Prolonged stirring pelleting coagulation for the enhanced treatment of fresh leachate: Removal performance and floc characteristics

Conventional coagulation (CC) is a widely adopted method for the pre-treatment of fresh leachate for enhancing subsequent advanced treatment processes. However, the formation of fluffy flocs in the CC process often results in limited separation and removal efficiency. In this study, a continuous alkali dosing prolonged stirring pelleting coagulation (C-PPC) process was proposed for the pre-treatment of fresh leachate. The C-PPC process consisted of three stages: prolonged stirring, reaction, and pelleting, achieving higher suspended solid (SS), turbidity, and organic removal efficiency compared with both CC and PPC processes. At a prolonged stirring intensity of 1500 r/min (G = 4241 s⁻¹) and a final pH of 12, with polyaluminum chloride (PAC) and polyacrylamide (PAM) dosages of 2000 mg/L and 25 mg/L, impressive removal efficiency of 93.7 % SS, 99.7 % turbidity, and 35.7 % organic were obtained. In the prolonged stirring stage, the primary particle size decreased from 16.848 μm to 6.755 μm due to intensive stirring for 30 min, facilitating the compact floc formation. In the subsequent reaction stage, PAC was dosed once while NaOH was continuously dosed to achieve a final pH of 12 within 10 min. The floc size initially decreased due to fragmentation and small metal crystal generation. Subsequently, the generated small metal crystals served as the nuclei for organic matter adsorption and were entrapped by generated flocs, resulting in a significant increase in floc size with higher sphericity. Furthermore, compact pellets were formed during the pelleting stage after the addition of PAM. Floc restructuring also occurred during the pelleting stage, leading to a decreased pore volume of mesopores, achieving final floc densification and superior separation performance.