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Aquaculture Environment Interactions

    AEI prepress abstract   -  DOI: https://doi.org/10.3354/aei00411

    Nutrient removal in a constructed wetland treating aquaculture effluent at long hydraulic retention time

    Johanne Dalsgaard*, Mathis von Ahnen, Per Bovbjerg Pedersen

    *Corresponding author:

    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, the current study examined longitudinal and seasonal nutrient removals in a MTF wetland with a hydraulic retention time (HRT) of 1.7 d, a free water surface (FWS) area of 7510 m2, and a volume (V) 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 coefficients (k1,A, m d-1) 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 N, P, 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-coefficients 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 including periods of net removal and release, resulting in minimal annual net removal. In summary, the study showed 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 reducing the amounts of ammonia and dissolved phosphorus entering the wetland.