Abstract
Constructed storm-water wetlands with different types of vegetation are capable of removing organics and nutrients from water while providing other important environmental and ecological services. The objectives of this study were to compare the efficiencies of organic and nutrient removal from a simulated storm-water runoff between the duckweed ponds (DWPs) and the algae ponds (APs) and to establish a potential link between the population of ammonia-oxidizing organisms and effluent water quality. Two replicate ponds were operated under identical conditions in which duckweeds initially thrived off the water, but they were taken over by filamentous algae after the complete removal of duckweeds. The ponds were able to remove 74±4% chemical oxygen demand (COD); 90±3%NH4+-N; 94±2%NO3−-N; and 47±7% orthophosphate in the DWPs in contrast to only 64±6% COD; 77±6%NH4+-N; 85±4%NO3−-N; and 21±6% orthophosphate in the APs. The dissolved oxygen (DO) levels were significantly different between the ponds. In the DWPs, the diurnal DO pattern (from 0.9 to 3.9 mg/L) associated with anoxic-aerobic sequencing conditions was related to the higher phosphorus (P) removal than in the APs. Archaeal and bacterial amoA abundance in the DWPs ranged from (1.1±0.2)×106 to (2.1±0.2)×109 copies/g dry soil and from (1.6±0.3)×107 to (1.1±0.4)×1010 copies/g dry soil, respectively. The hydric soil in the DWPs harbored more ammonia-oxidizing bacteria (AOB) and oligotrophic ammonia-oxidizing archaea (AOA) than any other samples in the ponds by approximately three orders of magnitude. For comparison, the high DO concentrations (an average of 5.5 mg/L) in the APs were linked to the absence of AOA, whereas the abundance of amoA from AOB ranged from (1.3±0.3)×107 to (1.6±0.2)×109 copies/g dry soil. These results suggest that the duckweeds play an important role by creating different microbial niches and cyclic anoxic-aerobic conditions and thus lead to improved organic and nutrient removal.
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