A comprehensive understanding of saturation index and upflow velocity in a pilot-scale fluidized bed reactor for struvite recovery from swine wastewater

Fluidized bed reactor (FBR) has been applied in phosphorus recovery from wastewater due to its ability on producing struvite pellets with large size, high crushing strength and purity. However, the fine struvite crystals (fines) can be flushed out of the reactor, which may result in undesirable and variable phosphorus recovery efficiencies. In this study, the influences of key factors with respect to phosphorus recovery and product quality in a FBR were investigated for the treatment of swine wastewater. The effects of process parameters were first evaluated using response surface methodology (RSM), and later, long-term pilot-scale operations were tested. Results showed that fines retention was directly related to the in-reactor saturation index (SI) and upflow velocity (vup). Decreasing SI (from 2.42 to 1.65) or vup (from 75 to 25 mm s− 1) will reduce the fines entrainment and improve the phosphorus recovery profile by 14.9 and 18.5%, respectively. Moreover, these two factors governed the quality of the struvite pellets in the pilot-scale investigations—SI showed more crucial effect to size distribution, crushing strength and morphology than vup. As to the composition, struvite (35.5–95.3% of mass fraction) and CaCO3 (2.8 to 51.4% of mass fraction) were the two major components in the pellets. Decreasing SI or increasing vup and pellet size can enhance the struvite purity. Therefore, phosphorus recovery and pellet quality can be simultaneously improved by SI and vup control without additional trapping devices or reagents. The finding is constructive for FBR system set-up and operation in either pilot- or full-scale applications, where different phosphorus recovery rates and product qualities are required.