The problem and its opportunity
Produced water is salty wastewater that is brought to the surface during production of natural gas. Although the wastewater’s constituents occur naturally, they are considered contaminants and must be treated. Varying widely in concentration, these constituents include salt, organic compounds, and, in some cases, heavy metals and trace elements. A common disposal method is deep well injection; its cost depends on the volume of wastewater injected. What is needed is a simple treatment technique that reduces wastewater volume and removes contaminants.
Argonne scientists are investigating several plant-based approaches for treating the salty wastewater that is brought to that surface along with natural gas from gas wells.
Argonne has developed a low-cost, low-tech method for cleaning up wastewater and reducing the volume of produced water. Based on phytoremediation, the method uses green plants in an engineered plant ecosystem modeled on natural wetland ecosystems. The “ideal” phytoremediation plant would be a large, vigorous, salt-tolerant grass or grasslike species having a large area of aboveground leaves and stems and a dense, fibrous root system to act as a biological filter.
Large, green plants can move copious amounts of water through their roots and into the plant body. This water is evaporated from the leaves as pure water vapor – a process called transpiration. When selected, adapted plants are grown in contaminated wastewater by hydroponic techniques, and the root system functions as a biological filter. Contaminants taken up along with the water are degraded, metabolized, and/or stored in the plant body. Transpiration is enhanced by maximizing the leaf/stem area of the plant. Contaminant concentrations in plants can become thousands of times higher than those in the wastewater. The contaminated plant biomass can be digested or burned to reduce its volume. The residue can be processed to recover the contaminant, if valuable, or disposed of in environmentally appropriate ways.
Phytoremediation offers several major advantages over competing physico-chemical technologies (e.g., ion exchange). One advantage is the ability of selected plant species to adsorb contaminant ions from an extremely broad range of concentrations. Another advantage is selectivity. Selected plants have the ability to adsorb the target contaminant while ignoring other ions in solution. Other important advantages of phytoremediation: It is low-cost and low-tech.
Status of the investigation
Promising processes and plant species that were identified during the literature review and database development phases have been evaluated in greenhouse experiments. Over 80 species have been screened for salt tolerance and high evapotranspiration rates. For six species, detailed experimental data have been generated on transpiration rates, salt tolerance, nutrient requirements, and maximum salt concentration at which effective transpiration occurs. Transpiration rates are consistently above open-water evaporation rates at a saline concentration of at least 2% salt in the nutrient solution. For several species, these high evapotranspiration rates have been maintained up to 6% salinity.
An experimental batch-processing bioreactor with two compartments will be assembled in 1995 to simulate planned pilot and field installations. Each downstream compartment will process wastewater of increasing salinity with different plant species. The goal is to reduce the volume of saline production wastewater by 75% in less than eight days.