Using mass struvite precipitation to remove recalcitrant nutrients and micropollutants from anaerobic digestion dewatering centrate

Guidance on aqueous matrices for evaluating novel precipitants and adsorbents for phosphorus removal and recovery

Phosphorus (P) removal from water and recovery into useable forms is a critical component of creating a sustainable P cycle, although mature technologies for P removal and recovery are still lacking. The goal of this paper was to advance the testing of novel materials for P removal and recovery from water by providing guidance on the development of more realistic aqueous matrices used during materials development. Literature reports of "new" materials to remove P from water are often difficult to compare in terms of performance because authors use a myriad of water chemistries containing P concentrations, pH, and competing ions. Moreover, many tests are conducted in simplified matrices that do not reflect conditions in real systems. To address this critical gap, the research herein developed a systematic approach of identifying aqueous matrices relevant to P recovery, including key components in the aqueous matrices having the greatest influence on the mechanisms of P removal with emphasis on phosphate precipitation and phosphate adsorption, and providing guidelines on relevant "recipes" for aqueous solutions for testing novel materials. Key components in the aqueous matrices included hydrogen ion (i.e., pH), multivalent metal cations, and dissolved organic matter due to their influence on phosphate precipitation and adsorption mechanisms. Recipes for buffer solution and synthetic groundwater, surface water, anaerobic digestate, and stored urine are discussed in the context of P removal and recovery processes. Wherein the adoption of standard matrices in other fields have permitted direct comparison of processes or materials, it is anticipated that adoption of relevant aqueous matrix recipes for P removal and recovery will improve the ability to directly compare novel materials and processes.

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.

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.

Role of suspended solids on the co-precipitation of pathogenic indicators and antibiotic resistance genes with struvite from digested swine wastewater

Struvite recovered from wastewater contains high concentration of fecal indicator bacteria (FIB), porcine adenoviruses (PAdV) and antibiotic resistance genes (ARGs), becoming potential resources of these microbial hazards. Understanding the precipitation behavior of pathogenic indicators and ARGs with suspended solids (SS) will provide the possible strategy for the control of co-precipitation. In this study, SS was divided into high-density SS (separated by centrifugation) and low-density SS (further separated by filtration), and the role of SS on the co-precipitation of FIB, PAdV and ARGs was investigated. The distribution analysis showed that 35.5-73.0% FIB, 79.6% PAdV and 64.5-94.8% ARGs existed in high-density SS, while the corresponding values were 26.9-64.4%, 11.7% and 3.5-24.3% in low-density SS. During struvite generation, 82.7-96.9% FIB, 75.5% PAdV and 56.3-86.5% ARGs were co-precipitated into struvite. High-density SS contributed 20.7-68.5% FIB, 63.9% PAdV and 38.7-87.2% ARGs co-precipitation, and the corresponding contribution of low-density SS was 31.4-79.2%, 3.9% and 6.2-54.7%. Moreover, the precipitated SS in struvite obviously decreased inactivation efficiency of FIB and ARGs in drying process. These results provide a potential way to control the co-precipitation and inactivation of FIB, PAdV and ARGs in struvite through removing high-density SS prior to struvite recovery.

Co-precipitation of heavy metals with struvite from digested swine wastewater: Role of suspended solids

Struvite production can recover ammonia and phosphorous from digested wastewater as fertilizer. During struvite generation, most of the heavy metals was co-precipitated with ammonia and phosphorous into struvite. Understanding the precipitation behavior of heavy metals with suspended solids (SS) might provide the possible strategy for the control of co-precipitation. In this study, the distribution of heavy metals in SS and their role on the co-precipitation during struvite recovery from digested swine wastewater were investigated. The results showed that the concentration of heavy metal (including Mn, Zn, Cu, Ni, Cr, Pb and As) ranged from 0.05 to 17.05 mg/L in the digested swine wastewater. The distribution analysis showed that SS with particles > 50 μm harbored most of individual heavy metal (41.3-55.6%), followed by particles 0.45-50 μm (20.9-43.3%), and SS-removed filtrate (5.2-32.9%). During struvite generation, 56.9-80.3% of individual heavy metal was co-precipitated into struvite. The contributions of SS with particles > 50 μm, 0.45-50 μm, and SS-removed filtrate on the individual heavy metal co-precipitation were 40.9-64.3%, 25.3-48.3% and 1.9-22.9%, respectively. These finding provides potential way for controlling the co-precipitation of heavy metals in struvite.

Enhanced photoelectrocatalytic oxidation of hypophosphite and simultaneous recovery of metallic nickel via carbon aerogel cathode

Carbon aerogel (CA) cathode was adopted to an undivided-chamber photoelectrocatalytic system with TiO₂ nanotube arrays (TNA) photoanode to enhance the oxidation of hypophosphite (H₂PO₂^-) and simultaneous recovery of metallic nickel (Ni). Both the efficiencies of H₂PO₂^- oxidation and Ni recovery were significantly enhanced after replacing Ti or carbon fiber paper cathode with CA cathode. With 1.0 mM H₂PO₂^- and 1.0 mM Ni²⁺, the ratio of PO₄^3- production increased from ∼41% or ∼54% to ∼100%, and the ratio of Ni recovery increased from ∼20% or ∼ 37% to ∼93% within 180 min at 3.0 V. H₂PO₂^- was finally oxidized to PO₄^3- by •OH radicals, which was speculated to be generated from UV/H₂O₂ and bound on TNA photoanode. Meanwhile, Ni²⁺ was eventually electro-reduced to metallic Ni by a two-electron reduction reaction. The efficiencies of H₂PO₂^- oxidation and Ni recovery were favored at higher cell voltage, faintly acid conditions and larger H₂PO₂^- concentration. The stability of this system exhibited that the ratio of PO₄^3- production increased significantly in each cycle, which was attributed to the increase of H₂O₂ in-situ-generation via CA cathode caused by deposition of metallic Ni. Finally, the treatment of actual electroless nickel plating effluents was demonstrated.

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.

Biochar-seeded struvite precipitation for simultaneous nutrient recovery and chemical oxygen demand removal in leachate: From laboratory to pilot scale

Refuse transfer station (RTS) leachate treatment call for efficient methods to increase nutrient recovery (NH4+−N and PO43−−P) and chemical oxygen demand (COD) removal. In this study, the effects of various operational factors (seeding dose, pH, initial NH4+-N concentration, and reaction time) on biochar-seeded struvite precipitation were investigated at laboratory and pilot scales. Mealworm frass biochar (MFB) and corn stover biochar (CSB) were used as seeding materials to compare with traditional seed struvite. The maximum NH4+−N and PO43−−P recover efficiency of the MFB-seeded process reached 85.4 and 97.5%, higher than non-seeded (78.5 and 88.0%) and CSB-seeded (80.5 and 92.0%) processes and close to the struvite-seeded (84.5 and 95.1%) process. The MFB-seeded process also exhibited higher COD removal capacity (46.4%) compared to CSB-seeded (35.9%) and struvite-seeded (31.2%) processes and increased the average particle size of the struvite product from 33.7 to 70.2 μm for better sustained release. XRD, FT-IR, and SEM confirmed the orthorhombic crystal structure with organic matter attached to the struvite product. A pilot-scale test was further carried out in a custom-designed stirred tank reactor (20 L). In the pilot-scale test, the MFB-seeded process still spectacularly recovered 77.9% of NH4+−N and 96.1% of PO43−−P with 42.1% COD removal, which was slightly lower than the laboratory test due to insufficient and uniform agitation. On the whole, MFB-seeded struvite precipitation is considered to be a promising pretreatment method for rural RTS leachate.

Recovery of nickel from electroless nickel plating wastewater based on the synergy of electrocatalytic oxidation and electrodeposition technology

Nickel exists primarily as a stable complex in electroless nickel plating wastewater, and the Ni recovery from it cannot be achieved solely through electrodeposition. As the electrocatalytic oxidation has excellent oxidation potential to break down the complex, an efficient and stable electrochemical system using the synergy of electrocatalytic oxidation and electrochemical deposition technology was developed for the recovery of nickel from electroless nickel plating wastewater. In the present study, the effects of initial pH, current density, and initial nickel ion concentration on the treatment performance of the electrochemical system was investigated. The highest Ni recovery (94.84%) and total organic carbon removal (63.94%) were achieved at a current density of 83.3 mA/cm² , initial pH of 3.0, and initial Ni concentration of 0.01 M. At the same time, the recovered nickel product was confirmed by scanning electron microscopy, energy dispersive X-ray, X-ray powder diffraction, and X-ray photoelectron spectroscopy. Furthermore, the electrochemical system displayed good stability and economic benefits, thereby suggesting its excellent application potential for the treatment of electroless nickel plating wastewater. PRACTITIONER POINTS: An efficient and stable electrochemical system was developed for the recovery of nickel from electroless nickel plating wastewater. In an acidic medium, the nickel recovery rate and TOC removal ratio were 94.84% and 63.94%, respectively. The system displayed good stability, thereby suggesting its excellent application potential for the treatment of nickel plating wastewater.

Spontaneous nutrient recovery and disinfection of aquaculture wastewater via Mg-coconut shell carbon composites

Aquaculture wastewater contained large amounts of pathogenic microorganisms, nitrogen (N) and phosphorus (P). In this study, the nutrient recoveries and wastewater disinfection were simultaneously achieved using Mg-coconut shell carbon (Mg-CSC). The composites were prepared by a ball milling method. The hydrogen peroxide (H₂O₂) was in-situ generated by the dissolved oxygen reduction driven by Mg corrosion on the CSC surface, which inactivated the microorganisms. Besides that, Mg corrosion provided sufficient Mg ions and appropriate pH conditions for struvite formation. The results show that 5.4-log E.coli removal was achieved under different conditions. Improving the Mg/CSC ratio and composite dosage could shorten the time required for disinfection. In addition to H₂O₂, singlet oxygen played a critical role. Reactive oxygen species destroyed the cellular structure and killed the bacteria. The recoveries of NH₄⁺-Nand P under certain conditions were about 60% and 91%, respectively. An increased composite dosage could improve the recovery ratio of P. Excessive dosages were not beneficial for removing NH₄⁺-N. The characterization result revealed that struvite crystals were the main precipitates on the CSC surface. The Mg-CSC composites also revealed satisfied nutrient recovery and disinfection performances in the real aquaculture wastewater treatment process.

Electrochemical nutrient removal from natural wastewater sources and its impact on water quality

In this study, a suite of natural wastewater sources is tested to understand the effects of wastewater composition and source on electrochemically driven nitrogen and phosphorus nutrient removal. Kinetics, electrode behavior, and removal efficiency were evaluated during electrochemical precipitation, whereby a sacrificial magnesium (Mg) anode was used to drive precipitation of ammonium and phosphate. The electrochemical reactor demonstrated fast kinetics in the natural wastewater matrices, removing up to 54% of the phosphate present in natural wastewater within 1 min, with an energy input of only 0.04 kWh.m^-3. After 1 min, phosphate removal followed a zero-order rate law in the 1 min - 30 min range. The zero-order rate constant (k) appears to depend upon differences in wastewater composition, where a faster rate constant is associated with higher Cl^- and NH₄⁺ concentrations, lower Ca²⁺ concentrations, and higher organic carbon content. The sacrificial Mg anode showed the lowest corrosion resistance in the natural industrial wastewater source, with an increased corrosion rate (v_corr) of 15.8 mm.y^-1 compared to 1.9-3.5 mm.y^-1 in municipal wastewater sources, while the Tafel slopes (β) showed a direct correlation with the natural wastewater composition and origin. An overall improvement of water quality was observed where important water quality parameters such as total organic carbon (TOC), total suspended solids (TSS), and turbidity showed a significant decrease. An economic analysis revealed costs based upon experimental Mg consumption are estimated to range from 0.19 $.m^-3 to 0.30 $.m^-3, but costs based upon theoretical Mg consumption range from 0.09 $.m^-3 to 0.18 $.m^-3. Overall, this study highlights that water chemistry parameters control nutrient recovery, while electrochemical treatment does not directly produce potable water, and that economic analysis should be based upon experimentally-determined Mg consumption data. Synopsis Statement: Magnesium-driven electrochemical precipitation of natural wastewater sources enables fast kinetics for phosphate removal at low energy input.

Co-treatment of acid mine drainage and municipal wastewater effluents: Emphasis on the fate and partitioning of chemical contaminants

The co-management of different wastewater matrices can lead to synergistic effects in terms of pollutants removal. Here, the co-treatment of real municipal wastewater (MWW) and acid mine drainage (AMD) is comprehensively examined. Under the identified optimum co-treatment condition, i.e., 15 min contact time, 1:7 AMD to MWW liquid-to-liquid ratio, and ambient temperature and pH, the metal content of AMD (e.g., Al, Fe, Mn, Zn) was grossly (~95%) reduced along with sulphate (~92%), while MWW's phosphate content was practically removed (≥99%). The PHREEQC geochemical model predicted the formation of (oxy)-hydroxides, (oxy)-hydro-sulphates, metals hydroxides, and other mineral phases in the produced sludge, which were confirmed using state-of-the-art analytical techniques such as FE-SEM-EDS and XRD. The key mechanisms governing pollutants removal include dilution, precipitation, co-precipitation, adsorption, and crystallization. Beneficiation and valorisation of the produced sludge and co-treated effluent could promote resource recovery paradigms in wastewater management. Overall, the co-treatment of AMD and MWW appear to be feasible, yet not practical due to the excessive volume of MWW that is required to attain the desired treatment quality. Future research could focus on chemical addition for the control of the pH and the use of (photo)-Fenton for enhancing treatment efficiency.

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

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

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.

The Influence of Co-Fermentation of Agri-Food Waste with Primary Sludge on Biogas Production and Composition of the Liquid Fraction of Digestate

Energy self-sufficiency is a current trend in wastewater treatment plants. This effect can be achieved by increasing the production of electricity from biogas and by reducing energy consumption for technological processes. One idea, in line with the circular economy concept, is the use of waste rich in organic matter as co-substrates for the fermentation process. The aim of this study was to determine the effect of waste co-fermentation on biogas production and nitrogen concentration in the reject water. A co-fermentation process with flotate or flotate and vegetables increased biogas production compared to primary sludge by 162 and 180%, respectively. During the tests, there was no inhibition of the fermentation process. Hydrolysis of organic compounds contained in flotate and vegetables resulted in a significant increase in ammonium nitrogen (by 80–100%) and dissolved organic nitrogen concentration (by 170–180%). The biogas and methane production rate as well as the ammonium and total nitrogen release rate were calculated. An energy balance was made, which took into account the variable amount of electric energy production depending on the efficiency of the cogeneration systems and energy consumption for supplying oxygen necessary to remove nitrogen contained in the reject water. A positive energy balance was obtained for all analyses.

An Electrochemical Study of Ammonium Dihydrogen Phosphate on Mg and Mg Alloy Electrodes

In this work, the electrochemistry of ammonium dihydrogen phosphate in aqueous solution on Mg and Mg AZ31 alloy (Al 3 wt%, Zn 1 wt%, balanced Mg) electrodes was studied using electrochemical characterization similar to electro-agglomeration and physical characterization to shed some light on the electrochemical mechanism of struvite formation as phosphate precipitation in waste. It was found that the Mg AZ31 alloy exhibited higher corrosion current densities (jcorr), and thus higher Mg dissolution rates and corrosion rates (vcorr) when compared with pure Mg. This finding was confirmed by ion chromatography (IC) analysis. Results also showed a phosphorus removal efficiency (PRem) of 16.8% together with ~53 mL H2 production for pure Mg and 17.2% with ~61 mL H2 production for Mg AZ31 alloy. The precipitates formed on the two Mg electrode materials were physically characterized by SEM, XRD, XPS, and energy-dispersive X-ray spectroscopy, which indicated the formation of struvite (magnesium ammonium phosphate hexahydrate-MgNH4PO4*6H2O). It was found that (i) the thickness of the films was influenced by the applied scan rates, which also had a significant impact on the morphology, and (ii) hydrogen bubble formation influenced the precipitates, whereby the film thicknesses were negatively impacted.

Graphical abstract

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

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

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

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

Recovery of phosphorus and metallic nickel along with HCl production from electroless nickel plating effluents: The key role of three-compartment photoelectrocatalytic cell system

A three-compartment photoelectrocatalytic (PEC) cell system combined with ion exchange and chemical precipitation was proposed to recover phosphorus and nickel from electroless nickel plating effluents containing hypophosphite (H₂PO₂^-) and nickel ions (Ni²⁺). Ion exchange was used to concentrate and separate Ni²⁺ and H₂PO₂^-. As a key unit, the established PEC system consisted of TiO₂/Ni-Sb-SnO₂ photoanode and Ti cathode. With 25.8 mM NaH₂PO₂ and 500 mM NiCl₂, 100 % H₂PO₂^- was oxidized to PO₄^3- in the anode cell, 78 % Ni²⁺ was recovered as metallic Ni in the cathode cell, and 900 mM HCl was obtained in the middle cell within 24 h at 3.0 V. Based on quenching experiments and ESR technique, OH radicals were mainly responsible for H₂PO₂^- oxidation. In situ Raman spectroscopy indicated that Ni²⁺ initially reacted with OH^- to form α-Ni(OH)₂, which was gradually reduced to metallic Ni. Fortunately, a slight pH decrease in the cathode cell in the three-compartment cell system was beneficial for Ni²⁺ reduction to Ni°. The obtained PO₄^3- was recovered by chemical precipitation. Finally, recovery of phosphorus and metallic nickel along with HCl production from an actual electroless nickel plating effluents in terms of efficiency, cost-benefit, and stability assessment were demonstrated.

Mitigation of recalcitrant nutrients and organic pollutants from small- to medium-scale biological nutrient removal plant sludge by digester optimization

Digestion of biological nutrient removal (BNR) plant sludge can be challenging, particularly for small- to medium-sized wastewater treatment facilities (WWTF) which often lack the economies of scale, and/or expertise to make digestion feasible. This study compared various types of sludge digestion, sludge retention times (SRTs), and temperatures on the release of recalcitrant nutrients, digestion economics, and digester performance utilizing mixed primary and secondary sludge from a small- to medium-sized BNR facility. Mesophilic anaerobic digestion (AD), cycling aerobic/anoxic (AERO/ANOX) digestion, and sequential anaerobic/aerobic/anoxic (AD/AERO/ANOX) digestion at room and mesophilic temperatures were compared at SRTs between 5 and 20 days. AERO/ANOX digestion was very effective in treating recalcitrant forms of nitrogen and phosphorous by removing up to 87% of dissolved organic nitrogen (DON), up to 88 ± 2% of non-reactive dissolved phosphorous (NRDP). AERO/ANOX digestion also offered the lowest increase in sludge management costs versus the existing no-digestion baseline scenario. ADs removed up to 53 ± 1% of volatile solids (VS), whereas unheated AERO/ANOX digesters were less effective, removing up to 39 ± 1% of VS. Sequential AD/AERO/ANOX digesters with a mesophilic second-stage removed up to 61 ± 3% of VS but had the highest operational and capital costs. Experiments also indicated that significant amounts of orthophosphate (PO₄^3-) may be released from digested AERO/ANOX sludge during on-site storage, with longer SRTs releasing PO₄^3- more rapidly than shorter ones. These results are important as more WWTFs deploy BNR to meet increasingly stringent nutrient limits.

Mass fluxes of nitrogen and phosphorus through water reclamation facilities: Case study of biological nutrient removal, aerobic sludge digestion, and sidestream recycle

At water reclamation facilities, recycling of nutrients (nitrogen and phosphorus) from solids-handling processes to the mainstream treatment process can have detrimental effects on biological nutrient removal systems. In this study, mass fluxes of nitrogen and phosphorus were quantified through the treatment trains at the Northwest Regional Water Reclamation Facility (NWRWRF) and the adjoining Biosolids Management Facility (BMF), which receives sludge from several water reclamation facilities in Hillsborough County, Florida. The driving objectives were to determine (a) whether the return stream from BMF to NWRWRF (i.e., the "sidestream") represents a significant source of nitrogen and phosphorus to NWRWRF, and (b) whether the sidestream return from BMF is interfering with biological nutrient removal processes at NWRWRF. We determined that nearly half of the overall phosphorus flux into NWRWRF is recycled from the BMF sidestream. This leads to an increased cost of treatment, for example, for alum used in phosphorus removal at NWRWRF. In contrast to phosphorus, the flux of nitrogen from BMF to NWRWRF is small (~3%) compared with the flux of nitrogen entering NWRWRF in raw wastewater. However, nitrogen in the sidestream is mostly in the form of nitrate, which prevents anaerobic conditions from developing in the fermentation basin at NWRWRF, and thereby interferes with the enhanced biological phosphorus removal (EBPR) process. Some measurements suggest that fermentation and release of phosphorus may occur in the return activated sludge line (despite the relatively short residence time in that line), which supports EBPR and may partially compensate for anoxic (denitrifying) conditions in the fermentation basin. Therefore, overall, NWRWRF is able to meet its permit limits for phosphorus through a combination of EBPR and alum addition. Although the fluxes measured here are particular to the treatment systems under consideration, the general trends observed are likely to apply to many similar facilities that employ biological nutrient removal, aerobic digestion, and sidestream recycle, particularly those with regional biosolids management facilities. We recommend that such facilities consider (a) removal or recovery of phosphorus from their sidestreams and (b) returning sidestreams downstream of fermentation basins to avoid inhibition of EBPR processes. PRACTITIONER POINTS: Sidestreams from aerobic digestion can represent significant sources of phosphorus to mainstream wastewater treatment. Recycle of nitrate in aerobic digestion sidestreams can interfere with enhanced biological phosphorus removal (EBPR) during mainstream treatment. Fermentation of return activated sludge (RAS) can support EBPR, even under short average hydraulic residence times (minutes).

Performance of a forward osmotic membrane bioreactor for anaerobic digestion of waste sludge with increasing solid concentration

A forward osmotic membrane bioreactor for sludge anaerobic digestion (ad-OMBR) could realize high-solid digestion via drawing moisture out by forward osmosis (FO). Methane production and microbial community evolution were monitored in an ad-OMBR as the total solids (TS) content was gradually increased. With magnesium chloride (MgCl₂) and cellulose triacetate (CTA) membrane as a draw solution and FO membrane, respectively, the ad-OMBR exhibited better performance than the conventional digester, with higher solid content, organic degradation and methane content in biogas. The conductivity of the ad-OMBR did not increase, potentially because of the formation of struvite crystals aided by the reverse-fluxed Mg²⁺ ions. Microbial diversity increased along with the increase in solid content based on the Shannon index, while the most operational taxonomic units were obtained in the 8% TS sludge Although phylum Firmicutes decreased when the TS content was raised to 11%, the relative abundance of Proteobacteria, Chloroflexi, Actinobacteria, and Bacteroidetes, which could also degrade organic matter, increased with increasing TS in ad-OMBR. FO membrane fouling in ad-OMBR was highly reversible.

Fractionating various nutrient ions for resource recovery from swine wastewater using simultaneous anionic and cationic selective-electrodialysis

Different from current nutrient recovery technologies of recovering one or two nutrient components (PO₄^3- or NH₄⁺) from wastewater, this study aimed to fractionate various nutrient anions and cations simultaneously, including PO₄^3-, SO₄^2-, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺, into several streams. The recovered streams could be further paired together to produce high-value products. A novel electrodialysis process was developed by integrating monovalent selective anion and cation exchange membranes into an electrodialysis stack. Results revealed that nutrient recovery was achieved effectively by fractionating PO₄^3- and SO₄^2- into the anionic product stream, whereas bivalent cations (Mg²⁺ and Ca²⁺) were extracted in the cationic product stream and the monovalent cations (K⁺ and NH₄⁺) were concentrated in the brine stream. For the permeation capabilities of anions, SO₄^2- and Cl^- possessed the higher preference, whereas PO₄^3- permeated the membrane more difficult. As to the cations, the permeation sequence was: NH₄⁺≈K⁺ >Ca²⁺>Mg²⁺≈Na⁺. Enhancing voltage values not only promoted ion migration rates, but also led to the increase of energy consumption. Although elevating initial phosphate concentration in the anionic product streams from 60 mg/L to 470 mg/L did not influence phosphate fractionation significantly, the current efficiency decreased from 3.55% to 0.65% and a remarkable increased of energy consumption from 29.42 kWh/kg NaH₂PO₄ to 160.13 kWh/kg NaH₂PO₄ was observed. Further experiments were conducted for phosphorus recovery by pairing two recovered product streams, which revealed that phosphate precipitation could be achieved by using inherent Ca²⁺ and Mg²⁺ in the wastewater without dosing external cation sources.

Anaerobically-digested sludge conditioning by activated peroxymonosulfate: Significance of EDTA chelated-Fe2

This study investigates the efficiency of anaerobically-digested sludge (ADS) conditioning using peroxymonosulfate (PMS) activated by EDTA chelated-Fe²⁺ process (PMS-EDTA-Fe²⁺), and the roles and mechanism of EDTA are explored by investigating the capillary suction time (CST), specific resistance to filtration (SRF), bound water content (BWC), extracellular polymeric substances (EPS), iron species transformation, sludge morphology, and active radicals formation. PMS-EDTA-Fe²⁺ process exhibits better performance in sludge dewatering than PMS-Fe²⁺ process. Upon the introduction of EDTA at optimum molar ratio of EDTA to Fe²⁺ [Formula: see text] of 1 : 3, CST, SRF, and BWC are decreased by 60.0%, 70.6%, and 1.5%, and the cell viability is decreased from 75.4% to as low as 24.7%. EDTA introduction significantly enhances the yield of active radicals (i.e., SO₄^-· and O⋅H) by nearly 50% as compared to PMS-Fe²⁺ process. This effect contributes to stronger cells lysis as indicated by SEM and CLSM, and the contents of polysaccharide and protein in total EPS fractions are increased by 19.2% and 37.4% accordingly. Additionally, EDTA increases the iron mobility by forming Fe^II/III-EDTA complexes and potentially inhibits ADS coagulation at high doses. The total dissolved iron (Fe_{Tot_dis}) concentration increases from 203.4 mg/L to 601.4 mg/L due to the introduction of EDTA at 0.4 mmol/g VSS, and the dominant iron species in supernatant transformed from Fe²⁺ to Fe³⁺ accordingly. The optimization of EDTA doses is significant to balance oxidation and coagulation effects and to achieve better dewatering performance.

Influence of irrigation with microalgae-treated biogas slurry on agronomic trait, nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage

Biogas slurry (BS) is a main byproduct of biogas production that is commonly used for agricultural irrigation because of its abundant nutrients and microelements. However, direct application of BS may cause quality decline and nitrate and heavy metal accumulation in crops. To address this issue, a microalgae culture experiment and an irrigation experiment were performed to evaluate the removal efficiencies of nutrients and heavy metals from diluted BS by microalgae Scenedesmus sp. and to investigate the effects of irrigation with microalgae-treated BS (MBS-25, MBS-50, MBS-75, and MBS-100) on nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage. After 8 days of continuous culture, a ratio of 1/1 for BS/tap water mixture (BS-50) was the optimal proportion for microalgal growth (3.73 g dry cell L^-1) and efficient removal of total nitrogen (86.1%), total phosphorus (94.3%), COD (87.5%), Cr (50%), Pb (60.7%), and Cd (59.7%). The pH in MBS-50 medium recovered to the highest level in a shorter period of time and accelerated the gas stripping of ammonia nitrogen and the formation of insoluble phosphate and metals, which partly contributed to the high removal efficiencies. MBS irrigation significantly promoted crop growth; improved nutritional quality, edible taste, and oxidation resistance; and reduced nitrate and heavy metal residues in Chinese cabbage at a large scale. Therefore, microalgae culture was beneficial to reduce negative impacts of BS irrigation in crop growth and agricultural product safety. This study may provide a theoretical basis for the safe utilization of BS waste in agricultural irrigation.

Evaluating Microbial and Chemical Hazards in Commercial Struvite Recovered from Wastewater

Controlled struvite (NH₄MgPO₄·6H₂O) precipitation has become a well-known process for nutrient recovery from wastewater treatment systems to alleviate the pressures of diminishing, finite rock phosphate reservoirs. Nonetheless, coprecipitation of potential microbial and chemical hazards is poorly understood. On the other hand, antimicrobial resistance (AMR) is a major global public health concern and wastewater is thought to disseminate resistance genes within bacteria. Fecal indicator bacteria (FIB) are typically used as measures of treatment quality, and with multiresistant E. coli and Enterococcus spp. rising in concern, the quantification of FIB can be used as a preliminary method to assess the risk of AMR. Focusing on struvite produced from full-scale operations, culture and qPCR methods were utilized to identify FIB, antibiotic resistance genes, and human enteric viruses in the final product. Detection of these hazards occurred in both wet and dry struvite samples indicating that there is a potential risk that needs further consideration. Chemical and biological analyses support the idea that the presence of other wastewater components can impact struvite formation through ion and microbial interference. While heavy metal concentrations met current fertilizer standards, the presence of K, Na, Ca, and Fe ions can impact struvite purity yet provide benefit for agricultural uses. Additionally, the quantified hazards detected varied among struvite samples produced from different methods and sources, thus indicating that production methods could be a large factor in the risk associated with wastewater-recovered struvite. In all, coprecipitation of metals, fecal indicator bacteria, antimicrobial resistance genes, and human enteric viruses with struvite was shown to be likely, and future engineered wastewater systems producing struvite may require additional step(s) to manage these newly identified public health risks.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.

The Economics of Wastewater Treatment Decentralization: A Techno-economic Evaluation

The existing wastewater treatment infrastructure has not adequately established an efficient and sustainable use of energy, water, and nutrients. A proposed scheme based on source separation and water-efficient use is compared to the current wastewater management paradigm (one largely based on activated sludge) using techno-economic terms. This paper explores the economic viability of adopting more sustainable management alternatives and expands the understanding of the economics of decentralization and source-separation. The feasibility of three different potential types of source-separation (with different levels of decentralization) are compared to the conventional centralized activated sludge process by using recognized economic assessment methodologies together with widely accepted modeling tools. The alternatives were evaluated for two common scenarios: new developments and retrofit due to the aging of existing infrastructures. The results prove that source-separated alternatives can be competitive options despite existing drawbacks (only when countable incomes are included), while the hybrid approach resulted in the least cost-effective solution. A detailed techno-economic evaluation of the costs of decentralization provides insight into the current constraints concerning the paradigm shift and the cost of existing technologic inertia.

The Role of Nanobubbles in the Precipitation and Recovery of Organic-Phosphine-Containing Beneficiation Wastewater

Dissolved air flotation (DAF) is broadly applied in wastewater treatment, especially for the recovery of organic pollution with low concentration. However, the mechanism of interaction between nanoscale gas bubbles and nanoparticles in the process of DAF remains unclear. Here, we investigated the role of nanobubbles in the precipitation of styryl phosphoric acid (SPA)-Pb particles and recovering organic phosphine containined in beneficiation wastewater by UV-vis (ultraviolet-visible) spectra, microflotation tests, nanoparticle tracking analysis, dynamic light scattering, and atomic force microscopy measurements. As suggested from the results, nanobubbles can inhibit the crystallization of SPA-Pb precipitation, which makes the sediment flotation recovery below 20%. After the precipitation crystallization is completed, nanobubbles can flocculate precipitated particles, which can promote the flotation recovery of precipitated particles to 90%. On the basis of the results, we proposed a model to explain the different roles of nanobubbles in the process of precipitation and flotation of SPA-Pb particles. This study will be helpful to understand the interaction between nanobubbles and nanoparticles in the application of flotation.