While geothermal energy is, at its heart, a clean and sustainable resource, the extraction and use of geothermal fluids inevitably generate liquid and solid byproducts, collectively referred to as geothermal waste. These byproducts, primarily brines and sludge/scale, pose contamination risks to the natural environment. Proper containment is essential to ensure the long-term viability of geothermal energy projects. This chapter will explore the challenges associated with liquid and solid geothermal wastes, the importance of effective containment strategies using geotextile liners, and how advanced geotextile solutions are used to create environmentally sound waste management systems.
Types of Geothermal Waste
Geothermal Brines
Found in reservoirs deep underground, geothermal brines are highly saline solutions formed as water circulates through mineral-rich rock formations.
Characteristics
Brines are characterized by their complex chemical composition. Depending on the reservoir’s location, they typically contain a range of dissolved minerals and salts, including heavy metals, silica, and other compounds.
Environmental Hazards
Brines can contaminate surface and groundwater resources, harm aquatic ecosystems, and make water unsuitable for drinking or irrigation. A brine’s high salinity and heavy metal content can also damage vegetation and soil structure.
Management Strategies
Brine waste is typically managed by reinjecting it underground where it can’t contaminate the surface. In some cases, evaporation, precipitation, or filtration processes may be used to remove contaminants or recover valuable minerals. Lined evaporation ponds concentrate the brine, which is disposed of or used for mineral extraction. Lined impoundments are also used for temporary storage or to contain treated brines before reinjection or disposal.
Geothermal Sludge and Scale
Geothermal sludge and scale accumulate in pipelines, wells, and surface equipment as dissolved minerals precipitate from geothermal fluids during cooling or pressure changes.
Characteristics
Sludge and scale are solid or semi-solid materials composed of minerals precipitating from the fluids as they cool or experience pressure changes. Since different geological formations contain varying types and concentrations of minerals, the composition of sludge and scale varies significantly based on the geological source of the geothermal fluids.
Environmental Hazards
Sludge and scale can contain the same heavy metals and other contaminants found in brines and pose similar risks of leaching into soil and water if not properly contained.
Management Strategies
Sludge and scale are removed from pipelines, wells, and surface equipment during routine cleaning and maintenance and are typically transported to dedicated, lined impoundments or storage facilities. Like tailings ponds in mining or coal ash storage facilities, these impoundments prevent contaminants from leaching into soil and groundwater. In some cases, the sludge and scale may undergo treatment before disposal to separate valuable minerals or reduce the contaminant concentration.
Spent Geothermal Fluids
Even after the extraction of heat energy, the remaining geothermal fluids can still present significant environmental threats. Since even spent fluids can retain significant residual heat and dissolved solids, they require careful management.
Characteristics
Depending on the efficiency of the power plant and the cooling processes employed, spent geothermal fluids can still be significantly warmer than ambient temperatures. The fluids retain dissolved minerals, salts, and other compounds from the original geothermal reservoir. The concentration of these solids may even increase due to evaporation or other processes during energy extraction. The chemical composition of spent geothermal fluids varies depending on the geological formation of the geothermal reservoir, and they may contain heavy metals, arsenic, boron, and other potentially harmful substances.
Environmental Hazards
Discharging hot, spent geothermal fluids into surface water bodies can raise water temperatures, disrupting aquatic ecosystems and harming sensitive organisms, causing thermal pollution. The dissolved solids in spent geothermal fluids can contaminate surface and groundwater resources, making them unsuitable for drinking, irrigation, or other uses. If the fluids are improperly disposed of, heavy metals, salts, and other compounds can contaminate the soil, affect plant growth, and even enter the food chain.
Management Strategies
Reinjecting spent geothermal fluids into the subsurface is the most common and environmentally sound management strategy, helping maintain reservoir pressure, minimize land subsidence, and prevent surface contamination. Before discharge or reinjection, spent geothermal fluids are often cooled using cooling towers or other methods to reduce thermal pollution. In some cases, spent geothermal fluids may require treatment to remove contaminants or adjust their chemical composition before discharge or reinjection. When reinjection is not feasible, or while fluids are being treated, lined impoundments are used to store spent fluids, preventing contamination of the surrounding environment.
Environmental Regulations
Geothermal waste management is subject to multiple environmental regulations, which vary depending on local, regional, and national jurisdictions. Key regulatory considerations include water quality regulations governing fluid discharge, including limits on contaminant concentrations and thermal pollution; hazardous waste regulations for handling contaminated materials; air quality regulations for non-condensable gases; land use regulations for facility siting; and reinjection regulations for subsurface fluid management.
Geothermal operators are responsible for complying with all applicable regulations and implementing best practices for waste management, including thorough environmental assessments, effective containment systems, and monitoring waste streams to ensure compliance and minimize ecological impact.
Geomembrane Solutions for Waste Containment
Geomembrane use is crucial to assure safe and effective containment of geothermal waste. In lined evaporation ponds, geomembranes prevent highly saline brines from seeping into soil and groundwater. Similarly, in lined impoundments designed for geothermal sludge, scale, and spent fluids, geomembranes provide essential containment and minimize the risk of environmental contamination.
In each case, containment integrity is critical, so geomembrane liners must feature high chemical resistance and durability to withstand the challenging conditions of geothermal waste.
Looking Ahead
This chapter has looked at the fundamentals of geothermal waste management and geomembranes’ valuable role. In Chapter 5, we’ll focus on the expanding frontier of geothermal energy, Enhanced Geothermal Systems (EGS). Let’s take a look.
