Which residual test for chlorine is considered the least desirable?

The residual test for chlorine that is considered the least desirable is the OTA (Ozone, Trihalomethanes, and Aldehydes) test. This is because the OTA test does not specifically measure chlorine residuals but instead assesses byproducts that may arise from chlorine and other disinfectants used in water treatment processes.

While the OTA test can provide some information about disinfection byproducts, it does not accurately indicate the remaining chlorine available for disinfection, which is crucial for ensuring water quality and safety. By contrast, tests for free chlorine, total chlorine, and combined chlorine directly measure the concentrations of chlorine present in the water. Free chlorine is particularly important as it is the most effective form for disinfection, while total chlorine gives a complete picture of all forms of chlorine present, and combined chlorine helps understand the amount of chlorine that has reacted with substances in the water.

In summary, the OTA test is not desirable for the assessment of chlorine residuals because it does not specifically track the efficacy of chlorine in treating water for microbial contaminants, which is essential for maintaining safe drinking water.