Long-term performance of subsurface-flow constructed wetlands treating Cd wastewater

Abstract

This study was conducted to investigate the long-term performance of subsurface-flow constructed wetland (SFCW) units treating a wastewater containing cadmium (Cd). The hydraulic retention time (HRT) was found to have significant effects on the SFCW performance, especially on Cd removal. During the 320 days of Cd feeding, the HRTs of 1, 3, 5 and 8 days resulted in the Cd removal efficiencies of 50, 90, 99 and 99%, respectively. The actual Cd breakthrough times in the SFCW effluent were found to be longer than the theoretical values calculated from the maximum adsorption capacities only, especially at the HRTs longer than 1 day, and were dependent on the operating HRT and Cd loading. Other mechanisms such as filtration, sedimentation and plant uptake were also responsible for Cd removal in the SFCW beds. The extents of Cd plant uptake were 21 and 6% of the Cd inputs for the SFCW units operating at the HRTs of 3 and 1 days, respectively. Based on Cd mass balance and fractionation analysis, the SFCW media were found to be most effective in Cd removal through adsorption of the residual and Fe/Mn oxide bound fractions. The results of this long-term study re-affirmed the necessity to determine actual breakthrough times of Cd or other heavy metals in the SFCW effluent which are dependent on HRT and Cd loading and are usually longer than the theoretical values calculated from the maximum adsorption capacity only.

Keywords:

• Cd removal
• subsurface-flow constructed wetland
• mass balance
• adsorption
• plant uptake
• Cd fractionation
• long-term performance

Acknowledgments

This study was supported by Inter University Post-Graduate Education Program on Environmental Toxicology, Technology and Management, Chulabhorn Research Institute and Asian Institute of Technology, Thailand. Special thanks are expressed to Mr. K.C. Balkrishna for his assistance in some data analysis.

Notes

^a = Langmuir equation in linear form.

^a Total adsorption capacity of SFCW media (mg Cd/g media).

^b Theoretical amount of Cd adsorbed by sand and gravel in each SFCW unit.

^a From Table 3.

^b Maximum amount of Cd adsorbed (g)/influent Cd loading (g/day).

^a Based on 170 days of Cd feeding.

^b Based on 320 days of Cd feeding.

^c () represents % of Cd input.

^d Represents the amount of Cd retained in the SFCW beds by filtration, sedimentation, adsorption on site walls of the SFCW units and other unaccounted mechanisms.

^a Based on 170 days of Cd feeding.

^b Based on 320 days of Cd feeding.

^c () Represents % of total fractions.