Effects of intermittent loading on nitrogen removal in horizontal subsurface flow wetlands

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Removal of CBOD5 and nitrogen from septic tank effluent was evaluated in four horizontal subsurface flow (HSSF) wetlands. An intermittently loaded cell was compared to a continuously loaded control cell, with both treatments receiving the same weekly volume. The intermittent cell was rapidly drained and “rested” for 24-hr, then refilled in steps, twice weekly. Two media with different particle sizes but similar porosities were also compared. The two media, light weight expanded shale and gravel, were both continuously loaded. As hypothesized, the wetland cell that was intermittently loaded had higher dissolved oxygen, greater ammonia removal, and greater nitrate production than the continuously loaded cells. Areal NH3-N removal for the intermittently loaded cell was 0.90 g m−2 d−1 compared to 0.47 g m−2 d−1 for the control. Ammonia removal was also higher in continuously loaded gravel cells than in cells with expanded shale. Ammonia-N removal was an order of magnitude lower in a similar SSF wetland that had been in operation for 3 years. However, CBOD5, total suspended solids, and total nitrogen did not vary substantially among the treatments.

Keywords: CBOD, intermittent loading, nitrification, nitrogen removal, subsurface flow, treatment wetlands

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