The effect of pH on the efficiency of chlorine disinfection and virus enumeration

In the light of increasing world-wide water pollution, the controlled and effective chlorination of water is of paramount importance in the treatment of drinking water supplies. A recent publication indicated that hypochlorite ion (OCl⁻) which is formed at high pH levels appeared to be a more effective virucidal form of free chlorine than hypochlorous acid (HOCl), which occurs at lower pH levels. This was a reversal of the findings of many other research workers. The present paper reports on a further study which confirms that HOCl is a more effective virucide than OCl⁻. It also reports on comparative studies between the PFU and TCID₅₀ methods for virus evaluation at two pH levels in an attempt to find the reasons for the reversal in the results obtained.