SEWAGE LOADS FROM A DRAINAGE BASIN, ESTIMATED FROM SPATIAL DISTRIBUTION OF DEMOGRAPHY

Untreated sewage is a relevant problem in large coastal cities, generating restrictions for recreation bathing and other nautical activities. Although a sewage separate collection system is desirable, besides expensive, it depends largely on people’s willingness to connect their output to the system. In many places the experience shows that reduction in pollution is only attained after long periods and huge amounts of investment. In many large cities worldwide, like Paris, New York, and London, the sewage collection systems are associated with rainwater drainage known as combined sewers. A separated sewage system cannot be constructed in these cities because they have functioned this way for more than one hundred years. It is based on the direct disposal of houses sewage into the stormwater systems, and most of these drainages are collected in wastewater treatment plants. In dry weather, the sewage is normally treated because the concentrations are elevated and the volume is small, but in rainy weather, storm water volume is way too large, overflowing to the rivers and coastal waters. Developing a dry weather sewage collection system is a difficult task, but models based on the size of population in drainage basins may help to reduce the overflowing of mixed sewage and rainwater, improving the water quality dumped into natural water. The present research aims to apply a sewage/rainwater model (dry weather collection system DWCS) in a small overpopulated drainage basin in the Baixada Fluminense, state of Rio de Janeiro, showing the possibility of reducing the inputs of DBO₅ in the natural environment. The model was applied to a small drainage basin (Botas River, 3 km²) in the Municipality of Nova Iguaçu, located in Rio de Janeiro Metropolitan region. From a georeferenced Google Earth image, treated in the free software QGIS, and IBGE cartographic bases, the drainage limits were determined and data from 2010 census were plotted in the whole area, giving the whole population in the basin. Forested and urban areas were also determined, and regionalization models for each type of soil cover were applied to calculate the drainage streamflow. Pluviometric data from station Santa Teresa, a few kilometers away from the studied basin with measurements from 1998 to 2024, showed a maximum rainfall of 248mm in 24 hours, so calculations were based on this maximum value. With the data obtained from the evaluation of the river basin it was possible to calculate the loads of BOD₅, the concentrations in the sewage, as well as flux of the river with a regionalization model, based on a study of a neighboring region.  A population in the drainage basin of Botas River of 22,414 inhabitants generates a load of 14,008 mg s-1 in the conflux. The total area of the basin was 2,981,949 m², with 415,360 m² of vegetated areas (13.93%) and 2,566,589 m² of urban area (86.07%). Considering the maximum rainfall of 248mm in 24 hours, the expected maximum flow was 0.662 m³ s^-1, including the baseline sewage inputs from the residences. A straight line of flow as a function of rainfall was obtained and is attributed to the model conception and an approximation of reality, but it gives a good idea of the response of the river, mainly considering that the basin is 86.07% urban area, where a Hortonian-like overland flow is occurring. On the other hand, a logarithm decay aspect of DBO₅ concentration is an indication that in this basin, large amounts of rainfall are necessary to dilute BOD₅. The minimum DBO₅ concentration calculated in a maximum rainfall of 248 mm for 24 hours was 21,04 mg L^-1, which is four times higher than the limit established by the National regulation for class 2 freshwaters. In an attempt to conceive a dry weather sewage collection system, it is conceivable that in the area of the conflux of the basin of the Botas River should preview pipes able to transport volumes in the order of 350 L s^-1, which would correspond to an approximate DBO₅ concentration of 40 mg L^-1, which is more or less the performance of sewage treatment plants in Rio de Janeiro. The works in the area of collection should include installation of a limnometric rule that measures the height of the water as a function of the flow. A concrete barrier should be constructed with the height measured for 350 L^-1, diverting the waters to a pipe leading to a wastewater treatment plant. Whenever the flow is higher than these values, it will overflow to river Iguaçu, although part of the water may continue to feed the wastewater treatment plant. In the present study, the concept of evaluating flows and concentrations of BOD₅ from an overpopulated and small drainage basin was presented, allowing to carry out calculations for a dry weather collection system that can significantly reduce natural waters contamination in large metropolitan areas. Although it is understood that the construction of the proposed system wouldn’t be able to solve the sewage contamination problem in Guanabara Bay (the final water receptor in the region), the application of this model to every drainage basin in the region would probably improve water quality considerably (by 90%).