THE IMPACT OF SEWAGE TREATMENT PLANT ON THE AMOUNT OF HEAVY METALS IN WATER OF THE SUPRAŚL RIVER CATCHMENT AREA

The main purpose of this study was to evaluate the effect of treated sewage flowing from sewage treatment plants located in the basin of the Supraśl river on the concentration and load of metals in river waters and its main tributaries. Three measuringcontrol points were chosen, on the river and its tributaries, located near Gródek, Sokółka and Dobrzyniewo. Selected points were located behind the discharge of treated wastewater from sewage treatment plants respectively – Gródek, Sokółka and Bialystok. The samples of treated sewage and water were collected in a period from May to November, once a month in 2014. Each individual sample was examined for the content of dissolved form of the following metals: Pb2+, Cu2+, Cd2+, Ni2+, Zn2+, Fe2+/3+. After taking into account water flow of the Biała, Sokołda and Supraśl in every month, metals loads expressed in mg·h-1, transported by the Supraśl and its tributaries waters were calculated. In the study monthly metals loads discharged into the Biała, Sokołda and Supraśl by sewage treatment plants in Białystok, Sokółka and Gródek were also calculated. The studies have shown the impact of metals load in treated wastewater on metals loads in waters of studied rivers based on the obtained correlation. Most of the searched relations between loafs of Pb2+ – r = 0,88; Cd2+ – r = 0,98; Fe2+/3+ – r = 0,45; Ni2+ – r = 0,55; Zn2+ – r = 0,86 were obtained in case of wastewater treatment plant in Gródek and Supraśl waters. In the study period we observed a diversity in concentration of Cd2+, Fe2+/3+, Ni2+ and Zn2+ in treated sewage and in river waters, which affected loads of this metals.

The content of metals in natural waters is low.The most important source is land leaching, containing metals in its composition [Duffy and vanLoon 2008].They can penetrate waters with their runoffs.An important anthropogenic source are industrial waste water mainly from the chemical, metallurgical and galvanic facilities [Kociołek-Balawejder and Stanisławska 2012].The source of increased metal content can also be corroded pipelines and tanks, made of raw materials, which was composed from these elements [Hermanowicz et al. 1999].Increased metals amounts in environment contributes to the intensification of agriculture and transport development [Duffy andvan Loon 2008, Piekutin 2008].Metals occurrence in surface waters are mostly affected by the type and degree of catchment urbanization.
The aim of this study was to evaluate the effect of the treated sewage flowing from sewage treatment plants located in the Supraśl basin on the concentration and metals loads in river water and its main tributaries.

METHODS
The Supraśl length is 93.8 kilometers, making it the second largest river in Podlaskie [Skorbiłowicz 2010].It is a tortuous and meandering lowland river, which is characterized by a small decline (0.76‰) with an average flow of 4.18 m 3 •s -1 .It flows in a well-shaped, broad valley.It begins its run near the village of Topolany and heads eastwards.By Michałowo it turns north and runs through Gródek, followed by a large part of Knyszyńska Forest.Then, it passes Supraśl and Wasilków and in the village of Złotoria it falls into the Narew on 299.8 kilometers of its course.
Its two main tributaries are the Słoja and Sokołda.Płoska and Biała also falls to the Supraśl river, which is main receiver of the treated sewage from plant in Białystok.In Wasilkow the Supraśl waters are used as drinking water for Białystok, which contributed to protection zone embrace.Forests cover 30% of the catchment area.A large share of which are also wetlands and peat bogs.On the Supraśl river and its tributaries, 3 measuring -control points were chosen, located near Gródek, Sokółka and Dobrzyniewo.Selected points were located behind the discharge of treated wastewater from sewage treatment plants Gródek, Sokółka and Białystok (Figure 1).
The sewage treatment process in Bialystok is based on activated sludge method.In the technological wastewater treatment stands out mechani-cal and biological node.Wastewater treatment plant in Bialystok is one of the largest facilities in Poland.The throughput is about 100,000 m 3 •d -1 .The operation principle of sewage treatment plants in Sokółka is based on activated sludge method, which is implemented in sequential biological reactors.The reactor operating cycle is about 8 hours.The capacity of sewage treatment plant in Sokółka is up to 6000 m 3 •d -1 .In that facility domestic wastewater with small amount of industrial wastewater is treated.Similarly to the Sokółka, wastewater treatment plant in Gródek is based on activated sludge method which is implemented in sequential biological reactors.Reactor cycle is about 1.5-2 hours.The capacity of sewage treatment plants in Gródek is about 80 The samples of treated sewage and water were collected in period from May to November, once a month in 2014.
Each individual sample was tested for concentration of dissolved form of the following metals: Pb 2+ , Cu 2+ , Cd 2+ , Ni 2+ , Zn 2+ , Fe 2+/3+ .After taking into account water flow of the Biała, Sokołda and Supraśl in every research month metal loads expressed in mg•h -1 , transported by the Sypraśl waters and its tributaries were calculated.In the study monthly metals loads discharged into the Biała, Sokołda and Supraśl by sewage treatment plants in Białystok, Sokółka and Gródek were also calculated.
The contents of the analyzed elements was determined by atomic absorption spectrometry (AAS) using a Varian SpectrAA type -100.The source of the oxidizing flame was air-acetylene.Each sample before the assay was acidified to pH ≈ 2.
Statistical analysis was performed with Statistica software licensed version 10.0.The scope of the analysis included descriptive basic measures, the arithmetic mean, median, minimum, maximum and standard deviation.Pearson cor- relation coefficients were determined between the metal loads in the Biała, Sokołda and Suprasl and purified wastewater from individual waste water treatment plant.The flow of river waters at some points was defined complementary based on a mathematical method proposed by Brykała [2009].

RESULTS AND DISCUSSION
The studies have shown the highest concentration of Pb 2+ in Sokołda waters at the point of Sokółka, which amounted to 0.015 mg•dm -3 .In contrast, concentrations observed in Gródek and Dobrzyniewo were similar and were respectively equal to 0.010 and 0.011 mg•dm -3 (Tab.1).Slightly higher range of this element concentrations was observed by Trawczyńską et al. [2009] in the waters of Bzura rivers.Depending on the measurement point authors report Pb 2+ concentration from 0.001 to 0.033 mg•dm -3 and point out the pollution of Poland rivers with lead.Therefore, it is difficult to determine the natural amount of the element present in the surface waters.
The concentration of Cd 2 + in each point was relatively uniform.The smallest concentration of this element was observed at the point of Sokółka and the highest in Dobrzyniewo (Table .1).The observed values were equal to 0.004 and 0.007 mg•dm -3 respectively.Slightly higher concentration compared to that observed in Sokółka was achieved in Gródek (0.005 mg•dm -3 ).Much lower concentration of Cd 2+ was observed by Palapa and Maramis [2015] in studies on the quality of water in a small stream flowing through Talawaan, which is located in the vicinity of a gold mine in Indonesia.Researchers obtained the con-centration of Cd 2+ ranging from 0.0001 to 0.0004 mg•dm -3 .As highlighted the values obtained by them are typical for the average concentration of this element in waters of other rivers in the region.
The concentrations of Cu 2+ were very similar in each measuring point and amounted to 0.040, 0.050 and 0.060 mg•dm -3 in Sokółka, Gródek and Dobrzyniewo (Table 1).
The concentration of Fe 2+/3+ in Supraśl water observed in Gródek was equal to 0.86 mg•dm -3 .Slightly less iron was in the waters around Sokółka (0.77 mg•dm -3 ), and the least in Dobrzyniewo (0.51 mg•dm -3 ) (Table 1).A greater range of concentrations of this element was obtained by Kaniuczak and Augustyn [2011] in the studies conducted in the river Wisłok.The observed amounts of Fe 2+/3+ ranged between 0.080 and 2.870 mg•dm -3 .
The concentration of Ni 2+ and Zn 2+ were the least diverse from all the analyzed elements.In the case of nickel, the same concentration in water was reported in Gródek and Dobrzyniewo (0.040 mg•dm -3 ), while in Sokolka Ni 2+ concentration reached almost twice as much (0.070 mg dm -3 ).In the waters of Gródek and Sokółka the same concentration of Zn 2+ were observed amounting to 0,030 mg•dm -3 in water of Dobrzyniewo Zn 2+ concentraion was two times higher reaching a value of 0.060 mg•dm -3 .
Lead concentrations in treated wastewater from all the objects was similar (Table 2).The least of this element occurred in wastewater from treatment plant in Białystok and Gródek (0.010 mg•dm -3 ).Slightly more lead was recorded in treated wastewater from the Sokółka (0.011 mg•dm -3 ).Cd 2+ concentration in treated wastewater from individual facilities was varied.Least Cd 2+ was in treated wastewater from Gródek (0.009 mg•dm -3 ), while the highest in wastewater from sewage treatment plant in the Sokółka (0.014 mg•dm -3 ).In turn, the concentration in treated wastewater from Bialystok was equal to 0.011 mg•dm -3 .Concentrations of Cu 2+ in particular objects were very similar and ranged from 0.050 to 0.060 mg•dm -3 .The concentration of Fe 2+/3+ varied.The least Fe 2+/3+ was recorded in treated wastewater discharged from treatment plant in Białystok (0.330 mg•dm -3 ).In wastewater discharged from Gródek, Fe 2+/3+ level was more than twice as high as in treated wastewater from Bialystok (0.700 mg•dm -3 ), but most of this element was in the wastewater from the Sokółka (1,070 mg•dm -3 ).The concentrations of Ni 2+ and Zn 2+ were characterized by a similar distribution in treated wastewater in each facility.The maximum concentration of both elements were observed in wastewater from the Sokółka (0.040 mg•dm -3 for Ni 2+ and 0.190 mg•dm -3 for Zn 2+ ).In turn, the minimum concentration of both elements occurred in sewage from Białystok equal respectively to Ni 2+ and Zn 2+ 0.010 and 0.050 mg•dm -3 .However, in treated wastewater from Gródek the concentration of these elements for Ni 2+ was 0,040 mg•dm -3 , while 0,120 mg Zn 2+ •dm -3 .For all the studied elements, none of the studied facilities exceeded the limits set by the Regulation of the Minister of Environment of 18 November 2014 on the conditions to be met when introducing sewage into water or soil and on substances particularly harmful to the aquatic environment.
Loads of individual elements within the study period were relatively balanced, as evidenced by similar values of arithmetic mean and median (Table 3).Wastewater treatment plant in Gródek discharged into the waters of Supraśl mostly Fe 2+/3+ , whose average amount was equal to 76.48 mg·h -1 .Pb 2+ and Cd 2+ were the least discharged loads and were equal to 1.10 and 0.93 mg·h -1 respectively.Cu 2+ charge was a few mg·h -1 , Zn 2+ dozen and Ni 2+ about 2 mg·h -1 .A number of discharged loads of the elements can be presented as follows: Fe 2+/3+ >Zn 2+ >Cu 2+ >Ni 2+ >Pb 2+ > Cd 2+ .In the Supraśl behind the discharge of sewage from a sewage treatment plant in Gródek, the average charge of Fe 2+/3+ (0.178 mg·h -1 ) was the largest and the lowest of Cd 2+ (0.001 mg·h -1 ) (Table 4).Dispersion of results from the mean values was small as evidenced by the value of standard deviation.
Loads of metals in wastewater from sewage treatment in Sokółka were higher in relation to wastewater from treatment plants in Gródek (Table 5).This is related directly to the size of each plant.Sewage treatment plant in Gródek represents a relatively small group of objects.Similar to wastewater treatment plants in Gródek, Sokółka discharged largest load of Fe 2+/3+ and Zn 2+ , and the smallest of Pb 2+ and Cd 2+ (Tables 3 and 5).In the case of wastewater treatment plant in Sokółka a slightly different number of discharged loads was observed, which was as follows: Fe 2+/3+ >Zn 2+ >C u 2+ >Ni 2 >Cd 2+ >Pb 2+ .In Sokołda waters the high-  est average load was from Fe 2+/3+ (0.487 mg·h -1 ) and the lowest from Cd 2+ (0.003 mg·h -1 ) (Table 6).The standard deviation does not indicate a large deviation from the average.Similarly, as in the case of the two earlier facilities, the largest discharges load from Bialystok was from Fe 2+/3+ and Zn 2+ (Table 7).The lowest discharge from the wastewater load was Pb 2+ and Cd 2+ .The loads of other elements were more than ten times greater than those discharged from the treatment plant in Sokolka and more than one hundred times greater than those form Gródek.The values of metal loads, as in other objects, were directly related to the volume of wastewater and the average volume of monthly wastewater inflow.The sewage treatment plant in Bialystok showed the same number of discharged loads as wastewater treatment plant in Sokółka (Fe 2+/3+ > Zn 2+ >Cu 2+ >Ni 2+ >Cd 2+ >Pb 2+ ).In the Biała waters the greatest load was form Fe 2+/3+ (1,354 mg·h -1 ) and the lowest form Cd 2+ (0.017 mg·h -1 ) (Table 8).Loads of metals in waters of the Biała in Dobrzyniewo were greater than those observed in the Supraśl waters (point Gródek) and the Sokołda (point Sokółka).It is related to load values in wastewater discharged by the analyzed facilities.
Comparing to all the studied treatment plants, significantly greater loads of the individual elements were obtained by Deycarda et al. [2014] in a sewage treatment plant in Fargue Louis, France.The studies of wastewater from treatment plant was characterized by the equivalent number of in-habitants at 287,000 and average daily flow in the range of 80,000 to 200,000 m 3 ·d -1 .
Statistical calculations revealed high Pearson correlation coefficients between the charges of most metals in wastewater discharged by the plant in Gródek and their loads in the Supraśl waters.These relationships were as follows: Pb 2+ -r = 0.88; Cd 2+ -r = 0.98; Fe 2+/3+ , -r = 0.45; Ni 2+ -r = 0.55; Zn 2+ -r = 0.86.The calculation results indicate a high metal charges connection in the Supraśl waters from the Gródek area with loads of metals in wastewater discharged from sewage treatment plant in Gródek.
Loads of Cd 2+ and Cu 2+ in the wastewater discharged from wastewater treatment plant in Sokółka shaped the loads of these metals in Sokołda waters at the point of Sokółka.A strong dependence was achieved at the level for Cd 2+r = 0.89 and Cu 2+ -r = 0.69.In this case only two strong dependences were obtained in accordance to Gródek.
Loads of Cd 2+ and Zn 2+ in Biała waters at the point of Dobrzyniewo are strongly dependent to loads of these metals in wastewater discharged from sewage treatment plant in Bialystok.The dependence show Pearson coefficients for: Cd 2+ -r = 0.77; Zn 2 + -r = 0.81.For other metals there was no sufficiently strong relations observed.
The values of the studied metals loads were affected by the flow of water and the concentration of the metal.It is worth to note differences in metal concentrations in treated wastewater and in the waters of the receiver, such asin Gródek vicinity.In the case of Fe 2+/3+ the observed differences were quite large, in the case of Cd 2+ and Ni 2+ the differences were almost twice as large, and in the case of Zn 2+ (up to four times).The Suprasl river flow is comparable in points Gródek and Sokołda (Sokółka) and Biała (Dobrzyniewo) and Gródek wastewater treatment plant by the smallest throughput.The smallest loads of metals in the treated wastewater from sewage treatment plant in Gródek of all studied also considering the issue of the achieved correlation.Probably, improper work of sewage treatment plants in Gródek is the cause of correlations between the studied charges which have also been confirmed in other studies conducted by the authors of the study.Correlated loads of Cd 2+ in all three cases are confirmed in large differences in the concentration of the metal in the treated wastewater and water loads.A similar situation also occurs partly in the case of Zn 2+ .

CONCLUSIONS
1.During the studies the concentration difference detected for Cd 2+ , Fe 2+/3+ , Ni 2+ and Zn 2+ in the treated wastewater and the water receivers affect the formation of the loads of these metals.
2. Studies have shown the impact of metals loads in treated wastewater on loads of metals in the receivers waters based on the calculated correlation.The highest correlation was found between charges of Pb 2+ -r = 0.88; Cd 2+ -r = 0.98; Fe 2+/3+ , -r = 0.45; Ni 2+ -r = 0.55; Zn 2+r = 0.86 in the case of wastewater treatment in Gródek and Supraśl water.
3. Loads of certain metals in the wastewater from sewage treatment plant in Bialystok were more than ten times larger than the loads discharged by the plant in Sokółka, and more than one hundred times larger than the loads from Gródek.The loads depended on the size of sewage treatment technologies.

Table 1 .
Average concentrations of metals in water behind the discharge of treated sewage

Table 2 .
The average concentration of metals in treated wastewater

Table 3 .
Loads of metals in treated wastewater from sewage treatment plants in Gródek

Table 4 .
Loads of metals in water of the Supraśl river behind discharge of sewage from sewage treatment plant in Gródek

Table 5 .
Loads of metals in treated wastewater from sewage treatment plants in Sokółka

Table 6 .
Loads of metals in Sokołda river behind the discharge of sewage from a sewage treatment plant in Sokółka

Table 7 .
Loads of metals in treated wastewater from sewage treatment plants in Białystok

Table 8 .
Loads of metals in the Biała river behind discharge of sewage from a sewage treatment plant in Białystok