Electrocoagulation EC is a technology useful for the removal of sulfate and other anions from brackish groundwater. The process feeds brine into an EC chamber and uses an applied voltage to flow current through the system. In addition to this technology, the use of reverse osmosis RO alone and in conjunction with EC were evaluated as two alternatives for brine desalination. RO is a cost effective means of seawater desalination; it has been extensively implemented in other parts of the world, especially Europe and the Middle East.
On a purely scientific level, EC as pretreatment for RO produced potable water; however, this system is not economical long term on an industrial scale due to its high yearly operating costs. When implemented on a smaller scale or when the brackish water feed has a lower sulfate concentration than specified for the task, EC has the potential to be economical due to its lower yearly operating and waste disposal costs. Chemical Engineering Undergraduate Honors Theses.
Advanced Search. Skip to main content. It originates mainly from domestic, industrial, groundwater, and meteorological sources, and these forms of wastewater are commonly referred to as domestic sewage, industrial waste, infiltration, and storm- water drainage, respectively.
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For instance, domestic sewage results from people's day- to-day activities, such as bathing, body elimination, food preparation, and recreation, averaging about litres about 60 gallons per person daily. Where, the quantity and character of industrial wastewater is highly varied, depending on the type of industry, the management of its water usage, and the degree of treatment the wastewater receives before it is discharged. A steel mill, for example, might discharge anywhere from to , litres about to 40, gallons per ton of steel manufactured.
Less water is needed if recycling is practiced. A typical metropolitan area discharges a volume of wastewater equal to about 60 to 80 percent of its total daily water requirements, the rest being used for washing cars and watering lawns, and for manufacturing processes such as food canning and bottling Karadi, et al. Wastewater, specifically referring to all kinds of polluted water generated by human activities is now, not only a main cause of irreversible damage to the environment but a contributor to the depletion of our fresh water reserves, posing a major threat to the upcoming generations.
We carry out a lot of activities involving the use of large amounts of water, ranging from domestic and agricultural processes, to industrial activities. These are often carried out at the expense of plenty fresh water which is exhausted as a wastewater, and needs to be treated properly to reduce or eradicate the pollutants and achieve he purity level for its reuse Ali et al. Heavy metals are defined as metallic elements that have a relatively high density compared to water Fergusson, et al.
While the Encarta dictionaries defined them as metals having high relative densities, usually of 5. These heavy metals such as copper, lead, mercury, and selenium, get into water from many sources, including industries, automobile exhaust, mines, and even natural soil. Like pesticides, heavy metals become more concentrated as animals feed on plants and are consumed in turn by other animals.
When they reach high levels in the body, heavy metals can be immediately poisonous, or can result in long-term health problems similar to those caused by pesticides and herbicides. The generation of wastewater containing heavy metals is ever on the increase, due to the growing population of various industries employing processes that produce these contaminants as waste. Industries that carry out activities such as paint and pigment production, battery production, fertilizers and herbicides production, metals processing, etc.
This wastewater is usually treated by techniques including biological processes for nitrification, denitrification, and phosphorous removal and physico-chemical treatment processes for filtration, air stripping, ion-exchange, chemical precipitation, oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis, volatilization and gas stripping. The common physico-chemical processes such as coagulation and flocculation require addition of chemicals.
Electrochemical technologies which include electrocoagulation, electrofloatation, and electrodecantation do not require chemical additions Mollah et al. Presently, the techniques for the removal of heavy metals, such as chromium, cobalt, copper, lead, and nickel, from industrial wastewater include chemical coagulation, precipitation, ion exchange, adsorption, advanced oxidation, electrodialysis and filtration Abdel-Ghani et al. The unreliable results offered by these classical techniques and the need for eco-friendliness as a desired feature of water treatment technology have led to increasing global interest in electrocoagulation EC as a research subject REF.
In recent times, from the past few decades, various literary works in the environmental science field have indeed shown a growing interest towards the treatment of different types of wastewater by electrocoagulation EC. Electrocoagulation EC is an emerging technology that combines the functions and advantages of conventional coagulation, electro-flotation, and electrochemistry in water and wastewater treatment REF.
Electrocoagulation can be defined as the process of destabilizing suspended, emulsified, or dissolved contaminants in an aqueous medium by introducing an electric current into the medium Emamjomeh and Sivakumar, ; Top et al. It is considered to be potentially an effective tool in the treatment of various wastewaters and has shown to be highly efficient in the removal of heavy metals from aqueous medium Bazrafshan et al. It is an electrochemical technique for treating polluted water using electricity instead of expensive chemical reagents.
The chemistry behind the EC process in water is such that the positively charged ions are attracted to the negatively charged hydroxides ions producing ionic hydroxides with a strong tendency to attract suspended particles leading to coagulation. The use of electricity to treat water was first proposed in in England as documented by Chen et al, The application of electrolysis in mineral beneficiation was patented by Elinore in Electrocoagulation EC with aluminium and iron electrodes was patented in the united states in The electrocoagulation of drinking water was first applied on a large scale in the United states in I Tamer, At that time because of the relatively large capital investment and the expensive electricity supply, electrochemical water or wastewater technologies did not find wide application worldwide.
However, in the United States and former USSR extensive research during the following half century has accumulated abundant amount of information Tamer, With the ever increasing standard of drinking water supply and the stringent environmental regulations regarding the wastewater discharge, electrochemical technologies have regained their importance worldwide during the past two decades and processes such as electrochemical metal recovery electrocoagulation EC, electrofloatation EF and electrooxidation EO can be regarded nowadays as established technologies Butler et al. Electrocoagulation is a complex process, with many synergistic mechanisms operating to remove water pollutants metals, anions, organic compounds, etc.
Zaleschi et al. This technology is a treatment process which applies electrical current to treat and flocculate contaminants without having to add coagulants. The process involves the simultaneous removal of heavy metal ions, solids in suspension, organic emulsions and many others water pollutants, using electric energy and sacrificial metallic plates electrodes instead of expensive chemical reagents.
After the polymeric metal hydroxide species neutralize negatively charged particles, the particles bind together to form aggregates of flocs, resulting in pollutant removal by adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these flocs are removed easily from aqueous medium by sedimentation or flotation. Additionally, electrolytic gas As a result of their dissolution, the anodes disappear during the treatment, reaching a time when it is necessary to replace the anodes.
In the electrocoagulation process it is important to use soluble anodes made of aluminium, iron or other material, and cathodes made of the same material, or steel Zaleschi et al. Several studies have investigated the use of EC to improve the quality of industrial wastewater Niam et al. The process has been employed successfully to decontaminate waste streams of toxic cations and anions, as well as heavy metals, foodstuff, oil wastes, textile and dyes fluorine, polymeric wastes, organic matter from landfill leachate, suspended particles, chemical and mechanical polishing wastes, aqueous suspension of ultrafine particles, nitrates, phenolic waste, arsenic, and refractory organic pollutants including lignin Charturvedi, Also and importantly, electrocoagulation is applicable for the treatment of drinking water.
Generally, the EC process has been positively documented to treat the wastewater from commercial laundry services, textile manufacturing, metal plating, fish and meat processing, mining operations, municipal sewage system plants, and palm oil industrial effluent Ali et al. Electrocoagulation EC consists of number of benefits which include: environmental compatibility, ease of operation, amenability to automation, cost effectiveness, energy efficiency, and high sedimentation velocity, reduced amount of sludge, safety, and versatility Rajeshwar et al. These are all in addition to it removing pollutants, and producing hydrogen gas simultaneously as revenue to compensate the operational cost.
This is because these heavy metals find intense application in industrial processes in the form of construction Heavy metals, such as Copper, Lead, Mercury, and Selenium, get into water from many sources, including industries, automobile exhaust, mines, and even natural soil. For example, Cadmium in fertilizer derived from sewage sludge can be absorbed by crops. If these crops are eaten by humans in sufficient amounts, the metal can cause diarrhoea and, over time, liver and kidney damage.
Lead can get into water from lead pipes and solder in older water systems; children exposed to lead in water can suffer mental retardation. Conventional water treatment techniques are basically burdened with a number of drawbacks in the removal of heavy metals from wastewater. Thus, it becomes the purpose of this work to attempt to investigate the effectiveness of electrocoagulation in the removal of heavy metals in wastewater and solutions in general. Reutilization of water in the manufacturing cycle also has been identified as an effective means of monitoring environmental pollution and electrocoagulation provides an effective and viable means of achieving this end.
From the environmental perspective, the discharge of wastewater into the natural environment has been implicated as a major cause of environmental pollution REF. As the need for sustainability of the environment increases globally, electrocoagulation represent an effective tool towards meeting this need. As the awareness on the challenges of global warming increases globally in a world that has been ravaged by the menace of climate change, there is need to transcend to an eco- friendly water treatment technology, which is a standout feature of electrocoagulation that makes it inevitable within the water treatment circle.
The electrocoagulation process also can serve as a field of learning to students, researchers and industries. The work would proceed to investigate the effects of the various process variables including; electrode distance, initial and varied pH, electrolysis time, varied initial concentration, current, and water temperature on the efficient removal of the pollutants that characterize the wastewater. The energy consumption during the course of each experimental run will be determined. It is thus undeniable that 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.
Removal of Some Heavy Metals by Electrocoagulation
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 countries. Highly developed countries, such as the US, are also experiencing a critical need for wastewater cleaning because of an ever-increasing population, urbanization and climatic changes.
The reuse of wastewater 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 additions.
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. A host of very promising techniques based on electrochemical technology are being developed and existing ones improved that do not require chemical additions. These include electrocoagulation, electrofloatation, electrodecantation, and others. Even though one of these, electrocoagulation, has reached profitable commercialization, it has received very little scientific attention.
This process has the potential to extensively eliminate the disadvantages of the classical treatment techniques. Moreover, the mechanisms of EC are yet to be clearly understood 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. Organic and nitrogenous compounds in the discharge can serve as nutrients for rapid microbial growth under aerobic, anaerobic, or facultative conditions.
The three conditions above differ in the way they use oxygen. Aerobic microorganisms require oxygen for their metabolism. Whereas, anaerobic microorganisms grow in the absence of oxygen: the facultative microorganism can proliferate either in the absence or presence of oxygen, although using different metabolic processes. Most of the microorganisms present in wastewater treatment use the organic content of the wastewater as a source of energy to grow, and are thus classified as heterotrophs from a nutritional point of view.
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Biological treatment systems can convert approximately one-third of the colloidal and dissolved organic matter into stable end products and convert the remaining two-thirds into microbial cells that can be removed through gravity separation. The organic load present is incorporated in part as biomass by the microbial populations, and almost all the rest is liberated gas.
Carbon dioxide CO2 is produced in aerobic treatments, whereas anaerobic treatments produce both carbon dioxide and methane CH4. Systems that are not operated continuously have reduced efficiency because of changes in nutrient loads to the microbial biomass. The biological treatment systems also generate a consolidated waste stream consisting of excess microbial biomass, which must be properly disposed.
The commonly used chemical treatment processes are air stripping, ion-exchange, chemical precipitation, chemical oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis and chemical coagulation. In chemical coagulation, the process involves the removal of colloids and is commonly used for water purification and wastewater treatment. Coagulation is the most widespread and practical method of removing colloidal solids from wastewater. This is a process of destabilizing colloids, aggregating them, and joining them together for ease of sedimentation.
It entails the formation of chemical flocs that adsorb, entrap, or otherwise bring together suspended matter, more particularly suspended matter that is so finely divided as to be colloidal.
The chemicals used are: aluminium sulphate, Al2 SO4 3. Aluminium sulphate is commonly used for coagulation. The use of chemical coagulants, able to act as either negatively or positively charged ions, has highly improved the effectiveness of removal of colloids by coagulation Nemerow and Agardy, The coagulation mechanisms, depending on the physical and chemical properties of the solution, pollutant and coagulant, include charge neutralization, double layer compression, bridging and sweep Holt et al.
The process of coagulation separation consists of four steps. The initial step is simple: the chemical is added to wastewater. This is followed by the second step, where the solution is mixed rapidly in order to make certain that the chemicals are evenly and homogeneously distributed throughout the wastewater. In the third step, the solution is mixed again, but this time in a slow fashion, to encourage the formation of insoluble solid precipitates, the process known as "coagulation".
Natural coagulation is another area to be looked at. It is desirable to have a progressive replacement of these chemical coagulants with alternative coagulants and flocculants preferably from natural and renewable sources. Even though, scientific community is researching new natural coagulant sources as Nirmali seeds Strychnos potatorum , tannins cactus and specially Moringa oleifera Deepa et al.
The history of the use of natural coagulants is long. Natural organic polymers have been used for more than years in India. Africa and China as effective coagulants and coagulant aids at high water turbidities. They may be manufactured from plants seeds. Leaves and roots Deepa et al, These natural organic polymers are interesting because comparative to the use of synthetic organic polymers containing acrylamide monomers, no human health danger and the cost of these natural coagulants would be less expensive over to the conventional chemicals like since it is locally available most rural communities.
Natural coagulants have bright future and are concerned by many researchers because of their abundant source, low price, environment friendly, multifunction and biodegradable nature in water purification. Mineral treatment processes generally produce wastewaters including suspended and colloidal particles, such as clay particles. Dewatering of waste clay mineral tailings is an important part of mining and mineral processing activities worldwide. For instance, clay tailings which arise from hydrometallurgical processing of mineral ores are always seen but cause problems in waste treatment and disposal McFarlane et al.
Dewatering of the clay tailings is commonly achieved through flocculated, gravity- assisted thickening processes Mpofu et al. Most colloidal particles are stable and remain in suspension, and thus lead to pollution in water into which they are discharged or degrade re-circulation water in processing plants Rubio et al. The mutual repulsion among colloidal particles owing to the same sign of their surface charges is the main reason for the stability of the system. It is difficult to remove colloidal particles in gravitational sedimentation ponds or devices without any size enlargement treatment.
Size enlargement treatment may involve destabilization of particles or collision of particles to form aggregates. Destabilization means either a rise in ionic strength of the medium or a neutralization of the surface charge of particles by the addition of chemicals called coagulants or flocculants.
These chemicals promote different processes involved in the charge destabilization as they increase ionic strength, Electrocoagulation EC has thus been suggested as an advanced alternative to chemical coagulation in pollutant removal from raw waters and wastewaters. Wastewaters usually contain suspended solids and dispersed particles that do not sediment easily, mainly colloids. The colloidal systems are stable when they have the same charges on their surface, which cause repulsion between them Zaleschi et al. Due to this fact, colloids do not aggregate to each other and therefore do not form bigger particles that can be able to precipitate by themselves Riera — Torres et al.
It is observed that the quality indicators present significant removal efficiency, making this technology suitable for treatment of wastewater especially after conventional treatment Zaleschi et al. According to Can et al. While an infographic from Brian Mikelson, Halliburton explained the basic principles of the electrocoagulation process. Figure 2. According to shivayogimath et. Electrocoagulation technique is a technology for water and wastewater treatment which uses an electrochemical cell, where a DC voltage is applied to the electrodes that are corroded to generate a coagulant in which the electrolyte are usually water effluents.
This process has proven very effective in removing contaminants from water and is characterized by reduced sludge production, no requirements for chemical use and ease of operation. Vik et. At the turn of the nineteenth century, the electrocoagulation system was seen as a promising technology. In , A.
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E Dietrich was the first to patent the principle of electrocoagulation which was used to treat bilge water from ships. In the United States, a patent for the purification of wastewater using an electrocoagulation treatment using sacrificial aluminium and iron electrodes was awarded by J. T Harris, since then a wide range of water and wastewater applications followed under a variety of conditions. In the following decade, the process was used for purifying drinking water and was first applied in the United States in Treatment of wastewater by electrocoagulation has been practiced for most of the twentieth century with limited success and popularity.
More investigations in and showed that electrocoagulation technology was not developed for other industrial purposes because of the low level environmental awareness and insufficient financial incentives were probably reasons for abandoning the technology. However, since the concept became popular in North America, electrocoagulation has been used primarily to treat wastewater from pulp and paper industries, mining and metal processing industries.
In the last decade, this technology has been increasingly used in South America and Europe for treatment of industrial wastewater containing metals. Further studies have showed the possibility of treating natural water in small systems by two-stage filtration under the influence of aluminium ions which produced electrolytic dissolution. The inter-relationship between these three is shown in figure 2. In an electro coagulator, electrolysis is based on applying an electric current through the solution to be treated by electrodes.
The anode is a sacrificial metal usually aluminium or iron that withdraws electrons When the electrodes are immersed into a solution to allow a direct current to flow through the solution, a chemical change occurs at the electrodes. The fundamentals of the chemical change depend on the type of electrodes, the potential difference or electromotive force and the type of wastewater. The material used at the anode determines the type of coagulant released into the solution depending on the type of electrodes. Different electrode materials that could be used for this process includes: Aluminium, iron, stainless steel and platinum which have been reported by other researchers.
To pass current to each electrode and release the coagulant, a potential difference and a current flow is required.
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The potential difference can be assumed from the electrochemical half-cell reactions occurring at each electrode, which differ depending on the PH and species present in the solution. A half-cell is an electrochemical reaction from an electrode containing an oxidized and reduced species. Coagulation simply means a process used to cause the destabilization and initial coalescing of colloidal particles whereas flocculation is an aggregation of smaller particles into larger particles. In order to overcome the stability of particles in treated water a coagulant can be added either with chemical as shown above or electricity.
The coagulants released by the passage of electric current causes the aggregate in the particles to form into larger heavier mass known as flocs, which can be more easily removed by settling and filtration. Precipitation pathways describe the interaction of the pollutant with the metal hydroxide precipitates. When the coagulants precipitates, it can react with particles of pollutants binding them to the precipitate. The coagulant dose is a function of the chemistry of the treatment water, particularly the PH, alkalinity, hardness, ionic strength and temperature Binnie et al.
Since the overall density of the bubble particle complex is significantly less than the liquid, it rises to the surface where the floated material scum is skimmed off. There are main methods of floatation namely: Air floatation, Dissolved air floatation and Electrofloatation. The main difference between electrofloatation and more conventional floatation methods is the method of producing bubbles.
The basis of electrolytic or electrofloatation is the generation of hydrogen bubbles in dilute aqueous solution by passing direct current between two electrodes Chen et al. The effectiveness of the floatation process for removing pollutants depends on the type of electrodes, current effects on the mixing within the reactor, possible contacts between individual pollutants particles, coagulant and bubbles. Thus, the pollutant removal rate by floatation is expected to increase accordingly when the current in the electrocoagulator increased.
Electrophoretic motion tends to concentrate negatively charged particles in the region of the anode and positively charged particles in the region of the cathode Chaturvedi, The consumable metal anodes are used to continuously produce polyvalent metal cations in the region of the anode. These cations neutralize the negative charge of the particles moved towards the anodes by production of polyvalent cations from the oxidation of the sacrificial anodes Fe or Al and the electrolysis gases like hydrogen evolved at the anode and oxygen evolved at the cathode Chaturvedi, It is possible to use the decantation as a technique to eliminate the maximum amount of particles.
This remark is especially valid for colloids. Due to electrophoretic action negative ions which are produced from the cathode moves towards the anode and the combination of the metal cations with these negative particles At the anode small bubble of oxygen and at the cathode small bubble s of hydrogen are generated which are responsible for electrolysis of water thus water becomes electrolyzed as the process is carried out continuously. The flocculated particles are attracted by these bubbles and these flocculated particles float due to the natural buoyancy towards the surface.
The quantity of electricity passed through is actually responsible for dissolution and deposition of metal ions at the electrodes. Electrocoagulation operating conditions are mostly dependent on the chemistry of the aqueous medium, mainly conductivity and PH. Also other important characteristics are particle size, type of electrodes, retention time between plate, plate spacing and chemical constituent concentrations.
Generally, oxidation of organic matter by electrochemical treatment can be classified as direct oxidation at the surface of the anode and indirect oxidation from the anode surface which are influenced by the anode material Chaturvedi, It is generally believed that there are three other possible mechanisms involved besides electrocoagulation, which are electrofloatation, electrochemical oxidation and adsorption Kobya, et al.
Formation rates of these different species depend on PH of the medium and types of ions present and play an important role in the EC process. Simplified oxidation and reduction mechanisms at the anode and cathode of the iron electrodes as represented as follows Parga et al. The produced ferrous ions hydrolyse to form monomeric hydroxide ions and polymeric hydroxide complexes that depend on the pH of the The polymeric hydroxides, which are highly charged cations, destabilize the negatively charged colloidal particles allowing their aggregation and formation of flocs.
GR is recognized as an important intermediate phase in corrosion of FeO. Oxidation will cause the anode material to undergo electrochemical corrosion, whereas the cathode will be subjected to passivation, when the cell is connected to an external power sour. But since electrodes with large surface area for a workable rate of metal dissolution, the afore-mentioned arrangement is generally not suitable for the treatment of pollutant liquid medium. This requirement was satisfied by use of monopolar electrodes either in parallel or series connections.
In this set-up, a resistance box is necessary to The sacrificial electrodes may be made up of the same or of different materials as anode. An arrangement of an electro coagulation cell with monopolar electrodes in series is shown in Figure 2. Since cells that are connected in a series mode have higher resistance, a higher potential is necessary for a given current flow, although the same current would, however, flow through the electrodes.
On the other hand, cells connected in a parallel mode have their electric current divided between all the electrodes in relation to the individual resistance of the cell. The use of bipolar electrodes in a parallel connected cell is also possible. In such case as shown in Figure 2C, two parallel electrodes that are connected to the electric power source are situated on either side of the sacrificial electrode, with no electrical connection to the sacrificial electrode.
This way, conducting maintenance during use becomes easier in comparison due to the simple set-up. If an electric current is passed through the electrodes, the neutral sides of the connected plate will be transformed to charged sides, which have opposite charged compared to the parallel side beside it.
In this setup, the sacrificial electrodes are referred to as bipolar electrodes Mollah et al. Thus, during electrolysis, the positive side undergoes an anodic reaction, whereas a cathodic reaction takes place on the negative side. There are several methods how electrodes can be arranged in the EC system. Flow between the electrodes can follow a vertical or horizontal direction.
Electrodes can be monopolar or bipolar.
In the monopolar systems Fig. In the bipolar systems Fig. In the bipolar systems the side of the electrode facing the anode is negatively polarized and vice versa on the other side facing the cathode. The pollutant removal efficiencies and operating costs of monopolar and bipolar configurations have been compared in several studies Golder et al.
Slaughterhouse wastewaters have been treated with mild steel and aluminium electrodes arranged in bipolar or monopolar configurations Bagramoglu et al. Current efficiency for the dissolving of the mild steel electrodes was lower when electrodes were in bipolar configuration This is probably due to the higher electrode potential of the electrodes in bipolar arrangement and competing reactions taking place on the electrodes.
However, treatment cost was lower with a monopolar arrangement when the treatment was continued to the discharge limit.
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Similar results were reported when EC was used for the removal of fluoride from drinking water. As well as some disadvantages it has. The process has the capability to remove a large number of pollutants under a variety of conditions. Above all it is a low sludge producing techniques. If this water is reused, the low TDS level contributes to a lower water recovery cost. These technologies can be considered competing technologies and therefore the comparisons of treatment eficiencies are important.
As previously mentioned, reliable comparisons are difficult to conduct due to the dynamic nature of the process. Change of pH during the process and its effect on aluminium species formed has been studied by various authors. The formation of monomeric and polymeric aluminium hydroxides were compared when aluminium was added as AIC13 or by electrocoagulation.
According to results, there are no significant differences in the speciation of aluminium obtained by these two methods. The difference between electrocoagulation and chemical coagulation is mainly in the way of which aluminium or iron ions are delivered Avsar et al. The comparison between electrocoagulation and chemical coagulation is reported in Table 2. Table 2.