Struvite formation, control and recovery

The present review provides an understanding of principles of struvite crystallization and examines the techniques and processes experimented to date by researchers at laboratory, pilot, and full-scale to maximize phosphorus removal and reuse as struvite from wastewater effluents. Struvite is mainly known as a scale deposit causing concerns to wastewater companies. Indeed, struvite naturally occurs under the specific condition of pH and mixing energy in specific areas of wastewater treatment plants (e.g., pipes, heat exchangers) when concentrations of magnesium, phosphate, and ammonium approach an equimolar ratio 1:1:1. However, thanks to struvite composition and its fertilizing properties, the control of its precipitation could contribute to the reduction of phosphorus levels in effluents while simultaneously generate a valuable by-product. A number of processes such as stirred tank reactors and air-agitated and -fluidized bed reactors have been investigated as possible configurations for struvite recove...

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Phosphorus Recovery from Wastewater by Struvite Crystallization: A Review

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

Production of slow release crystal fertilizer from wastewaters through struvite crystallization – A review

Impact of calcium on struvite crystal size, shape and purity

Predicting struvite formation in digestion

The kinetics of struvite (MgNH\d4PO\d4·6H\d2O) formation was studied to determine the cause of preferential accumulation at certain locations within the anaerobic digestion and postdigestion processes in wastewater treatment. Mechanisms controlling nucleation were also studied in laboratory induction time experiments. A field study of struvite accumulation on postdigestion process appurtenances was conducted to evaluate mechanisms affecting crystal growth rates. Struvite nucleation was found to be reaction-controlled, with nucleation strongly dependent on the struvite supersaturation level. Increased mixing intensity at constant supersaturation had a small effect on induction times. The crystal growth rate was found to be transport-limited, with the growth rate a function of mixing energy input. Evaluation of the effect of pH change resulting from CO/d2 evolution in high turbulence areas showed that it has a minor influence on the crystal growth rate. Preferential struvite accumulation was found to be primarily controlled by growth kinetics, which was strongly dependent on mixing energy. Preferential accumulation of struvite on selected construction materials was also studied, and experimental evidence suggested that surface roughness exerted the greatest control over preferential struviite accumulation on various materials under similar mixing and supersaturation conditions.

Kinetics Effects on Preferential Struvite Accumulation in Wastewater

The goal of this study was to determine the removal efficiencies of chromium, copper, lead, nickel, and zinc from raw wastewater by chemically enhanced primary treatment (CEPT) and to attain a total suspended solids removal goal of 80%. Operating parameters and chemical doses were optimized by bench-scale tests. Locally obtained raw wastewater samples were spiked with heavy metal solutions to obtain representative concentrations of metals in wastewater. Jar tests were conducted to compare the metals removal efficiencies of the chemical treatment options using ferric chloride, alum, and anionic polymer. The results obtained were compared with those from other studies. It was concluded that CEPT using ferric chloride and anionic polymer is more effective than CEPT using alum for metals removal. The CEPT dosing of 40 mg/L ferric chloride and 0.5 mg/L polymer enhanced heavy metals removal efficiencies by over 200% for chromium, copper, zinc, and nickel and 475% for lead, compared with traditional primary treatment. Efficient metals capture during CEPT can result in increased allowable headworks loadings or lower metal levels in the outfall.

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Enhanced removal of heavy metals in primary treatment using coagulation and flocculation.

Controlled precipitation of struvite from postdigestion, sludge lagoon supernatant was studied using a pilot scale fluidized bed reactor (FBR). The process effectively and rapidly removed struvite constituents from the supernatant and produced effluent significantly undersaturated for struvite. Recently published information on struvite thermodynamic and kinetic properties was incorporated into the FBR system design and selection of components. Struvite crystals were selected as seed media, which enabled precipitation of nearly pure crystals and allowed nucleation, with its associated lag time, to be bypassed. Mixing energy in the FBR was demonstrated to be sufficient to overcome transport limitations to struvite growth, allowing growth kinetics to be governed by the surface integration rate of constituent ions into the growing crystal lattice structure. Struvite removal was optimized by incorporating pH elevation into the FBR system. Process fluid was circulated in the pH adjustment tank/FBR system, which was configured as a continuous feed stirred-tank reactor. Struvite removal efficiency exceeded 80% in continuous feed experiments where the hydraulic detention time exceeded 1 h.

Postdigestion struvite precipitation using a fluidized bed reactor

The use of reclaimed water has become an option to reduce the demand for potable water. In the past, membrane technology has not always been economically feasible for this application due to high energy requirements needed to maintain membrane performance (i.e. flux). An increase in energy requirements and a reduction in throughput are a direct result of membrane fouling because the pressure requirements increase as particulate matter accumulates on the membrane surface. Membrane fouling is dependent on the type of constituents found in the feedstream, therefore there are different methods to reverse these types of fouling. To evaluate and study fouling potential and methods for limiting/reversing membrane fouling for the production of reclaimed water, three plant-scale experiments were conducted at a California wastewater treatment plant using secondary effluent from a high-purity oxygen, activated sludge/secondary clarifier process. The results of the experiments help establish the best methods for limiting/reversing fouling based on operating parameters (i.e. flux), production methods, and membrane configuration. The first experiment series was used to evaluate the impacts of flux on fouling and determined the effectiveness of backwashing and chemical maintenance washes on a membrane operating under a cross-flow configuration with 10% re-circulation. To determine the best method to limit/reverse fouling on two different membrane process/operating configurations, a second experiment was conducted that evaluated the average permeability decrease between backwashes and maintenance washes. Selected maintenance wash processes were conducted in a third experiment to determine which method was the most effective in permeability recovery (e.g. reducing irreversible fouling).

Kurt N. Ohlinger

Papers

Predicting struvite formation in digestion

Kinetics Effects on Preferential Struvite Accumulation in Wastewater

Enhanced removal of heavy metals in primary treatment using coagulation and flocculation.

Postdigestion struvite precipitation using a fluidized bed reactor

An Evaluation of Fouling Potential and Methods to Control Fouling in Microfiltration Membranes for Secondary Wastewater Effluent