Achieving an extraordinary high organic and hydraulic loadings with good performance via an alternative operation strategy in a multi-stage constructed wetland system.

In this study, a high organic loading rate of 58-146 g BOD5 /m2  day with a hydraulic loading rate (HLR) of 1.63 m3 /m2  day and retention time (RT) of 16 h was achieved to maximize the treatment capacity of a four-stage alum sludge-based constructed wetland (CW) system. An alternative operation strategy, i.e., the first stage anaerobic up-flow and the remaining stage tidal flow with effluent recirculation, was investigated to achieve the goal with good treatment performance of 82% COD, 91% BOD5 , 92% SS, 94% NH4 -N, and 82% TN removal. Two kinetic models, i.e., first-order model and Monod plus continuous stirred-tank reactor (CSTR) flow model, were employed for predicting the removal dynamics. The results showed that the tidal flow strategy enhances oxygen transport and diffusion, thus improving reduction of organics and NH4 -N. Effluent recirculation could further increase elimination of organics by extending the interaction time and also benefit the denitrification process. In addition, denitrification could be further enhanced by anaerobic up-flow in the first stage.