The information provided is not sufficient to determine the correct answer. The procedure for re-chlorinating a water trailer may depend on various factors, including the specific guidelines or regulations in place. Additionally, the concentration of chlorine required for water disinfection can vary based on the water quality and intended use.
If there are specific guidelines or regulations governing the re-chlorination of water trailers in your context, it's important to follow those guidelines. Alternatively, you may need to consult relevant authorities or resources to obtain accurate information on the proper procedure for re-chlorination in your specific situation.
