Improving sludge settling performance of secondary settling tank and simultaneously adsorbing nitrate and phosphate with surfactant modified zeolite (SMZ) ballasted flocculation

Poor sludge settling characteristics limit the enhancement of treatment capacity of municipal wastewater treatment plants, and excessive nitrogen and phosphorus discharge will further aggravate water eutrophication, but the existing ballasting agents cannot promote sludge settling and adsorb nitrate and phosphate simultaneously. In this study, surfactant modified natural zeolite (SMZ) was used as a novel ballasting agent to improve settling performance of activated sludge and simultaneously adsorb nitrate and phosphate. The results of settling experiments showed that 75–150 µm size SMZ had the best effect in promoting sludge settling, and the zone settling velocity (ZSV) of sludge increased to 3.38 m/h at 1 g/L dosage, which reached 2.11 times of the original sludge. The results of microscopic observation, floc size determination, and density test demonstrated that finer particle size SMZ facilitated the formation of larger and denser flocs, thus promoting sludge settling, but excessively fine SMZ (45–75 µm) reduced the ability to promote sludge settling due to the lower density of the flocs formed. SMZ promoted sludge settling performance by attracting sludge to agglomerate through charge neutralization, forming larger and denser agglomerates. In addition, the SMZ removed 47.05% and 53.25% of nitrate and phosphate, respectively, while increasing the sludge settling velocity to more than twice that of the original sludge. This study expected to provide useful information for upgrading municipal wastewater treatment plants.