Influence of pH and Temperature on Struvite Purity and Recovery from Anaerobic Digestate

Abstract

The precipitation of struvite (MgNH4PO4.6H2O) from wastewater streams simultaneosuly recovers nitrogen (N) and phosphorus (P) for reuse as fertilisers. Struvite crystallisation is controlled by pH, saturation index, temperature and other ions in the solution (e.g., Ca2+, Mg2+ and CO32−). This work studies the effect of pH and temperature on phosphorus and nitrogen removal via struvite precipitation and the quality of the resulting precipitate product (i.e., crystal size, morphology and purity). Struvite was precipitated in batch reactors from the supernatant produced during anaerobic sludge dewatering at a wastewater treatment works, under controlled pH (8, 9 and 10) and temperature (25, 33 and 40 °C) conditions. The optimal P removal as struvite, reduction of the co-precipitation with Ca and the increase in particle size of the struvite precipitates were determined. The results showed that temperatures of 33 °C and 40 °C are not recommended for struvite precipitation—i.e., at 33 °C the purity is lower, and at 40 °C the ammonia losses are induced by volatilisation. At all pH-tests, the P removal efficiency was >93%, but the highest phosphate content and purity as struvite were obtained at a pH of 9.0. The optimum pH and temperature for the formation of large crystals (84 µm) and a high purity (>70%) of the struvite precipitates were 9 and 25 °C, respectively.

Three Birds with One Stone: N/P Recovery, Dewaterability Improvement, and Facilitating Liquid Digestate Treatment of Anaerobically Digested Sludge

“…After the struvite was formed, the recovery efficiency of PO 4 3– -P could be over 99%, and the residual concentrations of PO 4 3– -P in the aqueous phase decreased to 0.26 mmol/L (initial phosphate concentration: 70% of its solubility) and 0.71 mmol/L (initial phosphate concentration: 100% of its solubility) (Figure b). The crystallization of struvite was carried out under alkaline conditions, which led to the conversion of NH 4 + into NH 3 ·H 2 O . The nitrogen could escape from the aqueous phase as the gaseous NH 3 under agitation conditions, so that the residual NH 4 + -N had a lower molar concentration than that of PO 4 3– -P (Figure b).…”

“…After the addition of phosphate for the salting-out operation, the molar concentration of phosphate (0.50 mol PO 4 3– -P/L) in the aqueous phase became much higher than that of NH 4 + -N. Therefore, NH 4 + -N was only supplemented into the filtrate for the N and P recovery. As shown in Figure a, the molar ratio of PO 4 3– and NH 4 + in the filtrate could basically reach 1:1 by further supplementing (NH 4 ) 2 SO 4 . , The mass flow of the whole process is shown in Figure S5 of the Supporting Information. The formation of precipitates could be induced by adjusting the filtrate pH to 9, and the XRD patterns of precipitates indicated the successful formation of struvite crystals (Figure S6 of the Supporting Information).…”

“…The crystallization of struvite was carried out under alkaline conditions, which led to the conversion of NH 4 + into NH 3 • H 2 O. 39 The nitrogen could escape from the aqueous phase as the gaseous NH 3 under agitation conditions, so that the residual NH 4 + -N had a lower molar concentration than that of PO 4 3− -P (Figure 3b). Besides, the lower molar concentration of residual NH 4 + -N than that of PO 4…”

“…3− and NH 4 + in the filtrate could basically reach 1:1 by further supplementing (NH 4 ) 2 SO 4 . 21,39 The mass flow of the whole process is shown in Figure S5 of the Supporting Information. The formation of precipitates could be induced by adjusting the filtrate pH to 9, and the XRD patterns of precipitates indicated the successful formation of struvite crystals (Figure S6 of the Supporting Information).…”

Recovery of High Concentrations of Phosphorus and Ammonium through Struvite Crystallization by CO2 Repelling

“…Numerous studies have been conducted to optimize the favorable conditions for the precipitation of struvite. Indeed, struvite precipitation depends on various physical and chemical parameters such as temperature, [ 10 ] pH, [ 10,11 ] concentrations of the constituent ions, [ 12,13 ] stirring, [ 14 ] and supersaturation. [ 11,15 ] Struvite precipitation depends also on the presence of foreign ions in the solution such as heavy metals [ 16,17 ] and phenolic organics.…”