Removal of bacterial and viral indicators in vertical flow constructed wetlands and intermittent sand filters

The removal of faecal coliforms (FC), Escherichia coli, somatic coliphages and F-specific bacteriophages was monitored over a period of 2 years in two vertical flow constructed wetlands (VFCWs) and four intermittent sand filters (ISFs) that treat pond effluent. Tracer tests (NaCl) were performed to estimate the hydraulic retention time (HRT) of the filters. The contribution of Phragmites australis to the removal of the faecal indicators was examined. The effect of media characteristics (depth, type of sand), hydraulic load and water application schedule on disinfection was also addressed. Planted and non-planted vertical sand filters were equally effective in removing microorganisms. The disinfection capacity was very sensitive to the depth and operation of the filters. HRT was the key parameter regarding filter disinfection capacity. Bacterial indicators were removed at a higher rate than viral ones. Somatic coliphages were removed at higher rates than F-specific bacteriophages.

The removal of microbial indicators in VFCWs and ISFs depends mainly on the water retention time in the filter, which in turn depends on the depth of the filter, the HL and the dose volume per batch. The presence of plants did not significantly affect the removal of indicator microorganisms in our study, which indicates that the presence of Phragmites is of minor importance for the removal of microorganisms in VFCWs intermittently dosed. No significant differences were found between the two kinds of sand tested (crushed and river sand). Low temperatures did not limit the removal of indicator microorganisms in ISFs and VFCWs. Bacterial indicators were removed at a higher rate than viral ones. Somatic coliphages were removed at higher rates than F-specific bacteriophages.