For a chlorine contact basin's effluent to be considered good, the "Secchi" disk should be readable at which depth?

In wastewater treatment, particularly in evaluating the performance of chlorine contact basins, the clarity of the effluent is an important measure of its quality. The Secchi disk is a tool used to determine water transparency, which is indicative of the levels of suspended solids and overall water quality.

For a chlorine contact basin's effluent to be considered good, a Secchi disk being readable at a depth of 6 feet signifies that the water is sufficiently clear for proper disinfection to occur. This depth indicates that the concentration of particulate matter in the water has been adequately reduced, allowing chlorine to effectively disinfect and kill pathogens present in the effluent. Clear effluent indicates that the treatment processes have been effective in removing suspended solids that could hinder disinfection.

If the Secchi disk is readable at shallower depths—like 2 or 4 feet—it suggests that the effluent may still contain higher levels of suspended solids, which can affect disinfection efficacy. Conversely, if it were readable at a depth greater than 6 feet, while it may indicate excellent clarity, it is not typically listed as a standard for evaluating chlorine contact basin performance. Therefore, the standard establishing readability at 6 feet reflects an optimal balance between effective treatment and operational feasibility.