Electrocoagulation


Electrocoagulation

Electrocoagulation is also known as Radio Frequency Diathermy or Short Wave Electrolysis. A fine wire probe or other delivery mechanism is used to transmit radio waves to tissues near the probe. Molecules within the tissue are caused to vibrate which lead to a rapid increase of the temperature, causing coagulation of the proteins within the tissue, effectively killing the tissue. At higher powered applications, full desiccation of tissue is possible.

Electrocoagulation in Water Treatment

Although electrocoagulation (EC) is an evolving technology that has for the past 100 years been effectively applied in waste water treatment, the paucity of scientific understanding of the complex chemical and physical processes involved as well as the limitations (in terms of size and cost) of the needed power supplies in the past, have curbed large scale applications and hindered progress. In addition, the powerful manufacturers of chemicals have been able to restrict the market penetration of this effective, environmentally friendly non-chemical procedurefact|date=July 2008.

With the latest technologies, reduction of electricity requirements, and miniaturization of the needed power supplies, EC systems have now become within reachvague of water treatment plants and industrial processes worldwide.

Theory

The need for clean water is particularly critical in developing countries. Rivers, canals, estuaries and other water-bodies are being constantly polluted due to indiscriminate discharge of industrial effluents as well as other anthropogenic activities and natural processes. In the latter, unknown geochemical processes have contaminated ground water with arsenic in many counties. Highly developed countries are also experiencing a critical need for wastewater cleaning because of an ever-increasing population, urbanization and climatic changes.

Both the treatment of wastewater prior to discharge and the reuse of wastewater have become absolute necessities. There is, therefore, an urgent need to develop innovative, more effective and inexpensive techniques for treatment of wastewater.

A wide range of wastewater treatment techniques are known, which includes biological processes for nitrification, denitrification and phosphorus removal, as well as a range of physico-chemical processes that require chemical addition. The commonly used physico-chemical treatment processes are filtration, air stripping, ion exchange, chemical precipitation, chemical oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis, volatilization, and gas stripping.

Technology

Treatment of wastewater by EC has been practiced for most of the 20th century with limited success and popularity. In the last decade, this technology has been increasingly used in South America and Europe for treatment of industrial wastewater containing metalsfact|date=July 2008. It has also been noted that in North America EC has been used primarily to treat wastewater from pulp and paper industries, mining and metal-processing industries. In addition, EC has been applied to treat water containing foodstuff waste, oil wastes, dyes, suspended particles, chemical and mechanical polishing waste, organic matter from landfill leachates, defluorination of water, synthetic detergent effluents, mine wastes, and heavy metal containing solution.

Coagulation

Coagulation is one of the most important physio-chemical reactions used in water treatment. The precipitation of ions (heavy metals) and colloids (organic and inorganic) are mostly held in solution by electrical charges. By the addition of ions with opposite charges, these colloids can be destabilized; coagulation can be achieved by chemical or electrical methods. The coagulant is added in the form of suitable chemical substances. Alum [Al2(SO4)3.18H2O] is such a chemical substance, which has been widely used for ageswhen for wastewater treatment.

The mechanism of coagulation has been the subject of continual review. It is generally accepted that coagulation is brought about primarily by the reduction of the net surface charge to a point where the colloidal particles, previously stabilized by electrostatic repulsion, can approach closely enough for van der Waals forces to hold them together and allow aggregation. The reduction of the surface charge is a consequence of the decrease of the repulsive potential of the electrical double layer by the presence of an electrolyte having opposite charge. In the EC process, the coagulant is generated in situ by electrolytic oxidation of an appropriate anode material. In this process, charged ionic species - metals or otherwise - are removed from wastewater by allowing it to react with an ion having an opposite charge, or with floc of metallic hydroxides generated within the effluent.

Electrocoagulation offers an alternative to the use of metal salts or polymers and polyelectrolyte addition for breaking stable emulsions and suspensions. The technology removes metals, colloidal solids and particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species. These species neutralize the electrostatic charges on suspended solids and oil droplets to facilitate agglomeration or coagulation and resultant separation from the aqueous phase. The treatment prompts the precipitation of certain metals and salts.

"Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitation of soluble metals 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." - Benefield L.D., Judkins J.F. and Weand B.L. 1982. Process chemistry for Water and Wastewater Treatment.

"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 electrocogulated flock tends to contain less bound water, is more shear resistant and is more readily filterable" - Woytowich D.L., Dalrymple C.W., Britton M.G. 1993. Electrocoagulation (CURE) Treatment of Ship Bilge Water for the US Coast Guard in Alaska. Marine Technology Society Journal, Vol 27, No. 1 p.62, Spring 1993.

Description of the technology

In its simplest form, an electrocoagulation reactor is made up of an electrolytic cell with one anode and one cathode. When connected to an external power source, the anode material will electrochemically corrode due to oxidation, while the cathode will be subjected to passivation.

An EC system essentially consists of pairs of conductive metal plates in parallel, which act as monopolar electrodes. It furthermore requires a DC power source, a resistance box to regulate the current density and a multimeter to read the current values. The conductive metal plates are commonly known as "sacrificial electrodes". The sacrificial anode lowers the dissolution potential of the anode and minimizes the passivation of the cathode. The sacrificial anodes and cathodes can be of the same or of different materials.

The arrangement of monopolar electrodes with cells in series is electrically similar to a single cell with many electrodes and interconnections. In series cell arrangement, a higher potential difference is required for a given current to flow because the cells connected in series have higher resistance. The same current would, however, flow through all the electrodes. On the other hand, in parallel arrangement the electric current is divided between all the electrodes in relation to the resistance of the individual cells.

During electrolysis, the positive side undergoes anodic reactions, while on the negative side, cathodic reactions are encountered. Consumable metal plates, such as iron or aluminum, are usually used as sacrificial electrodes to continuously produce ions in the water. The released ions neutralize the charges of the particles and thereby initiate coagulation. The released ions remove undesirable contaminants either by chemical reaction and precipitation, or by causing the colloidal materials to coalesce, which can then be removed by flotation. In addition, as water containing colloidal particulates, oils, or other contaminants move through the applied electric field, there may be ionization, electrolysis, hydrolysis, and free-radical formation which can alter the physical and chemical properties of water and contaminants. As a result, the reactive and excited state causes contaminants to be released from the water and destroyed or made less soluble.

Reactions within the electrocoagulation reactor

Within the electrocoagulation reactor, several distinct electrochemical reactions are produced independently. These are:

* Seeding, resulting from the anode reduction of metal ions that become new centers for larger, stable, insoluble complexes that precipitate as complex metal ions.

* 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.

* 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.

* Bleaching by the oxygen ions produced in the reaction chamber oxidizes dyes, cyanides, bacteria, viruses, biohazards, etc.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.

* Oxidation Reduction reactions are forced to their natural end point within the reaction tank which speeds up the natural process of nature that occurs in wet chemistry.

* Electrocoagulation Induced pH swings toward neutral.

Optimizing EC Reactions

Careful selection of the reaction tank’s material is essential along with control of the amperage, flow rate and pH. Electrodes can be made of iron, aluminum, titanium, graphite or other materials, depending upon wastewater to be treated and contaminants to be removed. Temperature and pressure have little effect on the process. In the EC process the water-contaminant mixture separates into a floating layer, a mineral-rich sediment, and clear water. The floating layer is removed by means of a patented overflow/removal method, and moved to a sludge collection tank. The aggregated mass settles down due to gravitational force, and is subsequently removed through a drainage valve at the bottom of the EC reaction tank, and moved to a sludge collection tank. The clear, treated water is pumped to a buffer tank for later disposal and/or reuse in the plant’s designated process.

Advantages of EC

* EC requires simple equipment and is easy to operate with sufficient operational latitude to handle most problems encountered on runningfact|date=July 2008.
* Wastewater treated by EC gives palatable, clear, colorless and odorless waterfact|date=July 2008.
* Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides.
* Flocs formed by EC are similar to chemical floc, except that EC floc tends to be much larger, contains less bound water, is acid-resistant and more stable, and therefore, can be separated faster by filtration.
* EC produces effluent with less total dissolved solids (TDS) content as compared with chemical treatments. If this water is reused, the low TDS level contributes to a lower water recovery cost.
* The EC process has the advantage of removing the smallest colloidal particles, because the applied electric field sets them in faster motion, thereby facilitating the coagulation.
* The EC process avoids uses of chemicals and so there is no problem of neutralizing excess chemicals and no possibility of secondary pollution caused by chemical substances added at high concentration as when chemical coagulation of wastewater is used.
* The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can be more easily concentrated, collected and removed.
* The electrolytic processes in the EC cell are controlled electrically and with no moving parts, thus requiring less maintenance.

Disadvantages of EC

* The sacrificial electrodes are dissolved into wastewater streams as a result of oxidation, and need to be regularly replaced.
* The use of electricity may be expensive with some EC systemsfact|date=July 2008.
* In some EC systems an impermeable oxide film can form on the cathode, leading to loss of efficiency of the EC unit.


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