Water Treatment Technologies: Membrane Separation, Chlorination, and Activated Sludge
Water Treatment Technologies
Membrane Separation
Membrane separation techniques, such as electrodialysis reversal (EDAR), are used to remove dissolved matter from water, achieving high levels of quality. However, these methods can be expensive, involving semipermeable membranes and dialysis.
Ultrafiltration, a process employing semipermeable membranes, separates contaminants from water under a pressure gradient. It is suitable for macromolecules and industrial effluent treatment.
Reverse Osmosis (RO) is a crucial method that reverses the natural direction of osmosis. In osmosis, when two solutions of different concentrations are separated by a semipermeable membrane, solvent molecules move from the less concentrated to the more concentrated solution to achieve equilibrium. This generates osmotic pressure. RO applies pressure greater than or equal to the osmotic pressure, forcing the solvent to move in the opposite direction, separating solutes/contaminants from purified water.
Electrodialysis involves applying an electric field to an ion-rich fluid. By placing two electrodes and applying an electric field, ions migrate to the anode (positive pole), and cations migrate to the cathode (negative pole). By intercalating selective anionic and cationic dialysis membranes, high water quality can be achieved.
Chlorination
Chlorination involves adding chlorine to water, leading to a double equilibrium of dissociation and hydrolysis of chlorine and hypochlorous acid. The predominant species depend on the pH. At highly acidic pH (2-5), chlorine (Cl) dominates. As pH increases from 5 to 6, hypochlorite appears. At pH 10, only hypochlorite exists, with no hypochlorous acid. The normal range is between pH 5 and 10, resulting in a balance between the two. Hypochlorous acid is a more effective disinfectant chemically. Therefore, working below pH 6 yields greater efficiency. This principle also applies when using hypochlorite instead of chlorine.
Breakpoint Chlorination: Chlorine is a highly oxidizing chemical compound that reacts with oxidizable inorganic compounds. Chlorine is converted into chlorides, which are not disinfectants. The initial chlorine dose is consumed in oxidizing these compounds. Once all such compounds are oxidized, disinfection begins. If the chlorine dose is increased in the presence of ammonia, chloramines are formed. Similarly, organochlorine compounds are formed in the presence of organic matter.
Mixed Liquor (Activated Sludge Treatment)
Mixed liquor treatment, also known as activated sludge treatment, is a continuous process in which wastewater is biologically stabilized in aeration tanks or ponds under aerobic conditions. The effluent from primary clarifiers passes into these activated sludge ponds, which require oxygen for the metabolic activity of microorganisms. Oxygen is supplied through turbines or diffusers placed inside the tank.
The system involves developing a bacterial culture fed with wastewater for purification. Agitation homogenizes sediments and prevents the mixing of bacterial flocs. The wastewater is aerated to provide the oxygen needed for digestion (mixed liquor). After sufficient contact time, the mixed liquor is sent to a secondary clarifier designed to separate the treated water from the sludge. A portion of the sludge is recycled to the aeration tank to maintain a sufficient concentration of active biomass.
The system requires essential nutrients, mainly nitrogen and phosphorus, to function properly.
After the influent passes through the aeration tanks and undergoes bacterial digestion, the effluent passes through the secondary clarifiers. Although biological treatment reduces the BOD (Biochemical Oxygen Demand) of the effluent water by 75-90%, the sludge is reduced to a lesser extent, often requiring subsequent treatment.
A balance must be maintained between the microorganisms in the reactor and the food content in the wastewater. New microorganisms are generated to maintain this balance and must be purged to prevent excessive organic matter.
Bacteria can develop in circular or stringy, filamentous forms. Filamentous bacteria create large networks that do not settle easily. To avoid this problem, conditions favoring circular forms should be created, either by providing less oxygen or by installing zig-zag walls to prevent the union of filamentous bacteria and oxygenating the reactor.