Figure 20.2 shows the surface equipment used in a typical subsurface waste-disposal system. Detailed discussion of surface treatment methods can be found in Warner and Lehr.6 The individual elements are listed in the following:
1. Sump tank. A sump tank or an open 113,550 to 189,250 L (30,000 to 50,000 gal) steel tank is commonly used to collect and mix wastestreams. An oil layer or, in a closed tank, an inert gas blanket is often used to prevent air contact with the waste. Alternatively, large, shallow, open ponds may provide sufficient detention time to permit sedimentation of particulate matter. Such ponds are often equipped with cascade, spray, or forced-draft
aerators to oxidize iron and manganese salts to insoluble forms that precipitate in the aeration ponds.
2. Oil separator. An oil separator is used when the waste contains oil, because oil tends to plug the disposal formation. The waste is passed through a settling tank equipped with internal baffles to separate the oil from the waste.24
3. Clarifier. A clarifier removes such particulate matter as polymeric flocs, dirt, oil, and grease. It is often a tank or a pond in which the detention time is long enough to allow suspended particles to settle gradually.25 The process may also be accelerated by adding a flocculating agent such as aluminum sulfate, ferric sulfate, or sodium aluminate.26 Tank clarifiers are often equipped with a mechanical stirrer, sludge rake, and surface skimmer that continuously remove sludge and oil.
4. Filter. A filter is used in some cases when coagulation and sedimentation do not completely separate the solids from the liquid waste in areas where sand and sandstone formations are susceptible to plugging. Filters with a series of metal screens coated with diatomaceous earth or cartridge filters are typically used.27 Where limestone formations with high solution porosity are used for injection, filtration is usually not required.
5. Chemical treater. A chemical treater is used to inject a bactericide if microorganisms could cause fouling of injection equipment and plugging of the injection reservoir.
6. Clear-waste tank. An unlined steel clear-waste tank is typically used to hold clarified waste before injection. The tank is equipped with a float switch designed to start and stop the injection pump at predetermined levels.
7. Injection pump. An injection pump is used to force the waste into the injection zone, although in very porous formations, such as cavernous limestone, the hydrostatic pressure of the waste column in the well is sufficient. The type of pump is determined primarily by the well-head pressures required, the volume of liquid to be injected, and the corrosiveness of the waste. Single-stage centrifugal pumps are used in systems that require well-head pressures up to about 10.5 kg/cm2 (150 psi), and multiplex piston pumps are used to achieve higher injection pressures.
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