Search

To remove nitrogen in cold conditions, we studied new nature-based treatment solutions using six pilot-scale reactors. The pilots were woodchip bioreactor (WBR), aquatic floating hook-moss (Warnstorfia fluitans) (MBR), and a combined woodchip and floating hook-moss hybrid unit (HBR) with an improved hydraulic design. The experiment was run in a climate room at temperatures of 10 °C and 5 °C and using mine water from two sites located in northern Finland. Unlike traditional horizontal flow woodchip bioreactors, in this study the hydraulic efficiency was improved from poor (λ = 0.06) in the woodchip bioreactor to satisfactory (λ = 0.51) in the hybrid unit by inserting two inner plates along the water flow and adding floating hook-moss. The hybrid bioreactor revealed the highest capability of nitrogen removal in all inorganic forms at T ≤ 10 °C with a mean HRT of 70.5 h. On average, 30–78 % of dissolved inorganic nitrogen was removed in the hybrid unit, which was 2 and 3 times more than in units consisting only of woodchip or floating hook-moss. The hybrid bioreactor revealed a maximum NO−3-N removal rate of 1.0–5.2 g m⁻³ d⁻¹ and a 21.8–99.7 % removal efficiency for an average incoming NO−3-N load of 40 g d⁻¹. The maximum NH⁺ ₄-N removal efficiency of 75.6 and 53 % took place in HBR and MBR, respectively, when the incoming NH⁺ ₄-N load was 23.6 ± 0.7 g d⁻¹ at 10 °C. Over the 154 days of the experiment, the hybrid unit removed a total of 2.95 kg DIN-N, which was 0.8 kg higher than the sum of the DIN-N mass removed in the individual woodchip (1.7 kg) and moss units (0.55 kg). The nitrogen content of the aquatic moss was higher in the hybrid unit compared to the moss unit, showing a higher contribution of N plant uptake. Overall, our results suggest that combining woodchips and aquatic moss in a hybrid unit with improved hydraulic efficiency using inner walls may enhance nitrogen removal in cold climate conditions.

Woodchip bioreactors are being successfully applied to remove nitrate from commercial land-based recirculating aquaculture system (RAS) effluents. In order to understand and optimize the overall function of these bioreactors, knowledge on the microbial communities, especially on the microbes with potential for production or mitigation of harmful substances (e.g. hydrogen sulfide; H2S) is needed. In this study, we quantified and characterized bacterial and fungal communities, including potential H2S producers and consumers, using qPCR and high throughput sequencing of 16S rRNA gene. We took water samples from bioreactors and their inlet and outlet, and sampled biofilms growing on woodchips and on the outlet of the three full-scale woodchip bioreactors treating effluents of three individual RAS. We found that bioreactors hosted a high biomass of both bacteria and fungi. Although the composition of microbial communities of the inlet varied between the bioreactors, the conditions in the bioreactors selected for the same core microbial taxa. The H2S producing sulfate reducing bacteria (SRB) were mainly found in the nitrate-limited outlets of the bioreactors, the main groups being deltaproteobacterial Desulfobulbus and Desulfovibrio. The abundance of H2S consuming sulfate oxidizing bacteria (SOB) was 5–10 times higher than that of SRB, and SOB communities were dominated by Arcobacter and other genera from phylum Epsilonbacteraeota, which are also capable of autotrophic denitrification. Indeed, the relative abundance of potential autotrophic denitrifiers of all denitrifier sequences was even 54% in outlet water samples and 56% in the outlet biofilm samples. Altogether, our results show that the highly abundant bacterial and fungal communities in woodchip bioreactors are shaped through the conditions prevailing within the bioreactor, indicating that the bioreactors with similar design and operational settings should provide similar function even when conditions in the preceding RAS would differ. Furthermore, autotrophic denitrifiers can have a significant role in woodchip biofilters, consuming potentially produced H2S and removing nitrate, lengthening the operational age and thus further improving the overall environmental benefit of these bioreactors.

This study investigated the effect of salinity on microbial composition and denitrification capacity of woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. Twelve laboratory-scale woodchip bioreactors were run in triplicates at 0, 15, 25, and 35 ppt salinities, and water chemistry was monitored every third day during the first 39 days of operation. Microbial communities of the woodchips bioreactors were analyzed at the start, after one week, and at the end of the trial. Woodchip bioreactors removed nitrate at all salinities tested. The highest NO3-N removal rate of 22.0 ± 6.9 g NO3-N/m3/d was obtained at 0 ppt, while 15.3 ± 4.9, 12.5 ± 5.4 and 11.8 ± 4.0 g NO3-N/m3/d were obtained at salinities of 15, 25 and 35 ppt, respectively. Nitrate removal rates thus decreased with salinity, being 54–69% lower than at 0 ppt. Leaching of total ammonia nitrogen (TAN) and orthophosphate (PO4-P) from woodchips was initially higher at saline treatments compared to 0 ppt, while initial leaching of BOD5 appeared to be similar across all treatments. Production of alkalinity per g NO3-N removed was higher at 0 (3.6 ± 0.5 gCaCO3/gNO3-N) and 15 ppt (3.5 ± 0.8) than at the more saline treatments (25 ppt: 2.0 ± 0.9, 35 ppt: 1.12 ± 0.5 gCaCO3/gNO3-N), indicating that heterotrophic denitrification was the dominant nitrate removing process at 0 and 15 ppt, while autotrophic denitrification processes probably interfered with the alkalinity balance at 25 and 35 ppt. In the woodchip reactors, Gammaproteobacteria was the most abundant taxa. However, salinity shaped the woodchip microbiome, resulting in an increase in the abundance of sulfide oxidizing autotrophic denitrifiers, but decrease in the overall abundance of denitrifying microbes at higher salinities, which presumably explained the reduced nitrate removal rates at elevated salinities. This study demonstrates that woodchip bioreactors can be applied to remove nitrate from saline RAS effluents albeit at lower nitrate removal rates compared to freshwater installations.

The effect of bioreactor design on nitrification efficiency has been well studied, but less is known about the overall impacts on water quality. Besides nitrification, submerged fixed bed bioreactors (FBBR) trap fine solid particles, whereas moving bed bioreactors (MBBR) grind solids, possibly increasing solids and particle accumulation in the system. In this experiment, the effects of different combinations of fixed bed and moving bed bioreactors on water quality, solids removal, particle size distribution, fish health based on histopathological changes and nitrification efficiency were studied in laboratory scale recirculating aquaculture systems (RAS) with rainbow trout (Oncorhynchus mykiss). Three set-ups with triplicate tanks were used: 1. two consecutive fixed bed bioreactors (FF); 2. a fixed bed bioreactor followed by a moving bed bioreactor (FM) and 3. two consecutive moving bed bioreactors (MM). Fish performance was not influenced by the design of the bioreactor, specific growth rate (SGR) being between 1.59 and 1.64% d−1 and feed conversion ratio (FCR) between 0.95 and 0.98. Water nitrite concentration was higher in the FF systems compared to FM and MM systems, whereas the average total ammonia nitrogen concentration (TAN) was not influenced by the treatments. Nitrification rate, which was measured in the laboratory, followed the water nitrite levels, indicating highest total ammonium oxidation rates in the MM systems. UV254 absorbance and total organic carbon (TOC) concentrations were higher in the groups with moving bed systems, indicating accumulation of organic substances in the circulating water. The total volume of particles was higher in the MM systems as compared to the FF systems. The total solids balance was similar in all the bioreactor groups, since the removal of solids by the FBBR backwash was compensated by the drum filter in the FM and MM systems. In general, no significant histopathological difference in gill, kidney, heart and liver tissue were observed between the RAS treatment groups and the flow-through treatment.

Aquaculture needs practical solutions for nutrient removal to achieve sustainable fish production. Passive denitrifying bioreactors may provide an ecological, low-cost and low-maintenance approach for wastewater nitrogen removal. However, innovative organic materials are needed to enhance nitrate removal from the low carbon effluents in intensive recirculating aquaculture systems (RAS). In this study, we tested three additional carbon sources, including biochar, dried Sphagnum sp. moss and industrial potato residues, to enhance the performance of woodchip bioreactors treating the low carbon RAS wastewater. We assessed nitrate (NO3−) removal and microbial community composition during a one-year in situ column test with real aquaculture wastewater. We found no significant differences in the NO3− removal rates between the woodchip-only bioreactor and bioreactors with a zone of biochar or Sphagnum sp. moss (maximum removal rate 31–33 g NO3−-N m−3 d−1), but potato residues increased NO3− removal rate to 38 g NO3−-N m−3 d−1, with stable annual reduction efficiency of 93%. The readily available carbon released from potato residues increased NO3−-N removal capacity of the bioreactor even at higher inflow concentrations (>52 mg L−1). The microbial community and its predicted functional potential in the potato residue bioreactor differed markedly from those of the other bioreactors. Adding potato residues to woodchip material enabled smaller bioreactor size to be used for NO3− removal. This study introduced industrial potato by-product as an alternative carbon source for the woodchip denitrification process, and the encouraging results may pave the way toward growth of blue bioeconomy using the RAS. peerReviewed

This study investigated the effect of salinity on microbial composition and denitrification capacity of woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. Twelve laboratory-scale woodchip bioreactors were run in triplicates at 0, 15, 25, and 35 ppt salinities, and water chemistry was monitored every third day during the first 39 days of operation. Microbial communities of the woodchips bioreactors were analyzed at the start, after one week, and at the end of the trial. Woodchip bioreactors removed nitrate at all salinities tested. The highest NO3-N removal rate of 22.0 ± 6.9 g NO3-N/m3/d was obtained at 0 ppt, while 15.3 ± 4.9, 12.5 ± 5.4 and 11.8 ± 4.0 g NO3-N/m3/d were obtained at salinities of 15, 25 and 35 ppt, respectively. Nitrate removal rates thus decreased with salinity, being 54–69% lower than at 0 ppt. Leaching of total ammonia nitrogen (TAN) and orthophosphate (PO4-P) from woodchips was initially higher at saline treatments compared to 0 ppt, while initial leaching of BOD5 appeared to be similar across all treatments. Production of alkalinity per g NO3-N removed was higher at 0 (3.6 ± 0.5 gCaCO3/gNO3-N) and 15 ppt (3.5 ± 0.8) than at the more saline treatments (25 ppt: 2.0 ± 0.9, 35 ppt: 1.12 ± 0.5 gCaCO3/gNO3-N), indicating that heterotrophic denitrification was the dominant nitrate removing process at 0 and 15 ppt, while autotrophic denitrification processes probably interfered with the alkalinity balance at 25 and 35 ppt. In the woodchip reactors, Gammaproteobacteria was the most abundant taxa. However, salinity shaped the woodchip microbiome, resulting in an increase in the abundance of sulfide oxidizing autotrophic denitrifiers, but decrease in the overall abundance of denitrifying microbes at higher salinities, which presumably explained the reduced nitrate removal rates at elevated salinities. This study demonstrates that woodchip bioreactors can be applied to remove nitrate from saline RAS effluents albeit at lower nitrate removal rates compared to freshwater installations. peerReviewed

The effect of bioreactor design on nitrification efficiency has been well studied, but less is known about the overall impacts on water quality. Besides nitrification, submerged fixed bed bioreactors (FBBR) trap fine solid particles, whereas moving bed bioreactors (MBBR) grind solids, possibly increasing solids and particle accumulation in the system. In this experiment, the effects of different combinations of fixed bed and moving bed bioreactors on water quality, solids removal, particle size distribution, fish health based on histopathological changes and nitrification efficiency were studied in laboratory scale recirculating aquaculture systems (RAS) with rainbow trout (Oncorhynchus mykiss). Three set-ups with triplicate tanks were used: 1. two consecutive fixed bed bioreactors (FF); 2. a fixed bed bioreactor followed by a moving bed bioreactor (FM) and 3. two consecutive moving bed bioreactors (MM). Fish performance was not influenced by the design of the bioreactor, specific growth rate (SGR) being between 1.59 and 1.64% d−1 and feed conversion ratio (FCR) between 0.95 and 0.98. Water nitrite concentration was higher in the FF systems compared to FM and MM systems, whereas the average total ammonia nitrogen concentration (TAN) was not influenced by the treatments. Nitrification rate, which was measured in the laboratory, followed the water nitrite levels, indicating highest total ammonium oxidation rates in the MM systems. UV254 absorbance and total organic carbon (TOC) concentrations were higher in the groups with moving bed systems, indicating accumulation of organic substances in the circulating water. The total volume of particles was higher in the MM systems as compared to the FF systems. The total solids balance was similar in all the bioreactor groups, since the removal of solids by the FBBR backwash was compensated by the drum filter in the FM and MM systems. In general, no significant histopathological difference in gill, kidney, heart and liver tissue were observed between the RAS treatment groups and the flow-through treatment. peerReviewed

The current study combined P and N removal using organic flocculant chemicals and woodchip bioreactors in both freshwater and brackish water (7 ppm) recirculating aquaculture systems (RAS). The use of carbon (C) containing flocculant chemicals in the process was hypothesized to further stimulate C-demanding N removal (denitrification) in bioreactors. The trial of combined P and N removal consisted of four treatments: freshwater and brackish water RAS with and without the addition of supernatant from flocculation process to the woodchip reactor. Duplicate woodchip reactors were used per treatment and the trial was run for six weeks. 56 % and 49 % of P was removed from fresh and brackish sludge water, respectively. The nitrate-N (NO3-N) removal rate was improved in the treatment when supernatant from flocculation process was used together with RAS discharge water when compared against the control. In brackish water RAS, the improvement was more pronounced (from 6.6 to 16.5 g NO3-N m-3 d-1) than in freshwater RAS (from 5.1 to 6.5 NO3-N m-3 d-1). In the freshwater bioreactors using supernatant, N was largely discharged as a nitrite-N (NO2-N). High NO2-N concentrations in freshwater reactors allude to incomplete denitrification reactions taking place. The results suggest that the organic flocculants did provide an additional C source for denitrification, which improved the N-removal process. However, in freshwater RAS this might have been partly due to untargeted processes such as DNRA (dissimilatory nitrate reduction to ammonium), and/or insufficient denitrification reactions taking place (excessive NO2-N production). peerReviewed