Chlorine

Chlorine is still the most widely used disinfectant in North America. It is very effective against a wide range of pathogens including bacteria and viruses. Chlorine is stable and it is capable of providing the necessary residual protection in the distribution system. Chlorination is also a highly economical process.

Chlorination has several disadvantages as well. As a disinfectant, it is not effective against protozoan oocysts like Cryptosporidium. Chlorine reacts with natural organic matter in water and forms halogenated by-products, which can cause long-term health effects. Application of gaseous chlorine in water is a hazardous process requiring special safety measures. High doses of chlorine can also cause taste and odour problems. Chlorine at a lower concentration is commonly used as a secondary disinfectant in most water systems in order to provide residual protection in the distribution system.

Chloramines

Chloramines are formed by the reaction of free chlorine and ammonia. They are more stable than free chlorine and are very effective for providing residual protection in the distribution system. They also form fewer halogenated by-products as compared to chlorine.

Monochloramine is the preferred chloramine species for use in water treatment because it causes less taste and odour problems compared to the other chloramines species. Monochloramine residual is very effective in controlling biofilms, which reduces coliform concentrations and corrosion in the distribution system. The normal dosage range for monochloramine is in the range of 1.0 to 4.0 mg/L (USEPA 1999). Excess ammonia used during the chloramination process in water treatment may cause nitrification. Nitrification can have an adverse effect on water quality such as the loss of total chlorine, excess ammonia residuals and an increase in bacteria concentration.

The germicidal effectiveness of monochloramine is significantly less than that of free chlorine. Monochloramine is generally not used as a primary disinfectant as it is weak in the inactivation of viruses and protozoa. Its effectiveness against Cryptosporidium is not practically feasible. However, monochloramine is a good choice for secondary disinfection because of its stability and persistence, and because it generally produces significantly lower levels of DBPs. It can provide the necessary residual protection in the distribution system.

Ozone

Ozone was first used for drinking water disinfection in Europe in the late 19th century. It took several years for ozone to transfer to North America for the purpose of water disinfection. Early application of ozone in North America was primarily for colour removal and taste or odour control. Due to its powerful oxidizing ability, ozone gained popularity significantly in the late 20th century.

Ozone is a powerful disinfectant, which is able to achieve effective disinfection with less contact time and concentration. Several studies have demonstrated that ozone has high germicidal effectiveness against bacteria, viruses, and protozoan cysts. However, because of its short half-life ozone can only be used as a primary disinfectant as it is unable to maintain a residual in the distribution system. A secondary disinfectant such as chlorine, chloramines or chlorine dioxide is usually used with ozone for a complete disinfection system.

Ozone is not a halogen and therefore does not form any halogenated disinfection by-products (DBPs) during its reaction with natural organic matter in water. However, in the presence of bromides, the major ozone by-product of concern is bromate (BrO3-). The Guidelines for Canadian Drinking Water Quality established an interim maximum acceptable concentration (IMAC) level of 10µg/L for bromate. USEPA (1999) reported that bromate ion formation is an important consideration for waters containing more than 0.10 mg/L bromide ion. Ozone can also form other DBPs by reacting with aldehydes and ketones.

The principle advantages of using ozonation systems in drinking water treatment are as follows:

• More effective as a biocidal agent than chlorine, chloramines, and chlorine dioxide for inactivation of viruses, and protozoan species like Cryptosporidium and Giardia
• Highly efficient, as it requires less concentration and contact time
• Can control colour, taste and odour in drinking water
• Can oxidize iron, manganese, and sulfides
• Does not form halogenated disinfection by-products (THMs and HAA5s)

The principle disadvantages of ozonation systems in drinking water are as follows:

• Harmful by-products like bromates, aldehydes, and ketones can be formed if the raw water has high concentrations of bromides and organic compounds
• Capital as well operational and maintenance costs are high for ozonation equipment
• Provides no residual protection and hence secondary disinfection is necessary
• Requires high level of maintenance and operator skill
• Requires off-gas destruction or quenching
• Tends to promote re-growth due to generation of BDOC/AOC unless BAF is used

It should be noted that because of the wide variation in system size, quality of raw water, and dosage of disinfectants applied, some of these advantages and disadvantages may not be applicable for certain water systems  (USEPA 1999).