Impact of Total Suspended Solids on Struvite Precipitation from Source-Diverted Blackwater

Source-diverted blackwater, especially concentrated blackwater collected from vacuum toilet systems, has been demonstrated to be an ideal source for struvite (NH4MgPO4·6H2O) production. However, the impacts of total suspended solids (TSS) in blackwater toward struvite quality are not well addressed. In this study, a fluidized bed reactor (FBR) was operated in a batch mode to examine the influences of TSS in raw concentrated blackwater on phosphorus recovery efficiency as well as the purity and size of the produced struvite. The produced struvite was characterized via X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The metal contents of the struvite product were determined by inductively coupled plasma mass spectrometry (ICP-MS). The results revealed that at appropriate water quality (e.g., pH=8.9) and operating conditions [e.g., magnesium to phosphorus (Mg/P) molar ratio = 1.5], neither blackwater TSS nor FBR upflow velocity significantly affected the phosphorus recovery efficiency. TSS had a significant impact on the purity and size of the struvite product. The struvite purity increased with the decrease of TSS concentration within the studied range of 120-1,600 mg/L. At higher TSS levels (520-1,600 mg/L), the struvite crystal remained small (<5 μm) with the FBR upflow velocity ranging from 18 to 90 m/h. When the TSS concentration was lower than 220 mg/L, the struvite crystal was able to grow continuously (up to 100 μm), and its size was directly impacted by the FBR upflow velocity. A greater upflow velocity was unfavorable to the agglomeration of struvite crystals.