Electrocoagulation-Electroflotation (ECF)

Electrocoagulation-Electroflotation (ECF)

The electrocoagulation flotation technique is a combination of different processes, including electrolysis, flocculation and flotation.

Compared to a chemical coagulation process that doses the amount of flocculent, electrocoagulation flotation is a technique that removes more contaminant, reacts quickly, features a larger pH application range, subsides in a more compact manner, and has a better clarification effect.

The electrocoagulation flotation machine is widely used in the wastewater treatment in textile printing, dye chemicals, medical, leather and electroplating industries.

Design principle

By placing the iron or aluminum on the positive pole in the purified water, the positive pole produces an oxygen bubble, while the negative pole will produce hydrogen bubbles in a process known as electrolysis. In the rising process of the bubbles, the suspended solids will be removed from the water and form a residue layer. On the other side, the tervalence aluminum ion (bivalence iron ion), its hydrolyzing polymerisate and the suspended residue will interact with each other, and thus become flocculated.

Main structure of the electrocoagulation flotation machine

The electrocoagulation flotation machine includes the transmission organ, transmission shaft, impeller and sand suction system.
Electrocoagulation-Flotation (ECF), which is the electrochemical process used for treating small suspended particle by supplying metal ions, e.g. Al3+ or Fe2+, from sacrificial anode to form coagulant and also generating hydrogen micro-bubble to proceed the flotation . This process has been applied to treat various kind of wastewater including oily wastewater.
It was reported that the ECF can produce more compact sludge, which solve the problems on the large sludge production of the chemical coagulation. To investigate the oily wastewater treatment by the ECF, effects of operating conditions on the treatment performance have to be considered.

The electrodes are made from either aluminum or iron. In case of aluminum cations dissolve from the anodes according to, Rincon and La Motta, 2014.
Al(s) → Al3+(aq) + 3e- and/or
Al(s) → Al2+(aq) + 2e-

Also the following water electrolysis reaction may take place:
2H2O(l) → 4H++O2(g) + 4e-

Simultaneously, at the cathode water is reduced to hydrogen gas and hydroxyl ion OH- according to:
2H2O(l)+ 2 e- → H2(g)+2OH-

Aluminum cations are reacting with hydroxyl to form various aluminum hydroxides:
Al3+ + 3OH− → Al(OH)3(s)

Also hydroxides polymers can be built up according to:

nAl(OH)3(s) → Aln(OH)3n(s)
Other dissolved ionic species, like Al(OH)2+, Al2(OH)24+, Al(OH)2+, Al(OH)4−, Al6(OH)153+, Al7(OH)174+ … etc. may also occurr in the system. These ionic hydroxides have very strong attraction force toward destabilized pollutants caused removal of these substances from the system by electrostatic attraction and then coagulation. The continually small bubbles of hydrogen and oxygen gases generated in the EC process make the pollutants floats to the wastewater surface in the reactor tank, Balla, 2010. Fig.1 shows a representation of the electrocoagulation/flotation process.
The interaction between the two phenomena; electrocoagulation (EC) and electroflotation (EF) gives unique technique for removal of several pollutants from wastewater before disposal or reuse, Deghles and Kurt, 2016.
Airlift reactors have been widely utilized in the wastewater treatments to implement many applications including two or three phase. Airlift reactors are a special case of bubble column. It consists of two distinctive zones the riser and the downcomer. The air or gas may be sparged in the riser zone which leads to a difference in gas holdup between riser and downcomer. As a result of the gas holdup differences and thus densities differences between the two sections, there will be a static pressure difference between the riser and downcomer, therefore this leads the liquid circulation between them. There are two main configurations for airlift: internal loop and external loop airlift reactors, Chisti, 1989.

Features

(1) The electrocoagulation flotation machine does not need the any extra chemical coagulant.
(2) The device can remove the germ through the removal of suspended solids in water, and adapt to special poles.
(3) The electrocoagulation flotation machine can improve the treatment biodegradability of the wastewater.
(4) The device can decrease the COD, SS, oils, turbidity, chroma, germ and virus amounts found in the wastewater.

ECF process contaminant removal efficiency

Electrocoagulation

Electrocoagulation (EC) is a well established technology for the treatment of industrial waste water without the need for process chemicals such as Ferric, PAC or polymers. It serves industries such as dairy, metal plating, oil & gas, food processing, mining, truck & car wash, cooling water, ground remediation, mining and potable water treatment.
A wide range of pollutants can be efficiently removed up to 98% including heavy metals, COD, BOD, suspended and colloidal solids, FOGs, bacteria, viruses, hydrocarbons, pesticides and herbicides. EC will not remove dissolved solids such as salts.
EC is a good pre treatment to membrane technologies where high quality water re-use is required.

Typical Contaminant Removal Performance using EC Technology
The removal rates for the contaminants listed below are typical and are intended to provide a guide. Most waste waters are complex products and it may only a single contaminant that needs removal to meet discharge or re-use standards. The EC technology can remove multiple contaminants from waste water or target specific elements and this is done by selection of electrode material, residence time and current density applied. It every case, it is advisable to run trials to determine the most efficient operation procedure to maximise removal rate to minimise operational costs.

The Electrocoagulation Process

Electrocoagulation EC is the process of applying a direct current voltage to the waste water to be treated using submerged electrodes which act as the anode and cathode. Typically, these electrodes are made from mild steel and aluminium. A DC voltage is applied to the electrodes and due to the conductivity of the water passes between the electrodes. The electrical current acts on the suspended particles in the water, neutralising their charges and allowing the very fine solids to precipitate and settle. The electrical current also makes the electrodes sacrificial and in doing so, they give up their metal ions into solution in water. These ions acts a similar manner to the chemical coagulants used in DAF systems. Suspensions and emulsions are destabilised, solids coagulate and separate out and and hydrocarbons coalesce. The EC reaction time is typically between 20 and 120 seconds depending upon the contaminants being treated. The consumables are electricity and the sacrificial electrodes. Both these directly effect the EC operational cost. Energy consumption is typically 1.0 kWhr per cubic meter (1000 litres) treated with a metal electrode consumption of between 5 and 20 grammes per cubic meter treated. Compared with chemical dosing systems, EC produces significantly less sludge with much lower sludge handling costs. Unlike chemically produced sludges, EC produces a broadly neutral pH, easy to dewater and non-leaching, oxide sludge.

Electrocoagulation – Electrical Description

EC requires a DC voltage to be applied to the submerged electrodes in the reaction chamber. The incoming voltage is AC and therefore needs rectifying before it can be applied. The incoming electrical supply is typically 400V three phase but for smaller EC plants 230V single phase can be used.
A 1:1 isolation transformer for safety and then the AC supply is converted to DC via a fully configurable and controllable digital DC drive. This delivers a fully variable DC voltage to the electrodes from 0 – 480 V DC. The EC process depends upon the electrical Current Density (CD) applied to the electrodes and the available surface areas of the electrodes. The EC plant provides a very large electrode surface area relative to the flow rate meaning the CD can be minimised for the specified reaction time. The current drawn is dependent on the conductivity of the water which can vary extensively.

The EC reaction chamber is configured using a bipolar electrical connection and multiple, close spaced electrodes to reduce the resistance in the chamber. This make the energy consumption very efficient because no excess voltages used as this would lead to wasted heat. Because the operator of the EC plant is able to fully tailor the voltage applied, the absolute minimum electrical energy and metal electrodes are consumed.


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