To ensure success of a subsurface waste-disposal operation, surface pretreatment of the waste water is generally required. Pretreatment can be quite expensive, but it can make the difference between a successful operation and one subject to repeated difficulties and even failure.

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 directly influenced by formation structure, interstitial-water properties, and waste characteristics, including particle size of solids, 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. The latter 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, and treatment to correct for corrosiveness or biologic growths. These procedures are followed by additional storage and final pumping to the disposal well.

A thorough chemical and physical analysis of the waste water and the receiving formation will result in an optimum design. Simplicity should be the aim. Although difficult, it may be possible to define and classify the various types of wastes that are deemed suitable for deep-well injection.