Chemical inactivation of microbiological contamination in natural or untreated water is usually one of the final steps to reduce pathogenic microbes in water/wastewater treatment. As an extra measure a second disinfection step is applied in order to protect the water from microbiological contamination in the process.
Chlorine is a widely used disinfectant for the deactivation of pathogens in drinking water, swimming pool water, wastewater and in textile bleaching. Chlorine can be easily applied, measured and controlled. It is a safe, reliable and cost-effective chemical used in the treatment process in compliance to water quality regulatory requirements.
Benefits
Terms and Significance
Common Applications
Measurement Methods
Benefits
- Bactericide effect
- Virucidal effect
- Effect against parasites
- Effect towards reductive compounds (iron, sulphides, nitrites)
- Effect towards nitrogenous compounds
- Generation of chloramines (wastes which needed to be withdrawn from the process)
- Chloramines are destroyed in case of chlorine over concentration
- Persistence effect (ability to keep, through time, changes already done)
Terms and Significance
- Free chlorine is an important parameter in direct relation with the efficiency of expected disinfection. When free chlorine is added to the water, it will form hypochlorous acid and hypochlorite ions, which are the active disinfecting compounds. These compounds rid the water of bacteria, viruses, and microorganisms by damaging their cell membranes and essentially, disturbing the processes necessary for their replication and survival.
- Combined chlorine is an intermediate product that is formed when the water is being disinfected. It is called combined chlorine as it is essentially free chlorine that has bound itself to a contaminant or organic material, such as oil from the skin, sweat, or urine. Sometimes, combined chlorine is being called chloramine. The presence of combined chlorine is an indicator that the free chlorine is actively neutralizing the bacteria and contaminants in the water.
Common Applications
- Free chlorine is a common disinfectant in water treatment due to its easy handling and strong disinfecting effect. Free chlorine sensors are applied in:
- Drinking water – to ensure sufficient disinfection
- Food – to provide hygienic bottling and packaging
- Pool water – to dose disinfectant efficiently
- Total chlorine is a good indicator of residual disinfectants in discharge water. The sensors are used in WWTPs:
- To measure the effluent water’s disinfection status
- To control reuse of water
- Chlorine dioxide is more and more becoming a disinfectant of choice since it is less corrosive and independent from the pH value. Chlorine dioxide sensors are applied in:
- Cooling systems or towers
- Drinking water
- Wash water for packed vegetables
- Desalination plants to prevent ClO2 from disturbing reverse osmosis
Measurement Methods
- Online amperometric sensor
The amperometric method is a newer technology designed for process control, offering continuous measurement and convenience because it does not require taking samples. Amperometric measurement feature a working electrode, which is separated from the medium by a thin membrane. Chlorine coming from the medium diffuses through this membrane and is reduced at the working electrode. The circuit is completed by means of the counter electrode and the electrolyte. The electron reduction at the working electrode is proportional to the concentration of chlorine in the medium. This process works in a wide pH and temperature range.
- Measuring Cell
Amperometric chlorine sensors should be installed where there is sufficient flow to meet the flow velocity specification for the sensor (generally, 0.5 to 1.0 linear feet per second). Free chlorine measurements are highly dependent on flow, pressure and pH. Special flow cells are used to maintain a stable and constant flow and pressure for the chlorine sensor. A pH sensor is used for compensation of the chlorine free measurement.
- Colorimetric method with DPD reagents
DPD is historically the most common method used to measure chlorine in water samples. DPD stands for N,N-diethyl-p-phenylenediamine, and is the name of a colourless compound that will turn a bright pink colour when in the presence of chlorine as well as other oxidising agents. The DPD colorimetric method is the result of extensive work carried out by Dr Palin in the 1960s to standardise a method for chlorine measurement. Visual and photometric testing options are available using DPD reagents. Photometric testing such as Palintest offer DPD reagents in the form of tablets or liquids.
- Note: Factors which determine chlorine disinfection effectivity: Chlorine concentrations, contact time, temperature, pH, number and types of microorganisms, concentrations of organic matter in the water.
