Operation optimization of ammonia nitrogen removal process in coking wastewater treatment

Abstract

Abstract One of the key steps of coking wastewater treatment is ammonia nitrogen removal. It is normally realized by a process combining distillation operation with bio-treatment, in which the high cost distillation is designed to reduce ammonia nitrogen low enough for the following low cost bio-treatment. Many efforts have being done to optimize the structure and operating parameters of this process. However, to the authors’ best knowledge, little work has been done considering the interactions between them. In this paper, a mathematical model of the overall ammonia nitrogen removal process, which combines distillation with shortcut nitrification/ANAMMOX/denitrification (short as O1-A-O2), is proposed for its operation and structural optimization. The proposed model is formulated as a mixed integer nonlinear programing (MINLP) problem with aim of minimizing overall operating cost. The main decisions include determining ammonia-nitrogen concentration of distillation bottom outlet, reaction recycle time and passby flowrate of O1 reactor subject to compliance with the maximum allowable inlet ammonia nitrogen concentration limits of the bio-treatment and discharge regulations. For simplicity, we adopt shortcut models for the ammonia distillation and bio-treatment process based on rigorous model calculation data and theoretical analysis to enforce solution convergence. Computational studies are carried out via GAMS/DICOPT using industrial operating parameters of the typical coking wastewater treatment system as input. The results indicate that the minimum operating cost is obtained at ammonia-nitrogen concentration of 128 mg/L from distillation bottom outlet, 10 h reaction recycle time of O1 and passby flowrate of 11.448 m3/h when the ammonia-nitrogen inlet concentration of distillation column is 4,000 mg/L.