GEOTHERMAL WASTEWATER TREATMENT AND DISPOSAL
Robert A. Ryder, Chief Engineer
Richard C. Casias, Hydrogeologist
Collins & Ryder Consulting Engineers
Larkspur, California 94939
INTRODUCTION
The development of geothermal heat sources for the production of energy is a rapidly increasing
activity in the Western United States that parallels worldwide interest. Major geothermal developments are occurring in usually volcanic-fault regions of the Earth where the underlying 1,000 C heated
magma is closer to the Earth's crust. These areas include: Italy, Iceland, New Zealand, Indonesia,
Japan, the Phillipines, Mexico, Central America, Hawaii and the United States [1]. In the search for
and development of alternative energy sources that has occurred in the past dozen years the stimulant
has been rapidly escalating oil prices and the decline in construction of nuclear power facilities.
Geothermal energy is often seen as an economically viable choice, particularly where energy produced
is purchased at more than 7 cents a kilowatt hour. The heat sources are of several types:
• Vapor dominated—dry steam that occurs at the geysers in Northern California, by far the most extensively developed U.S. geothermal field, and a few other places in the world, including Italy.
• Water dominated—Hot water that occurs much more often and is usually superheated and at high pressure.
Power generation from dry steam is by direct recirculation through steam turbines where the steam
loses pressure and heat. Two types of power generation are typical for water dominated geothermal
conditions. First by single or multiple flash of the liquid into steam that affords economical energy
recovery when the temperature is above 200 C. Organic-Rankine binary cycle heat extractors using
either isobutane or Freon as the vaporizing fluid are usually the economical generation processes used
if temperatures of hot water are below 200 C. These may be utilized with heat source temperature
as low as 70 C and the technology is similar to waste heat recovery or heat pump units.
There are other more experimental types including down hole, biphase turbines, and helical expander turbines that have been utilized in experimental development tests [2].
An overview of the known geothermal areas in the Western United States is shown in Figure 1.
The power production potential of this area is estimated to be about 30,000 megawatts of known
high temperature sources, and 32,000,000 megawatts of medium temperature sources. In perspective
this compares to the electrical generating capacity in the United States that is in excess of 500,000
megawatts so geothermal energy is by no means insignificant and within the past dozen years there
have been many experimental geothermal test programs sponsored by the U.S. Department of Energy
to further the use of geothermal energy.
California and Nevada are states where most of the commercial exploitation of geothermal energy
has occurred. The estimated power potential of some of the geothermal areas in these states are:
The Geysers 1,600 MW (dry steam)
Long Valley 6,000
Imperial Valley 4,500
Dixie Valley 600
The largest and probably easiest commercial development has occurred at The Geysers where there
are now about twenty power plants in operation or under construction that can produce about 1,400
MW of power. More recently there are power plants of 10 to 50 MW capacity being constructed in
Long Valley and the Imperial Valley areas. Several major constraints in the development of these
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