a horizontal circular filter is provided which includes plural filter pans arranged in a circle and rotatable as a group around a fixed central vacuum box . the filter pans are connected to respective sections of the vacuum box by respective manifolds which rotate with the filter pans . each of the filter pans comprises a pan having a flat bottom wall including plural filtrate drainage troughs therein . a removable , disposable , plastic channel - forming grid is positioned on the bottom wall between the side walls of the pan . a filter cloth is disposed over the grid , and a cloth &# 34 ; caulking &# 34 ; arrangement secures the filter cloth in place so as to also hold the grid in place in the pan . a method of retrofitting existing punched plate filter pans is also provided .