Effects of step-feeding and intermittent aeration on organics and nitrogen removal in a horizontal subsurface flow constructed wetland.

The effect of step feed strategy and intermittent aeration on removal of chemical oxygen demand (COD) and nitrogen was investigated in a laboratory scale horizontal subsurface flow constructed wetland (HSSFCW). Wetland was divided into four zones along the length (zone I to IV), and influent was introduced into first and third zones by step feeding. Continuous study was carried out in four phases. In phases I to III, 30% of influent was bypassed to zone III for denitrification along with organics removal. Intermittent aeration was provided only in zone II at 2.5 L/min for 4 h/day, during phases II, III and IV. In phase I, 87% COD and 43% NH-N (ammonia-nitrogen) removal were obtained from influents of 331 and 30 mg/L, respectively. In phase II study, external aeration resulted in 97% COD and 71% NH-N removal in the wetland. In phase IV, 40% of feed was delivered to zone III. Higher supply of organic in zone III resulted in higher denitrification, and total nitrogen removal rate increased to 70% from 56%. In the final effluent, concentration of NO-N was 9-11 mg/L in phase I to III and decreased to 4 mg/L in phase IV. Batch study showed that COD and NH-N removal followed first order kinetics in different zones of wetland.