Electrochemical oxidation and coagulation of process waters
Electro-oxidation is an effective electrochemical technique which is used for oxidation of different kind of wastewaters. Many organic substances (like phenols) have been degraded successfully and inactivation of bacteria and bacteriophages in domestic waters has given promising results. The basic principle in electrochemical oxidation is that ozone and hydroxyl radicals are produced on the surface of the anode material and they will oxidize harmful compounds. This technique is also environmentally friendly because any chemicals in the oxidation process need be used.
Powerful oxidants (etc. hydroxyl radicals, ozone and chlorine) can be formed on anode by splitting water electrochemically (direct oxidation):
or by oxidizing chloride ions on anode (indirect oxidation):
Electrochemical coagulation is a promising wastewater purification technique which is also already in use in some applications. Electrodes are already known (Fe and Al) but still lots of research can be done optimising the parameters and finding out effectiveness in different applications. In EC treatment Fe- and Al-electrodes are used to release Al3+ and Fe2+ -ions which will coagulate organic and inorganic material from wastewater. Also some disinfection will happen if microbes are present in wastewater.
In electrocoagulation metal electrodes dissolve and produce coagulant. Dissolving reactions happen mainly at the anode surface as follows:
Main cathodic reactions are hydrogen evolution and pH change as follows:
Electrochemical oxidation experimental set-up
Sludge electro-dewatering
Dewatering and disposal of waste sludge is a major economical factor in the operation of wastewater treatment plants. 30–50% of the annual operating costs to be related to sludge dewatering alone. The mechanical dewatering methods can not reach a sufficiently high potential to drive the interstitial water through the very narrow pore spaces, therefore dewatering using mechanical means is extremely limited and requires substantial further treatment. Electric field application through the sludge segment causes electro-osmotic phenomenon which is fluid flow in charged particle matrix. Due to electro-dewatering, electro-osmosis is reported to reduce the interstitial and some of the vicinal water, thus resulting in a dryer sludge cake.
Electrokinetic soil remediation
Electrokinetic remediation is an innovative technology, which aims to remove heavy metals, organics and radionuclides from contaminated soil, sludge, wood and other, contaminated media. It involves an application of a low level direct current (40 – 100 mA, which induces the movement of ionic species in the media-water solution towards the oppositely charged electrode (electro-migration), a bulk flow of moisture present in the contaminated media from the anode to the cathode (electro-osmosis) and a movement of charged particles or colloids in the applied electric field (electrophoresis). The success of electrochemical remediation depends on the specific conditions encountered in the field, including the types and amount of contaminant present, soil type, pH and organic content. Once the remediation process is over, extraction and removal of heavy metal contaminants are accomplished by electroplating at the electrode, precipitation or co-precipitation at the electrode, pumping water near the electrode, or complexing with ion exchange resins.
Further development of electrokinetic remediation technologies, especially coupling them with other technologies, is somewhat attractive, especially for international markets: field-scale applications are limited and there is still potential for developing better understanding of the processes and improving the cost-efficiency.
Electrofenton process for soil remediation
Electrofenton process is a technique which can be used for the remediation of soil contaminated with organic contaminants. The success of the process has already been proved by various researchers who have carried out laboratory scale electrofenton experiments in its various forms including auto generation of oxidants as well as external addition of oxidants to treat contaminated soil.
In electrokinetic Fenton process, hydrogen peroxide passes through low permeability soil from anode to cathode by electroosmosis and produces hydroxyl radicals in the presence of iron in soil. Finally, hydroxyl radicals oxidize or decompose organic wastes.
The technique is found to be attractive due to the following facts:
1. Its an effective remediation technique for treating organic compounds in soil.
2. It involves strong and aggressive reactions which in most cases lead to the mineralization of organic contaminants.
3. The oxidant used in this process is environmentally friendly and leaves no residues in the soil.
4. Post treatment of soil is not necessary.
5. Can be applied to low permeable and heterogeneous layer of soil.
Partners
- Savcor Forest Oy, Mikkeli
- Aalto University
- Tsinghua University, China
- Hong Kong University of Science and Technology, China
Funding
- Tekes, The Finnish Funding Agency for Technology and Innovation
- The Regional Council of South Savo
- Maa- ja vesitekniikan tuki ry
- The City of Mikkeli