Calibração do modelo de decaimento da DBO5,20 associado à qualidade da água do Lago Igapó I em Londrina/PR

Abstract

This paper presents a theoretical concept of water quality indicators in different classes established by legislation (CONAMA Resolution nº 357/2005). The goal of this study was to obtain hydraulic data (water flow), physical parameters (turbidity, temperature, total solids, suspended solids, apparent color) and chemical parameters (dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, alkalinity, electrical conductivity, pH), of water quality in Igapó Lake I and its tributaries (Igapó Lake II – Tributary 1, Leme Stream - Tributary 2 and Circular stream and reservoir - Tributary 3) along with analysis of dissolved oxygen and temperature profile in the lake, at depths of 0,15 m; 0,5 m; 1 m; 2 m and 3 m and calibrate a concentration decay model of BOD5,20 made by the mathematics department of the State University of Londrina (UEL). In order to evaluate the behavior of water quality parameters, the methodologies described by Apha (2005) were used, being performed in 3 campaigns in the tributaries and 2 campaigns along the Igapó Lake I. Besides the analysis of the parameters in the tributaries, flow rates were also measured, using the method of water current meter and half section in tributary 1, salt concentration curve method in tributary 2, the Mannig-Stricker equation to the circular channel, and the rectangular weir equation with lateral contraction for the reservoir, both present in tributary 3. The oxygen and temperature profile versus depth was measured with the assistance of a portable oximeter, in 22 points of the lake. The calibration of the mathematical model followed according to the analyzes of the initial and final concentrations of BOD5,20 and its behavior in relation to the Hydraulic Detention Time (HDT) along the lake. It was registered that the tributary 1, among all the tributaries analyzed, contributed the most with the liquid mass flow to the lake in study, about 91% and in relation to the water quality parameters, this tributary presented higher average values, in other words, poorer quality. The parameter dissolved oxygen of Igapó Lake I appeared acceptable to the resolution, only in greater depths, about 3 meters, there was a decay in the concentration of oxygen in mg/L. On the lake surface (0,15 m; 0,5 m; 1 m - profiles of depth), the concentration of DO decays according to the increase of temperature. In accordance to the CONAMA Resolution nº 357/2005, the concentrations of BOD5,20 analyzed in the tributaries and in the middle of Igapó Lake I, all the campaigns showed higher values than the allowed. In spite of the concentration decay model of BOD5,20 consider only the central flow of the Igapó Lake I, with the analyzed values of initial (12,6 mg/L) and final (6,5 mg/L) concentration, and its behavior towards the average HDT of 10 days, along with its coefficient of deoxygenation (K1 = 0,4469 days-1), it was possible to calibrate the model and obtain satisfactory results with the real behavior of the lake in study.