Nutrient removal and greenhouse gas emissions in duckweed treatment ponds

Stormwater treatment ponds provide a variety of functions including sediment retention, organic and nutrient removal, and habitat restoration. The treatment ponds are, however, also a source of greenhouse gases. The objectives of this study were to assess greenhouse gas (CH4, CO2 and N2O) emissions in duckweed treatment ponds (DWPs) treating simulated stormwater and to determine the role of ammonia-oxidizing organisms in nutrient removal and methanogens in greenhouse gas emissions. Two replicated DWPs operated at a hydraulic retention time (HRT) of 10 days were able to remove 84% (±4% [standard deviation]) chemical oxygen demand (COD), 79% (±3%) NH4+-N, 86% (±2%) NO3−-N and 56% (±7%) orthophosphate. CH4 emission rates in the DWPs ranged from 502 to 1900 mg CH4 m−2 d−1 while those of nitrous oxide (N2O) ranged from 0.63 to 4 mg N2O m−2 d−1. The CO2 emission rates ranged from 1700 to 3300 mg CO2 m−2 day−1. Duckweed coverage on water surface along with the continued deposit of duckweed debris in the DWPs and low-nutrient influent water created a low dissolved oxygen environment for the growth of unique ammonia-oxidizing organisms and methanogens. Archaeal and bacterial amoA abundance in the DWPs ranged from (1.5 ± 0.2) × 107 to (1.7 ± 0.2) × 108 copies/g dry soil and from (1.0 ± 0.3) × 103 to (1.5 ± 0.4) × 106 copies/g dry soil, respectively. The 16S rRNA acetoclastic and hydrogenotrophic methanogens ranged from (5.2 ± 0.2) × 105 to (9.0 ± 0.3) × 106 copies/g dry soil and from (1.0 ± 0.1) × 102 to (5.5 ± 0.4) × 103 copies/g dry soil, respectively. Ammonia-oxidizing archaea (AOA) appeared to be the dominant nitrifiers and acetoclastic Methanosaeta was the major methanogenic genus. The results suggest that methane is the predominant (>90%) greenhouse gas in the DWPs, where the relatively low stormwater nutrient inputs facilitate the growth of K-strategists such as AOA and Methanosaeta that may be responsible for ammonia removal and greenhouse gas emissions, respectively.

123Highlights► Two replicated duckweed ponds (DWPs) were operated at a hydraulic retention time of 10 days. ► DWPs removed an average of 84% COD, 79% NH4+-N, 86% NO3−-N and 56% orthophosphate. ► Methane is the predominant greenhouse gas with its emission rate ranging from 502 to 1900 mg m−2 d−1. ► K-strategists such as ammonia-oxidizing archaea (AOA) and Methanosaeta were dominant. ► Relatively low stormwater nutrient inputs facilitate the growth of K-strategists in the DWPs.