In light of the results of numerous studies demonstrating an increase in in the frequency of extreme precipitations and floods [1
] and of soil sealing due to urbanization [2
] all over Europe, stormwater management is assuming a crucial role in the preservation of urban catchments in the face of two undesired phenomena: (i) pluvial floods and (ii) combined sewer overflows (CSOs) [3
]. Engineers should provide solutions to minimize the impact of these phenomena which negatively affect human life, economic assets, and the environment [4
] without forgetting sustainability [5
]. In fact, the “sustainable management of water” is a pillar of the 2030 Agenda (https://sustainabledevelopment.un.org/
In the last few decades, European, national, and local authorities addressed these problems promoting mainly the use of three families of solutions: (i) “end-of-pipe solutions” (EOPSs), (ii) “source control technologies” (SCTs), and (iii) “real-time control” (RTC). The first includes construction techniques used to reduce already formed volumes of water and contaminants such as tanks and detention ponds. They are normally implemented as the last stage of a sewer system before the water is discharged into a water body [6
]. Because of the high cost of end-of-pipe stormwater management solutions and the lack of space in which to build them, particularly in highly urbanized urban basins, many water utilities are currently investigating the feasibility of reducing runoff with decentralized SCTs distributed throughout the urban watershed. This second family of solutions includes a range of approaches and techniques for local, on-site management and control of stormwater runoff at the point of rainfall [8
]. Examples include green roofs, bioswales, rain gardens, permeable pavements, vegetated strips, wet/dry ponds, and others [10
]. “Rainwater harvesting systems” can be considered a further example of SCTs. Despite being designed to meet other needs such as fighting water scarcity, reducing water withdrawal from traditional sources, or promoting water saving in buildings, they reduce and delay the peak of runoff conveyed into the sewer systems, thereby contributing to CSO reduction [13
Approaches to water pollution control that focus on wastewater prevention and minimization should be given priority over traditional EOPS whenever possible [14
]. In Italy, because of decades of design processes influenced by polices which incentivized the use of SCTs, most cities currently integrate both types of solutions. Moreover, there is also a third family of technologies, known as RTC, which allows minimizing overflows by optimizing water volumes stored in the sewer system. This group of technologies can be realized by efficiently operating the regulators of the system, such as pumps, gates, or others. RTC can be applied to manage the overall drainage systems including both EOPSs and SCTs. Most studies have been field-based and focused mainly on monitoring and then simulating a single type of technology at a site or a block scale. For example, some authors [15
] built and monitored some green roofs to reduce peak flows, while other estimated the storm water mitigation potential of rainwater harvesting systems at the urban scale. Others have shown that permeable pavements were effective in retaining precipitation during storm events [18
], that SCTs can alleviate water quality problems associated with diffuse pollution [22
], that storm detention tanks are efficient even under rainfall variability due to climate change [6
], and that RTC is not only a cost-efficient measure to mitigate the impact of CSOs but also a solution offering the most flexibility for further system upgrade [23
]. No study has focused on evaluating the possible benefits deriving from the combined application of the technologies mentioned above simulated with long-time rainfall series. Therefore, the optimal management of a current urban drainage system cannot neglect the possibility of studying the influence of a complementary use of EOPSs, SCTs, and RTC in terms of reducing the volumes of water and masses of pollutants discharged by CSOs, as well as urban flooding. To address the abovementioned lack of knowledge, the long-term behavior of a small urban catchment (48 ha) was studied using a scenario-based approach. Runoff quantity and quality were modeled by means of the Environmental Protection Agency (EPA) Storm Water Management Model (SWMM). Simulations were carried out using 15 years of weather data (rainfall and air temperature data with a 15 min time step). Scenarios evaluated the single impact of the abovementioned technologies and of their combination.
3. Results and Discussion
Twelve scenarios with different green, gray, and RTC installations were modeled under 15 years of rainfall and temperature data. Simulations results were evaluated with respect to the C1real benchmark scenario in terms of runoff volumes (m3) and total suspended solids (TSS) (kg) spilled by the CSO into the receiving water body. Long-term simulation results were analyzed on an annual basis and divided by the total surface. This allowed analyzing specific values of runoff volume (m3/ha) and TSS (kg/ha). The CSO specific volume and TSS of overflow spilled into the river, computed as the medium values over the 15 years, were fairly significant, ranging from 864 m3/ha to 24 m3/ha and from 88 kg/ha to 1 kg/ha, respectively.
Compared to the benchmark scenario (C1real
), the C1RTC
scenario reduced runoff and TSS by 34% and 65%, respectively, and confirmed the possibility of increasing the efficiency of the wastewater treatment system to which the most polluted water is sent (Table 5
). Figure 9
shows, year by year, the reduction in both runoff volume and TSS [36
depicts a bar chart of the simulation results, in terms of specific average runoff volume (a) and TSS (b), for all scenarios considered, while Table 5
shows the corresponding numerical values.
scenario (green roof) reduced runoff by 20% and TSS by 39% compared to the baseline scenario. The increased percentage of porous pavement in the C3real
scenario reduced stormwater runoff by 43% and TSS by 25%, as compared to the C2real
scenario because the C3real
scenario increased infiltration of stormwater runoff. Figure 11
shows the differences in terms of the total volumes of water infiltrated, drained, and lost through evapotranspiration for cases C1, C2, and C3. In the green roof scenario (C2), 27% of the rain evaporated, 40% infiltrated, and the remaining 33% drained to the sewer system. In scenario C3, however, the predominant phenomenon was that of infiltration (65%), followed by drainage (25%) and evapotranspiration (10%). The ability to infiltrate great volumes of water, typical of the permeable pavements used in the C3 scenario, allowed reducing volumes drained by the sewer system and, therefore, those spilled by the CSO toward the water body. However, these technologies, unlike the green roofs present in the C2real
scenario, are unable to filter the water. This is the reason why TSS removal was more pronounced in scenario C2real
(39%) than in scenario C3real
(25%). In both cases (C2 and C3), the additional presence of the RTC system determined a marked improvement in performance in terms of volume runoff. Furthermore, since the presence of an RTC system optimizes the storage capacity of the network and, therefore, increases the volumes of wastewater sent to depuration, the C3RTC
scenario could guarantee a clear improvement in terms of TSS removal respect to C3Real
. In fact, there was a decrease in the difference in TSS removal between the C2 and C3 scenarios, which went from an average of 14% to 2% in the real and RTC configurations, respectively. Figure 12
shows, year by year, the differences in volumes and TSS among C1real
, C2, and C3 cases for both configurations (real and RTC).
The C4real and C4RTC scenarios (rainwater harvesting systems) reduced runoff by 67% and 78% and TSS by 77% and 93%, compared to the baseline scenario. These systems stored varying volumes of precipitation which were then used by people for nonpotable purposes. In fact, they reduced the volumes of rain entering the sewer system, and this reduction led to less volume and mass spilled from CSO. The efficiency of these systems depended greatly on the type of rainy events; well-distributed low-intensity events led to a higher efficiency than a few very intense rainfall events. The use of 15 years of rainfall data allowed us to evaluate the average behavior, including the frequency of heavy rainfall. Simulation results showed that rainwater harvesting systems are a valuable technology not only to reduce potable water consumption but also to prevent CSO spills. Furthermore, in this case, the presence of an RTC system contributed significantly to lowering the peak flow rate below the threshold of the CSO and, consequently, the volume of runoff and the mass of TSS discharged.
Case C5a and C5b scenarios were those equipped with 450 and 2245.5 m3 of detention ponds, respectively, which guaranteed the hydraulic safety of the catchment. Compared to C1real, C5areal reduced stormwater runoff and TSS mass by 56% and 54%, respectively, while C5breal reduced stormwater runoff and TSS mass by 61% and 56%, respectively. This means that increasing the holding volume by five times increased the system performance by only 5% for volume and 2% for TSS mass. On the contrary, the inclusion of an RTC system in scenario C5a led to a net improvement in performance. In fact, C5aRTC showed a reduction of 97% for volume and 99% for TSS. This means that the joint insertion of detention systems of 10 m3/ha and an RTC system practically eliminated all CSO spills. With scenario C5bRTC, there were identical results.
The results of the simulations showed that the inclusion of an RTC system led to a significant hydraulic and environmental benefit, both in the base scenario and in all scenarios equipped with SCT or EOPS technologies. In particular, the simulation results showed that RTC systems are better at reducing the mass of TSS than the volumes. However, it should be noted that the use of RTC technology requires an important economic and management effort on the part of multiple utilities and does not lead to secondary benefits such as biodiversity, hydrological regeneration, or reduction in water withdrawal. The realization of an RTC system requires a high level of technology in terms of sensors, alarm systems, and controls. Furthermore, the technological complexity of these systems leads to a need to create detailed maintenance plans. EOPSs have proven to be an excellent tool for reducing outflows; however, their ability to remove TSS is adequate and they do not provide any further ecosystem services. On the other hand, SCTs show different behavior depending on the predominant mechanism. In fact, from the comparison of C2 and C3, it emerged that technologies such as permeable pavements, in which infiltration is predominant, have better performance in terms of a reduction in outflows, while those in which evapotranspiration predominates remove TSS better. This is mainly attributable to the vegetation that filters the water by retaining the particles of pollutants. In addition to the hydraulic benefits measured using the model, it is important to underline how these solutions also contribute to the increase in biodiversity, the creation of public spaces, the mitigation of the heat island effect, hydrological regeneration, etc. [37
]. On the other hand, these spaces are generally private; thus, the maintenance and construction operations fall on a single individual and not on multiple utilities, which can represent both an advantage and a disadvantage linked to the difficulties in controlling efficiency over time. Of all the technologies analyzed, rainwater harvesting systems appear to be the most efficient in terms of both volume reduction and TSS mass removal. These systems also make it possible to create onsite water supply sources by reducing the withdrawal from the traditional water supply. They, therefore, contribute to reducing the consumption of precious water resources while increasing the resiliency of the entire supply system. Furthermore, in this case, the costs of such interventions are often borne by individual citizens, and it is difficult to monitor the functioning status of these plants over time. While many local authorities promote their use through incentives, there is often no way to verify that these remain in place and, without incentives, the cost of water is often so low that it is not possible to reach the breakeven point of the investment in a reasonable time.