When sludge is withdrawn, the first sludge to reach the pump at the beginning of each new pumping cycle is the dilute sludge that remained in the suction line at the end of the previous pumping cycle. This is followed immediately by a high consistency sludge from the bottom of the sludge blanket. Here’s where the problems begin. The higher percent sludge is pumped out causing upper stratospheres of lower percent solids to be drawn due to the slow clarifier rake motion. The net result is more and more dilution as the pumping cycle continues. Call it what you will – dilution, post-holing, rat-holing, coning – it becomes a serious and an expensive overall process problem.