Treatment of water discharged from a Yellow River water purification plant: optimization and application of enhanced coagulation technology

In this study, the effects of the coagulant type, compounding ratio, dosage, hydraulic conditions and water temperature on the intensive coagulation process were systematically investigated for the treatment of discharge water with high turbidity and high sediment characteristics from a Yellow River water purification plant (YRWPP). Results indicated that the PFS-PDMDAAC composite coagulant achieved optimal treatment performance under the following conditions: a blending ratio of 6 : 1, a dosage of 13.65 mg L^-1, rapid mixing at 300 rpm for 1.5 min, slow mixing at 120 rpm for 7 min, and a water temperature of 20 °C. Employing these parameters, supernatant turbidity was reduced to 46.2 NTU, the specific resistance to filtration (SRF) of the sludge was less than 0.94 × 10¹² m kg^-1, and the solid content exceeded 8%, enabling direct dewatering. With the increase of the concentration of the discharged mud water, the effect of enhanced coagulation is weakened. Specifically, when the concentration was lower than 4.14%, a dosage of 13.65 mg L^-1 could be treated effectively; when the concentration was in the range of 4.14-9.12%, the dosage needed to be increased to 21 mg L^-1; and when the concentration was more than 9.12%, it was difficult to achieve the discharge standard. Furthermore, zeta potential analysis showed that the absolute zeta potential value was the lowest when the compounding ratio was 6 : 1, and the effect of colloid destabilization was the best. According to floc morphology observation, the volume of floc increased and the structure of floc was loose and porous after enhanced coagulation, which was conducive to settling and dewatering. The analysis of the floc particle size showed that the average floc particle size increased from 38.3 μm to 238 μm at a dosage of 21 mg L^-1, and the settling performance was significantly improved; when the dosage was more than 21 mg L^-1, the floc particle size decreased, and the settling performance declined. This study provides technical support for the treatment and resource utilization of mud water discharged from the Yellow River water purification plant.