Selection of cost-effective magnesium sources for fluidized struvite crystallization

Mg²⁺ and magnesium ammonium phosphate (MAP)-induced anammox granulation for comparable nitrogen removal: Implementation pathways and microbial mechanisms

Anaerobic ammonium oxidation (anammox) process is a highly effective and economic technology for nitrogen removal from wastewater. However, the slow growth of anammox bacteria and sludge flotation often hinder its field application. Ion adsorption and crystal precipitation can potentially promote the sludge granulation and hence address the above issues. This study investigated two approaches to support anammox granulation through Mg2+ adsorption and magnesium ammonium phosphate (MAP) precipitation. Mg2+ addition improved the specific anammox activity (SAA) by 4.09 to 4.75-fold compared to MAP-mediated ones, which could be explained by the upregulations of nitrogen and inorganic carbon metabolisms. The active extracellular polymeric substances generation at metabolites level may also favor the granulation in Mg2+-mediated anammox. However, sludge loss halted the continuous size increase of sludge. Differently, MAP promoted granulation by physically increasing the granular density, which allowed for a greater retention of sludge within the reactor. However, the co-growth of MAP precipitates with anammox may lead to mass transfer limitations, resulting in down-regulated gene expressions and metabolites in inorganic carbon metabolism, which negatively impacted the SAA. Overall, both strategies achieved comparable nitrogen removal capacities. Nevertheless, the co-growth of MAP and anammox was promising for effectively mitigating sludge flotation. Our study provided strategies and omics-based evidences for anammox granulation and activity variations, benefiting anammox practical applications.

Co-precipitation of Cd with struvite during phosphorus recovery

During the struvite recovery process, Cd, a hazardous metal commonly found in waste streams, can be sequestered by struvite. This study investigated the influence of Cd2+ on the precipitation of struvite. Quantitative X-ray diffraction (QXRD) results showed that the purity of struvite decreased from 99.1% to 73.6% as Cd concentration increased from 1 to 500 μM. Scanning electron microscopy (SEM) revealed a roughened surface of struvite, and X-ray photoelectron spectroscopy (XPS) analysis indicated that the peak area ratio of Cd-OH increased from 19.4% to 51.3%, while the area ratio of Cd-PO4 decreased from 86.6% to 48.7% as Cd concentrations increased from 10 to 500 μM. The findings suggested that Cd2+ disrupted the crystal growth of struvite, and mainly combined with -OH and -PO4 to form amorphous Cd-bearing compounds co-precipitated with struvite. Additionally, Mg-containing amorphous phases were formed by incorporating Mg2+ with -OH and -PO4 during struvite formation.

Optimization of a dual-chamber electrolytic reactor with a magnesium anode and characterization of struvite produced from synthetic wastewater

Diminishing phosphorus resources worldwide requires developing new technologies to recover phosphorus (P) from wastewaters. A lab-scale electrolytic reactor with a magnesium anode was investigated to remove NH4+ and PO43- from synthetic wastewater by producing struvite. The effects of mixing speed, pH, and applied current on struvite yield, NH4+, and PO43- removal efficiencies were first evaluated using a factorial design. Then, the two most significant parameters were further optimized using Central Composite Design (CCD) coupled with Response Surface Methodology (RSM). The struvite was characterized by SEM, XRD, and FT-IR. A 5.7-fold increase in struvite yield was achieved by increasing the applied current from 0.1 to 0.5 A. The three regression equations generated by the CCD/RSM design with applied current and mixing speed as the two independent parameters were highly correlated with the response variables (struvite yield, NH4+ and PO43- removal efficiencies). The desirability analysis showed the best operating condition: current, 0.5 A and mixing speed, 414 rpm, for the reactor system, under which the optimal struvite yield and NH4+ and PO43- removal efficiencies were 4.75 g/L, 93.0%, and 58.4%, respectively. The SEM, XRD, and FT-IR analyses confirmed the high purity and quality of the struvite produced by the electrolytic reactor system.

A review of struvite crystallization for nutrient source recovery from wastewater

Nutrient recovery from wastewater not only reduces the nutrient load on water resources but also alleviates the environmental problems in aquatic ecosystems, which is a solution to achieve a sustainable society. Besides, struvite crystallization technology is considered a potential nutrient recovery technology because the precipitate obtained can be reused as a slow-release fertilizer. This review presents the basic properties of struvite and the theory of the basic crystallization process. In addition, the possible influencing variables of the struvite crystallization process on the recovery efficiency and product purity are also examined in detail. Then, the advanced auxiliary technologies for facilitating the struvite crystallization process are systematically discussed. Moreover, the economic and environmental benefits of the struvite crystallization process for nutrient recovery are introduced. Finally, the shortcomings and inadequacies of struvite crystallization technology are presented, and future research prospects are provided. This work serves as the foundation for the future use of struvite crystallization technology to recover nutrients in response to the increasingly serious environmental problems and resource depletion.

Applying struvite as a N-fertilizer to mitigate N2O emissions in agriculture: Feasibility and mechanism

Nitrous oxide (N₂O) is an effective ozone-depleting substance and an important greenhouse gas in the atmosphere. Fertilization is a major factor that dictates agricultural N₂O emissions. In this work, as opposed to the commonly-seen highly-soluble nitrogen (N) fertilizers, the feasibility of using struvite as a slow-releasing N-fertilizer and its mechanism for mitigating N₂O emissions were investigated. During the 149-d field cultivation of water spinach (Ipomoea Aquatica Forsk), struvite exhibited comparable crop yields, with a 40.8-58.1% N₂O reduction compared with commercial fertilizers. In addition, struvite fertilization increased soil bacterial diversity and denitrification genes levels (narG, nirS, nirK, norB and nosZ) effectively, but decreased nitrification genes contents (amoA). By conducting partial least-square path modeling, it was found that the use of struvite would satisfy the soil N control and pH regulation, which altered N-cycling related bacteria and ultimately mitigated N₂O emissions. From an economic aspect, using struvite as a N-fertilizer may increase the struvite market price from 50 to 131.7 €/ton. These findings help change the inherent impression that struvite is only suitable as a P-fertilizer, the application of struvite as N-fertilizer could effectively mitigate the agriculture N₂O emission and inspire the application of struvite-based P-recovery technologies.

Impact of Magnesium Sources for Phosphate Recovery and/or Removal from Waste

As the population continues to rise, the demand for resources and environmentally friendly management of produced wastes has shown a significant increase in concern. To decrease the impact of these wastes on the environment, it is important to utilize the wastes in producing and/or recovering usable products to provide for the sustainable management of resources. One non-renewable and rapidly diminishing resource is phosphorus, which is used in several products, the most important being its use in manufacturing chemical fertilizer. With the increase in demand but reduction in availability of naturally occurring mineral phosphorus, it is important to investigate other sources of phosphorus. Phosphorus is most commonly recovered through struvite (magnesium ammonium phosphate) precipitation. The recovery of phosphorus from various wastewater has been well established and documented with recovery rates mostly above 90%. However, one of the major drawbacks of the recovery is the high cost of chemicals needed to precipitate the phosphorus. Since the external magnesium needed to achieve struvite precipitation accounts for around 75% of the total chemical cost, applicability of low-cost magnesium sources, such as bittern or seawater, can help reduce the operational cost significantly. This paper investigates the different magnesium sources that have been used for the recovery of phosphorus, highlighting the different approaches and operating conditions investigated, and their corresponding phosphorus recovery rates. An investigation of the economic aspects of the magnesium sources used for removal/recovery show that costs are dependent on the raw waste treated, the source of magnesium and the location of treatment. A review of published articles on the economics of phosphorus removal/recovery also indicates that there is a lack of studies on the economics of the treatment processes, and there is a need for a comprehensive study on life cycle assessment of such processes that go beyond the technical and economical aspects of treatment processes.

Phosphorus and nitrogen forms in liquid fraction of digestates from agricultural biogas plants

ABSTRACTThe novelty of the presented research is the determination of the nitrogen and phosphorus fraction in the liquid fraction of digestate from agricultural biogas plants. This information is important because it can help in proposing possible further liquid fraction management or developing of new technologies for their purification. The research covered digestates from agricultural biogas plants, obtained from the fermentation of three different groups of waste, i.e. agricultural lignocellulosic waste (where corn silage was a monosubstrate or a dominant co-substrate), food waste (where fruit and vegetable waste or distillery residue were monosubstrate or dominant co-substrate) and livestock manure (where cow manure was a monosubstrate or a dominant co-substrate). Concentrations of nutrients in the liquid fraction of digestates varied within a wide range (230.9-649.1 mg PO₄^3-/L and 1363-3211 mg N/L), and their content was determined by the feedstock characteristic. The highest concentrations of organic phosphorus were found in the liquid fraction of digestates from the fermentation of distillery brew and livestock manure, and the lowest in the fermentation of fruit and vegetable waste. In the liquid fraction of digestates from agricultural biogas plants, regardless of the composition of the feedstock, the dominant nitrogen form was ammonium nitrogen (from 60% to 90% TN). Organic nitrogen was dominated by CON fraction, which was from 35% to 54% of ON. It was 1.3-1.6 times higher than the DON fraction.

Technologies to recover nitrogen from livestock manure - A review

Today, the livestock industry is considered to be one of the biggest emitters of ammonia in the world. The nitrogen present in livestock manure has been linked to the contamination of water bodies. Livestock manures contain a significant quantity of recoverable nitrogen. Recovering nitrogen from livestock manure can minimize negative environmental consequences. This also presents an opportunity to generate some revenue by converting the captured nitrogen to marketable nitrogenous fertilizers. Substantial research efforts have been made toward recovering nitrogen from raw as well as digested livestock manures over the last decade. Many novel technologies as well as ones that have already been implemented to recover nitrogen from municipal wastewaters have been studied for their use in the livestock sector. This paper reviews the common manure nitrogen-recovery technologies reported in the literature, summarizes their efficiencies, discusses their pros and cons, and identifies the areas for future research. Owing to their higher ammonia recovery efficiencies, relatively fewer drawbacks, lower costs, and ability to produce ammonium fertilizers, air stripping by direct aeration, thermal vacuum stripping, and gas-permeable membrane stripping appear to be the most viable choices for livestock farmers. Further studies should focus on the economic feasibility, long-term performance on the manure of varying strengths, and the quality of recovered nitrogenous products.

Second-Generation Phosphorus: Recovery from Wastes towards the Sustainability of Production Chains

Phosphorus (P) is essential for life and has a fundamental role in industry and the world food production system. The present work describes different technologies adopted for what is called the second-generation P recovery framework, that encompass the P obtained from residues and wastes. The second-generation P has a high potential to substitute the first-generation P comprising that originally mined from rock phosphates for agricultural production. Several physical, chemical, and biological processes are available for use in second-generation P recovery. They include both concentrating and recovery technologies: (1) chemical extraction using magnesium and calcium precipitating compounds yielding struvite, newberyite and calcium phosphates; (2) thermal treatments like combustion, hydrothermal carbonization, and pyrolysis; (3) nanofiltration and ion exchange methods; (4) electrochemical processes; and (5) biological processes such as composting, algae uptake, and phosphate accumulating microorganisms (PAOs). However, the best technology to use depends on the characteristic of the waste, the purpose of the process, the cost, and the availability of land. The exhaustion of deposits (economic problem) and the accumulation of P (environmental problem) are the main drivers to incentivize the P’s recovery from various wastes. Besides promoting the resource’s safety, the recovery of P introduces the residues as raw materials, closing the productive systems loop and reducing their environmental damage.

Nitrogen recovery from pig slurry by struvite precipitation using a low-cost magnesium oxide

Ammonia nitrogen management is a recurrent problem in intensive livestock areas. Struvite precipitation stands as a mature technology to recover ammonia nitrogen and prevent associated environmental problems. However, the feasibility of struvite technology to recover ammonia nitrogen from pig manure is limited by the reagents cost. This research aimed to optimise the formulation of a stabilizing agent (SA) synthesised using an industrial low-grade MgO by-product (LG-MgO) and phosphoric acid for efficient TAN recovery via struvite precipitation. Experimental results showed that the H₃PO₄/LG-MgO ratio controls the magnesium phosphate mineral phase of the SA (bobierrite and/or newberyite). Newberyite-rich SA showed the highest TAN removal efficiency from pig manure (66-73%) compared to the SA formed by a mixture of newberyite and bobierrite (51-59%) and by bobierrite (26%). Particle size reduction of LG-MgO did not improve the SA's TAN removal efficiency, although XRD patterns showed that the precipitates from the TAN removal experiments contained some unreacted newberyite. The economic analysis showed that the higher reactivity of the SA formulated using higher H₃PO₄/LG-MgO ratios compensated reagent costs. The SA synthesised with a H₃PO₄/LG-MgO ratio of 0.98 showed the most economical treatment cost, which was estimated at 7.5 € per kg of ammonia nitrogen from pig manure. Finally, the optimum SA was successfully synthesised in a 200-L pilot plant, with a TAN removal capacity only 10% lower than the one synthesised at lab-scale.

Improving Life Cycle Economic and Environmental Sustainability of Animal Manure Management in Marginalized Farming Communities Through Resource Recovery

A growing world population with increasing levels of food consumption will lead to more dairy and swine production and increasing amount of manure that requires treatment. Discharge of excessive nutrients and carbon in untreated animal manure can lead to greenhouse gas emissions and eutrophication concerns, and treatment efforts can be expensive for small scale farmers in marginalized communities. The overall goal of this study was to determine the environmental and economic sustainability of four animal manure management scenarios in Costa Rica: (1) no treatment, (2) biodigesters, (3) biodigesters and struvite precipitation, and (4) biodigesters, struvite precipitation, and lagoons. Life cycle assessment was used to assess the carbon footprint and eutrophication potential, whereas life cycle cost analysis was used to evaluate the equivalent uniform annual worth over the construction and operation and maintenance life stages. Recovery of biogas as a cooking fuel and recovery of nutrients from the struvite reactor reduced the carbon footprint, leading to carbon offsets of up to 2,500 kg CO₂ eq/year. Offsets were primarily due to avoiding methane emissions during energy recovery. Eutrophication potential decreased as resource recovery processes were integrated, primarily due to improved removal of phosphorus in effluent waters. Resource recovery efforts led to equivalent uniform annual benefits of $825 to $1,056/year, which could provide a helpful revenue source for lower-income farmers. This research can provide clarity on how small-scale farmers in marginalized settings can utilize resource recovery technologies to better manage animal manure, while improving economic and environmental sustainability outcomes.

Dissolving the high-cost with acidity: A happy encounter between fluidized struvite crystallization and wastewater from activated carbon manufacture

Production of wood-based activated carbon (WAC) generates large volume of highly acidic and phosphate-rich wastewater. Currently, the routine treatment (i.e. lime precipitation) creates significant secondary pollution, leading to extra economic and environmental burdens. Here, by exploiting the strong acidity of WAC wastewater, we successfully demonstrate fluidized struvite crystallization as a low-cost treatment alternative. Based on a 12 m³/d on-site pilot-scale system, four different fluidized struvite crystallization scenarios are evaluated from technical, economic, and environmental perspectives. The results show that using MgO with MgCl₂ supplement saves 42.8% of the reagent cost when treating phosphate-rich wastewater (i.e. P = 3125.2 mg/L), and also maintains ideal P removal rate and struvite product purity. Meanwhile, the internal circulation mode exhibits higher P recovery (99.2%) than the external mode (55.3%-89.3%), while also demonstrates superior economic and environmental benefit because of less chemical consumption. In addition, the struvite morphology can be turned between pellets with strong crushing strength (external mode) to powder (internal mode). By Life cycle cost (LCC) analysis, we find that, on a treatment scale of 500 m³/d, struvite-based technology saves up to 31.33 million Chinese Yuan (CYN) during a 20-year lifespan, with relative payback period of 2.60 year. The technical, economic, and environmental assessments confirm that the struvite technology is a promising alternative in solving the bottleneck of WAC wastewater treatment.

Effect of sodium alginate on phosphorus recovery by vivianite precipitation

There are good prospects for phosphorus recovery from excess sludge by vivianite crystallization while a large number of extracellular polymeric substances in sludge will have impact on vivianite precipitation. In this study, as a representative of extracellular polymeric substance, the effect of sodium alginate (SA) on phosphorus recovery by vivianite precipitation under different initial SA concentrations (0-800 mg/L), pH values (6.5-9.0) and Fe/P molar ratios (1:1-2.4:1) was investigated using synthetic wastewater. The results showed that SA in low concentrations (≤400 mg/L) had little inhibitory effect on the phosphorus recovery rate. However, when the concentration of SA was larger than 400 mg/L, the phosphorus recovery rate decreased significantly with increasing SA concentrations. The inhibition rate of 800 mg/L SA was about 3 times as large as that of 400 mg/L SA. It was worth noting that the inhibitory effect of SA on vivianite precipitation decreased with increasing initial pH and Fe/P molar ratios. Additionally, SA has no obvious influence on the composition of products, but the morphology of harvested crystals was transformed from branches to plates or rods in uneven sizes.

Performance investigation of struvite high-efficiency precipitation from wastewater using silicon-doped magnesium oxide

In this study, a new adsorbent of silicon-doped magnesium oxide (SMG) was developed for the recovery of nutrients from wastewater. The adsorption conditions including adsorbent dosage, initial solution pH, contact time, coexisting substances, N/P molar ratios, and reaction temperature were investigated. Analysis of field emission scanning electron microscopy-energy dispersive spectrometer (FESEM-DES) and specific surface areas (BET) showed that SMG was a mesoporous adsorbent with S_BET of 108.31 m²/g. The recycled sediment (RS) was identified as almost pure struvite via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The recovery efficiencies of SMG reached 43.25% of ammonia nitrogen and 97.31% of phosphate at dosage of 0.3 g/L, initial solution pH of 7.0, contact time of 20 min, and temperature of 298 K. Under the optimal reaction conditions, the maximum adsorption capacities of SMG were 170.93 mg/g of ammonia nitrogen and 420.89 mg/g of phosphate at N/P molar ratio of 1.5:1. Coexisting humic acid (HA), calcium (Ca²⁺), acetic acid (AA), and ferric ions (Fe³⁺) in nutrient solution hindered the struvite ordered precipitation. The adsorption process followed pseudo-second-order and Elovich kinetic models and was well described by both the Langmuir and Freundlich isotherms at room temperature. All results indicated that the most likely mechanism of nutrients recovery from wastewater was chemical precipitation and proved that SMG was a high-efficiency adsorption material in a wide pH range of 3.0-9.0 for simultaneous recovery of nutrients from wastewater.

Effect of Ca:Mg ratio and high ammoniacal nitrogen on characteristics of struvite precipitated from waste activated sludge digester effluent

This study revealed the relationship between the presence of calcium impurities and ammoniacal nitrogen concentration upon crystallization of struvite. The research hypothesis was that the presence of both calcium and high concentrations of ammoniacal nitrogen (328-1000 mg/L) in waste activated sludge may influence the struvite quality and acid stability. Hence, we studied the impact of Ca:Mg ratio upon morphology, particle size, purity and dissolution of struvite, in the presence of varying levels of excess ammoniacal nitrogen. X-ray diffraction revealed that up to 31.4% amorphous material was made which was assigned to hydroxyapatite. Increasing the ammoniacal nitrogen concentration and elevation of the Mg:Ca ratio maximized the presence of struvite. Struvite particle size was also increased by ammoniacal nitrogen as was twinning of the crystals. Tests with dilute solutions of organic acid revealed the sensitivity of struvite dissolution to the physical characteristics of the struvite. Smaller particles (21.2 μm) dissolved at higher rates than larger particles (35.86 μm). However, struvite dissolved rapidly as the pH was further reduced irrespective of the physical characteristics. Therefore, addition of struvite to low pH soils was not viewed as beneficial in terms of controlled nutrient release. Overall, this study revealed that waste activated sludge effluent with high ammoniacal nitrogen was prospective for synthesis of high quality struvite material.

Mass balance and bacterial characteristics in an in-situ full-scale swine wastewater treatment system occurring anammox process

A spontaneous development of full-scale anaerobic ammonium oxidation (anammox) process was seldom reported, and its operational parameters could supply references in actual applications. This engineered case indicated that anammox process was suitable for treating relatively high-strength ammonium and organics wastewater due to niche differentiation of biofilm. Results of isotope labelling showed that anammox contributed approximately 40% to N-loss in aerobic unit, but this value increased to 78.3% in anoxic tank. Mass balance showed that N-removal via anammox and denitrification pathways were 38.1 and 23.9 g m^-3 d^-1, and anammox rate was 1.6 times higher than denitrifiaction. The wild-type anammox granules had a high purity, with anammox accounting for 92.2%. Candidatus Brocadia was the predominant species. Mixing sludge had a higher oxygen tolerance compared with granules, although the latter had a higher anammox activity under anaerobic conditions. Moreover, physicochemical precipitation on the surface of granules may be related to granulation mechanism.

Bioelectrochemical acidolysis of magnesia to induce struvite crystallization for recovering phosphorus from aqueous solution

A novel struvite crystallization method induced by bioelectrochemical acidolysis of magnesia (MgO) was investigated to recover phosphorus (P) from aqueous solution using a dual-chamber microbial electrolysis cell (DMEC). Magnesium ion (Mg²⁺) in the anolyte was firstly confirmed to automatically migrate from the anode chamber to the cathode chamber, and then react with ammonium (NH₄⁺) and phosphate (PO₄^3-) in the catholyte to form struvite. Recovery efficiency of 17.8%-60.2% was obtained with the various N/P ratios in the catholyte. When MgO (low solubility under alkali conditions) was added into the anolyte, the bioelectrochemical acidolysis of MgO naturally took place and the released Mg²⁺ induced struvite crystallization in the cathode chamber for P recovery likewise. Besides, there was a strong linear positive correlation between the recovery efficiency and the MgO dosage (R² = 0.935), applied voltage (R² = 0.969) and N/P ratio (R² = 0.905). Increasing the applied voltage was found to enhance the P recovery via promoting the MgO acidolysis and the released Mg²⁺ migration, while increasing the N/P ratio in the catholyte enhanced the P recovery via promoting the struvite crystallization. Moreover, the electrochemical performance of the system was promoted due to more stable anolyte pH and lower pH gradient between the two chambers. Current density was promoted by 10%, while the COD removal efficiency was improved from 78.2% to 91.8% in the anode chamber.

An innovative method for synergistic stabilization/solidification of Mn2+, NH4+-N, PO43- and F- in electrolytic manganese residue and phosphogypsum

Electrolytic manganese residue (EMR) contains large quantities of manganese (Mn²⁺) and ammonia nitrogen (NH₄⁺-N). Phosphogypsum (PG) contains plenty of phosphate (PO₄^3-), fluorine (F^-) and some heavy metals. Separate storage of EMR and PG could seriously damage the ecological environment. In this study, synergistic stabilization/solidification (S/S) of EMR and PG was studied. The effects of EMR:PG mass ratio, S/S pH, solid-liquid ratio and temperature on the concentrations of NH₄⁺-N, PO₄^3-, Mn²⁺ and F^- in the leaching solution, and the characteristics of EMR and PG were studied. Meanwhile, the synergistic S/S mechanisms of EMR and PG, and leaching test were investigated. The results showed that the concentrations of F^-, PO₄^3-, NH₄⁺-N and Mn²⁺ in the leaching solution were 4.5 mg/L, 13.6 mg/L, 55.5 mg/L and 0.8 mg/L, respectively, when the mass ratio of EMR to PG was 1:2 and the pH was 9.0 adjusted by MgO after 20 days S/S. Manganese was mainly solidified as Mn₃(PO₄)₂·7H₂O and Mn(OH)₂, and ammonia nitrogen was mainly stabilized as struvite; fluorine was mainly stabilized as (Mn, Ca, Mg)F₂, and phosphate was mainly solidified as (Mn, Ca, Mg)₃(PO₄)₂ and (Mn, Ca, Mg)HPO₄. The leaching test results showed that PO₄^3- and NH₄⁺-N were reduced to 13.6 mg/L and 55.5 mg/L, respectively, and the concentrations of all the measured heavy metals and F^- were within the permitted level for the GB8978-1996 after 20 days S/S.