Production of slow release crystal fertilizer from wastewaters through struvite crystallization – A review

Co-fertilization of Sulfur and Struvite-Phosphorus in a Slow-Release Fertilizer Improves Soybean Cultivation

In face of the alarming world population growth predictions and its threat to food security, the development of sustainable fertilizer alternatives is urgent. Moreover, fertilizer performance should be assessed not only in terms of yield but also in root system development, as it impacts soil fertility and crop productivity. Fertilizers containing a polysulfide matrix (PS) with dispersed struvite (St) were studied for S and P nutrition due to their controlled-release behavior. Soybean cultivation in a closed system with St/PS composites provided superior biomass compared to a reference of triple superphosphate (TSP) with ammonium sulfate (AS), with up to 3 and 10 times higher mass of shoots and roots, respectively. Root system architectural changes may explain these results, with a higher proliferation of second order lateral roots in response to struvite ongoing P delivery. The total root length was between 1,942 and 4,291 cm for plants under St/PS composites and only 982 cm with TSP/AS. While phosphorus uptake efficiency was similar in all fertilized treatments (11-14%), St/PS achieved a 22% sulfur uptake efficiency against only 8% from TSP/AS. Overall, the composites showed great potential as efficient slow-release fertilizers for enhanced soybean productivity.

Recovered phosphorus for a more resilient urban agriculture: Assessment of the fertilizer potential of struvite in hydroponics

Urban agriculture (UA) is a means for cities to become more resilient in terms of food sovereignty while shortening the distance between production and consumption. However, intensive soilless UA still depends on the use of fertilizers, which relies on depleting non-renewable resources such as phosphorous (P) and causes both local and global impact for its production and application. With the aim to reduce such impacts and encourage a more efficient use of nutrients, this study assesses the feasibility of using struvite precipitated from an urban wastewater treatment plant as the unique source of P fertilizer. To do so, we apply various quantities of struvite (ranging from 1 to 20 g/plant) to the substrate of a hydroponic Phaseolus vulgaris crop and determine the yield, water flows and P balances. The results show that treatments with more than 5 g of struvite per plant produced a higher yield (maximum of 181.41 g/plant) than the control (134.6 g/plant) with mineral fertilizer (KPO₄H₂). On the other hand, P concentration in all plant organs was always lower when using struvite than when using chemical fertilizer. Finally, the fact that different amounts of struvite remained undissolved in all treatments denotes the importance to balance between a correct P supply to the plant and a decrease of P lost through the leachates, based on the amount of struvite and the irrigated water. The findings of this study show that it is feasible for UA to efficiently use locally recovered nutrients such as P to produce local food.

From pollutant removal to resource recovery: A bibliometric analysis of municipal wastewater research in Europe

Municipal wastewaters are abundant low-strength streams that require adequate treatment and disposal to ensure public and environmental health. This study aims to provide a comprehensive summary of municipal wastewater research in Europe in the 2010s in the form of bibliometric analysis. The work was based on the Science Citation Index Expanded (Web of Science) and carried out using the R-package bibliometrix for bibliometric data analysis and the software VOSviewer for science mapping. Analysing a dataset of 5645 publications, we identified the most influential journals, countries, authors, institutions, and publications, and mapped the co-authorship and keyword co-occurrence networks. Spain had produced the most publications while Switzerland had the highest average citations per publication. China was the most collaborative country from outside of Europe. Analysis of the most cited articles revealed the popularity of micropollutant removal in European municipal wastewater research. The keyword analysis visualized a paradigm shift from pollutant removal towards resource recovery and circular economy. We found that current challenges of resource recovery from municipal wastewater come from both technical and non-technical (e.g., environmental, economic, and social) aspects. We also discussed future research opportunities that can tackle these challenges.

A review on the incorporation and potential mechanism of heavy metals on the recovered struvite from wastewater

Phosphorus, as a non-renewable element, is flowing out too fast in the past decades. To sustain the development of this globally scarce resource, efficient measures were taken to recover more phosphorus in the struvite form from wastewater. However, heavy metals in the wastewater might produce an inhibitory effect on phosphorus recovery, and even worse, pollutants might be incorporated in/onto the crystals precipitated. Impurities on struvite will reduce the quality of struvite as a potential slow-release fertilizer and affect the safe application of struvite in agriculture. This review aims to identify the trends in the literature to present the residues of heavy metals in struvite. It summarizes the current status in the residues of main metal elements on crystals and its response to wastewater properties, composition, and oxidation state of metals. The adsorption process and potential adsorption mechanism of heavy metals during the struvite crystallization are deeply explored, which might determine the latter release rate of metals when applying into the soil. Possible solutions are further provided to minimize the amounts of heavy metals mainly through adjusting operational conditions or employing pretreatment methods. Finally, this review critically analyzes the limitation gap between theory and actual generalization and potential application of struvite products in the market, and corresponding perspectives in the future are given to safely utilize the phosphorus resource from wastewater in the form of struvite.

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.

Nutrients Recovery from Dairy Wastewater by Struvite Precipitation Combined with Ammonium Sorption on Clinoptilolite

Struvite precipitation from Wastewater involves an excess of ammonium to create a supersaturated initial solution. The remaining fraction can be a threat to the environment. This work combined struvite precipitation and ammonium sorption using natural zeolite to decrease the ammonium level in the effluent. Two approaches of estimation of feed sample doses were used. One consisted of gradient experiments for ammonium precipitation to the asymptotic level and was combined with clinoptilolite to lower the ammonium level in the effluent. This approach used doses of 0.05:1.51:0.61:1 of Ca:Mg:NH₄⁺:PO₄³⁻ mole ratios, respectively. In contrast, three level design with narrowed NH₄⁺:PO₄³⁻ range reached 0.25:1.51:0.8:1 for Ca:Mg:NH₄⁺:PO₄³⁻ mole ratios. The addition of zeolite decreased effluent ammonium concentration. In both ways, the P and N recoveries were higher than 94% and 72%, respectively. The complexity of the precipitation mixture decreased the ammonium sorption capacity (Q_e) of clinoptilolite from Q_e of 0.52 to 0.10 meq∙g⁻¹ in single and complex solutions, respectively. Thermodynamically, the addition of 1.5 % of clinoptilolite changed the struvite precipitation spontaneity from ∆G of −5.87 to −5.42 kJ·mol⁻¹ and from 9.66 to 9.56 kJ·mol⁻¹ for gradient and three level experimental procedures, respectively. Thus, clinoptilolite demonstrated a positive effect on the struvite precipitation process and its environmental impact.

Raw and modified palygorskite in water treatment applications for low-concentration ammonium removal

Raw palygorskite (Pal) samples went under acid (H-Pal), NaCl (Na-Pal), and CaCl₂ treatment (Ca-Pal) in order to be examined as ammonium (NH₄ ⁺ ) sorbents from aqueous solutions. The samples were characterized by XRD and FT-IR techniques to examine potential structural differences after modifications, and batch kinetic experiment series were applied to determine the optimal conditions for NH₄ ⁺ removal. According to thermodynamic analysis, the removal reaction for sodium- and calcium-treated samples was endothermic (ΔΗ⁰  > 0, 1.65 kJ/mol and 24.66 kJ/mol, respectively), in contrast with the exothermic reactions of raw and acidic-treated palygorskite samples (ΔΗ⁰  < 0, -37.18 kJ/mol and -27.56 kJ/mol respectively). Moreover, each sample presented a different order of sorbed ions preference, whereas the strong affinity for Ca²⁺ sorption was common in all cases since the NH₄ ⁺ removal inhibited. Nevertheless, a similar pattern was followed for raw and modified samples at isotherm study, rendering the linear form of Freundlich isotherm to express better the NH₄ ⁺ sorption on palygorskite sample, indicating that it is a heterogeneous procedure. In all cases, the NH₄ ⁺ maximum uptake was within 15 min using 8 g/L of each sorbent, especially for the Na-Pal sample, which could reach almost 100% removal of low concentration NH₄ ⁺ . PRACTITIONER POINTS: Modified palygorskite samples were tested for NH₄ ⁺ removal from aqueous solutions. NaCl-treated palygorskite had the higher removal efficiency, which could reach almost 100% removal of low concentration NH₄ ⁺ . NH₄ ⁺ maximum uptake was within 15 minutes using 8 g/L of each sorbent. NH₄ ⁺ adsorption was an endothermic reaction for NaCl- and CaCl₂ -treated palygorskite sorbents. NH₄ ⁺ adsorption was an exothermic reaction for raw and acid-treated palygorskite sorbents.

Phosphorus recovery from wastewater by struvite in response to initial nutrients concentration and nitrogen/phosphorus molar ratio

Nutrients recovery from wastewater using struvite crystallization has received significant attention because the recovered nutrients can be used as a fertilizer in agriculture. However, the wastewater with different composition of nutrients might exert distinct struvite characteristics. The objective of this study is to explore the influence of nitrogen/phosphorus (N/P) molar ratio and initial nutrient concentrations on the phosphorus (P) recovery from wastewater and the crystallographic properties of struvite through batch experiments. Both the higher nutrients concentrations and N/P molar ratio in the wastewater resulted in higher recovery of P. However, initial nutrients concentration in the wastewater posed a larger effect on the formation of struvite than that of initial N/P ratio. The ratio of N to P consumed during the process varied with varying the N/P ratio or the initial nutrients concentration, indicating the existence of different crystallization mechanisms under different elemental compositions in the wastewater. The decline in solution pH was negatively correlated with the N/P ratio, whereas it was positively correlated with the nutrient concentration. The crystals produced were mainly rough (dendritic) under the N/P ratio of 2, while the morphology evolved from a regular coffin-like to irregular columnar with the increase in N/P ratio from 4 to 10. Meanwhile, the shape of the irregular cylinder gradually changed from elongated needle shape to the thick rods appearance with the increase in the nutrients concentration. Higher N/P ratio increased the nucleation rate, resulting in finer particles and an uneven crystal size distribution, but purer struvite crystals. The purity of struvite was also positively correlated with the initial nutrients concentration. The Visual MINTEQ modelling analysis demonstrated that the supersaturation ratio of struvite gradually increased with the increase in the N/P ratio or nutrients concentration, which led to a positive effect on struvite crystallization.

Recovery and applications of ammoniacal nitrogen from nitrogen-loaded residual streams: A review

Total ammoniacal nitrogen (TAN) is considered to be a pollutant, but is also a versatile resource. This review presents an overview of the TAN recovery potentials from nitrogen (N)-loaded residual streams by discussing the sources, recovery technologies and potential applications. The first section of the review addresses the fate of TAN after its production. The second section describes the identification and categorisation of N-loaded (≥0.5 g L^-1 of reduced N) residual streams based on total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), TAN, and TAN/TKN ratio. Category 1 represents streams with a low TAN/TKN ratio (<0.5) that need conversion of organic-N to TAN prior to TAN recovery, for example by anaerobic digestion (AD). Category 2 represents streams with a high TAN/TKN ratio (≥0.5) and high TSS (>1 g L^-1) that require a decrease of the TSS prior to TAN recovery, whereas category 3 represents streams with a high TAN/TKN ratio (≥0.5) and low TSS (≤1 g L^-1) that are suitable for direct TAN recovery. The third section focuses on the key processes and limitations of AD, which is identified as a suitable technology to increase the TAN/TKN ratio by converting organic-N to TAN. In the fourth section, TAN recovery technologies are evaluated in terms of the feed composition tolerance, the required inputs (energy, chemicals, etc.) and obtained outputs of TAN (chemical form, concentration, etc.). Finally, in the fifth section, the use of recovered TAN for three major potential applications (fertilizer, fuel, and resource for chemical and biochemical processes) is discussed. This review presents an overview of possible TAN recovery strategies based on the available technologies, but the choice of the recovery strategy shall ultimately depend on the product characteristics required by the application. The major challenges identified in this review are the lack of information on enhancing the conversion of organic-N into TAN by AD, the difficulties in comparing the performance and required input of the recovery technologies, and the deficiency of information on the required concentration and quality of the final TAN products for reuse.

Siderophore mediated mineralization of struvite: A novel greener route of sustainable phosphate management

Efficient and sustainable removal of phosphate ions from an aqueous solution is of great challenge. Herein we demonstrated a greener route for phosphate recovery through struvite formation by using bacterial siderophore. This method was efficient for removal of phosphate as low as 1.3 mM with 99% recovery efficiency. The siderophore produced by Pseudomonas taiwanensis R-12-2 act as template for the nucleation of struvite crystals and was found sustainable for recycling the phosphorous efficiently after twenty cycles. The formation of struvite crystals is driven by surrounding pH (9.0) and presence of Mg²⁺ and NH₄⁺ ions along with PO₄^3- and siderophore which was further validated by computational studies. The morphology of struvite was characterized by scanning electron microscopy, followed by elemental analysis. Furthermore, our results revealed that the siderophore plays an important role in struvite biomineralization. We have successfully demonstrated the phosphate sequestration by using industrial waste samples, as possible application for environmental sustainability and phosphate conservation. For the first time electrochemical super-capacitance performance of the struvite was studied. The specific capacitance value for the struvite was found to be 320 F g^-1 at 1.87 A g^-1 and retained 92 % capacitance after 250 cycles. The study revealed the potential implications of siderophore for the phosphate recycling and the new mechanism for biomineralization by sequestering into struvite.

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.

Efficient recovery of phosphorus and sulfur from Anaerobic Membrane Bioreactor (AnMBR) permeate using chemical addition of iron and evaluation of its nutrient availability for plant uptake

Anaerobic membrane bioreactors (AnMBRs) represent an emerging environmental biotechnology platform with the potential to simultaneously recover water, energy, and nutrients from concentrated wastewaters. The removal and beneficial capture of nutrients from AnMBR permeate has yet to be fully explored, therefore this study sought to foster iron phosphate recovery through a tertiary coagulation process, as well as characterize the recovered nutrient product (RNP) and assess its net phosphorus release, diffusion, and availability for plant uptake. One of the primary goals of this study was to optimize the dose of the coagulant, ferric chloride, and coagulant aid, aluminum chlorohydrate (ACH), for continuous application to the coagulation-flocculation-sedimentation (CFS) unit of an AnMBR pilot plant treating municipal wastewater, through controlled bench-scale jar tests. Anaerobic systems present unique challenges for nutrient capture, including high, dissolved hydrogen sulfide concentrations, along with settleability issues. The addition of the coagulant aid increases settleability, while enhancing phosphorus removal by up to 20%, decreasing iron demand. Water quality analysis indicated that a variety of factors affect nutrient capture, including the COD (chemical oxygen demand) concentration of the permeate and the limiting coagulant dose. COD >200 mg/L was shown to decrease the phosphorus removal efficiency by up to 15%. A combination of inductively coupled plasma optical emission spectrometer (ICP-OES) elemental analysis, inductively coupled plasma mass spectrometer (ICP-MS) elemental analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) spectroscopy analysis was used to characterize the P-rich RNP which revealed a 2.58% w/w phosphorus content and the lack of a well-defined crystalline structure. Detailed studies on resin extractable phosphorus to assess the plant uptake potential also demonstrated that iron-based P-rich RNPs may not be an effective fertilizer product, as they can act as a phosphorus sink in some agricultural systems instead of a source.

Potential of yak dung-derived hydrochar as fertilizer: Mechanism and model of controlled release of nitrogen

Improving fertilizer efficiency with assistance of biochar has drawn much attention in sustainable agriculture. Promoting slow-release properties of biochar itself with cost-effective production technology is a pressing demand. In this study, hydrochar derived from nutrition-enriched yak dung (HC) and corresponding controlled release nitrogen fertilizer (HCRNF) via HCl modifying were studied, and the slow release performance as well as mechanisms were investigated. The results show that HCRNF presents a better N controlled-release performance with cumulative N release amounts of 56.01%-70.30% compared with 72.60%-78.45% of HC. The specific surface area reached highest 47.161 m²·g^-1 in HCRNFs with the pore volume of 0.098 cm³·g^-1. Hydrochloric acid modification treatment increases the surface acid group contents such as phenolic hydroxyl group content increasing to 1.48 mmol·g^-1 in HCRNF250. Because the porous structure and stable internal force between N and O-containing functional groups are improved, the N desorption from HCRNF is retarded, which shows a controlled release behavior. We concluded that the HCRNF via HCl modification in this work has a great application potential as slow released N fertilizer in sustainable green agriculture.

Nutrient recovery from swine wastewater at full-scale: An integrated technical, economic and environmental feasibility assessment

In this study, the technical, economic and environmental attributes of a full-scale nutrient recovery process connected to the centralized swine wastewater treatment facility (CSWTF) were evaluated. The performance of the process was assessed by introducing influent to the recovery reactor from different components of the CSWTF such as sedimentation tank (swine wastewater) and biological treatment reactor (biologically oxidized material and supernatant of the biologically oxidized material). The results of technical performance assessment revealed that the O-P recovery (87.1-90.7%) and NH₄-N removal (66.9-72.1%) efficiencies from the influent of biological treatment reactor were significantly higher than the influent from sedimentation tank (81.7 and 19.8%, respectively, p < 0.05). The economic evaluation elucidated that by increasing the treatment capacity of the recovery reactor from 30 m³/d to 100 m³/d, operating expenses could be covered through the commercialization of struvite, while it would take around seven years to get back the capital investment. Additional economic savings could also be possible when using the recovered struvite as a fertilizer raw material along with other environmental benefits. Considering the current farming practices in Korea, the complete recovery of O-P from CSWTFs as struvite could drop the soil phosphorus surplus by 40%, minimize the phosphatic fertilizer consumption by 6.4% and ultimately reduce CO₂ equivalent emissions of 6522 tons/year in comparison to chemical fertilizer production. However, during the continuous operation of the full-scale nutrient recovery process, influent characteristics need to be incessantly monitored and adjusted to the optimum conditions to improve the economics of recovered products. Overall, the nutrient recovery process at full-scale not only solves the problem of treating highly polluted swine wastewater but also helps to ensure societal and environmental sustainability.

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

Environmental tradeoffs in municipal wastewater treatment plant upgrade: a life cycle perspective

Municipal wastewater treatment plants (WWTPs) play an indispensable role in improving environmental water quality in urban areas. Existing WWTPs, however, are an important source of greenhouse gas (GHG) emissions and may not be able to treat increasingly complicated wastewater or meet stringent environmental standards. These WWTPs can be updated to address these challenges, and different technologies are available but with potentially different environmental implications. Life cycle assessment (LCA) is a widely used approach to identify alternatives with lower environmental footprint. In this study, LCA was applied to an actual urban WWTP, considering four scenarios involving upgrading and energy-resource recovery. The environmental performance with respect to life cycle GHG emissions and eutrophication impact was analyzed. The environmental benefits of reduced water pollution and energy and material displacement associated with energy-resource recovery process were also considered. The results showed tradeoffs among the four scenarios. Although upgrading the studied WWTP would meet discharge standard for total phosphorus and reduce total eutrophication impact by about 19%, it would increase GHG emissions by at least 16%. Besides, the energy-resource recovery mode for existing WWTP (S2) performs the best in terms of GHG emissions. For different biogas utilization methods, combined heat and power (CHP) system is superior to the existing method of delivering biogas to gas grid, in terms of energy recovery or reduction of GHG emissions and eutrophication impact. Our research results may provide a reference for plant managers to select the most environmentally friendly upgrade scheme and energy-resource recovery technique for future upgrade projects.

Land application of sewage sludge incinerator ash for phosphorus recovery: A review

Phosphorus (P) is essential for all living things and an integral part of food production. However, significant amounts of P are functionally lost when wastewater byproducts, such as biosolids or sewage sludge incinerator ash (SSA), are not beneficially reused. Around 20% of sewage sludge produced in the US is incinerated and nearly 25% of sewage sludge is incinerated in European Union member countries. SSA contains significant amounts of P (up to 14% total P) and other beneficial elements but is typically sent to landfills for disposal. However, SSA has also been explored as one method of capturing and redirecting P back into the food system. Research investigating SSA characterization, P availability, and contaminant concentrations and behavior in soil is required to understand the effects of SSA land application on soil chemical properties and crop production. Several approaches for recovering P from SSA have been investigated that consider these factors. Ultimately, the opportunity for land application of SSA depends on the individual characteristics of a given SSA, ex. total P and contaminant concentrations, and the requirements and regulations of the region where it is produced and applied. In this review, we address the history of P recovery from SSA and discuss research regarding characterization, contaminants, P availability, and land application of SSA.

Improving the Fertigation of Soilless Urban Vertical Agriculture Through the Combination of Struvite and Rhizobia Inoculation in Phaseolus vulgaris

Soilless crop production is a viable way to promote vertical agriculture in urban areas, but it relies extensively on the use of mineral fertilizer. Thus, the benefits of fresher, local food and avoiding the transportation and packaging associated with food import could be counteracted by an increase in nutrient-rich wastewater, which could contribute to freshwater and marine eutrophication. The present study aimed to explore the use of mineral fertilizer substitutes in soilless agriculture. Phaseolus vulgaris (common bean) was fertilized with a combination of slow-releasing fertilizer struvite (a source of N, P, and Mg), which is a byproduct of wastewater treatment plants, and inoculation with Rhizobium (a N₂-fixing soil bacteria). The experiment included three bean-production lines: (A) 2 g/plant of struvite and rhizobial inoculation; (B) 5 g/plant of struvite and rhizobial inoculation, both irrigated with a Mg-, P-, and N-free nutrient solution; and (C) a control treatment that consisted of irrigation with a full nutrient solution and no inoculation. Plant growth, development, yields, and nutrient contents were determined at 35, 62, and 84 days after transplanting as well as biological N₂ fixation, which was determined using the ¹⁵N natural abundance method. Treatments A and B resulted in lower total yields per plant than the control C treatment (e.g., 59.35 ± 26.4 g plant^-1 for A, 74.2 ± 23.0 g plant^-1 for B, and 147.71 ± 45.3 g plant^-1 for C). For A and B, the nodulation and N₂ fixation capacities appeared to increase with the amount of initially available struvite, but, over time, deficient levels of Mg were reached as well as nearly deficient levels of P, which could explain the lower yields. Nevertheless, we conclude that the combination of struvite and N₂-fixing bacteria covered the N needs of plants throughout the growth cycle. However, further studies are needed to determine the optimal struvite quantities for vertical agriculture systems that can meet the P and Mg requirements throughout the lifetime of the plants.

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.

A novel approach for nutrients recovery from municipal waste as biofertilizers by combining electrodialytic and gas permeable membrane technologies

The recovery of valuable materials from waste fits the principle of circular economy and sustainable use of resources, but contaminants in the waste are still a major obstacle. This works proposes a novel approach to recover high-purity phosphorus (P) and nitrogen (N) from digestate of municipal solid waste based on the combination of two independent membrane processes: electrodialytic (ED) process to extract P, and gas permeable membranes (GPM) for N extraction. A laboratory ED cell was adapted to accommodate a GPM. The length of waste compartment (10 cm; 15 cm), current intensity (50 mA; 75 mA) and operation time (9 days; 12 days) were the variables tested. 81% of P in the waste was successfully extracted to the anolyte when an electric current of 75 mA was applied for 9 days, and 74% of NH₄⁺ was extracted into an acid-trapping solution. The two purified nutrient solutions were subsequently used in the synthesis of a biofertilizer (secondary struvite) through precipitation, achieving an efficiency of 99.5%. The properties of the secondary struvite synthesized using N and P recovered from the waste were similar to secondary struvite formed using synthetic chemicals but the costs were higher due to the need to neutralize the acid-trapping solution, highlighting the need to further tune the process and make it economically more competitive. The high recycling rates of P and N achieved are encouraging and widen the possibility of replacing synthetic fertilizers, manufactured from finite sources, by secondary biofertilizers produced using nutrients extracted from wastes.

Reactive electrically conducting membranes for phosphorus recovery from livestock wastewater effluents

We present a novel 'proof-of-concept' electrochemically based membrane filtration process for the recovery of nitrogen and phosphorus from livestock wastewater following an anaerobic digestion step. Reactive electrically conducting membranes are shown to precipitate and separate struvite, an eco-friendly fertilizer from synthetic livestock wastewater, resulting in the production of a solid fertilizer and a high-quality water stream, fit for irrigation. The recovery process is based on electrochemical hydrolysis and control of local pH in proximity to the surface of the membrane, and therefore, does not require chemical additives for pH adjustment. The system was assessed at varying concentrations of nitrogen and phosphorus corresponding to diluted and concentrated livestock wastewater (up to 1000 mg/L of N and P). Experimental results show up to 65% removal of phosphorus and nitrogen in the first 30 min of electrochemical filtration, and the precipitates were analytically confirmed to be struvite. In addition, the recovery process was shown efficient as it resulted in limited membrane fouling and flux reduction. Fouling and precipitation results were explained by a mathematical model describing the concentration of N, P, Mg ions in the presence of an external electric field. Accordingly, precipitation takes place in proximity to the membrane's surface but not directly on it, thus, limiting surface fouling. The electrochemical filtration system does not require chemical additives for pH adjustment, and the cost associated with electrochemical membrane-based struvite recovery was calculated to be $158 per ton of dry struvite, which is about 1.4 times lower in comparison to conventional recovery approaches. Overall, the electrochemical filtration system may be a promising alternative for nutrient recovery from livestock wastewater in terms of operational costs, recovery efficiency, and fouling mitigation.

Synergy of Phosphate-Controlled Release and Sulfur Oxidation in Novel Polysulfide Composites for Sustainable Fertilization

The development of smart and eco-friendly fertilizers is pivotal to guarantee food security sustainably. Phosphate rock and struvite are promising alternatives for P fertilization; nevertheless, the solubility of these sources is a challenge for consistent use efficiency. Here, we propose using a polysulfide obtained via inverse vulcanization as a novel controlled-release fertilizer matrix in a system containing either Bayóvar rock (Bay) or struvite (Str). The polysulfide provides S for plants after being biologically oxidized to sulfate in soil, generating local acidity for P solubilization. After 15 days of soil incubation, the composites with 75 wt % Str and 75 wt % Bay achieved, respectively, 3 and 2 times the S oxidation from the elemental sulfur reference. Results indicated that P content stimulates the soil microorganisms' activity for S oxidation. The matrix had a physical role in improving Bay dissolution and regulating the rapid release from Str. Moreover, the available P in soil was 25-30 mg/dm³ for Bay composites, while for pure Bay, it was 9 mg/dm³.

Economic Model Predictive Control for optimal struvite recovery

Resource recovery from municipal wastewater has been a prime focus for a decade. Although several recovery processes already exist in the market today, the high cost of material, inherent disturbance in the influent quality, lack of real time monitoring of critical parameters, and lack of a robust automation system may result in suboptimal performance. This work attempts to construct a model based predictive control for optimal operation of a struvite recovery unit in a full scale WRRF. A multi-parameter based predictive control has been developed by implementing an Economic Model Predictive Controller (EMPC) for optimal dosing of magnesium hydroxide in a struvite recovery unit. The EMPC used customized objective function for real-time optimization of performance and economical parameters of the crystallization unit. The effectiveness of the proposed EMPC controller is verified through tests conducted on the Benchmark Simulation Model No. 2 (BSM2d.). The results obtained from the simulator-based evaluation of EMPC demonstrate a significant improvement in resource recovery at reduced operational costs. The economic advantages of implementing an EMPC compared to proportional and constant magnesium dosage has also been enumerated.

Urea-functionalized amorphous calcium phosphate nanofertilizers: optimizing the synthetic strategy towards environmental sustainability and manufacturing costs

Nanosized fertilizers are the new frontier of nanotechnology towards a sustainable agriculture. Here, an efficient N-nanofertilizer is obtained by post-synthetic modification (PSM) of nitrate-doped amorphous calcium phosphate (ACP) nanoparticles (NPs) with urea. The unwasteful PSM protocol leads to N-payloads as large as 8.1 w/w%, is well replicated by using inexpensive technical-grade reagents for cost-effective up-scaling and moderately favours urea release slowdown. Using the PSM approach, the N amount is ca. 3 times larger than that obtained in an equivalent one-pot synthesis where urea and nitrate are jointly added during the NPs preparation. In vivo tests on cucumber plants in hydroponic conditions show that N-doped ACP NPs, with half absolute N-content than in conventional urea treatment, promote the formation of an equivalent amount of root and shoot biomass, without nitrogen depletion. The high nitrogen use efficiency (up to 69%) and a cost-effective preparation method support the sustainable real usage of N-doped ACP as a nanofertilizer.

Development of a Self-Sustaining Wastewater Treatment with Phosphorus Recovery for Small Rural Settlements

Global trends such as climate change and the scarcity of sustainable raw materials require adaptive, more flexible and resource-saving wastewater infrastructures for rural areas. Since 2018, in the community Reinighof, an isolated site in the countryside of Rhineland Palatinate (Germany), an autarkic, decentralized wastewater treatment and phosphorus recovery concept has been developed, implemented and tested. While feces are composted, an easy-to-operate system for producing struvite as a mineral fertilizer was developed and installed to recover phosphorus from urine. The nitrogen-containing supernatant of this process stage is treated in a special soil filter and afterwards discharged to a constructed wetland for grey water treatment, followed by an evaporation pond. To recover more than 90% of the phosphorus contained in the urine, the influence of the magnesium source, the dosing strategy, the molar ratio of Mg:P and the reaction and sedimentation time were investigated. The results show that, with a long reaction time of 1.5 h and a molar ratio of Mg:P above 1.3, constraints concerning magnesium source can be overcome and a stable process can be achieved even under varying boundary conditions. Within the special soil filter, the high ammonium nitrogen concentrations of over 3000 mg/L in the supernatant of the struvite reactor were considerably reduced. In the effluent of the following constructed wetland for grey water treatment, the ammonium-nitrogen concentrations were below 1 mg/L. This resource efficient decentralized wastewater treatment is self-sufficient, produces valuable fertilizer and does not need a centralized wastewater system as back up. It has high potential to be transferred to other rural communities.

Investigation of struvite crystals formed in palm oil mill effluent anaerobic digester

The formation of struvite crystals or magnesium ammonium phosphate (MgNH₄PO₄) in palm oil mill effluent (POME) occurs as early as in the secondary stage of POME treatment system. Its growth continues in the subsequent tertiary treatment which reduces piping diameter, thus affecting POME treatment efficiency. Hypothesis. The beneficial use of the crystal is the motivation. This occurrence is rarely reported in scientific articles despite being a common problem faced by palm oil millers. The aim of this study is to characterize struvite crystals found in an anaerobic digester of a POME treatment facility in terms of their physical and chemical aspects. The compositions, morphology and properties of these crystals were determined via energy dispersive spectroscopy (EDS), elemental analysis, scanning electron microscopy (SEM) and x-ray diffraction (XRD). Solubility tests were carried out to establish solubility curve for struvite from POME. Finally, crystal growth experiment was done applying reaction crystallization method to demonstrate struvite precipitation from POME. Results showed that high phosphorous (P) (24.85 wt%) and magnesium (Mg) (21.33 wt%) content was found in the struvite sample. Elemental analysis detected carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) below 4 wt%. The crystals analysed by XRD in this study were confirmed as struvite with 94.8% struvite mineral detected from its total volume. Having an orthorhombic crystal system, struvite crystals from POME recorded an average density of 1.701 g cm⁻³. Solubility curve of struvite from POME was established with maximum solubility of 275.6 mg L⁻¹ at pH 3 and temperature 40 °C. Minimum solubility of 123.6 mg L⁻¹ was recorded at pH 7 and temperature 25 °C. Crystal growth experiment utilizing POME as the source medium managed to achieve 67% reduction in phosphorous content. This study concluded that there is a potential of harnessing valuable nutrients from POME in the form of struvite. Struvite precipitation technology can be adapted in the management of POME in order to achieve maximum utilization of the nutrients that are still abundant in POME. At the same time maximization of nutrient extractions from POME will also reduce pollutants loading in the final discharge.

Phosphate and Ammonium Removal from Water through Electrochemical and Chemical Precipitation of Struvite

Batch electrocoagulation (BEC), continuous electrocoagulation (CEC), and chemical precipitation (CP) were compared in struvite (MgNH4PO4·6H2O) precipitation from synthetic and authentic water. In synthetic water treatment (SWT), struvite yield was in BEC 1.72, CEC 0.61, and CP 1.54 kg/m3. Corresponding values in authentic water treatment (AWT) were 2.55, 3.04, and 2.47 kg/m3. In SWT, 1 kg struvite costs in BEC, CEC, and CP were 0.55, 0.55, and 0.11 €, respectively, for AWT 0.35, 0.22 and 0.07 €. Phosphate removal in SWT was 93.6, 74.5, and 71.6% in BEC, CEC, and CP, respectively, the corresponding rates in AWT were 89.7, 77.8, and 74.4%. Ammonium removal for SWT in BEC, CEC, and CP were 79.4, 51.5, and 62.5%, respectively, rates in AWT 56.1, 64.1, and 60.9%. Efficiency in CEC and BEC are equal in nutrient recovery in SWT, although energy efficiency was better in CEC. CP is cheaper than BEC and CEC.

New insights into interactions of organic substances in poultry slurry with struvite formation: An overestimated concern?

The high content of organic substances in strength agro-industrial wastewater has been documented to be among the major barriers hampering nutrient recovery efficiency of struvite precipitation. However, our results in this study show that the previously reported negative impacts of organic substances in high-strength agricultural wastewater on struvite precipitation might be overestimated. This study is the first to test the influence of three forms of organic substances from real high-strength wastewater that contains a complex of particulate, colloidal and soluble organic substances, on nutrient recovery efficiency and product quality through struvite precipitation at varying pH conditions. Our results demonstrated that the inhibition of organic substances on struvite formation only happens at the pH levels of <9.0 with recovery reduction of PO₄^3- (5-15%) and NH₄⁺ (6-13%). The inhibitory effect of the organic substances at the optimal pH range (9.5-10) reported from the literature review is only ≤5%. Moreover, the transformation in the contents of humic- and protein-like substances with an increment in pH was characterized and may contribute to mitigate the inhibition of nutrient recovery. Even though the particulate and colloidal organic substances slowed the precipitation reaction, they substantially increased the particle size (i.e., 70% and 40%, respectively) of the formed struvite. The presence of organic substances in all tested forms does not significantly influence the purity and crystalline structure of struvite which can still be used as a slow-releasing fertilizer. Regarding the relocation process of organic substances during struvite precipitation under varying pH conditions, understanding the interaction between organics and heavy metals which in turn affect the dynamics of heavy metals in solution and precipitates remains limited; thus, additional research is needed.

Struvite production from anaerobic digestate of piggery wastewater using ferronickel slag as a magnesium source

This study aimed to fully recover ammonia contained at a high concentration in anaerobic digestate of piggery wastewater (ADPW) by forming struvite. As magnesium and phosphorus sources, ferronickel slag (FNS) and K₂HPO₄ were used, respectively. By leaching 200 g L^-1 of FNS with 3.0 M H₂SO₄, 10,309 mg L^-1 of magnesium ions were extracted, and this acid-leachate of FNS (FNSL) also contained 5965 mg L^-1 of total iron. In order to simultaneously remove both high concentrations of organic matters in ADPW and iron in FNSL which were known to hinder struvite formation, the mixture of ADPW and FNSL was added with H₂O₂ at the H₂O₂/Fe molar ratio of 0.75 and pH 4.0. After Fenton reaction, removal efficiencies of COD and total iron reached 77.36% and 99.89%, respectively. Then COD and an iron-reduced mixture of ADPW and FNSL were added with K₂HPO₄ satisfying Mg:N:P molar ratio of 1.2:1:1.15 at pH 9.5 to produce struvite for 1 h. From 1 L of ADPW (2.21 g NH₃-N), 0.65 L of FNSL (4.65 g Mg²⁺), and 5.63 g of PO₄ ^3-P, 46.7 g of precipitates were obtained. Overall removal efficiencies of magnesium, NH₃-N, and phosphorus were 98.59%, 94.25%, and 99.97%, respectively. Obtained precipitates were analysed by using XRD, XRF, SEM-EDX and found to be struvite with impurities of potassium and metals. Additionally, the economic feasibility of FNS was assessed by estimating chemical costs of various magnesium sources.

Recovery phosphate and ammonium from aqueous solution by the process of electrochemically decomposing dolomite

The cost-effective recovery of phosphate is of great significance to the mitigation of phosphorus resource depletion crisis. The electrochemical-decomposition of dolomite was developed to recover phosphate and ammonium from aqueous solution. The dolomite ore is mainly composed of CaMg(CO₃)₂ (53.73%), CaCO₃ (28.93%) and SiO₂ (16.59%). The continuous release of Mg²⁺ and Ca²⁺ were achieved by electrochemically decomposing dolomite ore, accompanied by the generation of base solution (9.0-10.5). The main factors affecting the recovery performance of phosphate (PO₄-P) and ammonium (NH₄-N) are current, initial concentration of PO₄-P and NH₄-N, initial pH of feed solution and feed rate. For a 30-d operation, the recovery rate of PO₄-P was maintained at 90-97% and that of NH₄-N at 50-60% under optimized operating conditions. The recovered product had low water solubility but high citric-acid-soluble, and was proposed as a slow-release fertilizer for crops. The proposed process as a simple, effective and green route may serve as a new strategy for recovering PO₄-P and NH₄-N from wastewaters.

The incorporation of Pb2+ during struvite precipitation: Quantitative, morphological and structural analysis

Struvite precipitated from wastewaters is an important fertilizer. However, struvite derived from wastewater usually contains toxic Pb, which can bring contamination to soil and even plants. Thus, understanding the incorporation mechanisms of Pb²⁺ during struvite precipitation is critical to its safe application. Here the influence of Pb concentration on struvite precipitation was assessed. When the initial Pb concentrations were at the range of 0.1-1 mg/L, the formation of pitting and roughening on struvite crystal surfaces was observed by scanning electron microscopy (SEM), indicating a surface interaction between Pb and struvite. Combined with X-ray photoelectron spectra (XPS), the results confirmed that the formed Pb-enriched layer with Pb-OH and Pb-PO₄ bonds was absorbed on struvite surface during precipitation. When Pb concentrations were increased to 10-100 mg/L, the precipitation of dominating Pb phase, crystalline Pb₁₀(PO₄)₆(OH)₂, was confirmed by X-ray diffraction (XRD). Combined with XPS, the amorphous Pb hydroxide/phosphate and Mg phosphate were also detected in struvite solids. Our findings revealed that at low concentrations (0.1-1 mg/L), Pb can affect the mineral surface by surface absorption, whereas Pb precipitated as separated phase(s) (e.g. Pb₁₀(PO₄)₆(OH)₂, Pb hydroxide and/or phosphate) at high Pb concentrations (10-100 mg/L). Thus, the initial Pb²⁺ concentrations in wastewater will dictate final struvite contents and Pb-bearing phases in recovered solids.

A Novel Ionic Exchange Membrane Crystallizer to Recover Magnesium Hydroxide from Seawater and Industrial Brines

A novel technology, the ion exchange membrane crystallizer (CrIEM), that combines reactive and membrane crystallization, was investigated in order to recover high purity magnesium hydroxide from multi-component artificial and natural solutions. In particular, in a CrIEM reactor, the presence of an anion exchange membrane (AEM), which separates two-compartment containing a saline solution and an alkaline solution, allows the passage of hydroxyl ions from the alkaline to the saline solution compartment, where crystallization of magnesium hydroxide occurs, yet avoiding a direct mixing between the solutions feeding the reactor. This enables the use of low-cost reactants (e.g., Ca(OH)₂) without the risk of co-precipitation of by-products and contamination of the final crystals. An experimental campaign was carried out treating two types of feed solution, namely: (1) a waste industrial brine from the Bolesław Śmiały coal mine in Łaziska Górne (Poland) and (2) Mediterranean seawater, collected from the North Sicilian coast (Italy). The CrIEM was tested in a feed and bleed modality in order to operate in a continuous mode. The Mg²⁺ concentration in the feed solutions ranges from 0.7 to 3.2 g/L. Magnesium recovery efficiencies from 89 up to 100% were reached, while magnesium hydroxide purity between 94% and 98.8% was obtained.

Synthesis and characterization of struvite derived from poultry manure as a mineral fertilizer

Animal manure is an important source of nutrients for crop production, but environmental issues can restrict its direct use. Thermochemical processing these manures may be an alternative to concentrate nutrients and reduce the final volume for agriculture application. We aimed here to evaluate the viability of extracting nutrients from chicken manure using a thermochemical process which reduces the volume of transported nutrients, targeting phosphorus (P) recovery as precipitated struvite, without add external source of P. The extraction of nutrients from poultry manure was performed in water, followed by a thermochemical treatment of the solid phase by incineration and acidulation of the resulting ash. Struvite was produced from the acidified ash extract after supplementation with Mg and regulating the pH (~8.5) by KOH addition. The recovery efficiency of P from the poultry manure and incorporation into struvite was 90%. The final product was a multi-nutrient fertilizer with high macronutrient levels (P, K, Mg and S) and low micronutrient content when compared to fresh manure, as well as lower levels of heavy metals, potentially harmful for the environment. The precipitated product obtained here is composed of struvite-NH₄ and struvite-K, alongside appreciable quantities of potassium sulphate and hydroxyapatite carbonate. Overall, we conclude that poultry manure represents a viable source of P and N for struvite production resulting in a nutrient-rich, pathogen-free inorganic fertiliser suitable for widespread use in agriculture.

Development of fertilizers for enhanced nitrogen use efficiency - Trends and perspectives

Despite nitrogen (N) being the most important crop nutrient, its use as fertilizer is associated with high losses. Such losses pollute the environment and increase greenhouse gas production and other environmental events associated with high ammonia volatilization and nitrous oxide emission. They also cause soil nitrate leaching and run-off that pollute surface and underground waters, with human health implications. The net outcomes for the plant are reduced N uptake and crop productivity that, together, increase the costs associated with fertilization of agricultural lands and dampen farmers' confidence in the efficacy and profitability of fertilizers. To address these problems, enhanced efficiency fertilizers (EEFs) are continuously being developed to regulate the release of N from fertilizers, allowing for improved uptake and utilization by plants, thereby lowering losses and increasing crop productivity per unit of fertilizer. The EEFs are classified based on whether they are inorganic- bio- or organic-coated; their mode of action on different N forms, including urease activity and nitrification inhibition; and the technologies involved in their development, such as targeted compositing of multiple nutrients and nanotechnology. This review is a critical revisit of the materials and processes utilized to coat or formulate enhanced efficiency N-fertilizers for reducing N losses, including their shortcomings, advances made to address such shortcomings, and effects on mitigating N losses and/or enhancing plant uptake. We provide perspectives that could assist in further improving promising and potentially effective and affordable coating or formulation systems for scalable improvements that allow for reducing the rate of N-fertilizer input in crop production. It is especially critical to develop multi-nutrient fertilizers that provide balanced nutrition to plants and humans, while improving N use efficiency and mitigating N-fertilizer effects on human and environmental health.

Crystallization of struvite in a hydrothermal solution with and without calcium and carbonate ions

Hydrothermal experiments with magnesium, ammonium, and phosphate (MAP) solution at a temperature of 120 ^OC for 24 h and pH (9 and 10), whilst effects of varying Mg²⁺/Ca²⁺/HCO^-₃ ratios on struvite crystallization were examined. The study was performed to investigate their effects on the quality and quantity of crystals using the XRPD Rietveld refinement and SEM method. Obviously, the struvite crystallization was inhibited through the forming of calcite, dolomite, hydroxyapatite, sylvite, and Mg-whitlockite under different pH conditions. In the absence of Ca²⁺ and HCO^-₃ ions, struvite and dittmarite were formed at pH solutions (9 and 10). Struvite proportion reduced with pH (9 and 10) under Mg²⁺/Ca²⁺/HCO^-₃ ratios (1:1:1 and 2:1:1), and depleted under the Mg²⁺/Ca²⁺/HCO^-₃ ratio of 1:2:2. An obvious change in morphologies of crystals into nanosized particles was observed. Results of the low proportion of struvite for experiments with Mg^2+/Ca²⁺/HCO^-₃ molar ratios may be a drawback for phosphate recovery.

Environmental and Economic Sustainability of Swine Wastewater Treatments Using Ammonia Stripping and Anaerobic Digestion: A Short Review

One of the most promising systems to treat swine wastewater is air stripping. This system simultaneously recovers nitrogen salts, to be used as fertiliser, and reduces the organic pollutant load in the effluents of swine breeding farms. Several reviews have discussed the air stripping as a treatment for many types of industrial wastewater or nitrogen-rich digestate (the liquid effluent derived from the anaerobic digestion plants) for the stripping/recovery of nutrients. However, reviews about the use of air stripping as treatment for raw or anaerobically digested swine wastewater are not available in literature. To fill this gap, this study: (i) Summarises the experiences of air stripping for recovery of ammonium salts from both raw and digested swine wastewater; and (ii) compares air stripping efficiency under different operational conditions. Moreover, combined systems including air stripping (such as struvite crystallisation, chemical precipitation, microwave radiation) have been compared. These comparisons have shown that air stripping of raw and digested swine wastewater fits well the concept of bio-refinery, because this system allows the sustainable management of the piggery effluent by extracting value-added compounds, by-products, and/or energy from wastewater. On the other hand, air stripping of raw and digested swine wastewater has not been extensively studied and more investigations should be carried out.

Simultaneous Removal of Pollutants and Recovery of Nutrients from High-Strength Swine Wastewater Using a Novel Integrated Treatment Process

Simple Summary

Due to the increasing trend of swine consumption in recent decades, swine husbandry practices have become more intensive in Korea. Intensive swine farming practices inevitably result in an increment of wastewater production. Treatment of high strength swine wastewater (SWW) is therefore becoming a matter of concern in Korea. Moreover, with the increasing number of swine heads, swine farms are having issues with malodor, sanitation, and disease control. In this study, a novel integrated treatment process was tested for the simultaneous removal of pollutants and nutrient recovery from high strength swine wastewater. The integrated treatment process used in this study successfully removed the nutrients and other pollutants through biological treatment, recovered the nutrients using struvite crystallization process and decolorized as well as disinfected the effluent before discharge into water bodies by electrochemical treatment. Therefore, using the proposed integrated treatment process, it might be possible to ensure efficient SWW management along with societal and environmental sustainability.

Abstract

In this study, a novel treatment approach combining biological treatment, struvite crystallization, and electrochemical treatment was developed and its efficiency for the simultaneous removal of pollutants and recovery of nutrients from high strength swine wastewater (SWW) was verified. For all the parameters, maximum removal efficiencies in the lab-scale test were obtained in the range of 93.0–98.7% except for total solids (TS) (79.4%). Farm-scale process showed overall removal efficiencies for total nitrogen (TN), total phosphorus (TP), soluble total organic carbon (sTOC), and color as 94.5%, 67.0%, 96.1%, and 98.9%, respectively, while TS, suspended solids (SS), ammonium nitrogen (NH₄-N), and ortho-phosphate (O-P) concentrations were reduced by 91.5%, 99.6%, 98.6%, and 91.9%, respectively. Moreover, the struvite recovered from SWW showed heavy metal concentrations within the range of the Korean standard for fertilizers and feedstocks and thus, suggesting its potential application as fertilizer and in animal feed production. Using the proposed process, the SWW was converted to liquid compost as a quick-acting fertilizer, struvite as a slow-release fertilizer, and the decolorized and disinfected effluent after electrochemical treatment was safe for discharge according to Korean standard. Therefore, the novel integrated treatment process used in this study can be considered as a solution for SWW management and for the simultaneous removal and recycling of nutrients (N and P).

Nitrogen release of hydrothermal treatment of antibiotic fermentation residue and preparation of struvite from hydrolysate

Hydrothermal treatment (HT) is an appropriate treatment method for organic hazardous wastes such as antibiotic fermentation residue (AR). However, there is no effective way to recycle hydrolysate with high nitrogen content. In this study, penicillin fermentation residue (PR, a type of AR) was used as raw material to study the release and redistribution of N during hydrothermal process. And the influences of pH, ion ratio and reaction time on the preparation of struvite were analyzed. The results showed that the nitrogen in PR consists of Inorganic-N and Amino-N. Most of N (~70%) that entered hydrolysate was converted into org-N, NH₄⁺-N and NO₃^--N. At 260 °C, the NH₄⁺-N concentration was 2842.78 mg/L, accounting for 45.2% of total nitrogen. The remaining amino-N in the hydrochar was gradually converted to pyridine-N, pyrrole-N and quaternary-N with the increasing of temperature. At pH = 9.5, Mg²⁺: NH₄⁺: PO₄^3- = 1.3: 1: 1.15, struvite was prepared by hydrolysate. And over 95% removal rate of NH₄⁺-N could be achieved. XRD analysis showed that the main component of the product was struvite, which was further confirmed by SEM-EDX and FT-IR. It was found that there was trace amount of MgKPO₄·H₂O precipitation in the product. In addition, Mg₃(PO₄)₂ precipitation might also be formed at pH = 10.

Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions

Kinetic parameters describing continuous reaction crystallization of struvite from aqueous solutions containing also calcium ions (from 100 to 2000 mg Ca2+/kg) were estimated. Test results were compared with kinetic data of struvite manufactured from real cattle liquid manure. Kinetic model for ideal MSMPR (Mixed Suspension Mixed Product Removal) crystallizer was used assuming dependence of crystal growth rate G on its size L (size-dependent growth, SDG MSMPR model). Based on nonlinear regression and statistical analysis, one from within five considered G(L) models was selected – Rojkowski exponential (RE) model – rendering the experimental population density distributions the best. It was concluded, that calcium ions influenced all components of struvite manufacturing process disadvantageously. A rise of Ca2+ concentration in a feed from 100 to 2000 mg/kg increased nucleation rate ca. 160-time, whereas growth rate of nuclei up to macroscopic size G0 decreased more than 10-time. Linear (larger) crystal growth rate G¥ was nearly two-times smaller: 1.71·10–8 m/s (100 mg Ca2+/kg) – 9.10·10–9 m/s (2000 mg Ca2+/kg). Resulting in a product with deteriorated quality. Mean size of the crystals decreased nearly two-times (to 18.4 μm), non-homogeneity within product population enlarged and calcium fraction in the product increased. The product, beside struvite MgNH4PO4·6H2O, also contained hydrated amorphous calcium phosphate(V) Ca3(PO4)2·nH2O (ACP). It was observed, that 5-times smaller concentration of phosphate(V) ions in a feed and magnesium ions excess in relation to phosphate(V) and ammonium ions (1.2 : 1 : 1) influenced all kinetic parameters of continuous struvite reaction crystallization advantageously.

Design and Optimization of Fluidized Bed Reactor Operating Conditions for Struvite Recovery Process from Swine Wastewater

Struvite crystallization using fluidized bed reactors (FBRs) is one of the most commonly used methods for nutrient recovery from different waste streams. However, struvite recovery from swine wastewater containing much higher solids using FBR has not been studied extensively. In this study, we therefore designed and optimized the key operating conditions parameters, i.e., pH (9.0, 9.5, and 10.0), circulation rate (CR) (1.5, 3.0, and 4.5 L/Lreactor·h), and hydraulic retention time (HRT) (1, 3, and 5 h) of FBR to ensure efficient nutrient removal and struvite crystallization from swine wastewater using response surface methodology (RSM) with central composite design (CCD) as experimental design. A magnesium/phosphorus (Mg/P) molar ratio of 1.3 was maintained with MgCl2 according to ortho-phosphate (O-P) concentration of influent and an air diffuser was set to supply air with 0.03 L air/Lreactor·min. The O-P recovery efficiency of over 91% was achieved through the entire runs. Among the operational parameters, pH did not show any significant effect on NH4-N recovery, particle size, and struvite production rate (SPR). The optimal CR over 2.94 L/Lreactor·h was found to be appropriate for efficient removal of nutrients and struvite crystallization. While optimizing the HRT, priority of the process operation such as the production of larger struvite particles or increased struvite productivity should be considered. Therefore, the optimal operational parameters of pH 9.0, CR > 2.94 L/Lreactor·h, and HRT of 1 or 5 h were chosen to obtain better responses through RSM analyses. The findings of this study would be useful in designing and operating either pilot- or full-scale FBR for struvite crystallization from swine wastewater.

Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds

This study investigates the recovery of phosphorus from the process water obtained through hydrothermal carbonisation (HTC) of a 'wet' biomass waste, namely spent coffee grounds. HTC was shown to liberate more than 82% of the total phosphorus in the grounds in the form of dissolved ortho-phosphate. Nanofiltration was used to concentrate the inorganic nutrients of the HTC process water, achieving a mass concentration factor of 3.9 times. The natural stoichiometry of phosphorus, magnesium and ammoniacal nitrogen in the nanofiltration retentate was favourable for struvite precipitation. 92.8% of aqueous phosphorus was recovered as struvite through simple pH adjustment, yielding a total phosphorus recovery of 75% from the feedstock spent coffee grounds.

The potential of adopting struvite precipitation as a strategy for the removal of nutrients from pre-AnMBR treated abattoir wastewater

Large volume of wastewater consisting complex forms of organics, lipids and nutrients, is discharged from the abattoir (red meat) processing industry. In this study, nutrient rich pre-Anaerobic Membrane Bioreactor (AnMBR) treated abattoir effluent was fed to a struvite (MgNH₄PO₄.6H₂O) precipitator to evaluate the possibility of developing an innovative environmentally sustainable treatment technology to produce nutrient free high-quality treated effluent. A series of continuous and batch experiments were conducted to investigate the influence of pH and presence of Ca²⁺ on struvite precipitation. The study found that Mg²⁺:Ca²⁺ molar ratio of 0.8 (or high Ca²⁺) impacts on the production and quality of struvite significantly. Pre-AnMBR treated abattoir wastewater with negligible Ca²⁺ (Mg²⁺:Ca²⁺ molar ratio > 20) showed over 80% removal of phosphorus via struvite precipitation. The highest removal rates of both nitrogen and phosphorus were achieved at pH 9.5 with Mg²⁺:PO₄^3- molar ratio of 2:1.

Resource recovery assessment at a pulp mill wastewater treatment plant in Uruguay

Using a mathematical model, a resource recovery assessment was carried out at a pulp mill activated sludge wastewater treatment plant (WWTP) located in Uruguay. Through the evaluation of different scenarios, the potential production of methane from secondary sludge, with its inherent energy savings, and the recovery of phosphorus (P) as struvite were estimated. Considering the current WWTP configuration with a sludge retention time (SRT) of 32 days, and according to the model, which is a simplification of reality, the assessment indicates that the implementation of an anaerobic digester (AD) to treat the excess sludge can lead to a methane production of approximately 1736 m³ CH₄ d^-1, being a promising alternative to increase the WWTP treatment performance. Furthermore, the model predictions suggest that by shortening the SRT from 32 to 5 days, the methane production could increase by up to 5568 m³ CH₄ d^-1. If the methane produced is used to generate electrical energy to operate the WWTP, energy savings of about 88% can be achieved. Regarding the potential recovery of P as struvite, the addition of a struvite reactor could be an efficient option to recover approximately 1611 mg L^-1 of struvite (corresponding to a load of about 433 kg d^-1). By optimizing the process performance, these findings highlight the potential recovery of resources in pulp mill WWTP, while complying with stringent effluent discharge standards. In addition, further research activities such as pilot-test or detailed laboratory studies may be needed to validate the previous recommendations for industrial scale application.

Selective and efficient sequestration of phosphate from waters using reusable nano-Zr(IV) oxide impregnated agricultural residue anion exchanger

There is an urgent need to develop low-cost and effective adsorbents for enhanced removal of phosphate from contaminated waters. In this study, nanosized Zr(IV) oxide particles were immobilized on the amino modified corn staw (MCS) to fabricate a novel nanocomposite (Zr@MCS) with superior application capability. Compared with the widely used commercial anion exchangers in previous studies, the modified agricultural residue was empolyed as the host to avoid the high costs and secondary pollution in the preparation. Zr@MCS displayed remarkable selective removal of phosphate from water even in the presence of coexisting anions (Cl^-, SO₄^2-, NO₃^-) at high levels, as well as with a high adsorption capacity, fast adsorption kinetics and high availability in the wide range of pH 2-8 toward phosphate. The excellent adsorption performance of Zr@MCS is attributed to the synergistic effect of the electrostatic attraction of the quaternary ammonium groups fixed on the host skeleton and the specific adsorption of phosphate derived from the hydroxyl functional groups of Zr(IV) oxide. The exhausted Zr@MCS can be effectively regenerated by 5% NaOH-NaCl solution for sustainably utilized, and phosphorus in the desorption effluent could be recovered as high-quality struvite by a simple struvite recovery process. Furthermore, the considerable treatment volume for the synthetic solution and real wastewater in a fixed-bed flow system indicated that Zr@MCS is of great potential for phosphate removal in practice.

Environmental performances of production and land application of sludge-based phosphate fertilizers-a life cycle assessment case study

Phosphorus (P) is a non-renewable resource extracted from phosphate rock to produce agricultural fertilizers. Since P is essential for life, it is important to preserve this resource and explore alternative sources of P to reduce its criticality. This study aimed to assess whether fertilizing with sludge-based phosphate fertilizers (SBPF) can be a suitable alternative to doing so with fertilizers produced from phosphate rock. Environmental impacts of production and land application of SBPF from four recovery processes were compared to those of two reference scenarios: triple super phosphate (TSP) and sewage sludge. To avoid bias when comparing scenarios, part of the environmental burden of wastewater treatment is allocated to sludge production. The CML-IA method was used to perform life cycle impact assessment. Results highlighted that production and land application of SBPF had higher environmental impacts than those of TSP due to the large amounts of energy and reactants needed to recover P, especially when sludge had a low P concentration. Certain environmental impacts of production and land application of sewage sludge were similar to those of SBPF. Sensitivity analysis conducted for cropping systems highlighted variability in potential application rates of sewage sludge or SBPF. Finally, because they contain lower contents of heavy metals than sewage sludge or TSP, SBPF are of great interest, but they require more mineral fertilizers to supplement their fertilization than sewage sludge. Thus, SBPF have advantages and disadvantages that need to be considered, since they may influence their use within fertilization practices.

Mechanochemical approach to synthesize citric acid-soluble fertilizer of dittmarite (NH4MgPO4·H2O) from talc/NH4H2PO4 mixture

In this study, a citric acid-soluble fertilizer of dittmarite (NH₄MgPO₄·H₂O) was synthesized by balling talc with NH₄H₂PO₄. The effects of ball milling speed and milling time on the dissolution rates of N, P and Mg in deionized water and 2% citric acid were explored. Characterization technologies such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG) and Scanning electron microscopy (SEM) were applied to test the prepared samples. In water, the prepared dittmarite was changed into struvite (NH₄MgPO₄·6H₂O) with almost no N, P or Mg release, while the dissolution rates of nutrient elements reached almost 100% in 2% citric acid. The proposed work presented a facile and environmentally friendly method to produce CASF with high agricultural and ecological value.

Dittmarite synthesis by a mechanochemical route for application as a citric acid-soluble fertilizer.

Rational budgeting approach as a nutrient management tool for mixed crop-swine farms in Korea

Objective

Due to rapid economic return, mixed crop-swine farming systems in Korea have become more intensive. Intensive farming practices often cause nutrient surpluses and lead to environmental pollution. Nutrient budgets can be used to evaluate the environmental impact and as a regulatory policy instrument for nutrient management. This study was conducted to select a nutrient budgeting approach applicable to the mixed crop-swine farms in Korea and suggest an effective manure treatment method to reduce on-farm nutrient production.

Methods

In this study, we compared current and ideal gross nutrient balance (GNB) approaches of Organisation for Economic Co-operation and Development and soil system budget (SSB) approach with reference to on-farm manure treatment processes. Data obtained from farm census and published literature were used to develop the farm nutrient budgets.

Results

The average nitrogen (N) and phosphorus (P) surpluses were approximately 11 times and over 7 times respectively higher in the GNB approaches than the SSB. After solid-liquid separation of manure, during liquid composting a change in aeration method from intermittent to continuous reduced the N and P loading about 50% and 47%, respectively. Although changing in solid composting method from turning only to turning+aeration improved the N removal efficiency by 30.5%, not much improvement in P removal efficiency was observed.

Conclusion

Although the GNB approaches depict the impact of nutrients produced in the mixed crop-swine farms on the overall agricultural environment, the SSB approach shows the partitioning among different nutrient loss pathways and storage of nutrients within the soil system; thus, can help design sustainable nutrient management plans for the mixed crop-swine farms. The study also suggests that continuous aeration for liquid composting and turning+aeration for solid composting can reduce nutrient loading to the soil.

Recovery of phosphates(V) from wastewaters of different chemical composition

Phosphate(V) ions were recovered from wastewaters of different chemical compositions in a continuous reaction crystallization of struvite. Two real wastewaters were represented by an effluent sample from the phosphorus mineral fertilizer industry and by liquid manure derived from a cattle-breeding farm. Two other wastewaters were prepared under laboratory conditions. Impurities present in the wastewaters caused the precipitation of solid products of distinctly different quality. The mean size of struvite crystals varied from ca. 18 to ca. 40 μm. Homogeneity within their populations, quantified by the coefficient of variation CV, varied from satisfactory (CV ca. 75%) to unfavorable (CV ca. 100%), which resulted in the elongation of necessary filtration time. Calcium ions in wastewater precipitated as amorphous hydrated calcium phosphates ACP. Their content in the products varied from 33.4 up to 73.1 mass %. Also 28.32 – 32.74 mass % of P2O5, 6.35 – 14.12 mass % of MgO and 4.35 – 16.94 mass % of CaO were confirmed in the products, together with hydroxides of some metals and salts of other impurities. Based on the chemical composition of the products derived from the investigated wastewaters, their application in agriculture as valuable mineral fertilizers is recommended.