Sistemas alagados construídos do tipo Bio-Rack como tratamento simplificado de esgoto sanitário para soluções individuais

Abstract

The adoption of decentralized, efficient and low-cost solutions for wastewater treatment is a necessary measure since a large part of the population in Brazil located in small municipalities and rural areas is not served by sewage collection and treatment services. The Bio-Rack Constructed Wetlands (BR-CW) are a new configuration developed to reduce the area demand, increase the useful life and provide decentralized sewage treatment in a single step. However, it is still necessary to investigate the best operating conditions, especially for tropical regions. Thus, the aim of the present study was to evaluate the performance of BR-CW in the treatment of sanitary sewage arising from the preliminary system of the Sewage Treatment Station at the Federal University of Lavras. For that, the research was carried out in two stages; in the first, the ideal hydraulic retention time (HRT) for a BR-CW in tropical conditions was evaluated. Four BR-CW were made on an experimental scale with HRT of 6, 12, 24 and 48 hours; BR-CW A, B, C and D, respectively. Analyses of pH, electrical conductivity (CE), BOD, COD, total nitrogen Kjeldahl (NTK) and total phosphorus (PT) were performed weekly, and evaluations of thermotolerant coliforms (CTer) occurred every two weeks. Removal efficiencies for BR-CW A, B, C and D, respectively, reached 27%, 50%, 58% and 67% BOD, 20%, 29%, 65% and 74% COD, 1.1%, 11%, 15% and 32% NTK, 1.4%, 12%, 18%, and 29% PT, and 15%, 39%, 59%, and 84% CTer. Based on the results obtained, it was observed that the BR-CW D (HRT = 48 h) performed better, with efficiencies similar to those of septic tanks. Based on the HRT with the best results in the first phase, changes in the BR-CW were evaluated, inferring about the influence of the presence of plants and the introduction of gravel as a support medium (second phase). Thus, four different configurations were used: systems with only tubes inside; with tubes and plants; with pipes and gravel; and with tubes, gravel and plants, respectively, called BR-CW T, TP, TB and TPB. The same variables from the previous step were analyzed, keeping the performance of the tests equal. Removal efficiencies for the BR-CW T, TP, TB, and TBP, respectively, reached 65%, 70%, 54% and 57% BOD, 56%, 59%, 61% and 56% COD, 30%, 36%, 38% and 40% NTK, 22%, 26%, 23%, and 39% PT, and 90%, 81%, 94%, and 81% CTer. Based on the results obtained, it was observed that the BR-CW with 48-hour HRT, containing gravel as a support and planted medium, presented a better performance. In addition to the performance of the BR-CW, cuts were also made to the aerial part of the vetiver grass and the productivity of dry matter, nutrient content and extraction capacity of nitrogen (N) and phosphorus (P) were evaluated. The high density of plants in the units may have caused lower grass yields to be obtained compared to the literature, with BR-CW A being the one with the highest productivity, with an average value of 226 kg ha-1 d-1 dry matter. The participation of plants in the removal of nutrients reached efficiencies of total removal of N and P of 15% and 6.9%; BR-CW D, TP and TPB, with higher HRT and that received lighter loads, were the ones which presented better performance.