Remoção de Matéria Orgânica Carbonácea e Nitrogenada de Esgoto Sanitário Real em Reatores de Bancada de Leito Fixo Ordenado e Empacotado

Abstract

The biological treatment of wastewater is widely used in sewage treatment plants (STPs) for the removal of carbonaceous organic matter. However, in Brazil, traditionally employed systems do not effectively remove total nitrogen. The main objective of this study was to evaluate the removal of both carbonaceous and nitrogenous organic matter from real sanitary sewage using different ratios of raw sewage (RS) to anaerobic effluent (AE) from a Upflow Anaerobic Sludge Blanket (UASB) reactor. Two bench-scale reactors with two different configurations of fixed bed were used: ordered bed (SFBR) and packed bed (PFBR), under intermittent aeration. The support medium for biomass fixation was the device called Mini Biobob®, made of high-density polyethylene (HDPE) and polyurethane foam (PU). The reactors were operated with a hydraulic retention time (HRT) of 10 hours and intermittent aeration, consisting of 120 minutes of aeration followed by 60 minutes without aeration, in cycles of 180 minutes. Three operating conditions were evaluated, differing in the RS to AE ratio (v/v): Condition 1: 50/50; Condition 2: 75/25; and Condition 3: 25/75. The results showed no statistical difference in the efficiency of chemical oxygen demand (COD) and total nitrogen (TN) reduction between the reactors. COD reduction efficiencies were above 60% in all three studied conditions. Condition 1 showed the best results with 87% and 88% COD reduction in SFBR and PFBR, respectively, and Condition 2 demonstrated the best TN removal results, with 70.8% and 73.3% in SFBR and PFBR, respectively. In Condition 2, nitrification was the limiting step, while in Condition 3, denitrification was limiting, affecting TN removal efficiency in both. The best kinetic rates were also identified in the assays of Condition 1. Scanning electron microscopy (SEM) images revealed bacteria-like structures in the shapes of cocci, bacilli, and filamentous bacteria colonizing the support material. Ecotoxicity assays indicated that raw sewage compromised root growth and cell replication, but after treatment in the reactors, aberration and mutagenicity rates decreased. Metataxonomic analysis identified the dominance of the Nitrospira genus, responsible for nitrification. Genera associated with denitrification, such as Chujaibacter, Rhodoblastus, Clostridium, Romboutsia, and Hypomicrobium, were also identified but in low proportions. Hydrodynamic assays showed that both reactors exhibited complete mixing, with N values close to 1. The results suggest that the studied reactor models allowed for the removal of organic matter and nitrogen, making them a viable alternative to enhance STP treatment capacity and serve as a post-treatment stage.