Phosphorus recovery from source-diverted blackwater through struvite precipitation

Phosphorus recovery from wastewater through struvite precipitation is becoming a promising strategy to both mitigate eutrophication risk due to excess phosphorus discharge into water bodies and alleviate the global phosphorus crisis by producing value-added fertilizer. However, the composition and quality of wastewater differ among regions and home to home. Source-diverted blackwater, especially concentrated blackwater collected from vacuum toilet systems, typically has a moderate phosphate-phosphorus (PO₄-P) content, high ammonia-nitrogen (NH₄-N) content, strong buffering capacity as a result of high alkalinity, and a high pH close to 9. Thus, concentrated blackwater is a good source for phosphorus recovery through struvite precipitation. In this study, we examined the feasibility of recovering phosphorus from concentrated blackwater through struvite precipitation by conducting batch experiments. The characterization of the struvite produced from concentrated blackwater was performed via X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) techniques. The metal contents of the struvite product were analyzed by inductively coupled plasma mass spectrometry (ICP-MS), and its purity was determined. A phosphorus removal efficiency exceeding 90% was achieved as a result of both an appropriate supersaturation ratio of struvite (>4.0) and a high initial pH. The high quality of the produced struvite was demonstrated by a purity of 94.9%, the Mg/P/N mass ratio of 10:12:4.7 based on the EDS analysis, and low heavy-metals contents. This feasibility study provides evidence for the efficacy of phosphorus recovery from concentrated blackwater through struvite precipitation in decentralized wastewater management systems.