Electrocoagulation (EC) - Science and ApplicationsBy Dr. Abe BeaglesMay 2004The Author is the President of Cal-Neva Water Quality Research Institute, Inc. in Newcastle, California.AbstractAlthough electrocoagulation is an evolving technology that is being effectively applied today for wastewater treatment, the paucity of scientific understanding of the complex chemical and physical processes involved is limiting future design and hindering progress in the mining and industrial sector of this country. The objective of this review and explanation is to explain the Haivala Targeted Water Fusion Technology and how it applies to already existing treatment process being used around the world and to bring the chemistry and physical processes involved into prespective.
Electrocoagulation (EC) - Science and Applications
By Dr. Abe Beagles
May 2004
The Author is the President of Cal-Neva Water Quality Research Institute, Inc. in Newcastle, California.
Abstract
Although electrocoagulation is an evolving technology that is being effectively applied today for wastewater treatment, the paucity of scientific understanding of the complex chemical and physical processes involved is limiting future design and hindering progress in the mining and industrial sector of this country. The objective of this review and explanation is to explain the Haivala Targeted Water Fusion Technology and how it applies to already existing treatment process being used around the world and to bring the chemistry and physical processes involved into prespective.
1. Introduction
One of the major challenges facing mankind today is to provide clean water to a vast majority of the population around the world. The need for clean water is particularly critical in Third-World 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, such as the US, are also experiencing a critical need for wastewater cleaning because of an everincreasing population, urbanization and climatic changes. The reuse of wastwater has become an absolute necessity. 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 phosphorous removal; as well as a range of physico-chemical processes that require chemical addition. The commonly used physico-chemical treatment processes are filtration, airstripping, ion-exchange, chemical precipitation, chemical oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis, volatililization and gas stripping. A host of very promising techniques based on electrochemical technology are being developed but are not yet to the commercial stage. One more process has been developed to the commercial stage and is being used in city wastewater treatment plants all over Europe and a few US cities have adopted parts of this technology. This process is known as the Harness Targeted Electric Water Fusion Technology or Electrocoagulation, we will refer to it as EC for simplicity in this review. One Man has patented this technology and to this day, the Finnish Scientist Erkki Haivala is the only man who really understands all of the processes that are occuring within the cells as they do their job. The scientific community has yet to understand this process and there has been very little consideration of the factors that influence the effective removal of ionic species, particularly metal ions, from wastewater by this technique. In the brief review, we wish to address these issues.
2. 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 industiral wastewater containing metals. 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 landfull leachates, defluorination of water, synthetic detergent effluents, mine wastes and heavy metal containing solution.
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3. Coagulation and Electrocoagulation
Coagulation is a phenomenon in which the charged particles in colloidal suspension are neutralized by mutual collision with counter ions and are agglomerated, followed by sedimentation. 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 ages 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 coloidal particles, previously stabilized by electrostatic repulsion, can approach closely enough for van der Waal’s 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 opposite charge, or with floc of metallic hydroxides generated withing the effluent.
The EC technology offers an alternative to the use of metal salts or polymers and poly-electrolyte addition for breaking stable emulsions and suspensions. The technology removes metals, colloidal solids and particles, and soluble inorganic pollutants from aqueoues media by introducing highly charged polymeric metal hydrozide 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. The advantages and disadvantages of EC technology are discussed below.
4. Advantages of EC
EC requires simple equipment and is easy to operate with sufficient operational lattitutde to handle most problems encountered on running.
Wastewater treated by EC gives palatable, clear, colorless and odorless water.
Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides. Above all, it is a low sludge producing technique.
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 lowTDS 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.
The EC technique can be conveniently used in rural areas where electricity is not available, since a solar paned attached to the unit may be sufficient to carry out the process.
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5. 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 in many places may be expensive.
An impermeable oxide film may be formed on the cathode leading to loss of efficiency of the EC unit. However, this does not occur in the Haivala unit for the process water is forced into turbulance and this oxide is never allowed to form.
High conductivity of the wastewater suspension is required. This is compensated for in the Haivala unit.
6. Descri ption of the Technology
In its simplest form, an electrocoaglating reactor may be 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. But, this arrangement is not suitable for wastewater treatment, because for a workable rate of metal dissolution, the use of electrodes with large surface area is required. This has been achieved by using cells with monopolar electrodes either in parallel or series connections. A simple arrangement of an EC cell with a pair of anodes and a pair of cathodes in parallel arrangement is shown in Fig. 1.
Figure 1
It essentially consists of pairs of conductive metal plates placed between two parallel electrodes and a dc power source as shown in Fig. 1. This setup requires 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 electrodes may be made up of the same or of different materials as the anode.
An arrangement of an EC cell with monopolar electrodes in series is shown in Fig. 2. As can be seen from Fig. 2, each pair of sacrificial electrodes is internally connected with each other, and has no interconnections with the outer electrodes.
Figure 2
This 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.
In one of the setups of the Haivala cell bipolar electrodes with cells in parallel have been used. In this instance the sacrificial electrodes are place between the two parallel electrodes without any electrical connections. Only the two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes. This cell arrangement provides a simple set-up, which facilitates easy maintenance during use. When an electric current is passed through the two electrodes, the neutral sides of the conductive plate will be transformed to charged sides, which have opposite charge compared to the parallel side beside it. The sacrificial electrodes in this case are also known as bipolar electrodes.
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Thus, during electrolysis, the positive side undergoes anodic reactions, while on the negative side, cathodic reaction is encountred. Consumable metal plates, such as iron or aluminum, are usually used as sacrificial electrodes to continuously produce ions in the system. The released ions neutalize the charges of the particles and thereby inititate coagulation. The released ions may remove the undersirable contaminants either by chemical reaction and precipitation, or by causing the colloidal materials to coalesce and then be removed by electrolytic 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 may alter the physical and chemical properties of water and contaminants. As a result, the reactive and excited state causes contaminants to be released from water and destroyed or made less soluble. Inert electrodes, such as titanium and the passage of alternating current, have been observed to remove metal ions from solution and to inititate coagulation of suspended solids. To ensure more effective removal of the undersirable ions, wastewater may be passed through a series of cells containing electordes made up of various metals. In such cases, the contaminated wastewater is passed through the annular spaces between the electrodes and is exposed to sequential positive and negative electrical fields. To optimize the removal efficiencies, the water characteristics such as pH, oxidation-reduction potential, and conductivity can be adjusted for specific contaminants.
In the EC process, an electric field is applied to the medium for a short time, and the treated dispersion transferred to an integrated clarifier system where the water-contaminant mixture separates into a floating layer, a mineral-rich sediment, and clear water. The aggregated mass settles down due to gravitational force. The clear water can be extracted by conventional methods.
7. AC versus DC Electrocoagulation
The direct current electrocoagulation (DCE) technology is inherent with the formation of an impermeable oxide layer on the cathode as well as deterioration of the anode due to oxidation. This leads to the loss of efficiency of the EC unit. In the Haivala cell this effect has been eliminated by the design of the cell inself and the addition of small holes drilled in the electrodes with the addition of a pressurized chamber on the outside of the electrodes so that a small jet of pressurized process water is constantly introduced into the process water flow creating a turbulence within the flow of the process water that continually washes the sides of the electrodes and prevents the buildup of this oxide. This in itself has eliminated the need to use AC current in the Haivala Cell.
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8. Alternating Current Electrocoagulation (ACE)
The US EPA has applied ACE technology for remediation of aqueous waste streams at Superfund Sites all over the United States. The ACE separator consists of either a paralled electrode unit in which a series of vertically oriented aluminum electrodes form a series of monopolar electrolytic cells through which the effluent stream passes, or a fluidized bed unit with nonconductive cylinders equipped with nonconsumable metal electrodes between which a turbulent fluidized bed of aluminum pellets is maintained. Compressed air is introduced into the EC cell to maintain a turbulent fluidized bed and to enhance the aluminum dissolution efficiency by increasing the anodic surface area. The basic flow diagram for the ACE separator with fluidized bed of alumimum alloy pellets entrained between a series of noncomsumable metal electrodes is shown in Fig 3. As can be seen from Fig. 3, an AC electric field is applied to the aqueous strem as it flows through the unit. As a result, a low concentration of aluminum dissolves from the fluidized bed and neutralizes the charges on suspende or emulsified particles. Once the charge species are electrically neutraized, they tend to coagulate and separate from the aqueous phase. The treated water is then transferred to a product separator where the water and solid phases are removed separately for reuse, recycling, additional treatment or disposal.
Figure 3
In a recent publication, (D. Mills, Am. Water Works Association 92 (2000)), describes what he calls a new process for electrocoagulation which is another prototype cell of the Haivala process. He states, the unit is made up of a ladder series of electrolytic cells containing iron anodes and stainless-steel cathodes. The electrolytic cells are constructed in such a way that a norrow concentric gap is maintained between the central anode and the surrounding cathode. Wastewater is allowed to flow through the ladder of cell, by way of a labyrinth of holes in the cathodes. Application of a low-voltage DC source to the cells produces iron hydroxide flocculant. This is the method that Haivala uses when treating water that is laden with Arsenic or Cynaide
9. Conclusions
The fact that electrocoagulation is now being successfully applied to contanimated water around the world is testament to its potential which is yet to be fully realized here in the US. The Haivala cell has clearly preformed some of the more complex requirements needed to totallly remove a wide range of contaminents from water. Some of these are: Hydrogen evolution has been controlled by the size of the cathodic reaction area and the electrode overpotential of hydrogen evolution. At the same time an anode has been designed to carry out several fundamental processes at the highest possible efficiencies. These include a corrodable part, that supplies the polyvalent coagulant ions to the solution at the lowest overpotential, a part that is efficient as an lectro-oxidation catalyst to form charged organics by partial oxidation, and an oxygen evolution part, that generate the oxygen at the highest efficiency bit in controlled amounts. The latter has been achieved by limiting the oxygen evolution electrode area. Electro-oxidation catalysts are available for shallow oxidation. These may be achieved by composite electrodes or unique multi-electrode arrangements. In addition the presence of sacrificial surfactants at low concentration is helpful to improve the efficiency of the coagulation process and ad/absorption processes. In addition the ionic make up of the solution is adjustable for optimization of the processes involved.
不好意思,没有办法整理,只能这样发了
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呵呵，谢谢，我需要啊
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