The paper discusses how citizen science within an ecosystem services (ESS) framework may enhance evaluation of de-centralized water solutions. In a demonstration case in Oslo, citizens were engaged in long-term monitoring and evaluation of two solutions for treatment of combined sewer overflows. The citizens participated in the design of the study, systematic observations, and final evaluation, via interviews and workshops. A wealth of real-time information was collected, supplementing simulation data and water sampling results. The concept of ESS drew attention to benefits that tend to be ignored in standard evaluations. It is, however, most elaborate for ecological services, and less developed for social aspects. Involving the citizens complemented the framework in these areas, while providing new insights into the contextual interactions influencing ESS and benefits of local treatment. Both solutions, a cross-flow lamella settler and a high-rate filtration system, were quite efficient in removing suspended solids, with a strong impact on visual appearance. A range of wider benefits were identified. These were difficult to monetize, but the citizens' evaluation provided an alternative measure. The study highlights the benefits of citizen science in local water management and suggests the need for more research on beneficiaries in ESS evaluation.