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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION <br />Design Basis <br />The proposed nanofiltration system will have a feed flowrate of 250 gpm which meets the 60% treated <br />permeate requirement at 90% recovery. The East Taylor Pond will provide some surge capacity for the <br />treatment plant for flows exceeding the 250 gpm. Overflow from the pond will be blended with treated <br />water at the discharge. <br />Pretreatment <br />Pretreatment is required to achieve higher recoveries for the NF treatment to minimize calcium carbonate <br />scaling on the membrane surfaces. Carbonate and sulfates are known to have scale forming tendencies. <br />Unlike sulfate, carbonate can be easily removed from the Elk Ridge water in a pretreatment step by pH <br />adjustment and air stripping. With a pKa for carbonic acid of 6.35, reducing the raw water pH to 6.0 will <br />shift most of the carbonate species to carbonic acid which forms an equilibrium between water and <br />gaseous CO2. <br />H2CO3 H H2O + CO2 <br />As the bubbled air replaces the CO2, more carbonic acid will convert until the carbonate ions are reduced <br />or depleted. While the CO2 stripping will slightly increase the pH of the water, additional pH adjustment <br />following the membrane treatment is expected to bring the water back into the neutral range, if necessary. <br />Following the air stripping, suspended solids from the East Taylor Pond must be removed in order to <br />extend the life of the membranes. The proposed system uses a pressurized mixed bed, granular -media <br />filtration system. The media filtration system will supply enough pressure to meet the feed pressure <br />requirements of the NF system. Solids from the media backwashing will be sent back to East Taylor Pond <br />on the opposing side from the plant water intake. <br />Membrane Separation <br />Pretreated, filter water will then be sent to the NF feed pump which will raise the pressure up to 300 psi. <br />The NF skid will be configured as an 8:4:2 arrangement with 4 elements per vessel and will include <br />recirculation of concentrate to achieve higher recoveries. This configuration allows for an overall recovery <br />of 87.5% and a rejection of 91 % of the sulfate and TDS. Permeate from the NF system will need to be pH <br />adjusted back into the 7.0 to 7.5 range with sodium hydroxide prior to discharge. <br />At higher recoveries, antiscalants must be considered for a membrane system because as the <br />concentrate stream flows decrease the concentrations can increase exponentially and exceed solubility. <br />The primary mineral scalant after the carbonate treatment is gypsum (CaSO4) which is expected to <br />exceed its solubility limit at the target recovery and must be managed with addition of an antiscalant to <br />inhibit precipitation. While this may be sufficient to keep the majority of scaling under control, fouling of <br />the membranes is likely to occur and periodic cleaning will be required. <br />A clean -in -place (CIP) tank with an immersion heater is included in the conceptual design for periodic <br />cleaning of membrane elements. Dry chemicals will be added to the CIP tank, mixed and heated prior to <br />use. The NF system is isolated into a closed loop condition and the heated solution is circulated through <br />the membrane system to remove scaling foulants. After required cleaning, the NF system is purged to <br />dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.5 <br />