Effect of external organic matter on nutrient removal and growth of Phragmites australis in a laboratory-scale subsurface-flow treatment wetland.

Coconut dust, which is used intensively in horticultural applications, was tested as an external organic additive in a series of laboratory-scale subsurface-flow constructed wetlands planted with Phragmites australis. The systems were fed with a mixture of NO3(-)-N, NH4+-N, and SRP in tap water to simulate high nutrient loads. In the absence of plants, TN removal efficiency was 66%, and the efficiency increased to > 80% in the microcosm wetlands. TN and NO3- removal efficiencies were marginally increased by coconut-dust treatment in comparison with sand-bed microcosms. Analysis by ANOVA showed that the TN removal from a coconut dust-supplemented sand-bed microcosm was significantly different from a sand-bed microcosm (0.0437 < p < 0.05). All the systems showed an equal capacity to treat NH4+ nitrogen under low influent concentration levels. Phosphorus removal efficiencies were > 98% in all three systems, and a difference between planted and unplanted systems was not observed. Shoot height and shoot densities of P. australis grown in the coconut dust-supplemented medium were significantly higher than those grown in the sand-bed medium. The difference in P. australis growth in response to the coconut dust addition revealed that the added material has the potential to create favourable conditions for plant growth.