In urbanising catchments the variability of particulate matter (solids) and oxygen demanding constituents of wet weather and dry weather characteristics result in variable treatment effectiveness from physical unit operations and biological unit processes. Higher organic concentrations during dry weather flow are more amenable to biological unit processes while the higher inorganic loads generated largely by urban wet weather constituents and much larger flows can be more effectively treated or conditioned by physical unit operations, including ideally preceded by hydrologic controls. This study examines the relationships between total chemical oxygen demand (COD)(t), dissolved chemical oxygen demand (COD)(d), particulate chemical oxygen demand (COD)(p) and solids, measured as total suspended solids (TSS). Results also provide an index for selection and optimisation of treatment operations for combined sewer overflow (CSO) loads.