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 matterk1,Afluctuated more or less randomly through the year, reflecting that particulate nutrient removal predominantly takes place by sedimentation. In contrast, dissolved nitrogen, phosphorus, and organic matterk1,Afluctuated seasonally, demonstrating that dissolved nutrient removal relies on biologically mediated processes. Temperature oscillations probably governed the observed seasonal fluctuations in nitrate-Nk1,Aand 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.
Highlights
Within the last 10−15 yr, many Danish fish farmers have reconstructed their land-based flow-through systems into model trout farms (MTFs; Jokumsen & Svendsen 2010), and approximately half of the Danish rainbow trout Oncorhynchus mykiss production (15 070 t yr−1) currently takes place in these systems (Statistics Denmark 2020)
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
An MTF wetland differs from most other constructed wetlands applied for wastewater treatment in that the hydraulic loading rate (HLR) is more or less constant throughout the year, the waste nutrient composition is well known, and nutrient loadings within a MTF are within the same magnitude throughout the year, reflecting that fish are produced year round (Svendsen et al 2008a, Kadlec & Wallace 2009, Jokumsen & Svendsen 2010)
Summary
Within the last 10−15 yr, many Danish fish farmers have reconstructed their land-based flow-through systems into model trout farms (MTFs; Jokumsen & Svendsen 2010), and approximately half of the Danish rainbow trout Oncorhynchus mykiss production (15 070 t yr−1) currently takes place in these systems (Statistics Denmark 2020). An MTF wetland differs from most other constructed wetlands applied for wastewater treatment in that the hydraulic loading rate (HLR) is more or less constant throughout the year, the waste nutrient composition is well known, and nutrient loadings within a MTF are within the same magnitude throughout the year, reflecting that fish are produced year round (Svendsen et al 2008a, Kadlec & Wallace 2009, Jokumsen & Svendsen 2010) To document their effect, Svendsen et al (2008a) measured inlet and outlet concentrations and flows for 2 yr in 8 of the firstever constructed MTF wetlands. Mass-balance calculations showed that total nitrogen (TN), total phosphorus (TP), total biochemical oxygen demand (BOD5-TOT), and total chemical oxygen demand (CODTOT) were reduced by 50, 76, 93, and 87%, respectively, corresponding to mass removal rates of 2.7 g TN, 0.18 g TP, 4.4 g BOD5-TOT, and 13.1 g CODTOT m−2 d−1
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