Protecting bathing water quality from sewage overflow in Rimini, Italy

Poor microbial water quality on recreational beaches can cause a variety of conditions such as gastrointestinal, eye, ear, skin, and respiratory diseases (Haile, et al., 1999; Wade et al., 2003; EEA, 2024).The main sources of bacterial contamination of coastal waters are sewage discharges, including sewage outfalls, sewer overflows, stormwater discharges, sewage network failures, polluted river discharges and agricultural and urban runoff (Penna et al., 2021; Claessens et al., 2014). In Rimini, heavy precipitation events caused sewage overflows which resulted in contamination of bathing water along the city’s coast on average around 10 times per year (Venier, 2018). The bathing bans usually lasted between 1 and 4 days, and in some cases up to 7 days (ARPAE, 2023). The combined sewage collection system could not keep pace with the growth in population, the number of tourists and the increasing rates of urbanization and soil sealing. In the northern part of the municipality, several small drainage channels originally used only for collection of rainwater were progressively culverted and integrated into the wastewater collection system. At the end of the channels, watertight floodgates with pumping stations prevented water from flowing into the sea and conveyed it towards the wastewater treatment plant. In cases of heavy rainfall overloading the sewage system, these channels were opened to the sea to avoid local sewage overflows. These occasional discharges of diluted sewage into the sea resulted in temporal bathing bans due to the presence of pollution in the water close to the beaches. Under changing climate conditions, the frequency of intense precipitation events is expected to increase in the region (SNPA, 2021), thereby raising the probability of sewage overflows with negative consequences for bathing water quality.

The creation of a separated sewage collection system (where feasible), improvement of the sewage treatment system, and installation of storage tanks for collecting and storing overflow wastewater and storm water (in areas with a combined sewage collection system) aim to reduce or stop contamination of sea water originating from urban sewage overflows during the increasingly frequent and intense rainfall events.

Following the Optimized Seawater Protection Plan, adopted in 2013, the city’s sewage system and the wastewater treatment plant have been renewed and upgraded, increasing and improving collection and treatment capacities in the whole area. The combined sewage system was replaced with a separated system in most parts of the city. In the most densely built-up areas, where the dual system was not possible, a pumping system including storage for the collection of runoff and sewage water was created or expanded. The system includes an underground plant, completed in 2020, for the collection of runoff water during extreme rainfall events. The runoff water is released successively into the sewage system and conveyed to a treatment plant once the pressure on the urban system has decreased. If storage capacities are reached, the diluted wastewater is released into the sea. The plant consists of two storage tanks located 40 meters below ground; a first tank with a capacity of 14,000 cubic metres collects the initial rainfall water, while the second, with a capacity of 25,000 cubic metres, is dedicated to storage and limiting the amount of mixed wastewater and stormwater conveyed to the sea. This stored water is discharged into the sea 1,000 meters from the shoreline to further reduce any health risks for beach users. The public works necessary for installing new sewage pipes and underground collectors were used as an opportunity to renew the city’s waterfront, substituting streets and parking lots with a park. The park’s walkways, green areas and sports facilities are slightly elevated to provide protection for the city against storm surges and sea level rise. The pumping plants and the water storage tanks are located beneath an attractive building which is used as a belvedere with an accessible elevated terrace overlooking the beaches.

The overall costs (until late 2023) for the improvements in the sewage system are approximately 200 M€. Upgrading of the sewage system has been financed by 3 entities: the Municipality of Rimini (33%) using inter alia regional and EU funding, the water utility (33%), and through tariffs paid by water users (33%). Whilst the latter led to some critique (Rimini 2.0, 2018), according to the Italian water pricing regulations, improvements of water and sewage system services can be co-financed by an increase in consumer payments for the water services. The creation of the urban park has been financed by the municipality which has been able to use, further to the municipal budget, additional national and regional funding. The completion of the Seawater Protection Plan will benefit the tourism industry, improving the reputation of Rimini as seaside resort and allowing for continuous use of beaches. The economic value of the avoided losses caused by health risks in sea water and consequent bathing bans for the bathing establishments is estimated at about 1.5 M€/year (Venier, 2018, p. 233).

Monitoring of bathing water quality as required by the Bathing Water Directive (EC, 2006) is done by the Regional Environment Agency (ARPAE) and takes place close to the shoreline, in a water depth between 80 and 120 centimeters. Monitoring points are pre-defined and correspond to the points where sewage water can potentially be released. The monthly controls of Enterococci and E. coli bacteria during the bathing season (from the beginning of May to the end of September) are complemented by ad-hoc monitoring in cases of extreme precipitation events or discharges from the sewage system into the sea, communicated by the utility company. Such events trigger precautional bathing bans, and additional monitoring activities are carried out to determine the end of the contamination event. From 2020 onwards, when the northern part of the new sewage plant was nearly complete (in the very northern sections some parts of the new system became operative later), the number of sewage discharges into the sea and bathing bans have reduced and, in some places, gone down to zero. The park, created between the beaches and the waterfront, raises the level of the promenade by 80 centimeters, providing an example of how strongly urbanized sections of the coast can be protected against some level of sea level rise without compromising the touristic use of beaches. At the same time, the level of protection against storm surges will not be sufficient under scenarios of higher sea level rise and additional measures might be needed to protect the city. The sewage system is protected against overflows for 1-in-50 year rainfall events. Under climate change, less frequent but more intense rainfall is projected for the Emilia Romagna Region (Sabelli, 2023). Therefore, today’s 1-in-50 year events are likely to occur more frequently and bathing bans are not excluded in the future.