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WET Tests and the Perils of Polymer
Hugh G. Tozer, PE Woodard & Curran Inc. 2015 Annual Conference
§ Industrial Wastewater Treatment System § Whole Effluent Toxicity Overview § Evaluation of Toxicity § Conclusions
Dewatering the underground metals mine generates up to 3000 gpm wastewater
Mine’s wastewater contains heavy metals and exhibits chronic toxicity
Field tests of 10 precipitants identified a treatment process to meet metal and toxicity limits § Design flow = 2600 gpm § Max flow = 3000 gpm § Metals precipitated as § § § §
sulfides and hydroxides Coagulation with iron salts Ballasted sedimentation Flocculation used emulsion anionic polymer Used dry polymer in pilot tests but emulsion polymer in full-scale system due to flow and convenience
Effluent met metal limits from startup
Predict the eﬄuent concentra;ons that interfere with normal growth, development and reproduc;on
§ Freshwater organism found in littoral zones throughout the world § Females reproduce by cyclic parthenogenesis (asexually) § They reproduce when they molt § Typical clutch is 4 to 10 eggs in brood chamber § Three broods in 7-day chronic WET test
Technicians randomly distribute neonates to 10 cups at each dilution Control
The number of neonates produced in 7-day period are statistically compared to control group
Inhibi;on concentra;on, such as IC25 = eﬄuent concentra;on that causes a 25% reduc;on in growth or reproduc;on IC25 is compared to in-‐stream waste concentra;on (IWC) of eﬄuent If IC25 < IWC, there is a poten;al to inhibit aqua;c organisms
WET test results from treated mine water PASSING TESTS
History of WET Tests at the Mine
§ Mine had typically failed its WET tests for C.
dubia § In pilot study, treated mine dewatering water passed WET tests § 1st WET test after full-scale startup passed both C. dubia and minnows § 2nd and 3rd WET tests failed C. dubia § Operating at higher pH produced passing WET tests, but scale formation and costs were impractical
Average concentrations of heavy metals and ions in tests that passed and failed were similar
Metal concentrations in 100% effluent of failed tests were below those reported to be toxic in literature
Spiked 100% effluent with Ni and Zn → No toxicity observed at higher concentrations
Correla;ons ≠ Causa;on
Emulsion polymer appeared to be associated with failed WET tests FAILED WET TESTS
PASSING WET TESTS
25 of 28 passing WET tests used dry polymer 1 of 6 failing WET tests used dry polymer
Flocculant aid was a medium molecular weight, medium ANIONIC charge density emulsion
§ Emulsion polymers are
common in smaller systems § Researchers (e.g., Stover) have reported toxicity with polymers § Toxicity typically associated with cationic charged polymers § Emulsions use mineral oil and surfactants
System changed to dry polymer feed system with similar molecular weight and charge density
§ All WET tests have passed
since changing to dry polymer, even at order of magnitude higher metal concentrations § Dry polymer requires more attention by operators
Why did some tests using emulsion polymer pass?
§ Effluent metal and ion
concentrations were similar to other passing tests § Passing tests operated at higher pH § Large amounts of solids generated by softening water § Excess polymer may have been tied up with solids
§ Correlation ≠ Causation – just because one can
easily measure metals does not mean they are the source of toxicity § Convenience of emulsion polymers may not warrant their use if plant is subject to chronic WET tests § Emulsifying agents appeared to be source of toxicity and not the polymer