The diaphragm pump will be sealed and much lees offensive when it is working properly. Most of the older piston pumps had well thought-out shear pins and ball-check maintenance options. Make sure the diaphragm pump you select is as well executed. While the diaphragm style pump is correct for your application, don’t forget the peristaltic or hose pump. I have used 4-inch peristaltic pumps for primary sludge (and scum) transfer. These pumps offer low speed/high discharge pressure while using simple rotary motion, unlike a piston or diaphragm unit that employs a crankshaft. Another option would be a recessed impeller pump (Wemco Model “C”), but these are typically designed for a higher flow rate that you may need.
Steve