Phosphorus recovery from wastewater: needs, technologies and costs

Recovery of phosphorus from chemical-enhanced phosphorus removal sludge: Influence of sodium sulfide dosage on phosphorus fractionation, sludge dewaterability, and struvite product

Despite the potential of sodium sulfide (Na2S) for phosphorus (P) recovery from iron-phosphate waste, the underlying mechanism regarding its impact on P conversion and product quality has not been well addressed. In this study, the effects of Na2S addition on P release and recovery from a chemical-enhanced phosphorus removal (CEPR) sludge during anaerobic fermentation were systematically investigated. The results revealed that the effective mobilization of P bound to Fe (Fe-P) by Na2S dominated the massive P release from the CEPR sludge, while the organic P (OP) release was not significantly enhanced during anaerobic fermentation. Due to the rapid reaction of Na2S with Fe-P and the prevention of Fe(II)-P precipitation by excess S2-, the Fe-P was decreased by 9.7%, 15.2% and 24.9% at S:Fe molar ratios of 0.3, 0.5 and 1, respectively. After anaerobic fermentation, the released P mainly existed as soluble phosphate (SP), P bound to Ca (Ca-P) and P bound to Al (Al-P). The nitrogen and P contents in the fermentation supernatant significantly increased with higher S:Fe ratios, facilitating the efficient recovery of P as high-purity struvite. However, the increased Na2S dosage deteriorated the sludge dewaterability because of the dissolution of hydrophilic extracellular polymeric substances and the looser secondary structure of proteins. Comprehensively considering the P recovery, sludge dewaterability and economic cost, the optimal Na2S dosage was determined at the S:Fe ratio of 0.3. These findings provide novel insights into the role of Na2S in P recovery as struvite from CEPR sludge.

Phosphorus recovery from ultrafiltered membrane wastewater by biochar adsorption columns: The effect of loading rates

The present study used bench scale columns filled with biochar for phosphorous (P) recovery from real ultrafiltered wastewater. No studies are available about the potentiality of biochar using ultrafiltered real wastewater. Therefore, this study aimed to assess phosphate (PO4 3-) recovery by biochar-packed columns employing real treated wastewater from an ultrafiltration process. Three flow rates were tested, specifically 0.7, 1.7 and 2.3 L h-1, to gain insights into the optimal working conditions. Results revealed that the maximum amount of PO4 3- recovery (namely, 3.43 mg g-1 biochar) can be achieved after 7 h by employing the highest tested flow rate. Furthermore, the phosphorus exchange capacity (PEC) was inversely correlated with the feeding flow rate (FFR), with PEC values equal to 35, 25 and 9 % for FFR of 0.67, 1.7 and 2.3 L h-1, respectively. The pseudo-first order model best approximated the adsorption kinetics, thus suggesting that the adsorption of phosphate by biochar depends on its concentrations (i.e. physiosorption mechanism).

Phosphate recovery from urban sewage by the biofilm sequencing batch reactor process: Key factors in biofilm formation and related mechanisms

The biofilm sequencing batch reactor (BSBR) technique has been deployed in the laboratory to enrich phosphorus from simulated wastewater, but it is still not clear what its performance will be when real world sewage is used. In this work, the effluent from the multi-stage anoxic-oxic (AO) activated sludge process at a sewage plant was used as the feed water for a BSBR pilot system, which had three reactors operating at different levels of dissolved oxygen (DO). The phosphorus adsorption and release, the biofilm growth, and the extracellular polymeric substances (EPS) components and contents were examined. The microbial communities and the signaling molecules N-acyl-l-homoserine lactones (AHLs) were also analyzed. Gratifyingly, the BSBR process successfully processed the treated sewage, and the biofilm developed phosphorus accumulation capability within 40 days. After entering stable operation, the system concentrated phosphate from 2.59 ± 0.77 mg/L in the influent to as much as 81.64 mg/L in the recovery liquid. Sludge discharge had profound impacts on all aspects of BSBR, and it was carried out successfully when the phosphorus absorption capacity of the biofilm alone was comparable to that of the reactor containing the activated sludge. Shortly after the sludge discharge, the phosphate concentration of the recovery liquid surged from 50 to 140 mg/L, the biofilm thickness grew from 20.56 to 67.32 μm, and the diversity of the microbial population plunged. Sludge discharge stimulated Candidatus competibacter to produce a large amount of AHLs, which was key in culturing the biofilm. Among the AHLs, both C10-HSL and 3OC12-HSL were significantly positively correlated with EPS and the abundance of Candidatus competibacter. The current results demonstrated BSBR as a viable option to enrich phosphorus from real world sewage with low phosphorus content and fluctuating chemistry. The mechanistic explorations also provided theoretical guidance for cultivating phosphorus-accumulating biofilms.

A comprehensive investigation to the fate of phosphorus in full-scale wastewater treatment plants using aluminum salts for enhanced phosphorus removal

This study investigated the fate of phosphorus (P) in 8 full-scale municipal wastewater treatment plants (WWTPs) in Shanghai, China, in which both biological nutrient removal and aluminum-based chemical phosphorus removal were used. The results showed that 83.8-98.9 % P was transferred to the sludge in the 8 WWTPs by both chemical and biological reactions. P speciation analysis indicated that chemical P precipitates accounted for 84.3 % in the activated sludge, of which crystalline AlPO4 and amorphous iron‑phosphorus compounds (FePs) were the main components. Sludge with more water-soluble and weakly adsorbed P was generated in the anaerobic-anoxic-oxic (A/A/O) process than in other processes. Among the 8 WWTPs, the one with the largest flow rate and relatively short sludge retention time (SRT) had the best potential to release P from all types of sludge. The recovery potential of P from thickened sludge can be improved by separately thickening the sludge produced in the high-efficiency sedimentation tank or feeding it into the dewatering process directly. Different P removal chemicals and dosing points changed the amount of chemical precipitate formed but had little effect on the composition of P accumulating organisms (PAOs) at the genus level. Although aluminum-based coagulants were applied in the investigated WWTPs, Fe in wastewater had the most positive effect on the proliferation of PAOs. The synthesis of polyphosphate was also related to the metabolism of PAOs as it affected transmembrane inorganic phosphate (Pi) transport and polyhydroxybutyrate (PHB) synthesis. The in-depth understanding of the fate of P is beneficial to improve P recovery efficiency in WWTPs.

Evaluating the opportunities for mainstream P-recovery in anaerobic/anoxic/aerobic systems

Mainstream P-recovery can help wastewater treatment plants (WWTPs) to effectively maintain good enhanced biological phosphorus removal (EBPR) while helping to recover P. In this study, a pilot-scale anaerobic-anoxic-aerobic (A2O) process was operated for simultaneous COD/N/P removal and P-recovery under different operational conditions. The operation with conventional extraction of waste activated sludge (WAS) from the aerobic reactor was compared to the mainstream P-recovery strategy of WAS extraction from the anaerobic reactor. Successful nutrient removal was obtained for both scenarios, but the anaerobic WAS extraction results improved polyphosphate accumulating organisms (PAOs) activity by increasing almost 27 % P concentration in the anaerobic reactor. WAS fermentation was also evaluated, showing that anaerobic WAS required only 3 days to reach a high P concentration, while the aerobic WAS fermentation required up to 7 days. The fermentation process increased the amount of soluble P available for precipitation from 24.4 % up to 51.6 % in the fermented anaerobic WAS scenario. Results obtained by precipitation modelling of these streams showed the limitations for struvite precipitation due to Ca2+ interference and Mg2+ and NH4+ as limiting species. The optimum precipitation scenario showed that P-recovery could reach up to 51 % of the input P, being 90 % struvite.

Biological recovery of phosphorus (BioP-Rec) from wastewater streams using brewer's yeast on pilot-scale

Most recent advances for phosphorus (P) recovery using brewery yeast on laboratory scale were used to scale up to a pilot-scale process (BioP-Rec module) and applied in a full-scale wastewater treatment plant (WWTP). A P balance was established for WWTP Markranstädt according to two thresholds: (1) the economic feasibility threshold for P recovery of 0.05 kg/m3 of free P, and (2) the German Sewage Sludge Ordinance (GSSO) threshold, which demands that all WWTPs with a P content in dry matter (DM) of biosolids of 20 gP/kgDM or higher in the coming years must perform mandatory P recovery. In terms of defined thresholds, return and excess sludges were identified as the most feasible WWTP process streams for P recovery. In a 1 m3 BioP-Rec module a 3 stage process was established. From the P-rich water-phase of the return sludge produced in stage 1, which contained 0.051 kg/m3 of free P, 77.56% was taken up by P-depleted brewer's yeast Saccharomyces pastorianus in 3 h in stage 2. In stage 3, the yeast was concentrated in 1 h to produce yeast sludge as a fertilizer product. We demonstrated a novel pilot-scale process for the production of bio-based P-rich fertilizer.

Impact of coagulation on SARS-CoV-2 and PMMoV viral signal in wastewater solids

Wastewater surveillance (WWS) of SARS-CoV-2 has become a crucial tool for monitoring COVID-19 cases and outbreaks. Previous studies have indicated that SARS-CoV-2 RNA measurement from testing solid-rich primary sludge yields better sensitivity compared to testing wastewater influent. Furthermore, measurement of pepper mild mottle virus (PMMoV) signal in wastewater allows for precise normalization of SARS-CoV-2 viral signal based on solid content, enhancing disease prevalence tracking. However, despite the widespread adoption of WWS, a knowledge gap remains regarding the impact of ferric sulfate coagulation, commonly used in enhanced primary clarification, the initial stage of wastewater treatment where solids are sedimented and removed, on SARS-CoV-2 and PMMoV quantification in wastewater-based epidemiology. This study examines the effects of ferric sulfate addition, along with the associated pH reduction, on the measurement of SARS-CoV-2 and PMMoV viral measurements in wastewater primary clarified sludge through jar testing. Results show that the addition of Fe3+ concentrations in the conventional 0 to 60 mg/L range caused no effect on SARS-CoV-2 N1 and N2 gene region measurements in wastewater solids. However, elevated Fe3+ concentrations were shown to be associated with a statistically significant increase in PMMoV viral measurements in wastewater solids, which consequently resulted in the underestimation of PMMoV-normalized SARS-CoV-2 viral signal measurements (N1 and N2 copies/copies of PMMoV). The observed pH reduction from coagulant addition did not contribute to the increased PMMoV measurements, suggesting that this phenomenon arises from the partitioning of PMMoV viral particles into wastewater solids.

Recovering Phosphate from Complex Wastewater Using Macroporous Cryogel Composited Calcium Silicate Hydrate Nanoparticles

Since currently used natural, nonrenewable phosphorus resources are estimated to be depleted in the next 30-200 years, phosphorus recovery from any phosphorus-rich residues has attracted great interest. In this study, phosphorus recovery from complex wastewater samples was investigated using continuous adsorption on cryogel column composited calcium silicate hydrate nanoparticles (CSH columns). The results showed that 99.99% of phosphate was recovered from a synthetic water sample (50 mg L-1) using a 5 cm CSH column with a 5 mL min-1 influent flow rate for 6 h while 82.82% and 97.58% of phosphate were recovered from household laundry wastewater (1.84 mg L-1) and reverse osmosis concentrate (26.46 mg L-1), respectively. The adsorption capacity decreased with an increasing flow rate but increased with increasing initial concentration and column height, and the obtained experimental data were better fitted to the Yoon-Nelson model (R2 = 0.7723-0.9643) than to the Adams-Bohart model (R2 = 0.6320-0.8899). The adsorption performance of phosphate was decreased 3.65 times in the presence of carbonate ions at a similar concentration, whereas no effect was obtained from nitrate and sulfate. The results demonstrate the potential of continuous-flow phosphate adsorption on the CSH column for the recovery of phosphate from complex wastewater samples.

New insight into the granule formation in the reactor for enhanced biological phosphorus removal

While granulated activated sludge exhibits high productivity, the processes of granule formation are incompletely studied. The processes of granule formation and succession of communities were investigated in a laboratory sequencing batch reactor (SBR) under conditions for enhanced biological phosphorus removal (EBPR) using microbiological and molecular techniques. Active consumption of acetate, primarily by the phosphate-accumulating organisms (PAO), commenced at day 150 of cultivation. This was indicated by the high ratio of molar P-released/acetate uptake (0.73-0.77 P-mol/C-mol), characteristic of PAO. During this period, two types of granule-like aggregates formed spontaneously out of the activated sludge flocs. The aggregates differed in morphology and microbial taxonomic composition. While both aggregate types contained phosphorus-enriched bacterial cells, PAO prevailed in those of morphotype I, and glycogen-accumulating organisms (GAOs) were predominant in the aggregates of morphotype II. After 250 days, the elimination of the morphotype II aggregates from the reactor was observed. The subsequent selection of the community was associated with the development of the morphotype I aggregates, in which the relative abundance of PAO increased significantly, resulting in higher efficiency of phosphorus removal. Metagenomic analysis revealed a predominance of the organisms closely related to Candidatus Accumulibacter IС and IIС and of Ca. Accumulibacter IIB among the PAO. Based on the content of the genes of the key metabolic pathways, the genomes of potential PAO belonging to the genera Amaricoccus, Azonexus, Thauera, Zoogloea, Pinisolibacter, and Siculibacillus were selected. The patterns of physicochemical processes and the microbiome structure associated with granule formation and succession of the microbial communities were revealed.

Phosphorus release and recovery by reductive dissolution of chemically precipitated phosphorus from simulated wastewater

Chemically mediated recovery of phosphorous (P) as vivianite from the sludges generated by chemical phosphorus removal (CPR) is a potential means of enhancing sustainability of wastewater treatment. This study marks an initial attempt to explore direct P release and recovery from lab synthetic Fe-P sludge via reductive dissolution using ascorbic acid (AA) under acidic conditions. The effects of AA/Fe molar ratio, age of Fe-P sludge and pH were examined to find the optimum conditions for Fe-P reductive solubilization and vivianite precipitation. The performance of the reductive, chelating, and acidic effects of AA toward Fe-P sludge were evaluated by comparison with hydroxylamine (reducing agent), oxalic acid (chelating agent), and inorganic acids (pH effect) including HNO3, HCl, and H2SO4. Full solubilization of Fe-P sludge and reduction of Fe3+ were observed at pH values 3 and 4 for two Fe/AA molar ratios of 1:2 and 1:4. Sludge age (up to 11 days) did not affect the reductive solubilization of Fe-P with AA addition. The reductive dissolution of Fe-P sludge with hydroxylamine was negligible, while both P (95 ± 2%) and Fe3+ (90 ± 1%) were solubilized through non-reductive dissolution by oxalic acid treatment at an Fe/oxalic acid molar ratio 1:2 and a pH 3. With sludge treatment with inorganic acids at pH 3, P and Fe release was very low (<10%) compared to AA and oxalic acid treatment. After full solubilization of Fe-P sludge by AA treatment at pH 3 it was possible to recover the phosphorus and iron as vivianite by simple pH adjustment to pH 7; P and Fe recoveries of 88 ± 2% and 90 ± 1% respectively were achieved in this manner. XRD analysis, Fe/P molar ratio measurements, and magnetic attraction confirmed vivianite formation. PHREEQC modeling showed a reasonable agreement with the measured release of P and Fe from Fe-P sludge and vivianite formation.

Evaluating Sustainable Development Pathways for Protein- and Peptide-Based Bioadsorbents for Phosphorus Recovery from Wastewater

Recovering phosphate (P) from point sources such as wastewater effluent is a priority in order to alleviate the impacts of eutrophication and implement a circular economy for an increasingly limited resource. Bioadsorbents featuring P-binding proteins and peptides offer exquisite P specificity and sensitivity for achieving ultralow P concentrations, i.e., <100 μg P L-1, a discharge limit that has been implemented in at least one treatment facility in nine U.S. states. To prioritize research objectives for P recovery in wastewater treatment, we compared the financial and environmental sustainability of protein/peptide bioadsorbents to those of LayneRT anion exchange resin. The baseline scenario (reflecting lab-demonstrated performance at a full-scale implementation) had costs that were 3 orders of magnitude higher than those for typical wastewater treatment. However, scenarios exploring bioadsorbent improvements, including increasing the P-binding capacity per unit volume by using smaller P-selective peptides and nanoparticle base materials and implementing reuse, dramatically decreased median impacts to $1.06 m-3 and 0.001 kg CO2 equiv m-3; these values are in line with current wastewater treatment impacts and lower than the median LayneRT impacts of $4.04 m-3 and 0.19 kg CO2 equiv m-3. While the financial viability of capturing low P concentrations is a challenge, incorporating the externalities of environmental impacts may provide a feasible path forward to motivate ultralow P capture.

Wastewater Treatment for Carbon Dioxide Removal

Wastewater treatment is notorious for its hefty carbon footprint, accounting for 1-2% of global greenhouse gas (GHG) emissions. Nonetheless, the treatment process itself could also present an innovative carbon dioxide removal (CDR) approach. Here, the calcium (Ca)-rich effluent of a phosphorus (P) recovery system from municipal wastewater (P recovered as calcium phosphate) was used for CDR. The effluent was bubbled with concentrated CO2, leading to its mineralization, i.e., CO2 stored as stable carbonate minerals. The chemical and microstructural properties of the newly formed minerals were ascertained by using state-of-the-art analytical techniques. FTIR identified CO3 bonds and carbonate stretching, XRF and SEM-EDX measured a high Ca concentration, and SEM imaging showed that Ca is well distributed, suggesting homogeneous formation. Furthermore, FIB-SEM revealed rhombohedral and needle-like structures and TEM revealed rod-like structures, indicating that calcium carbonate (CaCO3) was formed, while XRD suggested that this material mainly comprises aragonite and calcite. Results imply that high-quality CaCO3 was synthesized, which could be stored or valorized, while if atmospheric air is used for bubbling, a partial direct air capture (DAC) system could be achieved. The quality of the bubbled effluent was also improved, thus creating water reclamation and circular economy opportunities. Results are indicative of other alkaline Ca-rich wastewaters such as effluents or leachates from legacy iron and steel wastes (steel slags) that can possibly be used for CDR. Overall, it was identified that wastewater can be used for carbon mineralization and can greatly reduce the carbon footprint of the treatment process, thus establishing sustainable paradigms for the introduction of CDR in this sector.

Effect of recirculation and hydraulic loading rate on ammonium and phosphate recovery from fertilizer industry wastewater

This study aimed to determine the optimum recirculation ratio and hydraulic loading rate for the recovery process of ammonium and phosphate, and study the formation, morphology, and structure of struvite crystals. This study provides a crystallization method to treat fertilizer wastewater using a continuous system and recirculation, including a fluidized bed reactor (FBR). Ammonium and phosphate were recovered from fertilizer industry wastewater using a continuous FBR under different recirculation ratios (3, 6, and 9) and loading rates (0.39 L/min, 0.59 L/min, and 0.79 L/min). MgCl2 was used as the precipitant with a 1.5:1 molarity ratio [Mg2+]:[PO43-]. The results showed that 55.71% phosphate and 49.69% ammonium could be recovered at recirculation ratio and hydraulic loading rate were 9 and 0.39 L/min, respectively. Scanning electron microscopy-energy diffraction X-ray analysis showed that the formed crystals comprised 80% struvite. Further, a high recirculation ratio and low loading rate resulted in higher ammonium and phosphate recovery efficiencies. These results indicated that a long detention time of treated wastewater can increase the nutrient recovery.

Phosphorus recycling from waste activated sludge using the hydrothermal platform: Recovery, solubility and phytoavailability

To address the grand challenge of increasing the sustainability of wastewater treatment plants, hydrothermal carbonization was studied as a nutrient recovery platform, transforming sludge into a valuable hydrochar. Carbonization was achieved at different temperatures (200-300 °C) and durations (30-120 min). The highest mass recovery (73%) was observed in the lowest temperature, while the lowest (49%) was obsereved at the highest temperature. Under all reaction conditions, phosphorus recovery values exceeded 80%, with the dominated fraction of inorganic-P in the hydrochar being HCl-extractable. Although HCl-extractable P is considered a moderately labile P fraction, P phytoavailability assays indicate that sewage sludge hydrochar is an excellent source for P, surpassing soluble P, likely due to its slow-release nature. We postulate that polyphosphates constitute a significant portion of this P pool. Overall, we emphasize the benefits of using HTC as a circular economy approach to convert sludge into a valuable hydrochar.

Short-Term Impact of Recycling-Derived Fertilizers on Their P Supply for Perennial Ryegrass (Lolium perenne)

Various nutrient recycling technologies are currently under development in order to alleviate the dependency of non-renewable raw material for the production of mineral phosphorus fertilizers commonly used in agriculture. The resulting products, such as struvites and ashes, need to be assessed for their application as so-called recycling-derived fertilizers (RDFs) in the agricultural sector prior to commercialization. Here, we conducted a short-term (54 days) trial to investigate the impact of different phosphorus fertilizers on plant growth and the soil P cycling microbiota. Lolium perenne was grown with application of superphosphate (SP) as inorganic fertilizer, two ashes (poultry litter ash (PLA) and sewage sludge ash (SSA)), and two struvites (municipal wastewater struvite (MWS) and commercial CrystalGreen® (CGS)) applied at 20 and 60 kg P ha-1 in four replicates. A P-free control (SP0) was also included in the trial. Struvite application increased plant dry weights, and available P acid phosphatase activity was significantly improved for struvites at the high P application rate. The ash RDFs showed a liming effect at 60 kg P ha-1, and PLA60 negatively affected acid phosphatase activity, while PLA20 had significantly lower phoD copy numbers. P mobilization from phosphonates and phytates was not affected. TCP solubilization was negatively affected by mineral SP fertilizer application at both P concentrations. The bacterial (16S and phoD) communities were only marginally affected by the tested P fertilizers. Overall, struvites appeared to be a suitable substitute for superphosphate fertilization for Irish L. perenne pastures.

Isolation of phosphorus-hyperaccumulating microalgae from revolving algal biofilm (RAB) wastewater treatment systems

Excess phosphorus (P) in wastewater effluent poses a serious threat to aquatic ecosystems and can spur harmful algal blooms. Revolving algal biofilm (RAB) systems are an emerging technology to recover P from wastewater before discharge into aquatic ecosystems. In RAB systems, a community of microalgae take up and store wastewater P as polyphosphate as they grow in a partially submerged revolving biofilm, which may then be harvested and dried for use as fertilizer in lieu of mined phosphate rock. In this work, we isolated and characterized a total of 101 microalgae strains from active RAB systems across the US Midwest, including 82 green algae, 9 diatoms, and 10 cyanobacteria. Strains were identified by microscopy and 16S/18S ribosomal DNA sequencing, cryopreserved, and screened for elevated P content (as polyphosphate). Seven isolated strains possessed at least 50% more polyphosphate by cell dry weight than a microalgae consortium from a RAB system, with the top strain accumulating nearly threefold more polyphosphate. These top P-hyperaccumulating strains include the green alga Chlamydomonas pulvinata TCF-48 g and the diatoms Eolimna minima TCF-3d and Craticula molestiformis TCF-8d, possessing 11.4, 12.7, and 14.0% polyphosphate by cell dry weight, respectively. As a preliminary test of strain application for recovering P, Chlamydomonas pulvinata TCF-48 g was reinoculated into a bench-scale RAB system containing Bold basal medium. The strain successfully recolonized the system and recovered twofold more P from the medium than a microalgae consortium from a RAB system treating municipal wastewater. These isolated P-hyperaccumulating microalgae may have broad applications in resource recovery from various waste streams, including improving P removal from wastewater.

Characteristic of phosphorus rich compounds in the incinerated sewage sludge ashes: a case for sustainable waste management

Growing concern over mineral resources supply forces us to search for alternative sources of Phosphorus. The possibility to recover phosphorus from incinerated sewage sludge ashes appears to be an important aspect in anthropogenic phosphorus cycle and sustainable economy. To make phosphorus recovery efficient it is important to learn the chemical and mineral composition of ash and phosphorus speciation. The phosphorus content in the ash was over 7%, what corresponds to medium rich phosphorus ores. The main phosphorus rich mineral phases were phosphate minerals. The most widespread was tri-calcium phosphate Whitlockite with various Fe, Mg and Ca proportions. In minority Fe-PO₄ and Mg-PO₄ were detected. Whitlockite commonly overgrown with hematite, influences negatively mineral solubility and thus recovery potential and indicates low bioavailability of phosphorus. Considerable amount of phosphorus was found in the low crystalline matrix where phosphorus content was around 10 wt% however low crystallinity and dispersed phosphorus also does not strengthen the potential to recover this element.

Prediction of calcium phosphate generation and behaviors of metals during phosphorus recovery using a modified thermodynamic model

In this study, behaviors of metals and their effects on phosphorus recovery by calcium phosphate were investigated by the laboratory and pilot experiments as well as by the modified thermodynamic model. Batch experimental results indicated that the efficiency of phosphorus recovery decreased with the increase in metal content and more than 80% phosphorus can be recovered with a Ca/P molar ratio of 3.0 and a pH of 9.0 for the supernatant of an anaerobic tank in the A/O process with the influent containing a high metal level. The mixture of amorphous calcium phosphate (ACP) and dicalcium phosphate dihydrate (DCPD) was assumed to be the precipitated product with an experimental time of 30 min. A modified thermodynamic model was developed using ACP and DCPD as the precipitated products, and the correction equations were incorporated to simulate the short-term precipitation of calcium phosphate based on the experimental results. From the perspective of maximizing both the efficiency of phosphorus recovery and the quality or purity of the recovered product, the simulation results showed that a pH of 9.0 and a Ca/P molar ratio of 3.0 were the optimized operational condition for phosphorus recovery by calcium phosphate when the influent metal content was at the level of actual municipal sewage.

Nutrient recovery from municipal solid waste leachate in the scope of circular economy: Recent developments and future perspectives

Holistically considering the current situation of the commercial synthetic fertilizer (CSF) market, recent global developments, and future projection studies, dependency on CSFs in agricultural production born significant risks, especially to the food security of foreign-dependent countries. The foreign dependency of countries in terms of CSFs can be reduced by the concepts such as the circular economy and resource recovery. Recently, waste streams are considered as a source in order to produce recovery-based fertilizers (RBF). RBFs produced from different waste streams can be substituted with CSFs as input for agricultural applications. Municipal solid waste leachate (MSWL) is one of the waste streams that have a high potential for RBF production. Distribution of the published papers over the years shows that this potential was noticed by more researchers in the millennium. MSWL contains a remarkable amount of nitrogen and phosphorus which are the main nutrients required for agricultural production. These nutrients can be recovered with many different methods such as microalgae cultivation, chemical precipitation, ammonia stripping, membrane separation, etc. MSWL can be generated within the different phases of municipal solid waste (MSW) management. Although it is mainly composed of landfill leachate (LL), composting plant leachate (CPL), incineration plant leachate (IPL), and transfer station leachate (TSL) should be considered as potential sources to produce RBF. This study compiles studies conducted on MSWL from the perspective of nitrogen and phosphorus recovery. Moreover, recent developments and limitations of the subject were extensively discussed and future perspectives were introduced by considering the entire MSW management. Investigated studies in this review showed that the potential of MSWL to produce RBF is significant. The outcomes of this paper will serve the countries for ensuring their food security by implementing the resource recovery concept to produce RBF. Thus, the risks born with the recent global developments could be overcome in this way besides the positive environmental outcomes of resource recovery.

Whey permeate as a phosphorus source for algal cultivation

Microalgal cultivation for biodiesel and feed requires recycled nutrient resources for a sustainable long-term operation. Whey permeate (WP) from dairy processing contains high organic load (lactose, oils, and proteins) and nitrogen (resources tested for microalgal cultivation) and organic phosphorus (P) that has not yet been tested as a P source for microalgal cultivation. We explored the potential of green algae strains (brackish) and polyculture (freshwater) in exploiting P from WP added to a medium based on either seawater (7 psu) or landfill leachate. Both strains showed a capacity of using organic P in WP with equal growth rates (0.94-1.12 d^-1 ) compared with chemical phosphate treatments (0.88-1.07 d^-1 ). The polyculture had comparable growth rate (0.25-0.57 d^-1 ) and biomass yield (152.1-357.5 mg L^-1 ) and similar or higher nutrient removal rate in the leachate-WP medium (1.3-6.4 mg L^-1  day^-1 nitrogen, 0.2-1.1 mg L^-1  day^-1 P) compared with the leachate-chemical phosphate medium (1.2-4.7 mg L^-1  day^-1 nitrogen, 0.3-1.4 mg L^-1  day^-1 P). This study showed that WP is a suitable P source for microalgal cultivation over a range of salinities. To date, this is the first study demonstrating that raw WP can replace mineral P fertilizer for algal cultivation. PRACTITIONERS POINTS: Whey permeate is a comparable phosphorus source to standard fertilizers used in algal cultivation. Green algae removed phosphorus effectively from whey permeate. Microalgal cultivation is a good approach for treatment of whey permeate in combination with a nitrogen-rich wastewater.

Achieving phosphorus recovery at pilot-scale anaerobic anoxic/nitrifying-induced crystallization (A2N-IC) process: Performance, assessment, and challenges

A pilot-scale anaerobic-anoxic/nitrifying/induced crystallization (A₂N-IC) process was established for phosphorus (P) recovery and nutrient removal from municipal wastewater with a treatment capacity of 80 m³d^-1. Results show that the A₂N-IC process can operate stably on a pilot scale; the recovery efficiency of influent P reached 62.2%, and the total P removal efficiency of the IC section was 65.4%. The IC section had little effect on the removal of chemical oxygen demand (COD) and nitrogen (N), and the P removal efficiency was improved. Soluble non-reactive P (sNRP) was the key factor affecting P recovery efficiency. Although P recovery increases the construction and maintenance costs, the process can be profitable if a market for P recovery products is established. To improve the P recovery efficiency, attention should be paid to the effects of sNRP and dissolved organic matter (DOM) on P recovery, and P-rich sludge should be considered.

Macro-nutrients recovery from liquid waste as a sustainable resource for production of recovered mineral fertilizer: Uncovering alternative options to sustain global food security cost-effectively

Global food security, which has emerged as one of the sustainability challenges, impacts every country. As food cannot be generated without involving nutrients, research has intensified recently to recover unused nutrients from waste streams. As a finite resource, phosphorus (P) is largely wasted. This work critically reviews the technical applicability of various water technologies to recover macro-nutrients such as P, N, and K from wastewater. Struvite precipitation, adsorption, ion exchange, and membrane filtration are applied for nutrient recovery. Technological strengths and drawbacks in their applications are evaluated and compared. Their operational conditions such as pH, dose required, initial nutrient concentration, and treatment performance are presented. Cost-effectiveness of the technologies for P or N recovery is also elaborated. It is evident from a literature survey of 310 published studies (1985-2022) that no single technique can effectively and universally recover target macro-nutrients from liquid waste. Struvite precipitation is commonly used to recover over 95 % of P from sludge digestate with its concentration ranging from 200 to 4000 mg/L. The recovered precipitate can be reused as a fertilizer due to its high content of P and N. Phosphate removal of higher than 80 % can be achieved by struvite precipitation when the molar ratio of Mg²⁺/PO₄^3- ranges between 1.1 and 1.3. The applications of artificial intelligence (AI) to collect data on critical parameters control optimization, improve treatment effectiveness, and facilitate water utilities to upscale water treatment plants. Such infrastructure in the plants could enable the recovered materials to be reused to sustain food security. As nutrient recovery is crucial in wastewater treatment, water treatment plant operators need to consider (1) the costs of nutrient recovery techniques; (2) their applicability; (3) their benefits and implications. It is essential to note that the treatment cost of P and/or N-laden wastewater depends on the process applied and local conditions.

The assessment of phosphorus recovery potential in sewage sludge incineration ashes - a case study

A sewage sludge incineration ash contains large amounts of phosphorus, which are considered as a novel anthropogenic waste-based substitute for phosphorus natural resources. Phosphorus is accumulated at most in phosphate minerals of whitlockite structure, that contain Fe, Ca, and Mg and in the matrix composed of Si, Al, Fe, Ca, P, Mg, K, Na in various proportions. The goal of this study was to estimate phosphorus recovery potential. A four-stage sequential extraction, following the modified Golterman procedure, was applied. Separation of four independent fractions enabled to understand better the manner of phosphorus occurrence in the studied ash. The results of the extraction indicated the greatest release of phosphorus combined with organic matter using sulfuric acid. The release was on average at the level of 64%. The chelating Na-EDTA compound indicated lower ability to extract phosphorus (at the level of 35%), and the highest ability to extract heavy metals and potentially toxic elements (As, Zn, Mo). The sequential extraction led to the total recovery of phosphorus of around 40-60.

An integrated strategy for nutrient harvesting from hydrolyzed human urine as high-purity products: Tracking of precipitation transformation and precise regulation

The nutrient recovery from source-separated urine is of great significance for a sustainable and closed nutrient loop. However, common urine-processing techniques have several constraints, including inefficient recovery, low product purity and incapability of simultaneously harvesting multiple nutrients. In this study, an integrated strategy of P precipitation and N stripping was first proposed to harvest nutrients from hydrolyzed human urine as high-purity products via precisely regulating Ca/P dosing ratio. Ca(OH)2 was utilized to trigger Ca-P precipitation and elevate pH level. Different from the previously reported conventional struvite method, P recovery was oriented to calcium phosphate. P harvesting behavior was investigated as a function of key factors including initial P concentration and the dosing ratio. A thermodynamic model was constructed to unveil the precipitation transformation mechanism and visualize P recovery for an enhanced controllability. For N harvesting, Ca(OH)2 was dosed to increase the pH of the urine to converts ammonium to ammonia. The resulting ammonia was stripped and then adsorbed by H2SO4 as high-purity ammonium sulfate. Moreover, the sulfate derived from acidification treatment was recovered as calcium sulfate in the interests of material recycling and mitigating secondary contaminations. Results exhibited P recovery efficiency could reach 100 % and purity for calcium phosphate could be above 90 % within a Ca/P ratio range of 1.67-2.0. Further boosting pH to 12, over 85 % of S and 95 % of N was retrieved. The comprehensive scheme provides an efficient approach towards the precise P and N harvesting from hydrolyzed urine and advances the knowledge of precipitation transformation mechanism.

Phosphorus harvesting from fresh human urine: A strategy of precisely recovering high-purity calcium phosphate and insights into the precipitation conversion mechanism

Phosphorus (P) harvesting from source-separated urine to optimize the overall nutrient loop is one of the most appealing benefits and is a global research interest in wastewater management and treatment. However, current P precipitation is mainly oriented to struvite, which is limited by the issues such as relatively low product purity and high cost of Mg source. Distinguished from previous conventional struvite precipitation, the strategy of precisely harvesting P from fresh human urine as high-purity calcium phosphate was first proposed in this study. This enhanced strategy can optimize P harvesting performance and product purity by simply regulating the consumption of calcium-based materials via model simulation and experimental validation. The thermodynamic model was constructed to probe the precipitation conversion mechanism, and visually predict the component and yield for products under various operating conditions. Batch experiments were conducted to investigate P recovery performance as a function of initial Mg²⁺ concentration, initial pH level, as well as degree of urine hydrolysis. Moreover, the alternative dosing scheme with different calcium salts and alkali was presented, diversifying the options for efficient P recovery. The results showed that, from the perspective of acidic storage for fresh urine, P recovery can be boosted along with eliminating urine hydrolysis. In urine with an initial pH=2.0, P can be completely recovered and purity for calcium phosphate can be optimized to 100% within a Ca/P ratio range of 1.67-2.3. Overall, this work is of great significance for precisely and efficiently harvesting P from urine and provides an integrated strategy for P resource recovery from urine.

Mechanisms governing the dissolution of phosphorus and iron in sewage sludge by the bioacidification process and its correlation with iron phosphate speciation

In the last two decades, phosphorus (P) recovery from sewage sludge liquors gained much interest for its high-quality product potential. However, the consistently reported constraints are the low phosphorus availability and the technical-economical difficulties to increase it through chemical acidification. This article discusses the mechanisms of phosphorus dissolution by the biological acidification process (Biological acidification or acidic fermentation) as an alternative to chemical acidification. In addition, we investigate the potential correlation between the phosphorus dissolution and iron phosphate speciation of several types of sludge from different sewage treatment plants and P removal technologies. The results show that the percentage of P dissolution by bioacidification is always higher than the P dissolution by chemical acidification at equal pH for all types of sludge except for the settled primary sludge. The highest P dissolution was recorded for the sludge from the Enhanced Biological P Removal process assisted with Chemical P Removal process (EBPR-CPR) with around 65% of P dissolution. Three mechanisms were identified as contributing to the increased P dissolution by bioacidification: P release by the Polyphosphate Accumulating Organisms (PAO), P dissolution by pH decrease, and P dissolution by a biological activity at acidic pH (3.7-4) that includes iron reduction and aluminum dissolution. The principal component analysis and Pearson's correlation indicate that P dissolution by bioacidification is negatively correlated with the P-bound to ferric iron, hence positively correlated with the P-bound to ferrous iron, which characterizes the sludge from the EBPR-CPR process. This study suggests that the choice of the P removal technology significantly influences the P recovery from sewage sludge liquors.

Phosphorus speciation in sewage sludge from three municipal wastewater treatment plants in Sweden and their ashes after incineration

Given the high efficiency in phosphorus removal at municipal wastewater treatment plants (MMWWTP), sewage sludge constitutes a promising resource for phosphorus (P) recovery. Sewage sludge is, however, a complex matrix and its direct use as fertiliser is limited by its content of metals/metalloids and organic pollutants. In order to increase its usability as a potential resource of P, there is a need for increased knowledge on phosphorus speciation in these matrices. The sludge composition is highly influenced by local conditions (i.e. wastewater composition and treatment method), and it is therefore important to study sludge from several MMWWTPs. In this study, three different protocols for sequential extraction were utilised to investigate the chemical speciation of phosphorus in sludge from three different MMWWTP sludges in Sweden, as well as in corresponding ashes following incineration. The results showed that the total amounts of phosphorus ranged from 26 to 32 mg g^-1 sludge (dry weight), of which 79-94% was inorganically bound (IP). In the sludge, 21-30% of the IP was associated with calcium (Ca-P), which is the preferred species for fertiliser production. Following incineration, this fraction increased to 54-56%, mainly due to transformation of iron-associated phosphorus (Fe-P), while aluminium-associated species of phosphorus (Al-P) remained unaltered. The results from this study confirm that incineration is a suitable treatment for sewage sludge in terms of potential phosphorus recovery.

Phosphorus removal and recovery: state of the science and challenges

Phosphorus is one of the main nutrients required for all life. Phosphorus as phosphate form plays an important role in different cellular processes. Entrance of phosphorus in the environment leads to serious ecological problems including water quality problems and soil pollution. Furthermore, it may cause eutrophication as well as harmful algae blooms (HABs) in aquatic environments. Several physical, chemical, and biological methods have been presented for phosphorus removal and recovery. In this review, there is an overview of phosphorus role in nature provided, available removal processes are discussed, and each of them is explained in detail. Chemical precipitation, ion exchange, membrane separation, and adsorption can be listed as the most used methods. Identifying advantages of these technologies will allow the performance of phosphorus removal systems to be updated, optimized, evaluate the treatment cost and benefits, and support select directions for further action. Two main applications of biochar and nanoscale materials are recommended.

Life Cycle Environmental Impacts of Wastewater-Derived Phosphorus Products: An Agricultural End-User Perspective

Recovering phosphorus from wastewater in more concentrated forms has potential to sustainably recirculate phosphorus from cities to agriculture. The environmental sustainability of wastewater-based phosphorus recovery processes or wastewater-derived phosphorus products can be evaluated using life cycle assessment (LCA). Many LCA studies used a process perspective to account for the impacts of integrating phosphorus recovery processes at wastewater treatment plants, while some used a product perspective to assess the impacts of producing wastewater-derived phosphorus products. We demonstrated the application of an end-user perspective by assessing life cycle environmental impacts of substituting half of the conventional phosphorus rock-based fertilizers used in three crop production systems with wastewater-derived phosphorus products from six recovery pathways (RPs). The consequential LCA results show that the substitution reduces global warming potential, eutrophication potential, ecotoxicity potential, and acidification potential of the assessed crop production systems in most RPs and scenarios. The end-user perspective introduced in this study can (i) complement with the process perspective and the product perspective to give a more holistic picture of environmental impacts along the "circular economy value chains" of wastewater-based resource recovery, (ii) enable systemwide assessment of wide uptake of wastewater-derived products, and (iii) draw attention to understanding the long-term environmental impacts of using wastewater-derived products.

Phosphorous removal and recovery from urban wastewater: Current practices and new directions

Phosphate rocks are an irreplaceable resource to produce fertilizers, but their availability will not be enough to meet the increasing demands of agriculture for food production. At the same time, the accumulation of phosphorous discharged by municipal wastewater treatment plants (WWTPs) is one of the main causes of eutrophication. In a perspective of circular economy, WWTPs play a key role in phosphorous management. Indeed, phosphorus removal and recovery from WWTPs can both reduce the occurrence of eutrophication and contribute to meeting the demand for phosphorus-based fertilizers. Phosphorous removal and recovery are interconnected phases in WWTP with the former generally involved in the mainstream treatment, while the latter on the side streams. Indeed, by reducing phosphorus concentration in the WWTP side streams, a further improvement of the overall phosphorus removal from the WWTP influent can be obtained. Many studies and patents have been recently focused on treatments and processes aimed at the removal and recovery of phosphorous from wastewater and sewage sludge. Notably, new advances on biological and material sciences are constantly put at the service of conventional or unconventional wastewater treatments to increase the phosphorous removal efficiency and/or reduce the treatment costs. Similarly, many studies have been devoted to the development of processes aimed at the recovery of phosphorus from wastewaters and sludge to produce fertilizers, and a wide range of recovery percentages is reported as a function of the different technologies applied (from 10-25% up to 70-90% of the phosphorous in the WWTP influent). In view of forthcoming and inevitable regulations on phosphorous removal and recovery from WWTP streams, this review summarizes the main recent advances in this field to provide the scientific and technical community with an updated and useful tool for choosing the best strategy to adopt during the design or upgrading of WWTPs.

Treatment of high-concentration phosphorus wastewater based on foamed concrete

Phosphorus is a nonrenewable resource, and the recovery of phosphorus from wastewater containing high concentrations of phosphorus is of great importance. In this work, a novel method for highly efficient treatment of high-concentration phosphorus-containing wastewater (50 mg/L, 100 mg/L and 150 mg/L) with low energy consumption was developed by using the block waste foam concrete (FC) as a potential phosphorus recovery material. The results showed that acid leaching significantly improved the accumulation efficiency of phosphorus in calcium hydroxyphosphate (HAP) via accelerating the release of calcium in wastewater. The recovery rate of phosphorus could reach 99.0% under the pH value of 9.0 at 25 °C, using 2.0 g FC. It was also found that the microporous structure of the surface of FC provided the adsorption sites for phosphorus, resulting in the adsorption rate in different concentrations of phosphorus-containing wastewater up to 14.5%. It indicated that FC achieved the recovery of phosphorus from high-concentration phosphorus-containing wastewater by coupling HAP crystallization and physical adsorption to polyphosphorus.

Phosphorus recovery from wastewater and bio-based waste: an overview

Phosphorus is one of the most important macronutrients needed for the growth of plants. The fertilizer production market uses 80% of natural, non-renewable phosphorus resources in the form of phosphate rock. The depletion of those deposits forces a search for other alternatives, including biological waste. This review aims to indicate the most important ways to recover phosphorus from biowaste, with particular emphasis on wastewater, sewage sludge, manure, slaughter or food waste. A comparison of utilized methods and directions for future research based on the latest research is presented. Combining biological, chemical, and physical methods with thermal treatment appears to be the most effective way for the treatment of wastewater sludge in terms of phosphorus recovery. Hydrothermal, thermochemical, and adsorption on thermally treated adsorbents are characterized by a high phosphorus recovery rate (over 95%). For animal by-products and other biological waste, chemical methods seems to be the most optimal solution with a recovery rate over 96%. Due to its large volume and relatively low phosphorus content, wastewater is a resource that requires additional treatment to recover the highest possible amount of phosphorus. Pretreatment of wastewater with combined methods seems to be a possible way to improve phosphorus recovery. A compressive evaluation of combined methods is crucial for future research in this area.

A review on the integration of mainstream P-recovery strategies with enhanced biological phosphorus removal

Phosphorus (P), an essential nutrient for all organisms, urgently needs to be recovered due to the increasing demand and scarcity of this natural resource. Recovering P from wastewater is a feasible and promising way widely studied nowadays due to the need to remove P in wastewater treatment plants (WWTPs). When enhanced biological P removal (EBPR) is implemented, an innovative option is to recover P from the supernatant streams obtained in the mainstream water line, and then combine it with liquor-crystallisation recovery processes, being the final recovered product struvite, vivianite or hydroxyapatite. The basic idea of these mainstream P-recovery strategies is to take advantage of the ability of polyphosphate accumulating organisms (PAO) to increase P concentration under anaerobic conditions when some carbon source is available. This work shows the mainstream P-recovery technologies reported so far, both in continuous and sequenced batch reactors (SBR) based configurations. The amount of extraction, as a key parameter to balance the recovery efficiency and the maintenance of the EBPR of the system, should be the first design criterion. The maximum value of P-recovery efficiency for long-term operation with an adequate extraction ratio would be around 60%. Other relevant factors (e.g. COD/P ratio of the influent, need for an additional carbon source) and operational parameters (e.g. aeration, SRT, HRT) are also reported and discussed.

Pilot-scale magnetic recovery of vivianite from digested sewage sludge

Phosphorus (P) is an essential resource for food production and chemical industry. Phosphorus use has to become more sustainable and should include phosphorus recycling from secondary sources. About 20% of P ends up in sewage sludge, making this a substantial secondary P source. There is currently a technological gap to recover P from sludge locally at wastewater treatment plants (WWTP) that remove P by dosing iron. Vivianite (Fe₃(PO₄)₂•8(H₂O)) is the main iron phosphate mineral that forms during anaerobic digestion of sewage sludge, provided that enough iron is present. Vivianite is paramagnetic and can be recovered using a magnetic separator. In this study, we have scaled up vivianite separation from lab-scale to bench- and pilot-scale. Bench-scale tests showed good separation of vivianite from digested sewage sludge and that a pulsation force is crucial for obtaining a concentrate with a high P grade. A pilot-scale magnetic separator (capacity 1.0 m³/h) was used to recover vivianite from digested sewage sludge at a WWTP. Recirculating and reprocessing sludge allows over 80% vivianite recovery within three passes. A concentrated P-product was produced with a vivianite content of up to 800 mg/g and a P content of 98 mg/g. P recovery is limited by the amount of P bound in vivianite and can be increased by increased iron dosing. With sufficient iron dosing, the vivianite content can be increased, and subsequently more P can be recovered. This would allow compliance with existing German legislation, which requires a P recovery larger than 50%.

Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review

Phosphorus (P) plays essential roles in crops growth. Natural mineral sources of phosphate are non-renewable, overexploited and unevenly distributed worldwide, making P a strategic resource for agricultural systems. The search for sustainable ways to secure P supply for fertilizer production has therefore become a critical issue worldwide. Sewage sludge (SS) is an organic waste material considered as a key alternative source of P. Switzerland and the European Union are about to make it mandatory to recover P from SS or its treatment residues. Among the many technical options to achieve this objective, SS thermochemical treatments spiked with Cl-donors appear as a promising approach to recover P from SS and separate it from mineral pollutants such as trace metal elements (TME). The purpose of Cl-donor additives is to fix P within the mineral residues, possibly in bioavailable P species forms, while promoting TME vaporization by chlorination mechanisms. This review paper compares the various thermochemical treatments investigated worldwide over the past two decades. The influence of process conditions and Cl-donor nature is discussed. The presented results show that, except for nickel and chromium, most TME can be significantly vaporized during a high temperature treatment (over 900 °C) with Cl addition. In addition, the fixation rate and solubility of P is increased when a Cl-donor such as MgCl₂ is added.

Indicators for resource recovery monitoring within the circular economy model implementation in the wastewater sector

The European Union is currently in the process of transformation toward a circular economy model in which different areas of activity should be integrated for more efficient management of raw materials and waste. The wastewater sector has a great potential in this regard and therefore is an important element of the transformation process to the circular economy model. The targets of the circular economy policy framework such as resource recovery are tightly connected with the wastewater treatment processes and sewage sludge management. With this in view, the present study aims to review existing indicators on resource recovery that can enable efficient monitoring of the sustainable and circular solutions implemented in the wastewater sector. Within the reviewed indicators, most of them were focused on technological aspects of resource recovery processes such as nutrient removal efficiency, sewage sludge processing methods and environmental aspects as the pollutant share in the sewage sludge or its ashes. Moreover, other wide-scope indicators such as the wastewater service coverage or the production of bio-based fertilizers and hydrochar within the wastewater sector were analyzed. The results were used for the development of recommendations for improving the resources recovery monitoring framework in the wastewater sector and a proposal of a circularity indicator for a wastewater treatment plant highlighting new challenges for further researches and wastewater professionals.

A new strategy for obtaining highly concentrated phosphorus recovery solution in biofilm phosphorus recovery process

Recovery of phosphorus (P) from wastewater is of great significance for alleviating the shortage of P resources. At present, the P recovery process is faced with the problem of excessive organic carbon consumption when obtaining a P-concentrated recovery solution. This study proposed a new strategy to obtain a more highly concentrated P recovery solution with minimal carbon consumption by strengthening the P storage capacity of the biofilm. A biofilm sequencing batch reactor (BSBR) process was modified to treat synthetic wastewater. The effect of the dissolved oxygen (DO) concentration on the P storage capacity of the biofilm was investigated at DO concentrations of DO 3.5 mg/L (P_L) and DO 6.5 mg/L (P_H). The results showed a maximum P storage of 101.2 and 149.6 mg-P/g-mixed liquid suspended solids under the two conditions. Strengthening the P storage capacity of the biofilm resulted in a net increase in the P recovery rate, which was as high as 66.96% in a harvesting cycle, and total soluble P>220 mg/L in the P recovery solution was successfully achieved. Meanwhile, the carbon cost of P recovery in the BSBR was reduced to 41.57 g-chemical oxygen demand/g-P, and the carbon utilization efficiency was enhanced. To highlight the new strategy, the P recovery performance of the BSBR was given and the relationship between P content and anaerobic P release was discussed. In addition, the changes in the microbial communities under P_L and P_H conditions were analyzed.

Achieving stably enhanced biological phosphorus removal from aerobic granular sludge system via phosphorus rich liquid extraction during anaerobic period

In order to sustainably manage wastewater treatment plants and the environment, enhanced biological phosphorus (P) removal (EBPR) was proposed to achieve P recovery through extracting P-rich liquid (i.e., Phostrip) from the bottom of aerobic granular sludge (AGS)-based sequencing batch reactors (SBRs) under no mixing during the anaerobic phase. Results showed both tested bacterial AGS (BAGS) and algal-bacterial AGS (A-BAGS) systems stably produced low effluent P (<0.05 mg-P/L) with little impact on their organics and NH₄⁺-N removals (>99%). The collected P-rich liquids (55-83 mg-P/L) from both systems showed great potential for P recovery of about 83.85 ± 0.57 % (BAGS) or 83.99 ± 0.77% (A-BAGS), which were contributed by the influent P (>95%) and P reserves in granules based on P balance analysis. This study suggests that the AGS-based SBRs coupling the Phostrip holds great potentials for P recovery profit and further reduction in energy consumption.

Strengthening anoxic glycogen consumption in SNEDPR-CW as a strategy to control PAO-GAO competition under carbon limited condition

Cooperation between Phosphate and Glycogen Accumulating Organisms (PAOs and GAOs) plays a pivotal role in nutrients removal in simultaneous nitrification endogenous denitrification and phosphorous removal (SNEDPR) systems. Recent findings have expanded the application of SNEDPR from activated sludge system to constructed wetland (CW). However, how to regulate competition between PAOs and GAOs in SNEDPR-based CW is still unclear. Here we showed that, GAOs could easily gain dominance over PAOs in SNEDPR-CW under alternating anaerobic/aerobic (A/O) operational mode. Shortening aerobic hydraulic retention time (HRT) at low oxygen concentration was benefit for simultaneous nitrification endogenous denitrification (SNED) and denitrifying dephosphatation but would reduce the overall phosphorus uptake rate and lead to high phosphorus effluent concentrations. Extended aerobic HRT promoted the proliferation of aerobic GAOs over PAOs, decreasing both enhanced biological phosphorus removal (EBPR) and SNED performance. Surprisingly, by switching the operation of system to alternating anaerobic/aerobic/anoxic (A/O/A) mode, an extraordinary nutrients removal performance with mean nitrogen and phosphorus removal efficiency of 84.57% and 89.37% was achieved under carbon sources limited condition. Stoichiometric analysis demonstrated that adding anoxic stage strengthened the intracellular glycogen oxidization of GAOs for denitrification which compromised its subsequent anaerobic carbon sources uptake and PHA storage and provided sufficient carbon sources for PAOs. Microbial community analysis showed that numerical ratio of GAOs to PAOs decreased from 6.67 under A/O to 4.89 under A/O/A mode, which further indicated strengthening glycogen denitrification of GAOs should be an effective way to regulate microbial competition in order to obtain a desired nutrients removal performance in SNEDPR-CW.

Electrochemically mediated precipitation of phosphate minerals for phosphorus removal and recovery: Progress and perspective

Phosphorus (P) is an essential element for the growth and reproduction of organisms. Unfortunately, the natural P cycle has been broken by the overexploitation of P ores and the associated discharge of P into water bodies, which may trigger the eutrophication of water bodies in the short term and possible P shortage soon. Consequently, technologies emerged to recover P from wastewater to mitigate pollution and exploit secondary P resources. Electrochemically induced phosphate precipitation has the merit of achieving P recovery without dosing additional chemicals via creating a localized high pH environment near the cathode. We critically reviewed the development of electrochemically induced precipitation systems toward P removal and recovery over the past ten years. We summarized and discussed the effects of pH, current density, electrode configuration, and water matrix on the performance of electrochemical systems. Next to ortho P, we identified the potential and illustrated the mechanism of electrochemical P removal and recovery from non-ortho P compounds by combined anodic or anode-mediated oxidation and cathodic reduction (precipitation). Furthermore, we assessed the economic feasibility of electrochemical methods and concluded that they are more suitable for treating acidic P-rich waste streams. Despite promising potentials and significant progress in recent years, the application of electrochemical systems toward P recovery at a larger scale requires further research and development. Future work should focus on evaluating the system's performance under long-term operation, developing an automatic process for harvesting P deposits, and performing a detailed economic and life-cycle assessment.

Rare earth elements (REE) for the removal and recovery of phosphorus: A review

There is little doubt that 'rock phosphate' reserves are decreasing, with phosphorus (P) peak to be reached in the coming decades. Hence, removal and recovery of phosphorus (P) from alternative nutrient-rich waste streams is critical and of great importance owing to its essential role in agricultural productivity. Adsorption technique is efficient, cost-effective, and sustainable for P recovery from waste streams which otherwise can cause eutrophication in receiving waters. As selective P sorption using rare earth elements (REE) are gaining considerable attention, this review extensively focuses on P recovery by utilising a range of REE-incorporated adsorbents. The review briefly provides existing knowledge of P in various waste streams, and examines the chemistry and behaviour of REE in soil and water in detail. The impact of interfering ions on P removal using REE, adsorbent regeneration for reuse, and life cycle assessment of REE are further explored. While it is clear that REE-sorbents have excellent potential to recover P from wastewaters and to be used as fertilisers, there are gaps to be addressed. Future studies should target recovery and reuse of REE as P fertilisers using real wastewaters. More field trials of synthesized REE-sorbents are highly recommended before practical application.

Biosolids towards Back-To-Earth alternative concept (BEA) for environmental sustainability: a review

Biosolids are a nutrient-rich stable substance obtained during wastewater treatment process. With amplifying population and industrial development, upsurge of biosolid generation is also speculated. Biosolids are endowed with essential plant nutrient (macro- and micro-nutrients) which qualifies them to be used as soil amendment and in turn dwindles the use of chemical fertilizers. The characteristics of biosolid depends on the nature of the treatment process. In this regard, it would be possible to recycle certain nutrients from the agricultural use of biosolids and could be a sustainable solution to the management of this waste. Biosolids may therefore serve as a key tool for farm utilization. It improves the soil health through nutrient supply and promotes the plant growth. Furthermore, they are slow-release fertilizer and hence, restrains from groundwater contamination. This review, in a nutshell, unravels the influence of biosolids on land application, its effect on soil properties, agricultural and horticultural crops, environmental ramification of biosolids in restoring the degraded land and carbon sequestration.

Phosphate recovery from the P-enriched brine of AnMBR-RO-IE treating municipal wastewater via an innovated phosphorus recovery batch reactor with nano-sorbents

Municipal wastewater is a very unique pool full of energy and useful substances. Though the innovative integrated anaerobic membrane bioreactor and reverse osmosis-ion exchange (AnMBR-RO-IE) process can produce high-grade reclaimed water with high energy efficiency, phosphorus resources recovery in the WWTPs has been rarely reported thus far. This study evaluated the feasibility of a phosphorus recovery batch reactor (PRBR) as an approach for the phosphate production from the P-enriched brine from AnMBR-RO-IE. With operating PRBR for 162 cycles, high to 85% of P recovery rate was obtained for 145 cycles, leading to a P production rate of 6.17 g/m³ domestic wastewater with nano-sorbents (NSs) consumption rate of 10.2 g/m³. Acidification pretreatment efficiently improved the adsorption capacity and reduced the NSs renewing frequency. High adsorption selectivity of NSs contributed to low impurities (<0.3%) in the P-enriched reclaimed solution. Moreover, the integrated AnMBR-RO-IE-PRBR process saved 47% of energy consumption compared to the present NEWater production process in Singapore. The innovative PRBR reactor was competitive compared to the commonly-used chemical precipitation methods in conventional WWTPs in terms of phosphorus recovery/loss and energy balance. It is expected that the proposed integrated process can offer new insights into the direction of phosphorus reclamation in the future WWTPs.

Technical, economic and environmental feasibility of resource recovery technologies from wastewater

An enormous amount of wastewater is generated across the world from different industrial or municipal sectors. Traditional wastewater treatment plants (WWTP) have primarily focused on the treatment of wastewater rather than the recovery of valuable resources. A shift from a linear to a circular economy may offer a unique platform for recovering valuable resources including energy, nutrients, and high-value goods from wastewater. However, transitioning from conventional frameworks to sustainable WWT systems remains a significant challenge. Thus, this review paper focuses on the avenues of resource recovery from WWTPs, by evaluating the potential for nutrients, water, and energy recovery from different types of wastewaters and sewage sludge. It discusses in detail a variety of available and advanced technologies for resource recovery. Further, the feasibility of these technologies from a sustainable standpoint is discussed, covering the technical, economic, and environmental facets.

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

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

Algal biomass production by phosphorus recovery and recycling from wastewater using amorphous calcium silicate hydrates

The phosphorous supply crisis is a major challenge for a sustainable society, and the algal industry is not unrelated to this crisis. Recycling phosphorus from sewage wastewater is a potential way to address this issue. We previously developed amorphous calcium silicate hydrates (aCSH) as excellent phosphorus recovery materials. In this study, we designed a phosphorus recovery process using aCSH in a pilot-scale facility connected to a sewage wastewater treatment plant, and demonstrated the production of microalgal biomass using phosphorous-containing aCSH (P_aCSH). As a result, high phosphorous recovery rates (>80%) were obtained throughout the year. The carbohydrate-rich microalga Pseudoneochloris sp. NKY372003 was cultivable with P_aCSH. The biomass and carbohydrate productivity of this microalga with P_aCSH was comparable to that with conventional media. Approximately 94% of the phosphorus in P_aCSH was recycled into the biomass. This study successfully demonstrated the recycling the phosphorus recovered from wastewater for microalgal cultivation by aCSH.

Phosphorus recovery from wastewater by struvite in response to initial nutrients concentration and nitrogen/phosphorus molar ratio

Nutrients recovery from wastewater using struvite crystallization has received significant attention because the recovered nutrients can be used as a fertilizer in agriculture. However, the wastewater with different composition of nutrients might exert distinct struvite characteristics. The objective of this study is to explore the influence of nitrogen/phosphorus (N/P) molar ratio and initial nutrient concentrations on the phosphorus (P) recovery from wastewater and the crystallographic properties of struvite through batch experiments. Both the higher nutrients concentrations and N/P molar ratio in the wastewater resulted in higher recovery of P. However, initial nutrients concentration in the wastewater posed a larger effect on the formation of struvite than that of initial N/P ratio. The ratio of N to P consumed during the process varied with varying the N/P ratio or the initial nutrients concentration, indicating the existence of different crystallization mechanisms under different elemental compositions in the wastewater. The decline in solution pH was negatively correlated with the N/P ratio, whereas it was positively correlated with the nutrient concentration. The crystals produced were mainly rough (dendritic) under the N/P ratio of 2, while the morphology evolved from a regular coffin-like to irregular columnar with the increase in N/P ratio from 4 to 10. Meanwhile, the shape of the irregular cylinder gradually changed from elongated needle shape to the thick rods appearance with the increase in the nutrients concentration. Higher N/P ratio increased the nucleation rate, resulting in finer particles and an uneven crystal size distribution, but purer struvite crystals. The purity of struvite was also positively correlated with the initial nutrients concentration. The Visual MINTEQ modelling analysis demonstrated that the supersaturation ratio of struvite gradually increased with the increase in the N/P ratio or nutrients concentration, which led to a positive effect on struvite crystallization.

Advanced nitrogen and phosphorus removal by combining endogenous denitrification and denitrifying dephosphatation in constructed wetlands

To achieve high-efficiency nutrient removal in constructed wetlands (CWs), a novel simultaneous nitrogen and phosphorus removal (SNPR) process was developed by combining nitrification, endogenous denitrification, and denitrifying phosphorus removal. In SNPR process, denitrifying glycogen-accumulating organisms (DGAOs) and denitrifying polyphosphate-accumulating organisms (DPAOs) utilized NO_x^--N(NO₃^--N or NO₂^--N) as electron acceptor and poly-beta-hydroxy-alkanoates (PHAs) as carbon sources for endogenous denitrification and denitrifying phosphorus removal processes. Results from 217 days of operation showed that a high-level of nitrogen removal efficiency of 83.73% was achieved with influent COD/N of 4. The success was attributed to the fact that most of influent carbon sources could be transformed into PHAs before nitrification via enriching DGAOs and DPAOs in CW, which simultaneously improved nitrification and denitrification due to reducing oxygen and carbon sources consumption by aerobic heterotrophs. Phosphorus was mainly removed via denitrifying phosphorus removal, and PO₄^3--P removal efficiency reached up to 87.84% with even common gravel used as substrate. Stoichiometry analysis revealed that DGAOs were the main organisms providing nitrite to DPAOs, suggesting that the effective PO₄^3--P removal under high DGAO abundance condition might be attributed to the coordination of DGAOs and DPAOs in SNRP processes.

An overview on deficit and requirements of the Irish national soil phosphorus balance

Phosphorus (P) is an essential life-supporting nutrient for which there is no substitute. Modern farming practice and food production are supported by the application of mineral P fertiliser derived from finite mined phosphate rock. The European Union does not have indigenous mineral phosphate reserves, which poses a significant issue to food security. This research paper assesses the potential of indigenous recycled P sources to replace imported P fertiliser within the Republic of Ireland. The research is undertaken at NUTS 3 (Nomenclature of Territorial Units) regional level, the nutrient soil P requirement is established, and the extent to which the regional production of indigenous recycled P sources can offset this requirement is determined. The soil P requirement was derived from analyzing the regional soil P indexes, stocking rate and land-use. It was established that to optimise Irish agricultural production, approximately 95,500 t of P fertiliser is required by Irish agriculture per annum. Indigenous P sources were reviewed to determine their contribution to the Irish P balance; the sources included sewage sludge, dairy processing waste, and animal manures. Regional indigenous P quantities vary greatly with the South-West Region producing the largest quantity of indigenous recycled P at 42.4% of required P than the Mid-West Region only producing 22.0% of its P requirement indigenously. Sources of indigenous P also vary greatly from region to region depending on population and industry, with the highest quantity of sewage sludge being produced in the Dublin plus Mid-East Region while the greatest contributor of dairy waste is the South-West Region. In total, over 28,500 t of P is recovered from indigenous sources per annum. This indicates that approximately 30% of the national P requirement could be met by indigenous P recycling.