Struvite crystallization under a marine/brackish aquaculture condition

Influence of Ultrafine Fly Ash and Slag Powder on Microstructure and Properties of Magnesium Potassium Phosphate Cement Paste

This study investigated the influences of ultrafine fly ash (UFA) and ultrafine slag powder (USL) on the compressive strengths, autogenous shrinkage, phase assemblage, and microstructure of magnesium potassium phosphate cement (MKPC). The findings indicate that the aluminosilicate fractions present in both ultrafine fly ash and ultrafine slag participate in the acid-base reaction of the MKPC system, resulting in a preferential formation of irregularly crystalline struvite-K incorporating Al and Si elements or amorphous aluminosilicate phosphate products. UFA addition mitigates early age autogenous shrinkage in MKPC-based materials, whereas USL exacerbates this shrinkage. In terms of the sustained mechanical strength development of the MKPC system, ultrafine fly ash is preferred over ultrafine slag powder. MKPC pastes with ultrafine fly ash show greater compressive strength compared to those with ultrafine slag powder at 180 days due to denser interfaces between the ultrafine fly ash particles and hydration products like struvite-K. The incorporation of 30 wt% ultrafine fly ash enhances compressive strengths across all testing ages.

Characteristics and Mechanisms of Phosphorous Adsorption by Peanut Shell-Derived Biochar Modified with Magnesium Chloride by Ultrasonic-Assisted Impregnation

Recovery of phosphate (P) from sludge, domestic wastewater, and industrial wastewater is beneficial for overcoming the problem of shortage of P rock resources. In this study, Mg-functionalized peanut shell-derived biochar was prepared by ultrasound-assisted impregnation. The obtained Mg-laden biochar had a higher content of Mg, a larger specific surface area, and more porosity. The prepared Mg-modified biochar exhibited excellent adsorption properties of phosphorus. Modified biochar has a higher amount of adsorbed P than raw biochar. The capacity of P adsorption by modified biochar was 30.48-114.24% higher than that by raw biochar. Moreover, the Mg-laden biochar can be applied in a wide working environment (pH: 2-10; temperature range: 15-40 °C). This study not only develops a new strategy for the preparation of high-capacity P adsorbents but also provides a new green use for agricultural peanut shells.

Investigation and Optimisation of the Rheological Properties of Magnesium Potassium Phosphate Cement with Response Surface Methodology

Magnesium phosphate cement (MPC) is a promising alternative cement. However, the rheological property of this new binder is still to be explored. In this study, Response Surface Methodology (RSM) was adopted with Central Composite Design (CCD) to establish mathematical models describing the rheological characteristics of MPC in terms of initial mini slump (Y₁), mini-slump loss (Y₂), yield stress (Y₃) and plastic viscosity (Y₄), as a function of three independent variables, namely, water-to-solid ratio (W/S ratio, X₁), MgO to MKP ratio (M/P ratio, X₂) and borax dosage (X₃). The results show that the M/P ratio and borax dosage could significantly affect the yield stress and mini-slump loss of MPC, while the W/S ratio was the significant coefficient influencing plastic viscosity and initial mini slump. The numerical optimised values of X₁, X₂ and X₃ were 0.280, 7.528 and 0.170, respectively, and an MPC paste with desirable rheological characteristics (Y₁ 161.858 mm, Y₂ 11.282, Y₃ 0.680 Pa, Y₄ 0.263 Pa·s) with the highest desirability of 0.867 can be obtained.

A clean process for phosphorus recovery and gallium enrichment from phosphorus flue dust by sodium carbonate roasting

Phosphorus flue dust (PFD) is a solid waste product from phosphorus (P) production that contains P and is enriched with gallium (Ga). The recovery of these valuable components not only protects the environment, but also reduces resource waste. This study aimed to develop a green and efficient method to recover P and enriched Ga from PFD. The effects of different parameters on the P leaching rate and Ga loss rate during Na₂CO₃ roasting and water leaching were investigated and optimized. The reaction mechanisms during the experiment were characterized, revealing that the P-containing compounds in PFD mainly transformed into water-soluble Na₃PO₄. Furthermore, the leaching rate of P reached 85.38%, while Ga was mainly concentrated in the residue and its loss rate was only about 1%. Ga content in the residue reached about 0.1%. An attempt was made to recover Na⁺ and PO₄^3- from the aqueous solution by evaporative crystallization and XRD analysis showed that the main phase of the crystallization product was Na₂HPO₄. The proposed process is technically simple, only Na₂CO₃ is added and no hazardous substances are generated, and represents a new method for recovering P and enriching Ga from PFD.

Dynamic and equilibrium precipitation of struvite from the concentrated cellulosic ethanol stillage

The phosphate rock mineral is the main source of P-fertilizer production. It is estimated to become depleted in next century. Thus, the recovery of phosphorus from waste streams has attracted great interest. The cellulosic ethanol production is seen as more and more important in future. During the production of cellulosic ethanol, the phosphorus element is released from lignocellulosic biomasses and ends up dissolved as phosphate ions in the stillage stream. In this study, the struvite (MgNH₄PO₄ · 6 H₂O) recovery from the concentrated cellulosic ethanol stillage (ES) was conducted under room conditions with an initial pH at 7-9. The effect of Mg²⁺, PO₄^3-, NH₄⁺ and Ca²⁺ during struvite precipitation tests was investigated. The optimized pH value for struvite recovery is estimated at 8.5, by which 85% of PO₄^3- and 46% of Mg²⁺ are removed from the liquid stream. The mass fraction of struvite in recovered crystal sample reaches 82 wt.%. The economic evaluation of struvite recovery from ES was also investigated. This work proves that the struvite is potentially to be recovered with high purity from the concentrated cellulosic ethanol stillage.

The design of multi-stage open-loop hollow fiber membrane contactor and its application in ammonia capture from hydrolyzed human urine

Hollow fiber membrane contactor (HFMC) is a promising technology for removing or recovering wastewaters' volatile components. Developing a rational design method is very important for guiding its further application. In this study, we proposed a method to design the multi-stage open-loop hollow fiber membrane contactor (HFMC) employing shell-side influent. In addition, a three-stage HFMC was designed to capture ammonia from real hydrolyzed human urine. A continuous 1344 h performance was conducted. The results showed that the experimental effluent total ammonium nitrogen (TAN) concentration and ammonia mass transfer coefficient matched the predicted results well, which indicated that the design method was feasible and accurate. The three-stage HFMC showed excellent ammonia capture capacity with a TAN recovery efficiency of 93.29%, and the final effluent TAN concentration was 30.98±14.70 mg/L which met our design requirement (lower than 50 mg/L). More than 98.92% of the inorganic ions and 96.85% of the organic matter were retained in the effluent. The stripping solution after ammonia capture was the high-purity ammonium sulfate solution with low concentration of small molecular weight hydrophilic organic substances. The inorganic and organic membrane fouling was mild and randomly distributed. The inorganic membrane fouling was attributed to the deposition of calcium-, magnesium-, phosphate-related inorganic compounds, while the organic membrane fouling was mainly protein and carbohydrate. After the ammonia capture process, the surface hydrophobicity and pore properties of the membranes had no significant changes. These results demonstrated that the multi-stage open-loop HFMC could be a potential alternative for ammonia recovery from the high concentration of ammonium nitrogen wastewater.

Crystal seed-enhanced ammonia nitrogen and phosphate recovery from landfill leachate using struvite precipitation technique

Nitrogen and phosphorous are limiting and crucial elements for all living organisms. The recovery of nitrogen and phosphorous as struvite gained attention due to its ecofriendly fertilizer application. In the present study, feasible recovery of NH₄⁺ -N available in the landfill leachate with addition of economically viable waste resources like sewage sludge and Mg²⁺ source as struvite is investigated. However, the fertilizer application of struvite depends upon its purity, which in turn is influenced by pH, molar ratio, and presence of other ions. Laboratory scale studies are conducted to find optimum pH and molar ratio. The results of the studies demonstrated the optimum pH being 9.5 along with PO₄^3- -P: Mg²⁺: NH₄⁺ -N molar ratio of 1:1.3:1 is the best condition for struvite formation. To further augment the struvite precipitation kaolinite seed is added to the solution and optimized seed dose is 20 g/L. Existence of Ca²⁺ and Na⁺ ions in the solution exhibits a negative impact on struvite precipitation. Response surface methodology is employed to understand the interactive influence of parameters on recovery efficiency. The recovered precipitate consists of 82.5% struvite with PO₄^3- -P: Mg²⁺: NH₄⁺ -N ratio 1:1.1:0.9. Also, bioavailability of PO₄^3- -P in the recovered precipitate is 89.3%; this signifies high performance of precipitate as fertilizer. Economic assessment highlights that the struvite production is profitable and the profit gained is 159.5$/m³.

Phosphorous recovery from a novel recirculating aquaculture system followed by its sustainable reuse as a fertilizer

Phosphorus (P) is an essential element for life that is introduced through feed in modern aquaculture-the fastest growing food production sector. P can also be a source of environmental contamination and eutrophication if mistreated. Fish assimilate only 20-40% of the applied P; the rest is released into the water. The goals of this research were to study the fate of P in a novel intensive near-zero discharge (<1%) recirculating aquaculture system (RAS). We also tested means to recover and reuse the removed P. Water, sludge and the microbial communities in the different treatment units of the system were analyzed. The treated sludge was tested as a potential substitute for P fertilization in a planter experiment. Of the applied P, 29.5% was recovered by fish, 69.8% was found in the fish sludge and 3.8% was released into the water as soluble reactive P. The P concentration in the fish tank remained stable, likely due to its uptake by denitrifying polyphosphate-accumulating organisms and its precipitation in the RAS's anaerobic reactor. Thus, only 1.5% of the applied P was discharged as effluent, and 69% recovered. The dominant minerals were from the apatite group, followed by the struvite family. Differences in mineral abundance between thermodynamic prediction and actual findings were most probably due to biomineralization by bacteria. Similar plant biomass was recorded for the commercial and digested-sludge fertilization treatments. Biological P removal and recovery from RAS was successfully studied and demonstrated.

Struvite crystallization by using raw seawater: Improving economics and environmental footprint while maintaining phosphorus recovery and product quality

Seawater, as an alternative magnesium source, has the potential to improve the overall economics and environmental footprint of struvite production compared to the use of pure magnesium salts. However, the dilution effect and the presence of other ions in seawater can reduce the phosphorus recovery potential and the simultaneous precipitation of other compounds may reduce the quality of the produced struvite. This work presents a comparative study of seawater and MgCl₂ by performing a series of thermodynamic equilibrium modeling and crystallization experiments. The results revealed that acceptable phosphorus recovery (80-90%) is achievable by using seawater as the magnesium source for struvite precipitation. Further, the simultaneous precipitation of calcium phosphates was successfully controlled and minimized by optimum selection of reaction pH and seawater volume (i.e. Mg:P and Mg:Ca molar ratios). The increase of temperature from 20 °C to 30 °C reduced the phosphorus recovery by 15-20% while it increased the particle size by 30-35%. The presence of suspended solids in reject water did not have significant effects on phosphorus recovery but it made the struvite separation difficult as the obtained struvite was mixed with suspended solids. The experimental results and economic evaluation showed that the use of seawater can reduce the chemical costs (30-50%) and the CO₂-footprint (8-40%) of struvite production. It was concluded that seawater is a potential alternative to pure magnesium sources in struvite production, while studies in larger scale and continuous mode are needed for further verification before full-scale applications.

A comprehensive review of phosphorus recovery from wastewater by crystallization processes

The presence of phosphorus (P) in discharged wastewater can lead to water pollution events and eutrophication. Given the increasing consumption of phosphate (PO₄^3-) rocks, wastewater containing large quantities of P is deemed as a potential source of P recovery. Crystallization of P is an ideal way to recover P because of its simple design, ease of operation, high efficiency, and limited environmental impact. This paper provides a comprehensive review of P recovery by crystallization processes with respect to the mechanisms involved, operational parameters that influence the quality of the crystal, and available seed materials for inducing crystallization. Various operational parameters including pH, molar ratio of participating ions, mixing intensity, reactor type, and seeding conditions, were detailedly investigated. Different kinds of seeds were reviewed critically with regard to their principal properties, application, and long-term prospects. Crystallized products with a high P content can be used directly as slow-release fertilizers for agricultural production, and some test methods have been developed to determine their efficiency as a fertilizer and to evaluate their availability for plants. Further, the feasibility of P recovery by crystallization was evaluated in terms of economic benefits and environmental sustainability. This work serves as a basis for future research of P recovery by crystallization processes and responses to the increasingly stringent problems of eutrophication and the growing depletion of P resources.

Inhibition of ammonia on anaerobic digestion of synthetic coal gasification wastewater and recovery using struvite precipitation

Coal gasification wastewater (CGW) contains very high concentrations of phenols and ammonia. However, the potential impact of ammonia on the anaerobic digestion of phenols remained unclear. Firstly, the methanogens and phenols degraders had a good tolerance up to 1gL^-1 of total ammonia nitrogen (TAN), but the substrate utilization rate for phenol, and specific methanogenic activity of sludge were decreased by 89% and 67% at 5g TAN L^-1, and 94% and 100% at 10g TAN L^-1, respectively. Secondly, the optimum struvite crystallization conditions (pH=8.5, 10g TAN L^-1, n(Mg²⁺):n(TAN):n(PO₄^3-)=1:1:1) were obtained by the orthogonal tests. Thirdly, the removal efficiencies of chemical oxygen demand (COD) and phenols were recovered to around 82% and 66%, respectively in the upflow anaerobic sludge blanket reactor using the pretreatment of struvite precipitation at 10g TAN L^-1 and 1g phenols L^-1. Therefore, anaerobic digestion coupled with struvite precipitation was considered as an alternative way for CGW treatment.

Recovery of phosphorus from synthetic wastewaters by struvite crystallization in a fluidized-bed reactor: Effects of pH, phosphate concentration and coexisting ions

The crystallization of struvite in fluidized-bed crystallizer (FBC) was performed to treat synthetic wastewaters that contain phosphorous. Under optimal conditions (pH 9.5, molar ratio Mg/N/P = 1.3/4/1, struvite seed dose (53-297 μm) = 30 g L^-1, total flow rate = 12 ml min^-1, reflux = 120 ml min^-1), the removal of phosphate (PR) and the crystallization ratio (CR) were 95.8% and 93.5%, respectively. Based on a thermodynamic prediction, the supersaturation, which was obtained from the difference between the theoretical solubility and phosphate concentration, predominated the crystallization efficiency and the properties of the struvite pellets, such as their morphology, particle size and apparent density. Coexisting ions NO₃^- (80, 160 ppm), CH₂COOH^- (260, 520 ppm), F^- (650, 1300 ppm) and SO₄^2- (650, 1300 ppm), were utilized to prepare P-containing wastewaters. Of these ions, SO₄^2- (1300 ppm) remarkably reduced the capability of FBC to remove phosphate from solution. In the presence of NO₃^- and CH₃COO^- (for synthesizing TFT-LCD wastewater), and F^- and SO₄^2- (for synthesizing semiconductor wastewater), CR% was lower than in pure water, although the ultimate PR% did not differ significantly.

Clarification of phosphorus fractions and phosphorus release enhancement mechanism related to pH during waste activated sludge treatment

This study aimed to clarify phosphorus (P) fractions in waste activated sludge (WAS) and explore release performance and enhancement mechanism of different P species related to pH. Results showed that inorganic P (IP) was the major P fraction in raw sludge (87.86% of total solid P), and non-apatite inorganic P (NAIP), the most labile P forms, occupied 81.30% of IP, suggesting that WAS could be selected as potential substitution of phosphate rock. The optimized acid and alkaline conditions were pH=4 and pH=12 for molybdate reactive P accumulation, increased by 311.20mg/L and 479.18mg/L compared to raw sludge, which were 3.80 and 5.84 times higher than that of control, respectively. The mechanism study demonstrated that high pH promoted NAIP release, and apatite P was sensitive to low pH. Moreover, the releasable and recoverable P depended on both fractions of different P species in sludge and pH adjustment for sludge treatment.