Applicability of zirconium loaded okara in the removal and recovery of phosphorus from municipal wastewater

Recently, there is a new trend to consider wastewater as a precious resource. Since phosphorus is a limited non-renewable element, and MAP (Magnesium Ammonium Phosphate - MgNH₄PO₄.6H₂O) is a valuable slow-release fertilizer, the recovery of phosphorous as MAP has received special attention from scientists all over the world. However, the application of this process with municipal wastewater is still a challenge, due to low concentration of phosphorus and high volume of municipal wastewater. This study investigates the potential of reclaiming MAP from municipal wastewater by combination of adsorption and crystallization. Soybean milk residue (okara) was loaded with Zirconium (Zr) to prepare the adsorbent (ZLO). Adsorption and desorption experiments were conducted in a semi-pilot scale ZLO packed colum system. Effects of P: N: Mg molar ratios, chemical sources and temperature on the formation of MAP were examined in an attempt to identify the optimal crystallization conditions. The attained precipitate was characterized using XRD, SEM, FTIR techniques. It was found that the ZLO packed column adsorption-desorption system could pre-concentrate phosphorus from municipal wastewater up to 28.36 times, fitting well the minimum requirement (50 mg P/L) for the economical MAP recovery. Up to 95.19% of dissolved phosphorus in desorption solution was recovered at pH = 9, Mg: N: P molar ratio = 2:2:1, using a combination of MgCl₂.6H₂O and NH₄Cl. The harvested MAP exhibited high purity (92.59%), high P-availability (89% by mass), and extremely low levels of heavy metals. The results prove that it is viable to recover MAP from municipal wastewater by employing ZLO as adsorbent, followed by crystallization. This paves the way for mining phosphorus from municipal wastewater and reducing okara as an agricultural byproduct in a green way.