DETOXIFICATION OF OUTFALL WATER USING NATURAL ORGANIC MATTER

To protect organisms in an ephemeral stream at the Savannah River Site, a proposed National Pollutant Discharge Elimination System (NPDES) permit reduced the copper limit from 25 μg/l to 6 μg/l at the H-12 Outfall. Numerous traditional and natural treatment methods, which act to remove copper from the wastewater, were considered including ion exchange, wetlands, and peat bed treatments. The low target copper concentration and highly variable outfall conditions presented a significant challenge for all of the technologies considered. In addition, costs and energy use for most of these alternatives were high and secondary wastes would be generated.

The Savannah River National Laboratory (SRNL) developed a new “detoxification” approach to treat the outfall water based on the EPA’s Biotic Ligand Model (BLM) and dissolved organic carbon (DOC). The BLM calculates copper toxicity based on water quality parameters such as pH, DOC, and percentage of the DOC. The BLM predicts how DOC in the water binds with copper in the water, limiting the availability of copper to interact with “biotic ligands,” and reducing impacts to aquatic organisms.

Modeling results from the BLM confirmed that an addition of DOC in concentrations similar to the receiving stream could reduce copper toxicity in the H-12 water allowing the goal of the proposed NPDES permit to be met even if copper is present at concentrations greater than 6 μg/l. SRNL tested seven organic carbon amendments to determine the best candidates. These products are high in humic compounds or similar molecules and are typically used as soil additives in organic farming. The amendment chosen was a commercial potassium humate. Based on the laboratory tests, SRNL developed a process control equation used in the detoxification system at the H-12 Outfall. The South Carolina Department of Health and Environmental Control approved the treatment concept and allowed the outfall to maintain the 25 μg/l copper limit. The resulting system includes two double-walled 5500 gal storage tanks, recirculation pumps, pH and flow meters, program logic controllers, and solar panels. The detoxification system, which began operating in June 2009, protects and improves the environment while significantly reducing energy use, land disturbance, and costs, as well as eliminating generation of secondary waste.