Circulating residual salt monitoring in the water tank of the underground pit. Continuous measurement while circulating the water drawn from the submersible pump. We have built a system that monitors to maintain the target value of residual chlorine and controls drug injection. There was a problem that the water quality in the water receiving tank was not stable due to the reuse of equipment drainage and rainwater, but we were able to stably monitor residual chlorine by installing filters and selecting sensors.