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

We examined the longitudinal and seasonal removal of dissolved and particulate nutrient components in a free water surface (FWS) constructed wetland treating all the effluent from a commercial recirculating rainbow trout Oncorhynchus mykiss farm. The wetland consisted of a meandering, 0.7 m deep channel with a total FWS area of 5811 m(2), a total hydraulic loading rate (HLR) of 2.23 m d(-1), and a total hydraulic retention time (HRT) of 0.32 d.

Bi-weekly, 24 h composite samples were obtained along the wetland for 1 yr and analysed for dissolved and particulate nutrient components. Furthermore, a short sampling campaign assessed the sedimentation of particles (5 to 200 mu m). A first order kinetic plug flow model was fitted to the longitudinal data, and a first set of area-based removal rate constants (k(A)) for this wetland type was derived.

Sedimentation led to particulate nutrient removal, but there was no annual net removal of dissolved nutrients aside from an infinitesimal removal of phosphorus. Microbial removal processes were substrate-limited, and removal rate constants followed an annual cycle presumably coupled to available plant surface area and temperature.

Denitrification was limited by low carbon availability and high oxygen concentrations, and the wetland became a net producer of nitrate at times due to oxygenation of ammonia. In summary, dissolved nutrients were largely not removed and the wetland was over-dimensioned for particulate nutrient removal.

This new insight should be taken into account in future efforts to improve the treatment performance of similar types of aquaculture wetlands operated at short hydraulic retention times.