Electrocoagulation: An Overview
Electrocoagulation (EC), the passing of electrical current through water, has proven very effective in the removal of contaminants from water. Electrocoagulation systems have been in existence for many years (Dietrich, patented 1906), using a variety of anode and cathode geometries, including plates, balls, fluidized bed spheres, wire mesh, rods, and tubes. Powell Water Systems Inc. has taken a quantum leap in refining the EC process to increase removal rates and to lower capital and operating costs.
“The Electrocoagulation process is based on valid scientific principles involving responses of water contaminants to strong electric fields and electrically induced oxidation and reduction reactions. This process is able to take out over 99 percent of some heavy metal cations and also appears to be able to electrocute microorganisms in the water. It is also able to precipitate charged colloids and remove significant amounts of other ions, colloids, and emulsions. When the system is in place, the operating costs including electric power, replacement of electrodes, pump maintenance, and labor can be less than $1 per thousand gallons for some applications.
Potential applications to agriculture and quality of rural life include removal of pathogens and heavy metals from drinking water and decontamination of food processing wash waters.”1
Coagulation is one of the most important physiochemical operations used in water treatment. This is a process used to cause the destabilization and aggregation of smaller particles into larger particles. Water contaminants such as ions (heavy metals) and colloids (organics and inorganics) are primarily held in solution by electrical charges. Schulze, in 1882, showed that colloidal systems could be destabilized by the addition of ions having a charge opposite to that of the colloid (Benefield et al., 1982). The destabilized colloids can be aggregated and subsequently removed by sedimentation and/or filtration.
Coagulation can be achieved by chemical or electrical means. Chemical coagulation is becoming less acceptable today because of the higher costs associated with chemical treatments (e. g. the large volumes of sludge generated, and the hazardous waste categorization of metal hydroxides, to say nothing of the costs of the chemicals required to effect coagulation).
“Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitation of soluble metal species, as well as other inorganic species from aqueous streams, thereby permitting their removal through sedimentation or filtration. Alum, lime, and/or polymers have been the chemical coagulants used These processes, however, tend to generate large volumes of sludge with high bound water content that can be slow to filter and difficult to dewater. These treatment processes also tend to increase the total dissolved solids content of the effluent, making it unacceptable for reuse within industrial applications.”2
Electrocoagulation can often neutralize ion and particle charges, thereby allowing contaminants to precipitate, reducing the concentration below that possible with chemical precipitation, and can replace and / or reduce the use of expensive chemical agents (metal salts, polymer).
“Although the electrocoagulation mechanism resembles chemical coagulation in that the cationic species are responsible for the neutralization of surface charges, the characteristics of the Electrocoagulated flock differ dramatically from those generated by chemical coagulation. An Electrocoagulated flock tends to contain less bound water, is more shear resistant, and is more readily filterable. “3
Electrocoagulation has reduced contaminated water volume by 98%; and lowered the treatment cost by 90% for bilge water containing heavy metals and oil emulsions. Although Electrocoagulated water may vary because of the individual chemistry of process waters, a few examples of water treated by electrocoagulation include:
- The reduction of bacteria from 110,000,000 (standard plate unit) in sewage waste water to 2,700 bacteria per milliliter;
- The contaminants in oily waste from steam cleaning operations, refineries, rendering plants, and food processors are generally reduced 95 to 99%;
- Dissolved silica, clays, carbon black, and other suspended materials in water are generally reduced by 98%
- Heavy metals in water such as arsenic, cadmium, chromium, lead, nickel, and zinc are generally reduced by 95 to 99%
Electrocoagulation through the Powell Water Systems reaction chamber produces several distinct electrochemical results independently. These observed reactions may be explained as:
A. Seeding resulting from the anode reduction of metal ions that become new centers for larger, stable, insoluble complexes, that precipitate as complex metal oxides;
B. Emulsion breaking resulting from the oxygen and hydrogen ions that bond into the water receptor sites of oil molecules creating a water in soluble complex separating water from oil, driller’s mud, dyes, inks, etc.;
C. Halogen complexing as the metal ions bind themselves to chlorines in a chlorinated hydrocarbon molecule resulting in a large insoluble complex separating water from pesticides, herbicides, chlorinated PCB’s, etc.;
D. Bleaching by the oxygen ions produced in the reaction chamber oxidizes dyes, cyanides, bacteria, viruses, biohazards, etc.;
E. Electron flooding of the water eliminates the polar effect of the water complex, allowing colloidal materials to precipitate, and the increase of electrons creates an osmotic pressure that ruptures bacteria, cysts, and viruses;
F. Oxidation – Reduction reactions are forced to their natural end point within the EC chamber which speeds up the natural process of nature that occurs in wet chemistry;
G. EC induced pH swings toward neutral.
The process is optimized by controlling reaction chamber materials (iron, aluminum, titanium, graphite, etc.), amperage, voltage, flow rate, and the pH of the water. The technology handles mixed waste streams (oil, metals, and bacteria), very effectively. Variables such as temperature and pressure have little effect on the process. The best way to understand what will happen with a specific water is to test that water in the EC reaction chamber.
The electrocoagulation process has been successfully used to:
· Harvest protein, fat, and fiber from food processor waste streams.
· Recycle water, allowing closed loop systems.
· Remove metals, and oil from wastewater.
· Recondition antifreeze by removing oil, dirt, and metals.
· Recondition brine chiller water by removing bacteria, fat, etc.
· Pretreatment before membrane technologies like reverse osmosis.
· Precondition boiler makeup water by removing silica, hardness, TSS, etc.
· Recondition boiler blow down by removing dissolved solids eliminating the need for boiler chemical treatment.
· Remove BOD, TSS, TDS, FOG, etc., from wastewater before disposal to POTW, thus reducing or eliminating discharge surcharges.
· De-water sewage sludge and stabilize heavy metals in sewage, lowering freight and allowing sludge to be land applied
· Condition and polish drinking water
· Remove chlorine and bacteria before water discharge or reuse
The operating costs of electrocoagulation vary dependent on specific water treated. For example, municipal sewage water was treated for $0.24/1,000 gallons, and steam cleaner water containing crude oil, dirt and heavy metals was treated for $0.05/gallon.
References:
1. Dieterich, A. E., Electric Water Purifier, United States of America Patent No. 823,671 June 19, 1906.
2. Benefield, L. D., Judkins J. F. and Weand, B. L. 1982. Process Chemistry for Water and Wastewater Treatment. Prentice – Hall Inc., p. 212.
3. Woytowich D. L.; Dalrymple C. W.; Britton M. G.; 1993. Electrocoagulation (CURE) Treatment of Ship Bilgewater for the U. S. Coast Guard in Alaska. Marine Technology Society Journal, Vol. 27, No. 1 p. 62, Spring 1993.
4. Renk, R. R. 1989. Treatment of hazardous wastewaters by electrocoagulation. In: 3rd Annual Conference Proceedings (1989). Colorado Hazardous Waste Management Society.
5. Duffey, J. G. 1983. Electrochemical Removal of Heavy Metals from Waste water, Product Finishing, p. 72, August 1983
6. Franco, N. B. 1974. Electrochemical Removal of Heavy Metals from Acid Mine Drainage. Environmental Protection Agency Report EPA-670 12-74-023. May 1974
(1) United States Department of Agriculture (USDA), Agricultural Research Service: 12/18/95
(2) EPA, a SITE Superfund Innovative Technology Evaluation: EPA/640/S-937504. EPA, a SITE Superfund