Total mercury and methylmercury migration and transformation in an A2/O wastewater treatment plant

Zhiqiang Gao; Limei Cai; Ming Liu; Zuo Zhang; Bo Gao; Wei Zhao; James V Cizdziel; Laiguo Chen

doi:10.1016/j.scitotenv.2019.136384

Total mercury and methylmercury migration and transformation in an A²/O wastewater treatment plant

Sci Total Environ. 2020 Mar 25:710:136384. doi: 10.1016/j.scitotenv.2019.136384. Epub 2019 Dec 30.

Authors

Zhiqiang Gao¹, Limei Cai², Ming Liu³, Zuo Zhang⁴, Bo Gao³, Wei Zhao³, James V Cizdziel⁵, Laiguo Chen⁶

Affiliations

¹ State Environmental Protection Key Laboratory of Urban Ecological Environment Simulation and Protection, South China Institute of Environmental Sciences, Ministry of Ecology and Environment of the People's Republic of China, Guangzhou 510655, China; Department of Chemistry and Biochemistry, University of Mississippi, MS 38677, United States.
² College of Resources and Environment, Yangtze University, Wuhan 430100, China.
³ State Environmental Protection Key Laboratory of Urban Ecological Environment Simulation and Protection, South China Institute of Environmental Sciences, Ministry of Ecology and Environment of the People's Republic of China, Guangzhou 510655, China.
⁴ Qingyuan Environmental Monitoring station, Qingyuan 511515, China.
⁵ Department of Chemistry and Biochemistry, University of Mississippi, MS 38677, United States.
⁶ State Environmental Protection Key Laboratory of Urban Ecological Environment Simulation and Protection, South China Institute of Environmental Sciences, Ministry of Ecology and Environment of the People's Republic of China, Guangzhou 510655, China. Electronic address: chenlaiguo@scies.org.

PMID: 31927291
DOI: 10.1016/j.scitotenv.2019.136384

Abstract

Municipal wastewater treatment plants (MWTPs) serve an essential role in reducing mercury (Hg) pollution. However, few studies quantified the transport and transformation of Hg through MWTPs, particularly plants based on the anaerobic-anoxic-aerobic (A²/O) process. Here, we present a mass balance for total mercury (THg) and total methylmercury (TMeHg) at the plant, and investigate the influence of pH, temperature, and dissolved oxygen on the occurrence and fate of methylmercury (MeHg) in the system. The concentrations of the THg and TMeHg in the raw sewage were 40.3 ± 26.6 ng/L (4.3 ± 2.7 g/day) and 1.9 ± 0.6 ng/L (193 ± 58 mg/day), respectively. Their concentrations in the plant's effluent water were 7.4 ± 1.5 ng/L (0.74 ± 0.2 g/day) and 0.04 ± 0.01 ng/L (3.9 ± 1.0 mg/day), corresponding to decreases of ~82% for THg and ~98% for TMeHg. Within the plant, only ~10% of the THg was removed with primary sedimentation, as Hg in the raw sewage was predominately in dissolved form. In contrast, a significant portion of TMeHg (~43%) was associated with incoming particulate matter. Much of the remaining Hg was removed in subsequent A²/O process and secondary clarifiers, with a ~78% of the THg entering the plant transferring to the dewatered sludge (concentration 1.05 ± 0.28 μg/g; 3.2 ± 0.8 g/day). These same steps decreased TMeHg in the water by ~95%, with <10% of that reduction being TMeHg transferred to the sludge (concentration 2.1 ± 1.1 ng/g; 6.2 ± 3.3 mg/day), suggesting >90% TMeHg degradation. In addition, the most important factor that impacted the variation of TMeHg concentrations was pH, then was temperature. Dissolved oxygen showed no relationship with TMeHg and DMeHg. Overall, this study demonstrates that A²/O MWTPs effectively remove MeHg from wastewater, however, sludge remains an important potential source of Hg to the environment.

Keywords: Anaerobic-anoxic-aerobic process; Influencing factors; Mass balance; Methylmercury.