Chemically Assisted Primary Sedimentation (CAPS) consists of adding chemicals in order to increase the coagulation, flocculation and sedimentation of raw urban wastewater. The CAPS process can be developed in order to increase the efficacy of primary sedimentation as well as avoid any interference with biological treatment processes. The application of CAPS is particularly suitable as a technique for the upgrading of urban wastewater treatment plants (UWWTPs). In fact, CAPS does not require any further significant structural intervention (so saving investment costs and territory portions). The aim of this contribution is to emphasise the role of CAPS as a green chemistry option available in a UWWTP. In particular, a specific aim of the chapter is to focus the attention on the energetic importance of CAPS due to its capacity to increase the production of the primary sludge and consequently the energy production with an anaerobic digestion treating separately primary and secondary sludge. The energetic convenience and ''green'' propensity of the application of CAPS is discussed by means of the presentation of a paradigmatic case study containing economic evaluations, as well.