Operational modifications of a full-scale experimental vertical flow constructed wetland with effluent recirculation to optimize total nitrogen removal

A full-scale Vertical Flow Constructed Wetland (VFCW) was designed, built, and put into operation in Oman to provide a sustainable wastewater treatment solution within the Middle Eastern context. The VFCW had two stages (vertical flow 1 and 2; called VF1 and VF2, respectively) with a total treatment area of 995 m² and was planted with common reeds (Phragmites australis), while the treated effluent was recirculated to an anoxic tank. The VFCW was continuously fed with high-strength domestic wastewater for 1.5 years (January 2017–June 2018). The study was divided into two phases: the first phase (Baseline) aimed at evaluating the treatment performance, and the second phase (Modification) aimed at testing different operational modifications to optimize the total nitrogen removal and deal with the nitrified treated effluent. Samples were collected at the inlet buffer tank and VF2 outlet daily during the first phase and weekly during the second phase. Overall, high removal rates obtained for total suspended solids (99.6%), biological oxygen demand (98.7%), chemical oxygen demand (97.8%), ammonia nitrogen (99.5%), total phosphorus (97.2%), and pathogens (99.9%) complied with the Oman Irrigation Standard A. However, limited denitrification was the main challenge since the effluent's total nitrogen value (25 mg/L) remained above the legal limit. Therefore, several operational modifications were tested, such as increasing the hydraulic retention time in the anoxic tank, increasing the saturation in VF2 beds, the step-feeding of raw wastewater, and the addition of an external carbon source. The step-feeding with artificial carbon dosing resulted in the desired nitrate reduction, i.e., below 11.3 mg/L according to the irrigation standard. This research study is among the first VFCW carried out on a full-scale to optimize its performance under a hot and arid climate.