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DocuSign Envelope ID: D07924E6-3D94-4398-AO66-B683COBAB33E February 24,2021 Angela Aalbers,Chantell Johnson Page 5 of 12 Reference: Biological Sulfate Reduction Bench-Scale Test Results-Revision 1 TEST OPERATION The two sets of test columns were constructed on September 26, 2020 by Stantec using clear, schedule 40 PVC, including required fittings and filled with the appropriate materials. A process flow diagram is included as Attachment 1. Before beginning the test, inoculation of the bacteria occurred. Colowyo provided sediment for Stantec to use as inoculant which was collected near the seep surface expression. The sediment was introduced into the influent water and column media to expedite the inoculation growth. The refrigerator was initially turned off so the columns could be at ambient temperature to encourage bacteria growth. Columns indicated readiness to begin the initial 4X and 8X bench-scale tests by the odor produced from the test systems. On October 8th, Stantec performed the first set of field analytical tests to verify successful inoculation and that adequate BSR was occurring. When analyzing the samples for sulfate at the various sampling points, the results showed that concentrations of sulfate had not decreased in the effluent. It was determined that the inoculation with the sediment from site was unsuccessful and an alternate inoculate was needed. This was later confirmed by measuring that the oxidation-reduction potential (ORP)was not in the necessary range (- 200 to-400 mV) and the conditions in the columns were not favorable for electrochemical reduction. The test was paused to find an alternate inoculate. A variety of alternative inoculants were identified, but the most readily available material was anaerobic digester sludge from the City of Broomfield Wastewater Treatment Plant. Approximately one liter of sludge was slurried with deionized water and pumped into the bottom ports of columns 2 and 3. Sludge deposited into the bottom of the columns was expected to be distributed throughout the columns when the continuous upward flow was started. Once columns 2 and 3 had been dosed, the normal column plumbing was re- connected and another half-liter of sludge was mixed with source water and pumped into column 1, effectively distributing bacterial inoculant into columns 1, 2, and 3. On October 19th, sulfate and ORP measurements showed that the re-inoculation was successful with ORP measurements in the appropriate range. The ORP measurements indicated that anaerobic conditions were established in the columns and sulfate reduction was approximately 1,000 mg/L. For the first round of testing, both trains were run in parallel with a mechanically interlocked pump running at roughly 2 mL per minute and at the target temperature. One test train was run at an 8X carbon dose and the other train at a 4X carbon dose. Sampling was performed twice within the first testing period to make sure that the reducing conditions remained favorable following the re-inoculation. Tests were run for 5 days to achieve steady state operation prior to collecting samples for the initial rounds of WET testing. Gathering treated water for the WET tests was performed over the course of 5 days after reaching steady state (a total of 14 days of operation.) This long collection time was necessary to gather the 12 liters of water that the lab requested. Samples were packed in ice and transported directly to GEI Consultants. A second round of testing was conducted identical to the first round, except carbon doses of 1X and 2X were applied. For this round of operation, WET testing was performed only for the 2X test condition. The third round of testing utilized this test assembly and followed the same overall procedure for the BSR with 9 days to achieve steady state operation and 5 days of sample collection. Each of these samples were submitted for WET testing on January 20th, 2020. Additional details of the chemical modifications for these two samples are described in the Supplemental Testing section. TEST RESULTS Table 3 and Table 4 show the results from the field analysis collected by Stantec for the 4X and 8X test conditions. Note that Column 3 effluent samples were collected at the midpoint rather than the effluent due to sedimentation issues with the columns. Stantec believes that this collection point is representative of the Desi� h community in nnh dpg llus0321-ppfss01lshared_projects\233001407\reports\6_bench_scale_report\rpLbench_scale_report_24feb2O20_final_rl.docx