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.     

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).     

Activated sludge denitrification in marine recirculating aquaculture system effluent using external and internal carbon sources

Stringent environmental legislation in Europe, especially in the Baltic Sea area, limits the discharge of nutrients to natural water bodies, limiting the aquaculture production in the region. Therefore, cost-efficient end-of-pipe treatment technologies to reduce nitrogen (N) discharge are required for the sustainable growth of marine land-based RAS. The following study examined the potential of fed batch reactors (FBR) in treating saline RAS effluents, aiming to define optimal operational conditions and evaluate the activated sludge denitrification capacity using external (acetate, propionate and ethanol) and internal carbon sources (RAS fish organic waste (FOW) and RAS fermented fish organic waste (FFOW)). The results show that between the evaluated operation cycle times (2, 4, and 6 h), the highest nitrate/nitrite removal rate was achieved at an operation cycle time of 2 h (corresponding to a hydraulic retention time of 2.5 h) when acetate was used as a carbon source. The specific denitrification rates were 98.7 ± 3.4 mg NO₃⁻-N/(h g biomass) and 93.2 ± 13.6 mg NO_x⁻-N/(h g biomass), with a resulting volumetric denitrification capacity of 1.20 kg NO₃⁻-N/(m³ reactor d). The usage of external and internal carbon sources at an operation cycle time of 4 h demonstrated that acetate had the highest nitrate removal rate (57.6 ± 6.6 mg N/(h g biomass)), followed by propionate (37.5 ± 6.3 mg NO₃⁻-N/(h g biomass)), ethanol (25.5 ± 6.0 mg NO₃⁻-N/(h g biomass)) and internal carbon sources (7.7 ± 1.6–14.1 ± 2.2 mg NO₃⁻-N/(h g biomass)). No TAN (Total Ammonia Nitrogen) or PO₄^3- accumulation was observed in the effluent when using the external carbon sources, while 0.9 ± 0.5 mg TAN/L and 3.9 ± 1.5 mg PO₄^3--P/L was found in the effluent when using the FOW, and 8.1±0.7 mg TAN/L and 7.3 ± 0.9 mg PO₄^3--P/L when using FFOW. Average sulfide concentrations varied between 0.002 and 0.008 mg S^2-/L when using the acetate, propionate and FOW, while using ethanol resulted in the accumulation of sulfide (0.26 ± 0.17 mg S^2-/L). Altogether, it was demonstrated that FBR has a great potential for end-of-pipe denitrification in marine land-based RAS, with a reliable operation and a reduced reactor volume as compared to the other available technologies. Using acetate, the required reactor volume is less than half of what is needed for other evaluated carbon sources, due to the higher denitrification rate achieved. Additionally, combined use of both internal and external carbon sources would further reduce the operational carbon cost.     

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).     

Start-up performance of a woodchip bioreactor operated end-of-pipe at a commercial fish farm—A case study

There is a need for simple, maintenance-free technologies for removing nitrogen (N) from aquaculture effluents. Denitrifying woodchip bioreactors have been used successfully to remove nitrate-N (NO₃-N) from ground and surface waters and may potentially be applied to dilute aquaculture effluents as well. Real-life applicability in commercial, outdoor fish farms including practical start-up issues such as e.g. time till stable performance and potential leaching are, however, unknown to the industry.This case study consequently investigated the temporal performance of a woodchip bioreactor (12.5 m³) during start-up. The bioreactor was operated end-of-pipe at a commercial, outdoor rainbow trout (Oncorhynchus mykiss) farm in Denmark operated at low recirculation intensity. Applying an empty bed contact time (EBCT) of 5 h, the specific objectives of the study were to resolve: i) how fast the bioreactor would start to remove NO₃-N; ii) how fast steady state was achieved; iii) which NO₃-N removal rates could be attained at the relatively low effluent temperature (∼8 °C) and iv) to which extent any concomitant leaching of phosphorous (P), ammonia or organic matter would occur.In- and outlet grab samples were obtained every 6 h until the bioreactor was in steady state (2 weeks) followed by weekly 24 h pooled samples for another 3 weeks (5 weeks in total). Additional grab samples were obtained from 9 sampling ports within the bioreactor on 3 consecutive days during steady state. Samples were analyzed for dissolved nutrients (total N, nitrate, nitrite, ammonium, total phosphorous, ortho-phosphorous, BOD₅ and COD). In addition, oxygen, temperature and pH were logged every 30 min while sampling and alkalinity were measured once a week.Removal of NO₃-N started immediately and remained stable at 7.06 ± 0.81 g NO₃-N/m³/d (n = 6) throughout the sampling period. Increased effluent NO₂-N concentrations (peaking at 1.14 mg NO₂-N/l after 4–5 days) were transiently observed during the initial 11 days. After that, the woodchip bioreactor was largely in steady state with respect to N-balances corroborated by a close match between filtered total-N (TN_diss) and NO₃-N removal rates. Measurements within the bed showed that the majority of the influent dissolved oxygen (DO) was consumed within the first part of the bioreactor and that NO₃-N removal thereafter proceeded gradually with distance within the bed. Leaching of non-structural, dissolved organic compounds were observed just after startup, causing a short-term (1 week) increase in effluent concentrations of COD, BOD₅, P and ammonium.Additional measurements carried out until 147 days after start-up showed that the woodchip bioreactor continued to remove TN_diss at an average removal rate of 7.81 ± 0.82 g N/m³/d, and that the initial leakage of P stopped altogether.In summary, the study demonstrated that woodchip bioreactors can effectively remove NO₃-N from dilute aquacultural effluents at low temperatures and commercial conditions and that stable performance is achieved within a few weeks.     

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.     

褐煤的碳缓释特征及其对池塘底泥脱氮作用的影响

厌氧氨氧化和反硝化作用是底泥生物脱氮的主要过程,碳源是调控厌氧氨氧化和反硝化作用的关键因子。本研究以褐煤为对象,对褐煤的静态碳释情况及其对池塘底泥中脱氮作用的影响进行了研究。结果显示,褐煤在室温条件下的碳释放规律符合二级动力学方程,具备作为反硝化碳源的可行性;在脱氮实验中,发现褐煤对底泥上覆水体中的亚硝酸盐氮(NNO₂^--N)的去除具有促进作用,NNO₂^--N的去除率随褐煤浓度的增加而升高,当褐煤质量浓度为40 g/L时,N\${\rm{O}}_2^ - $\-N去除率最高达99.61%,此时硝酸盐氮(NO₃^--N)的浓度也最低;同时发现,水体中氨氮(NH₄⁺-N)氧化的最适褐煤质量浓度为10 g/L,其去除率达99.39%;对底泥中的厌氧氨氧化菌群进行Illumina高通量测序发现,其中浮霉菌门占比最大(39.6%~71.8%),优势菌属为Candidatus Brocadia (13.9%~35.8%)和Desulfovibrio (17.1%~34.8%),添加褐煤组Candidatus Scalindua菌属比例高于未添加组;荧光定量PCR得出,随着褐煤质量浓度升高,底泥中的反硝化菌丰度呈增长趋势,而厌氧氨氧化菌丰度则低于无褐煤添加组,表明添加褐煤对底泥反硝化有促进作用,而对厌氧氨氧化有一定的抑制作用。研究表明,褐煤具备作为反硝化碳源的条件,可用于池塘养殖底泥脱氮作用。     

Determination of dosage of Azotobacter and organic fertilizer for optimum nutrient release, net primary productivity and fish growth in fresh water fish ponds

Four experiments were conducted in order to determine the optimum dosageof Azotobacter chroococcum vis-a-vis organic fertilizer(cow-dung) required for optimum pond productivity. Hydrobiological parameters ofpond water, Azotobacter survival (viable counts), netprimary productivity (NPP) and fish growth were monitored. Studies have revealedthat irrespective of the treatments, dissolved oxygen (DO) levels weresignificantly (P < 0.05) lowered on inoculating the ponds withAzotobacter. Alkalinity, O-PO₄,NO₃-N, turbidity, NPP, plankton population and fish growth weresignificantly (P < 0.05) enhanced in ponds inoculated withAzotobacter @ 100.0 ml pond^–1w^–2 in combination with cow-dung @ 10000 kgha^–1 y^–1. At higher or lower dosages offertilizers, the values in most of these parameters remained low. On the otherhand, total kjeldahl nitrogen and NH₄-N increased continuously. Ingeneral, viable bacterial counts decreased with increase in pH, however, therate of nitrogen fixation was not affected. Multivariate analysis of the data revealed a significantpositive correlation of nutrients (Total kjeldahl Nitrogen, NO₃-N andO-PO₄), with NPP and plankton populations. NH₄-N, however,showed a significant negative correlation with DO, NPP and plankton populations.Highest fish biomass and SGR also coincided with the highest NPP and planktonpopulations, revealing that a dose of 100.0 ml pond^–1w^–2 (for 25 m³ ponds) ofAzotobacter along with 10000 kg ha^–1y^–1 of cow-dung appears to be optimum for obtainingoptimum pond productivity and fish yield. Nutrients in the sediment(NO₃-N and O-PO₄) also followed similar trend. On theother hand, organic carbon increased continuously with each increase in thedosage of fertilizers. A decline in fish biomass and pond productivity at higherfertilizer dosages has been attributed to low DO, high NH₄-N and BOD.     

C/N ratio control and substrate addition for periphyton development jointly enhance freshwater prawn Macrobrachium rosenbergii production in ponds

The present research investigated the effect of carbon/nitrogen ratio (C/N ratio) control in ponds with or without substrate addition for periphyton development on production of giant freshwater prawn. C/N ratios of 10, 15 and 20 were investigated in 40 m^− 2 ponds stocked with 2 prawn juveniles (5.023 ± 0.02 g) m^− 2 with or without added substrates for periphyton development. The various treatment combinations of C/N ratio and periphyton substrate addition are abbreviated as ‘CN10’, ‘CN15’, ‘CN20’, ‘CN10 + P’, ‘CN15 + P’ and ‘CN20 + P’, P representing periphyton substrate. A locally formulated and prepared feed containing 30% crude protein with C/N ratio10 was applied. Tapioca starch was used as carbohydrate source for manipulating C/N ratio and applied to the water column separately from the feed. Increasing the C/N ratio from 10 to 20 reduced (P < 0.001) the total ammonia-nitrogen (TAN), nitrite–nitrogen (NO₂–N) and nitrate–nitrogen (NO₃–N) in water column and total Kjeldahl nitrogen (TKN) in sediment. The addition of substrates only influenced the NO₂–N concentration in the water column (P < 0.001). Increasing the C/N ratio raised the total heterotrophic bacterial (THB) population in the water column, sediment and periphyton (P < 0.001). It also increased the dry matter (DM), ash free dry matter (AFDM), and chlorophyll a content of periphyton (P < 0.001). The lowest specific growth rate (SGR), the highest food conversion ratio (FCR), and the lowest protein efficiency ratio (PER) were recorded in treatment CN10 (P < 0.05). The addition of substrates did not influence size at harvest (P > 0.05) but improved the survival from 62.8 to 72% (P < 0.001). Increasing the C/N ratio from 10 to 20 increased the net yield by 40% and addition of substrate increased the net yield by 23%. The combination of C/N ratio control and substrate addition increased the net yield by 75% from 309 (CN10) to 540 (CN20 + P) kg ha^− 1 (120 days)^− 1. This 75% higher production concurred with (1) a lower inorganic nitrogen content in the water column, (2) a higher THB abundance supplying additional single cell protein to augment the prawn production, and (3) an improved periphyton productivity and quality.     

Denitrifying granules in a marine Upflow Anoxic Sludge Bed (UASB) reactor

Marine land-based Recirculating Aquaculture Systems (RAS) are generally perceived as environmentally friendly aquatic production systems. To promote their sustainability even further and reduce the discharge of nutrients, there is a need for cost-effective end-of-pipe treatment technologies for removing nutrients. This includes nitrate-nitrogen (NO₃⁻-N) for which well-proven technologies for freshwater systems exists, while similar technologies for saltwater systems are less advanced. Granular technology has been developed since the 1970s in wastewater treatment under the upflow anaerobic sludge bed (UASB) concept. This concept is based on the enrichment of different bacterial aggregations into a compact granule, optimizing synergistic and syntrophic bacterial processes by reducing the diffusion distance of substrates between the different bacterial consortia forming the granule. The following study examined the: 1) granular formation; and 2) nitrate removal capacity of a marine Upflow Anoxic Sludge Bed (UASB) reactor operating at different up-flow velocities (0.40–2.11 m/h). The results showed that marine denitrifying granules developed within 27 days using preconditioned rainbow trout (Oncorhynchus mykiss) organic matter waste, and that the highest specific denitrification rate (321.9 ± 13.1 mg NO₃⁻-N/g Total Volatile Suspended Solids (TVSS)/d) was found at an upflow velocity of 0.97 m/h. The marine UASB denitrifying granule reactor had a total capacity of removing 14.9 kg NO₃⁻-N/m³ reactor volume per day at a hydraulic retention time of 1.9 h, making it a strong candidate for end-of-pipe denitrification of marine RAS effluent as well as for in-line treatment in marine systems.     

Dietary protein level and C/N ratio manipulation in zero‐exchange culture of Litopenaeus vannamei: Evaluation of inorganic nitrogen control,biofloc composition and shrimp performance

A 30‐day experiment was conducted to evaluate inorganic nitrogen control, biofloc composition and shrimp performance in zero‐exchange culture tanks for juvenile L. vannamei offered a 35% (P35) or 25% (P25) crude protein feed, each feed supplemented with additional carbohydrate to increase the C/N ratio to 20:1 (CN20) or 15:1 (CN15). Sucrose was used as a carbohydrate to manipulate the two C/N ratios based on the carbon and nitrogen content of both the feeds and sucrose. The four treatments were referred to as: P35 + CN20, P35 + CN15, P25 + CN20 and P25 + CN15. Each treatment consisted of four replicate tanks (125 L), each stocked with 28 shrimp (equivalent to 224 shrimp m^?3). Bioflocs formed and developed based on initial inoculation in all four treatments; and monitored water quality parameters were maintained within acceptable ranges for shrimp culture throughout the experiment. No significant effects (P > 0.05) of dietary protein level, C/N ratio or their interaction were observed on biofloc development (BFV, TSS and BFVI) and inorganic nitrogen (TAN, NO₂^?‐N and NO₃^?‐N) concentrations. At the end of the experiment, proximate analysis of the bioflocs collected from the four treatments showed crude protein levels of 21.3% ~ 32.1%, crude lipid levels of 1.6% ~ 2.8% and ash levels of 43.4% ~ 61.4%. Extracellular protease and amylase activities of the bioflocs were 9.9 ~ 14.4 U g^?1 TSS and 293.5 ~ 403.8 U g^?1 TSS respectively. Biofloc composition and enzyme activity were both affected by dietary protein level (P < 0.01) and C/N ratio (P < 0.05). Survival, per cent weight gain and protein efficiency ratio of shrimp were not affected (P > 0.05) by dietary protein level, C/N ratio or their interaction; however, the feed conversion ratios were significantly lower (P < 0.05) in treatments with high dietary protein (P35) compared with those in treatments with low dietary protein (P25). The results from this study demonstrate that dietary protein level and C/N ratio manipulation can have important implications for water quality, biofloc composition and shrimp performance in intensive, zero‐exchange biofloc‐based culture systems.     

Effect of Dietary Protein Concentration on Nitrogenous Waste in Intensively Fed Catfish Ponds

Channel catfish were fed five diets containing 24, 28, 32, 36 or 40% protein in intensively stocked earthen ponds over a 141 d growing season. Mean standing crop at harvest was 7,559 kg/ha, and maximum daily feed allowance was 105 kg/ha. Dietary protein concentration had a negative linear effect on weight gain. Total ammonia-nitrogen (TAN) in pond water increased linearly as dietary protein concentration increased and was positively correlated with total protein fed. However, unionized ammonia-nitrogen (NH₃-N) was not influenced by dietary protein concentration. Dietary protein had a positive linear effect on nitrite-nitrogen (NO₁^-_- -N) concentration, which was positively correlated with total protein fed and TAN. There was no significant correlation between NO₂^-_--N and fish weight gain, although there was a significant positive correlation between NO₂^-_- -N/Cl-molar ratio in pond water and concentration of methemoglobin in the fish. Results from this study indicate that when the feeding rate is as high as 100 kg/ha/d, or 3,000 kg protein/ha/season, dietary protein concentrations of 36% and above can result in harmful concentrations of NO₂^-_--N when Cl concentration in the ponds is 2–3 mg/L. Although the NO₂^-_--N/Cl- ratio in the ponds increased to harmful levels with protein concentration of the diets, this might not be the major cause of the reduction in fish growth rate as dietary protein increased because the greatest difference in weight gain occurred at the lower protein concentrations and the greatest difference in NO₂^-_--N occurred at the higher dietary protein concentrations.     

The “self cleaning inherent gas denitrification-reactor” for nitrate elimination in RAS for pike perch (Sander lucioperca) production

Denitrification reactors have proven their functionality in commercial recirculation aquaculture systems (RAS). Nevertheless, clogging occurs due to the low hydraulic loads necessary to accomplish anoxic conditions for a successful denitrification process in RAS, which hampers the adjustment of stable working conditions within fixed bed denitrification reactors. Reactors working on the basis of activated sludge demand careful hydraulic control and/or complex configurations for sludge retention.To develop a low-maintenance denitrification reactor, an enclosed moving bed filter, driven by recirculation of the inherent, oxygen poor gas was designed. A Self cleaning Inherent gas Denitrification reactor (SID-reactor) of 0.65 m³, which offered a moving bed volume of 0.39 m³ was connected with a RAS of semi-industrial scale for pike perch (Sander lucioperca) production. This species indicates suboptimal environmental conditions (as e.g. NO₃-N concentrations above approximately 68 mg l⁻¹) by prompt reduction of the feed intake. In different experimental series, the SID-reactor was operated with denatured ethanol, methanol, acetic acid or glycerin as carbon sources and changing operational modes.Clogging was prevented by a 40 second inherent gas recirculation twice an hour, which provided continuous, maintenance free operation with marginal energy demand. With inlet (RAS) and outlet NO₃-N concentrations in the range of 49 mg l⁻¹ and 12 mg l⁻¹, mean denitrification rates of 199 g to 235 g NO₃-N per m³ moving bed volume and day were determined for all tested carbon sources. Negative effects on the feed intake of the reared pike perch were detected with all carbon sources except methanol. Changing the mode of operation to continuous circulation of the filter bed at inlet NO₃-N concentrations of 26 mg l⁻¹, the denitrification performance reached 451 g NO₃-N per m³ moving bed volume and day. The SID-reactor allowed for the reduction of freshwater exchange in the pike perch RAS from 600 l to 70 l (−88%) and the sodium bicarbonate buffer from 182 g to 31 g (−83%) per kg of administered food. The easy and reliable operation of the SID-reactor could help to establish controlled denitrification as a routine purification step in RAS.     

Effects of C:N ratio and substrate integration on periphyton biomass, microbial dynamics and growth of Penaeus monodon juveniles

An outdoor growth trial was conducted for 75 days to investigate the effect of C:N ratio and bamboo substrate (S) in brackish water shrimp culture. Penaeus monodon juveniles (0.35 ± 0.03 g) were stocked in experimental tanks with and without bamboo substrate. C:N ratio of 10 and 20 was manipulated with shrimp feed containing 32 % crude protein and rice flour as carbohydrate source. Addition of substrate and higher C:N ratio (CN20+S) significantly reduced (p < 0.01) inorganic nitrogen NH₃–N by 48.2 %, NO₃–N by 41.6 %, NO₂–N by 42.7 % compared with CN10. Substrate addition significantly (p < 0.05) reduced the turbidity by 22.4 % (CN10+S against CN10) and 20.7 % (CN20+S against CN20). Periphyton biomass and total heterotrophic bacterial load were significantly higher (p < 0.05) in CN20+S in comparison with CN10+S treatment. The autotrophic index increased from 123.4 ± 0.62 to 158.9 ± 3.8 for CN10+S and, 121.8 ± 1.6 to 228 ± 9.11 for CN20+S system. Provision of substrate and carbohydrate addition resulted in the highest body weight, 4.87 ± 0.12 g in CN20+S against 3.66 ± 0.07 g in CN20 and 2.90 ± 0.12 g in CN10 through provision of natural food in the form of periphytic algae and heterotrophic bacterial community. Treatment with substrates showed a higher survival rate by 7 % in comparison with without substrate treatments as it acted as shelter to shrimp during molting and there by reduced cannibalism. Thus, our results demonstrated that high C:N ratio and substrate addition improved growth, reduced FCR and better water quality conditions.     

Effect of chloride concentration on the acute toxicity of nitrite to common carp, Cyprinus carpio L., fry

Abstract. Duplicate static bioassays were conducted for 168 h each to determine the median lelhal concentration (LC50) of nitrite (NO^?₂) for common carp, Cyprinus carpio L., fry at five different chloride (Cl^?) levels. The acute toxicity of nilrite ceased towards the end of 96 h at all levels of chloride concentration. There was a highly significant positive correlation between the chloride concentration tested and the 96-h LC50. The 96-h LC50 values are 2·55, 5·77, 14·41 27·26 and 48·70 mgl^?1 NO^?₂-N at chloride concentration of 1·0. 5·0, 10·5, 27·3 and 45·0 mgl^?1 Cl^? respectively. The linear relationship between chloride concentration and 96-h LC50 is best described by the equation: y= 1·03x+ 1·49 (r=+ 0·996; d.f. = 3;P <0·001), where y= 96 h LC50 of NO^?₂-N and x= concentration of Cl^?. A NO^?₂-N to Cl^? ratio of about 1:1·5–3·0 prevented complete mortality over the 168-h experimental period. A NO^?₂-N to Cl^? ratio of 1:5 is recommended for protection of carp fry against nilrite mortality in fish farms.     

氨、亚硝酸盐和硝酸盐对凡纳滨对虾幼虾的毒性作用

研究了 NH₃-N、NO₂ ^- -N与 NO₃ ^- -N对凡纳滨对虾幼虾的毒性作用。获得了 NH₃-N_t(NH₃-N_m) 与 NO₂ ^- -N对体长2．4cm幼虾的 24h、48h、72h、96h之 LC₅₀值，两者对幼虾的安全质量分数分别为1．30 (0．101)mg/L和3．80mg/L。当 NH₃-N_t(NH₃-N_m)质量分数在1．3(0．101)~4．3(0．333)mg/L时，存活率为71．4% ~92．9%，体长增长率为36．3% ~57．1%，体重增长率为188．5% ~322．3%。当 NO₂ ^- -N质量分数在3．00~21．00mg/L时，成活率为75．0% ~91．7%，体长增长率为21．2% ~59．2%，体重增长率为72．0% ~311．9%。NO₃ ^- -N对体长7．37cm幼虾的亚急性毒性效应：NO₃ ^- -N的质量分数在 30~195mg/L时，成活率为 35% ~100%，体长增长率为8．5% ~20．5%，体重增长率为29．6% ~56．8% 。三态氮在一定质量分数范围内均对幼虾的存活率和生长率产生影响。     

海水观赏鱼居室养殖循环系统中三态氮的变化规律及硝化细菌对水质的影响

于 2005年 3月 5日 ~5月 3日连续监测了海水观赏鱼居室养殖循环系统的水质，研究该系统中三态氮的变化规律以及添加硝化细菌后对水质的影响。结果表明：1)试验初期氨氮的质量分数迅速上升， 在 1周内达到高峰(峰值2．56mg/L)，并在1．50mg/L的范围内维持 1周左右，此后迅速下降至0．01mg/L 左右，并一直维持在该水平直至试验结束。亚硝态氮的质量分数在氨氮的质量分数迅速回落时(约试验开始后 2周)呈现出直线上升的趋势，并在 3~3．5mg/L左右的水平上维持 2~3周时间(峰值为3．65 mg/L)，此后迅速下降至0．01mg/L以下，并一直维持在低水平直至试验结束。而硝酸盐的质量分数在整个试验期间基本保持稳定上升的趋势，至本试验末期，NO₃ ^- -N的质量分数达到 15mg/L左右。2)系统的生物滤器需要 4~5周左右时间才能基本成熟，即氨氮和亚硝酸氮均降至 ＜0．01mg/L，到达安全的质量分数，适合海水观赏鱼健康生长。3)添加硝化细菌的试验组，氨氮和 NO₃ ^- -N的质量分数变化与对照组基本相似，而 NO₂ ^- -N的质量分数变化与对照组明显不同。试验组从高质量分数水平迅速下降的时间比对照组提前了约 1周。     

草鱼养殖水体中参与氮转化途径的异养菌分析

为分析草鱼池塘中参与氮代谢的异养细菌比例及其代谢途径,从杭州郊区取得4个草鱼池塘的水样,每个水样通过涂布随即挑选100株菌株进行定性显色试验,并据此选取11株异养菌进行16S rRNA序列分析。结果表明,4个草鱼养殖池塘中NH4+-N和NO2--N的平均水平分别为5.597 mg/L和0.135 mg/L。池塘中可培养的异养菌平均为3.26×105cfu/mL,其中的89.75%参与了氮的不同代谢途径,其中31.25%的氨化菌和33.50%NO3--N(NO2--N)还原菌参与了NH4+-N的生成,32.45%的氨氧化菌参与了NH4+-N的降低;NO2--N生成途径主要包括蛋白质直接转化(11.26%)、氨氧化(4.25%)和硝酸盐氮还原(10.75%),而NO2--N降低主要通过15.50%的亚硝酸氧化菌、8.75%的NO2--N还原菌和10.75%的反硝化菌实现。结果提示,草鱼养殖水体中存在大量的异养硝化菌参与不同的氮代谢途径,且产生氨氮的异养菌比例远高于去除氨氮的菌,这是草鱼养殖水体中氨氮含量易偏高的原因。同时,11株不同功能的异养菌16SrRNA鉴定结果为寡养食单胞菌(Stenotrophomonas)6株、假单胞菌(Pseudomonas)3株、克雷伯氏菌(Klebsiella)和肠杆菌(Enterobacter)各1株,而且细菌对氮源的利用具有菌株特异性。     

Evaluation of the impact of nitrate-nitrogen levels in recirculating aquaculture systems on concentrations of the off-flavor compounds geosmin and 2-methylisoborneol in water and rainbow trout (Oncorhynchus mykiss)

Aquatic animals raised in recirculating aquaculture systems (RAS) can develop preharvest “off-flavors” such as “earthy” or “musty” which are caused by the bioaccumulation of the odorous compounds geosmin or 2-methylisoborneol (MIB), respectively, in their flesh. Tainted aquatic products cause large economic losses to producers due to the inability to market them. Certain species of actinomycetes, a group of filamentous bacteria, have been attributed as the main sources of geosmin and MIB in RAS. Previous studies have demonstrated that certain nutritional factors can stimulate or inhibit bacterial biomass and geosmin production by certain actinomycetes. In the current study, the effects of two nitrate-nitrogen (NO₃^--N) levels (20–40 mg/L and 80–100 mg/L) on geosmin and MIB levels in culture water and the flesh of rainbow trout (Oncorhynchus mykiss) raised in RAS were monitored. Water and fish tissue samples were collected over an approximately nine-week period from six RAS, three replicates each of low and high NO₃^--N, and analyzed for geosmin concentrations using solid phase microextraction–gas chromatography–mass spectrometry. Results indicated no significant difference in geosmin concentrations in water or fish flesh between the low and high NO₃^--N RAS. Therefore, higher NO₃^--N levels that may occur in RAS will not adversely or beneficially impact geosmin-related off-flavor problems.     

凡纳滨对虾养殖塘叶绿素a与水质因子的多元回归分析

2009年4-9月期间,对上海市奉贤区某凡纳滨对虾养殖场22个养殖池塘水体叶绿素a、水温、pH、溶解氧、透明度、悬浮物(SS)、总有机碳(TOC)、五日生化需氧量(BOD₅)、高锰酸盐指数(COD_Mn)等15项水质因子进行测定。取164组测定数据,进行描述性统计,分析叶绿素a与各项因子的相关性系数。分析结果显示,与叶绿素a呈极显著线性正相关的水质因子为SS、TOC、BOD₅、COD_Mn、TN、TP;呈显著正相关的为DO;而叶绿素a与透明度呈极显著线性负相关,与PO^3-₄-P呈显著线性负相关;与水温、pH、NO^-₂-N、 NO^-₃-N、NH₃-N则未呈现显著相关性。根据多元线性回归选择自变量的原则,选择了TOC、TN、PO^3-₄-P和TP₄项水质因子,建立了叶绿素a与4项水质因子的逐步回归模型:Chl.a =-0.054 5+0.0034 9 TOC+0.015 3 TN-0.418 PO^3-₄-P+0.276 TP(r=0.715 5)。利用偏回归系数检验各水质因子对叶绿素a的影响,结果表明,对叶绿素a影响从大到小依次是TP、TOC、PO^3-₄-P和TN。研究结果对进一步探讨养殖池塘生态系统的变化规律及水环境质量保护提供了依据。