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To ensure success of a subsurface waste-disposal operation, surface pretreatment of the wastewater is generally required. Pretreatment can be expensive, but can make the difference between a successful operation and one subject to repeated difficulties and even failure.
Various problems can arise. Reduction of formation permeabilities and porosity, face plugging, and precipitation and polymerization reactions will all lead to diminished acceptance rates and excessive backpressure levels. Injection compatibility is of varying importance and is directly influenced by formation structure, interstitial water properties, and waste characteristics including solids particle size, pH, corrosiveness, viscosity, bacterial content, dissolved gases, temperature, and specific gravity.
Each disposal problem and its related solution must be evaluated separately. Basic pretreatment designs vary considerably and are usually tailored to the particular operation. Of basic importance is the minimizing of precipitate-producing reactions and the removal of suspended solids before injection into unconsolidated formations. This is less important in vugular or fractured hard rock areas.
Usually, a pretreatment operation will involve waste storage, separation of oil and/or suspended solids through flotation or gravity means, filtration through coarse sand or fine cartridge and diatomaceous earth, chemical fixation of pH, treatment to correct for corrosiveness or biological growths, followed by additional storage, and final pumping to the disposal well.
A thorough chemical and physical analysis of the wastewater and the receiving formation will result in an optimum design. Simplicity should be striven for. Although difficult, it may be possible to define and classify the various types of wastes which are deemed suitable for deep-well injection.
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