“Chlorine” is the most popularly used disinfectant for water treatment worldwide. It is widely used for disinfection because it is effective in killing harmful bacteria, viruses, and other microorganisms. The presence of these pathogens can cause water-borne diseases like typhoid, Cholera, Giardia, Dysentery, among others. When added to water, chlorine forms hypochlorous acid (HOCl). It is the HOCl that gives chlorine the ability to destroy pathogens and makes it essential for maintaining safe drinking water.
Note: Chlorination is not the only disinfection process in water industry. Ozone (O3) and UV irradiation are also common.
Forms of Chlorine and Its Derivatives
The process of treating water with chlorine is known as chlorination. Chlorination can be carried out using pure chlorine or chlorine derivatives like calcium hypochlorite (Ca(OCl)2), Chlorine dioxide, Chloramine, sodium hypochlorite (bleach) and chlorinated Lime (mixture of calcium hypochlorite, calcium hydroxide, and calcium chloride). While bleach is mostly used for household water treat, most industrial chlorination process use high-test calcium hypochlorite, chlorine gas or other alternatives listed here. Chlorine dioxide tastes better than pure chlorine and it is best for the removal of biofilms, which make it a great compound for sewage treatment.
Required Chlorine Amounts
While treating water with chlorine, it is important to consider the standard requirements. Although the initial chlorine treatment depends on several factors including the quality of the water which influences chlorine demand, there are set standards, especially, for residual chlorine. The amount of chlorine needed in drinking water can differ based on what country you are living. However, the world health organization recommends a minimum free chlorine residual of 0.5 mg/L (ppm) in treated water to ensure effective disinfection. This residual chlorine concentration is expected to be maintained throughout the distribution system. Other countries have their measure as well. For instance, most Nigerian standard like the Nigerian Industrial Standard (NIS) require at least 0.2ppm residual chlorine after treatment. You need to consider storage material when using chlorine. For instance, if your water is packaged in sachets, you can decide to have less than 0.2ppm residual. Why the value matters is that most chlorine test kits in the market are caliberated from 0.2ppm. Therefore, sticking to standard is necessary for the precision of measurement and quality assurance. You can make findings about your local regulatory requirements.
Note: Chlorination is not a compulsory process. Most modern water treatment facilities have provided different stages of water treatment that no longer require chlorination. However, for some water factories, swimming pools, and sewage treatment facilities, chlorination is still very much in application. Chlorine remians the cheapest and easily accessible chemical for water treatment both at homes and industries.
Efficiency of Chlorination
Factors that influence chlorination
Several factors influences chlorination process, however, the efficiency of chlorine disinfection depends mostly on its purity. For instance, gaseous chlorine boasts the highest purity, while household bleach contains a lower concentration that's suitable for smaller-scale applications. Many chlorine compounds like high-test calcium hypochlorite come in solid forms with percentage purity of 65%-90%. Knowing the percentage purity is very important for calculating chlorine dose and other information you need for water treatment. Other factors that influence chlorination which must be considered are:
- Water Chemistry: Higher pH levels and colder water temperatures can reduce chlorine's effectiveness.
- Contact Time: The longer chlorine interacts with the water, the more thorough the disinfection.
- Competing Demands: Organic matter and ammonia in the water can consume chlorine, reducing its availability for disinfection.
- Sunlight Exposure: Chlorine degrades in sunlight, so storage tanks need to be protected.
How to boost chlorine efficiency
Regular monitoring of chlorine residuals is crucial. If levels fall below the recommended amount, treatment plants can employ various strategies:
- Adjusting Chlorine Dose: Increasing the amount of chlorine added if demand is high.
- Enhancing Mixing: Ensuring proper mixing for uniform chlorine distribution.
- Pre-Chlorination: Applying chlorine before other treatment steps.
- Combination Disinfection: Using combined chlorine with ammonia (chloramines) for longer-lasting disinfection in specific cases.
- Alternative Methods: Considering ultraviolet (UV) light, ozone, or chlorine dioxide for certain scenarios.
- Rigorous Maintenance: Regularly cleaning tanks, pipes, and equipment to prevent biofilm buildup, which can harbor microbes.
Safety precautions and Hazards associated with the use of chlorine.
While chlorine is very useful for water treatment, it is a very dangerous chemical, hence, it must be used with caution, and only by experts. Avoid inhalation of chlorine gas because it is highly toxic. Store chlorine gas or compounds in resistant container, mostly plastics and ensure that it is not filled above 85% volume since it can build-up gas or expand. Store solid chlorine compounds like calcium hypochlorite pellets in a cool dry place and away from sunlight. Poor storage can lead to explosion or corrosion of the storage material. Avoid excess chlorine in drinking water, where there is shock chlorination, neutralize it before such water is consumed. Also, test that the raw water is good for chlorination before applying it. For instance, presence of organic compounds in water can lead to the formation of carcinogenic chlorine by-products such as chloroalkanes and carcinogenic BDA