Fate and Transport of Mercury in a Large Advanced Wastewater Treatment Plant

Wastewater samples were collected weekly from October 2004 to May 2007 from up to eight process locations and analyzed for mercury and methylmercury. Twenty four hour sampling at four hour intervals was performed on three separate days to characterize diurnal variation in mercury concentrations. Activated sludge Biological Nutrient Removal (BNR) treatment trains were sampled to examine the potential for methylmercury production. Sludge samples were collected from six locations for mass balance estimates. Total, dissolved, and methylmercury are reduced 99, 50, and 97%, respectively through the treatment process. Total mercury removal correlated well with total suspended solids. Most total mercury and TSS are removed during primary treatment, however most methylmercury is removed during the BNR secondary step. Average effluent concentrations of total, dissolved and methylmercury were 1.9, 1.4, and 0.04 ng/l, respectively. Net methylmercury mass was reduced, suggesting degradation, but nearly all total mercury is recovered in digested solids.