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. 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. 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 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, electro flotation, electro decantation, etc. Even though 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.[3]
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 is such a chemical substance which has been widely used for ages for wastewater treatment. 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 Vander 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. [3]
Electrocoagulation is a technique involving the electrolytic addition of coagulating metal ions directly from sacrificial electrodes. These ions coagulate with turbidity agents in the water, in a similar manner to the addition of coagulating chemicals such as alum and ferric chloride, and allow the easier removal of the pollutants. 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 (i) with an ion having opposite charge, or (ii) with floc of metallic hydroxides generated within the effluent. The EC technology 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. [4]
2. MECHANISM OF ELECTROCOAGULATION
The mechanism of EC is highly dependent on the chemistry of the aqueous medium, especially conductivity and also on other characteristics such as pH, particle size, and chemical constituent concentrations. In the EC system, there are multiple electrochemical reactions occurring simultaneously at the anodes and cathodes. These mechanisms can be divided into the main mechanisms that cause destabilisation of pollutants, and side reactions, such as hydrogen formation. The most important reactions are summarised in Figure 1.
Figure 1. Schematic representation of typical reactions during the EC treatment [2]
Electrodes which produce coagulants into water are made from either iron or aluminium. In addition, there can be inert electrodes, typically cathodes, which are sometimes used as counter-electrodes in the system.
Aluminium
The electrolytic dissolution of the aluminium anode produces the cationic monomeric species such as Al3+ and Al(OH)2+ at low pH, which at appropriate pH values are transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n according to the following reactions:
Al → Al3+ (aq) + 3e−
Al3+ (aq) + 3H2O → Al(OH)3 + 3H+ (aq)
n Al(OH)3 → Aln(OH)3n
However, depending on the pH of the aqueous medium other ionic species, such as Al(OH)2+, Al2(OH)2 4+ and Al(OH)4− may also be present in the system. Under appropriate conditions various forms of charged multimeric hydroxo Al3+ species may be formed. These gelatinous charged hydroxo cationic complexes can effectively remove pollutants by adsorption to produce charge neutralization, and by enmeshment in a precipitate. Defluorination of water can be achieved using aluminum electrodes.
Iron
Iron upon oxidation in an electrolytic system produces iron hydroxide, Fe (OH)n, where
n = 2 or 3. Two mechanisms have been proposed for the production of Fe(OH)n.
• Mechanism 1
Anode:
4Fe(s) → 4Fe2+(aq) + 8e−
4Fe2+(aq) + 10 H2O(l) + O2(g) → 4Fe(OH)3(s) + 8H+(aq)
Cathode:
8H+(aq) + 8e− → 4H2(g)
Overall:
4Fe(s) + 10 H2O(l) + O2(g) → 4Fe(OH)3(s) + 4H2(g)
• Mechanism 2
Anode:
Fe(s) → Fe2+(aq) + 2e−
Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s)
Cathode:
2 H2O(l) + 2e− → H2(g) + 2OH−(aq)
Overall:
Fe(s) + 2 H2O(l) → Fe(OH)2(s) + H2(g)
The Fe(OH)n(s) formed remains in the aqueous stream as a gelatinous suspension, which can remove the pollutants from wastewater either by complexation or by electrostatic attraction, followed by coagulation. Wastewater containing chromium ions can be removed by the EC technique using iron as the sacrificial anode. [3]
The H2 produced as a result of the redox reaction may remove dissolved organics or any suspended materials by flotation.
3. FACTORS AFFECTING ELECTROCOAGULATION
There are various parameters which have an effect on the efficiency of the EC in removing the pollutants from water. The factors which are known to have an effect are:
• Material of the electrodes can be iron, aluminium and/or inert material (typically cathodes). Iron and aluminium ions and hydroxides have different chemistries and applications.
• pH of the solution has a
n effect on the speciation of metal hydroxides in the solution and also on the zeta potential of the colloidal particles. It also affects the dissolution of aluminium cathodes.
• Current density is proportional to the amount of electrochemical reactions taking place on the electrode surface.
• Treatment time or electric charge added per volume is proportional to the amount of coagulants produced in the EC system and other reactions taking place in the system.
• Electrode potential defines which reactions occur on the electrode surface.
• Concentration of the pollutants affects the removal efficiency because coagulation does not follow zeroth-order reaction kinetics but rather pseudo second or first-order kinetics.
• Concentration of anions, such as sulphate or fluoride, affects the composition of hydroxides because they can replace hydroxide ions in the precipitates.
• Temperature affects floc formation, reaction rates and conductivity. Depending on the pollutant, increasing temperature can have a negative or a positive effect on removal efficiency.
• Other parameters, such as hydrodynamic conditions and inter-electrode distance, may have effect on efficiency of the treatment and electricity consumption. [2]
4. APPLICATIONS OF ELECTROCOAGULATION
This technology has been increasingly used in South America and Europe for treatment of industrial wastewater containing metals. In North America, electrocoagulation has been used primarily to treat wastewater from pulp and paper industries, mining and metal-processing industries. In addition, it has been applied to treat water containing foodstuff wastes, oil wastes, dyes, suspended particles, chemical and mechanical polishing waste, and organic matter from landfill leachates, defluorination of water, synthetic detergent effluent, mine wastes and heavy metal-containing solution. [3]
The application can be broadly divided as follows:
· Removal of metal ions and/or hydroxides from synthetic solutions, groundwater or wastewaters. Typically high or complete removal could be obtained when treatment parameters are optimised. Aluminium, iron and combination electrodes can be used in EC system.
· Removal of organic material from wastewaters or synthetic solutions. High removal (> 70%) is typically obtained with optimum parameters. Aluminium, iron and combination electrodes can be used. In general, iron electrodes give higher organic matter removal, whereas higher colour removal is obtained with aluminium electrodes.
· Purification of surface waters from natural organic matter, inorganic pollutants or microbes. Typically high removal of pollutants (> 90%). Aluminium electrodes are more commonly used than iron electrodes in these applications. [2]
5. ADVANTAGES OF ELECTROCOAGULATION
The advantages of electrocoagulation technology are given below:
1. EC requires simple equipment and is easy to operate.
2. Wastewater treated by EC gives palatable, clear, colourless and odourless water.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. The electrolytic processes in the EC cell are controlled electrically with no moving parts, thus requiring less maintenance.
10. The EC technique can be conveniently used in rural areas where electricity is not vailable, since a solar panel attached to the unit may be sufficient to carry out the process.[3]
6. DISADVANTAGES OF ELECTROCOAGULATION
The disadvantages associated with electrocoagulation technology are:
1. The ‘sacrificial electrodes’ are dissolved into wastewater streams as a result of oxidation, and need to be regularly replaced.
2. The use of electricity may be expensive in many places.
3. An impermeable oxide film may be formed on the cathode leading to loss of efficiency of the EC unit.
4. High conductivity of the wastewater suspension is required.
5. Gelatinous hydroxide may tend to solubilize in some cases. [3]
7.COMPARISON OF CHEMICAL COAGULATION AND ELECTROCOAGULATION
CHEMICAL COAGULATION
|
ELECTROCOAGULATION
|
Chemicals are used
|
No addition of chemicals
|
Colour removal not possible for all type of colours
|
Crystal clear water can be obtained irrespective of the colour
|
No BOD and COD reduction
|
BOD and COD reduced by more than 50 %
|
High volumes of sludge is produced per day
|
Very less sludge volume
|
Hardness is increased
|
Hardness remains same or decreased
|
Scaling of pipes and channels due to lime
|
No scaling
|
Labour intensive
|
Easy to operate
|
8. CASE STUDY
The dairy industry is generally considered to be the largest source of food processing wastewater in many countries. Water is used throughout all steps of the dairy industry, including cleaning, sanitization, heating, cooling, and floor washing; naturally, the industry’s need for water is huge. In general, wastes from the dairy processing industry contain a high concentration of organic material such as proteins, carbohydrates and lipids, high BOD5 and COD, and high concentrations of suspended solids and suspended oil grease. The electrocoagulation process using aluminium electrodes is a reliable technique for removal of pollutants from dairy wastewaters. [1]
Dairy wastewaters are generally treated usually using biological methods such as activated sludge process, aerated lagoons, aerobic bioreactor, trickling filters, sequencing batch reactor (SBR), up flow anaerobic sludge blanket (UASB) reactor, up flow anaerobic filters, and bio coagulation. Aerobic biological processes are high energy intensive, whereas anaerobic treatment of dairy wastewater rejects very poor nutrient removal, and effluents treated by anaerobic biological processes need additional treatment the other hand, the physical/chemical methods that have been proven to be successful are coagulation/flocculation. This case study discusses the wastewater taken from the local dairy factory in Iran with 25000 (mean value) kg milk per day processing capacity. The setup is shown in figure 2. [1]
Figure 2. Experimental set up of the EC unit [1]
In each run, wastewater (supernatant) after 12 h settling time was poured into the electrocoagulation cell. The experiments were performed in a bipolar batch reactor as shown in Figure 2, with six aluminium electrode connected in parallel. Only the outer electrodes were connected to the power source, and anodic and cathodic reactions occurred on each surface of the inner electrode when the current passed through the electrodes. The temperature of each system was maintained at 25 ± 2∘C. During the runs, the reactor unit was stirred at 150 rpm by a magnetic stirrer to allow the chemical precipitate to grow large enough for removal. In this study, the raw dairy wastewater was allowed to settle in a preliminary settling tank before the electrocoagulation process. The parameters considered in the study were COD, BOD5, TSS, TC and FC. Electrocoagulation is efficient and able to achieve 98.84% COD removal, 97.95% BOD5 removal, 97.75% TSS removal, and >99.9% bacterial indicators at 60 V during 60 min. The values of the parameters before and after the treatment are shown in table 2. [1]
Table 2. Influence of electrocoagulation process using aluminium electrodes on dairy wastewater quality parameters [1]
Parameters
|
Raw waste water
|
12h settled waste water
|
Treated effluent after EC at 60 V
|
Permissible levels ( Iran standard)
|
COD (mg/L)
|
7855
|
6114
|
70.92
|
60
|
BOD5 (mg/L)
|
3486
|
2919
|
43.45
|
30
|
TSS (mg/L)
|
1724
|
734
|
16.52
|
60
|
TC(MPN/100mL)
|
4.39 x 10^6
|
3.53 x 10^6
|
361
|
1000
|
FC(MPN/100mL)
|
3.27 x 10^6
|
2.75 x 10^6
|
28
|
400
|
REFERENCES
1. Edris Bazrafshan, Hossein Moein, et.al, 2013, Application of electrocoagulation process for dairy wastewater treatment, Journal of Chemistry
2. Mikko Vepsäläinen, 2012, Electrocoagulation in the treatment of industrial waters and wastewaters
3. M. Yousuf A. Mollah, Robert Schennach, et. al, 2001, Electrocoagulation (EC)- science and applications, Journal of Hazardous Materials 29–41
4. Dr N. N. Mahapatra, Electro-Coagulation Process for the Waste Water Treatment
5. Rahmani AR., 2008, Removal of water turbidity by the electrocoagulation method, Journal of research in health sciences
