Recovering phosphorus as struvite from the concentrated solution produced by the alternating aerobic/anaerobic biofilm system

Abstract

The recovery of phosphorus is so far limited to recovering phosphorus as struvite or vivianite from the supernatant of digester, but this process is not economically viable. In this study, a biofilm reactor was operated to directly enrich phosphorus from sewage to investigate whether phosphorus harvest affected the phosphorus uptake/release of phosphorus accumulating organisms (PAOs) and evaluated the potential of phosphorus recovery from concentrated liquid. The results showed that phosphorus was quickly released in the new recirculated solution by PAOs with saturated poly-phosphate after phosphorus harvest, which promoted the synthesis or degradation of polyhydroxyalkanoate (PHA) and glycogen in PAOs, restoring its ability to absorb phosphorus. Although the results of crystallization experiments demonstrated that there were traces of K-struvite and Na-struvite (<0.1 wt%) in precipitate, the simultaneous precipitation of compounds could not significantly affect the purity of products. Using the concentrated solution with a phosphorus concentration of 168.5 ± 2.7 mg·L−1 as phosphorus source for struvite precipitation achieved phosphorus recovery efficiency of 74.5–95.8% and obtained struvite crystals with a size of 83.5–114.3 µm, whereas the quantity of fine products (<10 µm) was rised due to an increase in saturation. Especially, the highest phosphorus recovery efficiency (95.8 ± 3.2%) was obtained at pH= 8.5 and Mg: P = 1.5. Moreover, life cycle assessment (LCA) analysis indicated that the CO2 equivalent emission of struvite production was 0.64–1.01 kg CO2 equivalent/kg struvite, and the phosphorus recovery efficiency and environmental impact could be balanced at pH= 8.5 and Mg: P = 1.5:1.