Application and mechanism of nucleation-induced pelleting coagulation (NPC) in treatment of fracturing wastewater with high concentration of dissolved organic matter

Abstract

To improve the separation efficiency of fluffy flocs during coagulation of fracturing wastewater with high concentration of dissolved organic matter, a novel nucleation-induced pelleting coagulation (NPC) process is proposed based on pelleting coagulation in this study. In the NPC process, nucleation agents were added to act as pellet seeds to generate high-density pellet flocs. The results implied that the necessary condition for the NPC process was to control the metastable state, i.e. zeta potential around −10 mV after addition of the coagulant, polyaluminium chloride (PAC). Diatomite, which was added after rapid mixing, was used as the nucleation agent, and its optimal dosage was about 100 mg/L with particle size 100–200 μm. In addition, the dosage of the coagulant aid, polyacrylamide (PAM), significantly affected the performance of the NPC process, and the optimal PAM dosage was 10 mg/L in this study. It was found that 60 rpm (G = 55.1 s−1) was the optimal hydraulic condition for pellet growth during slow mixing. The pellet floc settling velocity reached 14.9 mm/s and the particle size (d50) reached 4.6 mm with an effective density of 0.021 g/cm3 at the optimal condition, which was one order higher than that of conventional aluminium flocs.