Performances of a constructed wetland treating domestic wastewaters during a macrophytes life cycle

In this study, the performance of a combined subsurface vertical and horizontal flow constructed wetlands system, designed for rural domestic wastewaters treatment and with theoretical hydraulic retention time of 2 d and 3.6 d, respectively, was investigated. Several water quality parameters including pH, BOD5, COD, TSS, TKN and TP, and faecal bacteria’s number in both raw and treated wastewaters were monitored during a macrophytes life cycle. At the same time, the growth of the rooted plants, reeds and cattails was investigated through the measurement of the height and leaves number. The average influent characteristics were as follows: pH (7.98 ± 0.27), BOD5 (420 ± 144 mg/L), COD (1339 ± 352 mg/L), TSS (798 ± 302 mg/L), TKN (205 ± 70 mg/L), TP (30.7 ± 12.4 mg/L), faecal coliforms ([2.69 ± 4.14] × 107 CFU/100 mL) and faecal streptococci ([9.35 ± 1.81] × 106 CFU/100 mL). The main treatment performance results showed the following average removal rates: BOD5 (93 ± 2%), COD (89 ± 3%), TSS (98 ± 1.5%), TKN (38 ± 19%), TP (72 ± 16%). The average bacterial reduction between the inlet and the outlet was of the order of 4 ± 1 logarithmic units for faecal coliforms and 3 ± 1 logarithmic units for faecal streptococci. Analysis of the results reveals a temporal variation in the system’s performance depending on the primary treatment operation and on the macrophytes growth rate. Reeds and cattails start their life cycle at the beginning of the spring and continue their development during summer. During the autumn season, the plants reach their optimum growth rate, and after that, they enter the dormant phase. The quality of the treated wastewaters was evaluated according to Tunisian standards. The average effluent pH, BOD5, TSS and faecal bacteria were in agreement with the standards, but COD, nitrogen and phosphorus residual loads were still above the values required by the quality criteria.