(Ultrasonic-Aided Leachate Treatment)
The ultrasonic-aided leachate treatment process involves enhanced chemical treatment of acidic soil leachate solutions. These solutions, also known as acid mine drainage, are caused by the oxidation and dissolution of sulfide-bearing wastes, which produces sulfuric acid. The resulting acidic water leaches metal contaminants from the exposed waste rock and mine tailings, creating large volumes of toxic acidic leachates.
The ultrasonic-aided leachate treatment process uses an ultrasonic field to improve contaminant removal through precipitation, coprecipitation, oxidation, ion scavenging, and sorption (see figure below). These processes are followed by solid-liquid separation by filtration equipment using a filter press and a cross-flow microfilter connected in series. The time required for treatment depends on (1) the nature of acidic waste to be treated, (2) the treated water quality with respect to contaminant concentration, and (3) the rate at which the physical and chemical processes occur. The treatable leachate volume is scalable.
The major difference between this technology and conventional processes is the use of ultrasonic mixing instead of mechanical agitation in large tanks. Research indicates that an ultrasonic field significantly increases both the conversion rate of dissolved contaminants to precipitates and the rate of oxidation and ion exchange. Earlier studies by Atomic Energy of Canada Limited (AECL) revealed that the time required to precipitate heavy metals from aqueous solutions decreased by an order of magnitude in the presence of an ultrasonic field.
The ultrasonic-aided leachate treatment process is compact, portable, and energy-efficient. Safety and process controls are built in as necessary for handling mixed radioactive solutions. This process also generates minimal fugitive emissions and produces a treated effluent that meets applicable discharge limits. The process may be able to treat waste containing small amounts of dissolved or suspended organics.
The ultrasonic-aided leachate treatment process treats acid mine drainage contaminated with heavy metals and radionuclides. The process can also be combined with soil remediation technologies.
The ultrasonic-aided leachate treatment process was accepted into the SITE Emerging Technology Program in 1993. Under this program, AECL is developing and testing a pilot-scale unit to treat acidic soil leachate solutions containing low levels of metals and radionuclides.
The quality assurance and test plan was approved in October 1994. Laboratory-scale testing using acidic leachates from the Berkeley Pit in Butte, Montana and from Stanleigh Mines in Elliot Lake, Ontario, Canada is complete. The tests were designed to find optimal single and multistage treatment regimes to remove from the leachates a variety of dissolved species (iron, aluminum, manganese, magnesium, copper, zinc, uranium, radium, and sulfate), either as contaminants or as reusable resources.
Given optimum process chemistry, low energy (less than 5 kilojoules per liter) and low frequency (20 kilohertz) ultrasonic cavitation fields were sufficient to remove the dissolved species to levels meeting discharge requirements.
The energy input corresponds to a chemical conditioning time of a few seconds to tens of seconds. The underlying principles examined include lime and limestone precipitation, copper cementation, iron, and uranium oxidation, ion sorption, and ion scavenging.
A Phase 1 interim report summarizing the laboratory-scale results was issued in August 1995. A revised Phase 1 report was issued in February 1996. Testing of the pilot-scale system is scheduled to be complete in December 1996.
EPA PROJECT MANAGER:
Joan Mattox
U.S. EPA
National Risk Management Research
Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7624
Fax: 513-569-7676
TECHNOLOGY DEVELOPER CONTACT:
Shiv Vijayan or Les Moschuk
Atomic Energy of Canada Limited
Waste Technology
Chalk River Laboratories
Chalk River, Ontario, Canada K0J 1J0
613-584-3311, ext. 3220/6057
Fax: 613-584-9182