Mercury pathways in municipal wastewater treatment plants

A nine-week sampling and analysis program was completed at a large municipal wastewater treatment plant to characterize the fate of Hg entering the plant. Mercury removal in primary treatment averaged 79%, and the average Hg removal across the entire plant was approximately 96%. Mercury loadings on the secondary (activated sludge) treatment process were elevated to near plant influent levels due to the recycle of spent scrubber water from sewage sludge incinerator emissions control equipment. This internal recycle of spent incinerator scrubber water resulted in elevated Hg loadings to the incinerators, and effectively reduced the Hg control efficiency of the emissions control equipment to near zero. Measurements indicate that approximately 95% of the Hg mass entering the plant is discharged to the atmosphere via sludge incinerator emissions. These results indicate that municipal wastewater treatment facilities can remove Hg from wastewater quite effectively; however, where wastewater sludge is incinerated, almost the entire mass of Hg removed from the wastewater can be discharged to the atmosphere.