Struvite Crystal Formation Research Articles

Interplay of digester supernatant composition and operating pH on impacting the struvite particulate properties

Although the reactive crystallization of struvite to recover phosphorus from digester supernatant in wastewater treatment plants has been extensively investigated in the past, the process frequently confronts challenges in terms of consistent product quality and optimal operating costs. Therefore, further investigation is necessary to better understand the complex interaction of digester supernatant composition and operating parameters. The present work has focused on investigating the interplay of the digester supernatant composition and the operating parameters on the particulate properties of the struvite product obtained from a complex wastewater system. Under different set of conditions, the obtained particulate properties and the relevant phosphorus removal/recovery efficiency were examined. It has been found that smaller struvite crystals were preferable in forming round granules that can facilitate the transportation as well as spreading to the field. High concentration of polymers present in the supernatant with low supersaturation level (pH=7.5, N/P=6.3) supported the formation of smaller struvite crystals with narrow size distribution, by inhibiting the crystal growth. But, this effect became less significant, as the supersaturation level was increased or the polymer concentration was decreased, which led to the formation of irregular X-shaped crystals (with dendritic surface). These X-shaped crystals cannot pack tightly despite their larger particle sizes. The P-removal/recovery rates were between 87.5-94% for all conditions investigated in this study. Considering the cost-efficiency and the desired crystal size for the formation of struvite granules, low pH 7.5 and low N/P molar ratio 6.3 were suggested for the proposed struvite crystallization process.

Applying a chemical equilibrium model for optimizing struvite precipitation for ammonium recovery from anaerobic digester effluent

Ammonium (NH4+) removal and recovery from wastewater by struvite precipitation is a sustainable approach, generating valuable fertilizer and reducing environmental impact. This study applied a chemical equilibrium model-Visual MINTEQ to optimise the systematic operation parameters for struvite precipitation from anaerobic digester effluent in a wastewater treatment plant. Designed systematic parameters were used as inputs into the model with different molar ratios of magnesium (Mg2+)/phosphate (PO43−) and pH control approach. The results show that the Mg2+/PO43− molar ratio can significantly affect the NH4+ recovery efficiency, and over 96% NH4+ recovery was obtained at an Mg2+/PO43− ratio of 2:1. The experimental results corresponded well with the model outputs in terms of ammonium recovery efficiency, struvite crystal formation and alkaline dosages; indicating that this model can be applied to predict precipitation process in order to enhance ammonium recovery efficiency and struvite formation. The economic benefit was briefly analyzed considering chemical costs. Our results indicate that ammonium removal and recovery from waste streams by struvite precipitation is a sustainable approach, generating valuable fertilizer and reducing nutrient contaminant loading in the eco-system.

A design of experiments to assess phosphorous removal and crystal properties in struvite precipitation of source separated urine using different Mg sources

Phosphorus recycling and recovery has received special attention due to its non-replaceable and non-renewability. Phosphorus recovery from human urine in the form of struvite crystals is a potential alternative source. In this work, the efficiency of struvite precipitation from source separated human urine and struvite crystals size were analyzed using three different Mg2+ sources (MgCl2, Mg(OH)2 and MgO), individually evaluated using a statistical design of experiments to assess the combined effect of Mg2+:P molar ratio (1:1, 1.5:1 and 2:1) and stirring speed (30, 45 and 60rpm). Formation of struvite crystals was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive spectrometer (EDS). Using the optimal conditions determined, MgO as Mg2+ source at 2:1 molar ratio and a stirring speed of 30rpm, 99% of P was recovered as struvite crystals with a size of 50–100μm.

Effects of calcium and ferric ions on struvite precipitation: A new assessment based on quantitative X-ray diffraction analysis

The precipitation of struvite (MgNH4PO4·6H2O) from waste streams has attracted considerable attention due to its potential for recovering phosphorus for fertilization. As struvite is primarily acquired by means of precipitation and crystallization from aqueous solutions, it is important to evaluate the roles of common metal ions, particularly those that are commonly found in wastewater, in the struvite crystallization process. This study was performed to quantitatively evaluate the effects of calcium and ferric ions on struvite crystallization using the Rietveld refinement method, which is based on the analysis of X-ray diffraction data. The results indicate that both calcium and ferric ions significantly inhibit the formation of struvite crystals, and the effects vary under different pH conditions. There was a negative linear correlation between the struvite weight content in the precipitates and the Ca/Mg molar ratio in the initial solution. However, ferric ions were confirmed to be a more efficient inhibitor of struvite crystallization. Ca2+ and Fe3+ further modified the needle-like struvite into irregular shapes. An unambiguous and quantitative understanding of the effects of foreign ions on struvite crystallization will help to reliably improve the quality of struvite products recovered from wastewater and the control of struvite deposits in water and sludge piping systems.

Evaluation of the hydration process of an industrial magnesia compound to obtain struvite crystals: a technique for recovering nutrients

Struvite crystallization as a technique for recovering nutrients from wastewater streams has proven to be an attractive alternative to conventional treatments. The process becomes economically viable with the cost reductions related to the chemical reagents, especially magnesium ions. Thus, this study assessed the processing of a cheap and high-grade industrial magnesia compound in obtaining magnesium hydroxide slurries through a hydration reaction using different hydrating agents (magnesium acetate and sodium chloride solutions and water). The optimum hydration condition was achieved in acetate solution with a conversion rate to hydroxide about 67%. The efficiency of the obtained slurries as alternative reagents on the struvite crystallization process was evaluated in terms of the nutrient removal, NH4+-N and PO43--P. In experimental conditions of Mg2+:NH4+:PO43-ratio equal to 1.5:1.0:1.25 and pH equal to 8.5, the best results were achieved by using the magnesium hydroxide slurry obtained in acetate solution, which promoted removals of about 62% and 67% of NH4+-N and PO43--P, respectively. Finally, instrumental analysis confirmed the formation of struvite crystals in a purity degree that allows its use as a potential fertilizer.

Morphology of struvite crystals as an evidence of bacteria mediated growth

Influence of Proteus mirabilis on the growth, morphology and size of struvite crystals in artificial urine has been studied. The growth in the presence of Proteus mirabilis shows formation of struvite crystals of more regular habits compared with that in the absence of bacteria. This implies that in the presence of Proteus mirabilis, struvite crystals grow with relative growth rates within smaller range of variability compared to the absence of bacteria. The results show that bacteria affect size and struvite morphology while the habit remains almost unchanged. It is suggested that the changes in struvite crystal morphology are induced by the presence of Proteus mirabilis. Microorganism's negatively charged residues interact electrostatically with positive ions which outcrop given surfaces and lead to an enhanced expression of these surfaces. This means that microorganisms actively mediate in growth processes. Therefore, the investigations provide evidence for the importance of biological regulation in crystallization process.

The influence of various physico-chemical process parameters on kinetics and growth mechanism of struvite crystallisation

In response to struvite formation problems in wastewater treatment plants (WWTP), this mechanistic growth kinetic study of struvite crystal (MgNH4PO4·6H2O) was undertaken to determine the cause of preferential accumulation and to provide the remediation information to the designer of WWTP. Struvite is the mineral phase of one of the urinary tract stones of human and animals also. Here the authors presented the struvite crystal growth formation kinetics mechanism under different physico-chemical process parameters such as supersaturation (Mg2+,NH4+,PO43-), solution pH, stirrer speed, temperature, impurities and seeding conductions. To measure the growth rate of struvite crystals and to identify its various dependence system parameters, laboratory measurements were conducted in an isothermal batch 1l stirred seeded crystalliser. Supersaturation and pH have been found to be the most influential parameters for struvite crystallisation. It was found that growth rate increased with an increase in solution pH in the pH range of 8–9. The rate of change of ortho-phosphate concentration in the bulk solution increases with increasing supersaturation ratio. The growth rate increased with increase in temperature. Higher NaCl salt concentration and higher speed also produced higher struvite growth kinetics. Fundamentally struvite crystal formation followed a classical diffusion–reaction mass transfer mechanism and overall mass transfer coefficients (KL) under various physico-chemical process parameters have been determined. The kinetic rate constants were also calculated by fitting a first-order kinetic model and power law model to the experimental data obtained. It was also found that there was increase in product crystal size with increase in solution pH, stirrer speed and amount of salts due to slow nucleation followed by fast growth process. Struvite crystallisation may also have implication in an alternative sustainable and economical recovery of phosphorous from concentrated waste water stream.

Phosphorus recovery potential from a waste stream with high organic and nutrient contents via struvite precipitation

Recovery of and via struvite precipitation (SP) was evaluated from liquor of thermally pretreated waste activated sludge, containing high levels of nutrients (1500 mg and 650 mg ), organics (45.5 g COD/L) and suspended solids (3.5 g TSS/L), with reference to anaerobically digested sludge centrate. In a series of jar tests, the order of pH adjustment and chemical addition were first tested for the digested sludge centrate. The effects of MgCl2 and MgO, as Mg2+ sources, on SP were evaluated in both waste streams. Up to 80% of the dissolved was recovered using MgO (pH=9.2) from the pretreated sludge liquor and more than 86% of from the digested sludge centrate (pH=8.0–8.5) regardless of the Mg2+ source used. recovery from digested sludge centrate required the addition of alkali, Mg2+ source and , making the process less viable. The precipitates contained mostly struvite and some levels of Ca2+, Fe2+ and other Mg2+ phosphates. The levels of solids, inorganics and organics in the waste streams influenced SP, specifically struvite crystal formation and settleability in the pretreated sludge liquor, which suggests that the applicability of SP for nutrient recovery from complex waste streams requires case-by-case testing, and process optimization.

Cystone, a well-known herbal formulation, inhibits struvite crystal growth formation in single diffusion gel growth technique

Objective: The present study was aimed to evaluate the beneficial effect of Cystone® against struvite crystal growth in in vitro conditions. Methods: Various concentrations of Cystone® was prepared in 1 M magnesium acetate solution and evaluated for crystal growth inhibition assay by a well-known method called single diffusion gel growth technique in vitro. Results: Cystone®, a well-known polyherbal formulation, at 0.5, 1 and 2% concentrations showed significant and dose-dependent inhibition of struvite crystal growth formation in in vitro by reducing number, total mass and total volume of the struvite crystals formed and also caused fragmentation of grown struvite crystals in the gel matrix. Conclusion: The results of the present study indicate, Cystone® significantly retards the formation of struvite stones and also brings about its fragmentation. This could be one of the probable mechanisms behind the beneficial effect offered by Cystone® in the clinical management of urolithiasis and urinary tract infections.

Nucleation and growth kinetics of struvite in a fluidized bed reactor

Kinetics of struvite crystallization were studied to gain a better understanding of intentional struvite formation in fluidized bed reactors. Mechanisms controlling nucleation were studied in the laboratory by induction time experiments. pH monitoring proved to be an effective method of induction time determination, during the induction period. The induction period, when nucleation was the controlling process for struvite crystal formation, was found to be primarily reaction controlled, with minor transport influence. The metastable region for struvite was explored in this study. The solubility and supersolubility curves, which are the boundaries of the metastable region, were observed to be almost parallel straight lines in the concentration range studied. The growth rate of struvite determined in a fluidized bed reactor was mainly transport controlled. With the determination of the mass-transfer coefficient and surface-reaction coefficient for a specified condition, a two-step linear growth rate model for struvite growth determination in a fluidized bed reactor has been proposed.

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

Factors affecting struvite, a magnesium–ammonium–phosphate complex (MgNH4PO46H2O), 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 °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, NH3 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.

A Struvite Control and Phosphorus Removal Process for Centrate: Full-Scale Testing

The North End Water Pollution Control Centre (NEWPCC) in Manitoba, Canada serves a population of approximately 380,000. The dry weather flow to the NEWPCC is approximately 175 ML/d (46 MGD). The plant uses anaerobic digestion and centrifuge dewatering in its solids treatment train. The centrate stream comprises approximately 20 percent of the plant’s phosphorus load. The plant suffers from severe struvite (magnesium ammonium phosphate) deposition: centrifuges and centrate piping have to be regularly cleaned. The Province requires that the plant control the phosphorus load recycled in the centrate: the total phosphorus (TP) load must not exceed 119 kg/d, as determined by a thirty day rolling average. In order to meet the requirements of the Province, and to control struvite, a metal salt will be dosed into the digested sludge, upstream of the centrifuges. Prior to design of a metal salt dosing system, a full-scale trial was initiated. Two potential metal salts were identified; ferric chloride (FeCl3) and aluminum sulphate or alum (Al2(SO4)3·14H2O). Examination of struvite crystals in the digested sludge indicated that nearly complete disappearance of crystals occurred at all dosing rates used in these trials. The trials demonstrated that alum and ferric chloride were highly effective in meeting the removal objectives and controlling struvite crystal formation in the centrate.

Optimum doses of Mg and P salts for precipitating ammonia into struvite crystals in aerobic composting

It was previously reported that struvite crystals could be formed in the aerobic composting reaction provided that Mg and P salts are added [Bioresource Technology 79 (2001) 129]. The formation of struvite crystals significantly reduced gaseous loss of ammonia and resulted in substantial increase in the ammonia content in the compost, attaining 1.5%. In this context, the present study was conducted to determine the optimal doses of Mg and P salts for struvite crystallization. It was found that cumulative ammonia production was about 33–36% of the initial total nitrogen in the aerobic composting reaction, irrespective of the amounts of Mg and P salts added. The theoretical doses for complete conversion of ammonia into struvite crystals seemed to be about 33–36% of the initial nitrogen. The addition of Mg and P salts at this level, however, caused adverse effects on the degradation of organic materials. Therefore, it was concluded that the optimal doses of Mg and P salts should be about 20% of the initial nitrogen in the compost mixture not to cause any harmful effects on the composting reaction.

Synthesis of struvite by ion exchange isothermal supersaturation technique

Recovery and recycling of phosphorus (P) from effluents and waste waters of different types is among the most important tasks of modern environmental chemistry. The importance of this task is determined by the rapid depletion of phosphate ore deposits, which are intensively exploited. The most pessimistic forecasts predict almost complete disappearance of high quality rock phosphates by the beginning of the next century. Phosphates are removed and recovered from the water under treatment as struvite (MgNH 4PO 4 · 6H 2O) or other low solubility compounds. The advantage of P recovery in the form of struvite lies in the possibility of using it as valuable mineral fertiliser. Ion exchange can be used both to recover phosphates from waste waters and to synthesize struvite of sufficiently high purity. This paper describes the results obtained by studying the conditions of struvite synthesis by using the ion exchange isothermal supersaturation (IXISS) technique. The IXISS phenomenon is observed in a number of ion exchange systems when using certain ion exchange separation techniques enabling the target substance to be concentrated beyond its solubility level at a given temperature. Moreover, this supersaturated solution remains stable for a long period, and after leaving the column it crystallizes spontaneously. Stable supersaturated struvite solutions were obtained by stripping magnesium from Lewatite CNP80 cation exchange resin in Mg-form by using ammonium phosphate and dihydrophosphate solutions under batch and dynamic conditions. The pH of struvite formation is around 8.0 at all eluate concentrations and temperatures studied. The results of X-ray diffraction analysis of the precipitates obtained confirm the formation of struvite crystals.

A new method for conservation of nitrogen in aerobic composting processes

Several factors, such as pH, C/N ratio, temperature, mixing and turning, and aeration rate, could affect the loss of ammonia in composting reactions. Substantial loss of ammonia can reduce the nutrient value of the compost product and may lead to a severe odor problem in the composting facility. A new method for conservation of ammonia in composting was proposed and tested in this study. The ammonia being produced during the composting was precipitated into struvite crystals by addition of Mg and P salts. Ammonia volatilization was greatly reduced by this method and it also contributed to a remarkable increase in total ammoniacal-N (TAN) content in the compost, reaching up to 1.4% of dry mass. This value of TAN content was 3–5 times higher than that in normal compost. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses confirmed the formation of struvite crystals in the aerobic composting process.

THE EFFECT OF CALPROTECTIN ON THE NUCLEATION AND GROWTH OF STRUVITE CRYSTALS AS ASSAYED BY LIGHT MICROSCOPY IN REAL-TIME

Purpose To use light microscopy to observe the urease-induced growth of struvite crystals in real-time, and to compare the effects of various proteins on that growth. Materials and Methods Artificial urine, with and without citrate, and a minimal urine solution containing only urea and the components of struvite and apatite were incubated with urease and test proteins in the depressions of culture slides. The number and size of rectangular and X-shaped struvite crystals were recorded using a low-power phase contrast microscope. Results The formation of crystalline struvite appears to occur after the formation of an amorphous calcium- and magnesium-containing phase. The extent of this amorphous phase is dependent on the presence of calcium and citrate, both of which strongly promote its formation over the crystalline phase. alpha-globulin, gamma-globulin and chymotrypsin inhibitor all result in the same amount of crystalline struvite as bovine serum albumin which is used as a control. Calprotectin, on the other hand, causes consistent and significant reductions in the number and size of struvite crystals under a wide range of conditions. No changes in the morphology of the struvite crystals were observed. Conclusions Calprotectin, the dominant protein of infection stone matrix, has distinctive properties which affect the formation and growth of struvite crystals. The presence of citrate in synthetic urine dramatically reduces the number of struvite crystals observed. The present method for observing the effects of putative infection stone inhibitors appears to have merit.

Crystal growth and molecular modeling studies of inhibition of struvite by phosphocitrate.

The inhibition by phosphocitrate of struvite crystal formation and growth has been examined in the present study. Crystal growth in a gel matrix was controlled by phosphocitrate in a dose-dependent manner. The effects of inhibition were followed using scanning electron microscopy, optical microscopy, and single crystal X-ray analysis. The presence of phosphocitrate induced very strong, crystal face specific inhibition of struvite, leading to total cessation of crystal growth when sufficient concentration of the inhibitor was made available. Crystal growth studies and results from molecular modeling indicated strong affinity of phosphocitrate to (101) faces of struvite. This in turn led to an alteration in the expression of these faces and the development of a characteristic arrowhead struvite morphology. Similar changes were not observed in the presence of identical concentrations of citrate, acetohydroxamic acid, and N-sulfo-2 amino tricarballylate (an analog of phosphocitrate), emphasizing the unique interaction of phosphocitrate with the struvite crystal lattice.

Effects of choreito consumption on urine variables of healthy cats fed a magnesium-supplemented commercial diet

Abstract Objective To investigate the effect of choreito consumption (500 mg/kg of body weight/d) on struvite crystal formation and signs of lower urinary tract disease (LUTD) in cats consuming a commercial canned diet with 0.5% added inorganic magnesium. Sample Population 6 male and 6 female adult cats, all considered to be clinically normal on the basis of physical examination findings; results of CBC, serum biochemical analyses, urinalyses, and urine cultures; and freedom from urolithiasis on the basis of urethrocystoscopic (females) or urethrocystographic (males) findings. Procedure Diets were fed for 12 weeks, or until appearance of signs of LUTD, including dysuria, hematuria, urine pH > 7.0, and severe struvite crystalluria. Presence of at least 2 of these signs was required for removal from study. Urine specimens were examined for electrolytes, struvite crystal content, and hematuria. Results Results for urine variables were compared between groups at 4 weeks, because of reduction in cat numbers attributable to removal from study. Struvite crystal content of 24-hour urine specimens was significantly lower for cats fed the choreito-containing diet. Moreover, frequency and severity of hematuria were significantly decreased in cats fed the choreito-containing diet. Correlation between hematuria and struvite crystal content was not observed in either group. Additionally, all 6 cats fed the diet without choreito had been removed from study by day 58 because of signs of LUTD. Of the 6 cats fed the choreito-containing diet, 2 completed the 12-week study. Clinical Relevance Choreito may be beneficial for relief of some signs of struvite-associated LUTD disease in cats. (Am J Vet Res 1997;58:146–149)

Effects of choreito and takushya consumption on in vitro and in vivo struvite solubility in cat urine

Abstract Objective To determine the effects of the takushya portion of choreito, a traditional Chinese treatment for urolithiasis, on urine and struvite crystal variables in cats fed diets containing takushya. Sample Population 6 male and 6 female adult cats, all considered to be clinically normal on the basis of physical examination findings, results of CBC, serum biochemical analyses, urinalyses, and urine cultures; and freedom from urolithiasis on the basis of urethrocystoscopic (females) or urethrocystographic (males) findings. Procedure Cats were fed a commercial canned diet supplemented with 0.1-mg of takushya/kg of body weight, or with 0.5 mg of choreito/kg. Diets were fed, using a Latin-square design, to 3 groups of 4 cats (2 male, 2 female) each for 2 weeks, followed by blood and 24-hour urine sample collections. Results Consumption of takushya, which comprises 20% by weight of choreito, was not associated with adverse effects in cats at the amounts provided during the period of study. Moreover, takushya was responsible for most of the effect of choreito consumption on reduction of urine pH, and approximately half its ability to reduce struvite crystal formation in cat urine. Clinical Relevance Alternative treatments for struvite urolithiasis in cats may be feasible. (Am J Vet Res 1997;58:150–152)

Unique ability of the Proteus mirabilis capsule to enhance mineral growth in infectious urinary calculi.

Struvite (MgNH4PO4.6H2O) calculi are a common complication of Proteus mirabilis urinary tract infections. Although urease is a major virulence factor in calculus formation, the polysaccharide capsule (CPS) of this organism also enhances struvite crystallization and growth in vitro (L. Clapham, R. J. C. McLean, J. C. Nickel, J. Downey, and J. W. Costerton, J. Crystal Growth 104:475-484, 1990). We obtained purified CPS, of known structure and varying anionic character, from P. mirabilis ATCC 49565 and several other organisms. Artificial urine was added to CPS, and the pH was elevated from 5.8 to 8.5 by the addition of urease or titration with 0.25 M NH4OH to induce struvite crystallization. Crystallization was measured by particle counting (Coulter counter), and the morphology (crystal habit) was examined by phase-contrast microscopy. In the presence of partially anionic P. mirabilis CPS, struvite formation occurred at a lower pH than in the absence of CPS or in the presence of other neutral, partially anionic, or anionic CPS. At pH 7.5 to 8.0, significantly more struvite crystals formed in the presence of P. mirabilis CPS than under other experimental conditions. With the exception of one polymer (curdlan) which did not bind Mg2+, enhancement of struvite formation by CPS polymers was inversely proportional to their Mg2+ binding ability. We speculate that the structure and partial anionic nature of P. mirabilis CPS enable it to enhance struvite formation by weakly concentrating Mg2+ ions during struvite crystal formation. This illustrates a new virulence aspect of bacterial CPS during infection.