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

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

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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

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.

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.

The flow rate was obtained from measurement stations which were installed by the Regione Piemonte on all the stretches of water considered in this study in 2001, but some values only became available in 2002 or 2003. The values of the

River Basin Management III 395 hydroelectric plant abstractions were used to correct the river flow rate, and chloride calibration methods were used when some data were missing. As far as the flow rate of the SMAT plant is concerned, given the poor variability of the exiting discharge from the SMAT plant, which is always about 6 m3/s, it was decided to use the mean monthly discharge.

Table 2: The Po river flow rate in S. Mauro (m3/s).

Year Mean flow rate Min. flow rate Max. flow rate

2001 178,1 20,2 1943,5

2003 99,6 20,9 629,4

5 Comparison between the SMAT discharge concentrations and those measured in Brandizzo

In order to understand how the SMAT discharge quality influences the quality in Brandizzo, a comparison was made between the concentrations so as to have a statistical indication of the concentration that can be expected to be found in the river, as a consequence of a certain quantity of nitric nitrogen exiting from the plant. If the two years are analysed separately, it is first of all possible to note, for the nitric nitrogen, that the maximum 2003 concentrations are equal to the minimum concentrations in 2001, thus demonstrating a decisive improvement in the performance of the plant. The mean value of nitric nitrogen had in fact been 20.99 mg N/1 in 2001, while it was 15.84 mg N/1 in 2003. It should also be mentioned that only one of the four foreseen denitrification modules has so far started to operate. This improvement, however, did not always have a direct effect on the downstream station; the concentrations in 2003, (on average 4.29 mg/1), were not always lower than those that were measured each month in 2001, which on average were 5.29 mg/1. The improvement throughout the year is therefore evident, but not exceptional. It is therefore clear how the quality of water in Brandizzo suffers from factors that do not depend on the plant, above all the contamination load coming from San Mauro, and it is on this that the environmental situation downstream to Castiglione greatly depends. It was then attempted to linearly interpolate the data in order to have a statistical relation between the discharge and the measurement station. The situation changes from 2001 to 2003, as it can be seen that low concentrations of the discharge seem to have less influence on the quality in Brandizzo than higher concentrations, such as those of 2001. When interpolating the 2003 data, it can also be noted what would happen if the discharge were not there, that is, if the SMAT concentrations vanished. The nitric nitrogen concentrations that we would have would be about 2.4 mg N/1:

396 River Basin Management III this in fact is more or less the mean yearly concentration measured at San Mauro (2.6 mg N/1), which therefore means that, even if the plant were not present, it would not be possible to obtain lower concentrations in this stretch.

8 y = 0,1202x + 2,3845 2001 6 2003

Brandizzo mg N/l 4

0 0 5 10 15 20 25 30 SMAT mgN/l

Figure 3: Comparison between the exiting nitric nitrogen concentrations from the SMAT plant and those measured in Brandizzo.

6 Comparison between the loads calculated in Castiglione and those measured in Brandizzo

In consideration of the increase in the river flow rate in Brandizzo arising from an agreement between the Regione Piemonte and ENEL for DMV releases, it was of fundamental importance to analyse the advected loads rather than the concentrations; this analysis leads in fact to the possibility to distinguish the contribution given by the treatment module from that given by the increase in the minimum flow rate. Furthermore, the treatment plant is surely the main contamination factor for Brandizzo, therefore it is also interesting to establish a relationship between the discharge and the water body receptor from the point of view of the loads, apart from that of the concentrations. It is first of all possible to see from the graphs how an increase in the SMAT load would inevitably reflect on the load passing through Brandizzo; this sensitivity was slightly more evident in 2001when the loads at stake were higher. Apart from foreseeing what the load that flows past in Brandizzo will be with a variation of the load dumped in the Po, it is also possible to see if this relationship has changed over the years. In 2001, in fact, when the denitrification module was not yet operative, the removal of the total nitrogen by the plant was of about 22 %; with the changes that have been made, the removal has jumped to about 43 %, therefore there has been a remarkable diminishing of the flowing pollutant load and this will be even more obvious when the other three modules begin to function. It can be noted

River Basin Management III 397 that, in spite of the variations in loads, the trend of the loads over the two years can be practically interpreted by the same relations and there have not been any particular variations. Analogously to what has been seen for the concentrations, it is easy to note from the graphs that there was a clear decrease in the nitric nitrogen load (a reduction of the total yearly load of about two thirds) from 2001 to 2003.

500

450

400

350

300 2001 250 2003 200 Brandizzo, g N/s 150

100

0 0 50 100 150 200 SMAT, g N/s

Figure 4: Comparison between the nitric nitrogen loads calculated in Castiglione and those measured in Brandizzo.

7 Kinetic model for the nitric nitrogen

If a comparison is made between the nitric nitrogen concentration found immediately downstream to the plant discharge and that in Brandizzo, about nine kilometres further downstream, it is possible to verify whether the nitric nitrogen tends to decrease or increase along the river, in a section where no tributaries, derivations or plant discharges are present. As can be seen from fig. 5, the nitric nitrogen concentrations calculated for the SMAT node are systematically higher than those measured in Brandizzo, except for in rare cases. This inevitably implies that there are phenomena under way in that stretch of the river which, as a whole, lead to a decrease in the load of nitrates in the river. Furthermore, the behaviour is the same for both years; the decrease in nitrates in the Po occurs independently of the initial concentrations of nitric nitrogen and of the flow rate of the river, which were both very different for the two years. In 2001, there was, in some rare cases, a value of nitric nitrogen downstream to the treatment plant that was in fact slightly lower than that measured in Brandizzo. This was probably due to a measurement error or an incorrect estimation of the flow rate. It can also be seen that a remarkable decrease in the nitric nitrogen concentrations downstream to the plant occurred once the first denitrification module began to work. This concentration sometimes touched 23 mg N/1 in some months in 2001, but in 2003 it hardly ever passed 10 mg N/1. This must

398 River Basin Management III have helped the quality of the water in Brandizzo where the mean yearly nitric nitrogen concentration in 2001 was 5.19 mg N/1, but in 2003 never went over 4.29 mg N/1. Having therefore verified that the nitric nitrogen decreases downstream to the treatment plant, a kinetic study was carried out with the purpose of obtaining an expression that would allow the entity of this removal to be quantified: the single kinetic constant K was considered to consider first order kinetics.

25 /

2001 15 2003

5 Concentrations after the discharge, mg N 0 024681012 Concentrations in Brandizzo, mg N/l

Figure 5: Comparison between the concentrations in Castiglione and those in Brandizzo.

The considered equation is: dC v −= KC dx which, integrated and re-written in such a way as to isolate the kinetic constant, becomes: − lnln CC K = 0 f ⋅ v X where C0 is the concentration downstream to the discharge (mg N/1), Cf is the concentration measured in Brandizzo (mg N/1), X is the actual length of this section, which is 8750 m, v is the velocity of the current (m/s) and K is the kinetic constant (s-1). The hypothesis was then made that the velocity of the river does not change from the SMAT plant discharge onwards and the Castiglione value was assumed as the representative velocity of the entire stretch, which is not particularly meandering and is rather regular. The available data were interpolated in order to have a linear velocity - flow rate relation. It was, however, possible to calculate the kinetic constant for the behaviour of the nitric nitrogen in the river for each month of 2001 and 2003. The results obtained with the data from 2001, which were only estimated, should be considered as a further

River Basin Management III 399 confirmation of those from 2003. In order to use the equation, it is necessary to have C0 and Cf: the first, which is the concentration of the nitrates downstream to the plant, can be calculated with a simple balance between the flow from San Mauro and the mean monthly SMAT discharge. The second is instead known. The arithmetic mean of all the kinetic Ks obtained each month in the two years is K = 1.14 E –5 seconds –1. All the values, with the exception of three, indicate that the kinetics of the nitric nitrogen, all in all, are of a decreasing and not increasing type. The values for 2003 are much less dispersed than those of 2001. However, half of these values are close to the mean value. As the K kinetics depend on many phenomena, which are also influenced by the seasons in which they occur, it was interesting to see whether there is any difference in the kinetic constant between the coldest months and the warmest months of the year. January, February, March, October and December were considered as “cold” months while April, May, June, July, August and September were considered “warm”.

Table 3: Kinetic constants (s-1) calculated for the removal of nitric nitrogen.

K mean K warm K cold 2001 9,59 E-6 ± 4.06 E - 5 1.67 E – 5 6.5 E – 6 2003 1,30 E-5 ± 6,31 E - 6

It is therefore clear that when it is cold many phenomena are inhibited because of the low temperatures and this makes the removal mechanisms of the nitrogen two to three times less effective than in the warm months.

8 Environmental situations and future plans

As already seen, the situation in Brandizzo from 2001 to 2003 has improved, as far as nitrates is concerned. If, however, the general situation is analysed, it can be noted how this progress has led to very little change in the environmental quality of the Po river in that stretch. The quality classes, as foreseen by EEC directive 91/271, are in fact dependent on seven macro-descriptors (nitrates, ammonia nitrogen, total phosphorus, BOD, COD, E. Coli and dissolved oxygen) and the improvement of just one of these does not influence the total points to any great extent, if not accompanied by a general progress of all the others. In the present case, the Po obtained macro-descriptor value (160) in Brandizzo in 2001, dropped to 150 in 2003. Although the nitric nitrogen in fact led to a higher score, the same did not occur for the other parameters, such as COD or phosphorus, which instead became worse. It should be pointed out that this worsening was not caused by the plant, whose exiting parameters in 2003 had all improved compared to 2001, but by a deterioration of the situation upstream. These scores would have placed the Po River as a level 3 water body (sufficient environmental state) for both years. If, instead, we consider the future, the lowering that the plant will be able to perform when all four modules start to function is estimated to involve at least 73% of the total nitrogen entering the plant; it is therefore possible to hypothesise that the exiting concentration of

400 River Basin Management III nitrates after 2007 will be about 7 mg N/1. If all the denitrification modules had already been working in 2003, the nitric nitrogen concentration in Brandizzo would have been, on average, about 2.52 mg N/l, against the 4.29 mg N/1 that was instead measured; the concentration would thus have been halved. The improvements that are under way in the plant will therefore have a tangible result on the water downstream to the plant, but this consistent improvement, if it remains isolated, will not have any influence in the least on the passing from one class of quality to another and, being of 150 points, the water will remain class 3. A higher class for nitrates would in fact only be reached below 1.5 mg N/1, a concentration that is already exceeded to a great extent upstream to the plant.

Table 4: Past situations and future forecasts for the studied area: nitric nitrogen concentrations, mg N/1.

Nitric N Nitr. removal Nitric N Nitric N Quality class Year SMAT efficiency Castglione Brandizzo Brandizzo discharge 2001 20,99 0 % 7,94 5,18 3 2003 15,84 23 % 6,92 4,29 3 2007 7 73 % 4,10 2,52 3 Discharge in Cimena Canal 2,65 1,27 3

Apart from evaluating what will happen once the denitrification system has been completed, it is also possible to analyse what would happen if the discharge from the SMAT plant were to be performed in the Cimena Canal instead of in the Po (a prospect that has already been discussed). Even though a flow rate equal to the DMV will continue to be released, the discharges in various months of the year will in fact always be much lower in the stretch where SMAT discharges (22 m3 /s on average in 2003) than those of the Cimena Canal (61 cu.mt/s on average in 2003). If this option had already been put into operation in 2003, there would have been a mean concentration of nitric nitrogen equal to 1.27 mg N/1, compared to the 4.29 mg N/1 that instead was measured. Only the absence of the discharge would allow Brandizzo to fall below 1.5 mg N/1 (and to obtain 40 points as a macro-descriptor score) which is an excellent result: this would however not have allowed the water to pass to class 2 in 2003, but it would have instead have allowed it in 2001. It is necessary to mention that nothing would change downstream to the inflows. Changing the discharge would therefore appear more as a solution for the Brandizzo station rather than for the Po River water body on the whole.

9 Conclusions

In this study it was attempted to understand the fate of nitric nitrogen downstream to the SMAT treatment plant after the first of four treatment modules started working. Some statistical comparisons were carried out between the discharge and the water body receptor, both from the loads and concentration points of view. It has been ascertained that along the Po, between San Mauro and

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Brandizzo, a removal of nitric nitrogen occurs, mainly due to assimilation by plants and bacteria and this was quantified through a kinetic constant. It was also verified that when the first module started functioning, a certain decrease in the level of nitrates in the Po was obtained, but it is difficult to separate this contribution from that given by the greater release downstream by the Cimena Canal withdrawal plant, which has led to a remarkable improvement in the downflow conditions in Brandizzo (the flow rate in Brandizzo was higher than 10 m3/s for 274 days in 2003). It was also noted how the ENEL derivation greatly penalises the environmental situation in Brandizzo and makes the consistent progress made by the plant less important. Furthermore, the quality class the studied stretch belongs to depends not only on the nitrates, but also on several other parameters, which have not shown any sign of decreasing in the station upstream to Brandizzo since 2001: it will therefore be necessary to act both from the hydraulic point of view and in terms of contamination upstream.

References

[1] Scott J.A., Abumoghli I.,”Modelling nitrification in the river Zarka of Jordan”, Water resources, vol. XXIX, n. 4, 1995. [2] Warwick J.J., Mc Donnell A.J., “Simultaneous in-stream nitrogen and D. O. balancing”, Journal of Environmental Engineering, vol. 111, n. 4, 1985. [3] Genon G., Marchese F., “ Evoluzione dello stato di inquinamento del fiume Po presso Torino”, Inquinamento, n. 7/8, 1985.