Optimization of artificial wetland design for removal of indicator microorganisms and pathogenic protozoa

The enhancement of water quality by artificial wetland systems is increasin gly being employed throughout the world. Three wetlands were studied in Tuc son, AZ to evaluate their individual performance in the removal of indicato r bacteria (coliforms), coliphage, and enteric pathogens (Giardia and Crypt osporidium). A duckweed-covered pond, a multi-species subsurface Flow (SSF) and a multi-species surface flow (SF) wetland were studied. Removal of the larger microorganisms, Giardia and Cryptosporidium, was the greatest in th e duckweed pond at 98 and 89 percent, respectively. The lowest removal occu rred in the SF wetland, 73 percent for Giardia and 58 percent removal for C ryptosporidium. In contrast, the greatest removal of coliphage, total and f ecal coliforms occurred in the SSF wetland, 95, 99, and 98 percent respecti vely, whereas the pond had the lowest removals (40, 62, and 61 percent, res pectively). Sedimentation may be the primary removal mechanism within the d uckweed pond since the removal was related to size, removal of the largest organisms being the greatest. However, the smaller microorganisms were remo ved more efficiently in the SSF wetland, which may be related to the large surface area available for adsorption and filtration. This study suggests t hat in order to achieve the highest treatment level of secondary unchlorina ted wastewater, a combination of aquatic ponds and subsurface flow wetlands may be necessary. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rig hts reserved.