Field-scale zero liquid discharge of coking wastewater - Operating efficiency and environmental impact

Abstract

Coking wastewater is typical industrial wastewater produced in the process of coal coking. The notion of Zero Liquid Discharge (ZLD) presents a paradigm aimed at mitigating waste generation and optimizing resource recovery. However, the implementation of ZLD in the realm of industrial wastewater treatment remains inadequately investigated and researched. We undertook an investigation into a ZLD treatment system applied to coking wastewater within a steel plant, which had been in operation for a span of four years, where the core steps are membrane concentration separation and thermal crystallization. The treatment efficiency of each unit was studied using the method of on-site sampling and detection. In comparison to the initial phase, the separation effectiveness of NF unit on different valence ions demonstrated sustained stability, the average rejection rate of SO42− was approximately 96%, whereas the rejection rate of Cl− was near 0. Conversely, under the influence of long-term operation and membrane fouling, the efficiency of the membrane concentration unit had a downward trend, with problems such as the deterioration of membrane, loss of ion rejection capability, and degradation of reclaimed water. The water recovery rate has decreased to 48.9% and the average ion retention rate has decreased to 85% in the membrane concentration unit. Nonetheless, the resultant by-products continued to align with the steel plant's recycling requirements. In addition, the Life cycle Assessment (LCA) method was used to assess environmental impact during operation. Subsequent analysis led to that reagents and electricity consumed in pretreatment and advanced treatment had a significant impact for negative EI. The impact of pretreatment on ODP accounted for 27.64%, and the impact on ADP accounted for 20.72%. The impact of advanced treatment on ADP accounted for 16.25%, the impact on EP accounted for 21.74%, and the impact on ET accounted for 27.06%. Meanwhile, the recovered water and salt during operation generate considerable environmental benefits and dominate the evaluation of the entire operating process. This work furnishes a valuable point of reference for pertinent ZLD investigations, while unveiling the pronounced capacity of ZLD to contribute substantially to waste management and the betterment of environmental consequences.