Phosphorus recovery as hydroxyapatite (HAP) from urban wastewaters using nanofiltration and reverse osmosis brines

Abstract

The recovery of phosphorus from wastewater appears to be the best option to ensure the recyclability of this product and save the fertiliser industry from losing its key element. Chemical precipitation is an effective method to obtain calcium phosphate in the form of hydroxyapatite (HAP), which can be applied to the soil as a fertiliser. In a Waste Water Treatment Plant (WWTP), the liquid effluent from the secondary clarifier is rich in phosphorus. This effluent can be introduced in an ion exchange unit that, when being regenerated, produces a solution with a concentration up to 1,000 mg PO43-/L. This stream can undergo chemical precipitation thanks to the addition of calcium ions with alkaline media. Concentrate or reject stream obtained from nanofiltration (NF) or reverse osmosis (RO) processes can be used as a source of calcium, valorising both wastewater and brines. A laboratory scale study is conducted using a semi continuous stirred batch reactor to precipitate phosphate of a solution that simulates the regenerated stream in the ion-exchange unit of a WWTP, using nanofiltration and reverse osmosis brines with and without Mg2+ at different pH. Results indicate that the presence of Mg2+ enhances phosphate removal, but does not stimulate HAP formation. The conditions that achieve higher phosphate removal are the NF and RO brines with Mg2+ at pH=10.5, representing almost a 100 % of removal. In all four types of brines, higher removal is obtained at pH=10.5. The type of brine does not have as much effect as other factors such as the presence of Mg2+ or pH. In less time, NF brine removes the same amount of phosphate from the solution than RO brine when there is Mg2+.