Sistema de tanque séptico e wetland construído vertical com recirculação para remoção de matéria orgânica e nitrogênio de esgoto doméstico

Abstract

Vertical flow constructed wetlands (VFCW) are attractive systems for decentralized wastewater treatment, enabling the organic matter removal and nitrification. However, the denitrification is limited in these systems. An alternative is to recirculate the treated effluent from VFCW to the primary treatment unit, such as a septic tank (ST). The aim of this study was to evaluate the performance of ST and VFCW system operated with recirculation, applied for organic matter and nitrogen removal from domestic wastewater. The study was developed in three strategic steps (steps 1, 2 and 3) carried out over two years of operation of ST (4.7 m3) and VFCW (24.5 m2) under an influent flow of 1.5 m3 d-1. The VFCW was filled out with gravel and during this research, it was planted with Heliconia and Canna, each one in a specific period of time. In step 1 it was evaluated the hydraulic and treatment performances of ST/VFCW system for recirculation ratios of 0%, 50% and 60%, with hydraulic loading rate (HLR) of 60, 85 and 110 mm d-1 in the VFCW, respectively. The results showed that recirculation reduced the hydraulic detention times on ST and the percolation times on VFCW; however, it was not detrimental to treatment since recirculation also allowed new chances of contact between wastewater and microorganisms. Besides, the results indicated that the plants played an important role in increasing wastewater percolation time, mainly at the beginning of its development. Regarding to treatment, the recirculation showed potential to treat domestic wastewater with high concentrations of organic matter and N. The system operation with 50% recirculation, HLR of 85 mm d-1, and fractionated into eight batches per day (11.3 mm batch-1), demonstrated the best efficiencies for the joint removal of COD (80%), TSS (85%) and TN (42%). In step 2 of the study, the influence of recirculation on the denitrifying populations present in the ST sludge and the structure of the nitrifying and denitrifying bacterial community present in the WCFV were evaluated by means of new generation sequencing. The results showed that recirculation provided an increase in the abundance of denitrifying bacteria in ST. In the VFCW, denitrifying bacteria were present in high diversity and abundance, however, ammonia oxidizing bacteria were present in low abundance, being identified only in the superficial layer (0-20 cm), and nitrite oxidizing bacteria were not identified. The presence of high concentrations of NO3--N in the effluent suggests the occurrence of nitrification in the bottom of the VFCW (50-75 cm) or the participation of other groups of microorganisms in the nitrification process (in addition to nitrifying autotrophic bacteria). Finally, in step 3, the VFCW oxygen transfer capacity was evaluated through assays with gaseous tracer (propane), for HLR ranging from 60 to 120 mm d-1. For the operating conditions tested, the oxygen transfer rates (OTR) obtained were sufficient to meet VFCW oxygen demand for organic matter degradation and nitrification, and a higher OTR was obtained for the lower HLR (60 mm d -1). In view of the above, the use of recirculation in a ST/VFCW system, for the conditions evaluated in this study, proved to be a promising alternative for the joint removal of organic matter and N, in the context of the decentralized treatment of domestic wastewater.