The use of macrophytic plants for the treatment of municipal wastewater is growing rapidly. The Constructed Ecosystems Research Facility (CERF) is a joint project of the Pima County Wastewater Management Department and The University of Arizona's Office of Arid Lands Studies. The facility consists of five 30-mil-hyperlon-lined raceways measuring 61 m long, 8.2 m wide, and 1.4 m deep and one slightly larger raceway. Research has been and is being conducted with aquatic plants such as water hyacinth (Eichhornia crassipes), duckweed (Lemna spp.), giant reed (Arundo donax), bulrush (Scirpus olneyi), black willow (Salix nigra), and cottonwood (Populus fremontii). Composite water samplers are used to collect refrigerated samples from incoming secondary wastewater from Pima County's Sewage Treatment Facility and from the effluent leaving each raceway. Water samples have been analyzed for parameters such as pH, BOD5, and total nitrogen. Microbiological studies have concentrated on the removal of indicator coliform bacteria, as well as Giardia, Cryptosporidium and viruses from the wastewater. Data from the water quality studies indicate that these constructed ecosystems reduce BOD5 consistently to below the 10 mg/L BOD5 tertiary standard, remove nitrogen as well as decrease the concentration of pathogens significantly.
The enhancement of water quality by artificial wetland systems is increasingly being employed throughout the world. Three wetlands were studied in Tucson, AZ to evaluate their individual performance in the removal of indicator bacteria (coliforms), coliphage, and enteric pathogens (Giardia and Cryptosporidium). 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 the duckweed pond at 98 and 89 percent, respectively. The lowest removal occurred in the SF wetland, 73 percent for Giardia and 58 percent removal for Cryptosporidium. In contrast, the greatest removal of coliphage, total and fecal coliforms occurred in the SSF wetland, 95, 99, and 98 percent respectively, whereas the pond had the lowest removals (40, 62, and 61 percent, respectively). Sedimentation may be the primary removal mechanism within the duckweed pond since the removal was related to size, removal of the largest organisms being the greatest. However, the smaller microorganisms were removed more efficiently in the SSF wetland, which may be related to the large surface area available for adsorption and filtration. This study suggests that in order to achieve the highest treatment level of secondary unchlorinated wastewater, a combination of aquatic ponds and subsurface flow wetlands may be necessary.
Aims: To determine the ability of duckweed ponds used to treat domestic waste-water to remove Giardia and Cryptosporidium. Methods and Results: The influent and effluent of a pond covered with duckweed with a 6 day retention time was tested for Giardia cysts, Cryptosporidium oocysts, faecal coliforms and coliphage. Giardia cysts and Cryptosporidium oocysts were reduced by 98 and 89%, respectively, total coliforms by 61%, faecal coliforms by 62% and coliphage by 40%. There was a significant correlation between the removal of Giardia cysts and Cryptospordium oocysts by the pond (P < 0AE001). Influent turbidity and parasite removal were also significantly correlated (Cryptosporidium and turbidity, P ¼ 0AE05; Giardia and turbidity, P ¼ 0AE01). Conclusions: The larger organisms (parasites) probably settled to the bottom of the pond, while removal of smaller bacteria and coliphages in the pond was not as effective. Significance and Impact of the Study: Duckweed ponds may play an important role in wetland systems for reduction of Giardia and Cryptosporidium.
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