**4. Discussion**

In both case studies presented, benefits of an alternative stormwater management approach result from two factors, runoff delivery reduction, and reduction of discharge fees due to implementation of mitigation measures. Introduction of such solutions brings environmental benefits in addition to economic ones: the possibility of using water for irrigation (case 1) allows improved maintenance of green areas, even during long intervals between rainfall events; in addition, lower water consumption from the public supply network reduces not only water bills but also energy consumption and related emissions. Improved maintenance of green areas will reduce surface soil erosion during intense storms, decreasing the load of directly mobilized sediments (and associated pollutants). The use of storage to relieve the flow load on rainwater drainage during heavy rainfall can prevent network overload and local flooding. Both systems have been operated now for two years. The retention tank proved a good solution, notwithstanding its relatively higher cost, and the rain garden also proved to work efficiently. In the first two years of operation (one of which with 35% higher than average cumulative precipitation), no direct local overflows to the Sl˛eza river were observed from either site's outfall, ´ even during the most intense events. Prior to the introduction of these systems, overflows of various intensity occurred approximately 10 times per year, as per qualitative records of the sewer operator. While it is early to assess the long-term performance of these systems, they have shown a positive effect in their operation so far. In case 1, the implemented solution showed direct positive effects on receiving water visual quality near the outfall, preventing pollutants from impervious areas being discharged into the stream, and reducing hydrocarbon residues and suspended solids compared to previous conditions (control analyses carried out twice per year for compliance purposes showed no detectable traces of these pollutants).

The cost analysis carried out for the two rainwater management solutions allows the following conclusions:


According to the analyzed figures, the rain garden solution appears to be the most appropriate from the cost point-of view; however, site subsoil conditions must be conductive to rapid infiltration. Furthermore, this solution excludes any subsequent local water reuse. Application of a rain garden solution in Case 1 (although subsoil conditions were not ideal) would yield a scenario summarized in Table 5.


**Table 5.** Comparison of rain garden vs. local storage solution in Case study 1 site.

As illustrated, the lower cost of rain garden installation would be quickly offset by the remaining cost of water supply (as in the case of no intervention), since this solution does not allow water reuse. On the other hand, it can be seen that in the absence of discharge fees (and related incentives) the economic analysis of the two cases would be completely different, as shown in Table 6.


**Table 6.** Economic analysis in the case of no discharge fees.

In case 1, in fact, investment costs would be recovered in less than four years, with a subsequent accrued gain of 16,950 € due to water bill savings until project year 20, i.e., over 10 times the amount of the initial investment. This contradicts the conclusions of a previous review of rainwater collection and usage systems for single-family houses in Poland, conducted in 2009 and based on the offer from manufacturers' catalogues, with costs estimated at 265–1225 €/m<sup>3</sup> stored (1050–5000 PLN/m<sup>3</sup> ), much higher than in the case study presented. It is clear that under these assumptions, investments would show recovery after a period of 59–100 years, depending on water demand and capacity, and thus hardly be justifiable [44].

In case 2, no economic advantage would exist, and initial cost would never be recovered. This analysis confirms that a financial incentives policy are paramount to the achievement of sustainable stormwater management at the local level.

Reducing, on average, by 89% and 81.5% the runoff volumes from local site discharges in the two cases, considerable savings on sewer network construction could be achieved by the city, offsetting the missed income from discharge fees. An estimate of the overall cost/benefit balance of such citywide policies requires, however, a complex approach that goes beyond the purpose of this paper.

While non-potable reuse of rainwater may provide significant conservation of potable water supplies, the possible relationship between reuse and microbiological risks should be carefully considered. Very few studies are available to date on pathogen risk related to such onsite reuse, and clearly the contamination potential is highly dependent on the specific site. Generally, studies showed that a large percentage of roof runoff samples were non-detects with reference to

pathogens (95%–90%, depending on microorganism), while stormwater samples from residential and commercial/light industrial areas showed most probable number (MPN) lognormal organisms distributions in the range of 1.3 <sup>±</sup> (1.3–2.5) MPN 10 L−<sup>1</sup> [47]. While some studies indicated that ingestion of untreated, onsite-collected roof rainwater and stormwater may result in gastrointestinal infection risks occasionally greater than that traditionally acceptable (10−<sup>3</sup> ppy), they also determined that conventionally collected and treated wastewater pathogen log-reductions may be too restrictive when applied to stormwater, with conflicting evidence about the level of treatment (if any) required for health protection [48]. Decentralized treatment for stormwater may eventually be necessary in specific cases, in the direction of what has already been proposed for greywater reuse: applied technologies may include membranes and microbial fuel cell applications, both showing a compatible degree of pathogenic organism reduction [49,50]. Pathogen cells commonly range from about 1 to 10 microns in length, hence higher degrees of filtration than the one already used in Case 1 (25 µm) may be required.
