Operator was unplugging the underflow on TB-24 clarifier-The air ram, lake water and air was shut off to the 2""line from the cone to the mud pot. There is a 2"" street elbow on the end of the line that was removed. The operator inserted a clean out rod into the line and when he pulled it out, hot mud shot out, splashing the operator's right arm and the right side of his neck.