Abase metals mine and mill in the U.S. faced new discharge limits on heavy metals and whole effluent toxicity (WET) that it could not meet using its existing sedimentation pond. The facility discharged mining-influenced water generated from dewatering of its underground operations, process wastewaters from the mill, and surface runoff.
Metals Removal With a Catch
Extensive field tests identified a treatment chemistry that effectively reduced the water’s heavy metal concentrations and met WET test requirements. It combined sulfide and hydroxide precipitation, followed by coagulation, flocculation and ballasted sedimentation using the CoMag process.
The treatment plant met the metal limits from the first day of startup in December 2013. The operators were able to adjust the chemical doses to treat a range of influent conditions, including cadmium and zinc several times higher than the design basis.
The performance test also required three WET tests. WET tests evaluate the aggregate toxicity of the effluent to organisms such as Ceriodaphnia dubia (daphnia) and Pimephales promela (fathead minnows). Diverse conditions, such as total dissolved solids (TDS), ratios of cations, and concentrations of metals, can contribute to toxicity, making it difficult to determine the cause, particularly in the case of chronic toxicity, where the organisms survive but do not thrive.
It was thought that the mining effluent would pass the WET test if the treatment process adequately decreased the concentrations of heavy metals. The effluent passed the first WET test for both daphnia and fathead minnows, but failed the next two for daphnia. These were the first failures using the selected treatment chemistry; none of the WET tests during the pilot study had failed.
Although the effluent metal concentrations were well below the permit limits, the design team was concerned metals were still high enough to contribute to chronic toxicity in daphnia. An increase to the operating pH from 9.5 to 10.3 precipitated magnesium hydroxide, generated large quantities of solids, and decreased the effluent metal concentrations to non-detect or very low levels. The next WET passed, but the operating conditions were expensive and impractical.
Tracking Down the Problem
A laboratory that specializes in toxicity compared the treated mine effluent characteristics to its toxicity model, which drew from a massive database of WET tests. The model found that neither metals nor TDS were likely sources of the observed toxicity. Fractionation tests with the effluent also ruled out oxidizers and non-polar organics. Tests with ethylenediaminetetraacetic acid (EDTA) chelation suggested toxicity from divalent metals (e.g. nickel, cobalt and zinc) remained a possibility, or perhaps surfactants.
A review of the 34 WET tests performed during the pilot study and at full scale did not clearly identify metal concentrations above which the problem surfaced. Further lab tests of plant effluent spiked with nickel and zinc were inconclusive. The spiked samples did not exhibit greater toxicity than the effluent. It was time to start looking for other possible causes.
Rooting Out the Cause
The average concentrations of different constituents were similar when comparing passing and failing WET tests, and there was an overlap in metal concentrations for passing and failing WET tests; however, the data suggested C. dubia were more sensitive when an emulsion polymer was used. The maximum metal concentrations were higher for dry polymer compared to emulsion polymer when passing WET tests.
Of the 28 chronic WET tests using C. dubia that passed, 25 (89%) were from effluent where dry polymer had been used in treatment. Of the six chronic WET tests that failed, only one used dry polymer.
The design team had used an emulsion polymer because of its relative ease of operation at the full scale plant, which treated an average water flow of 2,600 gal per minute. The pilot study had used a dry polymer. The chemical supplier had reported that dry and emulsion polymer had the same active compound, medium molecular weight, medium charge density and anionic flocculant aid. The primary difference between the polymers was the mineral oil and surfactants used to emulsify the liquid polymer.
Past studies have reported toxicity associated with polymers used in wastewater treatment, particularly cationic polymers. The toxicity was typically attributed to the charge and not the form of the polymer.
The formation of light foam on the effluent surface suggested the presence of surfactants even at a polymer dose of 1 mg/L dry weight. The mill flotation chemistry also was adversely affected when it attempted to use the treated effluent in production, further supporting the presence of surface acting agents.
Enacting a Solution
The design-build team installed a dry polymer feed system to determine if it would eliminate the source of toxicity. Since its installation, the plant has completed more than 12 WET tests with no indication of chronic toxicity.
In the treatment of mining-influenced water, typical contributors to chronic toxicity are TDS and heavy metals. Past experience led the team to initially focus on metals, which likely were not the source of chronic toxicity observed in WET tests with C. dubia at this mine. Instead, the anionic polymer used as a flocculant aid appeared to inhibit reproduction. The source of toxicity appeared to be the emulsifying agents and not the polymer itself.