Assessment of Nutrient Removal in Surface Flow Constructed Wetland Treating Secondary Effluent with Low Organic, Nitrogen and Phosphorus Loads

Nutrient loads must be reduced to safe levels to protect sensitive receiving environments. This work presents the results of a 15-month monitoring program of a surface flow-constructed wetland (SFCW) in Queensland, Australia. The SFCW reduced the influent TN concentration by 54% and was able to retain 80% of the TN load, mainly due to the efficient removal of NOx and ammonium (92-100%). TP removal was negative due to the unaccounted loads from wildlife activity. During occasions of high loads, the wetland reduced TP concentrations by 77%. The hydraulic loading rate (HLR) correlated poorly to the TSS and TVS loads (r < 0.55); however, when adjusted to account for precipitation and evapotranspiration, stronger correlations (r > 0.78) were revealed. Strong correlations were revealed between adjusted HLR and TP (r = 0.87) and TN (r = 0.93). TN removal was highly governed by the inflow of TN concentration. TN removal could be predicted from the inflow concentration using the first-order plug-flow model (R-2 = 0.72). The model suggests that the system has an irreducible threshold TN load of 0.115 kg-N per m(2) per month. This work shows that SFCW can be effective in managing the nutrient loads even in systems that receive low organic and nutrient loads.