Activated carbon catalytic ozonation of reverse osmosis concentrate after coagulation pretreatment from coal gasification wastewater reclamation for zero liquid discharge

Abstract

To fulfil the requirement of zero liquid discharge in the coal gasification wastewater treatment plant, evaporation and crystallization are usually adopted to treat reverse osmosis concentrate (ROC). However, their efficacies and the quality of their products are greatly affected by the organic matter concentration in coal gasification-ROC (CG-ROC). In the present study, ozonation with a commercial activated carbon (AC) as catalyst was conducted to further remove the organic matter in the concentrate originated from a three-stage RO after coagulation pretreatment. The kinetic model calculations were used for the first time to quantitatively evaluate the effect of matrix components on the performance of ozonation of CG-ROC at initial pH 9 and 11. The TOC removal difference among ozonation alone, ozonation alone followed by AC adsorption, and AC catalytic ozonation indicates that AC has a certain catalytic effect during ozonation of CG-ROC. The ΔO3/ΔTOC can be significantly reduced from 43.2 mg O3/mg TOC of ozonation alone to 16.2 mg O3/mg TOC of AC catalytic ozonation under the goal of achieving a similar TOC removal. The change of initial pH has little effect on the TOC removal during ozonation with AC, however, it has a remarkable effect on the TOC removal during ozonation alone. According to the results obtained in kinetic model calculations, high concentration of CO3•- would be generated from the reaction between HO• and CO32− with a high concentration in solution, leads to the delay on the TOC removal in the early reaction stage during ozonation alone at initial pH 11. The results of A254 and molecular weight (MW) distribution indicate that ozonation with AC is more efficient in removing the low MW intermediates absorption at 254 nm compared to that of ozonation alone. Ozonation with AC is more suitable for CG-ROC treatment since it is more energy saving, more efficient in organic matter removal, and more tolerated pH change and wastewater matrix compared to ozonation alone.