Treatment of domestic wastewater in tropical, subsurface flow constructed wetlands planted with Canna and Heliconia

Constructed wetlands have a good potential for wastewater treatment in developing countries due to the simple operation and low implementation costs. Ornamental plants like Canna and Heliconia are used in the wetlands to increase their aesthetic value and these two species were compared in this study. Six pilot scale horizontal subsurface flow constructed wetland units were constructed at the Asian Institute of Technology (AIT) campus in Bangkok, Thailand, of which three were planted with Heliconia psittacorum L.f. × H. Spathocircinata (Aristeguieta) and three with Canna × generalis L. Bailey. The beds were loaded with domestic wastewater in four trials with hydraulic loading rates ranging from 55 to 440 mm d−1 corresponding to nominal detention times between 12 h and 4 days. Both plant species grew well in the systems and especially Canna had high growth rates (3100 ± 470 g DW m−2 yr−1) compared to Heliconia (550 ± 90 g DW m−2 yr−1). TSS mass removal rates were very high with efficiencies >88% even at hydraulic loading rates of 440 mm d−1. COD mass removal rates varied between 42 and 83% depending on the loading rates. The removal rate constants for COD as fitted by the first-order k-C* model were estimated to be 0.283 and 0.271 m d−1 for Canna and Heliconia beds, respectively (C* = 28.1 and 26.7 mg l−1). Removals of nitrogen (N) and phosphorus (P) were low compared to the loading rates, but removal of total-N was higher in the beds planted with Canna than in beds with Heliconia because of the higher growth rate of Canna. It is concluded that ornamental species like Canna and Heliconia can be used to enhance the aesthetic appearance and hence the public acceptance of wastewater treatment systems in tropical climates. Canna is the preferred species from a treatment perspective because of its more vigorous growth, but since Heliconia has an economic potential as cut flowers may be preferred in many cases.