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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 8 Waste Receivor Tank Waste streams from the ultrafilter, cooling tower, RO unit, and boiler are collected in the waste receivor tank (60,000 gallon capacity). This tank provides a place to mix the several waste streams,providing a more predictable and uniform injectate.It also provides a damping effect for surges due to boiler blowdown. As a reservoir, it helps accomodate changes in rate of waste water disposal. Effects related to surging and mixing depend on the time that incoming parcels of liquid actually spend in the tank. For any one parcel, passage out of the tank may be described as a logarithmic dissipation. At nominal conditions, the working volume of the tank (<60,000 gal.) provides a residence time of about an hour or more. More precisely, a nominal half-time for clearance of an initial liquid parcel from the tank will vary depending on fill level and inflow rateb. Half-times on the order of one hour may be expected6. Relatively steady conditions of input to the tank are punctuated by blowdowns from the boiler, two or three times per day, when the rate changes suddenly from 3 gpm to 250 gpm, and back. Combining the surges into the steady boiler blowdown yields about 8 gpm,as an average for longer time intervals. Insertion of boiler blowdown imparts a nominal, momentary average concentration decrease of 5 to 10 percent, relative to the mixture composition prior to the boiler addition. Nominal average temperature effect will be a rise of 6° to 12°F, depending on current fill level7. Over time, surge effects from the boiler addition will both mix across the tank contents and dissipate due to discharge from the tank, the latter in accordance with current conditions which define residence half-life. Depending on conditions, this would permit a maximum of five to ten percent of the original (average) impact to be still present when the subsequent surge occurs$. Prior to boiler-induced surges, water composition in the tank could be within ±1 percent (relative)of a computed average based on composition and inflow rates of the three major,steady waste streams. Deviations from that, induced by boiler discharges, will not be large, and larger deviations will necessarily be associated with shorter lifetimes and smaller affected volumes. Effects of boiler alkalinity on the tank mixture will yield less than a 0.02 pH unit increase on the average fluid. This is less than the precision(±0.1 pH unit)used when estimating pH in the fluid mixtures. Transients near the point of insertion of the boiler discharge will be stronger than elsewhere, and cannot be defined sufficiently for precise modelling. Effects several times the average could still be regarded as negligible. Boiler discharges would have detectable effects in the waste receiver tank and transients there may have locally important effects. But, those effects may be expected not to propagate to the injection well in any important way. Thus, review of mineral deposition potential for fluid discharges from the receiver tank could be represented usefully by mixture compositions based on blending the three main waste stream compositions in proportion to their flow rates. It is recognized that dilutions of ten percent and temperature increases of 12°F are plausible, but quickly transient. Those transients should be recognized as being of the same relative magnitude as variations in the input water composition and temperature on which this assessment is based (Table 1). DON MICHELS ASSOCIATES -Missoula,Montana USA