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Abstract: Recovery of phosphorus from wastewater as calcium phosphate could diminish the need for mining of scarce phosphate rock resources. This study introduces a novel approach to phosphorus recovery by precipitation of calcium phosphate granules in anaerobic treatment of black water. The granules formed in the Upflow Anaerobic Sludge Blanket (UASB) reactor at lab- and demonstration-scale were analyzed for chemical composition and mineralogy by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), Electron microprobe (EMP), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and micro X-ray Diffraction (XRD). The granules had a diameter of 1–2 mm, organic content of 33 wt%, and phosphorus content of 11–13 wt%. Three calcium phosphate phases were identified in the granules: hydroxyapatite, calcium phosphate hydrate and carbonated hydroxyapatite. Without any addition of chemicals, 7 gP/person/year can be recovered with the calcium phosphate granules, representing 2% of the incoming phosphorus in the UASB reactor. As the heavy metal content was lower compared to other phosphorus recovery products, phosphate rock and phosphorus fertilizer, the calcium phosphate granules could be considered as a new phosphorus product. [Copyright &y& Elsevier]

• CaP granules can replace mined phosphate rock for application as fertilizer. • 35% of PO 4 3− in CaP granules dissolved in citric acid and 85% in H 2 SO 4. • Glowed (550 °C) granules contained 15 wt% of p (35 wt% of P 2 O 5). • Heavy metals and organic micropollutants are below european regulatory limits. Phosphate is essential for food production. However, phosphate rock, which is the main natural source, is becoming worse in quality due to the depletion of the reserves and contamination with heavy metals and radioactive elements. Marketable phosphate contains more than 13 wt% of phosphorus (P), but run-of-the-mine phosphate rock is of lower grade (8 to 11 wt% of P). In this study, calcium phosphate granules (CaP granules) produced during anaerobic treatment of vacuum collected black water (feces and urine, BW) were assessed for their potential as a substitute for phosphate rock. The quality assessment was based on elemental composition (macro nutrients and heavy metals), crystallography and morphology analyses, dissolution tests, and quantification of micropollutants. CaP granules contained 10 wt% of P, from which 35% was dissolved within 5 min in citric acid and 85% in H 2 SO 4. The incineration of the CaP granules increased the P content to 15 wt%, by eliminating the remaining organics (29%), pathogens, and organic micropollutants. Heavy metals (Cu, Zn, Cr, Ni, Cd, As, and Pb) and organic micropollutants in CaP granules were below the Dutch and European regulatory limits for direct fertilizer application. Moreover, incinerated CaP granules complied with the requirements for use in the fertilizer industry. At the current process conditions, CaP granules produced from BW can potentially replace 12% of the phosphate rock used in agriculture as fertilizer. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Phosphorus (P) removal and recovery from sewage as calcium phosphate (CaP) by chemical precipitation is a widely used method. To avoid the addition of chemicals to increase the pH of the bulk solution and the need for a further separation step in conventional chemical precipitation process, we developed an electrochemical method, which can locally increase the pH near a Ti cathode. The separation of product and liquid then happens simultaneously by accumulating CaP at the electrode surface. The current density plays a crucial role in this system. A current density of 19 A/m 2 results in the formation of crystalline CaP rather than amorphous CaP, but it does not enhance the removal of P in 24 h. Moreover, the current efficiency decreases with increasing current density. Furthermore, the increased H 2 production at high current density may push the precipitated CaP back to the bulk solution, resulting in its dissolution. In the presence of bicarbonate (1–5 mM) or humic acid (1–20 mg/L), the removal of P was higher. This is probably due to the inhibited CaP precipitation in the bulk solution which in turn leaves more Ca and P ions available for the local precipitation on the cathode. However, bicarbonate at high concentration (10 mM) dropped P removal from 52 to 25%. This is caused by competition of carbonate and phosphate with the free Ca 2+ ions and also by buffering the producted hydroxide ions at the cathode. The study shows that P can be removed as CaP by electrochemical precipitation at low current densities at common concentrations of bicarbonate and humic acid. [ABSTRACT FROM AUTHOR]

Simultaneous recovery of calcium phosphate granules (CaP granules) and methane from vacuum collected black water (BW), using an upflow anaerobic sludge blanket (UASB) reactor was previously investigated. It was calculated that only 2% of the total phosphorus (P) fed was present as CaP granules whereas 51% of the P accumulated dispersed in the reactor, limiting the applicability of this process for recovery of phosphate. This study proposes adding calcium to increase the P accumulation in the reactor and the production of CaP granules. Calcium was added in a lab-scale UASB reactor fed with BW. An identical UASB reactor was used as reference, to which no calcium was added. The treatment performance was evaluated by weekly monitoring of influent, effluent and produced biogas. Sludge bed development and CaP granulation were assessed through particle size analysis. The composition and structure of CaP granules were chemically and optically assessed. Calcium addition increased accumulation of P in the reactor and formation and growth of granules with size > 0.4 mm diameter (CaP granules). Moreover, with calcium addition, CaP granules contained 5.6 ± 1.5 wt% of P, while without calcium a lower P content was observed (3.7 ± 0.3 wt%). By adding Ca, 89% of the incoming P from BW accumulated in the reactor and 31% was sampled as CaP granules (> 0.4 mm diameter). Addition of 250 mgCa L −1 of BW was the optimum loading found in this study. Furthermore, no significant reduction in COD Total removal (> 80%) and CH 4 production (0.47 ± 0.10 gCOD-CH 4 g −1 COD Total -BW) was observed. Therefore, adding calcium can significantly increase the CaP granulation without inhibiting the simultaneous CH 4 recovery. This further indicates the potential of this process for phosphate recovery. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an essential nutrient for living organisms and cannot be replaced or substituted. In this paper, we present a simple yet efficient membrane free electrochemical system for P removal and recovery as calcium phosphate (CaP). This method relies on in situ formation of hydroxide ions by electro mediated water reduction at a titanium cathode surface. The in situ raised pH at the cathode provides a local environment where CaP will become highly supersaturated. Therefore, homogeneous and heterogeneous nucleation of CaP occurs near and at the cathode surface. Because of the local high pH, the P removal behavior is not sensitive to bulk solution pH and therefore, efficient P removal was observed in three studied bulk solutions with pH of 4.0 (56.1%), 8.2 (57.4%), and 10.0 (48.4%) after 24 h of reaction time. While P removal efficiencies are not generally affected by bulk solution pH, the chemical-physical properties of CaP solids collected on the cathode are still related to bulk solution pH, as confirmed by structure characterizations. High initial solution pH promotes the formation of more crystalline products with relatively high Ca/P molar ratio. The Ca/P molar ratio increases from 1.30 (pH 4.0) to 1.38 (pH 8.2) and further increases to 1.55 (pH 10.0). The formation of CaP precipitates was a typical crystallization process, with an amorphous phase formed at the initial stage which then transforms to the most stable crystal phase, hydroxyapatite, which is inferred from the increased Ca/P molar ratio from 1.38 (day 1) to the theoretical 1.76 (day 11) and by the formation of needle-like crystals. Finally, we demonstrated the efficiency of this system for real wastewater. This, together with the fact that the electrochemical method can work at low bulk pH, without dosing chemicals and a need for a separation process, highlights the potential application of the electrochemical method for P removal and recovery. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ca2+ addition doubled the SRT and increased hydrolysis from 29 % to 67 %. • Methanization increased from 20 % to 46 % by adding Ca2+.. • Ca2+ effect on methane production depends on Ca:COD (optimum 0.03 – 0.05, limit 0.08). • Phosphorus removal increased from 38 % to 61 % by adding Ca2+ to the UASB reactor. • Ca2+ was captured by bicarbonate, limiting Ca x (PO 4) y formation and struvite dissolution. Cow manure has potential to serve as a sustainable secondary fuel and phosphorus resource and cut our reliance on finite primary resources. The efficient and sustainable valorization of cow manure faces compositional challenges because of a high solids content, the lack of soluble phosphorus, fine struvite particles being the main phosphorus species and high bicarbonate concentrations. Addition of calcium could result in higher methane production and conversion of struvite to calcium phosphate. To investigate this, cow manure was digested in two up-flow anaerobic sludge blanket reactors for 456 days, one with and one without CaCl 2 addition. A positive effect of calcium addition was found for hydrolysis (29 % without calcium and 67 % with calcium), methane production (136 L-CH 4 kgVS-1 without calcium and 301 L-CH 4 kgVS-1 with calcium) and sludge bed development. Although calcium was added in a 3:1 ratio to phosphorus, it did not result in recrystallization of struvite to calcium phosphate. Instead, it precipitated as calcium carbonate, which was further induced by additional bicarbonate production through higher hydrolysis and methane production. Still, calcium addition caused better phosphorous removal (from 38 % to 61 %), which is attributed to the enhanced sludge bed capturing and accumulating both the calcium carbonate and struvite fines. Higher methane production and improved phosphorus retention enables better valorization of cow manure. The access to resources from cow manure through this technology can contribute to the circularity of agriculture and save on finite natural resources. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phosphorus removal increased from 60% to 74% by adding Ca2+ to the UASB reactor treating pig manure. • Adding Ca2+ dissolved struvite and formed amorphous calcium phosphate. • Ca2+ addition increased solid retention and aggregation (63% of particles >0.4 mm). • Calcium phosphate granulation was stimulated by Ca2+ addition (79% of phosphorus in particles >0.4 mm). • 85% less phosphorus transport when moving as granules instead of raw pig manure. Phosphorus is an essential but finite and scarce element, which is used extensively in pig farming. For recovering phosphorus from pig manure, we hypothesize that calcium addition during anaerobic treatment can trigger calcium phosphate granulation and enable efficient phosphorus recovery. In this study, we tested the recovery of phosphorus from pig manure by adding calcium during anaerobic treatment. Size–separated (<200 µm) pig manure was treated in two identical up-flow anaerobic sludge blanket reactors for 456 days. Calcium was supplied to one of the reactors to study calcium phosphate granulation. Calcium addition showed a positive effect on treatment stability and efficiency, increasing chemical oxygen demand and phosphorus removal. Phosphorus removal increased from 60% without calcium addition to 74% with calcium addition. The addition of calcium increased the phosphorus concentration in significantly larger particles, calcium phosphate granules, (79% >0.4 mm compared with 11% without calcium addition) in the reactor with calcium addition, which enables separation and a transport reduction of 85% for the recovered phosphorus. The main phosphorus phase in the reactor sludge bed changed from struvite to calcium phosphate with calcium addition. The Mg:P molar ratio was 0.09 with calcium addition and 0.88 without calcium addition in particles >2.5 mm. Thus, calcium addition enhances the treatment of pig manure and enables phosphate recovery as calcium phosphate granules. [ABSTRACT FROM AUTHOR]

Phosphonates are an important type of phosphorus-containing compounds and have possible eutrophication potential. Therefore, the removal of phosphonates from waste streams is as important as orthophosphate. Herein, we achieved simultaneously removal and recovery of phosphorus from nitrilotris (methylene phosphonic acid) (NTMP) using an electrochemical cell. It was found that the C–N and C–P bonds of NTMP were cleaved at the anode, leading to the formation of orthophosphate and formic acid. Meanwhile, the converted orthophosphate reacted with coexisting calcium ions and precipitated on the cathode as recoverable calcium phosphate solids, due to an electrochemically induced high pH region near the cathode. Electrochemical removal of NTMP (30 mg/L) was more efficient when dosed to effluent of a wastewater treatment plant (89% in 24 h) than dosed to synthetic solutions of 1.0 mM Ca and 50 mM Na 2 SO 4 (43% in 168 h) while applying a current density of 28 A/m2 and using a Pt anode and Ti cathode. The higher removal efficiency of NTMP in real waste water is due to the presence of chloride ions, which resulted in anodic formation of chlorine. This study establishes a one-step approach for simultaneously phosphorus removal and recovery of calcium phosphate from non-orthophosphates. Image 1 • Electrochemical treatment achieves P removal and recovery from non-orthophosphate. • Non-orthophosphate was converted to orthophosphate at the anode. • Converted phosphate precipitates with calcium ions on the cathode. • Anode material and background electrolyte affect the recovery performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon-felt fenced cathode system enhanced P recovery at less energy consumption. • Carbon felt delays hydroxide diffusion from the cathode to bulk solution. • Ca-phosphate precipitation location can be adjusted by current and stirring rate. • Carbon felt fenced system is highly stable over long-term operation. Phosphorus removal and recovery from wastewater is crucial for environmental pollution control and sustainable phosphorus fertilizer production. Electrochemical technologies are promising, but disadvantages like high energy consumption remain. Here, we improved the electrochemical system's performance by placing carbon felt in front of the cathode at a distance of 3 mm with a current density as low as 1 A/m2, which separated the reactor into two parts: near the cathode area and bulk solution. The phosphorus removal near the cathode can reach 75 % in 1 h, while it takes 8 h for the bulk solution to achieve 75 % removal efficiency. The main reason for the enhancement was the retention of OH− ions between the cathode and carbon felt with enriched calcium ions, providing an appropriate environment for calcium phosphate precipitation. The functional groups on the carbon felt surface were another reason for the retention of OH− ions. Furthermore, our results show that the carbon felt system has considerable stability under different mass transfer efficiencies. By adjusting the stirring rate, we could control the proportion of precipitation on the carbon felt or cathode, thereby reducing the scaling on the cathode and avoiding significantly reduced removal efficiency. A long-term continuous flow experiment also illustrated this. Interestingly, we observed that the phosphorus removal had a specific correlation with pH and fit an empirical formula. Therefore, we could estimate the removal efficiency by simply measuring the effluent pH of the solution after treatment. In treating real phosphorus-rich wastewater, the carbon felt system removed 63 %–87 % of 1500 mg/L P after 8 h to 48 h treatment at 20 A/m2, respectively, resulting in lower energy consumption (31 kWh/kg P, 8 h) compared to the system without carbon felt (48 kWh/kg P, 8 h). This study provides a simple yet efficient method for enhancing electrochemical phosphorus recovery. [ABSTRACT FROM AUTHOR]

Adding calcium during anaerobic digestion of vacuum collected black water (BW) in an up-flow anaerobic sludge bed (UASB) reactor increased the retention of total phosphorus (P) in the reactor from 51% to 87%. However, the insufficient mixing in the reactor caused cementation and relatively high content of organics in the recovered calcium phosphate (CaP) granules, limiting the P recovery. In this study, the UASB reactor was mixed with an internal gas-lift (UASB-GL) to prevent cementation and to enhance the P content in CaP granules. The novel UASB-GL reactor operated for 300 days, treating concentrated BW. At steady state, the removal of total COD and P was 92% and 90%, respectively. The gas injection created a sludge bed with an average total suspended solids concentration of 73 ± 16 g/L at the bottom and 31 ± 5 g/L at the top of the reactor. The concentration of solid P at the bottom of the reactor was 4.58 ± 1.34 gP/L, while at the top a much lower concentration was obtained (0.75 ± 0.32 gP/L). 89% of the CaP granules was found at the bottom of the reactor. The harvested CaP granules (>0.4 mm diameter) contained on average 7.8 ± 0.6 wt% of P. A potential recovery of 57% of P in BW as CaP granules was calculated, considering actual application of the UASB-GL reactor in source separated sanitation. Image 1 • Phosphorus removal was >90% during stable operation at 0.96 gCOD/L ⋅ d. • The phosphorous content in recovered CaP granules increased 16% by gas-lift mixing. • 70% of the phosphorus at the harvesting location was as CaP granules with 7.8 wt% of P. [ABSTRACT FROM AUTHOR]

To address the issues of eutrophication and the potential risk of phosphorus (P) shortage, it is essential to remove and recover P from P-containing streams to close this nutrient cycle. Electrochemical induced calcium phosphate (CaP) precipitation was shown to be an efficient method for P recovery. However, the influence of natural organic matter (NOM) is not known for this treatment. In this paper, the behavior of NOM and its effect on CaP precipitation was studied. In contrast to studies where NOM hindered CaP precipitation, results show that the interaction of NOM with CaP improves the removal of P, independent of the types of NOM. The P removal at the average increased from 43.8 ± 4.9% to 58.5 ± 1.2% in the presence of 1.0 mg L −1 NOM. Based on the yellow color of the CaP product, NOM is co-precipitated. The bulk solution pH with and without buffers has totally different effects on the precipitation process. Without buffer, CaP precipitates on the cathode surface in a wide pH range (pH 4.0–10.0). However, the precipitation process is completely inhibited when the bulk solution is buffered at pH 4.0 and 6.0. This is probably due to neutralization of OH − by the buffers. Regardless of the presence or absence of NOM and solution pH, the recovered products are mainly amorphous CaP unless the electrolysis time was increased to seven days with 4.0 A m −2 , in which crystalline CaP formed. These findings advance our understanding on the interaction of Ca, P and NOM species for the application of electrochemical method for P recovery from real wastewater. [ABSTRACT FROM AUTHOR]

[Display omitted] • Acidification of cow manure (CM) to pH 5 solubilized>94% of P, Ca and Mg. • The dissolution of CaCO 3 and removal of HCO 3 – enabled CaP granulation from CM. • 83 ± 3 % P was removed from acidified CM in the UASB reactor with Ca addition. • CaP granules > 2.5 mm with 8.4 ± 1.3 %P w/w at the bottom of the UASB reactor. • Existing digestion processes could be converted into the CaP granulation process. Cow manure (CM) is one of the largest secondary sources of phosphorus, while mined phosphorus rock is becoming scarce and diminishing in quality. Phosphorus recovery from CM is essential for increasing the phosphorus use efficiency and, therefore, securing future food production. Calcium phosphate (CaP) granulation can enable efficient phosphorus recovery during anaerobic digestion. However, in CM, the absence of phosphorus in the ionic form and the high presence of inorganic carbon and magnesium inhibits CaP granulation. In this study, CaP granulation was achieved for the first time during CM treatment, using an up-flow anaerobic sludge blanket (UASB) reactor preceded by an acidifying continuously stirred tank reactor (CSTR). The UASB reactor and CSTR ran for 190 days at organic loading rates of 1 and 22.5 gCOD L reactor -1 d-1, respectively. The acidification of CM at pH 5 dissolved struvite and calcium carbonate, causing > 94% of phosphorus, calcium and magnesium to be present as ions. In the UASB reactor fed with the CSTR effluent, 83 ± 3% of phosphorus was removed and mainly present as CaP granules > 2.5 mm in diameter. The CaP granules had a phosphorus content of 8.4 ± 1.3%P w/w with amorphous and crystalline calcium phosphate being the dominant phases. The formation of large and concentrated CaP granules causes accumulation at the bottom of the reactor and allows for simple size separation leading to efficient phosphorus recovery The recovery of CaP granules improves CM valorization and can reduce the transport of manure by up to 87%. The acidification prior to digestion thus promotes a more efficient use of phosphorus from CM in agriculture and can replace phosphorus from primary phosphorus sources. [ABSTRACT FROM AUTHOR]

• Electrochemical mediated precipitation (EMP) of phosphate minerals were reviewed. • Mechanism and influences of EMP towards P recovery were discussed. • EMP has the unique advantage of recovering both ortho and non-ortho P. • Challenges and limitations of EMP in P recovery were identified. • Future research should focus on upscaling under environment relevant conditions. 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• Electrochemically deposited solids adhere well to vertically placed electrodes. • The deposition layer delays the diffusion of hydroxide ions. • The prototype achieves high stability and efficiency over 173 days of operation. • Calcium phosphate is the dominant phase in the recovered solids. • Recovered solids have high p content (> 9wt%) and negligible heavy metals. Phosphorus (P) is an irreplaceable element, playing a vital role in living organisms, yet has limited earth reserves. The possibility of P recovery from wastewaters by e lectrochemically-induced ca lcium p hosphate p recipitation (ECaPP) was demonstrated previously. The current study presents a novel scalable prototype consisting of a column-shaped electrochemical reactor, a tubular stainless-steel cathode, and a Pt coated Ti anode. The adhesion of solids to the cathode, important for product recovery, was shown not to be negatively impacted by electrodes' vertical placement. The influence of current (density), hydraulic retention time (HRT), and initial phosphate concentration in this prototype were examined under continuous flow operation. The system accomplished the highest P removal rate (1267 mg/day) at 1.5 d HRT and 800 mA in treating undiluted cheese wastewater with 48.5 kWh/kg P. Moreover, the prototype showed high stability and efficiency (> 50%) over 173 days of continuous operation without performing maintenance. After turning off the current (0 mA), the system realized a surprising P removal jump up to 97.3%, revealing the delayed diffusion of hydroxide ions by the deposition layer. The calculation of CAPEX and OPEX of ECaPP in treating 100 m3 cheese wastewater per week indicates that the ECaPP plant can realize net-positive from the 12th year. The recovered solids have relatively high P content (> 9wt%) and insignificant contamination of heavy metals. Overall, the proven suitability of the scalable prototype can pave the way towards the actual adoption of the ECaPP process. [Display omitted] [ABSTRACT FROM AUTHOR]

Phosphorus removal and recovery from waste streams are crucial to prevent eutrophication and sustain fertilizer production. As has been shown in our previous papers, electrochemical treatment has the potential to achieve this goal. However, the adoption of electrochemical approach is limited by its high energy consumption. Here, we investigate the possibility of electrochemical phosphorus removal at extremely low current density using graphite felt as the cathode. We found a current density as low as 0.04 A/m2 can enhance the removal of phosphate in our electrochemical system. The removal of phosphate at extremely low current density resulted from electrochemical induced calcium phosphate precipitation and not by electrochemical adsorption. Electrochemical treatment of real domestic wastewater at 0.2 A/m2 almost eliminates the precipitation of Mg(OH) 2 and limits the formation of CaCO 3. The recovered precipitates are dominated by calcium phosphate (59%), followed by 35% CaCO 3 and 6% Mg(OH) 2. The specific energy consumption of this newly electrochemical system is between 4.4 and 26.4 kW h/kg P, which is 2 orders of magnitude lower than our previous system (110–2238 kW h/kg P). Key factors for this improvement prove to be enlarged precipitation area and hydroxide flux retardation by graphite felt. Practically, our study offers a potential way to reduce the energy consumption in electrochemical removal of phosphate by using a graphite felt cathode and at a current density below 0.2 A/m2. Fundamentally, our study contributes to the understanding of adsorption and precipitation in electrochemical removal of phosphate at an extremely low current density and with carbon-based electrodes. Image 1 • Electrochemical removal of PO 4 –P at current density ≤0.2 A/m2 is achieved. • The combination of graphite felt with Pt–Ti cathode enhanced PO 4 –P removal. • The enhancement is built on enlarged precipitation area and lowered flux of OH−. • 85% of removed calcium phosphate precipitates on graphite felt. • Low current allows selectively precipitation of calcium phosphate in wastewater. [ABSTRACT FROM AUTHOR]