Possible disinfection interventions in rivers on Tuscany cost (Italy)

Aim: Marine waters used for recreational purpose in Tuscany coast (Italy) are sometimes interested by episodes of fecal contamination. During the last years, this led to short-term pollution and subsequently bathing prohibition that posed the problem of possible classification as “scarce” of these areas (according to the Bathing Water Directive 2006/7/EC). The recreational location into consideration is a 345 km2 area in north-western Tuscany, densely populated with 160000 residents along a 20 km seacoast. This region is dedicated to tourism - which represents the main economical activity - with a tourist flow of 600000 visitors per year, mainly in summer season. In this bathing area, the most important sources of fecal contamination are the riverine discharges that receive polluted water mainly when wastewater flow exceeds the capacity of sewer system and/or wastewater treatment plants (WWTPs). This event happens mainly during summer season as a result of increased municipal sewage discharges due to tourist impact, but also during wet weather, due to stormwater intrusion into the sewer system. Consequently, the overflow is discharged into surface water, without any adequate treatment. To avoid short term pollution episodes, various actions have been undertaken, such as control of private discharges, optimized control of drainage pumping and improvement of wastewater treatment. Meanwhile, as an emergency measure to prevent short term pollution events, the possibility of direct treatment of stream water before sea confluence has been explored. This option has been rarely reported in literature, especially in cases of streams with a wide flow variability, for the high uncertainty that can affect disinfection efficiency and the possible side-effect on river ecosystem. Two possible disinfection systems are being evaluated in terms of efficacy and environment harmlessness: peracetic acid (PAA) dosage and cold plasma injection. PAA is a very strong oxidizer, already used for the treatment of WWTP effluents and combined sewer overflows (CSOs). Cold plasma is a mixture of heavy (molecules, atoms, free radicals, ions) and light (electrons and photons) species generated by excitation of gas by electric discharges. This is a new technology, already applied as a disinfection method for biomedical purpose, but not yet applied for full scale water disinfection. After a wide analytical survey aimed at detecting the fecal pollution sources and ways of spreading, an experimental study has ben planned to evaluate the disinfection efficiency and the environmental impact of the two systems. The data collected until now are reported in this paper, while a further data collection is planned in the 2015 bathing season, immediately after the beginning of the experimentation. Three aspects were considered: - Study of contamination sources, and effects of meteorological conditions; - Seawater impact of polluted stream inputs and related environmental issues; - Efficiency evaluation of PAA disinfection at river mouths. Methods: During the 2012 and 2013 bathing seasons, weekly sampling were carried out in five points along the rivers to identify the most important sources of contamination. In the same periods, monthly samples were also collected from the sea at the mouths of these rivers by Tuscany Environmental Protection Agency. All the samples were analyzed for bacterial indicators (Escherichia coli and intestinal enterococci), determined by international standard methods: ISO 9308-3 and ISO 7899-1 respectively. Meteorological data (water temperature and rainfall) were also collected for the sampling dates. The effect of peracetic acid dosage at the last stretch of a stream was evaluated by weakly sampling in three points: before and after the plant and at the river mouth. Each sample was analyzed for enterococci according to the same method. Statistical analysis was conducted to test the relationship between fecal indicators and environmental factors: regression analysis to evaluate the effect of rainfall on bacteriological parameters, and t test to compare bacteriological parameters in dry and wet days. Results: During the two bathing seasons, at river mouth the mean bacterial level was on average 746 ±5.01 CFU/100 ml for E. coli and 634 ± 9.34 CFU/100 ml for enterococci, while in seawater the mean concentrations were 83 ± 5.22 CFU/100 ml and 20 ± 9.14 CFU/100 ml, respectively (figure 1). No relation was found with seawater temperature. The hypothesized dilution effect of sea appears different in different seasons, as it ranged from a minimum of 0,39 dilution log in June to a maximum of 2.30 dilution log in September. Consequently, the limits of European Bathing Directive (500 CFU/100 ml) were exceeded only twice for E. coli and once for enterococci, although their concentration in the river were very high. The association between microbiological concentrations and mm of rainfall was statistically significant. The mean concentration of E. coli was 3633.6 ±5.23 CFU/100 ml in wet days and 467.56 ± 3,44 CFU/100 ml in dry days; for enterococci these values were respectively 4518.71 ± 8.68 CFU/100 ml and 349.59 ± 2.99 CFU/100 ml. Nevertheless, microbiological pollution was highly variable also in dry weather periods and in different sample points along the river, thus suggesting the presence of further undetected sources of fecal contamination. The process of river disinfection with PAA was able to reduce the bacterial levels of the effluent by 1 ± 0.3 Log on average, but this low disinfection efficiency was however sufficient to bring seawater contamination levels below the European limit, since during the study enterococci reached 1100 CFU/100ml as their maximum level of in the river. Conclusion: The collected data allowed us to better figure out the pollution dynamics along the coast and, in particular, they confirmed the high impact of rainfall events. The initial evaluation of stream water disinfection was quite positive, as a slight efficacy in controlling bacterial (enterococcal) seawater pollution emerged. This result indicates the need of a careful setup of disinfection conditions (concentration and contact times) to obtain a sufficient microbial reduction during pollution peaks. Further investigations will be performed during the 2015 bathing season, with an extended microbiological analysis that also will take into account other parameters, such as Salmonella spp. for its sanitary interest, and Human Adenovirus (HAdV) for its resistance to disinfection processes.