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

In-depth characterization of phosphate intercalated Mg Al Layered double hydroxides and study of the PO4 release properties

Slow-release fertilizers (SRFs) form the core of innovative strategies in sustainable agriculture. Layered Double Hydroxides (LDH), known for their high capacity to sequester plant nutrients, especially phosphate, are emerging as promising candidates for SRF synthesis. The phosphate release properties of MgAl LDH (with a targeted Mg/Al ratio of 2.0) intercalated with HPO42- anions were assessed in various aqueous environments. A comprehensive analysis, including in-depth chemical and structural characterizations (ICP-OES, XRD, PDF, 27Al NMR, 31P NMR, FTIR, SEM) of the as-prepared phase unveiled a more intricate composition than anticipated for a pure or ideal Mg2Al-HPO4 LDH, encompassing an excess of intercalated phosphate in conjunction with K+. Beyond the intercalated phosphate, solid state 31P NMR speciation identified multiple HxPO4(-3+x) environments, indicating a portion of the phosphate reacting with intralayer Mg2+ to form K-struvite. Additionally, some phosphates were adsorbed onto the surface of amorphous aluminum hydroxide, a side phase formed during MgAl coprecipitation. The phosphate release demonstrated rapid kinetics, occurring within 6 days. Moreover, the released phosphate increased significantly when reducing the Solid/Liquid (S/L) ratio (58%) and further increasing in the presence of carbonate ions (90%). The released phosphate varied from 12% to 90% under different release conditions, transitioning from water to a 3.33 mM NaHCO3 aqueous solution at a low S/L ratio (from 20 mg LDH per mL to 0.02 mg LDH per mL). The simultaneous release of K+, Mg2+, Al3+ indicated the complete dissolution of the K-struvite and partial dissolution of phosphate intercalated MgAl LDH. These results enhanced our understanding of the mechanism governing phosphate release from MgAl LDH, paving the way for potential phosphate recovery by LDH or for the development of LDH-based SRFs.

Synthesis of struvite-enriched slow-release fertilizer using magnesium-modified biochar: Desorption and leaching mechanisms

To improve the retention and slow-release abilities of nitrogen (N) and phosphorus (P), an 82 %-purity struvite fertilizer (MAP-BC) was synthesized using magnesium-modified biochar and a solution with a 2:1 concentration ratio of NH4+ to PO43- at a pH of 8. Batch microscopic characterizations and soil column leaching experiments were conducted to study the retention and slow-release mechanisms and desorption kinetics of MAP-BC. The slow-release mechanism revealed that the dissolution rate of high-purity struvite was the dominant factor of NP slow release. The re-adsorption of NH4+ and PO43- by biochar and unconsumed MgO prolonged slow release. Mg2+ ionized by MgO could react with PO43- released from struvite to form Mg3(PO4)2. The internal biochar exhibited electrostatic attraction and pore restriction towards NH4+, while magnesium modification and nutrient loading formed a physical antioxidant barrier that ensured long-term release. The water diffusion experiment showed a higher cumulative release rate for PO43- compared to NH4+, whereas in soil column leaching, the trend was reversed, suggesting that soil's competitive adsorption facilitated the desorption of NH4+ from MAP-BC. During soil leaching, cumulative release rates of NH4+ and PO43- from chemical fertilizers were 3.55-3.62 times faster than those from MAP-BC. The dynamic test data for NH4+ and PO43- in MAP-BC fitted the Ritger-Peppas model best, predicting release periods of 163 days and 166 days, respectively. The leaching performances showed that MAP-BC reduced leaching solution volume by 5.58 % and significantly increased soil large aggregates content larger than 0.25 mm by 24.25 %. The soil nutrients retention and pH regulation by MAP-BC reduced leaching concentrations of NP. Furthermore, MAP-BC significantly enhanced plant growth, and it is more suitable as a NP source for long-term crops. Therefore, MAP-BC is expected to function as a long-term and slow-release fertilizer with the potential to minimize NP nutrient loss and replace part of quick-acting fertilizer.

Rapid characterization of sulfur and phosphorus in organic waste by near infrared spectroscopy

Near-infrared spectroscopy (NIRS) has recently emerged as a valuable tool for monitoring organic waste utilized in anaerobic digestion processes. Over the past decade, NIRS has significantly improved the characterization of organic waste by enabling the prediction of several crucial parameters such as biochemical methane potential, carbohydrate, lipid and nitrogen contents, Chemical Oxygen Demand, and kinetic parameters. This study investigates the application of NIRS for predicting the levels of Sulfur (S) and Phosphorus (P) within organic waste materials. The results for sulfur prediction exhibited a high level of accuracy, yielding an error of 1.21 g/Kg[TS] in an independently validated dataset, coupled with an R-squared value of 0.84. Conversely, the prediction of phosphorus proved to be slightly less successful, showing an error of 1.49 g/Kg[TS] with an R-squared value of 0.70. Furthermore, the disparities in performance seem to stem from the inherent correlation between the spectral data and the sulfur or phosphorus contents. Significantly, a variable selection technique known as CovSel was employed, shedding light on the differing approaches used for sulfur and phosphorus predictions. In the case of sulfur, the prediction was achieved through a direct correlation with wavelengths associated with sulfur-related functional groups (such as R - S(=O)2 - OH, -SH, and R-S-S-R) present in the NIR spectra. In contrast, phosphorus prediction relied on an indirect correlation with absorption bands related to organic matter (including CH, CH2, CH3, -CHO, R-OH, C = O, -CO2H, and CONH).

Enhanced nutrient recovery from anaerobically digested poultry wastewater through struvite precipitation by organic acid pre-treatment and seeding in a bubble column electrolytic reactor

Limited mineralization of organic phosphorus to phosphate during the anaerobic digestion process poses a significant challenge in the development of cost-effective nutrient recovery strategies from anaerobically digested poultry wastewater (ADPW). This study investigated the influence of organic acids on phosphorus solubilization from ADPW, followed by its recycling in the form of struvite using a bubble column electrolytic reactor (BCER) without adding chemicals. The impact of seeding on the efficiency of PO43- and NH3-N recovery as well as the size distribution of recovered precipitates from the acid pre-treated ADPW was also evaluated. Pre-treatment of the ADPW with oxalic acid achieved complete solubilization of phosphorus, reaching ∼100% extraction efficiency at pH 2.5. The maximum removal efficiency of phosphate and ammonia-nitrogen from the ADPW were 88.9% and 90.1%, respectively, while the addition of 5 and 10 g/L struvite seed to the BCER increased PO43- removal efficiency by 9.6% and 11.5%, respectively. The value of the kinetic rate constant, k, increased from 0.0176 min-1 (unseeded) to 0.0198 min-1, 0.0307 min-1, and 0.0375 min-1 with the seed loading rate of 2, 5, and 10 g/L, respectively. Concurrently, the average particle size rose from 75.3 μm (unseeded) to 82.1 μm, 125.7 μm, and 148.9 μm, respectively. Results from XRD, FTIR, EDS, and dissolved chemical analysis revealed that the solid product obtained from the recovery process was a multi-nutrient fertilizer consisting of 94.7% struvite with negligible levels of heavy metals.

Nutrient recovery from wastewater for hydroponic systems: A comparative analysis of fertilizer demand, recovery products, and supply potential of WWTPs

Nutrient recovery from wastewater treatment plants (WWTPs) for hydroponic cultivation holds promise for closing the nutrient loop and meeting rising food demands. However, most studies focus on solid products for soil-based agriculture, thus raising questions about their suitability for hydroponics. In this study, we address these questions by performing the first in-depth assessment of the extent to which state-of-the-art nutrient recovery processes can generate useful products for hydroponic application. Our results indicate that less than 11.5% of the required nutrients for crops grown hydroponically can currently be recovered. Potassium nitrate (KNO3), calcium nitrate (Ca(NO3)2), and magnesium sulfate (MgSO4), constituting over 75% of the total nutrient demand for hydroponics, cannot be recovered in appropriate form due to their high solubility, hindering their separated recovery from wastewater. To overcome this challenge, we outline a novel nutrient recovery approach that emphasizes the generation of multi-nutrient concentrates specifically designed to meet the requirements of hydroponic cultivation. Based on a theoretical assessment of nutrient and contaminant flows in a typical municipal WWTP, utilizing a steady-state model, we estimated that this novel approach could potentially supply up to 56% of the nutrient requirements of hydroponic systems. Finally, we outline fundamental design requirements for nutrient recovery systems based on this new approach. Achieving these nutrient recovery potentials could be technically feasible through a combination of activated sludge processes for nitrification, membrane-based desalination processes, and selective removal of interfering NaCl. However, given the limited investigation into such treatment trains, further research is essential to explore viable system designs for effective nutrient recovery for hydroponics.

An update on sustainabilities and challenges on the removal of ammonia from aqueous solutions: A state-of-the-art review

An insightful attempt has been made in this review and the primary objective was to meticulously provide an update on the sustainabilities, advances and challenges pertaining the removal of ammonia from water and wastewater. Specifically, ammonia is a versatile compound that prevails in various spheres of the environment, and if not properly managed, this chemical species could pose severe ecological pressure and toxicity to different receiving environments and its biota. The notorious footprints of ammonia could be traced to anoxic conditions, an infestation of aquatic ecosystems, hyperactivity, convulsion, and methaemoglobin, popularly known as the "blue baby syndrome". In this review, latest updates regarding the sustainabilities, advancements and challenges for the removal of ammonia from aqueous solutions, i.e., river and waste waters, are briefly elucidated in light of future perspectives. Viable routes and ideal hotspots, i.e., wastewater and drinking water, for ammonia removal under the cost-effective options have been unpacked. Key mechanisms for the removal of ammonia were grossly bioremediation, oxidation, adsorption, filtration, precipitation, and ion exchange. Finally, this review denoted biological nutrient removal, struvite precipitation, and breakpoint chlorination as the most effective and promising technologies for the removal of ammonia from aquatic environments, although at the expense of energy and operational cost. Lastly, the future perspective, avenues of exploitation, and technical facets that deserve in-depth exploration are duly underscored.

Nitrogen and phosphorus recovery from hemodialysis wastewater to use as an agricultural fertilizer

INTRODUCTION

Hemodialysis wastewater contains high concentrations of ammonia nitrogen and phosphorus. Recovery of these nutrients as soil fertilizers represents an interesting opportunity to ensure a sustainable fertilizer supply.

METHODS

In this paper, a simple method for recovering phosphorous and nitrogen as crystalline struvite [MgNH4PO4·6H2O] is presented. An integrated cost model is also presented in order to create a positive business case.

RESULTS

Recovery rates in form of struvite of 95% of PO43--P and 23% of NH4+-N were achieved with a profit.

CONCLUSION

To the best of our knowledge, this paper is the first to study the recovery of these naturally occurring minerals from hemodialysis wastewater. This offers great potential for the valorization of this type of wastewater.

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.

Phosphorus recovery from aqueous product of hydrothermal carbonization of cow manure

The work studies the recovery of nutrients (phosphorus and nitrogen) from the process water of acid-assisted hydrothermal carbonization (HTC) of cow manure. Three organic acids (formic acid, oxalic acid, and citric acid) and sulfuric acid were evaluated as additives in HTC. Using 0.3 M sulfuric acid, more than 99% of phosphorus and 15.6% of nitrogen from manure are extracted and dissolved during HTC at 170 °C with 10 min reaction time in a batch reactor. Nutrients (mainly phosphorus) were recovered through precipitation from process water by raising the ionic strength of the solution by addition of salts of magnesium and ammonia, and by raising the pH to 9.5. Subsequently, phosphorus-rich solids were recovered containing almost all (greater than 95%) of the dissolved phosphorus in the sulfuric and formic acid assisted runs. Morphology and qualitative chemical analysis of the precipitates were determined. It is shown by XRD that the precipitate formed from process water generated by HTC with oxalic acid is crystalline, although the diffraction pattern could not be matched with any expected substance.

Struvite separation from wastewater and its use with sulfur-oxidizing bacteria improves phosphorus utilization in alkaline soil

A major portion of phosphatic fertilizer comes from the limiting natural resource, rock phosphate, which demands a timely alternative. Struvite, a crystalline mineral of low solubility, is a worthwhile alternative. Evaluation of the local wastewater streams for their ability to precipitate struvite and its capability as phosphatic fertilizer under an alkaline soil environment was studied. Two stirring speeds, a pH range of 8.0-11.0, and a constant molar ratio were used to optimize local wastewater streams for struvite precipitation. Struvite was used in five different combinations to assess the release of phosphorus (P), including control (no P source), single superphosphate, struvite, struvite + sulfur, and rock phosphate with or without inoculation of sulfur-oxidizing bacteria (SOB). For struvite precipitation, low stirring speeds are ideal because the precipitates readily sink to the bottom once they form. Furthermore, the amalgamation of SOB with sulfur significantly improved P use efficiency under alkaline soils through increased phosphorus sources solubility and enabled optimum wheat production due to its low solubility in an alkaline soil condition. Due to its capacity to recycle phosphorus from wastewater, struvite is poised to emerge as a sustainable fertilizer and had an opportunity to capture a share of this expanding market.

Examining current trends and future outlook of bio-electrochemical systems (BES) for nutrient conversion and recovery: an overview

Nutrient-rich waste streams from domestic and industrial sources and the increasing application of synthetic fertilizers have resulted in a huge-scale influx of reactive nitrogen and phosphorus in the environment. The higher concentrations of these pollutants induce eutrophication and foster degradation of aquatic biodiversity. Besides, phosphorus being non-renewable resource is under the risk of rapid depletion. Hence, recovery and reuse of the phosphorus and nitrogen are necessary. Over the years, nutrient recovery, low-carbon energy, and sustainable bioremediation of wastewater have received significant interest. The conventional wastewater treatment technologies have higher energy demand and nutrient removal entails a major cost in the treatment process. For these issues, bio-electrochemical system (BES) has been considered as sustainable and environment friendly wastewater treatment technologies that utilize the energy contained in the wastewater so as to recovery nutrients and purify wastewater. Therefore, this article comprehensively focuses and critically analyzes the potential sources of nutrients, working mechanism of BES, and different nutrient recovery strategies to unlock the upscaling opportunities. Also, economic analysis was done to understand the technical feasibility and potential market value of recovered nutrients. Hence, this review article will be useful in establishing waste management policies and framework along with development of advanced configurations with major emphasis on nutrient recovery rather than removal from the waste stream.

Fate of antimicrobial resistance genes (ARG) and ARG carriers in struvite production process from human urine

Struvite, a human urine-derived fertilizer types, is characterized by its low water solubility that renders it a slow-releasing eco-friendly fertilizer. Knowing the fate of antibiotic resistance genes in struvite is important since human urine carries microorganisms, viruses and mobilomes. In this study, urine samples were collected and struvite production was done using MgCl2. From the fresh, stored urine and struvite, DNA was extracted and metagenomic sequencing was done using Illumina HiSeqX. Metagenome-derived genome sequence analysis revealed the dominance of phages of Streptococcus, Bacillus and Escherichia, with nearly 50% abundance of streptococcus phage in fresh urine. Increased antibiotic resistance genes were found in the stored urine than in fresh and struvite samples. The top five resistance genes in all the three samples were to aminoglycosides, carbapenem, chloramphenicol, erythromycin and efflux pump, with key carrying pathogens including Acinetobacter, Aeromonas and Enterococcus. The identified families for carbapenem, aminoglycoside resistance and efflux pump were shown persistent in struvite with a shift in gene families. The detection of resistance-gene-laden mobilomes, including the last-resort antibiotics in the struvite sample, requires due attention before the implementation of struvite as fertilizer. Further optimization of the struvite production process with regard to the minimization of mobilomes is recommended.

Interaction of divalent metals with struvite: sorption, reversibility, and implications for mineral recovery from wastes

Phosphorus (P) recovered from wastewater as struvite (MgNH₄PO₄·6H₂O) can meet high P demands in the agricultural sector by reuse as a P fertiliser. Heavy metals are prevalent in wastewaters and are common fertiliser contaminants, therefore struvite as a sorbent for metals requires evaluation. Struvite sorption experiments were conducted in model solutions with cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) at 1-5 μM concentrations from pH 7-10. The struvite metal loading increased with dissolved metal concentration and pH, ranging from 2 to 493 mg kg^-1. Highest loadings were observed for 5 μM Pb, which exceeded the 120 mg kg^-1 European Union (EU) struvite fertiliser limit at all pH values. At 5 μM concentrations, Ni and Cd loadings exceeded EU limits of 100 mg kg^-1 at pH 10, and 60 mg kg^-1 at pH 8-10, respectively. In desorption experiments, 10-85% metal was released after resuspension in metal-free solutions, with a positive correlation between initial loading and amount desorbed. Distortions of the struvite phosphate band, by Fourier transformation infrared (FTIR) spectroscopy, indicated lowered symmetry of phosphate vibrations with metal sorption. X-ray absorption fine structure spectroscopy (XAFS) analysis of pH 9 solids indicated tetrahedral coordination for Cu and Zn, octahedral coordination for Co and Ni, and Pb in 9-fold coordination. Precipitation of Pb-phosphate minerals was a primary mechanism for Pb sorption. The results provide insight into metal contaminant sorption with struvite in wastewaters, and the potential for metal desorption after recovery.

A Comprehensive Review on Wastewater Nitrogen Removal and Its Recovery Processes

Discharging large amounts of domestic and industrial wastewater drastically increases the reactive nitrogen content in aquatic ecosystems, which causes severe ecological stress and biodiversity loss. This paper reviews three common types of denitrification processes, including physical, chemical, and biological processes, and mainly focuses on the membrane technology for nitrogen recovery. The applicable conditions and effects of various treatment methods, as well as the advantages, disadvantages, and influencing factors of membrane technologies, are summarized. Finally, it is proposed that developing effective combinations of different treatment methods and researching new processes with high efficiency, economy, and energy savings, such as microbial fuel cells and anaerobic osmotic membrane bioreactors, are the research and development directions of wastewater treatment processes.

Vivianite recovery from high concentration phosphorus wastewater with mine drainage as iron sources

High concentration phosphorus wastewater has attracted much attention due to the safety of water ecology and the potential crisis of phosphorus resource, which is caused by large amounts of phosphorus discharging into natural water bodies. Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has been considered as an effective technology for phosphorus recovery. In this study, we develop a potentially low-cost, sustainable approach to recover phosphorus from high concentration phosphorus wastewater using mine drainage as iron source. Inoculated with both sewage and Geobacter, mine drainage was suitable for vivianite recovery from high concentration phosphorus wastewater with PO₄^3- concentration between 6 and 18 mM. When the PO₄^3- concentration increased gradually, both phosphorus removal efficiency (R_P) and vivianite recovery efficiency (R_V) decreased significantly. The highest R_V of 48 % was obtained with 9 mM PO₄^3- in Geobacter batches (CJ2 batches), which was 15 % higher than that in the paralleled sewage batches (33 % in HJ2). Simultaneously, vivianite accounted for 91 % of the solid phosphate compounds in CJ2 batches due to the enhancement of Geobacter.

Nutrient recovery from yellow water to soil-crop systems

The potentials of the nutrient recovered (NRM), via a facile green and sustainable approach from human urine, as a fertilizer in soil-crop system was studied. Nutrient was recovered using a highly decentralized modular reactor, with packed bed of granular gastropod shell. The cultivations of Zea mays (maize) and Solanum lycopersicum (tomato) were the cases studied. The total nutrient composition, the P-speciation, and the safety-risk assessment of the NRM were determined. Using NPK as the standard fertilizer, and a non-fertilized soil as the control, the fertilizing potential of the NRM was evaluated. The influence of the different fertilizer application regimes on the wet and dry biomass nutrient composition, after-harvest soil nutrient composition, and pH values was studied. The NRM contained 106 mg/g of TN and 374.6 mg/g of TP, and the P species identified were Ca₂-P (31.66%), Ca₈-P (14.99%), and Ca₁₀-P (53.32%). The growth rate of the NRM crops were lower than that of the NPK crops until the 17th day, when the NRM crops grew faster than that of the NPK and control (p < 0.05). The NRM is beneficial to acidic soils and also acts as a slow nutrient releasing fertilizer.

Macro-nutrients recovery from liquid waste as a sustainable resource for production of recovered mineral fertilizer: Uncovering alternative options to sustain global food security cost-effectively

Global food security, which has emerged as one of the sustainability challenges, impacts every country. As food cannot be generated without involving nutrients, research has intensified recently to recover unused nutrients from waste streams. As a finite resource, phosphorus (P) is largely wasted. This work critically reviews the technical applicability of various water technologies to recover macro-nutrients such as P, N, and K from wastewater. Struvite precipitation, adsorption, ion exchange, and membrane filtration are applied for nutrient recovery. Technological strengths and drawbacks in their applications are evaluated and compared. Their operational conditions such as pH, dose required, initial nutrient concentration, and treatment performance are presented. Cost-effectiveness of the technologies for P or N recovery is also elaborated. It is evident from a literature survey of 310 published studies (1985-2022) that no single technique can effectively and universally recover target macro-nutrients from liquid waste. Struvite precipitation is commonly used to recover over 95 % of P from sludge digestate with its concentration ranging from 200 to 4000 mg/L. The recovered precipitate can be reused as a fertilizer due to its high content of P and N. Phosphate removal of higher than 80 % can be achieved by struvite precipitation when the molar ratio of Mg²⁺/PO₄^3- ranges between 1.1 and 1.3. The applications of artificial intelligence (AI) to collect data on critical parameters control optimization, improve treatment effectiveness, and facilitate water utilities to upscale water treatment plants. Such infrastructure in the plants could enable the recovered materials to be reused to sustain food security. As nutrient recovery is crucial in wastewater treatment, water treatment plant operators need to consider (1) the costs of nutrient recovery techniques; (2) their applicability; (3) their benefits and implications. It is essential to note that the treatment cost of P and/or N-laden wastewater depends on the process applied and local conditions.

Technologies for pollutant removal and resource recovery from blackwater: a review

Blackwater (BW), consisting of feces, urine, flushing water and toilet paper, makes up an important portion of domestic wastewater. The improper disposal of BW may lead to environmental pollution and disease transmission, threatening the sustainable development of the world. Rich in nutrients and organic matter, BW could be treated for resource recovery and reuse through various approaches. Aimed at providing guidance for the future development of BW treatment and resource recovery, this paper presented a literature review of BWs produced in different countries and types of toilets, including their physiochemical characteristics, and current treatment and resource recovery strategies. The degradation and utilization of carbon (C), nitrogen (N) and phosphorus (P) within BW are underlined. The performance of different systems was classified and summarized. Among all the treating systems, biological and ecological systems have been long and widely applied for BW treatment, showing their universality and operability in nutrients and energy recovery, but they are either slow or ineffective in removal of some refractory pollutants. Novel processes, especially advanced oxidation processes (AOPs), are becoming increasingly extensively studied in BW treatment because of their high efficiency, especially for the removal of micropollutants and pathogens. This review could serve as an instructive guidance for the design and optimization of BW treatment technologies, aiming to help in the fulfilment of sustainable human excreta management.

Pyrolysis-induced phosphorus transformations for biosolids from diverse sources

Biosolids have been long used as a soil amendment to promote nutrient recovery. The readily releasable forms of nutrients present in this biowaste, such as phosphorus (P), along with their over application, can be detrimental to the environment, causing eutrophication. Pyrolysis, the thermal decomposition of organic materials at elevated temperature and low oxygen, seems to be a promising strategy to lower P release from biowastes such as biosolids. We pyrolyzed biosolids from various treatments and locations (Florida and Illinois; Galicia, Spain; and São Paulo, Brazil) to convert to biochar. Our objectives were (a) to use solid-state assessments, such as X-ray diffraction and scanning electron microscopy, and chemical assessments, such as water-soluble P (WSP), pH, Mehlich 3-extractable P (M3-P), total P (TP), and total Kjeldahl nitrogen, to evaluate changes caused by the conversion and (b) to compare P leaching potentials of biosolids and their corresponding biochars on two soils with varying P retention capacities. Pairwise comparisons indicated that biochar conversion significantly increased TP in the final material, but the absolute WSP decreased. However, M3-P remained the same after conversion to biochar. Cumulative P leached as a fraction of TP was greater for biosolids than their corresponding biochars. Two soils with contrasting P retention capacities predictably differed in P leaching behaviors as amended with biosolids and biochars. Differences suggest that future research could evaluate the efficacy of using mixtures of biosolids and biochar for a given soil to maintain soil fertility while reducing environmental P loss risk.

Preparation of Fe/C-MgCO3 micro-electrolysis fillers and mechanism of phosphorus removal

Iron-carbon micro-electrolysis is effective for the removal of phosphorus in wastewater; however, meeting the stringent emission standards required for treatment is difficult. To meet these treatment standards, modified micro-electrolytic fillers were prepared from iron dust, powdered activated carbon, clay, and additives using an elevated temperature roasting process under an inert atmosphere. The results show that among several additives, the modified micro-electrolytic (Fe/C-MgCO₃) fillers using MgCO₃ were the most effective at phosphorus removal. The preparation conditions for the Fe/C-MgCO₃ fillers and their effects on phosphorus removal performance were investigated. Under the optimal preparation conditions (calcination temperature: 800 °C, Fe/C = 4:1, clay content 20%, and 5% MgCO₃), the filler yielded a high compressive strength of 3.5 MPa, 1 h water absorption rate of 25.7%, and specific surface area and apparent density of 154.2 m²/g and 2689.2 kg/m³, respectively. The iron-carbon micro-electrolysis process removed 97% of phosphorus in the wastewater by using the Fe/C-MgCO₃ fillers, which was 14% more than the Fe/C filler. Electrostatic adsorption and surface precipitation were identified as the main phosphorus removal mechanisms, and the surface of the Fe/C-MgCO₃ filler was continuously updated. These results demonstrated that Fe/C-MgCO₃ is a promising filler for phosphorus removal in water treatment.

Separation and Concentration of Nitrogen and Phosphorus in a Bipolar Membrane Electrodialysis System

Struvite crystallization is a successful technique for simultaneously recovering PO₄^3- and NH₄⁺ from wastewater. However, recovering PO₄^3- and NH₄⁺ from low-concentration solutions is challenging. In this study, PO₄^3-, NH₄⁺, and NO₃^- were separated and concentrated from wastewater using bipolar membrane electrodialysis, PO₄^3- and NH₄⁺ can then be recovered as struvite. The separation and concentration of PO₄^3- and NH₄⁺ are clearly impacted by current density, according to experimental findings. The extent of separation and migration rate increased with increasing current density. The chemical oxygen demand of the feedwater has no discernible impact on the separation and recovery of ions. The migration of PO₄^3-, NH₄⁺, and NO₃^- fits zero-order migration kinetics. The concentrated concentration of NH₄⁺ and PO₄^3- reached 805 mg/L and 339 mg/L, respectively, which demonstrates that BMED is capable of effectively concentrating and separating PO₄^3- and NH₄⁺. Therefore, BMED can be considered as a pretreatment method for recovering PO₄^3- and NH₄⁺ in the form of struvite from wastewater.

Phosphorus recovery as K-struvite from a waste stream: A review of influencing factors, advantages, disadvantages and challenges

Currently, the depletion of natural resources and contamination of the surrounding environment demand a paradigm shift to resource recycling and reuse. In this regard, phosphorus (P) is a model nutrient that possesses the negative traits of depletion (will be exhausted in the next 100 years) and environmental degradation (causes eutrophication and climate change), and this has prompted the scientific community to search for options to solve P-related problems. To date, P recovery in the form of struvite from wastewater is one viable solution suggested by many scholars. Struvite can be recovered either in the form of NH₄-struvite (MgNH₄PO₄•6H₂O) or K-struvite (MgKPO₄•6H₂O). From struvite, K (MgKPO₄•6H₂O) and N (MgNH₄PO₄•6H₂O) are important nutrients for plant growth, but N is more abundant in the environment than K (the soil's most limited nutrient), which requires a systematic approach during P recovery. Although K-struvite recovery is a promising approach, information related to its crystallization is deficient. Here, we present the general concept of P recovery as struvite and details about K-struvite, such as the source of nutrients, factors (pH, molar ratio, supersaturation, temperature, and seeding), advantages (environmental, economic, and social), disadvantages (heavy metals, pathogenic organisms, and antibiotic resistance genes), and challenges (scale-up and acceptance). Overall, this study provides insights into state-of-the-art K-struvite recovery from wastewater as a potential slow-release fertilizer that can be used as a macronutrient (P-K-Mg) source for plants as commercial grade-fertilizers.

Donnan dialysis for phosphate recovery from diverted urine

There is a critical need to shift from existing linear phosphorous management practices to a more sustainable circular P economy. Closing the nutrient loop can reduce our reliance on phosphate mining, which has well-documented environmental impacts, while simultaneously alleviating P pollution of aquatic environments from wastewater discharges that are not completely treated. The high orthophosphate, H_xPO₄^(3-x)-, content in source-separated urine offers propitious opportunities for P recovery. This study examines the use of Donnan dialysis (DD), an ion-exchange membrane-based process, for the recovery of orthophosphates from fresh and hydrolyzed urine matrixes. H₂PO₄^- transport against an orthophosphate concentration gradient was demonstrated and orthophosphate recovery yields up to 93% were achieved. By adopting higher feed to receiver volume ratios, DD enriched orthophosphate in the product stream as high as ≈2.5 × the initial urine feed concentration. However, flux, selectivity, and yield of orthophosphate recovery were detrimentally impacted by the presence of SO₄^2- and Cl^- in fresh urine, and the large amount of HCO₃^- rendered hydrolyzed urine practically unsuitable for P recovery using DD. The detrimental effects of sulfate ions can be mitigated by utilizing a monovalent ion permselective membrane, improving selectivity for H₂PO₄^- transport over SO₄^2- by 3.1 × relative to DD with a conventional membrane; but the enhancement was at the expense of reduced orthophosphate flux. Critically, widely available and low-cost/waste resources with sufficiently high Cl^- content, such as seawater and waste water softening regenerant rinse, can be employed to improve the economic viability of orthophosphate recovery. This study shows the promising potential of DD for P recovery and enrichment from source-separated urine.

Efficient phosphorus recovery as struvite by microbial electrolysis cell with stainless steel cathode: Struvite purity and experimental factors

Phosphorus (P) recovery from waste streams is an essential choice due to the coming global P crisis. One promising solution is to recover P by microbial electrolysis cell (MEC). Both the P recovery effectiveness and product quality are of critical importance for application. In this study, a two-chamber MEC was constructed and the effects of applied voltage, NaAc concentration, Mg/P molar ratio, N/P molar ratio, and initial P concentration on P recovery and product purity were explored. The maximum P recovery efficiency of 99.64 % and crystal accumulation rate over 106.49 g/m³-d were achieved. Struvite (MAP) was confirmed as the final recovered product and the purity obtained could reach up to 99.95 %. Besides, higher applied voltage, N/P molar ratio and initial P concentration could promote P recovery efficiency, while the purity of MAP showed correlation with applied voltage, Mg/P molar ratio, N/P molar ratio and initial P concentration. The correlation between NaAc concentration and both of the above was not very significant. A lower energy consumption of 4.1 kWh/kg P was observed at the maximum P recovery efficiency. In addition, the efficiency of P recovery from real wastewater also could reach nearly 88.25 %. These results highlight the promising potential of efficient phosphorus recovery from P-rich wastewater by MEC.

Phosphorus retention and agronomic efficiency of refined manure-based digestate—A review

Digestate, a by-product from anaerobic digestion of organic materials such as animal manure, is considered a suitable plant fertilizer. However, due to its bulkiness and low economic value, it is costly to transport over long distances and store for long periods. Refinement processes to valorize digestate and facilitate its handling as a fertilizer include precipitation of phosphorus-rich mineral compounds, such as struvite and calcium phosphates, membrane filtration methods that concentrate plant nutrients in organic products, and carbonization processes. However, phosphorus retention efficiency in output products from these processes can vary considerably depending on technological settings and characteristics of the digestate feedstock. The effects of phosphorus in plant fertilizers (including those analogous or comparable to refined digestate products) on agronomic productivity have been evaluated in multiple experiments. In this review, we synthesized knowledge about different refinement methods for manure-based digestate as a means to produce phosphorus fertilizers, thereby providing the potential to increase phosphorus retention in the food production chain, by combining information about phosphorus flows in digestate refinement studies and agronomic fertilizer studies. It was also sought to identify the range, uncertainty, and potential retention efficiency by agricultural crops of the original phosphorus amount in manure-based digestate. Refinement chains with solid/wet phase separation followed by struvite or calcium phosphate precipitation or membrane filtration of the wet phase and carbonization treatments of the solid phase were included. Several methods with high potential to extract phosphorus from manure-based wet phase digestate in such a way that it could be used as an efficient plant fertilizer were identified, with struvite precipitation being the most promising method. Synthesis of results from digestate refinement studies and agronomic fertilizer experiments did not support the hypothesis that solid/wet separation followed by struvite precipitation, or any other refinement combination, results in higher phosphorus retention than found for unrefined digestate. Further studies are needed on the use of the phosphorus in the solid phase digestate, primarily on phosphorus-rich soils representative of animal-dense regions, to increase understanding of the role of digestate refinement (particularly struvite precipitation) in phosphorus recycling in agricultural systems.

Development of a New Hydrogel Anion Exchange Membrane for Swine Wastewater Treatment

We developed a proprietary anion exchange membrane (AEM) for wastewater treatment as an alternative to commercial products. Following the successful development of a hydrogel cation exchange membrane on a porous ceramic support, we used the same approach to fabricate an AEM. Different positively charged monomers and conditions were tested, and all AEMs were tested for nitrate and phosphate anion removal from buffers by electrodialysis. The best AEM was tested further with real swine wastewater for phosphate removal by electrodialysis and nitrate removal in a bioelectrochemical denitrification system (BEDS). Our new AEM showed better phosphate removal compared with a commercial membrane; however, due to its low permselectivity, the migration of cations was detected while operating a two-chambered biocathode BEDS in which the membrane was utilized as a separator. After improving the permselectivity of the membrane, the performance of our proprietary AEM was comparable to that of a commercial membrane. Because of the usage of a porous ceramic support, our AEM is self-supporting, sturdy, and easy to attach to various frames, which makes the membrane better suited for harsh and corrosive environments, such as swine and other animal farms and domestic wastewater.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.

Increasing resource circularity in wastewater treatment: Environmental implications of technological upgrades

A paradigm shift is needed in wastewater treatment plants (WWTPs) to progress from traditional pollutant removal to resource recovery. However, whether this transformation produces overall environmental benefits will depend on the efficient and sustainable use of resources by emerging technologies. Given that many of these technologies are still being tested at the pilot scale, there is a lack of environmental assessments quantifying their impacts and benefits. In particular, an integrated approach to energy and nutrient recovery can elucidate the potential configurations for WWTPs. In this study, we conduct a life cycle assessment (LCA) of emergent wastewater treatment technologies aimed at increasing resource circularity in WWTPs. We focus on increasing energy self-sufficiency through biogas upgrades and a more radical circular approach aimed at nutrient recovery. Based on a case-study WWTP, we compare its current configuration with (1) implementing autotrophic nitrogen removal in the mainstream and deriving most of the organic matter for biogas production, which increases the quality and quantity of biogas available for energy production; (2) implementing struvite recovery through enhanced biological phosphorus removal (EBPR) as a radical approach to phosphorus management, offering an alternative to mineral fertilizer; and (3) a combination of both approaches. The results show that incremental changes in biogas production are insufficient for compensating for the environmental investment in infrastructure, although autotrophic nitrogen removal is beneficial for increasing the quality of the effluent. Combined phosphorus and energy recovery reduce the environmental impacts from the avoided use of fertilizers and phosphorus and the nitrogen release into water bodies. An integrated approach to resource management in WWTPs is thus desirable and creates new opportunities toward the implementation of circular strategies with low environmental impact in cities.

Use of a wastewater recovery product (struvite) to enhance subtropical seagrass restoration

Seagrasses are in decline worldwide, and their restoration is relatively expensive and unsuccessful compared to other coastal systems. Fertilization can improve seagrass growth in restoration but can also release nutrients and pollute the surrounding ecosystem. A slow-release fertilizer may reduce excessive nutrient discharge while still providing resources to the seagrass's rhizosphere. In this study, struvite (magnesium ammonium phosphate), a relatively insoluble, sustainable compound harvested in wastewater treatment plants, was compared to Osmocote™(14:14:14 Nitrogen: Phosphorus: Potassium, N:P:K), a popular polymer coated controlled release fertilizer commonly used in seagrass restoration. Two experiments compared the effectiveness of both fertilizers in a subtropical flow-through mesocosm setup. In the first experiment, single 0.5 mg of P per g dry weight (DW) doses of Osmocote™and struvite fertilizers were added to seagrass plots. Seagrass shoot counts were significantly higher in plots fertilized with struvite than both the Osmocote™and unfertilized controls (p< 0.0001). A significant difference in total P concentration was observed in porewater samples of Osmocote™vs struvite and controls (p< 0.0001), with struvite fertilized plots emitting more than controls (p ≤ 0.0001), but less than 2% of the total dissolved P (TDP) of Osmocote™fertilized plots (100+ mg/L versus x > 5 mg/L). A subsequent experiment, using smaller doses (0.01 and 0.025 mg of P per gram DW added), also found that the struvite treatments performed better than Osmocote™, with 16-114% more aboveground biomass (10-60% higher total biomass) while releasing less N and P. These results indicate the relatively rapid dissolution of Osmocote™may pose problems to restoration efforts, especially in concentrated doses and possibly leading to seagrass stress. In contrast, struvite may function as a slow-release fertilizer applicable in seagrass and other coastal restoration efforts.

Struvite-based composites for slow-release fertilization: a case study in sand

Struvite (St) recovered from wastewaters is a sustainable option for phosphorus (P) recovery and fertilization, whose solubility is low in water and high in environments characterized by a low pH, such as acidic soils. To broaden the use of struvite in the field, its application as granules is recommended, and thus the way of application should be optimized to control the solubility. In this study struvite slow-release fertilizers were designed by dispersing St particles (25, 50, and 75 wt%) in a biodegradable and hydrophilic matrix of thermoplastic starch (TPS). It was shown that, in citric acid solution (pH = 2), TPS promoted a steadier P-release from St compared to the pure St pattern. In a pH neutral sand, P-diffusion from St-TPS fertilizers was slower than from the positive control of triple superphosphate (TSP). Nevertheless, St-TPS featured comparable maize growth (i.e. plant height, leaf area, and biomass) and similar available P as TSP in sand after 42 days of cultivation. These results indicated that St-TPS slow P release could provide enough P for maize in sand, achieving a desirable agronomic efficiency while also reducing P runoff losses in highly permeable soils.

Process optimization of struvite recovered from slaughterhouse wastewater and its fertilizing efficacy in amendment of biofertilizer

The intensive discharge of slaughterhouse waste into water bodies increases Nitrogen (N), Phosphorus (P) in the wastewater and leads to various environmental problems. On the other hand, the increasing treatment effort after the extraction of these valuable nutrients in the commercial fertilizer reduces the dependence on scarce phosphate resources. The viable solution is to recover N, P as struvite (magnesium ammonium phosphate) from nutrient rich waste water as a small scale treatment unit application. The main parameters that have a significant impact on the process, including pH, Mg: P ratio, and precipitation time, were investigated from slaughterhouse wastewater using a central composite design and the experimental data's were statistically analysed. The results indicated that pH and Mg/P ratio level had a significant impact and thus 85% struvite precipitation efficiency was achieved at 9.6 pH and 1.5 dose mol ratio (mol Mg per mol P), in an inexpensive, stirred tank batch reactor with a retention time of 70 min. The fertilization efficiency was tested on the growth of Solanum melongena L with the obtained struvite and the integration of struvite with the Azospirullum rhizobium and Bacillus megaterium. Treatment of struvite, struvite with Azospirillum rhizobium and Bacillus megaterium increased growth parameters by 10%, 20%, and 25%, respectively, over control. The assessment of growth factors showed the most amazing number of fruits, shoots, and root length in a standard ratio of 60:40 of struvite to bio-inoculants compared to sole struvite fertilizer. Findings of this study would be beneficial to determine the feasibility of slaughterhouse waste as a phosphorus source for struvite recovery.

Technical advances on current research trends and explore the future scope on nutrient recovery from waste-streams: a review and bibliometric analysis from 2000 to 2020

An exponentially growing global population has led to an increase in nutrient pollution in different aqueous bodies. Although different processes have successfully removed nutrients from wastewater on a large scale, a limited number of studies have been reported on efficiency, cost-effectiveness, and future potential of physical, chemical, and biological nutrient recovery methods to overcome the depletion of natural resources. Therefore, researchers need to understand current research trends by applying different approaches to investigate higher efficient nutrient recovery technologies. In this article, the research patterns and in-depth review of various nutrient recovery processes have been circumscribed with the application of bibliometric and attractive index (AAI) vs. activity index (AI) analysis. The performance, advantages, limitations, and future prospects of different nutrient recovery methods have also been addressed. More than 70% of study publications were published in the last decade in chemical and biological processes, which might be related to more rigorous effluent quality rules and increasing water pollution. The future prediction in the field of nutrient recovery has been predicted using S-curve analysis, and it was found that the number of publications in the saturated state in chemical methods was highest. However, the growth rate of the biological-based nutrient recovery methods is greater, which may be because of their huge research scope, cost-effectiveness, and easy operation methods. This study can assist researchers in understanding the current research scenario in nutrient recovery techniques and provide the research scope in nutrient recovery from wastewater in the future.

Effects of Calcium on the Removal of Ammonium from Aged Landfill Leachate by Struvite Precipitation

Ammonium in landfill leachates is a major contributor to environmental degradation if not effectively treated. However, it could be converted to a valuable fertilizer when it is co-precipitated with phosphate and magnesium as struvite. Low-cost magnesium and phosphate sources are sought to offset the co-precipitation treatment costs, but most of the identified alternative magnesium sources have significant amounts of calcium, which may negatively impact the ammonium removal rates. In this study, the effects of calcium on ammonium removal from high-strength aged field landfill leachate as struvite were investigated. Laboratory-scale batch tests were conducted to assess the effects of the pH, Mg2+:NH4+:PO43−, and Ca2+:Mg2+ molar ratios on ammonium removal. Magnesium chloride salt was used as a model dissolved magnesium source, whereas different compounds derived from dolomite (CaMg(CO3)2) were used as model solid-phase magnesium sources. X-ray powder diffraction and activity ratio diagrams were used to delineate the ammonium removal mechanisms and struvite stability. The ammonium removal rate of the magnesium salt decreased from approximately 97% to 70%, upon increasing the Ca2+:Mg2+ molar ratio from 0 to 1.0, for the Mg2+:NH4+:PO43− molar ratio of 1.25:1:1.25 and pH = 9.5. For similar pH values, as well as the Mg2+:NH4+:PO43− and Ca2+:Mg2+ molar ratios, the ammonium removal rates by the dolomite-derived compounds reached up to 55%, which highlighted the limited availability of magnesium in solid phases, in addition to the negative impacts of calcium. The diffractometric analysis and thermodynamic calculations revealed the stable regions of struvite in the presence of competing solid phases. The new findings in this study could aid in the design of ammonium and phosphate removal and recovery systems by struvite precipitation.

Recycling of nutrients from landfill leachate: A case study

The continuous increase in the consumption of natural resources requires different solutions directed to the recovery and recycling of different materials and products, including the nutrients used as fertilizers for food production. In this context, this research assessed the feasibility of using landfill leachate as a source of nutrients for the growth of maize. Leachate was treated to precipitate struvite, a rich magnesium, phosphate, and ammonium mineral that can be applied directly as fertilizer. It was used for the growth of maize, which was sowed in three different parcels. A commercial DAP + urea mixture was used to compare, and non-fertilized parcels were used as controls. Struvite was successfully obtained and applied in the fields. A marginal higher maize yield was achieved in two sites when using struvite (6.36% and 2.16%) compared to the commercial fertilizer, even if it was applied in a lower dose to weather conditions. An increase in N and Mg in soil could be observed, which allowed for the assimilation of nutrients in the plants. Concerning safety, the use of struvite did not produce the transfer of heavy metals or pathogens to the soil or plants. This research shows a promising way of dealing with leachate, which could be attractive in countries where organic waste is buried in landfills.

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Landfill leachate can be used as a source of nutrients for the grow of maize by precipitation of struvite.

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A field trial in real scale was performed.

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A marginal higher maize yield was achieved in two of the sites (6.36% and 2.16%) when compared to the commercial fertilizer.

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Struvite did not cause presence of pathogens or heavy metals in the crops.

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It offers an alternative to conventional leachate treatment options, aligned with the principles of the circular economy.

Recovery of phosphate and ammonium nitrogen as struvite from aqueous solutions using a magnesium-air cell system

The addition of alkaline and magnesium sources during the recovery of NH₄⁺ and PO₄^3- in the form of struvite using the traditional struvite precipitation method increases the production cost. To solve this problem, a magnesium-air cell (MAC) system was used herein to recover NH₄⁺ and PO₄^3- as struvite from wastewater using a magnesium strip (Mg²⁺) and the oxygen adsorbed on the surface of a titanium plate (OH^-) as the anode and cathode, respectively. Experimental parameters (i.e. initial solution pH, temperature, NH₄⁺/PO₄^3- molar ratio, NH₄⁺ and PO₄^3- initial concentrations and stirring intensity) were found to affect the removal rate of NH₄⁺ and PO₄^3-. The presence of Ca²⁺ decreased the struvite purity. At Ca²⁺/PO₄^3- ratios of 0:1 and 0.5:1, the purity of the obtained struvite after 6 h was 93.8% and 58.9%, respectively. Struvite with a purity of 95.7%, electricity with an average output power of 2.53 mW, and an energy density of 1.05 W/m² were obtained when the proposed system was used to recover NH₄⁺ and PO₄^3- from an actual supernatant of domestic sludge anaerobic digestion. Scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analyses showed that the obtained struvite exhibited almost the same physicochemical properties as commercial struvite. Thus, the MAC system can be regarded as an effective method for recovering NH₄⁺ and PO₄^3- in the form of struvite from wastewater.

Phosphorus recovery from wastewater and bio-based waste: an overview

Phosphorus is one of the most important macronutrients needed for the growth of plants. The fertilizer production market uses 80% of natural, non-renewable phosphorus resources in the form of phosphate rock. The depletion of those deposits forces a search for other alternatives, including biological waste. This review aims to indicate the most important ways to recover phosphorus from biowaste, with particular emphasis on wastewater, sewage sludge, manure, slaughter or food waste. A comparison of utilized methods and directions for future research based on the latest research is presented. Combining biological, chemical, and physical methods with thermal treatment appears to be the most effective way for the treatment of wastewater sludge in terms of phosphorus recovery. Hydrothermal, thermochemical, and adsorption on thermally treated adsorbents are characterized by a high phosphorus recovery rate (over 95%). For animal by-products and other biological waste, chemical methods seems to be the most optimal solution with a recovery rate over 96%. Due to its large volume and relatively low phosphorus content, wastewater is a resource that requires additional treatment to recover the highest possible amount of phosphorus. Pretreatment of wastewater with combined methods seems to be a possible way to improve phosphorus recovery. A compressive evaluation of combined methods is crucial for future research in this area.

Sustainable biochar-based soil fertilizers and amendments as a new trend in biochar research

Today's world is struggling with many environmental problems. Due to the ever-growing size of the population, it is necessary to produce more and more food. The consequence of such a large demand for food is excessive fertilization of soils, often in an uncontrolled manner. The paper presents an overview of the different types of biochar (BC) fertilizers obtained by: coating BCs with a protective layer, coating commercial fertilizers with a layer of BCs, or mixing BCs with commercial fertilizers. Although the use of these new types of fertilizers has a positive effect on soil properties and crop yields, the production and use of "simple" inorganic fertilizers are dominant. The solution to starting the change of this trend may be the use of BC-compost systems as an effective soil amendment, due to the fact that composts are still the most frequently used products by farmers. The review summarized two types of BC-compost soil amendments: BC mixed with ready-made compost and BC co-composted with compost raw material. These types of soil amendments contribute to a significant reduction in the consumption of commercial inorganic fertilizers, and thus less pollution of the natural environment, while allowing for a high yield of safe food.

Bioelectricity generation from human urine and simultaneous nutrient recovery: Role of Microbial Fuel Cells

Urine is a 'valuable waste' that can be exploited to generate bioelectricity and recover key nutrients for producing NPK-rich biofertilizers. In recent times, improved and innovative waste management technologies have emerged to manage the rapidly increasing environmental pollution and to accomplish the goal of sustainable development. Microbial fuel cells (MFCs) have attracted the attention of environmentalists worldwide to treat human urine and produce power through bioelectrochemical reactions in presence of electroactive bacteria growing on the anode. The bacteria break down the complex organic matter present in urine into simpler compounds and release the electrons which flow through an external circuit generating current at the cathode. Many other useful products are harvested at the end of the process. So, in this review, an attempt has been made to synthesize the information on MFCs fuelled with urine to generate bioelectricity and recover value-added resources (nutrients), and their modifications to enhance productivity. Moreover, configuration and mode of system operation, and factors enhancing the performance of MFCs have been also presented.

Wastewater in India: An untapped and under-tapped resource for nutrient recovery towards attaining a sustainable circular economy

Wastewater (WW) contains nitrogen (N) and phosphorus (P), where N oxidizes to nitrate followed by denitrification to release N₂ and P is accumulated in sludge. Higher concentrations of N and P leads to eutrophication and algal blooming, thereby threatening the aquatic life systems. Such nutrients could be potentially recovered avoiding the fertilizer requirements. Distinct nutrient recovery systems have been demonstrated including chemical precipitation, ion-exchange, adsorption, bio-electrochemical systems, and biological assimilation at various scales of volumes. This study focusses on the nutrient recovery possibility from wastewater in India. The resource estimation analysis indicates that at 80% recovery, 1 million liters per day (MLD) of sewage can generate 17.3-kg of struvite using chemical precipitation. When compared with traditional fertilizers, nutrient recovery from sewage has the potential to avoid 0.38-Mt/a in imports. Replacing conventional fertilizer with struvite recovered from WW avoids 663.2 kg CO_2eq/ha in emissions (53%). Prevailing WW treatment looks at maintaining the discharging standards while recovering nutrients is an advanced option for a self-reliant and sustainable circular economy. However, more detailed assessments are necessary from techno-economic and environmental perspective in realizing these technologies at an industrial scale.

A Simple Population Balance Model for Crystallization of L-Lactide in a Mixture of n-Hexane and Tetrahydrofuran

In this contribution, crystallization was performed to assess the kinetics of nucleation and crystal growth of L-lactide. In most common solvents, this compound shows very high solubility even at low temperatures, which could be challenging for crystallization process design. In the first part of this paper, the anti-solvent effects of n-hexane on solutions of L-lactide in tetrahydrofuran (THF) were investigated through studying the influence of solvent compositions on the solubility. Thanks to these effects, the solubility of the interested compound can be adjusted to desired degrees of supersaturation by adding suitable amounts of the anti-solvent. In the second part, a solvent composition at a mass ratio of 45/55 (n-hexane/THF) was chosen, and an isothermal seeded crystallization process was implemented. The evolution of the particle sizes and changes in the solute concentration profile of this process were monitored. Based on the obtained data, a widely used model, i.e., the population balance equation (PBE), was then utilized to model the crystal size distribution (CSD). Reasonable assumptions were made to reduce the mathematical complexity of the PBE. In the simplified model, only crystal growth and secondary nucleation were considered for model formulation, with assumptions of the size-independent growth rate and negligible size of nuclei. The kinetic parameters were estimated by using the seed and final-time crystal density functions in combination with variations in the concentration of the mother liquor. Indeed, the numerical solution for the one-dimensional problem of the L-lactide crystallization based on the estimated parameters gained a relatively good agreement with the determined CSD. Furthermore, the obtained model also correlated well with the variations in the solute concentration of the mother liquor. In short, this simple approach can be used for predicting the productivity and CSD of the L-lactide crystallization.

Membrane distillation crystallization technology for zero liquid discharge and resource recovery: Opportunities, challenges and futuristic perspectives

Water resources are getting limited, which emphasises the need for the reuse of wastewater. The conventional waste(water) treatment methods such as reverse osmosis (RO) and multi-effect distillation (MED) are rendered limited due to certain limitations. Moreover, the imposition of stringent environmental regulations in terms of zero liquid discharge (ZLD) of wastewater containing very high dissolved solids has assisted in developing technologies for the recovery of water and useful solids. Membrane distillation crystallization (MDCr) is an emerging hybrid technology synergising membrane distillation (MD) and crystallization, thus achieving ZLD. MDCr technology can be applied to desalinate seawater, treat nano-filtration, and RO reject brine and industrial wastewater to increase water recovery and yield useful solids. This manuscript focuses on recent advances in MDCr, emphasizing models that account for application in (waste)water treatment. MDCr has dual benefits, first the environmental conservation due to non-disposal of wastewater and second, resources recovery proving the proverb that waste is a misplaced resource. Limitations of standalone MD and crystallization are discussed to underline the evolution of MDCr. In this review, MDCr's ability and feasibility in the treatment of industrial wastewater are highlighted. This manuscript also examines the operational issues, including crystal deposition (scaling) on the membrane surface, pore wetting phenomenon and economic consequences (energy use and operating costs). Finally, opportunities and future prospects of the MDCr technology are discussed. MDCr technology can amplify natural resources availability by recovering freshwater and useful minerals from the waste stream, thus compensating for the relatively high cost of the technology.

Bio-membrane integrated systems for nitrogen recovery from wastewater in circular bioeconomy

Wastewater contains a significant amount of recoverable nitrogen. Hence, the recovery of nitrogen from wastewater can provide an option for generating some revenue by applying the captured nitrogen to producing bio-products, in order to minimize dangerous or environmental pollution consequences. The circular bio-economy can achieve greater environmental and economic sustainability through game-changing technological developments that will improve municipal wastewater management, where simultaneous nitrogen and energy recovery are required. Over the last decade, substantial efforts were undertaken concerning the recovery of nitrogen from wastewater. For example, bio-membrane integrated system (BMIS) which integrates biological process and membrane technology, has attracted considerable attention for recovering nitrogen from wastewater. In this review, current research on nitrogen recovery using the BMIS are compiled whilst the technologies are compared regarding their energy requirement, efficiencies, advantages and disadvantages. Moreover, the bio-products achieved in the nitrogen recovery system processes are summarized in this paper, and the directions for future research are suggested. Future research should consider the quality of recovered nitrogenous products, long-term performance of BMIS and economic feasibility of large-scale reactors. Nitrogen recovery should be addressed under the framework of a circular bio-economy.

Crystallization of struvite-K from pumpkin wastes

BACKGROUND

Use of slow-release fertilizers derived from biological sources is important in sustainable agricultural development. Struvite-K (KMgPO₄ ·6H₂ O) is magnesium potassium phosphate mineral that has high potential for use as fertilizer in agriculture. Struvite-K is particularly suitable for slow-release fertilizer systems since struvite-K crystals are sparingly soluble in water. Seeds of pumpkin Cucurbita pepo L. are recovered and consumed as food, but the remaining pulp has no economic value.

RESULTS

The present study evaluated the feasibility of struvite-K crystals recovery from pyrolysis products of pumpkin wastes. In the study C. pepo pulp was decomposed at high temperatures and potassium was extracted from the residue and then crystalized from the solution by addition of NaH₂ PO₄ ·2H₂ O and MgCl₂ ·6H₂ O salts. Struvite-K was characterized by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) spectroscopy and X-ray diffraction (XRD) analysis.

CONCLUSIONS

The study revealed pumpkin wastes can be evaluated as source of potassium and 80% of potassium could be recovered as struvite-K crystals, which have a potential use as a slow-release mineral fertilizer for sustainable agriculture operations. © 2021 Society of Chemical Industry.

Enhanced struvite generation and separation by magnesium anode electrolysis coupled with cathode electrodeposition

Adding magnesium ions (Mg²⁺) to produce struvite is an important method to recover nitrogen and phosphorus from wastewater. Both the Mg²⁺ source and subsequent separation of struvite are key factors for the utilization of struvite. In this study, we developed an efficient method to recover nutrient salts from wastewater using sacrificial Mg anodes to generate struvite, with its simultaneous separation through cathode electrodeposition. The anode-released Mg²⁺ reacted with NH₄⁺-N and PO₄^3--P in bulk solution to form struvite, which was more intense on the cathode surface due to the relatively higher pH environment from hydrogen evolution, resulting in most of the struvite being deposited on the cathode surface and simultaneously separated out of the bulk solution. Using a cathode with a higher solution-cathode interface area and relatively low current density facilitated struvite deposition. Results showed that under optimal electrolysis condition (5.76 A/m², pH 8.5, 180 min, and 1.2:1.0 Mg:P), 91% of the undissolved substances as the phosphate precipitation were deposited on the graphite cathode surface, and the proportion of struvite in the deposition reached 41.52%. This study provides a novel electrochemical method for struvite synthesis and separation for the recovery of nitrogen and phosphorus from wastewater.

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.