Chlorine disinfection

Chlorine is used in the treatment of water the world over. It removes harmful microorganisms and any remaining chlorine provides some safeguard against low-level contamination and growth within pipes and fittings.

Types of chlorine

Chlorination can be achieved by using liquefied chlorine gas, sodium hypochlorite solution or calcium hypochlorite granules. Chlorine gas is very reactive and highly toxic and must be carefully stored and handled.

Gas chlorination is generally not appropriate for private supplies as leaks of chlorine gas are very dangerous. A separate area is necessary for storage of chlorine gas and an alarm system to detect leakage of chlorine must be installed. It is common practice to install alarm systems to indicate failure of the chlorine injector system or carrier water flow. Liquefied chlorine gas is supplied in pressurised containers. The gas is withdrawn from the cylinder and is dosed into water by a chlorinator, which both controls and measures the gas flow rate.

Sodium hypochlorite solution (liquid) can be delivered to site in drums. No more than one month’s supply should be delivered at one time, as its decomposition (particularly on exposure to light) results in a loss of available chlorine and an increase in concentration of chlorate, relative to chlorine. Alternatively, sodium hypochlorite solution (0.5 to 1.0% mass per volume (m/V) available chlorine) can be generated on site by the electrolysis of brine (sodium chloride solution). A typical electro-chlorination system consists of a water softener, a salt saturator, a voltage rectifier, an electrolysis cell and a storage tank containing up to three day’s supply of hypochlorite solution. Hydrogen, which is produced during electrolysis, must be vented safely. These systems are compact and limit the need to store and handle the toxic and corrosive chlorination chemicals. There is a wide choice of equipment available for dosing sodium hypochlorite solution. Simple gravity fed systems in which sodium hypochlorite solution is dripped at a constant rate into a tank of water have been used successfully and have proven reliable provided that the rate of flow into supply and the chlorine demand of the water are constant. Where the flow is more variable, water powered hypochlorinators that adjust the flow of sodium hypochlorite proportionately to the flow of water may be suitable. Electric dosing pumps can operate under flow proportional or chlorine residual control and thus maintain a consistent chlorine residual under conditions of variable flow or chlorine demand.

Calcium hypochlorite can be supplied in powdered, granular or tablet form (65 to 70% m/m available chlorine). Calcium hypochlorite is stable when dry and several month’s supply can be stored. It will however react with moisture in the air to form chlorine gas. Calcium hypochlorite dosing equipment is relatively simple. Most allow calcium hypochlorite to dissolve in a known volume of make-up water, which is then mixed with the main supply. Tablets are most commonly used, as their rate of dissolution is predictable. Control of dosage (proportional to the rate of dissolution) is often limited to changing the depth of immersion of the tablets in the make-up water or to changing the proportion of the make-up water to total flow.

Chlorine chemistry

Chlorine, whether in the form of pure chlorine gas, sodium hypochlorite or calcium hypochlorite, dissolves in water to form hypochlorous acid (HOCl) and hypochlorite ion (OCl–). For example, chlorine gas dissolves rapidly in water, initially forming hypochlorous and hydrochloric acids:

Cl₂ + H₂O ⇔ HOCl + HCl

Hypochlorous acid is a weak acid which undergoes partial dissociation to produce a hydrogen ion (H⁺) and a hypochlorite ion (OCl^–):

HOCl ⇔ H⁺ + OCl^–

The total concentration of chlorine, hypochlorous acid and hypochlorite ions is referred to as the free available chlorine. If ammonia is present in the raw water, the hypochlorous acid can react to produce chloramines. The total concentration of the chloramines and any organic nitrogen chlorine containing compounds is referred to as the combined available chlorine. Combined available chlorine is a less powerful disinfectant than free available chlorine but gives a more persistent residual.

The formation of combined chlorine is due to a sequence of reactions whereby hydrogen in ammonia is progressively replaced by chlorine as follows:

NH₃ ⇒ NH₂Cl ⇒ NHCl₂ ⇒ NHCl₃

If a large chlorine dose is applied (relative to ammonia), as is practiced in breakpoint chlorination, then nitrogen is formed.

2NH₂Cl + Cl₂ ⇒ N₂ + 4HCl

The effectiveness of chlorine for disinfection depends on the form of chlorine, its concentration and the contact time. Hypochlorous acid is a more powerful disinfectant than the hypochlorite ion and chlorination is usually practiced at values of pH favourable to its formation. The World Health Organization recommends that for the effective disinfection of drinking water “the pH should preferably be less than 8.0 and the contact time greater than 30 minutes, resulting in a free chlorine residual of 0.2 to 0.5mg/l”. Chlorination processes need to be carefully controlled in order to minimise the formation of taste and odour forming compounds. There may also be a need to control the formation of disinfection by-products including Trihalomethanes (THMs). Therefore, for private supplies, consideration should be given to using alternatives to chlorination, such as UV.

Methods of chlorination

Several regimes of chlorination can be used, including marginal (simple) chlorination, breakpoint chlorination, superchlorination/dechlorination and chloramination. On private supplies, it is probable that only marginal chlorination would be used in most cases. Marginal chlorination involves the dosing of chlorine to produce a suitable residual free available chlorine concentration. Breakpoint chlorination can and is used for removal of ammonia. Sufficient chlorine is added to exceed the demand for chloramine production and to ensure a free available chlorine residual. The chlorine dose must be carefully controlled to avoid forming dichloramine and nitrogen trichloride which can cause taste and odour problems. Breakpoint chlorination requires a dose of around 10 mg/l chlorine dosed per mg/l ammonia removed. The actual dose depends on water quality and has to be determined for each water.
The design of the contact system is very important. Applied chlorine must be mixed rapidly with the water and then passed through a contact tank, ideally with baffles or a serpentine arrangement to prevent short circuiting or retention in dead zones.

Control of chlorination

Chlorine residual control is the most common method of control where chlorine is dosed continuously into the water. If the quality of the water and hence the chlorine demand varies appreciably, it is necessary to use a control system to maintain a constant chlorine residual. A sample of chlorinated water is withdrawn downstream of the chlorination system and the chlorine residual in the treated water is monitored continuously. The signal from the chlorine analyser system is used to adjust the chlorine dose thus maintaining the required residual chlorine concentration. Where water quality is consistent, constant rate control or flow proportional control may be appropriate. In the former, a constant dose of chlorine is applied and in the latter a chlorine dose proportional to the flow of water is applied automatically under control of a signal from the flow sensor. There is no standard for chlorine in the regulations, but excess levels give rise to complaints of taste and odour.