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

Abstract

Struvite (MgNH4PO4·6H2O) can precipitate from wastewater thereby recovering both ammonium and phosphorus while simultaneously producing a valuable fertilizer. In the present work, the precipitation of struvite by CO2 repelling at initial constituent ions concentrations in the range between 123.89 and 800 mg L−1 and different airflow rates is investigated. Struvite precipitation is induced by an increase of the solution supersaturation occurring concurrently with the removal of the dissolved CO2 from the solution. The precipitates obtained are characterized by powder X‐ray diffraction and scanning electronic microscopy. For all experiments, struvite is the only phase observed. Whatever the airflow used, the precipitation pH decreases with increasing the initial phosphorus concentration. Struvite precipitates at supersaturations in the range 6–18 and pH in the range 6.5–8.5. High phosphorus removal efficiencies (>70%) are obtained at high struvite supersaturations. The increase in the airflow rate results in a significant decrease in the phosphorus removal efficiency. When the initial phosphorus concentration increases, the particle size distribution becomes monomodal covering a broad range of particle sizes up to 140 µm. This is of utmost importance since larger struvite crystals are easier to separate from the liquid phase and release ammonium and phosphorus from struvite faster, promoting the use of struvite as a fertilizer.

A Comprehensive Review on Wastewater Nitrogen Removal and Its Recovery Processes

“…Struvite is a white crystal compound that commonly accumulates in post-anaerobic digestion pipes in wastewater treatment plants (WWTPs) [ 50 ]. Struvite is formed when magnesium, ammonium, and phosphate that are in equimolar concentrations in waste water [ 51 , 52 ]; the chemical reaction equation is as follows (Equation (1)): Mg 2+ + NH 4 + + PO 4 3− + 6H 2 O⇋MgNH 4 PO 4 ·6H 2 O↓ S = 0.169 (g/L) …”

Phosphorus Recovery from Sewage Sludge as Struvite

“…In contrast, the PHOSHPAQ process treats the liquid fraction of the dewatered sludge. Both treatments require the addition of Mg and, through aeration, achieve CO 2 stripping to reach a slightly basic pH (8-8.5), which causes the spontaneous precipitation of struvite in the presence of ammonium [27][28][29]. At the laboratory scale, the phosphorus recovery as struvite from a liquid effluent after WO treatment of municipal sewage sludge has been studied in several works [30,31] through processes similar to those applied in this work.…”

Circular Economy Model Through Phosphorus Recovery to Obtain Struvite from Sewage Sludge

“…In contrast, the PHOSHPAQ process, implemented in a WWTP in the Netherlands, treats the liquid fraction of the dewatered sludge. Both treatments require the addition of Mg and, through aeration, achieve CO2 stripping to reach a slightly basic pH (8-8.5) which causes the spontaneous precipitation of struvite in the presence of ammonium [24][25][26]. At the laboratory scale, phosphorus recovery as struvite was studied in a liquid effluent after WO treatment of municipal sewage sludge [27,28], process similar to the applied in this work.…”