Removal And Recovery Of Nitrogen And Phosphorus As Struvite From Swine Waste Biogas Digester Effluent

Anaerobic biogas digestion is the wildly accepted method for the treatment of swine wastes, but high concentration of nitrogen and phosphate in the digester effluent poses a potential risk on pollution of water bodies. The aims of this research were to study the feasibility of recovering ammonium nitrogen and phosphate from swine wastes biogas digester effluent (SWBDE) as struvite, to investigate the effects of factors including pH, NH₄⁺:Mg²⁺: PO₄^3- molar ratios, and Ca²⁺ on the crystallization process, to optimize the conditions for maximum struvite recovery and to design a device to isolate the precipitated crystal as a pure product.Precipitation experiments were conducted at pH 7.5, 8.0, 8.5 and 9.0 using SWBDE with NH₄⁺:Mg²⁺: PO₄^3- molar ratios of 1: 0.2: 0.08, 1: 1: 1, and 1: 1.5: 1.5, respectively. The highest recovery efficiency of NH₄⁺ and PO₄^3- were achieved at pH 9.0 in each experiment. For SWBDE with NH₄⁺:Mg²⁺: PO₄^3- molar ratios of 1: 0.2: 0.08, the elevation of pH to 9.0 alone could decrease the PO₄^3- concentration from 42 mg L^-1 to 4.7 mg L^-1 and 89.2% PO₄^3- recovery was achieved. When pH and NH₄⁺: Mg²⁺: PO₄^3- molar ratios were 9.0 and 1: 1.5: 1.5, respectively, 76.5% NH₄⁺ and 68.5% PO₄^3- were recovered from the SWBDE. The NH₄⁺:Mg²⁺: PO₄^3- molar ratio of 1:1:1 together with pH of 9.0 was determined as the optimum combination for both NH₄⁺ and PO₄^3- recovery. Under this optimal condition, the recovery efficiency of NH₄⁺ and PO₄^3- reached 70% and 97%, respectively, and struvite is the major component of the precipitate according to the X-ray diffraction (XRD) pattern. During the process of precipitation, pH was found decreasing until the termination of precipitation, which could be used to indicate the nucleation of struvite in the reactor.The effect of Ca²⁺ on the struvite formation was investigated at pH 9.0 using artificial wastewater with Ca²⁺:Mg²⁺:NH₄⁺:PO₄^3- molar ratios of 0: 1: 1: 1, 0.25: 1:1: 1, 0.5: 1: 1: 1 and 1: 1: 1: 1, and real SWBDE with Ca²⁺: Mg²⁺:NH₄⁺:PO₄^3- molar ratio of 0.075: 1: 1: 1 with the aid of scanning electron microscope (SEM), XRD and X-ray energy dispersive spectroscopy (EDS). In the experiment conducted with SWBDE with Ca²⁺: Mg²⁺:NH₄⁺:PO₄^3- molar ratio of 0.075: 1: 1: 1, the struvite was found as the major precipitate but contaminated with some calcium products. Similarly, the experiment with artificial wastewater showed that the presence of Ca²⁺ ions prolonged the induction time of struvite nucleation, and increased Ca²⁺ concentration adversely affected the shape of crystal and the purity of the struvite in the precipitate. For the artificial wastewater with Ca²⁺: Mg²⁺: NH₄⁺: PO₄^3- molar ratio of 1: 1: 1: 1, an amorphous substance was observed and the main components of the precipitate were Ca and P. Therefore, the control of Ca²⁺ concentration is an effective way to promote the recovery efficiency of struvite from SWBDE.To determine the accumulating efficiency of struvite by the introduced device and the effect of pH controlled condition on the precipitation, experiments were conducted using SWBDE under controlled pH (pH=9.0) and uncontrolled pH (initial pH=9.0) conditions with four modes of mixing: no-mixing, magnetic stirring at 500 rpm, magnetic stirring at 500 rpm plus rotating device at 50 rpm, and rotating device at 50 rpm without magnetic agitation, respectively. The average amount precipitated at four mixing modes under pH-controlled condition was 24.6±0.86 g, which is higher than that 19.8±1.86 g obtained under pH uncontrolled condition. The struvite accumulation rate on the introduced device ranged from 6.5～8.2 g m^-2h^-1 to 3.6～4.8 g m^-2h^-1 under pH controlled and pH uncontrolled condition, respectively. The highest amount of struvite was collected from the introduced device operated at 50 rpm along with 500rpm magnetic stirring. Although Cu²⁺ or Zn²⁺ ions existed in the SWBDE, none of them were detected in the struvite adhere to the device, and the purity of struvite reached around 96%. Thus, it is feasible to use the introduced precipitate accumulating device in the recovery of struvite from SWBDE .