Abstract
Formalin is applied in certain aquaculture systems to control parasites infestations as well as bacterial and fungal diseases. This study investigated the capacity of end-of-pipe denitrifying woodchip bioreactors to remove potentially harmful amounts of residual formaldehyde (FA) from aquaculture effluents. Formaldehyde was readily removed by experimental- and field-scale denitrifying woodchip bioreactors and the removal of FA was found to be a combination of an initial adsorption of FA to woodchip surfaces (52 ± 2.8 g FA/m3 woodchips) and microbial degradation. Volumetric FA removal rates reaching 261 ± 27 g FA/m3/d were found at FA inlet concentrations of 90 mg FA/L and hydraulic retention times (HRT) of 5 h. High FA removal efficiencies ranged from 88.3 ± 4.6–99.8 ± 0.2% found for FA inlet concentrations –up to 105 mg FA/L and HRTs between 3.4 and 15 h. Microbial FA degradation rates in woodchip bioreactors were positively correlated to temperature with a Q10 value of 2.27 and a corresponding Arrhenius temperature coefficient of 1.086 for the investigated temperature range of 7–23 °C. At a commercial, outdoor recirculating aquaculture system (RAS) three full-scale woodchip compartments, achieved an average volumetric FA removal rate of 29.4 ± 0.2 g FA/m3/d and a removal efficiency of 82.5 ± 0.8% during the first 24 h following addition of FA. The results demonstrated that woodchip bioreactors are efficient in removing residual FA from RAS effluents and that nitrate removal was transiently enhanced during FA removal.
Highlights
Formalin is an aqueous solution of formaldehyde (FA, CH2O) and is commonly used as a disinfectant in aquaculture systems to treat various bacterial and fungal diseases as well as parasite infections such as Ichthyophthirius multifiliis and Ichtyobodo necator (Costia) (Masters, 2004; Matthews, 2005, Henriksen and Plessner, 2007; Rowland et al, f 2008; Tavares-Dias, 2021)
The results demonstrated that woodchip bioreactors are efficient in removing residual FA from recirculating aquaculture system (RAS) effluents and that nitrate removal was transiently enhanced during FA removal
Formalin is still used in significant amounts in aquaculture, and p approximately 13 l of formalin (37%) are used per ton of rainbow trout (Oncorhynchus mykiss) - produced in Denmark (Henriksen et al, 2008)
Summary
Formalin is an aqueous solution of formaldehyde (FA, CH2O) and is commonly used as a disinfectant in aquaculture systems to treat various bacterial and fungal diseases as well as parasite infections such as Ichthyophthirius multifiliis (white spot disease) and Ichtyobodo necator (Costia) (Masters, 2004; Matthews, 2005, Henriksen and Plessner, 2007; Rowland et al, f 2008; Tavares-Dias, 2021). From the moment formalin is added to the r production unit of land-based fish farms, three processes determine the resulting concentration of u FA in the water over time: 1) an instant adsorption onto surfaces, 2) microbial degradation and 3) Jo dilution due to continuous water intake (Masters, 2004; Eiroa et al, 2005; Pedersen et al, 2007; Sortkjær et al, 2008a,b). In Danish field studies, instant adsorption of FA to surfaces caused initial FA concentrations of around 20 mg FA/L to be reduced by up to 10% in traditional flow-through farms and in Model Trout Farms (MTF) type 1 (hydraulic retention times, HRT, of 4-5 hours in production units). In MTFs type 3 ( HRT, of 5-12 hours in production units) containing biofilters FA reductions of some 30% were found (Sortkjær et al, 2008b)
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