Performance of two-stage batch loading inverse vertical subsurface flow constructed wetlands

Research was conducted on pilot-scale, two-stage batch-flow constructed wetland (CW) systems for domestic wastewater treatment. Synthetic domestic wastewater was treated in a pre-acidification reactor with a hydraulic retention time (HRT) of 3 hours and the average removal rate of chemical oxygen demand (COD) and ammonia-nitrogen (NH4-N) reached 30% and 13.6%, respectively. The first-stage constructed wetland operated with up-flow and batch feed and drain. In the same year, under the conditions of 12h feed/12h drain, 6h feed and 6h drain and 3h feed/9h drain, with HRT of 3 days, COD removal rates kept the same level. But COD removals in the second year were greater than that of the first year under the same batch period. The first CW NH4-N removal rates averaged 51% under the experimental conditions. The second-stage CW operated with down-flow, 12h feed/12h drain. Under maximum HLR of 10.5cm/d and OLR of 5.87gCOD/(m(2.)d) to 8.7 gCOD/(m(2.)d), COD removal averaged 73.3%; NH4-N average removal exceeded 99%. The spatial variation of COD and NH4-N in the first-stage CW demonstrated that entrainment of air during draining of constructed wetland could strengthen the removal of COD and NH4-N. Temperature had no significant effect on COD degradation while obviously affected the removal of NH4-N.