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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 7 Similarly, silica which threatens the RO membrane also threatens the injection well. In normal operations, waste from the RO units becomes mildly diluted in the mixture of waste streams which is eventually directed to the well. Chemicals added to prevent silica deposition in the RO membrane will appear as residuals in the injectate mixture. Whether or not those residuals are effective for preventing solids from forming in the waste mixture is an important factor not currently resolved. Even their effectiveness at the membrane, based on limited experimental testing, contains ambiguities. Those uncertainties could appear acceptable to in-plant operations, but are of greater concern for injections. Consequently this review assigns no effect to the residual dispersants. Cooling Tower The effect of evaporation on the concentrations of residual salts in cooling tower liquid is aimed at a factor of two. In that case, silica will not become a problem there. Nominal concentrations in blowdown from the cooling tower will be intermediate between those of ultrafilter waste and RO waste.Cooling tower blowdown carries residuals of corrosion inhibitors, algacides, and bactericides to mixtures in the waste receiver tank. Demineralizer The product stream from the RO unit goes to the demineralizer where remaining salts are removed by ion exchange.Output from the demineralizer is approximately as free of salts as distilled water, suitable as input for the boiler.Silica, at fractional mg/kg concentrations,leaves the demineralizer in product which feeds the boiler. Boiler Blowdown Additives to the boiler water contribute more to the its composition than do effects of evapo-concentration on the fluid's initial contents. Inflow water to the boiler contains very low levels of dissolved materials, mainly trace levels of silica. Corrosion inhibitors, etc., operate to maintain pH in the alkaline range, >9, nominally 10.4. This contrasts with the other waters which are maintained slightly acid, near pH 6.2. Higher pH strongly inhibits corrosion while enhancing solubility of silica. Normal liquid waste discharge rate from the boiler is continuous at 3 gpm(--4300 gal/day). In a steady-state that would result in about a 120-fold concentration effect between inflow and liquid discharge, but steady state is not expected. The low concentrations and generally low rate of contribution to the waste mixture cause this part of boiler blowdown to be an insignificant contribution to the overall waste composition and amount4. In addition to the continuous discharge, the boiler also will be discharged at a rate of 250 gpm two or three times per day for a few minutes each time (^-5000 to 7000 gal/day). Discharge at boiler temperature, (268°F, s�s25 psig) is associated with steam flashing which is controlled in a surge tank. Residual liquid at boiling temperature then enters the 60,000 gallon waste receiver tank. This inflow causes transients of temperature and concentration that cannot be readily modelled. Details of the transients depend on actual level of liquid in the tank, manner by which hot water enters the pool, and current inflow rates of other waste steams. DON MICHELS ASSOCIATES -Missoula,Montana USA