Organic matter affects phosphorus recovery during vivianite crystallization

Unlocking phosphorus recovery from microalgae biomass: The enhanced transformation and release of phosphorus species

The intertwined challenges of harmful algae blooms and the phosphorus (P) resource crisis have necessitated the recovery of P from algae biomass. For the first time, a co-pyrolysis strategy that incorporates NaHCO3 into the pyrolysis process of chlorella to efficiently recover P in the form of vivianite was proposed. The findings demonstrated that the addition of 20 wt.% NaHCO3 during pyrolysis significantly enhanced P extraction from biochar, increasing the extraction efficiency from 2.8 % to 94.37 %. A complementary array of techniques including chemical extraction, nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), as well as two-dimensional correlation spectroscopy (2D-COS), was employed to elucidate the transformation of hard-to-extract P in chlorella to easy-to-extract P during pyrolysis. It was observed that organophosphorus (OP), pyrophosphate (pyro-P), and polyphosphates (poly-P) reacted with NaHCO3 at 700 °C, undergoing depolymerization and hydrolysis, which led to the formation of orthophosphate (ortho-P) species (e.g., Na3PO4, NaCa(PO4)3, (Fe2(PO4)3), accounting for 98.88 % of the P species in biochar product. High-purity vivianite (∼98.13 %) was subsequently obtained without the need for impurity removal, as indicated by chemical equilibrium simulations, due to the minimal ions and dissolved organic matter (DOM) present in the leaching solution, a consequence of the simple and pure structure of microalgae biomass. The estimated economic profit of this strategy is $1.51 per kilogram of dry chlorella. Additionally, the resulting biochar exhibited a high surface area (518.40 m2/g) and a well-developed pore structure, make it a promising material for adsorption and catalytic applications. This study provides a novel perspective for addressing the P crisis while effectively mitigating harmful algal blooms.

External voltage regulates hydrogen and vivianite recovery from fermentation liquid in microbial electrolysis cell equipped with iron anode: Performance and mechanism

Employing an iron anode in microbial electrolysis cell (MEC) can promote hydrogen yield and vivianite recovery from waste biomass by accelerating electron transport, but the performance is highly dependent on the functional microbial community present and the ferrous ion content. An external voltage had a significant effect on enriching functional microbes and controlling the release of ferrous ions. In this study, the effects of different voltages, i.e., 0.4 V, 0.6 V, 0.8 V and 1.0 V, on hydrogen production and vivianite recovery were explored. The results indicated that an applied voltage of 0.8 V resulted in the maximum hydrogen productivity of 11.17 mmol/g COD, representing an increase of 18∼91 % compared with the other voltage conditions. The removal efficiency of phosphorus reached 100 % at 3 d in the 0.8 V group, with vivianite as the main product at a purity of 92.7 %. An external voltage of 0.8 V notably enhanced the electrochemical performance of the MEC. The relative abundances of bio-cathodic microbes, i.e., electrochemically active bacteria, anaerobic fermentation bacteria, dissimilatory iron-reducing bacteria and homoacetogens, greatly changed with different voltages, reaching 9.6 %, 3.2 %, 3.1 % and 23.7 %, respectively, in the 0.8 V group. The expression of key functional genes related hydrogen production, i.e., the ferredoxin-dependent hydrogenase pathway and pyruvate ferredoxin oxidoreductase pathway, was significantly upregulated, whereas that related to homo-acetogenesis was downregulated under 0.8 V. This work reveals the performance and mechanism of synergistic hydrogen production and phosphorus recovery under an applied voltage, and provides new insights and feasible measures for improving hydrogen production and phosphorus recovery in MECs.

Compatibility of vivianite-crystallization pathway of phosphorus recovery with anaerobic digestion systems of thermally hydrolyzed sludge

Phosphorus in sewage is mostly enriched in activated sludge in wastewater treatment plants, making excess sludge an appropriate material for phosphorus recovery. The potential of vivianite (Fe3(PO4)2·8H2O) crystallization-based phosphorus recovery during the anaerobic digestion of thermally hydrolyzed sludge was discussed with influences of organic compounds on the formation of vivianite crystals being investigated in detail. Bovine serum albumin, humic acids and alginate, as model compounds of proteins, humic acids and polysaccharides, all inhibited vivianite crystallization, with the influence of humic acids being the most significant. A sludge retention time of >12 d for effective degradation of organic compounds and a certain degree of FeII excess are suggested to decrease the organics resulting inhibition. The results demonstrate the compatibility of vivianite-crystallization pathway of phosphorus recovery with anaerobic sludge digesters, and reveal the complexity of vivianite formation in the sludge with further research warranted to minimize the inhibitory influences.

Improving nutrients ratio in class A biosolids through vivianite recovery: Insights from a wastewater resource recovery facility

Class A biosolids from water resource recovery facilities (WRRFs) are increasingly used as sustainable alternatives to synthetic fertilizers. However, the high phosphorus to nitrogen ratio in biosolids leads to a potential accumulation of phosphorus after repeated land applications. Extracting vivianite, an FeP mineral, prior to the final dewatering step in the biosolids treatment can reduce the P content in the resulting class A biosolids and achieve a P:N ratio closer to the 1:2 of synthetic fertilizers. Using ICP-MS, IC, UV-Vis colorimetric methods, Mössbauer spectroscopy, and SEM-EDX, a full-scale characterization of vivianite at the Blue Plains Advanced Wastewater Treatment Plant (AWTTP) was surveyed throughout the biosolids treatment train. Results showed that the vivianite-bound phosphorus in primary sludge thickening, before pre-dewatering, after thermal hydrolysis, and after anaerobic digestion corresponded to 8 %, 52 %, 40 %, and 49 % of the total phosphorus in the treatment influent. Similarly, the vivianite-bound iron concentration also corresponded to 8 %, 52 %, 40 %, and 49 % of the total iron present (from FeCl3 dosing), because the molar ratio between total iron and total incoming phosphorus was 1.5:1, which is the same stoichiometry of vivianite. Based on current P:N levels in the Class A biosolids at Blue Plains, a vivianite recovery target of 40 % to ideally 70 % is required in locations with high vivianite content to reach a P:N ratio in the resulting class A biosolid that matches synthetic fertilizers of 1:1.3 to 1:2, respectively. A financial analysis on recycling iron from the recovered vivianite had estimated that 14-25 % of Blue Plain's annual FeCl3 demand can potentially be met. Additionally, model simulations with Visual Minteq were used to evaluate the pre-treatment options that maximize vivianite recovery at different solids treatment train locations.

Effects of wet-dry alternation on organic phosphorus dynamics and sediment characteristics in the intertidal zone of Nansi Lake

The study of the fractions and distribution characteristics of organic phosphorus in the sediment of the water level fluctuating zone of Nansi Lake is conducive to revealing the transformation of phosphorus in the lake, and has important scientific significance for controlling the eutrophication of Nansi Lake. Based on the sediment of the water level fluctuation zone of Nansi Lake. The improved Hedley continuous grading extraction, ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscope were used to characterize the structural characteristics and stability of organic molecules in the sediment, and to reflect the differences in the structure and stability of organophosphate in the water level fluctuating zone. Principal component analysis (PCA), Redundancy analysis (RDA) and correlation heat map analysis were used to analyze the correlation between phosphorus and physicochemical index. The results showed that the alternation between wet-dry conditions was more favorable for the release of phosphorus from sediment, compared to continuous inundation conditions. Moreover, the higher the frequency of wet-dry alternations, the greater the release of phosphorus in different forms from the sediment. Wet-dry alternation resulted in a reduction of substituent on the aromatic rings of sediment DOM (dissolved organic matter), and the continuous drying would increase the molecular weight and humidification degree of DOM in the sediment. Correlation analysis showed that NaOH-Po content in sediment was significantly negatively correlated with TP, IP, OP and various organophosphorus forms, indicating a close transformation relationship between phosphorus forms in sediment. The results can provide a scientific basis for controlling the release of endogenous phosphorus and the risk of eutrophication in Nansi Lake.

Emerging electro-driven technologies for phosphorus enrichment and recovery from wastewater: A review

The recovery of phosphorus from wastewater is a critical step in addressing the scarcity of phosphorus resources. Electro-driven technologies for phosphorus enrichment have gathered significant attention due to their inherent advantages, such as mild operating conditions, absence of secondary pollution, and potential integration with other technologies. This study presents a comprehensive review of recent advancements in the field of phosphorus enrichment, with a specific focus on capacitive deionization and electrodialysis technologies. It highlights the underlying principles and effectiveness of electro-driven techniques for phosphorus enrichment while systematically comparing energy consumption, enrichment rate, and concentration factor among different technologies. Furthermore, the study provides a thorough analysis of the capacity of various technologies to selectively enrich phosphorus and proposes several methods and strategies to enhance selectivity. These insights offer valuable guidance for advancing the future development of electrochemical techniques with enhanced efficiency and effectiveness in phosphorus enrichment from wastewater.

Phosphorus recovery from synthetic anaerobic fermentation supernatant via vivianite crystallization: Coupling effects of various physicochemical process parameters

Recovery of phosphorus (P) via vivianite crystallization is an effective strategy to recycle resources from the anaerobic fermentation supernatant. However, the presence of different components in the anaerobic fermentation supernatant (e.g., polysaccharides and proteins) might alter conditions for optimal growth of vivianite crystals, resulting in distinct vivianite characteristics. In the present study, the effect of different components on vivianite crystallization was explored. Then, the reaction parameters (pH, Fe/P, and stirring speed) for P recovery from synthetic anaerobic fermentation supernatant as vivianite were optimized using response surface methodology, and the relationship between crystal properties and supersaturation was elucidated using a thermodynamic equilibrium model. The optimized values for pH, Fe/P, and stirring speed were found to be 7.8, 1.74, and 500 rpm respectively, resulting in 90.54 % P recovery efficiency. Moreover, the variation of reaction parameters did not change the crystalline structure of the recovered vivianite but influenced its morphology, size, and purity. Thermodynamic analysis suggested the saturation index (SI) of vivianite increased with increasing pH and Fe/P ratio, leading to a facilitative effect on vivianite crystallization. However, when the SI was >11, homogenous nucleation occurred so that the nucleation rate was much higher than the crystal growth rate, causing a smaller crystal size. The findings presented herein will be highly valued for the future large-scale application of the vivianite crystallization process for wastewater treatment.

Human urine: A novel source of phosphorus for vivianite production

Human urine contributes up to 50 % of the phosphorus load in domestic wastewater. Decentralized sanitation systems that separately collect urine provide an opportunity to recover this phosphorus. In this study, we leveraged the unique and complex chemistry of urine in favor of recovering phosphorus as vivianite. We found that the type of urine affected the yield and purity of vivianite, but the kind of iron salt used, and reaction temperature, did not affect the yield and purity. Ultimately, it was the urine pH that affected the solubility of vivianite and other co-precipitates, with the highest yield (93 ± 2 %) and purity (79 ± 3 %) of vivianite obtained at pH 6.0. Yield and purity of vivianite were both maximized when Fe:P molar ratio was >1.5:1, but <2.2:1. This molar ratio provided sufficient iron to react with all available phosphorus, while exerting a competitive effect that suppressed the precipitation of other precipitates. Vivianite produced from fresh urine was less pure than vivianite produced from synthetic urine, because of the presence of organics in real urine, but washing the solids with deionized water improved the purity by 15.5 % at pH 6.0. Overall, this novel work adds to the growing body of literature on phosphorus recovery as vivianite from wastewater.

A sustainable strategy for recovery of phosphorus as vivianite from sewage sludge via alkali-activated pyrolysis, water leaching and crystallization

A sustainable strategy for P recovery from sewage sludge via alkali-activated pyrolysis, water leaching and crystallization was proposed, and a high value-added product of vivianite was recovered. Effects of the type and dose of alkali activator on P transformation during sludge pyrolysis were investigated. 50 wt% dose of KHCO₃ was determined as the alkali-activated pyrolysis condition. The content of water-soluble P (referred to as Water-P) in biochar derived from raw sludge (referred to as RS) and ferric sludge (Fenton's reagent conditioned sludge, referred to as FS) by KHCO₃-activated pyrolysis at different temperatures was compared. The Fe element in the Fenton's reagent enhanced the content of Fe-bound P in the dewatered sludge, which was readily transformed into potassium phosphate during KHCO₃-activated pyrolysis, thus increasing the Water-P content in the biochar derived from FS. The proportions of Water-P to total P in the biochar samples obtained by KHCO₃-activated pyrolysis of RS and FS at 600 °C were 72.5% and 96.2%, respectively, which were notably higher than those in the biochar samples obtained by direct pyrolysis of RS and FS (3.5% and 0.5%), respectively. The water leaching solution of biochar obtained by KHCO₃-activated pyrolysis of FS at 600 °C was purified to remove impurity elements, and vivianite with high purity was finally recovered by crystallization. A total P recovery efficiency of 88.08% was achieved throughout the process from sewage sludge to the final vivianite product. This study proposes a promising and sustainable approach for realizing the recovery of high value-added product vivianite from sewage sludge.

Recovery of phosphorus from wastewater containing humic substances through vivianite crystallization: Interaction and mechanism analysis

Vivianite crystallization has been regarded as a suitable option for recovering phosphorus (P) from P-containing wastewater. However, the presence of humic substances (HS) would inevitably affect the formation of vivianite crystals. Therefore, the influences of HS on vivianite crystallization and the changes in the harvested vivianite crystals were investigated in this study. The results suggested the inhibition effect of 70 mg/L HS on vivianite crystallization reached 12.24%, while it could be attenuated by increasing the pH and Fe/P ratio of the solution. Meanwhile, the addition of HS altered the size, purity, and morphology of recovered vivianite crystals due to the blockage of the growth sites on the crystal surface. Additionally, the formation of phosphate ester group, hydrogen bonding, and COOH-Fe²⁺ complexes are the potential mechanisms of HS interaction with vivianite crystals. The results obtained herein will help to elucidate the underlying mechanism of HS on vivianite crystallization from P-containing wastewater.

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

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

Acid/alkali pretreatment enhances the formation of vivianite during anaerobic fermentation of waste activated sludge

Phosphorus (P) recovery from waste activated sludge (WAS) of wastewater treatment plants is significant in the world suffering from P shortage. Recently, vivianite crystallization has been regarded as an essential method of recovering P from anaerobic fermentation (AF) of WAS. This study performed acid/alkali pretreatment (pH 3/pH 10) on AF of WAS to improve iron reduction and vivianite formation. The results showed that the maximum iron reduction rate (R_max) in the pH 3 and pH 10 groups was increased by 1.9 and 1.7 times compared with that in the Control-Fe group, and the iron reduction efficiency (E_Fe) was increased by 17.5% and 12.0% respectively. The Fe bound P (Fe-P) proportion in the sludge in the pH 3 and pH 10 groups increased by 50.0% and 33.7%, respectively. Furthermore, the relative abundance of the iron-reducing bacteria Clostridium_sensusensu in the pH 3 group was higher; and the Fe-P proportion in the sludge and the size of vivianite crystal after AF were larger. With these results, pH 3 pretreatment was preferred for promoting Fe²⁺ release and vivianite formation during AF.

Vivianite precipitation for iron recovery from anaerobic groundwater

Iron in anaerobic groundwater is commonly removed by oxidation followed by sand filtration. This produces large volumes of iron(III)(hydr)oxide sludge with little value. Our research investigates the novel concept of anaerobic iron(II) recovery from groundwater as the valuable mineral vivianite (Fe₃(PO₄)₂ • 8 H₂O) by the addition of phosphate to the water. We found that vivianite precipitated both in synthetic and natural groundwater when the saturation index (SI) was higher than 4. The SI can be increased by elevating the pH, which allows for iron removal at lower concentrations. Anaerobic iron removal reached 93.7% in natural groundwater, which increased further to 99.9% after a subsequent aeration step. Vivianite precipitation followed second order kinetics with a rate constant of 2.3 M^-1s^-1 and the sludge volume decreased by two third compared to iron oxidation. We therefore conclude that anaerobic iron removal is a promising new approach towards sustainable groundwater treatment.