The performance of combined process of biological pond and constructed wetland for cadmium removal from irrigation water

The cadmium removal effect in irrigation water by the combined process of biological pond and constructed wetland was investigated through a field experiment. The influence of hydraulic retention time (HRT) on the cadmium removal effect and the daily variation rules of purification effect were further analyzed. Results show that the average concentration of total cadmium in irrigation water of the experiment area is 13.92 μg/L, which is 1.39 times of the cadmium limit in Standards for Irrigation Water Quality (GB 5084-2005). Most of them are dissolved cadmium, accounting for 77.8%. After the treatment by the combined process, the average removal rate of total cadmium reaches 42.0%, and the effluent in 85.2% of days during experiment can reach the standard. With the extension of the HRT, the cadmium removal rate of the combined process increases at first and then decreases, and the optimal HRT found to be 19 hours. In addition, the daily variation rules of cadmium removal rate by biological pond and constructed wetland both present as a single peak curve which shows lower value in the morning and evening but higher value at noon, and meanwhile the highest efficiency appears at 13:00.