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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 measuring- control 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. [ABSTRACT FROM AUTHOR]

• The relation between COD fractions and CAPS process. • The impact of CAPS process on the energy and economic balance of WWTPs. • COD fractions analysis is helpful in selecting coagulants for the CAPS process. • The models of Artificial Neural Networks reflect the effects of CAPS process. • Dosing of PIX and PAX with polyelectrolyte is profitable. The aim of the study was to supplement the information missing in the literature on the influence of organic compounds speciation (percentage of chemical oxygen demand (COD) fractions, readily biodegradable and non–biodegradable dissolved organic matter; slowly biodegradable and non‒biodegradable insoluble organic matter) in wastewater on chemically assisted primary sedimentation process and its potential impact on the energy and economic balance of the plant. The statistical modelling of the chemically assisted primary sedimentation process using artificial neural networks (ANN) was a novelty. It was also important to check the differences resulting from the use of coagulants, polyelectolites and their doses. For both large wastewater treatment plants (>500,000 people equivalent) located in the various regions of the country in Central Europe only ferric chloride had a positive balance. The addition of polyelectrolyte in the Wroclaw wastewater did not affect the chemically assisted primary sedimentation process but in the Bialystok wastewater it strengthened the process effect. In both plants only doses from 10 to 40 mgFe/dm3 are cost-effective. The results indicate that the addition of polyelectrolyte should be taken into account in this application and was correlated with the amount of colloidal COD fractions in wastewater. The developed ANN models reflected well the effectiveness of COD removal in both large WWTP while the sensitivity analysis of individual ANN models showed differences in the variables affecting the calculation process of both models, despite similar values of parameters characterizing wastewater in both wastewater treatment plants. [ABSTRACT FROM AUTHOR]