Electrod Assisted Soil Washing Essay, Research Paper
Soil washing is generally considered a media transfer technology.
Typical environmental problems involve contaminated soil, sludge, surface water, and groundwater, usually containing widely distributed contaminants such as heavy metals, organics and their byproducts/decomposition products, and low-level radioactive materials. To develop an effective treatment for a contact-contaminated soil or other waste, it is necessary to understand its physical and chemical characteristics, including the distribution of the contaminants. Soil washing process can be defined as a water-based process for scrubbing soils ex situ to remove contaminants. The process removes contaminants from soils in one of two ways:
The concept involves literally washing the contaminates from the soil using specially designed equipment.
1. By dissolving or suspending them in the wash solution (which can be sustained by chemical manipulation of pH for a period of time).
2. By concentrating them into a smaller volume of soil through particle size separation, gravity separation, and attrition scrubbing (similar to those techniques used in sand and gravel operations).
A novel soil washing process that is called Electrod Assisted Soil Washing (EASW) technology has been invented and demonstrated by Harry W. Parker, and the graduate student Ramesh Krishnan. These persons are working in the continued development of this process. EASW process technology is assigned to Toxic Environment Control Systems, Lubbock, Texas. This firm supplied the funding for the invention and development of the process over the past five years.
Advantage of the EASW Soil Washing Technology and Theory of Operation
Soil washing is frequently the most cost effective means of remediating soils contaminated with organics, or heavy metals. Commercial soil washing technologies are frequently ineffective on silts and clays due to their small particle sizes, and hence large specific surfaces. The EASW process causes boiling to be initiated on the surface of the particles. The violence of boiling being nucleated on particle surfaces directly dislodges contaminants from the particle surface. Such violent scrubbing of very small clay and silt particles can not be achieved by intense external mixing and shearing as practiced with other soil washing technologies. Intense mechanical shearing is not successful in washing small particles because the small particles move within the water film surrounding them during the mixing and shearing. In contrast, the EASW process initiates violent boiling on the particle’s surface directly. Removal and destruction of a chlorinated hydrocarbon, pentachlorophenol.
Boiling is nucleated on the particle surface by superheating the liquid water surrounding the particles. Superheating is achieved by the flow of electric current through the soil slurry being washed. The local intensity of energy release is increased by the geometry of electrodes and insulating orifices employed in the EASW soil washing apparatus. One such geometry that has been tested for continuous EASW soil washing is shown in Figure 1. The diameter of the central insulating orifice can be varied as desired to control the local intensity of energy release in its vicinity. The electrodes are connected to commercial 60Hz power via transformers. The present apparatus allows up to 400 volts to be applied across the electrodes. The process is self-regulating. When steam is present in the orifice the electrical resistance increases and the power input decreases. A patent (3) has been granted for this unique soil washing technology.
Integration of EASW process into conventional soil washing processes
The EASW unit is easily integrated into a conventional soil washing flowsheet as shown in Figure 2. The feed to the unit can either be whole soil, or just the contaminated fines stream from an existing soil washing process. Feeding only the contaminated fines stream would significantly reduce the required size of the EASW processing facility. First the soil to be treated is mixed with recycled water, plus any make-up chemicals required to adjust the pH and electrical conductivity. The soil slurry then flows through the EASW unit where it efficiently scrubbed by the mechanism described in the previous paragraphs. The resulting steam is condensed and volatile contaminants separated. The soil slurry continues to a separator unit where free water is separated from the slurry. A centrifuge was used in the laboratory investigation, but a settler may be desirable for large scale operations. The soil is then rinsed with water. This rinsing should be accomplished in a counter-flow mode so that the soil leaving the rinse unit only contains uncontaminated rinse water. The contaminated water stream is treated to remove the bulk of the contaminants. A variety of options are available to treat the contaminated water. These include decanting of oil phases, biological treatment to destroy organics, precipitation of soluble materials, etc. The resulting recycle water does not have to be treated to discharge standards. The water treatment process just has to be adequate to prevent excessive build-up of contaminants in the process streams.
Cost of using the EASW process
It has been found based on their research experiment that the cost of commercial electric power for the EASW process would be about $15 per ton of soil. This price is based on experimental data and confirmed with heat-balance calculations. The amount of electrical energy might be further reduced by efforts to conserve energy and to optimize the apparatus. The incremental capital investment is estimated as 10 to 20 percent over that for conventional soil washing. (2) On this basis the EASW would be the least costly remediation technology for soils which can not be washed with existing processes.
A computer simulation of the EASW soil is being developed. This simulation will map theoretical voltage and current distributions within the apparatus shown in Figure 1. These data will then be used to calculate local power release rates and temperatures in the apparatus. This computer simulation is needed to optimize the electrode and orifice geometry and to plan for higher capacity soil washing units. In the future, the cost of soil washing by this technology will be much cheaper than today by optimization using computer simulations.
(1) Krishnan, R., H.W. Parker and R.W. Tock, “Electrode Assisted Soil Washing,” Journal of Hazardous Materials, Vol. 48, pp. 111-119, (1996).
(2) Snyder, B.M., R.M. Dennis, M.J.S. Roth, R. Krishnan, and H.W. Parker, “Evaluation of soilwashing process for ‘unwashable’ clays and silts from the Palmerton zinc site,” Remediation, pp. 69-80, Winter 1995/96, (1995).
(3) Parker, H.W., “Process for washing contaminated soil,” U.S. Patent 5,391,018, Feb. 21, (1995).
(4) EPA, “Engineering Bulletin — Soil Washing Treatment,” EPA/540/2-90/017, Sept., (1990).