The determination of fertilizer quality of the formed struvite from effluent of a sewage sludge anaerobic digester

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.

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.

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

The aim of this study was to assess the potential of struvite precipitation to recover nutrients from anaerobically-processed poultry slurry and struvite's interactions with heavy metals (Zn, Cu, Pb, Cr, and Ni) and pathogens (total coliforms and Escherichia coli). The impacts of pH, Mg, N, and P molar proportion, reaction time, and mixing rate and duration were explored to determine the optimal conditions for nutrient recovery through struvite precipitation. A pH range of 9.5 to 10.5, was ideal for P and N removal and recovery, with a molar ratio of 1:1:1 for Mg:N:P. A mixing rate of 150rpm for 10min could allow nutrient recovery with little loss (3.32%) of NH₃ through volatilization, and also achieve an optimal struvite crystal size (50-60μm). The results of X-ray diffractometry and scanning electron microscopy confirmed that the precipitates generated at pH9 and 10 were orthorhombic struvite. Moreover, along with the recovery of nutrients, 40, 45, 66, 30, and 20% of Zn, Cu, Pb, Cr, and Ni, respectively, and 70% total coliforms and E. coli were removed by struvite precipitation from poultry slurry. This was observed despite that the levels of contaminants (heavy metals) detected in struvite were well below the permissible limits and free of pathogens. Consequently, it was inferred that the struvite quality was reasonable by virtue of its heavy metal and pathogen content, and therefore appropriate for application in the field. Similarly, struvite precipitation has multiple benefits as it can effectively recover nutrients as well as reducing pathogenic populations.

The Valorization of Ammonia in Manure Digestate by Means of Alternative Stripping Reactors

The proper recovery of resources such as nitrogen and phosphorus present in the manure from intensive livestock farming is essential in order to allow environmental sustainable zootechny especially in densely populated areas where these activities are historically prevalent. The experiences at full-scale established that the ammonia stripping allows recovery from 35% to 50% of nitrogen depending on the type of substrate treated with anaerobic digestion and on the nitrogen content/form in the digestate. This study focuses on the ammonia stripping on digestate derived from anaerobic digestion of livestock manure and corn silage. Two different full-scale plants are studied including a packed column and an air bubble reactor without filling material with the aim to reduce fouling issues due to the content of suspended solids in digestate. The main results suggest that the use of an air bubble reactor could treat digestate with high concentration of suspended solids. A deeper study based on a two-level factorial experiment highlights that the temperature is an important parameter that influences the ammonia removal yields. Thus, a proper management of available thermal energy is very important.

Recovery of phosphorous from industrial waste water by oxidation and precipitation

This paper describes the development of a method for recovery of phosphorous from one of the waste waters at an Akzo Nobel chemical plant in Ale close to Göteborg. It was found that it is possible to transform the phosphorous in the waste water to a saleable product, i.e. a slowly dissolving fertilizer. The developed process includes oxidation of phosphite to phosphate with hydrogen peroxide and heat. The phosphate is then precipitated as crystalline struvite (ammonium magnesium phosphate) by the addition of magnesium chloride. The environmental impacts of the new method were compared with those of the current method using life cycle assessment. It was found that the methodology developed in this project was an improvement compared with the current practice regarding element resource depletion and eutrophication. However, the effect on global warming would be greater with the new method. There could however be several ways to decrease the global warming effect. Since most of the carbon dioxide emissions come from the production of magnesium chloride from carbonates, changing to utilization of a magnesium chloride from desalination of seawater or from recycling of PVC would decrease the carbon footprint significantly.

Application of Struvite-MAP Crystallization Reactor for Treating Cattle Manure Anaerobic Digested Slurry: Nitrogen and Phosphorus Recovery and Crystal Fertilizer Efficiency in Plant Trials

Recycling and reusing the nutrient resources from anaerobic digested slurry is very promising for environmental pollution control and agriculture sustainable development. We focus here on nitrogen and phosphorus recycling in treating cattle manure anaerobic digested slurry by a magnesium ammonium phosphate (struvite-MAP) crystallization process and examine the impact of MAP precipitation on plant growth. The MAP crystallization process was studied by a combination of Design-Expert 8.0.6 software, mathematical modeling, and experiments. The influence of Mg/P, N/P and pH on nitrogen (N) and phosphorus (P) recovery was investigated. Then, the fertilizing efficiency of the MAP precipitate on the growth of three vegetables (water spinach (Swamp cabbage), amaranth and Brassica parachinensis) was also evaluated. The results showed that more than 89% of N and 99% of P could be recovered at pH = 10 with molar ratios of Mg/P = 1.6 and N/P = 1.2. Compared with the control pots and potassium chloridepots, the fresh weight, dry weight and average height of swamp cabbage in the MAP pots were obviously enhanced without burning effects. The results showed that MAP precipitation can promote the development of plants, which is promising for its use as a slow-release fertilizer for agricultural production.

Nutrient management via struvite precipitation and recovery from various agroindustrial wastewaters: Process feasibility and struvite quality

Improving environmental protection and finding sustainable and renewable resources of nutrients are core issues in circular bioeconomy. Thus, this study evaluated the efficiency of recovering struvite, MgNH₄PO₄·6H₂O, from different agro-industrial wastewaters (four highly loaded reject waters of anaerobically co-digested agro-industrial waste and a raw swine slurry) and assessed the quality of recovered struvite crystals and their reusability as fertilizer. The efficiency of crystallization (E_c 40-80%) and amount of struvite in the precipitate (P_p 55-94%) highly varied due to the characteristics of influent wastewaters, particularly to the content of competing elements, such as alkaline and heavy metals and total solids (TS). In particular, E_c (94, 75, 61%) and P_p (76, 66, 48%) decreased at increasing TS (0.57, 0.73, 0.99%), demonstrating the hindering effect of solid content on struvite recovery and quality. According to X-ray diffraction analysis, the structure of all isolated samples corresponded to crystalline, orthorhombic struvite, which exhibited high purity (32-48 g/kg_d N, 114-132 g/kg_d P, and 99-116 g/kg_d Mg) containing only a few foreign elements, whose amount depended on the characteristics of the influent wastewater. All struvite contained other plant macronutrients (K, Ca) and many micronutrients (Fe, Na, Cu, Mn, Co, Zn) that further enhance its agronomic value. Therefore, this study showed that struvite can be successfully recovered from a wide range of highly loaded agroindustrial wastewaters, and that the quality of the recovered struvite could be suitable for reuse in agriculture.

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

The primary objective of this research was to remove recalcitrant nutrients from anaerobically digested sludge dewatering centrate. A struvite precipitation methodology is proposed where salt crystals are encouraged to ballast colloidal particles through heterogeneous nucleation and subsequent crystal growth. The secondary objective was to assess presence of micropollutants in precipitates. Four biologically unique dewatering centrates were used to test the precipitation methodology on the variety of anaerobic digester configurations that can be expected from municipal wastewater treatment plant. The effect of digestion sludge retention time (2 day, 20 day) and digestion temperature (35 °C, 55 °C) on the removal of dissolved unreactive phosphorus (P) and nitrogen (N) was monitored. Averaged across all four centrates, the precipitation methodology resulted in dissolved unreactive P and N removal of 82.4% and 66.6%, respectively. Antimicrobial contaminants (triclosan, triclocarban) were observed in the precipitates at minute concentrations (<18 ng/g-dry solids). Therefore, mass struvite precipitation can provide a means of recalcitrant nutrient treatment and reactive nutrient recovery without the micropollutant burden of biosolids land application.

Easy and fast extraction methods to determine organochlorine pesticides in sewage sludge, soil, and water samples based at low temperature

Organochlorine pesticides present in sewage sludge can contaminate soil and water when they are used as either fertilizer or agricultural soil conditioner. In this study, the technique solid-liquid extraction with low temperature purification was optimized and validated for determination of ten organochlorine pesticides in sewage sludge and soil samples. Liquid-liquid extraction with low temperature purification was also validated for the same compounds in water. Analyses were performed by gas chromatography-mass spectrometry operating in the selective ion monitoring mode. After optimization, the methods showed recoveries between 70% and 115% with relative standard deviation lower than 13% for all target analytes in the three matrices. The linearity was demonstrated in the range of 20 to 70 µg L^-1, 0.5 to 60 µg L^-1, and 3 to 13 µg L^-1, for sludge, soil, and acetonitrile, respectively. The limit of quantification ranged between 2 and 40 µg kg^-1, 1 and 6 µg kg^-1, and 0.5 µg L^-1 for sludge, soil, and water, respectively. The methods were used in the study of pesticide lixiviation carried out in a poly vinyl chlorine column filled with soil, which had its surface layer mixed with sludge. The results showed that pesticides are not leached into soil, part of them is adsorbed by the sewage sludge (4-40%), and most pesticides are lost by volatilization.

Struvite formation and decomposition characteristics for ammonia and phosphorus recovery: A review of magnesium-ammonia-phosphate interactions

Struvite (MgNH₄PO₄·6H₂O) forms in aqueous systems with high ammonia and phosphate concentrations. However, conditions that result into struvite formation are highly dependent on the ionic compositions, temperature, pH, and ion speciation characteristics. The primary ions involved in struvite formation have complex interactions and can form different crystals depending on the ionic levels, pH and temperature. Struvite as well as struvite analogues (with substitution of monovalent cations for NH₄⁺ or divalent cations for Mg²⁺) as well as other crystals can form simultaneously and result in changes in crystal morphology during crystal growth. This review provides the results from experimental and theoretical studies on struvite formation and decomposition studies. Characteristics of NH₄⁺ or divalent cations for Mg²⁺ were evaluated in comparison to monovalent and divalent ions for formation of struvite and its analogues. Struvite crystals forming in wastewater systems are likely to contain crystals other than struvite due to ionic interactions, pH changes, temperature effects and clustering of ions during nucleation and crystal growth. Decomposition of struvite occurs following a series of reactions depending on the rate of heating, temperature and availability of water during heating.

Enhancing recovery of magnesium as struvite from landfill leachate by pretreatment of calcium with simultaneous reduction of liquid volume via forward osmosis

Landfill leachate contains substances that can be potentially recovered as valuable resources. In this study, magnesium in a landfill leachate was recovered as struvite with calcium pretreatment; meanwhile, the leachate volume was reduced by using a submerged forward osmosis (FO) process, thereby enabling significant reduction of further treatment footprint and cost. Without pretreatment, calcium exhibited strong competition for phosphate with magnesium. The pretreatment with a Ca²⁺: CO₃^2- molar ratio of 1:1.4 achieved a relatively low loss rate of Mg²⁺ (24.1±2.0%) and high Ca²⁺ removal efficiency (89.5±1.7%). During struvite recovery, 98.6±0.1% of magnesium could be recovered with a significantly lower residual PO₄^3--P concentration (<25mgL^-1) under the condition of (Mg+Ca_residual): P molar ratio of 1:1.5 and pH9.5. The obtained struvite had a similar crystal structure and composition (19.3% Mg and 29.8% P) to that of standard struvite. The FO process successfully recovered water from the leachate and reduced its volume by 37%. The configuration of calcium pretreatment - FO - struvite recovery was found to be the optimal arrangement in terms of FO performance. These results have demonstrated the feasibility of magnesium recovery from landfill leachate and the importance of the calcium pretreatment, and will encourage further efforts to assess the value and purity of struvite for commercial use and to develop new methods for resource recovery from leachate.

The influence of different pretreatment methods on biogas production from Jatropha curcas oil cake

Drought and pest resistance, together with high oil content in its seeds, make Jatropha curcas a good oil source for biodiesel. Oil cake from J. curcas is not suitable for animal feeding and thus may be profitably used for additional energy production by conversion into biogas; however, the anaerobic digestion process must be optimized to obtain good efficiency. We subjected oil cake to thermal and acidic pretreatment to deactivate protease inhibitors and partially hydrolyze phytate. We then digested the samples in batch conditions to determine the effects of pretreatment on biogas production. Thermal pretreatment changed the kinetics of anaerobic digestion and reduced protease inhibitor activity and the concentration of phytate; however, biogas production efficiency was not affected (0.281 m³ kg^-1). To evaluate the possibility of recirculating water for SSF hydrolysis, ammonium nitrogen recovery from effluent was evaluated by its precipitation in the form of struvite (magnesium ammonium phosphate).Concentration of ammonium ions was reduced by 53% (to 980 mg L^-1). We propose a water-saving concept based on percolation of J. curcas cake using anaerobic digestion effluent and feeding that percolate into a methanogenic bioreactor.

Carbon and Phosphorus Removal from Primary Municipal Wastewater Using Recovered Aluminum

In this work, recovery of aluminum from coagulated primary sludge and its reuse potential as secondary coagulant were investigated. The recovery process consisted of releasing the particle-bound aluminum from primary sludge by acidification (HCl or H₂SO₄), followed by separation using centrifugation for dissolved coagulant recovery. The recovered coagulant was then reused for treating primary wastewater and overall coagulation efficiency was determined. While with fresh alum, the removal efficiencies of total suspended solids, chemical oxygen demand, total phosphorus, and total nitrogen were 85%, 65%, 80% and 33%, respectively, a drop in removal efficiency of total suspended solids and chemical oxygen demand was observed for recovered aluminum (85-60% and 65-50%, respectively). Nitrogen concentration remained almost constant with each cycle, while phosphorus in the effluent increased by 1 mg/L and 3 mg/L in the first and second cycle, respectively. Precipitation of various aluminum species was modeled for determining the recovery potential of aluminum at low pH. Preliminary cost analysis indicates that optimum recovery of aluminum occurred at a pH of 1.5 for both acids. Struvite precipitation effectively removed increased phosphorus solubilized by acidification at the end of second cycle, however, it also decreased the amount of aluminum available for recycle.

Recovery of nutrients from digested sludge as struvite with a combination process of acid hydrolysis and Donnan dialysis

In this study, the effects of the combination of acid hydrolysis and Donnan dialysis on the high-quality struvite production from digested sewage sludge were examined. The Box-Behnken design was applied in the hydrolysis using oxalic acid for the optimization of the conditions that affect the nutrients and metal release from digested sludge. An optimal condition was obtained at oxalic acid 0.5 M, acid/sludge ratio (mL/g) 10/1, and reaction time 60 min. The separation of the metals from hydrolyzed sludge liquid was carried out with the Donnan dialysis using a Nafion 117 cation exchange membrane. At the end of the 4 hours of operating time, the recovery values for Zn, K, Na, Mg, Fe, and Al were obtained as 67.9%, 62.1%, 57.6%, 39.4%, 5.3%, and 2.5%, respectively. The produced struvite meets the legal limits for fertilizer use in terms of Cd, Cu, Ni, Pb, Zn, Hg, and Cr content specified by Turkish regulations. A high Ca/Mg molar ratio and the presence of K, Na, and Al affected the efficiency of struvite crystallization. The use of oxalic acid resulted in low Ca release, and the metal ions that affect struvite purity were eliminated using Donnan dialysis.

Prospect of recovering phosphorus in magnesium slag-packed wetland filter

Phosphorus recovery from wastewater not only reduces the unbearable impacts of excessive nutrient discharge on environmental systems but also favor the reuse of phosphorus resource. Based on the mechanism as well as technical analysis for major phosphorus recovery techniques including struvite precipitation and wetland substrate adsorption, a novel magnesium slag-packed wetland filter and corresponding operational procedures are proposed, which aim to reduce the dependence of using magnesium-containing chemical reagent as magnesium sources for struvite precipitation, and improve the accumulation and recovery performance for struvite precipitation within porous wetland substrate. Results from preliminary experiments indicated that magnesium slag particles with approximately 2 mm in diameter can recover 43.20-72.39% phosphorus from 1-25 mol/L PO₄^3- solution, and the presence of 5-50 mol/L NH₄⁺ contributed to 11.71-29.11% enhancement of phosphorus recovery mainly due to struvite precipitation. The detected generation of struvite via XRD spectrum analysis partly demonstrated the potential of phosphorus recovery in magnesium slag-packed wetland filter. The proposed phosphorus recovery technology is free of secondary pollution and solid waste generation; phosphorus-saturated (mainly due to struvite precipitation and adsorption) magnesium slag particles can be potentially used as phosphorus fertilizer and thus partly solved the traditional shortages of disposing phosphorus-saturated substrate due to low phosphorus contents.

Transformation of Phosphorus during (Hydro)thermal Treatments of Solid Biowastes: Reaction Mechanisms and Implications for P Reclamation and Recycling

Phosphorus (P) is an essential nutrient for all organisms, thus playing unique and critical roles at the food-energy-water nexus. Most P utilized by human activities eventually converges into various solid biowastes, such as crop biomass, animal manures, and sewage sludges. Therefore, integration of efficient P recovery practices into solid biowaste management will not only significantly reduce the dependence on limited geological P resources but also reduce P runoff and related water contamination issues associated with traditional waste management strategies. This study reviews the applications of (hydro)thermal techniques for the treatment of solid biowastes, which can greatly facilitate P recovery in addition to waste volume reduction, decontamination, and energy recovery. Research showed that P speciation (including molecular moiety, complexation state, and mineralogy) can experience significant changes during (hydro)thermal treatments, and are impacted by treatment techniques and conditions. Changes in P speciation and overall properties of the products can alter the mobility and bioavailability of P, and subsequent P reclamation and recycling efficiency of the treatment products. This review summarizes recent progresses in this direction, identifies the challenges and knowledge gaps, and provides a foundation for future research efforts targeting at sustainable management of nutrient-rich biowastes.

Nutrient recovery from airplane wastewater: composition, treatment and ecotoxicological assay

For the 2014 World Cup and the 2016 Olympic Games, Brazil has expanded its airport infrastructure. This will lead to an increase in wastewater generation from aircrafts. This wastewater is traditionally taken from the aircrafts and disposed in the public sewage collection system. However, this residual water may have a different composition than the usual sanitary sewage. Therefore, it is important to study an alternative to treat this kind of wastewater. Thus, the objective of this study was to characterize and analyze the treatment of wastewater from airplane toilets through chemical precipitation for the removal of ammonia in the form of struvite. The airplanes' effluent showed a composition similar to human urine with pH 8.9, ammonia nitrogen 4,215 mg L^-1, phosphorus 430 mg L^-1 and a very high acute toxicity (Vibrio fischeri). The best treatment for struvite formation was with pH 9.0 and molar ratio Mg:NH₄:PO₄ equal to 1.5:1.0:1.0. In this case, the removal of ammonia and phosphorus achieved 97.0% and 95.3%, respectively. After this procedure, the toxicity by Vibrio fischeri decreased.

In Situ Nanoscale Imaging of Struvite Formation during the Dissolution of Natural Brucite: Implications for Phosphorus Recovery from Wastewaters

As phosphorus (P) resources are diminishing, the recovery of this essential nutrient from wastewaters becomes an increasingly interesting option. P-recovery through the controlled crystallization of struvite (MgNH₄PO₄·6H₂O), a potential slow-release fertilizer, is highly attractive, but costly if large amounts of Mg have to be added. In this context, natural Mg-minerals like brucite (Mg(OH)₂) could provide more cost-effective Mg-sources compared to high-grade Mg-compounds such as MgCl₂. Here we used in situ atomic force microscopy (AFM) to study the interactions of ammonium phosphate solutions with brucite (001) cleavage surfaces. Brucite dissolution was strongly enhanced in the presence of H₂PO₄^- ions, most likely due to the formation of negatively charged surface complexes. Simultaneously with brucite dissolution, we directly observed the formation of a new phase that was identified as struvite by Raman spectroscopy. Our results suggest that brucite dissolution and struvite precipitation were coupled at the mineral-fluid interface within a thin fluid boundary layer. An interpretation is proposed where the heterogeneous nucleation and growth of struvite occurs via a particle-mediated process involving the formation of primary nanoparticles, followed by their continuous aggregation, fusion and possible transformation to crystalline struvite. These observations have implications for the feasibility of using brucite in phosphorus recovery processes.

Ammonia removal from landfill leachate by struvite formation: an alarming concentration of phosphorus in the treated effluent

This study examined the removal of ammonia nitrogen from the leachate of a landfill site using the chemical precipitation of struvite (MgNH₄PO₄.6H₂O). This procedure achieved a reduction in the ammonia concentration that was higher than 99% when the molar ratio of 1.8:1.0:1.4 for Mg₂⁺:NH₄⁺:PO₄^3- was adopted. The metal concentration found in the precipitate formed was lower than the limits set by Brazilian and American regulations (CONAMA 375/2006 and US EPA, 40 CFR 503.1993). This demonstrates the potential use for this practice in agriculture. However, the effluent obtained from the tests presented a phosphorus concentration higher than the one in the raw leachate. This shows that removing this compound from the effluent must be further studied. Otherwise, adopting the chemical precipitation of ammonia by the formation of struvite may become unfeasible.

Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies

Phosphorus (P) is an essential and limited resource. Municipal wastewater is a promising source of P via reuse and could be used to replace P derived from phosphate rocks. The agricultural use of sewage sludge is restricted by legislation or is not practiced in several European countries due to environmental risks posed by organic micropollutants and pathogens. Several technologies have been developed in recent years to recover wastewater P. However, these technologies target different P-containing flows in wastewater treatment plants (effluent, digester supernatant, sewage sludge, and sewage sludge ash), use diverse engineering approaches and differ greatly with respect to P recycling rate, potential of removing or destroying pollutants, product quality, environmental impact and cost. This work compares 19 relevant P recovery technologies by considering their relationships with existing wastewater and sludge treatment systems. A combination of different methods, such as material flow analysis, damage units, reference soil method, annuity method, integrated cost calculation and a literature study on solubility, fertilizing effects and handling of recovered materials, is used to evaluate the different technologies with respect to technical, ecological and economic aspects. With regard to the manifold origins of data an uncertainty concept considering validity of data sources is applied. This analysis revealed that recovery from flows with dissolved P produces clean and plant-available materials. These techniques may even be beneficial from economic and technical perspectives under specific circumstances. However, the recovery rates (a maximum of 25%) relative to the wastewater treatment plant influent are relatively low. The approaches that recover P from sewage sludge apply complex technologies and generally achieve effective removal of heavy metals at moderate recovery rates (~40-50% relative to the WWTP input) and comparatively high costs. Sewage sludge ash is the most promising P source, with recovery rates of 60-90% relative to the wastewater P. The costs highly depend on the purity requirements of the recycled products but can be kept comparatively low, especially if synergies with existing industrial processes are exploited.

Anaerobic biorefinery: Current status, challenges, and opportunities

Anaerobic digestion (AD) has been in use for many decades. To date, it has been primarily aimed at treating organic wastes, mainly manures and wastewater sludge, and industrial wastewaters. However, with the current advancements, a more open mind is required to look beyond these somewhat restricted original applications of AD. Biorefineries are such concepts, where multiple products including chemicals, fuels, polymers etc. are produced from organic feedstocks. The anaerobic biorefinery concept is now gaining increased attention, utilizing AD as the final disposal step. This review aims at evaluating the potential significance of anaerobic biorefineries, including types of feedstocks, uses for the produced energy, as well as sustainable applications of the generated residual digestate. A comprehensive analysis of various types of anaerobic biorefineries has been developed, including both large-scale and household level applications. Finally, future directives are highlighted showing how anaerobic biorefinery concept could impact the bioeconomy in the near future.

Characterization of morphology and component of struvite pellets crystallized from sludge dewatering liquor: Effects of total suspended solid and phosphate concentrations

A lab-scale struvite pellet crystallization system was used to study phosphorus (P) removal and recovery from sludge dewatering liquor (SDL). Influences of total suspended solids (TSS) and phosphate concentrations on P removal as well as the size, morphology, purity, and components of struvite pellets were investigated. The increase in TSS concentration resulted in not only the decreases in phosphate removal efficiency and struvite purity but also the irregular pellet morphology and broken struvite crystals. Increasing inlet PO4-P concentration enhanced PO4-P removal, average struvite pellet diameter, purity and crystal volume growth rate. Amorphous calcium phosphate (ACP), calcite, brucite and magnesium phosphate were formed as co-precipitates with struvite. However, species and quantity of co-precipitates could be variable. More calcium precipitates were easily formed at lower PO4-P concentration (48mg/L), while brucite was the main co-precipitate at higher PO4-P concentration (151mg/L). Organic compounds were involved in struvite pellets along with suspended solids during the formation of struvite. Higher TSS concentration resulted in both more species and higher contents of organic compounds in struvite pellets. Therefore, it is essential to remove suspended solids in advance so as to obtain high P-removal and harvest high-quality struvite pellets.

Ultrasonic waste activated sludge disintegration for recovering multiple nutrients for biofuel production

Waste activated sludge is a valuable resource containing multiple nutrients, but is currently treated and disposed of as an important source of pollution. In this work, waste activated sludge after ultrasound pretreatment was reused as multiple nutrients for biofuel production. The nutrients trapped in sludge floc were transferred into liquid medium by ultrasonic disintegration during first 30 min, while further increase of pretreatment time only resulted in slight increase of nutrients release. Hydrogen production by Ethanoligenens harbinense B49 from glucose significantly increased with the concentration of ultrasonic sludge, and reached maximum yield of 1.97 mol H2/mol glucose at sludge concentration of 7.75 g volatile suspended solids/l. Without addition of any other chemicals, waste molasses rich in carbohydrate was efficiently turned into hydrogen with yield of 189.34 ml H2/g total sugar by E. harbinense B49 using ultrasonic sludge as nutrients. The results also showed that hydrogen production using pretreated sludge as multiple nutrients was higher than those using standard nutrients. Acetic acid produced by E. harbinense B49 together with the residual nutrients in the liquid medium were further converted into hydrogen (271.36 ml H2/g total sugar) by Rhodopseudomonas faecalis RLD-53 through photo fermentation, while ethanol was the sole end product with yield of 220.26 mg/g total sugar. Thus, pretreated sludge was an efficient nutrients source for biofuel production, which could replace the standard nutrients. This research provided a novel strategy to achieve environmental friendly sludge disposal and simultaneous efficient biofuel recovery from organic waste.

Natural attenuation of toxic metal phytoavailability in 35-year-old sewage sludge-amended soil

Toxic heavy metals persist in agricultural soils and ecosystem for many decades after their application as contaminants in sewage sludge and fertilizer products This study assessed the potential long-term risk of cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) in land-applied sewage sludge to food crop contamination. A sewage sludge-amended soil (SAS) aged in the field more than 35 years was used in a greenhouse pot experiment with leafy vegetables (lettuce and amaranth) having strong Cd and Zn accumulation tendencies. Soil media with variable levels of available Cd, Zn, and Cu (measured using 0.01 M CaCl2 extraction) were prepared by diluting SAS with several levels of uncontaminated control soil. Despite long-term aging in the field, the sludge site soil still retains large reserves of heavy metals, residual organic matter, phosphorus, and other nutrients, but its characteristics appear to have stabilized over time. Nevertheless, lettuce and amaranth harvested from the sludge-treated soil had undesirable contents of Cd and Zn. The high plant uptake efficiency for Cd and Zn raises a concern regarding the quality and safety of leafy vegetables in particular, when these crops are grown on soils that have been amended heavily with sewage sludge products at any time in their past.

Assessment of fertilizer potential of the struvite produced from the treatment of methanogenic landfill leachate using low-cost reagents

Leachates generated in methanogenic landfills contain high strength of ammonium nitrogen which removal is hard to be accomplished by means of conventional techniques. The chemical precipitation of struvite, which is a mineral that could be reused as a slow-release fertilizer, is an effective process in the removal and recovery of NH4 amount of high-concentrated wastewaters. In this paper, a struvite precipitation process using unconventional reagents is proposed for a sustainable recovery of nitrogen content. In particular, seawater bittern, a by-product of marine salt manufacturing, and bone meal, a by-product of the thermal treatment of meat waste, have been used as low-cost sources of magnesium and phosphorus, respectively. The process enables the removal of more than 98 % ammonia load, the recovery about 99 and 95 % of phosphorus and magnesium, respectively, and the production of a precipitate containing struvite crystals. Heavy metals concentrations of produced precipitate were below the threshold values specified by the EC Directive for use of sewage sludges as fertilizers. Specific agronomic tests were conducted to investigate the fertilizing value of precipitate recovered from landfill leachate. The fertilizing effect of struvite deposit in cultivating Spinacia oleracea was compared with that of vegetable soil and commercial fertilizer. The growth of selected vegetable in the pots with struvite precipitate resulted significantly greater in both than those in the control pots and in the pots with the complex fertilizer. Furthermore, the struvite application as fertilizer did not result in more heavy metals in the vegetables respect those from soil and model fertilizer.

Phosphorus recovery as struvite from farm, municipal and industrial waste: Feedstock suitability, methods and pre-treatments

Global population growth requires intensification of agriculture, for which a sustainable supply of phosphorus (P) is essential. Since natural P reserves are diminishing, recovering P from wastes and residues is an increasingly attractive prospect, particularly as technical and economic potential in the area is growing. In addition to providing phosphorus for agricultural use, precipitation of P from waste residues and effluents lessens their nutrient loading prior to disposal. This paper critically reviews published methods for P recovery from waste streams (municipal, farm and industrial) with emphasis on struvite (MgNH4PO4·6H2O) crystallisation, including pre-treatments to maximise recovery. Based on compositional parameters of a range of wastes, a Feedstock Suitability Index (FSI) was developed as a guide to inform researchers and operators of the relative potential for struvite production from each waste.

Ammonia nitrogen removal from acetylene purification wastewater from a PVC plant by struvite precipitation

Acetylene purification wastewater (APW) usually contains high concentrations of ammonia nitrogen (NH4-N), which is generated during the production of acetylene in a polyvinylchloride manufacturing plant. In this study, a struvite precipitation method was selected to remove NH4-N from the APW. Laboratory-scale batch experiments were performed to investigate the effects of the initial APW pH, phosphate (PO4(3-)) concentration, magnesium (Mg(2+)) concentration, and sources of PO4(3-) and Mg(2+) on NH4-N removal. The results indicated that the initial APW pH had a significant effect on the removal of NH4-N, while the other factors had relatively minor effect. The NH4-N could be effectively removed at an optimum initial APW pH of 9.5, when Na2HPO4·12H2O and MgSO4·7H2O were both applied to NH4-N at a ratio of 1.2. Under these conditions, the efficiency of removal of NH4-N, total nitrogen and chemical oxygen demand were 85%, 84% and 18%, respectively. The X-ray diffraction analysis indicated that the precipitates were dominated by struvite. The scanning electron microscopy analysis of the precipitates showed a typical morphology of stick-like and prismatic crystals with coarse surface. The energy dispersive spectroscopy analysis indicated that the precipitates contained P, O, Mg and Ca.

Growth of mono- and mixed cultures of Nannochloropsis salina and Phaeodactylum tricornutum on struvite as a nutrient source

The suitability of crude and purified struvite (MgNH4PO4), a major precipitate in wastewater streams, was investigated for renewable replacement of conventional nitrogen and phosphate resources for cultivation of microalgae. Bovine effluent wastewater stone, the source of crude struvite, was characterized for soluble N/P, trace metals, and biochemical components and compared to the purified mineral. Cultivation trials using struvite as a major nutrient source were conducted using two microalgae production strains, Nannochloropsis salina and Phaeodactylum tricornutum, in both lab and outdoor pilot-scale raceways in a variety of seasonal conditions. Both crude and purified struvite-based media were found to result in biomass productivities at least as high as established media formulations (maximum outdoor co-culture yield ∼20±4gAFDW/m(2)/day). Analysis of nutrient uptake by the alga suggest that struvite provides increased nutrient utilization efficiency, and that crude struvite satisfies the trace metals requirement and results in increased pigment productivity for both microalgae strains.

Improvement of the trace metal composition of medium for nitrite-dependent anaerobic methane oxidation bacteria: Iron (II) and copper (II) make a difference

Nitrite-dependent anaerobic methane oxidation (n-damo) is a potential bioprocess for treating nitrogen-containing wastewater. This process uses methane, an inexpensive and nontoxic end-product of anaerobic digestion, as an external electron donor. However, the low turnover rate and slow growth rate of n-damo functional bacteria limit the practical application of this process. In the present study, the short- and long-term effects of variations in trace metal concentrations on n-damo bacteria were investigated, and the concentrations of trace metal elements of medium were improved. The results were subsequently verified by a group of long-term inoculations (90 days) and were applied in a sequencing batch reactor (SBR) (84 days). The results indicated that iron (Fe(II)) and copper (Cu(II)) (20 and 10 μmol L(-1), respectively) significantly stimulated the activity and the growth of n-damo bacteria, whereas other trace metal elements, including zinc (Zn), molybdenum (Mo), cobalt (Co), manganese (Mn), and nickel (Ni), had no significant effect on n-damo bacteria in the tested concentration ranges. Interestingly, fluorescence in situ hybridization (FISH) showed that a large number of dense, large aggregates (10-50 μm) of n-damo bacteria were formed by cell adhesion in the SBR reactor after using the improved medium, and to our knowledge this is the first discovery of large aggregates of n-damo bacteria.

Assessing the feasibility of N and P recovery by struvite precipitation from nutrient-rich wastewater: a review

Literature on recovery of nitrogen and phosphorous from wastewater in the form of value-added struvite fertilizer has been critically reviewed towards the evolution of a sustainable management strategy. Presence of nitrogen and phosphorus is widespread in both domestic as well as industrial wastewater streams such as swine wastewater, landfill leachate, urine waste, dairy manure, coke wastewater, and beverage wastewater. Where these nitrogen and phosphorus compounds cause eutrophication of water bodies and considered as harmful discharges to the environment, they can be turned useful through simple chemical conversion into struvite (MgNH4PO4·6H2O). In extensive studies on wastewater treatment, aspects of recovery of valuable materials remain dispersed. In the present article, almost all relevant aspects of sources of raw materials, chemistry and technology of struvite production, and its detailed characterization have been captured in a systematic and classified way so as to help in planning and designing an integrated scheme of struvite production through conversion of nitrogen and phosphorus components of waste streams. The study will help in formulating a new waste management strategy in this context by shifting focus from removal to recovery of nutrients from waste streams.

Reagent use efficiency with removal of nitrogen from pig slurry via struvite: A study on magnesium oxide and related by-products

Controlled struvite formation has been attracting increasing attention as a near mature technology to recover nutrients from wastewater. However, struvite feasibility is generally limited by the high cost of chemical reagents. With the aim to understand and control reagent use efficiency, experiments and equilibrium model simulations examined inorganic nitrogen (TAN) removal from pig manure via struvite with added magnesium and phosphate reagents. Four industrial magnesium oxide (MgO), a commercial product and three by-products from magnesite calcination, were tested with phosphate added as a highly soluble potassium salt. TAN removal extents with the MgOs ranged from 47 to 72%, with the highest grade MgO providing the greatest extent of TAN removal. However, model analysis showed that all the MgO reagents were poorly soluble (only about 40% of added magnesium actually dissolved). The model results suggested that this poor dissolution was due to kinetic limitations, not solubility constraints. A further set of additional reagents (termed stabilization agents) were prepared by pre-treating the MgO reagents with phosphoric acid, and were tested separately as a source of both magnesium and phosphate. Results showed that acid pre-treatment of moderate to highly reactive MgOs (soft to medium-burnt) primarily formed bobierrite as the stabilizing agent, whereas the pre-treatment of very low reactivity MgOs (dead-burnt) mostly formed newberyite. The newberyite stabilizing agents achieved very high TAN removal extents of about 80%, which is significant, considering that these were formed from dead-burnt/low-grade MgOs. However, the bobierrite stabilizing agents achieved a substantially lower TAN removal extent than their medium-to-high reactivity precursor MgOs. Again, model analysis showed that the bobierrite stabilizing agents were poorly soluble, due to kinetic limitations, not solubility constraints. In contrast, the model suggested that the newberyite stabilizing agents almost completely dissolved to very effectively form struvite. A mechanism was proposed by which conditions near a dissolving reagent particle surface causes unwanted struvite nucleation onto and overgrowth of the reagent particle, inhibiting further dissolution and markedly reducing reagent efficiency. The findings of the study could have implications for reagent efficiency with struvite in general, even when using other solid reagents such as magnesium hydroxide or other MgOs.

Effects of phosphate addition on methane fermentation in the batch and upflow anaerobic sludge blanket (UASB) reactors

Ammonia inhibition of methane fermentation is one of the leading causes of failure of anaerobic digestion reactors. In a batch anaerobic digestion reactor with 429 mM NH3-N/L of ammonia, the addition of 25 mM phosphate resulted in an increase in methane production rate. Similar results were obtained with the addition of disodium phosphate in continuous anaerobic digestion using an upflow anaerobic sludge blanket (UASB) reactor. While methane content and production rate decreased in the presence of more than 143 mM NH3-N/L of ammonium chloride in UASB, the addition of 5 mM disodium phosphate suppressed ammonia inhibition at 214 mM NH3-N/L of ammonium chloride. The addition prevented acetate/propionate accumulation, which might be one of the effects of the phosphate on the ammonia inhibition. The effects on the microbial community in the UASB reactor was also assessed, which was composed of Bacteria involved in hydrolysis, acidogenesis, acetogenesis, and dehydrogenation, as well as Archaea carrying out methanogenesis. The change in the microbial community was observed by ammonia inhibition and the addition of phosphate. The change indicates that the suppression of ammonia inhibition by disodium phosphate addition could stimulate the activity of methanogens, reduce shift in bacterial community, and enhance hydrogen-producing bacteria. The addition of phosphate will be an important treatment for future studies of methane fermentation.

Arsenic speciation in newberyite (MgHPO(4)·3H(2)O) determined by synchrotron X-ray absorption and electron paramagnetic resonance spectroscopies: implications for the fate of arsenic in green fertilizers

Newberyite (MgHPO4·3H2O), a biomineral and common constituent in guano deposits, is an important decomposition product of struvite that is an increasingly popular green fertilizer recovered from wastewaters. Two samples of newberyite containing 1099 and 25 ppm As have been obtained at pH = 6.4, by using Na2HAsO4·7H2O and NaAsO2 as the dopant, respectively (i.e., Synthesis 1 and Synthesis 2). Synchrotron arsenic K-edge X-ray absorption spectroscopic data of newberyite from Synthesis 1 show that As(5+) is dominant and has a local environment typical of the arsenate species. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated newberyite from Synthesis 1 contain two arsenic-associated oxyradicals: [AsO3](2-) and [AsO2](2-) derived from As(5+) and As(3+), respectively, at the P site. Quantitative analyses of powder EPR spectra allow determinations of the As(5+) and As(3+) contents in newberyite from Synthesis 1 and Synthesis 2. Elevated concentrations of arsenic also occur in natural newberyite transformed from struvite in guano deposits and record the accumulation of this metalloid in the food chain. Therefore, newberyite, which sequesters As during crystallization and retains this metalloid during the transformation from struvite, can attenuate arsenic contamination from green fertilizers in moderately acidic soils. Also, the capacity for accommodating both As(5+) and As(3+) in the crystal lattice coupled with simple chemistry and easy crystallization at ambient conditions makes newberyite an attractive material for remediation of arsenic contamination in aqueous environments.

Zinc interaction with struvite during and after mineral formation

Sorption of Zn with struvite was assessed both during and after mineral formation at pH 9.0 for 1-100 μM (0.065-6.54 mg L(-1)) aqueous Zn. The Zn loadings of recovered solids were lower when Zn was present during struvite precipitation compared to when Zn was added to struvite-bearing solutions. X-ray absorption fine structure spectroscopy confirmed that Zn added to struvite-bearing solutions at concentrations≤5 μM sorbed as both octahedral and tetrahedral complexes (Zn-O 1.98-2.03 Å), with evidence for bidentate configuration (Zn-P 3.18 Å). Bidentate complexes were incorporated into the near-surface structure, contributing to distortion of the struvite ν3 PO4(3-) band in the Fourier transform infrared spectra. At Zn concentrations>5 μM, tetrahedral monodentate adsorbates (Zn-O 1.98 Å) dominated, transitioning to a Zn-phosphate precipitate at 100 μM. When Zn is present during struvite precipitation, octahedral monodentate sorbates detected at 1 μM (Zn-O 2.08-2.10 Å; Zn-P 3.60-3.64 Å) polymerized at 5-50 μM, ultimately forming a Zn-hydroxide precipitate at 100 μM. The lowest initial Zn concentrations (0.065 mg L(-1)) and resultant solid loadings from precipitation experiments (13 mg kg(-1)) are consistent with those reported for struvite recovered from wastewater, suggesting that similar Zn sorption processes may occur in more complex systems.

Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents

Land spreading of digestates causes the discharge of large quantities of nutrients into the environment, which contributes to eutrophication and depletion of dissolved oxygen in water bodies. For the removal of ammonia nitrogen, there is increasing interest in the chemical precipitation of struvite, which is a mineral that can be reused as a slow-release fertilizer. However, this process is an expensive treatment of digestate because large amounts of magnesium and phosphorus reagents are required. In this paper, a struvite precipitation-based process is proposed for an efficient recovery of digestate nutrients using low-cost reagents. In particular, seawater bittern, a by-product of marine salt manufacturing and bone meal, a by-product of the thermal treatment of meat waste, have been used as low-cost sources of magnesium and phosphorus, respectively. Once the operating conditions are defined, the process enables the removal of more than 90% ammonia load, the almost complete recovery of magnesium and phosphorus and the production of a potentially valuable precipitate containing struvite crystals.

Optimization of struvite fertilizer formation from baker's yeast wastewater: growth and nutrition of maize and tomato plants

Struvite precipitate obtained from yeast industry anaerobic effluent with high ammonium nitrogen (NH4-N) was investigated for fertilizer effect on plant growth and nutrition according to applications of N, nitrogen/phosphorus/potassium (NPK), and control. Optimum struvite formation conditions were determined via Box-Behnken design. Optimum condition was obtained at pH 9.0 and Mg/N/P molar ratio of 1.5:1:1. Under these conditions, heavy metal concentrations in the obtained struvite precipitate (except Cu) were below the detection limits. In addition to high N, P, and Mg content, energy-dispersive X-ray (EDX) analysis showed that the struvite also included the nutritional elements Ca, K, Na, and Fe. X-ray diffraction (XRD) analysis revealed the complex structures of NaAl(SO4)2(H2O)12, NaMn(2+)Fe2(PO4)3, and (Na2,Ca)O2(Fe,Mn)O.P2O5 in the precipitate. High Na(+) and Ca(2+) concentrations in the anaerobic effluent reacted with phosphate during struvite precipitation. Different applications and struvite dosages significantly affected fresh and dry weights and nutrient element uptakes by plants (P < 0.05). N, P, and Mg uptakes of plants were significantly higher at struvite ×2, ×3, and ×4 dosages compared with NPK application. For adequate nutrition and supply of optimum dry weight, struvite ×2 dosage (5.71 g struvite/kg soil) was found appropriate for both maize and tomato plants.

Arsenic incorporation in synthetic struvite (NH4MgPO4·6H2O): a synchrotron XAS and single-crystal EPR study

Struvite, a common biomineral and increasingly important fertilizer recovered from wastewater treatment plants, is capable of sequestering a wide range of heavy metals and metalloids, including arsenic. Inductively coupled plasma mass spectrometric (ICPMS) analyses and microbeam synchrotron X-ray fluororescence (μ-SXRF) mapping show that struvite formed under ambient conditions contains up to 547 ± 15 ppm As and that the uptake of As is controlled by pH. Synchrotron As K-edge XANES spectra measured at 20 K show that As(5+) is the predominant oxidation state in struvite, irrespective of Na2HAsO4·7H2O or NaAsO2 as the source for As. Modeling of As K-edge EXAFS data suggest that local structural distortion associated with the substitution of As(5+) for P(5+) in struvite reaches up to 3.75 Å. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated struvite disclose five [AsO3](2-) radicals and one [AsO4](2-) radical. These arsenic-centered oxyradicals are all readily attributed to form from diamagnetic [AsO4](3-) precursors during irradiation, providing further support for exclusive incorporation and local structural expansion beyond the first shell of As(5+) at the P site in struvite.

Strategies for the recovery of nutrients and metals from anaerobically digested dairy farm sludge using cross-flow microfiltration

This work reports on the recovery of nutrients and metals from anaerobically digested manure sludge using a pilot scale microfiltration membrane system. Soluble nitrogen (N), phosphorous (P) and metals are valuable commodities which exist in high concentration in anaerobically digested manure sludge. The typical disposal of sludge on farmland can cause serious harm to the ecosystem due to eutrophication. The recovery of these materials in clarified solutions represents an added value product and a less contaminated sludge that is environmentally less hazardous. The objective of this study was to investigate the recovery of nutrients and metals using a pilot scale cross-flow membrane filtration system. A ceramic membrane of 0.22 m(2) and 0.2 μm pore size was used to perform solid-liquid separations and soluble materials were recovered in particle and bacteria free solutions. Strategies such as batch diafiltration (DF) and acid pre-treatment were investigated and the fractions collected compared against the initial permeate containing 686.2 mg NH3-N L(-1) and 41.51 mg PO4-P L(-1). Clarified fractions obtained through DF with no acid pre-treatment yielded N:P ratios of around 30 and relatively low levels of P (364.24 mg NH3-N L(-1) and 25.60 mg PO4-P L(-1)) and metals. Acid pre-treatment of the sludge resulted in a two-fold increase of P extracted (271.11 mg NH3-N L(-1) and 71.60 mg PO4-P L(-1)), altering N:P ratios to 8. Depending on the metal species, a 2-9 fold increase in concentration was also observed. Thus it has been demonstrated that different treatment strategies influence the removal and recovery of nutrients and metals from sludge. The best treatment conditions therefore depend on the targeted materials to be recovered. By careful manipulation of the treatment processes the production of specific nutrient compositions in terms of N:P ratios is possible.

Nitrogen and phosphorus recovery from anaerobic co-digestion residues of poultry manure and maize silage via struvite precipitation

Anaerobic digestion is commonly used for the stabilization of agricultural and animal wastes. However, owing to the stringent environmental criteria, anaerobic digester effluents need to be further treated to reduce nutrient loads to the receiving water bodies. Struvite precipitation is one of the promising techniques applied for this purpose. Yet, in the majority of cases, struvite precipitation is only applied to the liquid phase of anaerobic digester effluents. This study investigated the recovery of nutrients from both the liquid and the solid phases of the phase-separated effluent of a full-scale biogas plant co-digesting poultry manure and maize silage. Struvite precipitation in the liquid phase led to 72.1% and 95.1% average removal efficiencies of ammonium-nitrogen (NH4-N) and orthophosphate respectively. Changing the external phosphorus source did not make any statistically significant difference in nutrient removal. An acidic phosphorus-dissolution process was applied to the solid phase sample to obtain a phosphorus-enriched solution. More than 90.0% of both NH4-N and PO4-P were recovered from the phosphorus-enriched solution with the amendments of magnesium and phosphorus. In the experiments performed without any addition of external magnesium- and phosphorus-containing chemicals, almost complete (99.6%) PO4-P recovery and partial (14.6%) NH4-N recovery were obtained. The results of this study could contribute to the understanding of nutrient recovery from anaerobic digestion residues of manure and agricultural wastes by struvite precipitation.

Nucleation and growth kinetics of struvite crystallization

Struvite crystallization technology is being widely applied in full-scale due to a surge in nutrient demand and phosphate price increases. Past investigations on struvite crystallization focused on process efficiency and thermodynamics, and less on kinetics, while mainly using fluidized bed type crystallizer. In this work, nucleation and growth kinetic data were measured using stirred vessel. The primary and secondary nucleation was measured in synthetic wastewater, and crystal growth in digested supernatant. The measured kinetic data was correlated with solution supersaturation. The dependence of growth rate on relative supersaturation in the digested was higher compared to synthetic wastewater. The crystal polymorph in synthetic wastewater and real wastewater was comparable. Products from the growth studies showed narrow size distribution and favorable separation characteristics. The secondary nucleation rate showed second order dependence on relative supersaturation. The nucleation induction time decreased with an increase in supersaturation ratio with a clear distinction between homogenous and heterogeneous primary nucleation.

Optimization of struvite precipitation in synthetic biologically treated swine wastewater--determination of the optimal process parameters

A sustainable way to recover phosphorus (P) in swine wastewater involves a preliminary step of P dissolution followed by the separation of particulate organic matter. The next two steps are firstly the precipitation of struvite crystals done by adding a crystallization reagent (magnesia) and secondly the filtration of the crystals. A design of experiments with five process parameters was set up to optimize the size of the struvite crystals in a synthetic swine wastewater. More than 90% of P was recovered as large crystals of struvite in optimal conditions which were: low Mg:Ca ratio (2.25:1), the leading parameter, high N:P ratio (3:1), moderate stirring rate (between 45 and 90 rpm) and low temperature (below 20 °C).These results were obtained despite the presence of a large amount of calcium and using a cheap reactant (MgO). The composition of the precipitates was identified by Raman analysis and solid dissolution. Results showed that amorphous calcium phosphate (ACP) co-precipitated with struvite and that carbonates were incorporated with solid fractions.

Struvite-based fertilizer and its physical and chemical properties

This study describes a method to formulate struvite fine powder into pellets that are easy to spread on agricultural land. To evaluate the quality of produced pellets, some chemical and physical properties commonly measured for fertilizers were tested. The findings indicated that the salt index and heavy metal content ofstruvite pellets were significantly lower than those of commercial NPK fertilizers. In addition, the percentage of nutrient released from struvite pellets after 105 days was in the range of 9.6-23.2, 8.4-26.7 and 11.3-32.6% for nitrogen, phosphorous and magnesium, respectively, which is considerably lower than that of commercial NPK fertilizer. Among different formulations between struvite crystals and binders, starch and bentonite were the most efficient in agglomerating struvite powder, leading to an increase in the crush strength to over the recommended limit of >2.5 kgf for fertilizer hardness.

Sorption of chromium with struvite during phosphorus recovery

Struvite (MgNH(4)PO(4)·6H(2)O; MAP) precipitation is a viable means of phosphorus (P) recovery from animal and human wastes. The behavior of metal contaminants such as chromium (Cr) during struvite precipitation, however, requires consideration. Here the influence of both Cr concentration and oxidation state on sorption is assessed. The Cr content of struvite precipitated in the presence of 1-100 μM Cr as Cr(III) (22.3-3030.1 mg/kg) was higher than that of solids from Cr(VI) (4.5-5.1 mg/kg) solutions. For 1-20 μM Cr(III) solids, scanning electron microscopy (SEM) revealed etch pit formation on struvite crystal surfaces, indicative of a surface interaction. The formation of an adsorbate was confirmed by X-ray absorption fine structure spectroscopy (XAFS). At initial concentrations ≥20 μM Cr(III), XAFS confirmed the formation of a Cr(OH)(3)·nH(2)O(am) precipitate. Fourier transform infrared (FT-IR) spectroscopy revealed that both Cr(III) and Cr(VI) sorption resulted in distortion of the PO(4)(3-) tetrahedra in the mineral structure. This, combined with SEM results revealed that even at low sorbed concentrations, the Cr impurity can affect the mineral surface and structure. Thus, the initial Cr concentration and oxidation state in wastes targeted for P recovery will dictate the final Cr content, the mechanism of sorption, and impact on the struvite structure.

Influence of pH and oxidation state on the interaction of arsenic with struvite during mineral formation

Struvite (MgNH(4)PO(4)·6H(2)O) precipitated from animal and human wastes may be a sustainable source of fertilizer. However, arsenic, present in some wastes, may be removed with struvite. Here the sorption of As with struvite during mineral formation at pH 8-11 was assessed. The yield of struvite increased with pH, and was highest at pH 10. For recovered struvite, XRD indicated reduced crystallinity and particle size, and FT-IR suggested less distortion of phosphate tetrahedra with increased pH. The As impurity did not affect the crystallinity or particle size, but did contribute to phosphate distortion. Sorption of As(V) was observed at all pH values, and was highest at pH 10. As(III) sorption was consistently lower than that of As(V), but increased with pH. XAFS suggested coprecipitation of As(V), and adsorption of As(III) as the potential sorption mechanisms. Solids derived from As(III) solutions exhibited dual mechanisms due to the partial oxidation of As(III) to As(V) in solution prior to sorption. For struvite recovery in the presence of As, optimizing the pH to improve yields may increase the As content. Adsorbed As(III) could be removed prior to fertilizer application, however coprecipitated As(V) will release upon mineral decomposition, linking its cycling to that of phosphorus.