in Sewage Treatment. The International Council for Local Environmental Initiatives (ICLEI) has reported a waste-reduction program in Stockholm, Sweden, fundamentally intended to reduce "discharges of hazardous substances into the sewer system," because ultimately the city wished to assure that treated wastewater effluent would be "harmless to the receiving waters of the Stockholm archipelago and the Baltic Sea" (6:2). The purpose for attacking metal discharges at the source--prior to their entry to the sewer--was stated bluntly: ". . . modern sewage treatment facilities are not capable of eliminating these metals [lead, cadmium, and mercury] from wastewaters" (6:2).
This program worked, but with an odd twist. In 18 months of 1990 and 1991, the mercury content of digested sludge at the waste treatment plant was reduced by 50% (in mg/kg dry weight); cadmium was reduced between 62 and 89%, and chromium was reduced by 84% (6:3). The twist was, the biota (bacteria) in the plant were taking up these metals very nicely; they were removing them from the wastewater, and somehow they were retaining them through aerobic and anaerobic processes--in obvious conflict with the claim that sewage treatment could not remove metals.
Balogh and Liang found in St. Paul, MN, that standard biological municipal sewage treatment processes removed 96% of the influent mercury (1:1181). They followed influent mercury in a mass balance sense throughout all five of their plant's stages and determined that 79% of the mercury was removed in the primary plant, a higher percentage removal than was attained for suspended solids removal (70%)--the actual point of primary sedimentation (1:1187). While these authors had difficulty with their mercury mass balance through the secondary part of the process (1:1187-1188), they found that 96% of the influent mercury (248 g/day) was removed by the primary and (biological) secondary systems (1:118
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