Study of the Sedimentation Characteristics of Solids in Carwash Wastewater

Studies evaluating the sedimentation of solid particles in carwash wastewater (CWW) are scarce. This research is innovative because it is the first to study solid sedimentation specifically in CWW. The motivation lies in the fact that existing parameters (for sanitary sewage) are inadequate due to the peculiar physicochemical characteristics of CWW. This study evaluated the settleability of solids present in CWW, aiming to generate empirically validated parameters to support the optimized design of sedimentation units. Granulometric characterization of the settleable material and column settling tests for total suspended solids (TSS) were performed. The granulometric analysis of the settleable solids revealed a predominance of the sandy fraction (D_90% = 1.1 mm), with an average of 87.44%. This characteristic confirms the coarse texture of the retained material and its high sedimentation velocity during the first hour. The column settling tests for TSS demonstrated highly variable removal efficiency, which did not directly correlate with the initial concentration of solids or with rainfall conditions. Results indicated the need for hybrid sedimentation models to adequately represent TSS sedimentation. A surface application rate of 1.5 m·h^-1 is suggested, which corresponds to an average TSS removal efficiency of approximately 80%. The adoption of specific design parameters for CWW provides greater reliability in the sizing of treatment units, supporting both operational efficiency and the economic viability of the system.