Last year the limit for copper in our NPDES effluent was reduced from a daily average of 0.83 lb/day to 0.11 lb/day. The daily maximum was reduced from 1.37 lb/day to 0.11 lb/day. The concentration of copper in the discharge now could not exceed 10 to 15 ppb, depending on the flowrate. Much of the copper in the effluent comes from corrosion of copper alloy heat exchangers, even though the corrosion is low (0.1 mils per year). The copper in the effluent was being removed by hydroxide precipitation in settling ponds.
Sulfide-based precipitation was selected for removing the copper. Retubing with stainless steel would be very expensive and would reduce the heat transfer of the heat exchangers. Hydroxide precipitation could not reduce the copper to less than about 20 ppb.
Sodium sulfide and dithiocarbamate were ruled out because of their known toxicity to our biomonitoring species Mysidopsis bahia (grass shrimp). Dithiocarbamate attached to insoluble starch was non-toxic to the shrimp in laboratory tests but seemed to decompose to toxic compounds in the ponds, possibly due to sun light. Trisodium trimercapto-triazine was non-toxic in field tests but at times was difficult to flocculate. A proprietary sulfide-based precipitant called MR-3300 was found to be non-toxic and reduces the copper to 1 to 10 ppb. By using filtration in addition to clarification lower copper concentrations are expected. The MR-3300 is sold by Southern Water Treatment, located in Greenville, South Carolina.
Reducing the copper in our effluent is expected to improve the environmental quality of the Galveston Bay.
Details of Reductions
Additional Information :
Meeting the new copper limit increases our costs by $50,000 to $60,000 per year.