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.

In this study, vertical and horizontal SSF wetlands were constructed in series to treat domestic wastewaters, from a small rural community. This CWs system had a good capacity to reduce BOD5, COD, TSS and faecal bacteria, but only moderate removal efficiencies of total nitrogen and phosphorus were recorded during the monitoring period. The effluent quality was still compliant with the Tunisian standards in terms of BOD5, TSS and faecal bacterial levels. However, this artificial system was limited by insufficient removal of COD, nitrogen and phosphorus, with the residual loads greatly exceeding the required standards. The treatment of these elements may be achieved through the combination of different CW types, such as surface flow systems, or by increasing the HRT in the horizontal bed for a better denitrification and the size of the vertical bed for a better nitrification. Other treatment approaches, like adsorption on specific media or chemical reactions, are often advised for phosphorus reduction. Acknowledgments This work has been realized with the financial support provided by the Tunisian government (SERST). The authors wish to acknowledge the support of the personnel of the Centre International des Technologies de l’Environnement de Tunis (CITET), with special thanks to the laboratories technical staff for their kind help. We also acknowledge the meteorological data provided by the Direction des Ressources en Eau (DRE). Dr. Akintunde Babatunde of the University College Dublin (UCD), Ireland is very much thanked for his contributions while Dr. Yaqian Zhao, also of UCD is sincerely thanked for his referral. Abbreviations CW FWS SSF VFB HFB T Constructed wetlands free water surface subsurface flow vertical flow basin horizontal flow basin temperature DO EC BOD5 COD TSS TKN TP FC FS log U CFU dissolved oxygen electrical conductivity 5-day biochemical oxygen demand chemical oxygen demand total suspended solids total Kjeldahl nitrogen total phosphorus faecal coliforms faecal streptococci logarithmic unit colonies forming unit