Alkaline wastewaters (AWWs), such as those produced by chemical plants in the manufacture of several organic molecules of commercial interest including caprolactam, are characterized by very high alkalinity, salinity, and COD values (about 350 g/L), in addition to the presence of recalcitrant organic molecules; thus, they are very difficult to clean. A previously devised treatment process for such wastewaters consists of three main steps: an initial pretreatment to acidify the polluted stream, a successive extraction of the biorecalcitrant compound, and a final biological process. In particular, this study focuses on the first step, during which a black mud precipitate is formed. An optimization of this step is necessary to render such slurries more treatable for the usual solid-liquid separation units (i.e., filtration, centrifugation etc.), because of their serious fouling characteristics. In this work, CaCl2 was used to produce CaSO4 as a coprecipitator to improve the AWW filtration capacity, COD abatement, etc. The experimental analysis was carried out by factorial experiments to optimize this pretreatment process.