Evaluation of a constructed wetlands hybrid system with and without recirculation

Abstract

The objective of this study was to evaluate the organic matter behavior under the effect of the recirculation of treated domestic sewage in a hybrid system of constructed wetlands. This study was carried out at the sewage treatment plant in the Instituto Federal Goiano campus, in Rio Verde city, Goias, Brazil. The treatment system consists of a septic tank followed by a hybrid system of constructed wetlands (CW). Sewage monitoring was performed at 3 points: PTS (previously treated sewage), TS1 (VS-CW output) and TS2 (HS-CW output). Water quality analysis were carried out weekly, between May and December 2021, in two types of operation: no recirculation (10 weeks) and 100% recirculation (17 weeks). In this research, three kinetics models were evaluated (1st Order, Monod and Monod Multi) combined with two types of idealized dynamic flow: PFR and CSTR. Results show that the water behavior of the recirculation system increasing by 335% and 59.25% in inlet and outlet flows, respectively, from the VS-CW. The overall efficiency in BOD removing was 89.26% in linear system and 83.95% in the recirculating system. In the case of COD, an overall removal system was 69.7% in the linear and 25.7% in the recirculating system. The Monod kinetic model was the most representative to describe the BOD removal in the linear system (R² = 0.80) and in the recirculation (R² = 0.61). None of the models were included in the description of the linear COD removal processes in the system. In the recirculating system, the Monod kinetic model (R² = 0.81) was the most representative in terms of concentration. Sewage recirculation brought new treatment dynamics and was efficient in BOD reducing (concentration). It is noteworthy that the BOD and COD removal efficiency was lower than in the recirculation by sewage dilution in the suction well.