Polishing domestic wastewater on a subsurface flow constructed wetland: organic matter removal and microbial monitoring

Microbial monitoring of constructed wetlands (CWs) treating domestic wastewater is generally scarce, despite the need of more knowledge about its biocenosis. The sanitation quality of a wastewater treated in a CW is a crucial aspect, mainly when the receiving water body is used as a swimming and/or recreation area. The present study was carried out in a horizontal subsurface flow CW planted with Phragmites australis receiving pre-treated domestic wastewater (mean flow 50 m 3 day -1), from a population of about 300 inhabitants. The monitoring programme undertaken during the first year operation, revealed removal efficiencies of 61% BOD 5, 44% COD, and 65% TSS for inlet water with ca. 90 mg L -1 BOD 5, 157 mg L -1 COD, and 17 mg L -1 TSS. Total Coliform (TC) and Faecal Coliform (FC) bacteria were removed from wastewater (mean inlet values of 5 × 10 6 CFU 100 mL -1 TC and of 9 × 10 5CFU 100 mL -1 FC), with efficiencies of 92 and 97%, respectively. The dynamics of microbial communities established in the system assessed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), had revealed a high bacterial diversity within the system, with no relevant differences in composition at the CW inlet and outlet but exhibiting temporal differences in bacterial communities.