Overview of the Technology 2 2 2. Electrocoagulation
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1. ELECTROCOAGULATION – OVERVIEW OF THE TECHNOLOGY 2 1.1 Coagulation 2 1.2 Electrocoagulation - the process 2 2. ELECTROCOAGULATION (EC) – ADVANTAGES OF THE SYSTEM 3 2.1 Applications 4 2.2 Benefits of the technology 5 3. EXAMPLES OF PERCENTAGE REMOVAL RATES ACHIEVED IN VARIOUS ELECTROCOAGULATION TRIALS 5 3.1Treatment of abattoir processing line wastewater and stick water (wastewater from a low temperature rendering plant) 5 3.2 Copper products manufacturer 6 3.3 Nickel smelter 6 3.4 Concentrated wastewater from tannery 7 3.5 Concentrations of ions in a wastewater before and after electrocoagulation 7 4 OPERATING PROCEDURES AND COST 7 5 CONCLUSIONS 8 6 CONTACTS 8 1 Electrocoagulation – overview of the technology 1.1Coagulation Coagulation is one of the important methods used in water treatment. It is a process used to cause the destabilization and subsequent aggregation of smaller particles into larger complexes. Water contaminants such as ions (heavy metals) and colloids (organic and inorganic) are primarily held in solution by electrical charges. In 1882 Schulze showed that colloidal systems could be destabilised by the addition of ions of the charge opposite to that of the colloid (Benefield et al, 1982), The destabilized colloids can then aggregate and subsequently be separated from the wastewater. Coagulation can be achieved by both the chemical or electrical means. Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitation of soluble species and other pollutants from aqueous streams. Alum, lime and polymers are some of the chemical coagulants used. These processes, however, tend to generate large volumes of sludge with high bound water content which can be difficult to separate and dewater. The processes also tend to increase the total dissolved solids content of the effluent, making it unacceptable for reuse within industrial applications. Other aspects of chemical coagulation are becoming increasingly less acceptable. The disposal cost of the large volumes of sludge (generally of fairly high hazardous waste category), the cost of the chemicals required to achieve coagulation and the environmental issues associated with the process are critical problems in many industries. 1.2 Electrocoagulation – the process Electrocoagulation, the passing of the electrical current through water, has proven very effective in the removal of contaminants from water. Electrocoagulation systems have been in existence for many years (Dieterich, patented 1906),using a variety of anode and cathode geometries, such as plates, balls, fluidized bed spheres, wire mesh, rods, and tubes. Although the electrocoagulation mechanism resembles the chemical coagulation – the cationic species being responsible for the neutralization of surface charges – in many ways it is very different. Electrocoagulation is a process of destabilizing suspended, emulsified or dissolved contaminants in an aqueous medium by introducing electrical current into the medium. The electrical current provides the electromotive force causing the chemical reactions. Sacrificial electrodes supply either aluminium or iron cations that assist in a formation of more stable compounds – the contaminants form precipitating hydrophobic entities. Several distinct electrochemical processes occur during the electrocoagulation process independently. These observed reactions might be explained as: . Seeding resulting from the anode reduction of metals ions that become new centres for larger, stable, insoluble complexes, precipitating as complex metal oxides. Emulsion breaking – forming water soluble complexes and that way separating oil, drillers mud, dyes, inks, etc from the water. Halogen complexing – as the metal ions bind themselves to chlorine and form chlorinated hydrocarbon molecules – resulting in the formation of large insoluble complexes and isolating pesticides, herbicides, chlorinated PCBs, etc. Oxygen species that are produced in the electrocoagulation reaction chamber provide oxidization of chemical substances in dyes , cyanide containing chemicals, and also reducing bio-hazards through oxidization of bacteria, viruses, etc. Electron flooding in water affects its polarity, allowing colloidal materials to precipitate. The flood of electrons also changes the osmotic pressure resulting in the rupturing of cells walls of bacteria, cysts, and viruses. Oxidation and reduction reactions are forced to their natural end point. Electrocoagulation can speed up the naturally occurring processes. Electrocoagulation induced PH typically shifts towards neutral. The wastewater passes through a chamber with parallel vertical cathodes and anodes constructed of metals selected to optimize the removal process. The two most common plate materials are iron and aluminium. In accordance with the Faradays Law, the metal electrodes are sacrificed and slowly dissolve into the liquid medium. The principal + - cathodic reaction is the reduction of hydrogen ions o hydrogen gas (2H + 2e =H2). The principal anodic reaction is the release of metal ions ( depending on the anode material) into solution (eg. 3Al = Al3+ + 3e-). The metal ions tend to form metal oxides that electromechanically attract the destabilized contaminants. 2. Electrocoagulation (EC) – advantages of the system The EC system is the most cost effective and elegant solution for many water treatment problems. EC systems have a small footprint. EC systems require no use of additional chemicals to achieve the desired results. EC systems use a low amount of power. The technology handles mixed waste streams (for example water polluted with oil, metals, and bacteria) very effectively as different processes take place at the same time and wastewater characteristics (pH, standard redox potential, conductivity) can be varied and optimized for maximum removal efficiencies of specific material treated. The polarity of the electrodes is reversed periodically to assist in cleaning of the electrodes. After the treated wastewater leaves the electrocoagulation chamber, the destabilized colloids are allowed to flocculate in the “development” tank and then separated in an integrated system. Sometimes polymers can be added to enhance the flocculation, but in most cases they are not required. The sludge can be further de-watered using a filter press, settling pond, or other de-watering techniques. Since EC systems require no or minimal use of additional chemicals to achieve the desired results, electrocoagulation generates only a fraction of the sludge amount that is created through chemical coagulation. By using electricity to precipitate dissolved and suspended solids, electrocoagulation systems generate on average less than 0.5% added sludge as opposed to the chemical coagulation generating in excess of 49% added sludge. Compared with alum treatment, electrocoagulation provides approximately 83% less sludge volume and 78% improvement in filtration rate (22 EPA, SITE Superfund Innovative Technology Evaluation; EPA/ 640/ s-937505, Sept. 1993, USA). Another advantage is that the dissolved solids (salinity) of the treated wastewater does not increase during the treatment as it tends to when chemical coagulation is applied. Electrocoagulation typically decreases the total dissolved solids in liquid by 27% - 60%, whereas chemical precipitation adds dissolved solids to the wastewater. Electrocoagulation can be use to remove most suspended materials and metal ions from solution, It, therefore, has possible uses in treating mining, electroplating, and other process wastewater, as well as contaminated groundwater and pre-treatment of drinking water. The treatment system can be used as pretreatment before reverse osmosis systems because it removes suspended material, reduces calcium scaling, and generally reduces the total salt level in water and wastewater. 2.1 Applications The electrocoagulation process has been successfully used to: . Remove metals and oil from wastewater . Harvest protein, fat, and fibre from food processes waste streams . Remove BOD, TSS, TDS, FOG, etc from wastewater before its disposal to the municipal sewerage system . Recycle water, allowing closed loop systems . Pre-treatment, conditioning and polishing of drinking water . Recondition antifreeze by removing oil, dirt, and metals . Pre-treatment prior to applications of membrane technologies, such as reverse osmosis . Pre-conditioning of the boiler make-up water by removing silica, hardness, TSS, etc . Recondition of the boiler blow down by removing dissolved solids and eliminating the need for the boiler chemical treatment . De-watering of the sewage sludge and stabilisation of the heavy metals in sewage, lowering freight cost and allowing the sludge to be land applied 2.2 Benefits of the technology . Treats multiple contaminants . Capital cost significantly less than conservative technologies . Operating cost significantly less than conservative technologies . Electrodes – being the only consumables – provide independence from various shortages where multiple consumables are involved (shortages of chemicals etc) . Low power requirements . Generally no chemical additions . Low maintenance . Minimal operator attention . Consistent and reliable results . Sludge minimization Where other technologies are warranted for the wastewater treatment, technology can often be used effectively in the pre-treatment of the influent, or the final polishing of the effluent, prior to the reuse or discharge of the treated wastewater. This is particularly true with reverse