Comparative evaluation of hydraulic retention time effect in Bio-rack wetlands system operation in tropical climate

The aim of this study was to evaluate the effect of different hydraulic retention time (HRT) in Bio-rack Wetland Systems (BRWSs) planted with vetiver grass in tropical climate. For this purpose, four BRWSs were built on an experimental scale with HRTs of 6, 12, 24 and 48 h, denoted BRWSs A, B, C, and D, respectively. The removal efficiencies for BRWSs A, B, C, and D reached 27, 50, 58 and 67% for biochemical oxygen demand (BOD); 20, 29, 65, and 74% of chemical oxygen demand (COD); 1.1, 11, 15 and 32% of total Kjeldahl nitrogen (TKN); 1.4, 12, 18 and 29% of total phosphorus (TP); and 0.01, 0.1, 0.3, and 0.8 log units of thermotolerant coliforms (C_Ther), respectively. Based on results, BRWS D showed statistically greater performance than BRWS A for BOD, COD, TP, and CTher and efficiencies were similar to those typical found in septic tanks (with BRWS D being more effective), being able to be used for decentralized wastewater treatment. The high variability in sewage characteristics may have resulted in limitations in detecting statistical differences between treatments. From regression analysis, there is a suggestion that the HRT of 38 h would provide the best results.