Macroscopic and microscopic variation in recovered magnesium phosphate materials: Implications for phosphorus removal processes and product re-use

Phosphorus (P) recovery and re-use will become increasingly important for water quality protection and sustainable nutrient cycling as environmental regulations become stricter and global P reserves decline. The objective of this study was to examine and characterize several magnesium phosphates recovered from actual wastewater under field conditions. Three types of particles were examined including crystalline magnesium ammonium phosphate hexahydrate (struvite) recovered from dairy wastewater, crystalline magnesium ammonium phosphate hydrate (dittmarite) recovered from a food processing facility, and a heterogeneous product also recovered from dairy wastewater. The particles were analyzed using “wet” chemical techniques, powder X-ray diffraction (XRD), and scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (SEM–EDS). The struvite crystals had regular and consistent shape, size, and structure, and SEM–EDS analysis clearly showed the struvite crystals as a surface precipitate on calcium phosphate seed material. In contrast, the dittmarite crystals showed no evidence of seed material, and were not regular in size or shape. The XRD analysis identified no crystalline magnesium phosphates in the heterogeneous product and indicated the presence of sand particles. However, magnesium phosphate precipitates on calcium phosphate seed material were observed in this product under SEM–EDS examination. These substantial variations in the macroscopic and microscopic characteristics of magnesium phosphates recovered under field conditions could affect their potential for beneficial re-use and underscore the need to develop recovery processes that result in a uniform, consistent product.     

Removal of nutrients from piggery wastewater using struvite precipitation and pyrogenation technology

In this paper, removal of nutrients from piggery wastewater by struvite crystallization was conducted using a combined technology of low-cost magnesium source in struvite precipitation and recycling of the struvite pyrolysate in the process. In the present research, it was found that high concentrations of K⁺ and Ca²⁺ present in the solution significantly affected the removal of nutrients. When the struvite crystallization formed at the condition of dosing the magnesite pyrolysate at a Mg:N:P molar ratio of 2.5:1:1, and having a reaction time of 6 h, a majority of nutrients in piggery wastewater can be removed. Surface characterization analysis demonstrated that the main components of the pyrolysate of the obtained struvite were amorphous magnesium sodium phosphate (MgNaPO₄) and MgO. When the struvite pyrolysate was recycled in the process at the pH range of 8.0-8.5, the precipitation effect was optimum. When the struvite pyrolysate was recycled repeatedly at pH 8.5 or without any adjustment of pH, the outcome of the removal of the nutrients in both cases was similar. With the increase in the number of recycle times, the performance of struvite precipitation progressively decreased. An economic evaluation showed that the combination of using low-cost material and recycling of struvite was feasible. Recycling struvite for three process cycles could save the chemical costs by 81% compared to the use of pure chemicals.     

Enhancing phosphorus recovery by a new internal recycle seeding MAP reactor

Phosphorus is a depleting resource that needs recovery from wastewater streams. The magnesium ammonium phosphate (MAP) crystallization process could simultaneously recover ammonium nitrogen and phosphorus at equal molar basis to yield slow-release MAP fertilizer. However, the present MAP processes are not efficient in recovering phosphorus at low P concentrations. This work presented and tested the performance of a newly proposed MAP reactor, the internal recycle seeding reactor (IRSR) that comprised of a reaction zone and a settling zone connecting with an internal recirculation loop. Owing to the enhanced secondary nucleation rates of MAP crystals in reaction zone under controlled circumstance, the proposed IRSR recovered 78% of phosphorus from wastewater at a low level of 21.7 mg-PL(-1). The optimal operation parameters for the IRSR were investigated with synthetic wastewater and determined as that the Mg/PO(4)(3-)-P molar ratio was 1.3-1.5:1, THRT was up to or longer than 1.14 h, the seed concentration of reaction zone was 0.40-1.0 gL(-1). Further needs for the proposed IRSR strategies were also discussed.     

Removal and recovery of phosphorous from swine wastewater by demonstration crystallization reactor and struvite accumulation device

A demonstration crystallization reactor and struvite accumulation device for the removal and recovery of phosphorous was constructed and their performance was evaluated using actual swine wastewater for 3.5 years. The wastewater pH was increased by aeration, and the concentrations of total P and soluble PO(4)-P were reduced by a struvite crystallization reaction induced under a high pH condition. A 30% MgCl(2) addition was effective in enhancing the struvite crystallization reaction. The concentrations of suspended solids, total Zn and total Cu, were also decreased by the settling function of the reactor. On removing the efficiencies of these components, no noticeable seasonal fluctuation in performance was observed during the 3.5-year operation. In terms of maximum yield, 171g struvite was obtained from 1m(3) swine wastewater by the demonstration accumulation device for struvite recovery. The recovered struvite needed only air-drying before use since it was approximately 95% pure even without washing.     

An economic evaluation of phosphorus recovery as struvite from digester supernatant

Phosphorus can be recovered from wastewater through crystallisation of struvite, MgNH(4)PO(4).6H(2)O. Approximately 1 kg of struvite can be crystallised from 100 m(3) of wastewater. Crystallisation is profitable compared to chemical and biological removal of phosphorus due to savings from the reduction in (i) chemicals used for precipitation and sludge disposal; and (ii) downtime for cleaning unwanted struvite formed during chemical and biological removal. The struvite produced annually from a wastewater treatment plant that processed 100 m3/d, would be sufficient to apply on 2.6 ha of arable land, as fertilizer. If struvite were to be recovered from wastewater treatment plants worldwide, 0.63 million tons of phosphorus (as P(2)O(5)) could be harvested annually, reducing phosphate rock mining by 1.6%. Therefore, this technology could provide opportunities to recover phosphorus sustainably from waste streams and preserve phosphorus reserves.     

Struvite precipitation in anaerobic swine lagoon liquid: effect of pH and Mg:P ratio and determination of rate constant

Because of increased concern about surface water eutrophication from nutrient-enriched agricultural runoff, many swine producers are encouraged to decrease application rates of waste-based P. Precipitation and subsequent removal of magnesium ammonium phosphate (MgNH(4)PO(4) x 6H(2)O), commonly known as struvite, is a promising mechanism for N and P removal from anaerobic swine lagoon effluent. The objectives of this research were to (i) quantify the effects of adjusting pH and Mg:P ratio on struvite precipitation and (ii) determine the rate constant pH effect for struvite precipitation in anaerobic swine lagoon liquid. Concentrations of PO(4)-P in liquid from two anaerobic swine lagoons were determined after 24 h of equilibration for a pH range of 7.5-9.5 and Mg:P ratios between 1:1 and 1.6:1. Struvite formation reduced the PO(4)-P concentration in the effluents to as low as 2 mgl(-1). Minimum concentrations of PO(4)-P occurred between pH 8.9 and 9.25 at all Mg:P ratios. Struvite precipitation decreased PO(4)-P concentrations by 85% within 20 min at pH 9.0 for an initial Mg:P ratio of 1.2:1. The rate of PO(4)-P decrease was described by a first-order kinetic model, with rate constants of 3.7, 7.9, and 12.3 h(-1) at pH 8.4, 8.7 and 9.0 respectively. Our results indicate that induced struvite formation is a technically feasible method to remove N and P from swine lagoon liquid and it may allow swine producers to recover nutrients for off-farm sale.     

Partial nitrifying granule stimulated by struvite carrier in treating pharmaceutical wastewater

Aerobic granule was successfully cultivated in SBR (sequencing batch reactor) by struvite carrier (magnesium ammonium phosphate, MgNH₄PO₄), which can increase polysaccharides to 42.2 mg/gMLVSS (mixed liquor volatile suspended solid) versus only 28.4 mg/gMLVSS of the sludge without it. Meanwhile, it was found that struvite play a positive role in initial granulation and bacterial group distribution in treating pharmaceutical wastewater, involving effect of solid surface and special contents of struvite. The results of fluorescence in situ hybridization technique indicate that ammonia-oxidizing bacteria can dominate over nitrite-oxidizing bacteria in mature granules. COD removal efficiency of 90 % and NO₂ ^?–N:(NO₂ ^?–N?+?NO₃ ^?–N) accumulation efficiency of 89 % were achieved in stable state. Emphasis is placed on that struvite addition can be applied as a new-type carrier to promote formation of partial nitrification granular sludge.     

Isolation and metagenomic characterization of bacteria associated with calcium carbonate and struvite precipitation in a pure moving bed biofilm reactor-membrane bioreactor

A bench-scale pure moving bed bioreactor-membrane bioreactor (MBBR-MBR) used for the treatment of urban wastewater was analyzed for the identification of bacterial strains with the potential capacity for calcium carbonate and struvite biomineral formation. Isolation of mineral-forming strains on calcium carbonate and struvite media revealed six major colonies with a carbonate or struvite precipitation capacity in the biofouling on the membrane surface and showed that heterotrophic bacteria with the ability to precipitate calcium carbonate and struvite constituted ~7.5% of the total platable bacteria. These belonged to the genera Lysinibacillus, Trichococcus, Comamomas and Bacillus. Pyrosequencing analysis of the microbial communities in the suspended cells and membrane biofouling showed a high degree of similarity in all the samples collected with respect to bacterial assemblage. The study of operational taxonomic units (OTUs) identified through pyrosequencing suggested that ~21% of the total bacterial community identified in the biofouling could potentially form calcium carbonate or struvite crystals in the pure MBBR-MBR system used for the treatment of urban wastewater.     

Growth inhibition of struvite crystals by the aqueous root extract of Rotula aquatica

Formation of urinary stone is a serious and debilitating problem throughout the world. In the present study, the inhibitory effect of aqueous extract of root of Rotula aquatica was investigated against struvite crystals (one of the components of urinary stone) grown in vitro using single diffusion gel growth technique. For setting the gel, sodium metasilicate solution (specific gravity 1.05) and 0.5 M aqueous solution of ammonium dihydrogen phosphate were mixed, so that the pH of the mixture could be set at 7.0. Equal amounts of supernatant solution of magnesium acetate (1.0 M) prepared with 0.0%, 0.5% and 1% concentrations of the extract were gently poured on the set gels. It was observed that the number, dimension, total mass, total volume, growth rate and depth of growth of struvite crystals decreased with the increasing extract concentrations in the supernatant solutions. The enhancement of dissolution rate and fragmentation of struvite crystals suggested potential application of the extract for inhibition of struvite type urinary stone.     

Removal of ammonia as struvite from anaerobic digester effluents and recycling of magnesium and phosphate

A second order kinetic model was developed to predict the rate and extent of NH(4)(+) removal as struvite from anaerobic digester effluents. Alternative to this, NH(4)(+) can be recovered from struvite and the remaining Mg(2+) and PO(4)(3-) can be recycled back to the wastewater to fix more NH(4)(+). The NH(4)(+) solution was retained and the remaining Mg(2+) and PO(4)(3-) were returned back to be mixed with wastewater. In a five-step process, NH(4)(+) recovery was initially 92% and progressively decreased to 77% in the fifth stage, due to loss of Mg(2+) and PO(4)(3-) at each step in the supernatant. Finally, economic analysis of recycling nutrients was performed and compared to the one step process. The cost of NH(4)(+) recovery was calculated as $0.36/kgNH(4)-N which is lower than $7.7/kgNH(4)-N the cost of one step process without considering the market value of struvite obtained in one step process.     

Effect of mixing on spontaneous struvite precipitation from semiconductor wastewater

The objective of this study was to investigate on the effect of mixing intensity (G) and mixing duration (t(d)) on struvite precipitation in the chemical mechanical polishing (CMP) wastewater generated from the semiconductor manufacturing process. Batch-scale experiments revealed that struvite crystallization was affected by both G and t(d). The mixing effect was to enhance the mass transfer of solute to the crystals in the process, resulting in the improvement of struvite crystallization and growth. By forming struvite, removal efficiencies of N and P increased logarithmic with the multiple values of G and t(d), i.e., Gt(d). Insufficient mixing energy with the Gt(d) value less than 10(5) caused an increase in the formation potential of unexpected precipitate unlike to pure struvite, causing a decrease in removal efficiencies of N and P in the process. At the Gt(d) value over 10(6), struvite precipitation was not restricted by fluoride, of which high level inherently contained in the CMP wastewater. The study results can be taken into consideration in the design and operation of the struvite precipitation process for both nutrient (N and P) removal and recovery.     

Recovery of phosphorous from swine wastewater through crystallization

All the phosphate rock Japan needs must be presently imported from abroad because the country has no subterranean phosphorous resources. Therefore, there is a need to accelerate the development of and establish the technologies for phosphorous recovery from waste and wastewater. Swine wastewater has a high potential for phosphorous recovery in Japan. A reactor for removing and recovering phosphorous from swine wastewater was designed with dual functions, crystallization through aeration and separation of formed struvite by settling. However, a dehydration, composting and characterization process was first needed before using sediment sludge, including struvite, on farmland, since the struvite will settle along with huge amounts of other suspended solids (organic matter). For the recovery of pure struvite, an accumulation device was designed and its efficiency examined. The device has a struvite-accumulation face made of stainless steel wire mesh (1 mm in diameter, 1 cm(2) square) to reduce its total weight. During submergence in the aeration column of the demonstration reactor, struvite cross-bridged and accumulated on the face of the device. The struvite could be scraped off easily with only a light brushing, and was found to be approximately 95% pure. Because this device is a very simple structure, it is thought to be acceptable to swine farmers.     

Isolation of Leclercia adcarboxglata Strain JLS1 from Dolostone Sample and Characterization of its Induced Struvite Minerals

The biomineralization of struvites is becoming a very popular research subject. In this study, struvite crystals were precipitated and characterized in the presence of the bacterial Leclercia adcarboxglata strain JLS1 that was isolated from a typical dolostone sample. The JLS1 strain was identified as L. adcarboxglata using physiological and biochemical identification methods and by 16S rDNA phylogenetic analysis. The minerals obtained from the culture medium using experimental samples inoculated with L. adcarboxglata JLS1 were not found in the control samples without the bacteria. The experimentally derived minerals were determined to be pure struvites that exhibited primarily a uniform, prismatic-shaped morphology of various sizes. The results of the X-Ray Powder Diffraction (XRD) analysis of the minerals revealed that the preferred orientation and lattice distortion were characteristic of struvite crystals. In the experiments conducted using the L. adcarboxglata it was also found that the phosphate, ammonium and magnesium ions in solution were consumed to form the struvites in an alkaline environment. The results of this study can provide important insights into the understanding of the biomineralization process of microbial struvites and wastewater treatment applications, as well as the genesis and therapy of kidney stones.     

Factors affecting struvite (MgNH4PO4.6H2O) crystallization in feline urine

Factors affecting struvite, a magnesium-ammonium-phosphate complex (MgNH(4)PO(4).6H(2)O), in feline urine were evaluated. Incubation of just "urine mineral (UM)" solution, in which mineral concentrations are compatible with those in feline urine, for 4 h at 37 degrees C did not induce the formation of crystals. Similarly, incubation of urine alone did not produce crystals. However, struvite crystals were formed by the addition of urine to UM solution. Mg, NH(3) and P were all required for urine-induced struvite crystallization. The lower molecular weight (LMW) fraction of urine was essential for struvite crystal formation, and the higher molecular weight (HMW) fraction enhanced formation of LMW-induced struvite crystals. The effects of urine proteins further fractionated by column chromatography were examined. A protein at >250 kDa and cauxin, a major urine protein recently identified as a regulator of felinine production, potentiated struvite crystal formation induced by the LMW fraction. In contrast, Tamm-Horsfall glycoprotein, a urine protein thought to promote struvite crystallization, did not have this activity. The present study reveals a novel mechanism of feline struvite crystallization.     

Sewage sludge management for phosphorus recovery as struvite in EBPR wastewater treatment plants

The influence of separate and mixed thickening of primary and secondary sludge on struvite recovery was studied. Phosphorus precipitation in the digester was reduced from 13.7 g of phosphorus per kg of treated sludge in the separate thickening experiment to 5.9 in the mixed thickening experiment. This lessening of the uncontrolled precipitation means a reduction of the operational problems and enhances the phosphorus availability for its later crystallization. High phosphorus precipitation and recovery efficiencies were achieved in both crystallization experiments. However, mixed thickening configuration showed a lower percentage of phosphorus precipitated as struvite due to the presence of high calcium concentrations. In spite of this low percentage, the global phosphorus mass balance showed that mixed thickening experiment produces a higher phosphorus recovery as struvite per kg of treated sludge (i.e., 3.6 gP/kg sludge vs. 2.5 gP/kg sludge in separate thickening).     

Physical, X-ray diffraction and scanning electron microscopic studies of uroliths

Identification of chemical constituents of calculus is important in the diagnosis and management of urolithiasis. The compositional variability of uroliths has different etiologies and requires various modes of treatment and prophylaxis. In the present study, we report the chemical compositional analyses of calculi recovered from buck and bullock by X-ray diffraction (XRD) and energy dispersive X-ray (EDX) techniques and ultra-structure examination by scanning electron microscopy (SEM). XRD and EDX investigations conclusively established the chemical compositions of urinary calculi under investigation. The calculus from buck (sample I) had calcium oxalate monohydrate, a dominant salt phase and magnesium compound in significant amount. The calculus from bullock (sample II) had magnesium ammonium phosphate phase, with significant amount of calcium in apatite form and K+ ions. SEM study at higher magnification (X 1000) showed bipyramidal crystals in external zones of urolith (sample I). The struvite apatite calculus showed that basic unit of structure was lamination and the laminitis appeared to be made up of fine granules and high porosity. The bio-mineralization process of calculus formation was also studied, with a view to take preventive and therapeutic measures for amelioration of urinary stone diseases in animals and humans.     

Biotransformation of struvite by Aspergillus niger: phosphate release and magnesium biomineralization as glushinskite

Struvite (magnesium ammonium phosphate-MgNH₄PO₄·6H₂O), which can extensively crystallize in wastewater treatments, is a potential source of N and P as fertilizer, as well as a means of P conservation. However, little is known of microbial interactions with struvite which would result in element release. In this work, the geoactive fungus Aspergillus niger was investigated for struvite transformation on solid and in liquid media. Aspergillus niger was capable of solubilizing natural (fragments and powder) and synthetic struvite when incorporated into solid medium, with accompanying acidification of the media, and extensive precipitation of magnesium oxalate dihydrate (glushinskite, Mg(C₂O₄).2H₂O) occurring under growing colonies. In liquid media, A. niger was able to solubilize natural and synthetic struvite releasing mobile phosphate (PO₄³⁻) and magnesium (Mg²⁺), the latter reacting with excreted oxalate resulting in precipitation of magnesium oxalate dihydrate which also accumulated within the mycelial pellets. Struvite was also found to influence the morphology of A. niger mycelial pellets. These findings contribute further understanding of struvite solubilization, element release and secondary oxalate formation, relevant to the biogeochemical cycling of phosphate minerals, and further directions utilizing these mechanisms in environmental biotechnologies such as element biorecovery and biofertilizer applications.     

Struvite Precipitation Induced by a Novel Sulfate-Reducing Bacterium Acinetobacter calcoaceticus SRB4 Isolated from River Sediment

This article presents a study of struvite formation in liquid medium induced by the sulfate-reducing bacterium Acinetobacter calcoaceticus SRB4, a strain isolated from river sediment. We identified the bacterial strain A. calcoaceticus SRB4 and analyzed its micromorphology. The minerals formed were studied with an electroprobe microanalyzer, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray diffraction, thermogravimetry, differential thermogravimetry, and differential scanning calorimetry. Acinetobacter calcoaceticus SRB4 was found to induce struvite precipitation, whereas sterile control cultures did not. Many transparent stick-shaped struvite precipitates were distributed at the bottom of the conical flasks in the experimental group. Most bacteria were spherical and a large quantity of spherical struvite particles (less than 200 nm in diameter) adhered to the bacterial surface. An electron probe microanalysis showed that the precipitates contained C, O, P, Mg, and other elements. Fourier transformation infrared spectra showed that the precipitates contained crystalline water, NH₄⁺, and PO₄^3? groups. X-ray diffraction spectra showed that the precipitates were struvite crystals, with preferential orientation and lattice distortion. Thermogravimetry showed that the weight loss was caused by the evaporation of crystalline water at temperatures lower than 136°C and the release of ammonia from struvite at temperatures of 136–228.5°C. In this article, we discuss the possible mechanism of struvite formation and the possible role played by A. calcoaceticus SRB4. Our study extends our understanding of the phosphate biomineralization mechanism and should prove useful in recycling phosphorus in wastewater.     

Autotrophic denitrification and chemical phosphate removal of agro-industrial wastewater by filtration with granular medium

A novel granular medium consisting (1.5-5 mm in diameter) of inert perlite particles as nuclei and an effective surface layer containing sulfur, CaCO3 and Mg(OH)2 was developed for advanced treatment of agro-industrial wastewater. The performance of the medium was examined with a laboratory-scale down-flow fixed-bed column reactor using piggery wastewater, which had been treated by an upflow anaerobic sludge blanket reactor and a trickling filter. The removal efficiency of NOx- -N was more than 70% with a NOx- -N loading rate of less than approximately 0.3 kg Nm(-3) d(-1); the removal efficiency dropped due to the accumulation of nitrite when the loading rate exceeded that value. A significant drop of phosphate and Mg2+ concentrations occurred when the effluent pH exceeded 7.9. Ammonium was removed with an average removal efficiency of 12.4%. These results indicated that the crystalline reaction of PO4(3-), Mg2+ and NH4+ (MAP reaction) under alkaline conditions contributed to the removal of phosphate. This medium could be useful for the simultaneous reduction of nitrogenous and phosphorus compounds in biologically treated agro-industrial wastewater.     

Correlating on-line monitoring parameters, pH, DO and ORP with nutrient removal in an intermittent cyclic process bioreactor system

The paper presents the study correlating the profile of on-line monitoring parameters and nutrient removal in an intermittent cyclic process bioreactor (ICPBR) system, thereby utilizing the parameters as operational tool. A laboratory scale ICPBR was employed to treat low C/N ratio domestic wastewater from a township. The study was conducted for correlating biological nutrient removal and on-line monitored parameters pH, dissolved oxygen (DO) and oxidation-reduction potential (ORP). The results revealed that pH, DO and ORP related with the dynamic behavior of nutrient concentration (NH4-N, NO3-N, and PO4-P) during treatment in an ICPBR system. The variation in pH and ORP of the reactor liquor correlate to conversion of ammonia (NH4-N) and nitrate (NO3-N) concentrations, respectively. As the bioconversion of ammonia nitrogen and phosphorus are related to the varying profile of the on-line monitored parameters, the profiles could possibly be used as onsite process control parameters.