Toward Resource Recovery from Wastewater: Extraction of Phosphorus from Digested Sludge Using a Hybrid Forward Osmosis–Membrane Distillation Process

We demonstrate the simultaneous extraction of phosphorus and clean water from digested sludge centrate using a forward osmosis (FO)–membrane distillation (MD) hybrid process. In this FO–MD hybrid process, FO concentrates orthophosphate and ammonium for subsequent phosphorus recovery in the form of struvite (MgNH4PO4·6H2O), while MD is used to recover the draw solution and extract clean water from the digested sludge centrate. A decline in water flux was observed during the FO process, but fouling was largely reversible after a brief, simple membrane flushing using deionized water. The FO process also provides an effective pretreatment capacity to the subsequent MD process, which exhibited stable water flux. The use of MgCl2 as the draw solute for the FO process is another novel aspect of the system. The reverse salt flux of magnesium to the concentrated digested sludge across the FO membrane and the diffusion of protons away from the digested sludge create favorable conditions for the formation of struvite crystals. The precipitates obtained in the hybrid process were verified to be struvite crystals by examining the crystal morphology, element composition, and crystal structure. Results reported here highlight the potential and robustness of the FO–MD hybrid process for extracting phosphorus from wastewater.