Integrated Modelling of the Sewer System and the Receiving Waters for the Island of Ischia

The paper describes a comprehensive feasibility study carried out to resolve a principal conceptual dilemma concerning the Ischia's (Italy) wastewater problem: Should the wastewater be treated and disposed on the island or should the concept of remote wastewater treatment on the Italian mainland be applied? The study was based on modelling of the impact from the continuous wastewater emissions and combined sewer overflows (CSO) on the water quality in the sea surrounding the island. The simulations were carried out by the application of integrated generic simulation tool — Integrated Catchment Simulator (ICS) — for addressing the environmental impact from urban catchments on the receiving waters. The pollution loads from the present and future sewer network was simulated by use of a deterministic pollution transport model for urban drainage networks (MOUSE TRAP). This provided a detailed information on hydraulic and pollution loads at any point of interest in the island's sewer network. The highly variable pollution emissions from the wastewater system, together with the recipient dynamics (sea currents, wind, and tides) and the water quality processes in the sea (dispersion and fate of Coliform bacteria and dispersion and settlement of suspended solids) create a very dynamic situation concerning the impacts in the surrounding sea. Description of these effects required a 2-D hydrodynamic and water quality model (MIKE 21) to be setup for the waters around Ischia. ICS was applied in order to perform easy and consistent analyses of the entire system, facilitating the transfer of the urban pollution loads to the sea. Simulations were carried out for the present situation and for alleviation schemes, including local wastewater treatment in the island's municipalities, centralised treatment in one large WWTP on Ischia, and with the transport of wastewater to mainland.