Cultivation and particle characteristics of white phosphorus removal granular sludge

The strategy of combining particle-size selective pressure and settling-velocity selective pressure was used to successfully cultivate white phosphorus removal granular sludge within 120 days. After the granules matured, stratification appeared in the reactor due to the differentitation of particle size. According to the particle sizes, granules in the reactor were divided into four groups, which were R1(<2 mm), R2(2-4 mm), R3(4-7 mm) and R4(>7 mm). The pollutant removal efficiency and physicochemical properties of white granular sludge were investigated. The results showed that white granular sludge exhibited good COD removal performance across four particle size ranges. In addition, TP removal rate was the optimal with a particle size of 4-7 mm, reaching 99.94%. Moreover, the TN removal rate was positively correlated with the particle size, with granules larger than 7 mm achieving the best TN removal rate of 82.15%. The analysis of physicochemical properties showed that within the range of 0-7 mm, moisture content was negatively correlated with particle size, while wet density, average settling rate and VSS/SS were positively correlated with particle size. The EPS content also showed an overall increasing trend with the increase of particle size. When the particle size was between 2-7 mm, the PN/PS was larger, and the physicochemical structure was relatively stable. There were obvious hydroxyapatite cores inside of granules larger than 7 mm, which could be used as rich phosphorus resources for collection and utilization. Considering both granule stability and pollution removal efficiency, the particle size of 2-7 mm should be considered as the targeted regulation for granular sludge when treating phosphorus containing wastewater.