Environmentally sustainable land-based marine aquaculture

Reduced fishery harvests and increased consumer demand for seafood have precipitated an increase in intensive fish farming, predominantly in coastal and open ocean net-pens. However, as currently practiced, aquaculture is widely viewed as detrimental to the environment and typical operations are vulnerable to environmental influences, including pollution and endemic diseases. Here we report the development of a land-based, marine recirculating aquaculture system that is fully contained, with virtually no environmental impact as a result of highly efficient biological waste treatment and water recycling. Over 99% of the water volume was recycled daily by integrating aerobic nitrification to eliminate toxic ammonia and, for the first time, simultaneous, anaerobic denitrification and anaerobic ammonium oxidation, to convert ammonia and nitrate to nitrogen gas. Hydrogen sulfide generated by the separated endogenous organic solids was used as an electron source for nitrate reduction via autotrophic denitrification and the remaining organic solids were converted to methane and carbon dioxide. System viability was validated by growing gilthead seabream (Sparus aurata) from 61 g to 412 g for a total of 1.7 tons in a record 131 days with 99% fish survival. Ammonia nitrite and nitrate did not exceed an average daily concentration of 0.8 mg/l, 0.2 mg/l and 150 mg/l, respectively. Food conversion values were 16% lower than recorded levels for net-pen aquaculture and saltwater usage of less than 16 l/every kg of fish produced. The system is site-independent, biosecure, devoid of environmental contaminants and is not restricted to a single species.     

Heterotrophic denitrification of aquaculture effluent using fluidized sand biofilters

The ability to consistently and cost-effectively reduce nitrate-nitrogen loads in effluent from recirculating aquaculture systems would enhance the industry's environmental stewardship and allow improved facility proximity to large markets in sensitive watersheds. Heterotrophic denitrification technologies specifically employing organic carbon found in aquaculture system waste offer a unique synergy for treatment of land-based, closed-containment production outflows. For space-efficient fluidized sand biofilters to be used as such denitrification reactors, system parameters (e.g., influent dissolved oxygen and carbon to nitrogen ratios, C:N) must be evaluated to most effectively use an endogenous carbon source. The objectives of this work were to quantify nitrate removal under a range of C:Ns and to explore the biofilter bacterial community using three replicated fluidized sand biofilters (height 3.9 m, diameter 0.31 m; fluidized sand volume plus biofilm volume of 0.206 m³) operated at a hydraulic retention time of 15 min and a hydraulic loading rate of 188 L/min m² at The Conservation Fund Freshwater Institute in Shepherdstown, West Virginia, USA. Nitrate reduction was consistently observed during the biofilter study period (26.9 ± 0.9% removal efficiency; 402 ± 14 g NO₃-N/(m³ biofilter d)) although nitrite-N and total ammonium nitrogen concentrations slightly increased (11 and 13% increases, respectively). Nitrate removal efficiency was correlated with carbonaceous oxygen demand to nitrate ratios (R² > 0.70). Nitrate removal rates during the study period were moderately negatively correlated with influent dissolved oxygen concentration indicating it may be possible the biofilter hydraulic retention time was too short to provide optimized nitrate removal. It is reasonable to assume that the efficiency of nitrate removal across the fluidized sand biofilters could be substantially increased, as long as organic carbon was not limiting, by increasing biofilter bed depths (to 6–10 m), and thus hydraulic retention time. These findings provide a low-cost yet effective technology to remove nitrate-nitrogen from effluent waters of land-based closed-containment aquaculture systems.     

不同有机碳源及C/N对生物滤池净化效果的影响

利用生物滤池模拟装置,以实际养殖废水为处理对象,探讨了4种常见有机碳源(葡萄糖、乙醇、红糖和淀粉)及不同碳氮比对有机物去除、硝化反应和异养反硝化作用等生物滤池主要净化过程的影响.碳源初选结果显示,同种碳源下,当C/N从0升高至6过程中,生物滤池对TAN(总氨氮)的去除率呈先升高后降低趋势;当C/N较小时,各组对NO2--N的去除率差异性不显著(P＞0.05),随着C/N继续升高,NO2-N去除率则显著降低(P＜0.05);乙醇组除外,其他3组随着C/N升高,CODMn去除率先迅速增大然后趋于稳定;各组NO3-N和TN去除率呈先升高后降低趋势,且变化显著(P＜0.05),当C/N=4时,分别达到最高值.碳源复选结果显示,在C/N=4条件下,分别添加有机碳源(乙醇、淀粉、红糖和葡萄糖)的4组对TAN、NO3--N、TN和CODM的去除率显著高于对照组(P＜0.05);而对照组NO2--N的去除率最高,达到93.59％;添加乙醇,生物滤池对水体中TAN、NO2-N、NO3-N和TN的去除效果优于其他3种碳源.研究表明,当C/N=4时,乙醇作为外加碳源能很好地提高生物滤池的净化效率.     

Use of an internal fibrous biofilter for intermittent nitrification and denitrification treatments in a zero-discharge shrimp culture tank

To achieve water reuse in recirculating aquaculture systems, intermittent nitrification and denitrification processes using internal fibrous media was proposed. A pre-acclimated Biocord biofilter, with an initial nitrification rate of 17.1 ± 12.4 mg total ammonia nitrogen-N/m²/d was applied in a marine whiteleg shrimp (Litopenaeus vannamei) culture tank. Throughout the experiment, the aerobic nitrification activity of the biofilter was sufficient to control the ammonia and nitrite levels below 0.2 mg-N/L with an accumulation of nitrate up to 50 mg-N/L. The remaining nitrate was successfully removed after shrimp harvest with the same biofilter through anoxic denitrification in conjunction with a methanol supplement at a chemical oxygen demand: nitrate-N ratio of 5:1. With complete nitrogen removal, the water was re-aerated and the next crop of shrimp culture was initiated. In this study, a two-crop shrimp cultivation was performed in sequence in the same tank without water exchange. The microbial diversity was monitored using high-throughput sequencing on Illumina MiSeq, which demonstrated that Proteobacteria (45.3 %), Chloroflexi (18.4 %), and Bacteroidetes (17.1 %) were the most abundant phyla. With an emphasis on nitrogen removal, the family Nitrosomonadaceae and Nitrospiraceae were the dominant nitrifying bacteria during the aerobic nitrification, while a high relative abundance of the Methylophaga and Methylotenera genera was observed under the anoxic condition.     

Comparing denitrification rates and carbon sources in commercial scale upflow denitrification biological filters in aquaculture

Aerobic biological filtration systems employing nitrifying bacteria to remediate excess ammonia and nitrite concentrations are common components of recirculating aquaculture systems (RAS). However, significant water exchange may still be necessary to reduce nitrate concentrations to acceptable levels unless denitrification systems are included in the RAS design. This study evaluated the design of a full scale denitrification reactor in a commercial culture RAS application. Four carbon sources were evaluated including methanol, acetic acid, molasses and Cerelose™, a hydrolyzed starch, to determine their applicability under commercial culture conditions and to determine if any of these carbon sources encouraged the production of two common “off-flavor” compounds, 2-methyisoborneol (MIB) or geosmin. The denitrification design consisted of a 1.89 m³ covered conical bottom polyethylene tank containing 1.0 m³ media through which water up-flowed at a rate of 10 lpm. A commercial aquaculture system housing 6 metric tonnes of Siberian sturgeon was used to generate nitrate through nitrification in a moving bed biological filter. All four carbon sources were able to effectively reduce nitrate to near zero concentrations from influent concentrations ranging from 11 to 57 mg/l NO₃–N, and the maximum daily denitrification rate was 670–680 g nitrogen removed/m³ media/day, regardless of the carbon source. Although nitrite production was not a problem once the reactors achieved a constant effluent nitrate, ammonia production was a significant problem for units fed molasses and to a less extent Cerelose™. Maximum measured ammonia concentrations in the reactor effluents for methanol, vinegar, Cerelose™ and molasses were 1.62 ± 0.10, 2.83 ± 0.17, 4.55 ± 0.45 and 5.25 ± 1.26 mg/l NH₃–N, respectively. Turbidity production was significantly increased in reactors fed molasses and to a less extent Cerelose™. Concentrations of geosmin and MIB were not significantly increased in any of the denitrification reactors, regardless of carbon source. Because of its very low cost compared to the other sources tested, molasses may be an attractive carbon source for denitrification if issues of ammonia production, turbidity and foaming can be resolved.     

Microbial valorization of solid wastes from a recirculating aquaculture system and the relevant microbial functions

This study was performed to establish valorization technology for solid wastes from a seawater recirculating aquaculture system (RAS) by using beneficial microorganisms. An efficient microbial agent (KBM-1) was selected based on the degradation activity of the RAS solid wastes (20% slurry) in a lab-scale reactor system considering the removal rates of chemical oxygen demand, solid material, total nitrogen, ammonium-N, and nitrate-N and the production of organic acids as electron donors for denitrification. The microbial consortium KBM-1 was particularly efficient in the removal of ammonium-N and nitrate-N with removal efficiencies of 42% and 50%, respectively, in eight days and in the rapid production of organic acids (230 mg L⁻¹, 3.5 mM, 0.018 kg m⁻³ d⁻¹) after two days. There was a concomitant removal of NO₃--N (41%, 0.005 kg N m⁻³ d⁻¹) after two days when a significant production of organic acids occurred. Comamonas sp. was a dominant genus after eight days in all treatments. The level of nitrate-N in the treatments with KBM-1 decreased by 50.4% after eight days, as opposed to that of the control sample (27.7%), indicating the potential denitrification activity of Citrobacter freundii and Comamonas sp. The bioaugmented species (Sporolactobacillus inulinus, Lactobacillus mali, Lactobacillus casei, and Clostridium tyrobutyricum), constituting 41% of the total communities, appeared to facilitate the growth of indigenous microbial communities that were involved in the degradation (hence valorization) of solid wastes (mostly remaining fish feed and fish feces) into simple metabolites (organic acids and inorganic materials such as ammonium, nitrite, nitrate, and CO₂). The simultaneous generation of organic acids through the valorization of solid wastes and their subsequent reuse in the denitrification of an RAS biofilter system can provide a significant contribution to the eco-friendly management of RASs and provide meaningful economic merit to the solid wastes of RASs.     

Potential applications of biological filtration media in changing water properties and subsequent effects on common carp (Cyprinus carpio) in recirculating aquaculture systems

This work aims to develop a suitable aquaculture system to solve issues pertaining to water quality in recirculating aquaculture systems (RAS) thereby improving fish growth performance and their feed conversion ratio (FCR). It is known that the efficiency of nutrient decomposition depends on the amount and species of bacteria present in the filter, and these can in turn be regulated by the pH. Our study shows that the significant increase in pH from 7.45 to 7.51 and 7.47 is achieved after the water was filtered through sawdust and wood wool respectively. At the same time, nitrite concentrations were found to have significantly decreased from 0.06 ppm in the control to 0.01 ppm in the sawdust treatment. Changes in the physical–chemical properties of treated water affect the biological properties of the organisms. It was found that the average final body weight for fish in the wood wool treatment 30.96 g and fish in the sawdust treatment 31.44 g was significantly higher compared to that for the fish in the control treatment 28.42 g. The use of sponge, wood wool or sawdust resulted in significantly lower FCR and therefore better fish productivity in RAS. At the same time, the red blood cell count, plasma urea, plasma creatinine, plasma albumin, and GOT and GPT were significantly higher in fish in the plastic, sponge, wood wool or sawdust treatment compared to the control treatment. In summary, this study demonstrates the benefits of using sponge, wood, wool and sawdust as biofilter media for aquaculture.     

Discharge management in fresh and brackish water RAS: Combined phosphorus removal by organic flocculants and nitrogen removal in woodchip reactors

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 6.6–16.5 g NO₃-N m⁻³ d^-1) than in freshwater RAS (from 5.1–6.5 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).     

一株反硝化细菌的筛选及其反硝化特性的研究

从土壤中分离到一株高活性反硝化菌DNF409,经生理生化和16S rDNA序列分析,初步判断为芽孢杆菌属(Bacillussp.)。在生长的各个阶段,该菌株均具有较强的反硝化活性,最适反硝化碳源为乙醇。在天然养殖水体中,碳氮摩尔比达到8.0∶1、菌体浓度达到108cfu/L时,其反硝化活性即可充分发挥,硝态氮和亚硝态氮的降解率可分别达到94.79%和99.94%。试验表明该菌株在养殖水体的生物脱氮方面具有广阔的应用前景。     

End-of-pipe denitrification using RAS effluent waste streams: Effect of C/N-ratio and hydraulic retention time

Environmentally sustainable aquaculture development requires increased nitrogen removal from recirculating aquaculture systems (RAS). In this study, removed solids from a large commercial outdoor recirculated trout farm (1000 MT year⁻¹) were explored as an endogenous carbon source for denitrification. This was done by (1) a controlled laboratory experiment on anaerobic hydrolysis of the organic matter (from sludge cones, drumfilter, and biofilter back-wash) and (2) an on-site denitrification factorial experiment varying the soluble COD (COD_S)/NO₃-N ratio from 4 to 12 at hydraulic retention times (HRT) from 50 to 170 min in simple 5.5 m³ denitrification reactors installed at the trout farm.The lab-experiments showed that the major part of the readily biodegradable organic matter was hydrolyzed within 14 days, and the hydrolysis rate was fastest the first 24 h. Organic matter from the sludge cones generated 0.21 ± 0.01 g volatile fatty acids (VFA) g⁻¹ total volatile solids (TVS), and the VFAs constituted 75% of COD_S. Analogously, 1 g TVS from the drum filter generated 0.15 ± 0.01 g VFA, constituting 68% of the COD_S. Comparison of the laboratory hydrolysis experiments and results from the on-farm study revealed as a rough estimate that potentially 17–24% of the generated VFA was lost due to the current sludge management.Inlet water to the denitrification reactors ranged in NO₃-N concentration from 8.3 to 11.7 g m⁻³ and COD_S from 52.9 to 113.4 g m⁻³ (10.0 ± 1.2 °C). The highest NO₃-N removal rate obtained was at the intermediate treatments; 91.5–124.8 g N m⁻³_reactor d⁻¹. The effect of the C/N ratio depended on the HRT. At low HRT, the variation in C/N ratio had no significant effect on NO₃-N removal rate, contrary to the effect at the high HRT. The stoichiometric ratio of COD_S/NO₃-N was 6.0 ± 2.4, ranging from 4.4 (at the high HRT) to 9.3 (at the low HRT). A simple model of the denitrification reactor developed in AQUASIM showed congruence between modeled and measured data with minor exceptions. Furthermore, this study pointed to the versatility of the NO₃-N removal pathways expressed by the bacterial population in response to changes in the environmental conditions; from autotrophic anammox activity presumably present at low C/N to dissimilatory nitrate reduction to ammonia (DNRA) at high C/N, besides the predominate “normal” heterotrophic dissimilatory nitrate reduction (denitrification).     

Nitrate removal effectiveness of fluidized sulfur-based autotrophic denitrification biofilters for recirculating aquaculture systems

There is a need to develop practical methods to reduce nitrate–nitrogen loads from recirculating aquaculture systems to facilitate increased food protein production simultaneously with attainment of water quality goals. The most common wastewater denitrification treatment systems utilize methanol-fueled heterotrophs, but sulfur-based autotrophic denitrification may allow a shift away from potentially expensive carbon sources. The objective of this work was to assess the nitrate-reduction potential of fluidized sulfur-based biofilters for treatment of aquaculture wastewater. Three fluidized biofilters (height 3.9 m, diameter 0.31 m; operational volume 0.206 m³) were filled with sulfur particles (0.30 mm effective particle size; static bed depth approximately 0.9 m) and operated in triplicate mode (Phase I: 37–39% expansion; 3.2–3.3 min hydraulic retention time; 860–888 L/(m² min) hydraulic loading rate) and independently to achieve a range of hydraulic retention times (Phase II: 42–13% expansion; 3.2–4.8 min hydraulic retention time). During Phase I, despite only removing 1.57 ± 0.15 and 1.82 ± 0.32 mg NO₃–N/L each pass through the biofilter, removal rates were the highest reported for sulfur-based denitrification systems (0.71 ± 0.07 and 0.80 ± 0.15 g N removed/(L bioreactor-d)). Lower than expected sulfate production and alkalinity consumption indicated some of the nitrate removal was due to heterotrophic denitrification, and thus denitrification was mixotrophic. Microbial analysis indicated the presence of Thiobacillus denitrificans, a widely known autotrophic denitrifier, in addition to several heterotrophic denitrifiers. Phase II showed that longer retention times tended to result in more nitrate removal and sulfate production, but increasing the retention time through flow rate manipulation may create fluidization challenges for these sulfur particles.     

聚己内酯添加量对淡水养殖水体硝酸盐氮处理效果的影响

以可生物降解聚合物(Biological degradable polymers,BDPs)为有机碳源进行异养反硝化可以避免多次添加碳源、碳源不足或过量等问题。聚己内酯(Polycaprolactone,PCL)已被证明能够作为水产养殖用水异养反硝化的有机碳源。研究了聚己内酯添加量对水产养殖用水硝酸盐氮去除效率的影响。在进水硝酸盐氮(NO_3~--N)负荷为0.1 g/(L·d)条件下,200 m L水体中分别加入5 g、10 g、15 g、20 g、25 g和30 g的PCL颗粒进行反硝化,各组的NO_3~--N去除效率没有明显差异;出水中溶解有机碳的质量浓度随着PCL添加量的增加而增加;5 g组的PCL利用率明显高于其他组。结果显示:试验条件下,PCL添加量的增加并不会必然增加NO_3~--N的去除效率,反而会造成出水中溶解有机碳的增加;添加5 g PCL为最适添加量。     

Effect of different C/N ratios and hydraulic retention times on denitrification in saline,recirculating aquaculture system effluents

Data on operation and performance of cost-effective solutions for end-of-pipe removal of nitrate from land-based saltwater recirculating aquaculture systems (RAS) are scarce but increasingly requested by the aquaculture industry. This study investigated the performance of a (semi)commercial-scale fixed-bed denitrification unit using single sludge for treating effluent from a commercial, saltwater RAS used for production of Atlantic salmon (Salmo salar). A fixed-bed denitrification reactor was fed continuously with 3-days hydrolyzed sludge from the commercial RAS, and was operated at different hydraulic retention times (HRTs; 1.82, 3.64, 5.46, or 7.28 h) or influent C/N ratios (3, 5, 7, or 10). Twenty-four h pooled samples were collected from the inflowing RAS water and the hydrolyzed sludge as well as from the denitrification reactor outlet, and samples were analyzed for nutrients and organic matter content.Nitrate removal rates increased consistently with decreasing HRT (from 64.3 ± 5.2–162.7 ± 22.0 g NO₃-N/m³/d within the HRTs tested) at non-limiting C/N ratios, while nitrate removal efficiencies decreased (from 99.6 ± 0.3–58.2 ± 8.9 %). With increasing influent C/N ratios at constant HRT (3.64 h), nitrate removal rates increased until the removal efficiency was close to 100 % and nitrate concentration in the denitrification reactor became rate-limiting. A maximum nitrate removal rate of 162.7 ± 2.0 g NO₃-N/m³/d was achieved at a HRT of 1.82 h and an influent C/N of 6.6 ± 0.5, while the most efficient use of hydrolyzed sludge (0.19 ± 0.02 g NO₃-N removed/g sCOD supplied) was obtained with a HRT of 3.64 h and a C/N ratio of 2.9. Removal rates of organic matter significantly and consistently increased with decreasing HRT and increasing C/N ratio. In addition, reducing HRT and increasing C/N ratios significantly improved removal of total phosphorus (TP) and PO₄-P.In conclusion, optimal management of the operating parameters (HRT and C/N ratio) in a single-sludge denitrification process can significantly reduce the discharge of nitrogen, organic matter, and phosphorous from land-based saltwater RAS and thus contribute to increased sustainability.     

Denitrification in recirculating systems: Theory and applications

Profitability of recirculating systems depends in part on the ability to manage nutrient wastes. Nitrogenous wastes in these systems can be eliminated through nitrifying and denitrifying biofilters. While nitrifying filters are incorporated in most recirculating systems according to well-established protocols, denitrifying filters are still under development. By means of denitrification, oxidized inorganic nitrogen compounds, such as nitrite and nitrate are reduced to elemental nitrogen (N₂). The process is conducted by facultative anaerobic microorganisms with electron donors derived from either organic (heterotrophic denitrification) or inorganic sources (autotrophic denitrification). In recirculating systems and traditional wastewater treatment plants, heterotrophic denitrification often is applied using external electron and carbon donors (e.g. carbohydrates, organic alcohols) or endogenous organic donors originating from the waste. In addition to nitrate removal, denitrifying organisms are associated with other processes relevant to water quality control in aquaculture systems. Denitrification raises the alkalinity and, hence, replenishes some of the inorganic carbon lost through nitrification. Organic carbon discharge from recirculating systems is reduced when endogenous carbon sources originating from the fish waste are used to fuel denitrification. In addition to the carbon cycle, denitrifiers also are associated with sulfur and phosphorus cycles in recirculating systems. Orthophosphate uptake by some denitrifiers takes place in excess of their metabolic requirements and may result in a considerable reduction of orthophosphate from the culture water. Finally, autotrophic denitrifiers may prevent the accumulation of toxic sulfide resulting from sulfate reduction in marine recirculating systems. Information on nitrate removal in recirculating systems is limited to studies with small-scale experimental systems. Packed bed reactors supplemented with external carbon sources are used most widely for nitrate removal in these systems. Although studies on the application of denitrification in freshwater and marine recirculating systems were initiated some thirty years ago, a unifying concept for the design and operation of denitrifying biofilters in recirculating systems is lacking.     

Nitrogen removal characteristics of <Emphasis Type="Italic">Pseudomonas stutzeri</Emphasis> F11 and its application in grass carp culture

This study was designed to investigate the denitrification characteristics of Pseudomonas stutzeri F11 under different environmental conditions and to evaluate the effect of these characteristics on nitrogen removal and the water microbiome in an experimental grass carp aquaculture system. The results showed that the optimal growth conditions of strain F11 were (1) ammonia-nitrogen (N), nitrite-N, or nitrate-N as sole N source; (2) initial ammonia-N concentration of 10–30 mg N/l; (3), initial nitrite-N concentration of 200 mg N/l; (4) sodium citrate as carbon source; (5) rotation (r) speed of 200 r/min; a C:N ratio of between 2 and 10; (6) culture at 32–37 °C. The addition of P. stutzeri F11 to the experimental grass carp aquaculture system reduced the levels of ammonia-N, nitrite-N, and total N in the water over an extended range, but had no effect on nitrate-N level. Results of the 454 pyrosequencing analysis indicated that the structure of the microbial community in the aquaculture water changed significantly after the addition of P. stutzeri F11 preparations. The addition of P. stutzeri F11 to the aquaculture system also altered the microbiome metabolism in the water, especially the bacteria involved in nitrogen metabolism. These results suggest that the addition of P. stutzeri F11 to an experimental grass carp aquaculture system decrease nitrogen levels and alter the microbial community structure of the water; as such, this bacterial strain could be a potential candidate for the regulation of water quality in aquaculture systems.     

Nitrate removal efficiency and bacterial community of polycaprolactone-packed bioreactors treating water from a recirculating aquaculture system

A denitrification reactor packed with polycaprolactone (PCL) as a carbon source and biofilm carrier was developed to remove nitrate nitrogen (\( {\mathrm{NO}}_3^{-}-\mathrm{N} \)) from the water of a recirculating aquaculture system for 115 days. The hydraulic retention time was set to 6 h, and the water flow rate was 0.95 L h^?1. The removal rates of \( {\mathrm{NO}}_3^{-}-\mathrm{N} \) ranged from 34.67 to 155.7 g \( {\mathrm{NO}}_3^{-}-\mathrm{N} \) m^?3 day^?1 when the effluent \( {\mathrm{NO}}_3^{-}-\mathrm{N} \) concentrations ranged between 1.62 and 72.25 mg L^?1. No obvious changes in the Fourier transform infrared spectra of the PCL before and after use were observed. Bacterial community structure in the biofilm of the PCL granules was analyzed using Illumina MiSeq sequencing technology. Proteobacteria and Bacteroidetes were predominant in the biofilm, with relative abundances of 63.9 and 27.4%, respectively. Three genera, Acidovorax, Azospira, and Diaphorobacter, were capable of both denitrification and PCL degradation. This study indicates that PCL-packed reactors may be used and optimized for removing nitrate from aquaculture effluents.     

Performance characteristics of fluidised bed biofilters in a novel laboratory-scale recirculation system for rainbow trout: nitrification rates, oxygen consumption and sludge collection

A laboratory-scale recirculating aquaculture system for fluidised bed biofilter evaluation was engineered. The design included all components found in typical full-scale commercial production systems. The system included two identical units each with oxygenation, UV treatment, cooling, biofiltration and a particulates separation device. Water from the two systems was mixed in a degassing unit. A 1 month test period after biofilter maturation revealed stable concentrations of total ammonia nitrogen (TAN), nitrite and nitrate within the system. Mean nitrification rate was 0.27 and 0.21 g TAN m⁻² day⁻¹. Oxygen consumption in the biofilters ranged between 56 and 64% due to nitrifying activity. Mass balances on nitrogen indicated that 48%, added via the feed, was converted to nitrate within the system, with 6% of the added nitrogen being found in the sludge. The remaining 43% was either used during fish growth, left the system, as organic nitrogenous compounds (or unidentified nitrogenous compounds), via the outlet, or was lost to the atmosphere. At least 61% of the nitrate produced was generated by the biofilters. The system proved to be an exceptional set-up for evaluation of the performance of fluidised bed biofilters, allowing both pre- and post-filter measurements of various water quality criteria.     

End-of-pipe single-sludge denitrification in pilot-scale recirculating aquaculture systems

A step toward environmental sustainability of recirculat aquaculture systems (RAS) is implementation of single-sludge denitrification, a process eliminating nitrate from the aqueous environment while reducing the organic matter discharge simultaneously. Two 1700 L pilot-scale RAS systems each with a 85 L denitrification (DN) reactor treating discharged water and hydrolyzed solid waste were setup to test the kinetics of nitrate and COD removal. Nitrate removal and COD reduction efficiency was measured at two different DN-reactor sludge ages (high θ_X: 33–42 days and low θ_X: 17–23 days). Nitrate and total N (NO₃⁻ + NO₂⁻ + NH₄⁺) removal of the treated effluent water ranged from 73–99% and 60–95% during the periods, respectively, corresponding to an overall maximum RAS nitrate removal of approximately 75%. The specific nitrate removal rate increased from 17 to 23 mg NO₃⁻-N (g TVS d)⁻¹ and the maximal potential DN rate (measured at laboratory ideal conditions) increased correspondingly from 64–68 mg NO₃⁻-N (g TVS d)⁻¹ to 247–294 mg NO₃⁻-N (g TVS d)⁻¹ at high and low θ_X, respectively. Quantification of denitrifiers in the DN-reactors by qPCR showed only minor differences upon the altered sludge removal practice. The hydrolysis unit improved the biodegradability of the solid waste by increasing volatile fatty acid COD content 74–76%. COD reductions in the DN-reactors were 64–70%. In conclusion, this study showed that single-sludge denitrification was a feasible way to reduce nitrate discharge from RAS, and higher DN rates were induced at lower sludge age/increased sludge removal regime. Improved control and optimization of reactor DN-activity may be achieved by further modifying reactor design and management scheme as indicated by the variation in and between the two DN-reactors.     

Performance of a closed recirculating system with foam separation, nitrification and denitrification units for intensive culture of eel: towards zero emission

The development of a closed recirculating aquaculture system that does not discharge effluents would reduce a large amount of pollutant load on aquatic bodies. In this study, eel were reared in a closed recirculating system, which consisted of a rearing tank, a foam separation unit, a nitrification unit and a denitrification unit. The foam separation unit has an inhalation-type aerator and supplies air bubbles to the rearing water. The growth of eel, which were fed a commercial diet, was satisfactory, with gross weight increases of up three times in 3 months. The survival rate under the congested experimental conditions was 91%. The foam separation unit maintained oxygen saturation in the rearing water at about 80%. Furthermore, fine colloidal substances were absorbed on the stable foam formed from eel mucus and were removed from the rearing water by foam separation. Ammonia oxidation and the removal of suspended solids were accomplished rapidly and simultaneously in the nitrification unit. The ammonia concentration and turbidity were kept at less than 1.2 mg of N per litre and 2.5 units, respectively. When the denitrification process was operated, nitrate that accumulated in the rearing water (151 mg of N per litre) was reduced to 40 mg of N per litre. The sludge was easily recovered from the nitrification and denitrification tanks, and the components were found suitable as compost. Based on these results, the intensive aquaculture of freshwater fish such as eel can be achieved using a closed recirculating system without emission.     

Efficiency of producing bioflocs with aquaculture waste by using poly-β-hydroxybutyric acid as a carbon source in suspended growth bioreactors

With additional organic carbon, fish waste can be used as a substrate to produce bioflocs, a protein source for aquaculture animals. In choosing a carbon source, one should consider convenience, cost and biodegradability. This study investigates the efficiency of poly-β-hydroxybutyric acid (PHB), a biologically degradable polymer, as a carbon source to produce bioflocs in suspended growth bioreactors (SGRs), PHB-SGRs, compared with glucose (GLU-SGRs). The C:N ratio in PHB-SGRs could be maintained around 15:1. The volatile suspended solids (VSS) yield was 2.94 ± 0.72 gVSS/g fish waste for PHB-SGRS and 4.90 ± 0.23 gVSS/g fish waste for GLU-SGRs. The recycling rate of nitrogen in aquaculture solid waste was 56 ± 2% and 87 ± 7% for the PHB-SGRs and Glu-SGRs. No significant differences were found in the bioflocs produced and in the crude protein content of the produced bioflocs between PHB-SGRs and GLU-SGRs. PHB-SGRs and GLU-SGRs could remove dissolved inorganic nitrogen from aquaculture wastewater, with average values of 11.82 ± 8.95 and 16.27 ± 3.95 mg/g TSS/d. Because the calculation of the added amount of carbon and the multiple additions of carbon was avoided, PHB is considered to be a good choice as an organic carbon source for this process, even though not all parameters used for assessment were better than those of GLU-SGRs.