Advanced Treatment techniques

1. There are two basic treatment stages. The first stage consists of mechanical and physicochemical treatments followed by the advanced treatment of the pre-treated primary effluent. • Advanced treatment: • Electrocoagulation. • Chemical oxidation. • Biological techniques Advanced Treatment techniques

2. Electrocoagulation: • Electrocoagulation (EC) uses an electrochemical cell to treat polluted water. • Sacrificial anodes corrode to release active coagulant cation, usually aluminum or iron, to solution. • Accompanying electrochemical reactions are dependent on species present and usually evolve electrolytic gases. • Simultaneously, electrolytic gases are generated (typically hydrogen at the cathode).

3. EC has the capability to remove a wide range of pollutants including suspended solids, heavy metals, dyes, organic material (such as sewage), FOG’s (i.e. fats, oils and greases), ions and radionuclide. • Figure 2 depicts the complex, interdependent nature of the EC process. Clearly, the coagulant and its hydrolysis products can have numerous interactions with the pollutant, with other ionic species or with the electrolytic gas bubbles.

4. Interactions Occurring Within an Electrocoagulation Reactor

5. Theory of the process Electrocoagulation Formation of coagulants by electrolytic oxidation of the ‘sacrificial electrode’. Destabilization of the contaminants, particulate suspension, and breaking of emulsions. Aggregation of the destabilized phases to form flocs.

6. It is possible to identify three basic sciences - electrochemistry, coagulation, and flotation - that interact to make EC work. Venn Diagram of the Main Sciences Underlining Electrocoagulation

7. Electrochemistry • As noted, all EC reactors are electrochemical cells. • The electrode material used determines the coagulant type. • Thus, regardless of the electrode design employed, the electrode material determines the electrochemical reactions occurring, and hence the coagulant cation. • For this reason, electrochemistry is one of the foundations for EC. Aluminium, the most commonly used anode material, is used here as an exemplar (note that an analogous description can be developed for other metals).

8. Electrochemistry Electrocoagulation Aluminium’s anodic dissolution Oxygen evolution is also possible at the anode At neutral or alkaline pH, hydrogen is produced

9. Electrochemistry Electrocoagulation while under acidic conditions, electrochemical reaction of aluminum ions with water:

10. Predict the equations if the electrode made of iron: Anode : Fe(s) → Fe2+ (aq) + 2e− Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s) ◦ Cathode: 2H2O(l) + 2e− → H2(g) + 2OH−(aq) ◦ Overall: Fe(s) +2H2O(1)→ Fe(OH)2(s) +H2(g)

11. Flotation Electrocoagulation • For a given gas volume, a smaller bubble diameter results in both a greater surface area and more bubbles, thereby increasing the probability of collision and the ability to remove fine pollutant particles. One of the main advantages of flotation by electrolytic gases is the small size of the bubbles produced. .

12. Advantages Electrocoagulation Simple equipment and is easy to operate. Wastewater: palatable, clear, colorless and odorless water. Sludge: readily settable and easy to de-water. Flocs: much larger, and separated faster by filtration. Removing the smallest colloidal particles. Avoids uses of chemicals. The gas bubbles: carry the pollutant to the top of the solution. Require less maintenance.

13. A schematically Diagram of electrochemical cell Al anode ) Plexi-glass tank (3) Aluminum Cathode. (4) Voltammeter (5) Ammeter. (6) Power supply (7) Resistance.