A system and process for removing solids from raw, untreated liquid that combines mechanical techniques, such as via shakers, hydrocyclones author centrifuges, with an additive technique for removal of smaller solids. The additive is selected according to the application. In drilling mud applications, preferred additive embodiments are polyaluminum chloride or polyacrylamide flocculants. Preferably, liquid additive precursors are pre-mixed separately and are then blended before injection into the solids removal process. Some embodiments provide an externally-actuated rack and pinion mud screen lock for simplified screen lockdown on shakers. Some embodiments provide a separate preliminary processing and feed system for pretreatment of the raw, untreated liquid.