Septage dewatering in vertical-flow constructed wetlands located in the tropics

Constructed wetlands (CWs) have been proven to be an effective low-cost tre atment system, which utilizes the interactions of emergent plants and micro organisms in the removal of pollutants. CWs for wastewater treatment are no rmally designed and operated in horizontal-flow patterns, namely, free-wate r surface or subsurface flow, while a vertical-flow operation is normally u sed to treat sludge or septage having high solid contents. In this study, t hree pilot-scale CW beds, each with a surface area of 25 m(2), having 65 cm sand-gravel substrata, supported by ventilated-drainage system and plantin g with narrow-leave cattails (Typha augustifolia), were fed with septage co llected from Bangkok city, Thailand. To operate in a vertical-flow mode, th e septage was uniformly distributed on the surface of the CW units. During the first year of operation, the CWs were operated at the solid loading rat es (SLR) and application frequencies of, respectively, 80-500 kg total soli d (TS)/m(2).yr and 1-2 times weekly. It was found that the SLR of 250 kg TS /m(2).yr resulted in the highest TS, total chemical oxygen demand (TCOD) an d total Kjeldahl nitrogen (TKN) removal of 80, 96 and 92%, respectively. Th e TS contents of the dewatered septage on the CW beds were increased from 1 -2% to 30-60% within an operation cycle. Because of the vertical-flow mode of operation and with the effectiveness of the ventilation pipes, there wer e high degrees of nitrification occurring in the CW beds. The nitrate (NO3) contents in the CW percolate were 180-250 mg/L, while the raw septage had NO, contents less than 10 mg/L. Due to rapid flow-through of the percolates, there was little liquid retain ed in the CW beds, causing the cattail plants to wilt, especially during th e dry season. To reduce the wilting effects, the operating strategies in th e second year were modified by ponding the percolate in the CW beds for per iods of 2 and 6 days prior to discharge. This operating strategy was found beneficial not only for mitigating plant wilting, but also for increasing N removal through enhanced denitrification activities in the CW beds. During these 2 year operations, the dewatered septage was not removed from the CW beds and no adverse effects on the septage dewatering efficiency were obse rved.