Colorimeters
These kits can be more precise and accurate; however, they can be cumbersome to use. A sample must be taken and mixed with a packet to create a color change. The less expensive kits use a color chart to determine the results and the more expensive kits have a meter that provides a read out of the chlorine content.
Electrodes
This method includes an ion selective probe or amperometric device. This measures the movement of electrons due to oxidation and reduction of chlorine compounds. While ion selective probes can be very handy for quick measurements, periodic calibration is required using standards and they can be fouled by compounds in the water. Cleaning and preventive maintenance is required.
Chlorine Dosage
The dosage required to eliminate pathogens is a function of both chlorine concentration and contact time. Disinfection is not an instantaneous process and the chlorine must come in contact with the microorganisms. The greater the concentration, the more likely a chlorine molecule will come into contact with a microbe; increasing the contact time also increases the likelihood a chlorine molecule will come into contact with microorganisms. In regards to treatment of agricultural water, the contact time is the duration the water is in the pipeline after the chlorine is injected. The longer the contact time, the lower the concentration of chlorine needed to kill the microorganisms. Municipal water and wastewater disinfection allows for 20 minutes of contact time, which means a lower chlorine concentration. A typical irrigation system may be designed to have a water velocity between two and five feet per second. If our irrigation mainline is 1,000 feet long, then our contact time within the mainline will be between 200 and 500 seconds (three to eight minutes). Dosage or Ct is concentration (C) multiplied by time (t). For example, 50 mg/L with a 10 minute contact time is 500 mg min/L. This assumes that the water will be in the pipeline for 10 minutes. However, if the water is only in the pipeline for three minutes after the chlorine injection, then we need 167 mg/L of chlorine to get the same dosage. That is, 167 mg/L multiplied by 3 = 501 mg min/L.
Temperature
Temperature is another factor that impacts the efficacy of chlorine disinfection. Room temperature (or about 70oF) is standard when measuring chemical activity. Warmer temperatures increase activity and cooler temperatures decrease activity. In general, every 18oF drop in water temperature will double the required contact time. As temperatures drop, chemical reactions take longer, thus more time is needed to ensure the disinfection process takes place.
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