Chlorination and Disinfection:
Chlorine (Cl2) has been used for many years to treat potable water, municipal and industrial water and waste waters to control microorganisms because of its capacity to inactivate most pathogenic microorganisms quickly. The effectiveness of chlorine is dependent on the chlorine concentration, time of exposure, and the pH of the water. Chlorine is used for treating potable water where a residual chlorine concentration near 0.5 mg/L is commonly used. In an industrial water treatment scheme, fouling of water intake lines, heat exchangers, sand filters, etc., may be prevented by maintaining a free residual chlorine concentration of 0.5–1.0 mg/L or higher, dependent on the organic content of the incoming water.
Chorine in gaseous form is a good oxidising agent and extensively used in water & waste water treatment projects mainly for disinfection. It effectively kills pathogenic micro- organisms, is non toxic to living beings, easy and safe to store, provides residual protection in drinking water and offers economical disinfection. There are many applications of Chlorine, apart from Disinfection.
Chlorine is a gas at room temperature and pressure and is soluble in water.
Chlorine hydrolyses with water to form hypochlorous acid and hydrochloric acid according to the following equation:
Cl2 + H2O → HOCl + HCl
Sodium Hypochlorite reacts with water as follows:
2NaOCl + H2O → NaOH + NaCl
This hydrolysis feature makes chlorine suitable for water disinfection applications where turbulence is present, as chlorine is not present in water as dissolved gas like chlorine dioxide or ozone.
Chlorine becomes choice disinfectant because it is comparatively less expensive compared to other disinfectants available in market in various forms.
a. Calcium hypochlorite
Calcium hypochlorite is available as granules or tablet form. The tablets can be used with erosion feeder. Tablets are placed in a hopper where they are stored dry. Water is allowed to run past the bottom layer of tablets where slow erosion will occur. The chlorinated water then drops into a buffer storage tank where flushing and dilution occurs. The diluted (<500ppm) solution is continually pumped to the process water header for disinfection. Control of chlorination is by a residual chlorine controller (Chlorine Analyzer). This controller gives a pulse duration control signal to the feeder and the feeder controls the tablet erosion water valve. b. Sodium Hypochlorite Sodium Hypochlorite is a commercially available disinfectant chemical usually the concentration ranges from 10 – 12.5%. This liquid disinfectant is normally dosed with solenoid operated dosing pumps or motor driven metering pumps for smaller applications The dosing system comprises of chemical day tanks and metering pumps prefabricated in compact skids with all relevant accessories. Design and construction of these dosing skids could be made suitable for all industries including strict oil and gas field standards. The discharge rate from the dosing pumps could be varied to meet different dosing requirements. Sodium hypochlorite undergoes gasification as it decomposes to give off oxygen gas. This gasification may cause problems in chemical metering pumps as the gas will accumulate in the metering pump suction line, eventually making its way into the liquid end where there is loss of priming. Larger metering pumps (>20 L/hr) don’t usually suffer from loss of prime as the valves are comparatively bigger and the associated volume of chemical being pumped is also larger.
c. Gas Chlorination
Chlorine gas is dosed from bulk cylinders or drums using remote vacuum gas chlorination equipment. The vacuum regulator is mounted directly on the cylinder isolation valve using a lead gasket. Chlorine gas under pressure is fed to the inlet pressure reducing valve, the needle of which is seated on a diaphragm. At the water end of the system, pressure differential is applied across a venturi by a boost flow which causes the venturi to suck. This vacuum is applied to the diaphragm, causing the needle to be pushed into the spring loaded valve, thereby allowing chlorine gas to flow into the chlorinator. Chlorine Gas under vacuum then flow through a regulating device (needle valve V notch) and through the vacuum tube to the venturi where it is dissociated in water.
Electro-chlorination is a chemical process which uses water, common salt and electricity to produce Sodium Hypochlorite (NaOCl). The brine solution (or sea water) is made to flow through an electrolyzer cell, where direct current is passed which leads to Electrolysis. This produces Sodium Hypochlorite instantaneously which is a strong disinfectant. This is then dosed in water in the required concentration to disinfect water, or to prevent Algae Formation and Bio Fouling. The main advantage of electro chlorination is that dangerous chlorine gas can be avoided where brine or sea water is abundantly available.
- Public drinking water supply.
- Food and beverage industry.
- Paper industry.
- Treatment of water in Breweries.
- Pasteurizing and Rinsing in Breweries.
- CIP systems.
- Cooling water treatment.
- Cold sterile bottling.
- Condensate water treatment in the Milk industry. In Aqua culture
- Wash water treatment.
- Meat processing industry.