Phosphorous transformation during cattle farm biogas slurry treatment using struvite crystallization

Abstract: Struvite (or MAP) crystallization is a promising method for removing and recovering phosphorus from wastewater such as biogas slurry which is a wastewater high in phosphorus content. In this research, we analyzed different forms of phosphorus in cattle farm biogas slurry and phosphorous transformation in cattle farm biogas slurry during struvite crystallization. The transformation processes under pH value of 8.5, 9.0 and 9.5 were respectively observed based on measurement of the P content in liquid and solid phases which was mainly magnesium ammonium phosphate (MAP) obtained by centrifugation. Struvite was determined by the following steps: 1) The solid was recovered by centrifugation after the crystallization; 2) It was then washed and dried repeatedly; 3) It was dissolved by 1 mol/L of HCl; 4) The ammonium was detected; 5) The amount of struvite was calculated based on each MgNH4PO3?6H2O molecule which contained one ammonium molecule only. Phosphorus of liquid phase was classified into particulate phosphorus (PP), ortho phosphate (Ortho-P) and reductive dissolved phosphorus (RDP). In other phases, it was classified into phosphorus extracted by deionized water (H2O), NaHCO3, NaOH and HCl, respectively. Among them, H2O-P and NaHCO3-P was regarded as soluble phosphorus, while NaOH-P was regarded as a phosphorus compound which was combined with iron & aluminum (Fe-P/Al-P) and HCl-P was regarded as a phosphorus compound which was combined with calcium & magnesium (Ca-P/Mg-P). Before struvite crystallization, total phosphorus of (56.21±0.75) mg/L in liquid phase existed in the form of PP, Ortho-P and RDP with contents of 41.01%, 53.69% and 5.30%, respectively. Meanwhile, 5.69%, 25.45%, 12.32% and 32.16% of total phosphorus of (79.97±1.84) mg/L in solid phase was extracted successively by deionized water, 0.5 mol/L NaHCO3, 0.1 mol/L NaOH and 1.0 mol/L HCl, respectively, while the remained 24.38% was residual phosphorus (Re-P). During the struvite crystallization at about pH value 8.5, 9.0, and 9.5, the content of Ortho-P of (30.18±0.37) mg/L in liquid phase decreased to (17.98±0.30), (11.43±0.73) and (6.23±1.32) mg/L, respectively. The content of NaOH-P of (9.85±0.18) mg/L in solid phase decreased to (1.47±0.11), (1.29±0.07) and (1.44±0.27) mg/L, respectively. Meanwhile, the content of HCl-P in solid phase increased significantly (P＜0.05). In most cases, the fraction of other forms of phosphorus such as PP and RDP in liquid phase and H2O-P, NaHCO3-P and Re-P in solid phase did not vary significantly (P>0.05). According to the experiment, we can speculate that Ortho-P and NaOH-P (Fe-P/Al-P) were transformed into struvite and the other forms. Ortho-P and NaOH-P occupying 29.39% of biogas slurry phosphorus made greatest contribution to struvite formation among all forms, and 80.84% of them were removed from cattle farm biogas slurry, of which 91.28% of them were the struvite formed at about pH 9.5. After the crystallization, 10.96% (mass proportion) of the precipitation could be regarded as struvite at about pH 9.5, and the proportions were 6.97% and 9.25% at about pH 8.5 and pH 9.0, respectively. Thus, pH 9.5 was regarded as the proper pH for struvite crystallization among the values pH value 8.5, 9.0 and 9.5. In this paper, we clarified the transformation of different forms of phosphorus in cattle farm biogas slurry. In this way, it can be used as a reference for struvite crystallization condition optimization in the future.