An integrated process for struvite electrochemical precipitation and ammonia oxidation of sludge alkaline hydrolysis supernatant

Phosphorus recovery through struvite crystallization from real wastewater: Bridging gaps from lab to market

Food security will be jeopardized as reserves of the non-renewable phosphate rock, which is used to manufacture phosphorus fertilizers, are rapidly depleting. Meanwhile, excess nutrient (phosphorus and nitrogen) loads coming from improperly treated nutrient-rich wastewaters are disposed in soil and water resources, and significantly affect the atmosphere, lithosphere, hydrosphere and biosphere. Struvite crystallization (SC) offers a promising solution, recovering nutrients as a slow-release fertilizer. However, SC's application across various wastewater types remains limited, with few full-scale implementations. This review examines SC process, focusing on studies using real wastewaters. Key SC methodologies include chemical, electrochemical, ion-exchange precipitation, membrane crystallization, and microbial mineralization. Wastewater pretreatment methods are classified and critically evaluated, while SC-affecting factors like calcium and salinity are discussed. To enhance SC's economic sustainability, future research should prioritize cost-efficient reactor designs, energy recovery strategies, and regulatory compliance to develop market-ready, waste-derived fertilizers that are safe for agricultural applications.

Nitrogen-removed organic matters from cyanobacterial decomposition promote the release of nitrogen from lake sediment

Cyanobacterial blooms, which carry a lot of nitrogen (N) and phosphorus (P), have emerged as one of the most severe environmental issues in freshwater ecosystems. However, there are few studies on the effect of organic matters released during cyanobacterial decomposition in promoting N release from lake sediments that remain underexplored. An essential step is to eliminate the impact of the N contributions from cyanobacteria when evaluating sedimentary N release. The response surface methodology (RSM) was developed to optimize the struvite precipitation model, and the results indicated that 1.3 of Mg/N, 1.0 of P/N, and pH 9.5 were the optimum conditions for N removal from cyanobacterial pyrolysis liquid. Following this, calcium phosphate crystallization (at pH 10 and Ca/P = 4.98) removed residual P, and zeolite adsorption (at pH 8 and 10 g/L zeolite dosage) eliminated the remaining N. Ultimately, 99.3% of N was removed with the two methods in cyanobacterial pyrolysis liquid. The cyanobacterial pyrolysis liquid, stripped of N, was found to significantly enhance the release of N from lake sediment under anaerobic conditions, which can then be reutilized by cyanobacteria. These findings reveal that organic matter derived from cyanobacterial decomposition promotes sedimentary N release, creating a feedback loop that sustains cyanobacterial blooms in freshwater ecosystems.

Simultaneous removal of ammonium, phosphate, and phenol via self-assembled biochar composites CBCZrOFe3O4 and its utilization as soil acidity amelioration

ABSTRACTHigh concentrations of ammonium, phosphate, and phenol are recognized as water pollutants that contribute to the degradation of soil acidity. In contrast, small quantities of these nutrients are essential for soil nutrient cycling and plant growth. Here, we reported composite materials comprising biochar, chitosan, ZrO, and Fe3O4, which were employed to mitigate ammonium, phosphate, and phenol contamination in water and to lessen soil acidity. Batch adsorption experiments were conducted to assess the efficacy of the adsorbents. Initially, comparative studies on the simultaneous removal of NH4, PO4, and phenol using CB (biochar), CBC (biochar + chitosan), CBCZrO (biochar + chitosan + ZrO), and CBCZrOFe3O4 (biochar + chitosan + ZrO + Fe3O4) were conducted. The results discovered that CBCZrOFe3O4 exhibited the highest removal percentage among the adsorbents (P < 0.05). Adsorption data for CBCZrOFe3O4 were well fitted to the second-order kinetic and Freundlich isotherm models, with maximum adsorption capacities of 112.65 mg/g for NH4, 94.68 mg/g for PO4 and 112.63 mg/g for phenol. Subsequently, the effect of CBCZrOFe3O4-loaded NH4, PO4, and phenol (CBCZrOFe3O4-APP) on soil acidity was studied over a 60-day incubation period. The findings showed no significant changes (P < 0.05) in soil exchangeable acidity, H+, Mg, K, and Na. However, there was a substantial increase in the soil pH, EC, available P, CEC, N-NH4, and N-NO3. A significant reduction was also observed in the available soil exchangeable Al and Fe (P < 0.05). This technique demonstrated multi-functionality in remediating water pollutants and enhancing soil acidity.

Application of physicochemical techniques to the removal of ammonia nitrogen from water: a systematic review

Ammonia nitrogen is a common pollutant in water and soil, known for its biological toxicity and complex removal process. Traditional biological methods for removing ammonia nitrogen are often inefficient, especially under varying temperature conditions. This study reviews physicochemical techniques for the treatment and recovery of ammonia nitrogen from water. Key methods analyzed include ion exchange, adsorption, membrane separation, struvite precipitation, and advanced oxidation processes (AOPs). Findings indicate that these methods not only remove ammonia nitrogen but also allow for nitrogen recovery. Ion exchange, adsorption, and membrane separation are effective in separating ammonia nitrogen, while AOPs generate reactive species for efficient degradation. Struvite precipitation offers dual benefits of removal and resource recovery. Despite their advantages, these methods face challenges such as secondary pollution and high energy consumption. This paper highlights the development principles, current challenges, and future prospects of physicochemical techniques, emphasizing the need for integrated approaches to enhance ammonia nitrogen removal efficiency.

Optimization and kinetic modeling of phosphate recovery as struvite by electrocoagulation from source-separated urine

Phosphorus recovery is indispensable due to the rapid depletion of its natural reserves and excessive utility in agriculture. Though human urine has high nutrient content including phosphate, nitrogen and potassium; direct use as a fertilizer is restricted due to hygienic, environmental, social and ethical issues. To overcome these limitations, the nutrients are precipitated by the external addition of magnesium (Mg) to form a slow-releasing fertilizer called struvite. The present study aims to maximize phosphate recovery through optimizing struvite production by an emerging electrocoagulation technique. A maximum of 95% phosphate recovery was achieved using inter-electrode distance of 0.5 cm, 2 A current from undiluted urine using Mg-Mg electrodes in a reaction time of 30 min. Further, kinetic modeling of phosphate recovery through electrocoagulation was conducted to comprehend the intended mechanism through the order of kinetics. The results revealed that the data best correlated with first-order kinetics with a correlation coefficient of 0.95. Electrocoagulation improved the supernatant quality by reducing the ion concentrations other than phosphate (30-50%), salinity (40-45%), and microbial population (99%). Qualitative assessment of the precipitate through sophisticated analysis further confirmed the presence of struvite crystals.

Production of struvite by magnesium anode constant voltage electrolytic crystallisation from anaerobically digested chicken manure slurry

Nitrogen and phosphorus levels in livestock manure and digestive fluid are high, posing a threat to soil and water quality and necessitating nutrient removal and recovery. Phosphorus recovery has the potential to alleviate the global phosphorus resource crisis. This study proposed a magnesium anode constant voltage electrolysis method to crystallise struvite (magnesium ammonium phosphate hexahydrate, MgNH₄PO₄·6H₂O) from anaerobically digested chicken manure slurry using reaction kinetics at variable constant voltages ranging from 2 V to 12 V. The recovery of nitrogen and phosphorus was shown to be effective over a wide initial pH range (3.00 ± 0.03-7.90 ± 0.10) using synthetic digestion fluids. Moreover, the pH gradually increased during the reaction without any external chemical adjustments. The phosphorus recovery rates conformed to the first-order kinetic model, with a maximum rate constant of 2.13 h^-1. When the best voltage of 2 V was used at 25 ± 1 °C, the recovery rate reached 5.24 mg P h^-1cm^-2 in the synthetic digestion fluids during 90 min and 4.60 mg P h^-1cm^-2 in the anaerobically digested chicken manure slurry. The crystalline products recovered were identified as high-purity struvite by XRD and XPS. The purity of recovered struvite with an initial pH of 3.00 and 7.90 was 96.5% and 98.9%, respectively. These results demonstrated that the magnesium electrode could rapidly react with nitrogen and phosphorus to generate high-purity struvite.

Phosphate recovery and simultaneous nitrogen removal from urine by electrochemically induced struvite precipitation

The direct discharge of urine into water bodies leads to environmental pollution, and an increase in the water treatment cost, whereas recycling of the nutrients in urine is of significant economic value. A single-compartment reactor was investigated for the recycling of phosphate and simultaneous removal of nitrogen from urine wastewater by electrochemical magnesium induction, and electrochemical oxidation for the removal of residual nitrogen from the supernatant. The results demonstrated that phosphate recovery capacity was greater than 11 mg P cm^-2 h^-1 at a current density of 15 m A cm^-2 and anodizing time of 20 min; the removal rates of ammonium and total nitrogen in the synchronous electrochemical oxidation were 80% and 75%, respectively, at a current density of 45 m A cm^-2 and anodizing time of 60 min. The anodizing time and initial pH were determined to be critical control factors in the electrochemical struvite induction and nitrogen electrochemical oxidation. The on-site electrochemical nitrogen oxidation could rapidly utilize the alkaline supernatant following phosphate recovery. Thus, the integration of the single-compartment reactor, electrochemical magnesium dosage, and simultaneous nitrogen electrochemical oxidation demonstrates potential for application to decentralized reactors to treat source-separated urine.

Electroless Production of Fertilizer (Struvite) and Hydrogen from Synthetic Agricultural Wastewaters

The drive toward sustainable phosphorus (P) recovery from agricultural and municipal wastewater streams has intensified. However, combining P recovery with energy conservation is perhaps one of the greatest challenges of this century. In this study, we report for the first time the simultaneous electroless production of struvite and dihydrogen from aqueous ammonium dihydrogen phosphate (NH₄H₂PO₄) solutions in contact with either a pure magnesium (Mg) or a Mg alloy as the anode and 316 stainless steel (SS) as the cathode placed in a bench-scale electrochemical reactor. During the electroless process (i.e., in the absence of external electrical power), the open circuit potential (OCP), the formation of struvite on the anode, and the generation of dihydrogen at the cathode were monitored. We found that struvite is formed, and that struvite crystal structure/morphology and precipitate film thickness are affected by the concentration of the H_nPO₄^n-3/NH₄⁺ in solution and the composition of the anode. The pure Mg anode produced a porous 0.6-4.1 μm thick film, while the AZ31 Mg alloy produced a more compact 1.7-9.9 μm thick struvite film. Kinetic analyses revealed that Mg dissolution to Mg²⁺ followed mostly a zero-order kinetic rate law for both Mg anode materials, and the rate constants (k) depended upon the struvite layer morphology. Fourier-transform infrared spectrometry, X-ray diffraction, and scanning electron microscopy indicated that the synthesized struvite was of high quality. The dihydrogen and Mg²⁺ in solution were detected by a gas chromatography-thermal conductivity detector and ion chromatography, respectively. Furthermore, we fully demonstrate that the reactor was able to remove ∼73% of the H_nPO₄^n-3 present in a natural poultry wastewater as mainly struvite. This study highlights the feasibility of simultaneously producing struvite and dihydrogen from wastewater effluents with no energy input in a green and sustainable approach.