How long is the chlorine disinfection contact time?

Question

when use the gas chlorine for drinking water disinfection

Answer 1

The action of chlorine is rapid, but not always instantaneous; for example, some chemical constituents or bacteriological and viral contaminants will require time for the chemical reactions to complete, or for the micro-organisms to be inactivated.
This “reaction time” part of the disinfection process is usually termed the “Contact Time”, and varies with the chlorine concentration. (The term Ct may be encountered; this is concentration x contact time) usually specified within a range of pH values, and is expressed in units of mg.min/l.)