**2.3. Chlorination (solid calcium hypochlorite)**

Ca(OCl)<sup>2</sup> (calcium hypochlorite) is an essential solid that can be used in replacement of NaOCl (liquid). As a disinfectant, it has similarity with NaOCl but it is much safer to handle. Almost 70% of chlorine is available in commercial grades of Ca(OCl)<sup>2</sup> . It has applications in both waste water and drinking water [7].

## *2.3.1. Advantages*


## *2.3.2. Limitation*

Contamination or improper use of Ca(OCl)<sup>2</sup> may lead to explosion, fire, or the release of gases (toxic gases). Calcium hypochlorite should not be allowed to contact any foreign matter (including other water treatment products). If Ca(OCl)<sup>2</sup> is exposed to even very small amounts of water, it can react violently to produce toxic gases, heat, and spatter. Product should be added to water instead of adding water into the product. Exposure to heat can cause Ca(OCl)<sup>2</sup> to decompose rapidly, which may lead to explosion, intense fire, and the release of toxic gases. Dry, cool, well-ventilated area is needed for storing the product. Ca(OCl)<sup>2</sup> is used as a strong oxidizing agent. It increases the intensity of fire. Ca(OCl)<sup>2</sup> must be kept away heat, i.e., flame, heat, and any kind of burning materials [7].

### *2.3.3. Process*

The calcium hypochlorite chlorinator contains a cylindrical polyvinyl chloride (PVC) tank with a height of 0.6–1.2 m and a diameter of 230–610 mm. A sieve plate is present containing holes that support the 80 mm diameter Ca(OCl)<sup>2</sup> tablets. Tablet chlorinator systems can usually provide between 1 and 295 kg of chlorine per day. At the bottom, a side stream is piped into the chlorinator. The flow arises from the holes in the sieve plate that results in eroding the last layer of tablets. The amount of water entering the chlorinator is proportional to the rate at which the tablets erode. The rate of chlorine dosage can be calculated by controlling the water flow through the chlorinator. To meet the operational requirement, the chlorinator effluent is returned to the main stream, providing the desired level of available chlorine.

Variation in the dose and the contact time can be done to calculate the required disinfection. Different factors affect the chlorine dosage, i.e., wastewater characteristics, the demand of chlorine and discharge requirements. Mostly, the dose ranges from 5 to 20 mg/L. Different factors account for optimum disinfection that might include temperature, alkalinity, and nitrogen content. The pH of wastewater might affect the distribution of chlorine between hypochlorous acid and hypochlorite. A lower pH favors the hypochlorous acid: a better disinfectant. Higher concentrations of hypochlorous acid will lead to the formation of hazardous chlorine gas [2].
