A chopper pump has a sharpened edge on the impeller which acts as a chopper as solids come through the pump. The impeller is mounted against a static bar that creates a scissoring action by which the impeller chops large solids against the bar into a size that can travel through the volute and out the discharge piping. A grinder pump does not allow any solid to pass through the volute and out the discharge piping. All solids are ground into a chum before entering the volute. Grinder pumps can be much slower in processing influent, and they dramatically reduce the pipe size from the influent to the discharge.