METHYLMERCURY FATE AND TRANSPORT STUDY

Methylmercury (MeHg) is a highly toxic and bioaccumulative contaminant, which can be produced via methylation by sulfate reducing bacteria in natural waters. The fate and transport of MeHg at wastewater treatment plants is largely unknown. The Sacramento Regional County Sanitation District performed a study to investigate the fate and transport of MeHg throughout its treatment process and recycle streams. This study was conducted from September 2004 through May 2005 at the Sacramento Regional Wastewater Treatment Plant (SRWTP). The SRWTP uses conventional primary treatment followed by high purity oxygen secondary treatment, chlorine disinfection, and discharges treated effluent to the Sacramento River in the town of Freeport, south of the city of Sacramento. The study utilized a two-phased approach; Phase 1A and Phase 1B. Phase 1A focused on the principal liquid train locations and return flows. Phase 1B sampling plan was developed from the results of Phase 1A and focused on data needed to investigate MeHg production and removal mechanisms in more detail. The study's main objectives were to evaluate the fate and transport throughout the plant, and to identify key factors that affect MeHg. Results of this study concluded that approximately 75% of MeHg in the influent was removed by the treatment process, with the majority or the removal occurring in the secondary treatment process. Approximately 98% of total mercury (THg) was removed by the treatment process. The key difference between the two mercury forms is their particle affinity. The partition coefficient for MeHg (log Kp ≈ 4.0) is approximately ten-fold lower than for THg (Log Kp ≈ 5.0). Mass transport models show that this attribute is the dominant factor controlling the difference in removal efficiencies between MeHg and THg at the SRWTP. Methylation may be occurring in the anaerobic digesters, as is evident by higher concentrations of MeHg in particles in the digesters and solids storage basins. Return flows from the on site Biosolids Recycling Facility and Solids Storage Basins increase the influent MeHg loading by approximately 13%.