Electrochemical route outperforms chemical struvite precipitation in mitigating heavy metal contamination.

Electrochemically mediated struvite precipitation (EMSP) offers a robust, chemical-free process towards phosphate and ammonium reclamation from nutrients-rich wastewater, i.e., swine wastewater. However, given the coexistence of heavy metal, struvite recovered from wastewater may suffer from heavy metal contamination. Here, we systematically investigated the fate of Cu2+, as a representative heavy metal, in the EMSP process and compared it with the chemical struvite precipitation (CSP) system. The results showed that Cu2+ was 100% transferred from solution to solid phase as a mixture of copper and struvite under pHi 9.5 with 2-20 mg/L Cu2+ in the CSP system, and varying pH would affect struvite production. In the EMSP system, the formation of struvite was not affected by bulk pH, and struvite was much less polluted by co-removed Cu2+ (24.4%) at pHi 7.5, which means we recovered a cleaner and safer product. Specifically, struvite mainly accumulates on the front side of the cathode. In contrast, the fascinating thing is that Cu2+ is ultimately deposited primarily to the back side of the cathode in the form of copper (hydro)oxides due to the distinct thickness of the local high pH layer on the two sides of the cathode. In turn, struvite and Cu (hydro)oxides can be harvested separately from the front and back sides of the cathode, respectively, facilitating the subsequent recycling of heavy metals and struvite. The contrasting fate of Cu2+ in the two systems highlights the merits of EMSP over conventional CSP in mitigating heavy metal pollution on recovered products, promoting the development of EMSP technology towards a cleaner recovery of struvite from waste streams.

Basket anode filled with CaCO3 particles: A membrane-free electrochemical system for boosting phosphate recovery and product purity.

Phosphorus (P) is often regarded as the primary stimulant for eutrophication, while its importance as a crucial life element is also well acknowledged. Given its future scarcity, P recycling from waste streams is suggested and practiced. Electrochemically mediated precipitation (EMP) is a robust and chemical-free process for P removal and recovery, yet it requires further developments. The first generation of the CaCO3-packed electrochemical precipitation column successfully solved the problem of H+-OH- recombination, achieving enhanced P removal efficiency with less energy consumption but suffering from low Ca-phosphate purity in recovered products. Herein, a new concept of a basket-anode electrochemical system is proposed and validated to prevent direct H+-OH- recombination and enhance product purity. The CaCO3 pellets packed basket anode alleviates the OH- depletion by CaCO3-H+ interaction and provides extra Ca2+ for enhanced P removal. The novel structure of the basket anode, by its derived acidic anode region and alkaline cathode region, completely avoids the precipitation of Ca-phosphate on the packed CaCO3 and greatly facilitates the collection of high-quality Ca-phosphate product. Our results suggest that almost 100% of the removed P was in high-purity, highly crystalline Ca-phosphate on the cathode. The recovered products contained significantly more P (13.5 wt%) than in the previous study (0.1 wt%) at similar energy consumptions (29.8 kWh/kg P). The applied current density, pellets size, and influent P concentration were critical for P removal performance, product purity, and power consumption. We further demonstrated the long-term stability of this novel system and its technical and economic feasibility in treating real stored urine. Our study provides new cell architectural designs to enhance the performance of EMP systems and may inspire innovations and developments in other electrochemical water treatment processes.

Removal of small elemental sulfur particles by polysulfide formation in a sulfidic reactor.

For over 30 years, biological gas desulfurization under halo-alkaline conditions has been studied and optimized. This technology is currently applied in already 270 commercial installations worldwide. Sulfur particle separation, however, remains a challenge; a fraction of sulfur particles is often too small for liquid-solid separation with conventional separation technology. In this article, we report the effects of a novel sulfidic reactor, inserted in the conventional process set-up, on sulfur particle size and morphology. In the sulfidic reactor polysulfide is produced by the reaction of elemental sulfur particles and sulfide, which is again converted to elemental sulfur in a gas-lift reactor. We analyzed sulfur particles produced in continuous, long term lab-scale reactor experiments under various sulfide concentrations and sulfidic retention times. The analyses were performed with laser diffraction particle size analysis and light microscopy. These show that the smallest particles (< 1 µm) have mostly disappeared under the highest sulfide concentration (4.1 mM) and sulfidic retention time (45 min). Under these conditions also agglomeration of sulfur particles was promoted. Model calculations with thermodynamic and previously derived kinetic data on polysulfide formation confirm the experimental data on the removal of the smallest particles. Under the 'highest sulfidic pressure', the model predicts that equilibrium conditions are reached between sulfur, sulfide and polysulfide and that 100% of the sulfur particles <1 µm are dissolved by the (autocatalytic) formation of polysulfides. These experiments and modeling results demonstrate that the insertion of a novel sulfidic reactor in the conventional process set-up promotes the removal of the smallest individual sulfur particles and promotes the production of sulfur agglomerates. The novel sulfidic reactor is therefore a promising process addition with the potential to improve process operation, sulfur separation and sulfur recovery.

Effect of sulfide on morphology and particle size of biologically produced elemental sulfur from industrial desulfurization reactors.

We investigated the effect of polysulfide formation on properties of biologically produced elemental sulfur (S8) crystals, which are produced during biological desulfurization (BD) of gas. The recent addition of an anoxic-sulfidic reactor (AnSuR) to the BD process resulted in agglomerated particles with better settleability for S8 separation. In the AnSuR, polysulfides are formed by the reaction of bisulfide (HS-) with S8 and are subsequently oxidized to S8 in a gas-lift reactor. Therefore, sulfur particles from the BD are shaped (i.e. morphology and particle size) both by formation and dissolution. We assessed the reaction of HS- with S8 particles in anoxic, abiotic experiments in a batch reactor using two S8 samples from industrial BD reactors. Under these conditions, the sulfur particle surface became coarser and more porous, and in addition the smallest particles disappeared. Agglomerates initially fell apart but were reformed at a later stage. Moreover, we found different observed polysulfide formation rates for each S8 sample, which was related to the initial morphology and size. Our findings show that particle properties can be controlled abiotically and that settleability of S8 is increased by increasing both the HS--S8 ratio and retention time.

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.

Novel Agglomeration Strategy for Elemental Sulfur Produced during Biological Gas Desulfurization.

This article presents a novel crystal agglomeration strategy for elemental sulfur (S) produced during biological desulfurization (BD). A key element is the nucleophilic dissolution of S by sulfide (HS-) to polysulfides (S x 2-), which was enhanced by a sulfide-rich, anoxic reactor. This study demonstrates that with enhanced S x 2- formation, crystal agglomerates are formed with a uniform size (14.7 ± 3.1 µm). In contrast, with minimal S x 2- formation, particle size fluctuates markedly (5.6 ± 5.9 µm) due to the presence of agglomerates and single crystals. Microscopic analysis showed that the uniformly sized agglomerates had an irregular structure, whereas the loose particles and agglomerates were more defined and bipyramidal. The irregular agglomerates are explained by dissolution of S by (poly)sulfides, which likely changed the crystal surface structure and disrupted crystal growth. Furthermore, S from S x 2- appeared to form at least 5× faster than from HS- based on the average S x 2- chain length of x ≈ 5, thereby stimulating particle agglomeration. In addition, microscopy suggested that S crystal growth proceeded via amorphous S globules. Our findings imply that the crystallization product is controlled by the balance between dissolution and formation of S. This new insight has a strong potential to prevent poor S settleability in BD.

Density-dependent microbial calcium carbonate precipitation by drinking water bacteria via amino acid metabolism and biosorption.

Drinking water plumbing systems appear to be a unique environment for microorganisms as they contain few nutrients but a high mineral concentration. Interactions between mineral content and bacteria, such as microbial calcium carbonate precipitation (MCP) however, has not yet attracted too much attention in drinking water sector. This study aims to carefully examine MCP behavior of two drinking water bacteria species, which may potentially link scaling and biofouling processes in drinking water distribution systems. Evidence from cell density evolution, chemical parameters, and microscopy suggest that drinking water isolates can mediate CaCO3 precipitation through previously overlooked MCP mechanisms like ammonification or biosorption. The results also illustrate the active control of bacteria on the MCP process, as the calcium starts to concentrate onto cell surfaces only after reaching a certain cell density, even though the cell surfaces are shown to be the ideal location for the CaCO3 nucleation.

Electrochemical recovery of phosphorus from wastewater using tubular stainless-steel cathode for a scalable long-term operation.

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 electrochemically-induced calcium phosphate precipitation (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.

Electrochemical Recovery of Phosphorus from Acidic Cheese Wastewater: Feasibility, Quality of Products, and Comparison with Chemical Precipitation.

The recovery of phosphorus (P) from high-strength acidic waste streams with high salinity and organic loads is challenging. Here, we addressed this challenge with a recently developed electrochemical approach and compared it with the chemical precipitation method via NaOH dosing. The electrochemical process recovers nearly 90% of P (∼820 mg/L) from cheese wastewater in 48 h at 300 mA with an energy consumption of 64.7 kWh/kg of P. With chemical precipitation, >86% of P was removed by NaOH dosing with a normalized cost of 1.34-1.80 euros/kg of P. The increase in wastewater pH caused by NaOH dosing triggered the formation of calcium phosphate sludge instead of condensed solids. However, by electrochemical precipitation, the formed calcium phosphate is attached to the electrode, allowing the subsequent collection of solids from the electrode after treatment. The collected solids are characterized as amorphous calcium phosphate (ACP) at 200 mA or a precipitation pH of ≥9. Otherwise, they are a mixture of ACP and hydroxyapatite. The products have sufficient P content (≤14%), of which up to 85% was released within 30 min in 2% citric acid and a tiny amount of heavy metals compared to phosphate rocks. This study paves the way for applying electrochemical removal and recovery of phosphorus from acidic P-rich wastewater and offers a sustainable substitute for mined phosphorus.

Dynamic probabilistic material flow analysis of engineered nanomaterials in European waste treatment systems.

Knowledge on the material flows of engineered nanomaterials (ENMs) is crucial for assessing their environmental risks. Waste management processes constitute important parts of material flow analyses as they affect large fractions of the ENMs. Accordingly, their detailed representation could substantially improve the models. Our goal was to consider the temporal variations of wastewater and solid waste management in the dynamic probabilistic material flow analysis of carbon nanotubes (CNTs), nano-Ag, -TiO2 and -ZnO in Europe from 2000 to 2020. New input parameters included wastewater and solid waste management rates for each year. The uncertainties associated with these data were assessed based on the type of consulted source, the geographical representativeness and temporal concordance. Results show modal values of 10-27% of ENMs going from sorting to reprocessing. Large shares of environmental releases of nano-Ag and nano-ZnO end in surface water (4.9 t and 1700 t respectively in 2020), while sludge-treated soil as environmental compartment is receiving most of nano-TiO2 (22,000 t in 2020) and CNTs (8.8 t in 2020). Discharges from wastewater management to the subsurface soil make this compartment the largest environmental sink of nano-Ag and nano-ZnO (30 t and 3860 t accumulated in 2020, respectively). Landfills represent significant stocks of ENMs, with 105 t, 2077 t, 69,000 t and 1042 t of nano-Ag, nano-ZnO, nano-TiO2 and CNTs. This model includes detailed descriptions of waste management and sources of ENMs released at the European scale. However, a better understanding of the behaviour, i.e. fate and potential transformations of ENMs in reprocessing systems, is needed to complete the full assessment.

Electrochemical removal of phosphate in the presence of calcium at low current density: Precipitation or adsorption？.

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 CaCO3. The recovered precipitates are dominated by calcium phosphate (59%), followed by 35% CaCO3 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.

Electrochemically mediated calcium phosphate precipitation from phosphonates: Implications on phosphorus recovery from non-orthophosphate.

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 Na2SO4 (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.

Recovery of calcium phosphate granules from black water using a hybrid upflow anaerobic sludge bed and gas-lift reactor.

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 ±â€¯16 g/L at the bottom and 31 ±â€¯5 g/L at the top of the reactor. The concentration of solid P at the bottom of the reactor was 4.58 ±â€¯1.34 gP/L, while at the top a much lower concentration was obtained (0.75 ±â€¯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 ±â€¯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.

Calcium Carbonate Packed Electrochemical Precipitation Column: New Concept of Phosphate Removal and Recovery.

Phosphorus (P) is a vital micronutrient element for all life forms. Typically, P can be extracted from phosphate rock. Unfortunately, the phosphate rock is a nonrenewable resource with a limited reserve on the earth. High levels of P discharged to water bodies lead to eutrophication. Therefore, P needs to be removed and is preferably recovered as an additional P source. A possible way to achieve this goal is by electrochemically induced phosphate precipitation with coexisting calcium ions. Here, we report a new concept of phosphate removal and recovery, namely a CaCO3 packed electrochemical precipitation column, which achieved improved removal efficiency, shortened hydraulic retention time, and substantially enhanced stability, compared with our previous electrochemical system. The concept is based on the introduction of CaCO3 particles, which facilitates calcium phosphate precipitation by buffering the formed H+ at the anode, releases Ca2+, acts as seeds, and establishes a high pH environment in the bulk solution in addition to that in the vicinity of the cathode. It was found that the applied current, the CaCO3 particle size, and the feed rate affect the removal of phosphate. Under optimized conditions (particle size, <0.5 mm; feed rate, 0.4 L/d; current, 5 mA), in a continuous flow system, the CaCO3 packed electrochemical precipitation column achieved 90 ± 5% removal of phosphate in 40 days and >50% removal over 125 days with little maintenance. The specific energy consumptions of this system lie between 29 and 61 kWh/kg P. The experimental results demonstrate the promising potential of the CaCO3 packed electrochemical precipitation column for P removal and recovery from P-containing streams.

Influence of Cell Configuration and Long-Term Operation on Electrochemical Phosphorus Recovery from Domestic Wastewater.

Phosphorus (P) is an important, scarce, and irreplaceable element, and therefore its recovery and recycling are essential for the sustainability of the modern world. We previously demonstrated the possibility of P recovery by electrochemically induced calcium phosphate precipitation. In this Article, we further investigated the influence of cell configuration and long-term operation on the removal of P and coremoved calcium (Ca), magnesium (Mg), and inorganic carbon. The results indicated that the relative removal of P was faster than that of Ca, Mg, and inorganic carbon initially, but later, due to decreased P concentration, the removal of Ca and Mg became dominant. A maximum P removal in 4 days is 75% at 1.4 A m-2, 85% at 8.3 A m-2 and 92% at 27.8 A m-2. While a higher current density improves the removal of all ions, the relative increased removal of Ca and Mg affects the product quality. While the variation of electrode distance and electrode material have no significant effects on P removal, it has implication for reducing the energy cost. A 16-day continuous-flow test proved calcium phosphate precipitation could continue for 6 days without losing efficiency even when the cathode was covered with precipitates. However, after 6 days, the precipitates need to be collected; otherwise, the removal efficiency dropped for P removal. Economic evaluation indicates that the recovery cost lies in the range of 2.3-201.4 euro/kg P, depending on P concentration in targeted wastewater and electrolysis current. We concluded that a better strategy for producing a product with high P content in an energy-efficient way is to construct the electrochemical cell with cheaper stainless steel cathode, with a shorter electrode distance, and that targets P-rich wastewater.

Bulk pH and Carbon Source Are Key Factors for Calcium Phosphate Granulation.

Recovery of calcium phosphate granules (CaP granules) from high-strength wastewater is an opportunity to reduce the natural phosphorus (P) scarcity, geographic imbalances of P reserves, and eutrophication. Formation of CaP granules was previously observed in an upflow anaerobic sludge bed (UASB) reactor treating source separated black water and is enhanced by Ca2+ addition. However, the required operating conditions and influent composition for CaP granulation are still unknown. In this study, we have experimentally demonstrated that the carbon source and bulk pH are crucial parameters for the formation and growth of CaP granules in a UASB reactor, operating at relatively low upflow velocity (<1 cm h-1). Degradation of glucose yielded sufficient biomass (microbial cells and extracellular biopolymers) to cover crystal and amorphous calcium phosphate [Ca x(PO4) y], forming CaP granules. Influent only containing volatile fatty acids as the carbon source did not generate CaP granules. Moreover, bulk pH between 7.0 and 7.5 was crucial for the enrichment of Ca x(PO4) y in the granules over bulk precipitation. Bulk pH 8 reduced the Ca x(PO4) y enrichment in granules of >1.4 mm diameter from 9 to 5 wt % P. Moreover, for bulk pH 7.5, co-precipitation of CaCO3 with Ca x(PO4) y was reduced.

The Effect of Bioinduced Increased pH on the Enrichment of Calcium Phosphate in Granules during Anaerobic Treatment of Black Water.

Simultaneous recovery of calcium phosphate granules (CaP granules) and methane in anaerobic treatment of source separated black water (BW) has been previously demonstrated. The exact mechanism behind the accumulation of calcium phosphate (Ca x(PO4) y) in CaP granules during black water treatment was investigated in this study by examination of the interface between the outer anaerobic biofilm and the core of CaP granules. A key factor in this process is the pH profile in CaP granules, which increases from the edge (7.4) to the center (7.9). The pH increase enhances supersaturation for Ca x(PO4) y phases, creating internal conditions preferable for Ca x(PO4) y precipitation. The pH profile can be explained by measured bioconversion of acetate and H2, HCO3- and H+ into CH4 in the outer biofilm and eventual stripping of CO2 and CH4 (biogas) from the granule. Phosphorus content and Ca x(PO4) y crystal mass quantity in the granules positively correlated with the granule size, in the reactor without Ca2+ addition, indicating that the phosphorus rich core matures with the granule growth. Adding Ca2+ increased the overall phosphorus content in granules >0.4 mm diameter, but not in fine particles (<0.4 mm). Additionally, H+ released from aqueous phosphate species during Ca x(PO4) y crystallization were buffered by internal hydrogenotrophic methanogenesis and stripping of biogas from the granule. These insights into the formation and growth of CaP granules are important for process optimization, enabling simultaneous Ca x(PO4) y and CH4 recovery in a single reactor. Moreover, the biological induction of Ca x(PO4) y crystallization resulting from biological increase of pH is relevant for stimulation and control of (bio)crystallization and (bio)mineralization in real environmental conditions.

Is There a Precipitation Sequence in Municipal Wastewater Induced by Electrolysis?

Electrochemical wastewater treatment can induce calcium phosphate precipitation on the cathode surface. This provides a simple yet efficient way for extracting phosphorus from municipal wastewater without dosing chemicals. However, the precipitation of amorphous calcium phosphate (ACP) is accompanied by the precipitation of calcite (CaCO3) and brucite (Mg(OH)2). To increase the content of ACP in the products, it is essential to understand the precipitation sequence of ACP, calcite, and brucite in electrochemical wastewater treatment. Given the fact that calcium phosphate (i.e., hydroxyapatite) has the lowest thermodynamic solubility product and highest saturation index in the wastewater, it has the potential to precipitate first. However, this is not observed in electrochemical phosphate recovery from raw wastewater, which is probably because of the very high Ca/P molar ratio (7.5) and high bicarbonate concentration in the wastewater resulting in formation of calcite. In the case of decreased Ca/P molar ratio (1.77) by spiking external phosphate, most of the removed Ca in the wastewater was used for ACP formation instead of calcite. The formation of of brucite, however, was only affected when the current density was decreased or the size of cathode was changed. Overall, the removal of Ca and Mg is much more affected by current density than the surface area of cathode, whereas for P removal, the reverse is true. Because of these dependencies, though there is no definite precipitation sequence among ACP, calcite, and brucite, it is still possible to influence the precipitation degree of these species by relatively low current density and high surface area or by targeting phosphorus-rich wastewaters.

Interaction of calcium, phosphorus and natural organic matter in electrochemical recovery of phosphate.

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 ±â€¯4.9% to 58.5 ±â€¯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.

Calcium addition to increase the production of phosphate granules in anaerobic treatment of black water.

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 CODTotal removal (> 80%) and CH4 production (0.47 ± 0.10 gCOD-CH4 g-1CODTotal-BW) was observed. Therefore, adding calcium can significantly increase the CaP granulation without inhibiting the simultaneous CH4 recovery. This further indicates the potential of this process for phosphate recovery.