A full-scale comparison of two hybrid constructed wetlands treating domestic wastewater in Pakistan.

Objective of the current work was to monitor the year-round response of full-scale hybrid constructed-wetlands (CWs) treating domestic wastewater under variable continuous flow. Two systems were evaluated: system-I consisted of an anaerobic baffled reactor (ABR) followed by a saturated vertical subsurface-flow (VSSF) CW and a free-water-surface (FWS) CW as a tertiary treatment; system-II consisted of an ABR followed by a horizontal subsurface-flow (HSSF) CW and FWS. Maximum reduction of 80 and 78%, 81 and 82%, 63 and 69%, 79 and 89% for chemical oxygen demand (COD), biological oxygen demand (BOD), total kjeldahl nitrogen (TKN) and total suspended solids (TSS) was achieved in Systems I and II respectively. There was also effective removal (94% and 93%) of the bacterial population in both systems while more than 94% of pathogenic microorganisms were removed. Data from both systems were further used to compute the first-order rate constants for the k-C* model commonly used in CW design. The treatment performance was confirmed to follow a first-order reaction rate, in which the k20 values of chemical oxygen demand (COD), biological oxygen demand (BOD), total kjeldahl nitrogen (TKN), total phosphorus (TP) and total suspended solids were calculated as 165, 117, 133, 7.5 and 78 m yr-1 respectively for VSSF and 226, 134, 199, 22 and 73 m yr-1 respectively for HSSF. A positive correlation with temperature was discovered for all parameters in both systems.