The Quality of the Po River in Connection to a Denitrification Improvement in the City of Turin Treatment Plant

River Basin Management III 391 The quality of the Po River in connection to a denitrification improvement in the city of Turin treatment plant G. Genon & M. Vaccarono Politecnico di Torino DITAG , Italy Abstract The city of Turin collects urban wastewater and sends it to an important treatment plant and from there it is discharged into the river Po. Some problems arise from the fact that wastewater flow rate corresponds to 6–8 m3/s, and this flow rate, which is normally high enough to guarantee a good degree of mixing, can be reduced in some periods of the year at the connection point to approximately the same previously mentioned numerical value because of the withdrawal of water to a plant where it is used for hydroelectric production. These aspects have been evaluated in different periods, before and after insertion of a specific section for the removal of biological nitrogen. Some evaluations have been performed on the basis of regular monitoring data concerning the plant effluent characteristics and the quality of the river; a statistical analysis was performed of the correlation of the two data sets, while taking different hydrological aspects of the river into account. A modellistic approach, which is useful to define the capacity of the river for removing the emitted nitrogen, has also been studied. As a final result it was, on the one hand, possible to verify the effective improvement in quality of the river in connection with the increased efficiency due to the treatment and, on the other, to verify the heavy limit on the quality of the river water due to the low dilution ratio at the connection point. Keywords: river quality, nitric nitrogen, denitrification, treatment plant, Po River, nitrogen removal, model, first order kinetics. 1 Introduction Most of the contamination of the Po River, downstream to the city of Turin, is caused by discharges from the municipal treatment plant which is managed by WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 392 River Basin Management III SMAT (Società Metropolitana Acque Torino). The plant is located just after an important derivation for a hydroelectric plant which draws a large quantity of water in San Mauro, and then puts it back just upstream to Chivasso. The environmental conditions surveyed in the regional monitoring site on the Po in Brandizzo, situated some kilometres downstream to the point of discharge from the plant, have therefore been very precarious for several years, both due to the residual river flow rate but also because of the presence of the discharge itself. This study has concentrated on the fate of nitric nitrogen downstream to the plant in relation to the start of operations of a new anoxic denitrification module, the first of four, which has recently been inaugurated. As this first modification started working in 2002, the evolution of the nitrates was studied in 2001 and 2003 to show how the river has responded to this improvement and to see whether these systems could be determinant in resolving the criticality concerning the nitric nitrogen parameter in that stretch of the river. After having tried to reconstruct the hydrological situation in the area, for the two considered years, with as much detail as possible, some statistical analyses were carried out to relate the discharge to the water body receptor and to understand whether there were any phenomena under way in the river that could lead to a decrease or increase in the nitrates. It was then attempted to evaluate the entity of the removal of the nitric nitrogen from the river through a kinetic model in order to be able to forecast the concentration of nitrates in Brandizzo. This evaluation was made using the performances of the plant, but also in relation to the different seasons. Finally, mention is made of how the situation could evolve, from the point of view of 2007, when the treatment in the plant will be completed and the new hydrological state will be working at full regime. 2 Description of the studied area The studied stretch of the Po River, almost 9 kilometres long, is immediately downstream to the city of Turin, and goes from the municipality of San Mauro (1) to the municipality of Brandizzo (3). At the beginning of this stretch, the Po receives discharge treated wastewaters from the largest treatment plant in Italy, which is located in Castiglione Torinese (2). After having passed along the long stretch inside the city of Turin and having received the contribution from the Stura di Lanzo and Dora Riparia rivers, the Po encounters two important water derivations, used for hydroelectric purposes, in the municipality of San Mauro: the AEM plant in San Mauro and the ENEL plant in San Raffaele Cimena and then immediately after collects the municipal discharge wastewaters. This area is therefore particularly interesting as the quantity of water that is removed in San Mauro from the ENEL canal makes this a very critical stretch as far as the environmental state of the waters is concerned. The discharging is in fact made in a section of river that often suffers from low water conditions during the year, with the consequent result of high concentrations of contamination in Brandizzo. The Cimena Canal withdrawal plant is situated in San Mauro, immediately downstream to the AEM plant, and the discharge into the Po is made just after WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 393 Brandizzo: the maximum conceded capacity is 110 m3/s. A scheme of the studied stretch is shown in figure 1, together with the measurement locations. Figure 1: The studied area. 3 Description of the SMAT plant The SMAT treatment plant, in function since 1984, is situated between the municipalities of Castiglione Torinese and Settimo Torinese. The residual waters from the Turin area and numerous other municipalities are channelled there, for a total of more than 3 million equivalent inhabitants. The area, about 450 Kmq, though representing only 1.2% of the Po basin, contributes about 10% of the overall contamination load discharged into the river from civil and industrial sewage systems. This area has almost 1.5 million inhabitants and 1800 industries. Table 1: Situation of the SMAT discharge between 2001 and 2003. Year of discharge Parameter Measurement unit Mean yearly value 2001 Discharge 6,2 m3/s 2003 flow rate 6,2 2001 Ammonia 3,00 mg NH /l 2003 nitrogen 4 2,82 2001 20,99 Nitric nitrogen mg N/l 2003 15,84 2001 27,53 Total nitrogen mg N/l 2003 20,33 The inflows, which amount to 6 m3/s on average, are subjected to physical, chemical and biological purification treatments. In order to conform with CEE directive 91/271, an improvement in the treatment plant was necessary, as far as the abatement of nitrogen is concerned; the first denitrification module was started in July 2002. The other three modules are still working in a traditional manner, while waiting for the improvement works which, in a short time, will make it possible for the entire plant to remove nitrogen from sewage, in WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 394 River Basin Management III prevision of the probable future passage of the Turin area of the Po basin to a zone classified as being sensitive. It has been forecast that in 2007, once the interventions have been completed, there will be an expected lowering of the entering nutrient load of more than 70 %. The year 2002 should be considered as a transition year as far as the plant is concerned, and this is why it was decided to compare 2001, a year in which the lowering of the total nitrogen was fixed at about 23%, with 2003, when the efficiency increased to 44%. 4 Hydrological and qualitative situation of the considered River Po stretch This work was carried out using data obtained from the Regione Piemonte monitoring programme for the quality of water. This programme is based on monthly point measurement campaigns. The survey stations in the area are in San Mauro, upstream to the plant, and in Brandizzo, situated 8.75 kilometres downstream. Even before Brandizzo, the Po is far from being able to be classified as a good level water body. The critical parameters in the studied zone are E. Coli, ammonia, nitrates and above all IBE. The concentration values of the nitric nitrogen measured in Brandizzo during the monthly monitoring activity are reported in the graph for the 2001-2003 period. The level of nitrogen in the Po in this station has all and all diminished (mean value of 5.17 mg N/1 in 2001 and a mean value of 4.29 mg N/1 in 2003) but not in a drastic way, because of the brevity of the considered period (3 years). It is also necessary to consider that though part of the merit is due to the introduction of the treatment module, the measurements are significantly influenced by the hydrological conditions of the river bed which were different for the two years because of the new release constraint of the DMV (Deflusso minimo vitale, an Italian term concerning the minimum flow rate for the conservation of the aquatic life) in San Mauro. 14 12 10 8 g N/l m 6 4 2 0 1 1 1 1 2 2 2 3 3 3 4 0 0 0 0 0 0 - - 0 - 0 - 0 - - - 0 - - - - 0 1 3 5 7 9 - 01 1 3 5 7 - 02 9 - 02 1 3 5 - 03 7 - 03 9 - 03 1 1 - 04 3 11 - 01 11 - 02 1 Figure 2: Trend of the nitrates in Brandizzo from 2001 to 2003.

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