamine, tryptophan, phenylalanine, and lysine, respectively) but they were not significantly consumed (26.1, 15.0, 0, 7.20, and 1.10 mg TOC per L for arginine, glutamine, tryptophan, phenylalanine, and lysine, respectively). For an ideal P-extraction agent, it would be expected to induce a high P release with high kinetics, whereas its consumption should be as low as possible. Based on this criterion, glycine was determined to be the most promising P-extraction agent, which demonstrated the highest P release at an extremely low consumption. Although higher P-release rate was observed for acetate, it might not be a desirable P-extraction agent, since its high P-extraction performance was at the expense of its own high consumption (44.9 mg TOC per L). However, acetate is among the most readily usable carbon sources for PAOs (such as Ca. Accumulibacter, Chen et al., 2022).³⁷ The P release to acetate consumption ratio is typically in the range of 0.75–1.71 mg P per mg TOC.²⁸ In this study, the P release to acetate consumption ratio was observed to be 1.31 mg P per mg TOC, which is close to the model results for Ca. Accumulibacter.^35,37

The performance of P extraction and TOC consumption were strongly related to specific amino acids, which could be attributed to that the amino acids could be used by PAO community (e.g., Ca. Accumulibacter) for P recycling but with different preferences by individual ones.^35,38 However, in a more complex environment (e.g., WWTPs), not only PAOs but also glycogen-accumulating organisms (GAOs) could use amino acids without P recycling,³⁹ which may reduce the efficiency and effectiveness of the P-extraction agents. In this study, the bacterial community compositions in the activated sludge were analyzed using 16S rRNA gene amplicon sequencing. The results are shown in Fig. 1(d) and (e). At the phylum level, the communities were dominated by Bacteroidota (with relative abundances of 33.95%), followed by Proteobacteria (29.12%), Chloroflexi (11.35%), Actinobacteriota (5.86%), Patescibacteria (5.22%), Myxococcota (3.69%), Acidobacteriota (2.78%), Planctomycetota (1.69%), and Nitrospirota (1.47%). The relative abundance values of typical GAOs, i.e., Ca. Competibacter, Defluviicoccus, Micropruina, and Propionivibrio, were 0.67%, 0.10%, 0.07%, and 0.05%, respectively, whereas the relative abundance values of Ca. Accumulibacter, Dechloromonas, and Tetrasphaera were 0.62%, 0.66%, and 0.67%, respectively. Ca. Accumulibacter, Dechloromonas, and Tetrasphaera are commonly recognized PAOs in WWTPs,^40–42 but only Tetrasphaera and Ca. Accumulibacter are considered to take up amino acids as substrates.^35,43 According to Tian et al.,³³ glycine was taken up by PAOs under anaerobic conditions via a TRAP-type C4-dicarboxylate transport system at the expense of the sodium motive force; whereas glycine was not metabolized due to inactivation of the downstream glycine-cleavage system genes. The accumulation of glycine in the cell induced the drug/metabolite transporter efflux mechanism. As a result, glycine was not consumed in the liquid phase, but the activation of the uptake and efflux systems consumed ATP, resulting in P release. Despite the diverse microbial community of activated sludge in this study, glycine presented a strong ability for extracting P without being consumed, indicating that glycine is a highly applicable P-extraction agent for P recovery from WAS.

Overall, considering the P-extraction efficiency and the consumption characteristics of the amino acids, glycine is considered the most cost-effective P-extraction agent to obtain P-enriched solutions without being consumed. Thus, glycine was chosen to investigate its feasibility as a recyclable P-extraction agent.

3.2 Optimization of the glycine-based P-extraction process

3.3 Performance of glycine as a recyclable P-extraction agent

To study the feasibility of glycine as a recyclable P-release/extraction agent, repetitive P-extraction tests were performed with P recovery and glycine solution recycling. First, glycine (100 mg TOC per L) was added to the activated sludge with an 8.5 g L⁻¹ MLSS concentration to obtain a P-enriched supernatant. After using CaCl₂ (Ca/P molar ratio of 1.67) to precipitate the extracted P in the supernatant, the solution was filtered to obtain the CaP precipitates. The glycine-containing filtrate was reused in the next P-extraction test with the addition of fresh WAS at the same MLSS concentration (8.5 g L⁻¹). The experiment was conducted in four cycles.

The results are presented in Fig. 4. The final P concentrations obtained in the four P-extraction tests were 19.5, 20.9, 20.8, and 15.0 mg P per L, respectively, indicating that the P-extraction ability of the reused glycine-containing filtrate was not significantly impaired, whereas the use of acetate showed a 77.6% decrease (46.0 to 10.2 mg P per L) from the first to the fourth cycle, suggesting that glycine was more suitable as a recyclable P-extraction agent compared to acetate. With each extraction test, the TOC values in the glycine system remained unchanged before and after each test, demonstrating that the performance of glycine was not undermined by the P-precipitation and -recovery process at the end of each P-extraction experiment. The recovery efficiency of glycine was 83–90%, of which around 10% was consumed by the activated sludge and 3% in the CaP-precipitation process. Although there was a decrease in glycine concentration due to the dilution effect of the fresh activated sludge in each following cycle, the P-extraction rate achieved by the recovered glycine solution was largely unaffected (6.51. 6.96, 6.94, and 5.00 mg P per L per h from the first to the fourth cycle, respectively). The P precipitation at a Ca/P molar ratio of 1.67 resulted in a high P-recovery rate of up to 72% under a 30 min reaction time, meanwhile, the rest of the P (28%) was returned to the P-recovery system with the glycine recycling, which prevented the P from being wasted. Even though calcium hydroxide was considered to be a more effective calcium source,⁴⁵ the P-recovery rate in this study (72%) was comparable to the P-recovery rate (75.6%) obtained without Ca/P molar ratio adjustment using calcium hydroxide as the calcium source under pH 8.5 for synthetic wastewater.⁴⁶ Within the four-time-repeated experiment, the P-recovery rates in the glycine group (66–72%) were comparable with those in the acetate group (64–78%), suggesting that glycine had no effect on CaP precipitation.⁴⁷ The existence of glycine did not undermine the P-recovery efficiency, which would be a significant advantage of the glycine-assisted P-extraction and recovery process. Overall, glycine is considered to be a feasible, environmentally friendly, and recyclable P-extraction agent. A diagram showing the proposed glycine-assisted in situ P-extraction and recovery process for use in WWTPs is presented in Fig. 5.

3.4 Phosphorus recovery and precipitates analysis

3.5 Implications

This study proposed and validated a new method for sustainable P recovery from WAS utilizing glycine as a recyclable P-extraction agent. Via P-extraction potential tests of a range of amino acids, the unique property of glycine to result in P release from the WAS without being consumed was verified and showed its good sustainability and reusability characteristics for P recovery. The method works well at ambient temperatures. A high MLSS was found to be favorable for P extraction. Glycine-assisted P extraction could also effectively avoid the co-extraction of heavy metals, which is important and desirable for downstream applications of the P-recovery products.

P is commonly reclaimed in the form of struvite from WAS anaerobic digestion supernatant.⁵³ After digestion, the P concentration in the supernatant may reach 50–500 mg L⁻¹. The ammonium concentration may be a few times higher, which renders a possibility to simultaneously recover P and nitrogen in the form of struvite.^46,54 However, struvite precipitation is difficult to control because this method needs a strict ratio of phosphate and ammonium to magnesium.²⁴ Additionally, high levels of heavy metals (e.g., Ca, Mg, Zn, and Cu) in the supernatant may trouble the P-recovery process and the downstream application of the P-recovery products. Thus, it becomes necessary to remove these impurities before/during P recovery, which requires additional chemicals and/or energy inputs.^55,56 Comparatively, by using the glycine-assisted P-extraction method proposed in this study, P may be extracted and recovered from the WAS before anaerobic digestion, preventing potential undesirable P precipitation in the digester and potential resultant pipe/equipment clogging. The glycine-assisted P-extraction method proposed herein may be operated at ambient temperatures. The amount of P extracted is positively correlated with the concentration of WAS. The extraction efficiency may be controllable via sludge concentration manipulation.

Additionally, glycine as a P-extraction agent is recoverable and reusable, making the glycine-assisted P-extraction and recovery method an extremely sustainable alternative. For efficiency and economy considerations in practical applications, a necessarily high glycine concentration (100 mg L⁻¹) is favorable. A high MLSS concentration would also be desirable to achieve high P concentrations in the extract to facilitate downstream P precipitation in real applications. Additionally, further accelerating the P-extraction rate would be favorable. In this study, it was found that a few amino acids, such as threonine, serine, cysteine, valine, and alanine, in addition to glycine, also showed the capability to result in P release without being consumed. There is thus potential to formulate a composite agent composed of multiple amino acids to accelerate the P-extraction kinetics. The method proposed in this study may be further tested in extended spatial and temporal scales for real applications.

4. Conclusions

This study reported an innovative strategy for effective, sustainable, and green P extraction and recovery from WAS with the assistance of glycine. The addition of glycine resulted in high P-release rates linearly related to the MLSS concentrations without impacts from the temperature in the range of 25–35 °C. A high recyclability of glycine was observed, with <10% glycine loss in four repetitive P-extraction–P-recovery–glycine reuse cycles. Via calcium precipitation, high P-content (48% as phosphate) amorphous calcium phosphate was obtained with extremely low heavy metal contents. Glycine-assisted P extraction and recovery offers a promising alternative for sustainable P recovery from WAS.

Data availability

The data supporting this article have been included in the ESI.† 16S rRNA gene amplicon sequencing data in this article were deposited in the NCBI database under the BioProject No. PRJNA1076689. All other data is available upon request.

Author contributions

Sheqi Cen: conceptualization, methodology, investigation, formal analysis, writing – original draft. Yao Zou: supervision, resources, data curation, writing – review & editing. Hang Chen: methodology, writing – review & editing. Xuhan Deng: methodology, writing – review & editing. Fu Huang: methodology, investigation. Liping Chen: writing – review & editing. Le Li: investigation. Tenghui Jin: investigation. Chaohai Wei: project administration. Lichao Nengzi: supervision, project administration, funding acquisition. Guanglei Qiu: conceptualization, resources, supervision, project administration, funding acquisition, writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52270035 and 51808297), the Natural Science Foundation of Guangdong Province, China (No. 2021A1515010494), the Science and Technology Program of Guangzhou, China (No. 202002030340), the Pearl River Talent Recruitment Program (No. 2019QN01L125), the Guangdong Science and Technology Program (No. 2020B121201003), the Guangzhou Key Research and Development Program (2023B03J1334), and the Special Project for Research and Development in Key areas of Guangdong Province (2019B110209002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ew00158c

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