Nutrient removal in a slow-flowing constructed wetland treating aquaculture effluent

Abstract

Danish model trout farms (MTFs) use stream-like constructed wetlands for effluent polishing, and the industry is keen to improve wetland removal efficiency. To facilitate this, we examined longitudinal and seasonal nutrient removals in an MTF wetland with a hydraulic retention time (HRT) of 1.7 d, a free water surface (FWS) area of 7510 m2, and a volume of 6008 m3. Biweekly, 24-h composite water samples were obtained for 1 yr at 6 sampling stations along the wetland. Assuming plug flow conditions, reductions in particulate and dissolved nutrient concentrations were modelled as first-order removal processes, and removal rate constants (k1,A, m d-1) were plotted to reveal seasonal fluctuations. Particulate phosphorus and organic matter k1,A fluctuated more or less randomly through the year, reflecting that particulate nutrient removal predominantly takes place by sedimentation. In contrast, dissolved nitrogen, phosphorus, and organic matter k1,A fluctuated seasonally, demonstrating that dissolved nutrient removal relies on biologically mediated processes. Temperature oscillations probably governed the observed seasonal fluctuations in nitrate-N k1,A and could be approximated with an Arrhenius temperature coefficient of 1.07. Furthermore, denitrification appeared to be carbon-limited. Incoming dissolved phosphorous and ammonia became incorporated in the natural wetland growth cycle that included periods of net removal and release, resulting in minimal annual net removal. In summary, this study shows that improving nitrate removal in a slow-flowing MTF wetland would require some kind of carbon dosing, while further improving ammonia and phosphorus removal would require a reduction of the amounts of ammonia and dissolved phosphorus entering the wetland.