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

Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

HighlightsDenitrifying woodchip bioreactors treat nitrate-N in a variety of applications and geographies.This review focuses on subsurface drainage bioreactors and bed-style designs (including in-ditch).Monitoring and reporting recommendations are provided to advance bioreactor science and engineering. Denitrifying bioreactors enhance the natural process of denitrification in a practical way to treat nitrate-nitrogen (N) in a variety of N-laden water matrices. The design and construction of bioreactors for treatment of subsurface drainage in the U.S. is guided by USDA-NRCS Conservation Practice Standard 605. This review consolidates the state of the science for denitrifying bioreactors using case studies from across the globe with an emphasis on full-size bioreactor nitrate-N removal and cost-effectiveness. The focus is on bed-style bioreactors (including in-ditch modifications), although there is mention of denitrifying walls, which broaden the applicability of bioreactor technology in some areas. Subsurface drainage denitrifying bioreactors have been assessed as removing 20% to 40% of annual nitrate-N loss in the Midwest, and an evaluation across the peer-reviewed literature published over the past three years showed that bioreactors around the world have been generally consistent with that (N load reduction median: 46%; mean ±SD: 40% ±26%; n = 15). Reported N removal rates were on the order of 5.1 g N m-3 d-1 (median; mean ±SD: 7.2 ±9.6 g N m-3 d-1; n = 27). Subsurface drainage bioreactor installation costs have ranged from less than $5,000 to$27,000, with estimated cost efficiencies ranging from less than $2.50 kg-1 N year-1 to roughly$20 kg-1 N year-1 (although they can be as high as $48 kg-1 N year-1). A suggested monitoring setup is described primarily for the context of conservation practitioners and watershed groups for assessing annual nitrate-N load removal performance of subsurface drainage denitrifying bioreactors. Recommended minimum reporting measures for assessing and comparing annual N removal performance include: bioreactor dimensions and installation date; fill media size, porosity, and type; nitrate-N concentrations and water temperatures; bioreactor flow treatment details; basic drainage system and bioreactor design characteristics; and N removal rate and efficiency

Nitrate removal from aquaculture effluents using woodchip bioreactors improved by adding sulfur granules and crushed seashells

Abstract This study examined the effects on nitrate removal when adding sulfur granules and crushed seashells to a woodchip bioreactor treating aquaculture effluents. Using a central composite design, the two components were added at three levels (0.000, 0.125 and 0.250 m3/m3 bioreactor volume) to 13 laboratory-scale woodchip bioreactors, and a response surface method was applied to find and model the optimal mixture ratios with respect to reactor performance. Adding 0.125 m3/m3 sulfur granules improved the total N removal rate from 3.27 ± 0.38 to 8.12 ± 0.49 g N/m3/d compared to pure woodchips. Furthermore, the inclusion of crushed seashells together with sulfur granules helped to maintain the pH above 7.4 and prevent a production (i.e., release) of nitrite. According to the modeled response surfaces, a sulfur granule:crushed seashell:woodchip mixture ratio containing about 0.2 m3 sulfur granules and 0.1 m3 crushed seashells per m3 reactor volume would give the best results with respect to high N removal and minimal nitrite release. In conclusion, the study showed that N removal in woodchip bioreactors may be improved by adding sulfur granules and seashells, contributing to the optimization of woodchip performance in treating aquaculture effluents.</jats:p