This article presents the feasibility evaluation and preliminary design of a wastewater treatment plant upgrade supported by simulation. The existing facility was based on trickling filters, and the objective of the upgrade was to achieve nutrients removal. The proposed solution modifies the existing primary clarifier to host an anaerobic-anoxic suspended growth reactor, which is an alternative that, to our knowledge, has not been proposed or explored so far. The trickling filters would remain as aerobic reactors. In this study, the novel treatment scheme has been assessed for the first time, through model simulations. The modified treatment train was simulated, showing that the anoxic zone is able to denitrify satisfactorily achieving the required effluent nitrogen concentration. However, to promote biological phosphorus removal, an additional aerobic zone combined with a bypass of activated sludge from the anoxic zone to the first trickling filter is needed, in order to provide aerobic conditions to the phosphate accumulating organisms. Several combinations of additional aerobic volume and sludge bypass flowrate were found to successfully achieve both nitrogen and phosphorus removal, using the existing facilities without the need for new reactors neither implementing modifications that could put the trickling filters' physical integrity at risk. The novel treatment scheme could be applied in other cases with similar flowsheet in the same context.