Chlorine dosing in water purification is most frequently used to disinfect water. Its shortcomings as a cleaner are balanced by its benefits. However, unlike chlorination, other disinfection techniques like UV light and ozonation do not leave a residue to stop pathogen regrowth. Chlorination is the best method for providing end users with safe water when treatment facilities are far from the place of use. Measurements of "chlorine residual"—the quantity of chlorine still present in the water after it has reached its destination—are typically used by municipal water suppliers as a means of demonstrating safety. Although there are different residual needs, a normal residual goal would be for 0.2 to 1 mg/L.
Types of Chlorine Used in Water Treatment
Pure chlorine is rarely used to clean water. Chlorine gas, sodium hypochlorite, and calcium hypochlorite are the three chlorine-containing compounds used in water purification the most frequently. The selection of the chlorine type to be utilized frequently relies on cost, on the available storage alternatives and on the pH conditions required. The effects of pH on results are influenced by chlorination, a truth that is frequently forgotten in residential water treatment.
Chlorine Gas
Chlorine gas is denser than air and has a greenish golden color. Due to its extreme toxicity, it is both a great water disinfectant and a danger to handlers. Of course, using chlorine gas in chemical combat makes it a lethal weapon. It is a respiratory irritant, may irritation skin and mucous membranes, and, after prolonged contact, can be fatal. Chlorine gas is not more harmful to people than other types of chlorine when employed in disinfecting water for consumption because of chemical alterations that take place when it is introduced into water. The most popular form of chlorine for municipal water systems is chlorine gas, which is typically sold as an amber-colored compressed liquid. It is the least expensive form of chlorine.
Calcium Hypochlorite
Chlorine vapour is used to make calcium hypochlorite. It is most commonly used in domestic water treatment as chlorine pellets and granules. It must not be kept close to wood, textiles, or petroleum-based products because it is a white solid with a strong odor that can cause it to explode. Calcium hypochlorite raises the pH of the water being treated.
Sodium Hypochlorite
The chlorine-containing substance sodium hypochlorite is most commonly known as domestic bleach. It is a thin, light-yellow liquid with an indeterminate storage life. Of all the chlorine types, this one is the simplest to manage. The pH of the water being cleaned rises when sodium hypochlorite is added. Compared to calcium hypochlorite or chlorine gas, this form of chlorine requires a reduced concentration of chlorine to treat water.
How To Design Chlorine Dosing Tank Capacity in STP?
Wastewater is disinfected by adding chlorine using chlorine dosing tanks. The amount of time needed for the chlorine to interact with the effluent and disinfect it is known as the retention time of the tank. The wastewater flow rate, the intended retention period, and the chlorine dosage rate all affect the tank's capacity. The following equation can be used to determine a chlorine dosage tank's capacity:
Tank Capacity = (Chlorine Dosage Rate (mg/l) x Flow Rate (m3/hr) x Retention Time (hrs)) / Chlorine Strength (mg/l)
Assume, for instance, that the intended retention time is 2 hour and the wastewater flow rate is 200 m3/hr. The concentration of chlorine is 10 mg/l, and the dose rate is 3 mg/l. In that situation, the tank's volume would be:
Tank Capacity = (3 mg/l x 200 m3/hr x 2hr) / 10 mg/l = 80 m3
In order to accomplish a retention period of 2 hour, a chlorine dosage rate of 3 mg/l, and a chlorine strength of 10 mg/l, the chlorine dosing tank's capacity should be 80 m3.
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