Variation of mercury fractionation and speciation in municipal sewage treatment plant: effects of mercury on the atmosphere

The speciation of mercury in various processing units of sewage treatment in autumn and winter were studied to understand the conversion and fate of mercury. The results show that the average concentrations of total mercury (THg) in the influent were 130.5 ± 69.8 ng/L and 231.3 ± 107.2 ng/L in autumn and winter, respectively, and the particulate mercury was the main speciation (accounting for 59.3% and 86.9%, respectively). The proportion of dissolved mercury increased after treatment, and the total removal efficiencies of THg were 78.9% and 90.8%, respectively. The release of mercury into the atmosphere during wastewater treatment was studied for the first time. The dissolved gaseous mercury levels in the influent in autumn and winter were 0.60 ± 0.40 ng/L and 0.34 ± 0.21 ng/L, respectively. The average gaseous element mercury concentration in aerobic tank air was 6.34 ± 0.49 ng/m³. The estimated mercury from aeration was 6.4 kg per year in China. The dissolved gaseous mercury will be released to the air if the sewage treatment plant is open-air. Closed sewage treatment and collection of waste gas treatment can reduce the influence of released mercury. Mercury releases into the atmosphere in the process of sewage treatment are one of the fates of mercury in sewage. Closed sewage treatment and collection of waste gas treatment are necessary to reduce the influence of released mercury. The reactive mercury levels in the influents of autumn and winter were 1.28 ± 0.49 ng/L and 1.96 ± 0.43 ng/L, and these levels account for a small proportion of THg, only 1.7% and 0.8%. Hg²⁺ were released by the degradation of organic matter in the secondary biological treatment. The THg levels in dehydrated sludge were higher than those in biochemical sludge but lower than the maximum limit of THg in agricultural sludge.