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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 12 particulates, not as dissolved forms. Particulates would be removed by the filtration sequence. Solubility calculations presume dissolved forms, so using the apparent concentrations for computing mineral stabilities could yield results that are biased, probably strongly biased toward high apparent supersaturation. By using apparent concentrations of heavy metals in the solubility calculations, several iron- and copper-bearing minerals can be identified as "super-saturated"in the processed waters. These should be regarded as minerals capable of forming in the STIG environment, but probably it is false to consider that they will form, due to factors described above. Even accepting such computed indications of mineral deposition, the amounts implied are small. They would likely be of negligible operational concern in any case. These same, or related, iron-bearing minerals could also form if corrosion occurred to ferrous parts of the plant equipment. Thus, development of iron mineral deposits could remain an item of interest, but their presence would more reasonably be taken as an indicator of corrosion, rather than an indicator of deposition from water-born components. Some representative mineral names and compositions are listed in Table 2, where they are indicated to be of negligible concern to plant operations. C. Phosphate Minerals Orthophosphate, ionic units of composition H3-nPO4-n, are common, natural components of water at low concentrations. Polyphosphates and phosphonates are larger, more complicated molecular and ionic forms, commonly added as water treatment chemicals and some are programmed for addition in the STIG plant. Polyphosphates tend to partly decompose to orthophosphate.Orthophosphate combines with calcium and other ionic components, notably fluoride, carbonate, and hydroxyl, to form a variety of mineral forms collectively known as apatite.The most common scale variety, hydroxy-apatite, and a natural form, fluorapatite, were indicated by WATEQ4F as not being over saturated in any mixed or single waste water. However, an uncommon form,containing magnesium, fluoride, and carbonate, was indicated to be over saturated. The potential amount of deposition is large, but the likelihood of any deposition seems poorly definedls When water treatment chemicals are present, especially dispersants, which contain polyphosphate, etc., apatite minerals that manage to form tend to be fine-grained. These may accumulate as sludge, to be removed periodically from places of accumulation(e.g.stagnant parts of cooling tower), or as suspended particles which are removed continuously by filtration units. Since sludge removal and cartridge filters are part of the system design for STIG, the formation of apatite may be regarded as a normal part of plant operations,presenting no serious or unforseen risks beyond timely removal operations. Higher pH and warmer temperatures favor formation of apatite minerals,so they could be formed intermittently during transient conditions within the waste receiver tank, especially in the vicinity of boiler surge input. If particulates do form there and become suspended, they could be removed by cartridge filters maintained between tank and injection well. Potentially, these phosphate minerals could form as hard crusts (scale) but the planned dispersants tend to prevent such development. Because the amounts which might form are DON MICHELS ASSOCIATES - Missoula,Montana USA