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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 Filtration Initial steps in preparing water for plant use involve removal of particulates. This is done in three stages. A multimedia filter removes the bulk of suspended particles, specifically those larger than about 20 microns (>0.0008 inches diameter). Backwash of this filter and subsequent filter pressing operations are forecast to produce about 25 pounds per day(10%moisture)of sludge for disposal. The water stream passing the multimedia filter is divided. Part goes to the cooling tower and receives a selection of conventional chemicals, but no additional processing. The other part passes through cartridge filters to remove particulates of 5 microns and larger (0.0002 inches). Cartridges are disposed of as solid waste. Fluids passing through the cartridge filters receive chemical additives. Chlorination aims to prevent bacterial growth. Dispersants aim to keep suspended particles suspended so they may be carried away in the liquid stream instead of fouling subsequent units. Chemically modified fluid enters the ultrafilter which removes particulates as small as 0.1 micron(0.000004 inches).This step is helpful for reducing fouling of the RO unit. Ultrafiltration also causes a mild separation of dissolved materials. Permeate (product) passing the ultrafilter is reduced in dissolved components by a factor(multiplier)in the range of 0.91 to 0.93. The effluent (waste) discharge is more concentrated in dissolved materials by a factor of 1.2 to 1.4, relative to the input concentration. The numerical values of these concentration multipliers vary modestly with the ratio of product to waste and with the condition (age) of the filter material. This stream also carries particulates and enters the waste receiver tank where it is the least concentrated of the three major streams mixed there. It may be regarded as providing a mild dilution effect. Product from the ultrafilter goes to the reverse osmosis (RO) unit. Reverse Osmosis Molecular size holes in the RO membrane enable a separation in which about three fourths of the input water passes through the membrane, becoming product, while carrying only five to ten percent of initial amount(by weight)of dissolved salts. Effluent from the RO unit at STIG will be more concentrated than the input by factors near 2.9. This stream carries the largest single contribution of dissolved components to the mixture intended for injection/disposal, but is volumetrically smaller than the ultrafilter waste during normal operations. A variety of additives are used to control chemical deposition on the membranes where concentrations can be locally high.Internal concentration multipliers exceed 2.9,which represents an average related to gradients near the membrane. Some added chemicals are intended for this environment, to suppress deposition on and in the membrane. Selection of additives depends on the composition of input liquid and the concentration factor involved with the separation effects. Control of pH is important and usually involves reducing natural alkalinity by adding a small stream of sulfuric acid. Silica in the WSWCPF source water is the component which limits the concentration effect that is practical in the RO unit at STIG. Normal operation at STIG results in those silica concentrations reaching, and sometimes exceeding, concentrations which could yield silica scales. Such deposition could plug the RO membrane.Careful and continued avoidance of plugging is one essential part of the proposed operations, which involves added chemical dispersants. DON MICHELS ASSOCIATES -Missoula,Montana USA