Nutrient recovery from anaerobically digested chicken slurry via struvite: Performance optimization and interactions with heavy metals and pathogens

Fate of micropollutants in struvite production from swine wastewater with sacrificial magnesium anode

Struvite recovery shows significant potential for simultaneously recovering nitrogen (N) and phosphorus (P) from swine wastewater but is challenged by the occurrence and transformation of antibiotic residuals. Electrochemically mediated struvite precipitation with sacrificial magnesium anode (EMSP-Mg) is promising due to its automation and chemical-free merits. However, the fate of antibiotics remains underexplored. We investigated the behavior of sulfadiazine (SD), an antibiotic frequently detected but less studied than others within the EMSP-Mg system. Significantly less SD (≤ 5%) was co-precipitated with recovered struvite in EMSP-Mg than conventional chemical struvite precipitation (CSP) processes (15.0 to 50.0%). The reduced SD accumulation in struvite recovered via EMSP was associated with increased pH and electric potential differences, which likely enhanced the electrostatic repulsion between SD and struvite. In contrast, the typical strategies used in enhancing P removal in the EMSP-Mg system, including increasing the Mg/P ratio or the Mg-release rates, have shown negligible effects on SD adsorption. Furthermore, typical coexisting ions (Ca2+, Cl-, and HCO3-) inhibited SD adsorption onto recovered products. These results provide new insights into the interactions between antibiotics and struvite within the EMSP-Mg system, enhancing our understanding of antibiotic migration pathways and aiding the development of novel EMSP processes for cleaner struvite recovery.

Heavy metal sorption on struvite recovered from livestock wastewaters and release properties of granular forms

Phosphorus recovery from wastewater is receiving more attention due to its non-renewable property. As copper (Cu) and zinc (Zn) usually occur in livestock wastewater, this study focused on metal sorption in struvite from swine wastewater and the release properties of granular struvite in solution with varying pH conditions (2, 4, 7). The results demonstrated pH values presented a slightly decreasing trend with increasing Cu/Zn ratio, and Zn exhibited higher sorption performance on struvite crystals than that of Cu. Under the high content of metals in the wastewater, Cu/Zn ratios in the wastewater contributed to varying metal binding forms and mechanisms, resulting in the difference in the leaching properties of nutrients and metal. For the granular struvite manufactured with the adhesion of alginate, the P release percentage achieved 30.3-40.5% after 96 h in the wastewater of pH 2, whereas they were only 5.63-8.92% and 1.05-1.50% in the wastewater of pH 4 and 7, respectively. Acid wastewater contributed to the release of two metals, and the release amount of Zn was higher than that of Cu, which is associated with their sorption capacity in crystals. During the latter soil leaching test of adding granular struvite, the NH4+-N and PO43--P concentration in the effluent ranged from 0.34 to 1.26 and 0.62 to 2.56 mg/L after 96 h, respectively. However, the Cu and Zn could not be measured due to lower than the detection limit under varying treatments. Struvite might be accompanied by quicker metal leaching and slower nutrient leaching when surface sorption dominates in wastewater with lower metal concentrations.

Physico-chemical assessment of on-farm bioconversion of organic waste in dairy farms in context to sustainability and circular bioeconomy

On a milk-producing dairy farm, milk production is correlated with manure production and the number of cattle, and manure is widely used as a soil fertilizer. However, excessive dairy manure production is linked with greenhouse gas emissions and water quality issues. On-farm planning of manure storage and application to enhance soil nutrients are essential in a circular economy to reduce environmental impact, where manure is not landfilled and incinerated. Instead, it creates a nutrient resource for crops and soil. Dairy manure, which is rich in nutrients, is a valuable fertilizer that contains many nutrients such as nitrogen (N), organic matter (OM), phosphorous (P), Potassium (K) and micronutrients. In this work, a pilot field research was conducted between 2016 and 2018 in various parts of California, USA (San Joaquin Valley, Sacramento Valley, Shasta Cascade, and the North Coast of California) to assess physio-chemical characteristics of solid fractions of dairy manure among various dairy farms. A total of 156 samples were collected from the gut (n = 107) and toe (n = 49) of the manure piles across California for determining total solid (TS), volatile solid (VS), temperature, moisture content and carbon-nitrogen ratio (C: N). Here, using the observations of field study and analysis, we show that C: N, OM and MC of solid fractions of dairy manure vary significantly among dairy farms. The average C: N ratio of manure (26-32) among various regions was close to an ideal C: N value of 24:1 for soil microbes to stimulate nutrient release to crops. Manure pH ranged between 7.0 and 8.0, which was close to an optimal pH range for common crops (6.0-8.0). Moreover, considering less cost and surplus availability, manure will likely continue providing a cost-effective organic fertilizer resource compared to commercial chemical fertilizers.

Struvite-Driven Integration for Enhanced Nutrient Recovery from Chicken Manure Digestate

This study investigated the synergistic integration of clean technologies, specifically anaerobic digestion (AD) and struvite precipitation, to enhance nutrient recovery from chicken manure (CM). The batch experiments were conducted using (i) anaerobically digested CM digestate, referred to as raw sample (RS), (ii) filtered digestate sample (FS), and (iii) a synthetically prepared control sample (CS). The research findings demonstrated that the initial ammonia concentration variations did not significantly impact the struvite precipitation yield in the RS and FS, showcasing the materials inertness process's robustness to changing ammonia concentrations. Notably, the study revealed that the highest nitrogen (N) recovery, associated with 86% and 88% ammonia removal in the CS and FS, was achieved at pH 11, underscoring the efficiency of nutrient recovery. The RS achieved the highest nitrogen recovery efficiency at pH 10, at 86.3%. In addition, the research highlighted the positive impact of reducing heavy metal levels (Zn, Cu, Pb, Ni, Cd, Cr and Fe) and improving the composition of the microbial community in the digestate. These findings offer valuable insights into sustainable manure and nutrient management practices, emphasizing the potential benefits for the agricultural sector and the broader circular economy. Future research directions include economic viability assessments, regulatory compliance evaluations, and knowledge dissemination to promote the widespread adoption of these clean technologies on a larger scale. The study marks a significant step toward addressing the environmental concerns associated with poultry farming and underscores the potential of integrating clean technologies for a more sustainable agricultural future.

Co-precipitation of Cd with struvite during phosphorus recovery

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

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.

State, synthesis, perspective applications, and challenges of Graphdiyne and its analogues: A review of recent research

Carbon materials technology provides the possibility of synthesizing low-cost, outstanding performance replacements to noble-metal catalysts for long-term use. Graphdiyne (GDY) is a carbon allotrope with an extremely thin atomic thickness. It consists of carbon elements, that are hybridized with both sp. and sp2, resulting in a multilayered two-dimensional (2D) configuration. Several functional models suggest, that GDY contains spontaneously existing band structure with Dirac poles. This is due to the non-uniform interaction among carbon atoms, which results from various fusions and overlapping of the 2pz subshell. Unlike other carbon allotropes, GDY has Dirac cone arrangements, that in turn give it inimitable physiochemical characteristics. These properties include an adjustable intrinsic energy gap, high speeds charging transport modulation efficiency, and exceptional conductance. Many scientists are interested in such novel, linear, stacked materials, including GDY. As a result, organized synthesis of GDY has been pursued, making it one of the first synthesized GDY materials. There are several methods to manipulate the band structure of GDY, including applying stresses, introducing boron/nitrogen loading, utilizing nanowires, and hydrogenations. The flexibility of GDY can be effectively demonstrated through the formation of nano walls, nanostructures, nanotube patterns, nanorods, or structured striped clusters. GDY, being a carbon material, has a wide range of applications owing to its remarkable structural and electrical characteristics. According to subsequent research, the GDY can be utilized in numerous energy generation processes, such as electrochemical water splitting (ECWS), photoelectrochemical water splitting (PEC WS), nitrogen reduction reaction (NRR), overall water splitting (OWS), oxygen reduction reaction (ORR), energy storage materials, lithium-Ion batteries (LiBs) and solar cell applications. These studies suggested that the use of GDY holds significant potential for the development and implementation of efficient, multimodal, and intelligent catalysts with realistic applications. However, the limitation of GDY and GDY-based composites for forthcoming studies are similarly acknowledged. The objective of these studies is to deliver a comprehensive knowledge of GDY and inspire further advancement and utilization of these unique carbon materials.

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.

Comparison of phosphorus species in livestock manure and digestate by different detection techniques

Phosphorus (P) species characterize the effectiveness of the P fertilizer. In this study, the P species and distribution in different manures (pig manure, dairy manure and chicken manure) and their digestate were systematically investigated through combined characterization methods of Hedley fractionation (H₂OP, NaHCO₃-P, NaOH-P, HCl-P, and Residual), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) techniques. The results from Hedley fractionation showed that >80 % of P in the digestate was inorganic and the HCl-P content in manure increased significantly during anaerobic digestion (AD). XRD manifested that insoluble hydroxyapatite and struvite belonging to HCl-P were presented during AD, which was in agreement with the result of Hedley fractionation. ³¹P NMR spectral analysis revealed that some orthophosphate monoesters were hydrolyzed during AD, meanwhile the orthophosphate diester organic phosphorus like DNA and phospholipids content has increased. After characterizing P species by combining these methods, it was found that chemical sequential extraction could be an effective way to fully understand the P in livestock manure and digestate, with other methods used as auxiliary tool depending on the purpose of studies. Meanwhile, this study provided a basic knowledge of utilizing digestate as P fertilizer and minimizing the risk of P loss from livestock manure. Overall, applying digestates can minimize the risk of P loss from directly applied livestock manure while satisfying plant demands, and is an environmentally friendly P fertilizer.

Nutrient recovery and valorisation from pig slurry liquid fraction with membrane technologies

Livestock slurry has been reported to be a potential secondary raw material as it contains macronutrients ‑nitrogen, phosphorus and potassium-, which could be valorised as high-quality fertilizers if proper separation and concentration of valuable compounds is performed. In this work, pig slurry liquid fraction was assessed for nutrient recovery and valorisation as fertilizer. Some indicators were used to evaluate the performance of proposed train of technologies within the framework of circular economy. As ammonium and potassium species are highly soluble at the whole pH range, a study based on phosphate speciation at pH from 4 to 8 was assessed to improve the macronutrients recovery from the slurry, resulting in two different treatment trains at acidic and alkaline conditions. The acidic treatment system based on centrifugation, microfiltration and forward osmosis was applied to obtain a nutrient-rich liquid organic fertilizer containing 1.3 % N, 1.3 % P₂O₅ and 1.5 % K₂O. The alkaline path of valorisation was composed by centrifugation and stripping by using membrane contactors to produce an organic solid fertilizer -7.7 % N, 8,0 % P₂O₅ and 2.3 % K₂O-, ammonium sulphate solution -1.4 % N- and irrigation water. In terms of circularity indicators, 45.8 % of the initial water content and <50 % of contained nutrients were recovered - 28.3 % N, 43.5 % P₂O₅ and 46.6 % K₂O - in the acidic treatment resulting in 68.68 g fertilizer per kg of treated slurry. 75.1 % of water was recovered as irrigation water and 80.6 % N, 99.9 % P₂O₅, 83.4 % K₂O was valorised in the alkaline treatment, as 219.60 g fertilizer per kg of treated slurry. Treatment paths at acidic and alkaline conditions yield promising results for nutrients recovery and valorisation as the obtained products (nutrient rich organic fertilizer, solid soil amendment and ammonium sulphate solution) fulfil the European Regulation for fertilizers to be potentially used in crop fields.

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.

Response of phosphorus speciation to organic loading rates and temperatures during anaerobic co-digestion of animal manures and wheat straw

The world is facing huge phosphate (P) shortage and anaerobic digestion (AD) is a recognized technology to promote nutrient (N and P) recycling. The composition of P speciation in the digestate is essential for the fertilizing effect. However, how P speciation in the digestates interacts with the AD process conditions is unknown. Therefore, interaction of P speciation in digestates with AD process conditions was investigated by using a chemical sequential extraction method (Hedley fractionation) and X-ray diffraction; specifically, the effects of organic loading rate (OLR), temperature, and substrate composition were investigated. The results showed that OLR and feedstock affected P speciation in the digestate significantly due to different ion species and ionic strengths. The H₂O-P concentration in chicken manure with straw (CMS) and dairy manure with straw (DMS) digestates decreased by 44.04-48.76% and 48.88-50.49%, respectively, as the OLR increased from 2 to 4 kg VS m^-3 d^-1. Simultaneously, HCl-P increased by 38.02-44.01% in the CMS digestates due to Ca-P and Mg-P formation, indicating that Ca-P and Mg-P formation was positively correlated with OLR, whereas P mobility decreased. Further, thermophilic temperature conditions were more conducive for the formation of insoluble P than mesophilic temperature conditions in the digestates due to the thermodynamic driving force of the reactions. The results would facilitate the understanding of P transformation in the AD process under the influence of feedstock, OLR, and temperature. From the viewpoint of nutrient management, lower OLR and temperature are more beneficial for a fast P availability, whereas higher OLR and temperature are more helpful for storage and export because of P precipitated into solid phase of digestate.

Simultaneous removal of phosphate and antibiotic from hydrolyzed urine by novel spherical particles

Phosphate recovery from wastewater is regarded as promising strategy to achieve sustainable supply of non-renewable natural resources. In this study, a novel technique for spherical materials preparation was developed to achieve both phosphate recovery and antibiotic removal from urine. Phosphate removal and sulfamethoxazole (SMX) degradation performance of the synthesized spherical materials was studied in synthesized urine and real urine. MgB, made from magnesium oxide (10%) and biochar (10%), was the most effective in phosphate removal and SMX degradation. Struvite formation and radical production were the mechanisms of phosphate removal and SMX degradation, respectively. The phosphate removal capacity of MgB was 0.181 g/g and the removal cost was around 0.245 RMB/g phosphate. Meanwhile, the combination of MgB and persulfate could achieve a 98% degradation efficiency of SMX, which could eliminate the hazardous impurity in final product. Furthermore, this technique has also been validated useful in treating real hydrolyzed urine.

Unveiling the Potential of Novel Struvite–Humic Acid Composite Extracted from Anaerobic Digestate for Adsorption and Reduction of Chromium

A novel struvite–humic acid composite (S–HA) was derived from an anaerobic digestate and evaluated for the adsorption and reduction of chromium [Cr (VI)] in this study. The results indicated that the struvite–humic acid composite (S–HA) contains higher contents of oxygen-containing and aromatic functional groups (47.05% and 34.13%, respectively) and a higher specific surface area (19.3 m2 g−1). These special characteristics of S–HA contributed to its higher adsorption capacity (207.69 mg g−1 and 254.47 mg g−1 for pseudo-first and second-order kinetic models, respectively) for chromium. Furthermore, XPS analysis showed that a portion of the bonded Cr (VI) was reduced to Cr (III) by carboxyl and hydroxyl functional groups, which oxidized and changed into ketone and phenol functional groups. Based on the findings, it was concluded that the phosphate–humic acid composite has an outstanding chromium adsorptive and reduction capacity. However, more research is needed to fully understand the potential of the struvite–humic acid composite for chromium adsorption and reduction.

Quantitative, morphological, and structural analysis of Ni incorporated with struvite during precipitation

Struvite precipitation is a promising strategy for the simultaneous recovery of nitrogen and phosphorus from waste streams. However, waste streams typically contain high amounts of metal contaminants, including Ni, which can be easily sequestered by struvite, but the behavior of Ni during struvite precipitation remains unclear. Thus, this study investigates the influence of Ni concentrations on struvite precipitation. The quantitative X-ray diffraction (QXRD) results revealed that the purity of struvite decreased from 96.6 to 41.1% with the Ni concentrations increased from 0.1-100 mg·L^-1. At lower Ni concentrations of 0.1-1 mg·L^-1, scanning electron microscopy (SEM) showed a roughened surface of struvite crystal, and this was combined with X-ray absorption near edge structure (XANES) data that indicated a stack of Ni-OH and Ni-PO₄ on struvite surface. At Ni concentrations of 10-25 mg·L^-1, Ni primarily crystalized as Ni-struvite (NiNH₄PO₄·6H₂O), as detected by QXRD. At higher Ni concentrations of 25-100 mg·L^-1, the co-precipitation of amorphous Ni phosphate(s) (e.g., Ni₃(PO₄)₂) and Ni hydroxide (e.g., Ni(OH)₂) was identified by XANES. Specifically, the X-ray photoelectron spectroscopy (XPS) analysis detected the formation of amorphous Mg hydroxide(s) and phosphate(s) at Ni of 25-100 mg·L^-1. The overall results revealed that Ni formed Ni-OH and Ni-PO₄ on struvite surface at 0.1-1 mg·L^-1, whereas Ni precipitated as separated phases (e.g. Ni-struvite, Ni hydroxide and phosphate) at 10-100 mg·L^-1. The existence of Ni disturbed the crystal growth of struvite and promoted the formation of Ni-struvite, amorphous products during struvite formation.

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.

A Glimpse of the World of Volatile Fatty Acids Production and Application: A review

Sustainable provision of chemicals and materials is undoubtedly a defining factor in guaranteeing economic, environmental, and social stability of future societies. Among the most sought-after chemical building blocks are volatile fatty acids (VFAs). VFAs such as acetic, propionic, and butyric acids have numerous industrial applications supporting from food and pharmaceuticals industries to wastewater treatment. The fact that VFAs can be produced synthetically from petrochemical derivatives and also through biological routes, for example, anaerobic digestion of organic mixed waste highlights their provision flexibility and sustainability. In this regard, this review presents a detailed overview of the applications associated with petrochemically and biologically generated VFAs, individually or in mixture, in industrial and laboratory scale, conventional and novel applications.

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.

Quantitative characterization and effective inactivation of biological hazards in struvite recovered from digested poultry slurry

Struvite formed from digested poultry slurries can serve as an alternative to chemical fertilizers; however, the biological safety of such products is questionable. Therefore, quantification and inactivation of foodborne pathogens existing in struvite are important. Herein, the dynamics of foodborne pathogens' (Streptococcus faecalis, S. typhimurium, Clostridium perfringens, and Escherichia coli) living status, whether culturable and viable but non-culturable (VBNC) in struvite, were quantified for the first time. Meanwhile, inactivation technologies, namely high-humidity hot air impingement blanching (HHAIB), cold plasma, and hot air treatment, were evaluated and compared for their potential to inactivate/kill foodborne pathogens in struvite. An increase in precipitation pH from 9.0 to 11.0 decreased the culturable count of pathogens in the struvite from 75 to 86% to 7-20%, while the VBNC pathogen counts increased from 16 to 24% to 35-55%. Among the tested inactivation technologies, the HHAIB treatment at 130 °C for 120 s killed approximately 68-79% of foodborne pathogens in struvite precipitated at pH 9.0. VBNC pathogens increased from 16 to 24% to 57-68% after HHAIB treatment at 130 °C for 120 s. Struvite treatment with different inactivation technologies did not change its crystalline structure; however, it reduced functional group abundance. Therefore, further research on inactivation technologies is required to achieve better pathogen reduction efficiency in struvite to make it a biologically safe fertilizer for crop production.

Ultrasonic power combined with seed materials for recovery of phosphorus from swine wastewater via struvite crystallization process

Recovering P via struvite crystallization is an effective way to utilize the resources in swine wastewater. At present, the main challenges of traditional struvite crystallization process are the long reaction time and insufficient removal efficiency. In this study, a novel method to promote struvite crystallization process through ultrasound (US) combined with seed materials is proposed to overcome these defects. We systematically study the effects of US, seed materials, and ultrasonic power on nutrient recovery. The experimental results show that under the conditions of pH 9.5 and MgCl₂:P molar ratio1.4:1, the addition of 2 g/L pre-synthesized struvite as the seed materials can increase the P removal rate to 91.56%, whereas, the addition of 80 W ultrasonic power for 15 min can make the P removal rate reach 94.18%. Meanwhile, the combination of US and struvite seed crystals can achieve a maximum P removal efficiency value of 97.66% in which 10 min for the reaction time is enough. The products are characterized using XRD, SEM, and FTIR to determine the phosphorus removal mechanism of ultrasonic power combined with seed induction. The shearing effect of US is found beneficial to affect the surface morphology of the seed crystals, which provides more nucleation sites to enhance crystal nucleation and growth. The removal efficiency comparison reveals that this combined technology performs an excellent removal effect.

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.

Nutrient recovery from the digestate obtained by rumen fluid enhanced anaerobic co-digestion of sewage sludge and cattail: Precipitation by MgCl2 and ion exchange using zeolite

The aim of this study was to recover nutrients (NPK and other) from the liquid fraction of digestate obtained by rumen fluid enhanced anaerobic co-digestion of sewage sludge and cattail (Typha latifolia grass). Firstly, anaerobic digestion (AD) studies were performed to examine the biogas potential of selected substrates. The liquid fraction of digestate was then used in nutrient recovery experiments. Four methods were applied to recover nutrients: i) conventional struvite precipitation by MgCl₂, ii) simultaneous precipitation and ion exchange by Na-zeolite, and iii) two-step recovery using precipitation, followed by ion exchange with powdered or iv) granulated Na-zeolite. The products of nutrient recovery were characterised using different chemical methods and the cress seed germination test was performed to evaluate their fertility potential. The results show that co-digestion of sewage sludge with cattail enhanced biogas production by almost 50 vol%. The addition of rumen fluid positively contributed to the degradation of lignocellulosic materials and to biogas production. In all of the recovery methods tested, phosphorus was successfully recovered with efficiency of more than 99 wt%. Nitrogen recovery was less efficient than phosphorus recovery, 85-92 wt%. Simultaneous precipitation and ion exchange lowered nitrogen recovery efficiency compared to classical struvite precipitation, while sequential precipitation and ion exchange resulted in improvement. The most efficient method was two-step recovery using granulated zeolite. The precipitates consisted of different Mg and K-phosphates in quite irregular shapes. The struvite and K-struvite were detected in low quantities. The precipitates contained more than 25 wt% of macronutrients (NPK), exhibited effective utilization of nutrients by plants, and showed good fertility potential. Precipitate mixed with powdered Na-zeolite promises to be interesting for further agricultural use, as zeolite offers several potential improvements for soil. Both zeolites exhibited good performance in the recovery of K⁺ ions.

Co-precipitation of Cu and Zn in precipitation of struvite

Struvite recovered from wastewater can be used as a slow-release fertilizer. Nevertheless, hazardous metals easily precipitated with struvite would increase the ecological risk for its agricultural use. In this study, the influence of individual and coexistence of Cu and Zn on the precipitation of struvite was investigated. The loading of Cu and/or Zn in precipitates increased with the increase of initial metal concentrations (0.1-100 mg/L). Quantitative X-ray diffraction (QXRD) analysis revealed that the increase of Cu and/or Zn level in reaction solution disturbed crystal growth of struvite and promoted the formation of amorphous phase(s). Scanning electron microscopy (SEM) revealed the pit formation on struvite crystal surfaces, combined with X-ray photoelectron spectroscopy (XPS) data, the results indicated a surface interaction for the formation of Cu-OH and Cu-NH₃ on struvite surface at Cu of 0.1-10 mg/L. With the increase of Cu to 25-100 mg/L, the precipitation of amorphous Cu phosphate(s) was confirmed by XPS and QXRD. At Zn of 0.1-10 mg/L, the enrichment of Zn-PO₄ and Zn-OH on struvite surface was observed, whereas, the precipitation of amorphous Zn hydroxide(s) was confirmed at Zn of 25-100 mg/L. At Cu and Zn co-existed solution, the decrease of Cu-PO₄ and increase of Zn-PO₄ suggested the competitive binding of PO₄ between Cu and Zn. In addition, the formation of amorphous Mg hydroxide(s) and phosphate(s) was detected regardless of the addition of Cu in solutions. The overall results revealed that the existence of Cu and Zn during struvite formation can greatly affect its content by formation of different metal-containing products.

Phosphorus concentration and availability in raw organic waste and post fermentation products

The aim of the study was to determine the mobility of phosphorus forms in raw organic waste and from the solid and liquid fractions of digestate. To achieve the purpose of this study, the components (including livestock manure, agricultural waste, food waste, sewage sludge) and their post fermentation products were considered. Subsequently, the effect of the fermentation process on the mobility of phosphorus forms in post-fermentation fractions (solid and liquid) was investigated. Then, the evaluation of the fertilising potential of digestate fractions was assessed. The available organic and inorganic phosphorus forms were determined according to the Standards in Measurements and Testing (SMT) Programme extraction protocol and according to the acid molybdate spectrophotometric method. It has been shown that phosphorus in digestates occurred mainly in inorganic forms with Fe, Al, Mn, Mg and Ca ions. Its proportion in relation to total phosphorus ranged from 80 to 90%. The lowest phosphorus content was found in digestate from the fermentation of agricultural and food waste (fruit and vegetables), while digestate from livestock manure and sewage sludge fermentation was rich in phosphorus. It was shown that the solid fractions of digestate represented from 30 to 70% of highly labile phosphorus (i.e. phosphorus with organic matter and in bonds with Al, Fe, Mg and Mn oxides and hydroxides) in relation to total phosphorus. However, the share of labile phosphorus forms in the liquid fraction of digestates was much higher and accounted for 80-90% of the total phosphorus.

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.

Dynamic evolution of humic acids during anaerobic digestion: Exploring an effective auxiliary agent for heavy metal remediation

Information on the dynamic evolution of humic acid (HA) from anaerobic digestate and the potential of HA serving as an effective agent for remedying heavy metals is rather scarce. This study monitored the evolution of the structure and functional groups and metal-binding abilities of HA during chicken manure and corn stover anaerobic digestion (AD) processes. Higher increases in aromatic (41-66%) and oxygen-containing functional groups (37-45%) were observed in HA from the AD of corn stover, resulting in higher metal-binding abilities for Cu(II), Co(II), and Ni(II) than those of chicken manure AD. Moreover, HA extracted from fast (before day 12 for chicken manure and day 16 for corn stover), and slow (day 40) methane production stages performed different complexation capacities for the heavy metals. These results reveal the mechanisms of HA and heavy metal interactions, and confirm the potential of HA extracted from AD process for the remediation of heavy metals.

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.

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.

A novel magnesium-based oxygen releasing compound for eutrophic water remediation

Eutrophication of surface water bodies is a global problem in recent years. Dosing polluted water with oxygen releasing compounds (ORCs), especially those that can remove excessive nutrients simultaneously is regarded as one of the most economical and eco-friendly methods of treating eutrophic waters. In this study, a novel Mg-based ORC was synthesized and characterized as a magnesium hydroxide and hydrogen peroxide complex (MHHPC) with Mg to H₂O₂ ratio of 2:1. Oxygen-releasing, pH-adjusting and nutrient-removal potentials of MHHPC were evaluated in nano-pure and eutrophic water. The overall performance of MHHPC in preventing the eutrophic water from turning black and odorous was compared with the performance of other ORCs namely, MgO₂, CaO₂ and the combination of MgCl₂ and H₂O₂. The results showed that MHHPC was capable of constantly releasing oxygen to aqueous phase over a period of one week. Phosphate and ammonia nitrogen in synthetic buffered water can were removed as struvite and other precipitates from the aqueous phase. In the synthetic eutrophic water, all the ORCs tested were able to reduce aqueous ammonia nitrogen below 0.5 mM, while only CaO₂ and MHHPC successfully removed the aqueous phosphate. However, CaO₂ and MgCl₂+H₂O₂ significantly inhibited microbial activity.

Holistic Approach to Phosphorus Recovery from Urban Wastewater: Enhanced Biological Removal Combined with Precipitation

Combined phosphorus (P) removal and recovery from wastewater is a sensible and sustainable choice in view of potential future P-resource scarcity, due to dwindling primary global reserves. P-recovery from wastewater, notwithstanding the relatively small fraction of total global amounts involved (less than 1/5 of total global use ends up in wastewater) could extend the lifespan of available reserves and improve wastewater cycle sustainability. The recovery of the resource, rather than its mere removal as ferric or aluminum salt, will still allow to achieve protection of receiving waters quality, while saving on P-sludge disposal costs. To demonstrate the possibility of such a recovery, a strategy combining enhanced biological phosphorus removal and mineral P-precipitation was studied, by considering possible process modifications of a large treatment facility. Process simulation, a pilot study, and precipitation tests were conducted. The results demonstrated that it would be possible to convert this facility from chemical -precipitation to its biological removal followed by mineral precipitation, with minimal structural intervention. Considerable P-recovery could be obtained, either in form of struvite or, more sustainably, as calcium phosphate, a mineral that also has possible fertilizing applications. The latter would present a cost about one order of magnitude lower than the former.

Energy Issues in Sustainable Urban Wastewater Management: Use, Demand Reduction and Recovery in the Urban Water Cycle

Urban water systems and, in particular, wastewater treatment facilities are among the major energy consumers at municipal level worldwide. Estimates indicate that on average these facilities alone may require about 1% to 3% of the total electric energy output of a country, representing a significant fraction of municipal energy bills. Specific power consumption of state-of-the-art facilities should range between 20 and 45 kWh per population-equivalent served, per year, even though older plants may have even higher demands. This figure does not include wastewater conveyance (pumping) and residues post-processing. On the other hand, wastewater and its byproducts contain energy in different forms: chemical, thermal and potential. Until very recently, the only form of energy recovery from most facilities consisted of anaerobic post-digestion of process residuals (waste sludge), by which chemical energy methane is obtained as biogas, in amounts generally sufficient to cover about half of plant requirements. Implementation of new technologies may allow more efficient strategies of energy savings and recovery from sewage treatment. Besides wastewater valorization by exploitation of its chemical and thermal energy contents, closure of the wastewater cycle by recovery of the energy content of process residuals could allow significant additional energy recovery and increased greenhouse emissions abatement.

A promising strategy for nutrient recovery using heterotrophic indigenous microflora from liquid biogas digestate

Nutrient overloading resulting from digestate (effluent of anaerobic digestion process) application has become a major bottleneck for the development of the biogas industry and raised environmental concerns in regions with intensive animal husbandry. Due to this, it is imperative to find low cost and effective alternative to export nutrient from digestate. Among the numerous applications, indigenous microflora has recently been utilized successfully as a biofloc technology in aquatic systems for controlling ammonia and subsequent reduction of feeding cost. Accordingly, performance of the indigenous microflora in undiluted liquid digestate of chicken manure was evaluated in this study to recover nutrients and produce high-value biomass under aerobic heterotrophic mode in batch shaking experiments. The results showed that 68% of phosphate was recovered and 97% of total nitrogen was removed from the liquid digestate. Additionally, >6 g L^-1 of dry biomass was simultaneously produced and featured with up to 65% crude protein without pathogens, 10.9% lipids, 10.7% ash and 19.6 MJ kg^-1 gross energy. Therefore, the produced biomass could be used either as an alternative sustainable source for animal or fish feeding or as a substrate for energy applications.

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

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

P-recovery in a pilot-scale struvite crystallisation reactor for source separated urine systems using seawater and magnesium chloride as magnesium sources

Practical recovery of a non-renewable nutrient, such as phosphorus (P), is essential to support modern agriculture in the near future. The high P content of urine, makes it an attractive source for practicing the recovery of this crucial nutrient. This paper presents the experimental results at pilot-plant scale of struvite crystallisation from a source-separated urine stream using two different magnesium sources, namely magnesium chloride and seawater. The latter was chosen as sustainable option to perform P-recovery in coastal areas. Real seawater was used to assess in a more realistic way its efficiency to precipitate P as struvite, since its composition (with noticeable concentration of ions such as Ca²⁺, SO₄^2-, Na⁺, …) could lead to the formation of impurities and other precipitates. 0.99 g of struvite was obtained per litre of urine irrespective of the operational conditions tested. In all tested conditions, precipitation efficiencies exceeded 90% and recovery efficiencies were higher than 87%, with an average struvite crystal size higher than 110 μm (and up to 320 μm, depending on the experimental conditions) in the harvested struvite samples. Almost pure struvite was obtained when MgCl₂ was used as precipitant, while amorphous calcium phosphate and other impurities appeared in the precipitates using seawater as magnesium source. However, the lower settling velocity of the amorphous precipitates in comparison with the struvite precipitates suggests that their separation at industrial scale could be relatively straightforward.

Improved anaerobic co-digestion of food waste and domestic wastewater by copper supplementation - Microbial community change and enhanced effluent quality

Anaerobic co-digesters are biorefineries for energy recovery from food waste and domestic wastewater via methane production. Nonetheless, the performance of this technology was not always satisfied due to the long chain fatty acids (LCFAs) generation from food waste. Micronutrient supplementation is an effective strategy that could be applied during the anaerobic (co-)digestion to further enhance the digestion efficiency while treating food waste. In this study, supplementing copper (as CuSO₄ and CuCl₂) at 10, 30, and 50 mg/L Cu²⁺ was selected to further enhance the methane production of anaerobic co-digester while treating food waste and domestic wastewater. Overall, with the supplementation of copper, the chemical oxygen demand (COD) removal efficiency was over 90%, while higher methane yields (0.260-0.325 L CH₄/g COD _removed) were obtained compared to the control without supplementation (0.175 L CH₄/g COD _removed). For the cumulative methane yield, the highest increment of 94.1% was obtained when 10 mg/L of Cu²⁺ were added. The results showed copper as a cofactor of many microbial enzymes and coenzymes involved in the methane production further improved both methane production and COD removal efficiency. Meanwhile, the microbial community analysis verified the copper supplementation significantly changed the bacterial communities but with the limited effect on the diversity of archaea. Furthermore, since the anaerobic co-digester was not that much efficient on the nutrients removal, the effluent from the upflow anaerobic sludge blanket (UASB) reactor was further treated by the anaerobic/anoxic/oxic (A²O) rector and the resulting effluent reached the satisfying quality in terms of COD, total nitrogen (TN), and NH₃-N removal, meeting the regional effluent discharge limits.

Biochar seeding promotes struvite formation, but accelerates heavy metal accumulation

This study investigated the effects of biochar seeding (wheat straw biochar and rice husk biochar) on nutrient recovery via struvite formation, and improvements in the particle size of precipitated struvite from anaerobic digestate supernatant. Simultaneously, the influence of biochar seeding on heavy metal accumulation and elimination of pathogens (total coliforms and Escherichia coli) was evaluated under various operational factors, e.g., pH, supersaturation, reaction time, and seeding rates. Compared to the non-seeding process (maximum recovery efficiency of phosphate and ammonium 91% and 83%, respectively, with a particle size of 70 μm) and the struvite-seeding process (maximum recovery efficiency of phosphate and ammonium 97% and 94%, respectively, with a particle size of 100 μm), the process of biochar seeding improved nutrient recovery up to 7% and 11% for phosphate and ammonium, respectively, and increased struvite particle size by 43%, regardless of biochar type. XRD diffraction and FTIR analysis confirmed the prevalence of orthorhombic characteristics and an inner crystalline structure of the struvite formed by biochar seeding. About 75% of total coliforms and 70% of Escherichia coli were removed from the digestate supernatant through seeded struvite precipitation, regardless of the seeding materials. However, the biochar seeding process led to an accumulation of heavy metals in the acquired struvite product than that with non-seeded precipitation process. The concentrations of these metals were still well below permissible limits for application on agricultural land. It can be concluded that the inclusion of biochar as a seeding material might be a sustainable strategy to enhance struvite formation, intensify nutrient recovery, and yield high-quality struvite fertilizer with increased particle sizes.

Influence of process parameters on the heavy metal (Zn2+, Cu2+ and Cr3+) content of struvite obtained from synthetic swine wastewater

Struvite recovered from swine wastewater can be used as a good slow release fertilizer. Nevertheless, the presence of heavy metals would be easily precipitated with struvite and increase the ecological risk for its agricultural use. This paper investigated the possibility of using process variables for heavy metal (Cu²⁺, Zn²⁺ and Cr³⁺) minimization during struvite crystallization in swine wastewater. The heavy metal content, effect ratios (ER) of the citric acid concentration under varying conditions were tested and their SEM, EDS and XRD patterns were compared for morphology analysis. The results show that an increase in pH decreased the content of Cu, Zn and Cr in recovered precipitates. Heavy metal content in the precipitates increased markedly with their initial concentrations in the solution. The effect ratio calculation indicates that Cr has the strongest co-precipitation potential, followed by Zn and Cu. An increase in citric acid concentration reduced the heavy metal removal efficiency (14.3, 27.7 and 28.1% for Cu, Zn and Cr, respectively) but did not decrease their content in struvite precipitates. What is more, increase of total ammonia nitrogen (TAN) to soluble phosphate molar ratio significantly decreased Cu, Zn removal efficiency (52.2 and 50% respectively), while Mg:PO₄P molar ratio had much less effect.

Nutrient recovery technologies integrated with energy recovery by waste biomass anaerobic digestion

Anaerobic digestion widely considered as a promising waste biomass disposal treatment approach, is attracting increasing interest in all corners of the globe. However, due to the specific features of different types of waste biomass, the bioenergy conversion efficiency of this process is not ideal. Another problematic aspect of anaerobic digestion is that the nutrient rich effluent sometimes needs to be treated before discharge. This review presents the recent achievements of waste biomass digestion from the perspective of energy recovery and nutrient recovery. In this work, the anaerobic treatment characteristics of common types of waste biomass are summarized and compared. With a focus of nutrient recovery and post treatment issues, the challenges and technical hurdles encountered in the anaerobic digestion of waste biomass are critically reviewed. Finally, an integrated system of anaerobic digestion, anaerobic ammonia oxidation (anammox) and phosphorus recovery is proposed for efficient energy and nutrient recovery from waste biomass.

Phosphate removal from aqueous solution using iron oxides: Adsorption, desorption and regeneration characteristics

Dynamics of phosphate (PO₄^3-) adsorption, desorption and regeneration characteristics of three lab-synthesized iron oxides, ferrihydrite (F), goethite (G), and magnetite (M) were evaluated in this study. Batch experiments were conducted to evaluate the impact of several adsorption parameters including adsorbent dosage, reaction time, temperature, pH, and ionic strength. The results showed that PO₄^3- adsorption increased with reaction time and temperature while it decreased with an increase in solution pH. Adsorption isotherm data exhibited good agreement with the Freundlich and Langmuir model with maximum monolayer adsorption capacities of 66.6 mg·g^-1 (F), 57.8 mg·g^-1 (M), and 50.5 mg·g^-1 (G). A thermodynamics evaluation produced ΔG < 0, ΔH > 0, and ΔS > 0, demonstrating that PO₄^3- adsorption onto tested minerals is endothermic, spontaneous, and disordered. The PO₄^3- removal mostly occurred via electrostatic attraction between the sorbate and sorbent surfaces. Moreover, the PO₄^3- sorption was reversible and could be desorbed at varying rates in both neutral and alkaline environments. The good desorption capacity has practical benefits for potential regeneration and re-use of the saturated particles in wastewater treatment systems.