What is Residual Chlorine ?

Testing for Residual Chlorine is very common in water treatment. The word, "residual", means remainder or that which is left. This test is made to measure the amount of chlorine remaining in the sample at the time the test is made.

For example, 2 ppm of chlorine may be added to a certain water. After a certain contact time, the test is made and indicates that 0.3 PPM chlorine is present. The 0.3 PPM is the chlorine residual. The difference between 2.0 PPM and 0.3 PPM, 1.7 PPM chlorine, was used during the contact time in oxidizing organic matter (bacteria, vegetative matter, sewage, etc.) in the water. This is known as the "chlorine demand" for that period of time. There is no chlorine demand in distilled water.

It is important to note that due to the chlorine demand, a chlorine residual test indicating sufficient chlorine at the plant does not necessarily mean that a test at the end of a main will indicate sufficient chlorine.

There are a number of materials in the lines which can use up the chlorine, such as iron bacteria or foreign matter introduced by backsiphonage.

DKK-TOA's Model RC-24P is portable and can be easily brought to and used at end of distribution points.

Why Measure Residual Chlorine ?

Chlorination has long been the conventional method for disinfection as it is effective and inexpensive. The water company must ensure the water system has a suitable concentration of chorine for a suitable period of time to ensure effective inactivation of bacteria and viruses. Water companies need to monitor chlorine for their due diligence to ensure that their equipment is working and that they are providing potable water to their customers.

The Medical Health Officer may make monitoring a requirement. Monitoring residual chlorine is suggested for three critical areas:

1. Before the first customer to ensure potability.
2. At the chlorinator to ensure the equipment is functioning.
3. At the end of the distribution system to ensure all water is treated.

These checkpoints are typically monitored by small hand held devices. The advantage of DKK-TOA's Model RC-24P is that it can store data for later retrivel and it does not require any reagents to perform its measurement.

Increasingly stringent requirements, growing awareness and understanding of the effects of chlorine on the environment, water quality and greater need for process optimization are strongly prompting many progressive water utilities to more fully optimize the performance, reliability and safety of their chlorination programs.

Where tight control is required, water treatment plants can add an on-line residual control to their chlorinator, using analysers such as DKK-TOA's Model CLF-100. The residual control will measure the residual chlorine level at a set location then send a signal to the chlorine feed equipment. The feeder will automatically adjust the chlorine addition to ensure a constant level of chlorine exiting the treatment plant.

Residual control can be applied to both gas and liquid chlorine feed. Typically, the residual is combined with a flow signal for a “compound loop control”. Using this method, the chemical feed will increase or decrease as the water flow rate changes. The residual analyzer will measure the resulting chlorine dose then bias the dosage to meet a set concentration of chlorine residual. In addition to control, the residual analyzer can output an alarm of low residual. The alarm is immediate therefore it is possible for operation staff to resolve the situation before a boil advisory must be issued.

As the residual chlorine is the last element of a water treatment plant, the importance of tight control cannot be overemphasized.