Aquacultural Engineering 47 (2012) 16–26 Contents lists available at SciVerse ScienceDirect Aquacultural Engineering journal homepage: www.elsevier.com/locate/aqua-online Hydrodynamic characterization and performance evaluation of an aerobic three phase airlift fluidized bed reactor in a recirculation aquaculture system for Nile Tilapia production Iván Andrés Sánchez O. a,∗ , Tsunao Matsumoto b,1 a b Departamento de Recursos Hidrobiológicos, Universidad de Nariño, Ciudad Universitaria Torobajo, Carrera 22, No. 18-109 Pasto, Nariño, Colombia Faculdade de Engenharia de Ilha Solteira, FEIS/UNESP, Departamento de Engenharia Civil, Alameda Bahia 550, Ilha Solteira, SP CEP 15385-000, Brazil a r t i c l e i n f o Article history: Received 9 August 2011 Accepted 6 December 2011 Keywords: Three phase fluidized bed Biofilters Biological treatment Hydrodynamic characterization Recirculation Aerobic reactor a b s t r a c t The hydrodynamic characterization and the performance evaluation of an aerobic three phase fluidized bed reactor in wastewater fish culture treatment are presented in this report. The objective of this study was to evaluate the organic matter, nitrogen and phosphorous removal efficiency in a physical and biological wastewater treatment system of an intensive Nile Tilapia laboratory production with recirculation. The treatment system comprised of a conventional sedimentation basin operated at a hydraulic detention time HDT of 2.94 h and an aerobic three phase airlift fluidized bed reactor AAFBR operated at an 11.9 min HDT. Granular activated carbon was used as support media with density of 1.64 g/cm3 and effective size of 0.34 mm in an 80 g/L constant concentration. Mean removal efficiencies of BOD, COD, phosphorous, total ammonia nitrogen and total nitrogen were 47%, 77%, 38%, 27% and 24%, respectively. The evaluated system proved an effective alternative for water reuse in the recirculation system capable of maintaining water quality characteristics within the recommended values for fish farming and met the Brazilian standards for final effluent discharges with exception of phosphorous values. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Aquaculture has been growing more than other sectors of animal food production, with an average value of 8.8% per year since 1970. According to data provided by FAO (2006), the species with the highest growing production tendency, between 1991 and 2000, in Latin America and the Caribbean was Tilapia. Wheaton (1993) states that the three basic kinds of aquaculture systems are open, semi-closed, and closed systems. In closed systems water is reconditioned and recirculated to culture units. A recirculating aquaculture system (RAS) can be defined as an aquaculture closed system that incorporates the treatment and reuses water with less than 10% of the total water volume replaced per day (Hutchinson et al., 2004); it represents a compact alternative for intensive farming of different species, maximizes production requiring a smaller proportion of water and land to generate the same quantity of fishes than the required by other kinds of systems (Timmons et al., 2002). ∗ Corresponding author. Tel.: +57 2 7311449x239; fax: +57 2 7314482. E-mail addresses: ivansaor@hotmail.com, iaso@udenar.edu.co (I.A. Sánchez O.), tsunao@dec.feis.unesp.br (T. Matsumoto). 1 Tel.: +55 018 3743 1125; fax: +55 018 3743 1125. 0144-8609/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaeng.2011.12.006 Water quality is important in fishculture in order to optimize water conditions for the cultured species and to minimize environmental effects on the receiving water bodies produced by wastewaters. On RAS for tilapia culture, recommended values of some water quality parameters are: temperature: 24–29 ◦ C; oxygen: 4–6 mg/L; total suspended solids: <15 mg/L; total ammonia nitrogen (TAN): <3 mg/L; unionized ammonia NH3 –N: <0.6 mg/L (Timmons et al., 2002). The Nile tilapia (Oreochromis niloticus) was one of the first fish species cultured. Illustrations from Egyptian tombs suggest that Nile tilapia were cultured more than 3000 years ago (Popma and Masser, 1999). Tilapia is one of the most widely farmed freshwater fish in the world; on RAS with aeration it is common to produce 15–40 kg/m3 of fish (Kubitza, 2000). According to Naylor et al. (2000), to produce 1 kg of fish meat, 2 kg of dry food (adopting a food conversion ratio of 2 for Tilapia) are necessary; however, the values of feed conversion ratio FCR for fishes depend upon the species and variables as the diet formulation and its composition. For example, Mbahinzireki et al. (2001) evaluated the growth, feed utilization and body composition of Tilapia fed with cottonseed meal-based diets in a RAS and reported FCR values from 1.46 to 2.28; Azaza et al. (2009) evaluated the use of waste date fruit as partial substitute for soybean meal in practical diets of juvenile tilapia and registered FCR values from 1.62 to 1.80. I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 Nomenclature Cc D10 D60 HDT KL a KL a20 UC S carrier concentration on liquid (g/L) effective size, size of an opening which will pass only the smallest 10% of the granular media (mm) sieve size which will pass 60% of the granular media (mm) hydraulic detention time (h, min) volumetric oxygen mass transfer coefficient (1/s) volumetric oxygen mass transfer coefficient standardized to 20 ◦ C (1/s) uniformity coefficient density of a particle of media (g/cm3 ) List of abbreviations AAFBR aerobic three phase airlift fluidized bed reactor Biochemical oxygen demand (mg/L) BOD COD Chemical oxygen demand (mg/L) recirculating aquaculture systems RAS TAN total ammonia nitrogen (mg/L) dissolved oxygen (mg/L) DO UNESP Universidade Estadual Paulista GAC granular activate carbon total solids (mg/L) TS TVS total volatile solids (mg/L) TFS total fixed solids (mg/L) TSS total suspended solids (mg/L) FSS fixed suspended solids (mg/L) volatile suspended solids (mg/L) VSS TN total nitrogen (mg/L) total phosphorous (mg/L) TP ORP oxidation–reduction potential (mV) concentrations measured at influent of sedimentaI.Sed tion tank E.Sed concentrations measured at effluent of sedimentation tank E.EBR concentrations measured at AAFBR effluent platinum cobalt units PCU CFB cumulative feed burden, CFB = F/VFW (g feed/L of freshwater/day) daily mass of feed delivered (g) F VFW equivalent volume of freshwater replaced per day (m3 ) MBBR moving bed bioreactor fluidized bed reactors FBR From the food provided in a fish culture system about 36% is excreted in the form of BOD, 75% of nitrogen as ammonia (Brune et al., 2003) and almost 84.02% of P is released to the representative culture system as metabolite wastes as reported by Rafieea and Saad (2005). High levels of nutrients and organic matter cause environmental degradation of receiving water bodies, the level of the impact depends on the quantity or concentration evacuated and the assimilation capacity of the aquatic environment. The environmental legislation is one of the options to establish sustainable limits to the pollutant activities of the aquatic resources. In Brazil, the conditions and limits of effluent disposal are defined by the 357 CONAMA Resolution of 2005, and in the São Paulo by the state Decree number 8468 of 1976. Besides the environmental restrictions, other reasons as biosecurity in the collecting of natural waters; scarcity and high costs of the liquid; quality control and transparency demands, hard to achieve in semi intensive systems; and thermal control 17 (Avnimelech, 2006) justify the aquaculture effluents treatment, especially for intensive aquaculture recirculating systems. The RAS with biofilters treat the polluted waters internally with dissolved organic matter and ammonia and can reduce the water used quantity and throwing-away by aquaculture (Gutierrez-Wing and Malone, 2006). According to Crab et al. (2007), there is a wide variety of systems to execute the biological treatment of the aquaculture waters that could be grouped in two general types: the emerged fixed biofilm (rotating biological contactors and down flow filters) and the fixed submerged biofilm (as the fluidized sand biofilters and floating bead filters). The RAS submerged fixed film biofilters that display similar characteristics are clustered in three basic blocks: packed; expandable; and expanded bed biofilters as downflow microbead, moving bed bioreactor and fluidized sand biofilters (Malone and Pfeiffer, 2006). A microbead filter is a combination of trickling and granular type of biological filters. These filters are operated in a downflow configuration where influent water is distributed over the top of the media bed and the water then trickles down through the media and flows by gravity out of the reactor vessel (Timmons and Ebeling, 2010). Experiences with these biofilters were reported by Greiner and Timmons (1998) and Timmons et al. (2006). The moving bed bioreactor (MBBR) is an attached growth biological treatment process based on a continuously operating, non-clogging biofilm reactor with low head loss, a high specific biofilm surface area, and no requirement for backwashing. The bacterial biomass grows on the media carriers and moves freely in the water volume of the reactor (Timmons and Ebeling, 2010). The idea behind the development of the Kaldnes MBBR process was to adopt the best features of the activated sludge process as well as those of the biofilter processes, without including the worst (Rusten et al., 2006). In recent literature, Pfeiffer and Wills (2011) described the evaluation of different types of floating plastic media for total ammonia removal in a RAS. Fluidized bed reactors (FBR) are a kind of expanded bed biofilters in which the media is contained in a vertical vessel with a cylindrical or square cross-section (Lawson, 1995). The fluidized sand biofilters are FBR that use sand as a carrier and keep it in suspension or fluidized by the upward flow of water. Some of the main advantages of the FBR described by Timmons and Ebeling (2010) include the facts that fluidized bed biofilters are very economical to build from commercially available materials, raw filter media has very high specific surface area at low cost, and that they can be field built using a variety of proven methods. Some of the main disadvantages indicate that fluidized bed biofilters can have problems with media carryover on system start-up or with restarting if not designed to account for bed re-fluidization and manifold/lateral flushing distribution; media density changes over time with biofilm accumulation in fine sand filters which necessitates a bed growth management strategy. Nam et al. (2000) hold that in fluidized bed reactors the supporting media size, flow velocity and reactor hydrodynamic properties influence the biofilm development characteristics and state that the lesser the size of the grains the better the biofilm structure. In fish culture it is fundamental to maintain high dissolved oxygen (DO) levels to guarantee an optimal species growing. Fall of the DO produced by the animal respiration, by the organic matter degradation process and the transformation or oxidation of total ammonia nitrogen (TAN) is present in the RAS; Nicolella et al. (2000) asserted that to overcome the high recirculation rate problems when oxygen demand is high, this could be directly aerated becoming a three phase fluidized bed reactor by air injection. A brief description of the three phase fluidized bed reactors for use in aquaculture was presented by Lekang (2007). 18 I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 In aerobic biofilters, such as the fluidized bed reactors, oxygen is consumed as penetrated in the biofilm, until values that determine anoxic or anaerobic conditions are reached. Thus two biofilm layers – one aerobic that could potentially develop nitrification, and other anaerobic or anoxic that could develop denitrification processes – could be developed depending on the OD concentration in the external fluid and the biofilm thickness. In fluidized bed reactors with circulation produced by airlift, the three phases (liquid, solid and gas) circulation can be external or internal and is produced by hydrostatic pressure difference between two sections: riser and downcomer (Merchuk and Berzin, 1995). Owing to the fact that in airlift fluidized bed reactors part of the oxygen dissolved is necessary to the organic matter oxidation, the oxygen amount transferred during air injection is an important parameter that can be quantified by the measurement of oxygen transfer, whose methodology is defined by ASCE (1990) that reported the volumetric oxygen mass transfer coefficient (KL a). In the airlift reactors, the liquid velocity circulation is an important parameter, because it determines the liquid mixing, bubble recirculation and solids suspension. Van Benthum et al. (1999) described three bubble circulation regimes: in regime I, at very low input rates, no air bubbles are present in the downcomer, since the liquid velocity in the downcomer is lower than the average slip velocity of the air bubbles in the liquid; in regime II, the liquid velocity in the downcomer is equal to the slip velocity of the air bubbles, the downcomer can either be partly or fully filled with bubbles; finally, in regime III, the liquid velocity in the downcomer becomes higher than the slip velocity, the air bubbles recirculate with the liquid from the downcomer into the riser. When airlift reactors work at low velocities, the carrier could settle or undergo low shearing tension, increasing the biofilm thickness, causing anaerobic or anoxic layer conditions; when working at high velocities, the biofilm thickness could be reduced. In both cases the organic matter removal and nitrification processes are affected (Martins, 2005). The main objective of this research was to evaluate the organic matter, nitrogen and phosphorous efficiency removal in the wastewater treatment of a RAS by intensive Nile Tilapia production system. The treatment system was made up by a conventional sedimentation tank and an aerobic three phase airlift fluidized bed reactor with circulation. 2. Materials and methods The experiment was done in the Hydrology—Hydrometry and Sanitation Laboratories of the Engineering Faculty of the Universidade Estadual Paulista (UNESP), Ilha Solteira campus. The hydrodynamic research was done in a fluidized by airlift bed reactor with circulation in concentric tubes. The external diameter was of 0.25 m and the internal diameter was 0.10 m. The RAS evaluated during 56 days consists in three intensive Tilapia farming tanks with a biomass density higher than 30 kg/m3 whose effluents were treated by a conventional settling tank; an aerobic three phase aerobic fluidized by airlift bed reactor with circulation in concentric tubes, the reactor got a sedimentation unit to retain particles of the granular activated carbon used as biofilter media, and an outlet at the upper side of this unit; a CO2 removal and DO transfer reactor. The final effluent of the treatment system was recirculated by a centrifugal pump to the culture tanks. The RAS configuration, volumes and hydraulic retention times of the system elements is presented in Fig. 1. 2.1. Materials The experimental RAS system evaluated in the research project was composed of three plastic tanks containing 0.20 m3 of freshwater for Tilapia fish farming, with a bottom central drainage, interconnected by a semi-circular gutter made in a 100 mm diameter PVC tube that collected the tanks effluents and helped to maintain the water level on the tanks constant; a sedimentation Fig. 1. Profile view of the intensive Tilapia production recirculating system evaluated. I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 tank constructed in sheets of steel 2.0 m long by 1.0 m wide in the surface and 1.4 m3 of effective volume; an aerobic three phase airlift fluidized bed reactor in PVC concentric tubes with external diameter of 0.25 m and 2.60 m height; 0.10 m of internal diameter and 2.17 m of internal height. The reactor got a sedimentation unit and an outlet at the upper side of this unit, as showed in Fig. 1; a reactor to OD transfer and CO2 removal to the AAFBR effluent, made of a 0.40 m diameter and 1.60 m high PVC tube. Inside the reactor, in order to increase the residence time of the injected bubbles at the bottom of the tube, a 0.30 m layer filled with 2 cm PVC conduit flexible pieces of 3/8 in. diameter was arranged; a 7.5 HP compressor with a pressure regulation valve; two flow meters, one to control for the air injected in the central bottom part of the AAFBR (600 L/h) by an injector made in a PVC 40 mm diameter tube that had six lines of 36 holes of 1.0 mm diameter; and in the OD transfer and CO2 removal reactor (2000 L/h) by a micro bubbles injection membrane located at the base of that reactor; a 0.25 m3 suction tank to pump the treated effluent; two centrifugal pumps of 1/2 HP and 3/4 HP used to pump the treatment system effluent to a 0.050 m3 constant level distribution box which divided the treated flow to the three Tilapia farming tanks; A 25 mm diameter PVC tabulation system used to water distribution to the fish farming tanks and the transportation of the effluents between treatment units; a 1/2 HP blower with 25 mm diameter outflow pipe used to inject air in the fish farming tanks by diffusion stones, and dechlorination tanks of 1.0 and 0.5 m3 ; 66 Oreochromis niloticus units divided in the three fish farming tanks maintaining approximately 6 kg of biomass in each culture unit (farming density of 30 kg/m3 ). For the hydrodynamic experiments four probes to electric conductivity measurement, two located at internal and two located at external tube; and piezometers placed at the internal and external tubes were also used. 2.2. Methods In the hydrodynamic experiments the different amounts of injected air were regulated by two flow meters. The liquid velocity was measured in the reactor to 800; 1200; 1600; 2000; 3000; 4000; 5000; and 6000 L/h of air at a constant pressure of 2.5 MPa. The experiments in each reactor were done four times, and NaCl at 15 g/L concentration was used as tracer. The probes detected the tracer presence and transferred the conditioned signals, and then acquisition data software read the signals and traced curves in which peaks showed the highest detected tracer concentration moment. With the minimum tension peaks and the distance between probes the liquid circulation velocity was calculated. The expansion and gas holdup of the two-phase (liquid and air) condition values were registered at the reactor for different air flow amounts that varied between 300 and 7000 L/h. For the three-phase condition the gas holdup was measured at three different carrier concentrations 30; 70 and 100 g/L at an air flow of 2500 L/h. The expansion was expressed by the volume difference between aerated and without aeration condition, determined by piezometers. The three-phase gas holdup depended of the mentioned volume differences, the carrier concentration, and the liquid and carrier densities. The KL a determination was done for two-phase and three-phase conditions (carrier concentration of 100 g/L), at 1000, 1200, 1500, 1800 and 2100 L/h of air flow. The experimental methodology was the one described by ASCE (1990). During the RAS evaluation period, 600 L/h of air were used in order to guarantee the right quantity to transfer the DO and suspension and recirculation of granular activate carbon (GAC). The 19 physic characterization of GAC and two additional possible carriers (anthracite carbon and filter sand) were done based on the ABNT (1984a,b) methods for granulometric analysis and the specific mass of solids, by the NBR 7181 and NBR 6508 methods, respectively. At the inclusion of the GAC on the AAFBR an air flow of 2000 L/h was used to maintain the carrier in suspension. The carbon was added progressively at the top of the reactor by addition of mass quantities equivalent to 10 g/L concentration of GAC until the concentration of evaluation of the system 80 g/L was reached, after that, the air flow was reduced to 600 L/h. In the first week of inoculation of the AAFBR and filling up of the RAS system identical amounts of non-chlorine water and water from the Ipê Lake, localized in the municipality of Ilha Solteira SP were used. After GAC introduction into the reactor, during 1 week and a half three partial replacements of water to promote the removal of a black color produced by the hydration of the carrier and the migration of a little amount of GAC grains that passed through the bomb were performed. Finally, each day during 1 1/2 weeks, 200 L of siphoned water from ornamental fish farming tanks were added to the RAS. The RAS monitoring was done daily during 8 weeks. The samples collection and the in situ parameters measuring in situ were done at P1, P2, P3, P4 and P5 points arranged on the RAS as showed in Fig. 1. The measured parameters carried out twice a week were: total solids (TS), total volatile solids (TVS), total fixed solids (TFS), total suspended solids (TSS), fixed suspended solids (FSS), volatile suspended solids (VSS), by gravimetric method; turbidity with 2100NA Hach Turbidimeter; apparent color, COD, total nitrogen (TN) and total phosphorous (TP), nitrite and nitrate – only in the first four weeks of the experiment – in a Hach DR2500 spectrophotometer. The daily controlled parameters by an YSI Inc. 6920 probe were: dissolved oxygen (DO), pH, conductivity, temperature, oxidation reduction potential (ORP), ammonia NH3 , ammonium NH4 + ; water flow at the tanks was measured by volumetric method; there were also controlled and regulated water levels and air pressure. The carrier concentration was controlled by volumetric method weekly. Physical chemical parameter measurements followed the recommendations of APHA, AWWA and WEF (1998), in Standard Methods for Examination of Water and Wastewater and were done at Sanitation and Hydrology and Hydrometrics Laboratories of the Engineering Faculty on the UNESP Ilha Solteira’s campus. Once a week the sedimentation tank was totally drained and cleaned to remove the accumulated floating and settled solids; this activity represented a weekly replacement of 55% of total water volume of the RAS, equivalent to 7.8% of total volume per day; that was between the values recommended by authors as Hutchinson et al. (2004) and Timmons et al. (2002). At the farming tanks between 2% and 3% of the live weight in fish food with 30% of protein contents, divided in three meals were provided daily, as recommended by Kubitza (2000). In the fish tanks the oxygen concentration was monitored three times a day, in case of a reduction of the DO concentration a higher amount of air was provided by opening the respective globe valve. The pH variations did not justify some actions in order to control it because most of the time it was very near to neutrality. A treatment to reduce and eliminate the fungus presence on the fish epithelium was done. The treatment consisted in a 50% tank water volume reduction; the granulated and diluted marine salt added to the farming units; the manual agitation to unify the saline solution concentration; the action of the unified solution during 10 min; the siphoning on the bottom of the tanks, and refilled with the previously removed water. 20 I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 Fig. 2. Liquid velocity in the riser and downcomer zones of the reactor. 3. Results and discussions 3.2. Liquid velocity 3.1. Carrier physical characterization Fig. 2 shows the average measured liquid velocities in the riser and in the downcomer reactor zones. The measured velocities at the riser zone varied between three and eight times the velocities registered at the downcomer section, its ratio increases with the air flow increasing. The densities of the possible supporting media evaluated were: S = 2.57, 1.74, and 1.64 g/cm3 , and the uniformity coefficients (UC = D60 /D10 ) were 2.13, 2.17, and 1.68 to sand, anthracite carbon, and GAC, respectively. The GAC was selected as the carrier because it had predominance of certain material sizes that suggest their uniformity (represented by the smallest value of UC); besides GAC was the carrier with the smallest specific mass which made it of easier suspension requiring less air flow than the other materials. Those data and the high specific surface of the GAC (Lawson, 1995) made of this material with natural humidity of 13.9% and effective size D10 of 0.34 mm the best option for its use in the AAFBR and the subsequent biofilm development conditions. 3.3. Gas holdup at two-phase and three-phase reactors Fig. 3 shows the average measured gas holdup values of the two-phase condition reporting high gas holdup difference values between the riser and the downcomer zones. The values suggested that the reactor presented the bubble recirculation regimes II and III that produces the liquid circulation. The experiments registered higher gas holdup values in the riser than the observed in the Fig. 3. Gas holdup values in two-phase condition at the riser and downcomer zones. I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 21 Fig. 4. Gas holdup on the riser and downcomer at different carrier concentrations. downcomer; this means that some portion of the bubbles injected escaped to the atmosphere and the rest recirculated to the downcomer. The gas holdup values in three-phase configuration of 2500 L/h air flow and different carrier concentrations (Cc ) reported that at higher concentrations the gas holdup inside the reactor is reduced. According to Fig. 4, the carrier concentration increment causes a reduction of the gas holdup values and could affect the gas transfer efficiency in the three-phase internal loop airlift reactors as affirmed by Freitas and Teixeira (2001). 3.4. Oxygen transfer The KL a measured was standardized at a 20 ◦ C temperature. The KL a20 presented in Fig. 5, adapted from Araújo (2008), shows that higher air injected flows increase the oxygen mass transfer. The air mass transfer occurs mainly at the riser zone, and due to the increases of liquid velocities at high air flows the liquid–gas contact time will be reduced producing the stabilization of the KL a20 values. In three-phase experiments, it was difficult to maintain the carrier (sand) in suspension, mainly to low air flow values. It was noted that oxygen transfer in the three-phase condition was lower than the achieved at the two-phase experiments as reported by Freitas and Teixeira (2001). 3.5. Characterization of the waters used in the inoculation of the AAFBR The total solids mean values measured in the ornamental fish tank and the Ipê Lake waters were similar, however, the ornamental fish tank water presented higher dissolved solids concentration in the organic matter form, produced by the food that was not consumed, feces and fish secretions. Lake Ipê water presented a higher concentration of suspended solids and total volatile solids probably produced by the presence of some microalgae which is very common in this kind of natural waters (Table 1). The water of Lake Ipê had a higher DO concentration than the ornamental fish tank because DO was consumed by the fish in the tank. pH in the tank was lower because of the biological activity of the fishes and their secretions. In Table 2, there are the values of BOD, COD, total phosphorus, nitrite, TAN and TN, which were higher in the tank because the biological waste produced by the fishes metabolism and because of the food not consumed by the animals. 3.6. Monitored parameters Fig. 5. Comparison of the average values of KL a20 obtained in two-phase and threephase conditions. At the beginning of the monitoring period (first 9 days), there were important oscillations of the values of some parameters due to the adaptation process of the AAFBR to carbonaceous and nitrogenous organic matter removal. Parameters as the oxidation–reduction potential (ORP) and pH depended on the weekly exchanged water, and conductivity depended mainly on the fish treatments practiced. There was variability on some parameters in the culture units mainly due to the non-uniform removal of the produced solids at tanks 2 and 3. The small water outflow in the tanks did not collect and transport feces and non-consumed food appropriately because of the low velocity of the water; it causes the aggregation and accumulation of the solids in the tubes producing partial blocking, reduction of the out flow, and rising of water level in the 22 I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 Table 1 Values of the solids measured in the AAFBR inoculation waters. Water Origin TS (mg/L) FTS (mg/L) VTS (mg/L) SS (mg/L) FSS (mg/L) VSS (mg/L) DS (mg/L) DS (%) Lake Ipê Tank 103 109 81 93 22 16 21 8 14 7 7 1 82 101 79.6 92.7 Table 2 Measured parameters values in the AAFBR inoculation waters. Water Origin DO (mg/L) BOD (mg/L) COD (mg/L) TP (mg/L) Nitrite (mg/L) TAN (mg/L) TN (mg/L) pH Turbidity NTU Lake Ipê Tank 6.04 4.61 1.38 1.46 3.5 8 0.075 0.650 0.029 0.033 0.05 0.12 1.3 1.8 8.1 7.3 2.52 3.23 tanks. Such situation causes suddenly detachment of the material, increase of the flow, and dragging of the solids to the sedimentation tank changing the HRT in that unit and in the AAFBR temporarily. Another reason of the variability of the measured parameters could be the presence of a fine dust produced by the GAC hydration; their friction inside the AAFBR and the passing through the pumps rotors. This last condition was observed due to the loss of little amounts of carrier because the effluent flow velocity exceeds the sedimentation velocity of some particles at the decantation AAFBR zone. 3.6.1. TAN removal monitoring Table 3 illustrates the average and maximum values of ionized ammonium (NH4 + ) and non ionized ammonia (NH3 ) forms and their addition (TAN) registered during the project, including and excluding the mean concentrations measured during the reactor adaptation period. The division was made because the strong influence of the extreme values presented on the first day of operation of the system, while the biofilm developed on the carrier surface achieved the adaptation to the kind of wastewater of the RAS and maintained an apparent equilibrium between ammonium excreted by the fishes or produced by the bacterial decomposition of the non-consumed food (Durborow et al., 1997) and the amount of ammonium removed by the AAFBR. The high concentrations registered in the first 2 weeks of the experiment produced the 19 fishes death, representing 90% of total mortality registered during the project; the dead animals were replaced by new ones in order to keep the biomass in the tanks. Concentration variability of the different ammonium forms in the culture tanks depended on biological wastes produced by the fishes and food consumption. It tried to provide a uniform food quantity (3% of tank biomass, 1% three times a day) from the beginning of the research; nevertheless, the food consumption was not constant probably as a function of some water quality parameters as temperature and DO concentration. Fig. 6 shows the TAN concentrations at influent and effluent of sedimentation tank (I.Sed and E.Sed) and in the AAFBR effluent (E.FBR), the peak values were the result of the permanent production of that substance by the fish metabolism and their accumulation. TAN capacity removal of the AAFBR and the partial water substitution of the RAS practiced once a week promotes the control of the different ammonium forms, the reduction of the TAN levels can be appreciated in Fig. 6. After the AAFBR stabilization, TAN values observed in culture tanks were within the range recommended by authors as Timmons et al. (2002) and Colt (2006). TAN removal efficiency was calculated based on mean concentrations measured in the three culture tanks mixed effluents at sedimentation tank inlet; in its effluent and in the AAFBR effluent. The average efficiency measured after reactor stabilization was of 27.06%, with a maximum value of 51.33%. The results obtained with D10 = 0.34 mm GAC carrier were better than those reported at two phase fluidized sand reactors by Timmons et al. (2002) and Summerfelt (2006) with D10 between 0.45 and 0.80 mm; however, the calculated efficiency at AAFBR was lower than the obtained by using fine media as reported by Weaver (2006) and Davidson et al. (2008). 3.6.2. TN removal Most of the time, the AAFBR effluent (E.FBR) had lower TN values than those registered on the sedimentation tank effluent (E.Sed); it suggests nitrogen assimilation by the biofilm microorganisms and a possible denitrification as a part of the ammonium process removal. Fig. 7 represents the TN values measured. According to Van Rijn et al. (2006), denitrification can occur inside the RAS in anoxic environments and in presence of carbon and nitrogen inorganic compounds, or also in the same biofilm as observed by Dalsgaard and Revsbech (1992) in the form of a “passive denitrification”. TN of AAFBR influent and effluent mean values were 4.6 mg/L and 4.1 mg/L, respectively. As observed in Fig. 7, there were moments in which TN concentration at effluent was higher than the measured at influent; this situation could be explained for the thickness loss attached to carrier biofilm and its reactor output produced by the effluent flow. TN average removal efficiency during the project was of 24.32%, value related with the 27.06% of TAN removal efficiency demonstrated by the AAFBR in the same period. Table 3 Mean and maximum values of ammonium nitrogen forms registered in the AAFBR. Sampling point Tank 1 Tank 2 Tank 3 Sedimentation tank inlet Sedimentation tank outlet AAFBR outlet Values including adaptation period (mg/L) Values excluding adaptation period (mg/L) NH4 + NH3 TAN TAN max NH4 + NH3 TAN TAN max 0.133 0.200 0.189 0.145 0.139 0.101 0.004 0.004 0.005 0.004 0.003 0.003 0.136 0.205 0.193 0.149 0.143 0.104 0.338 0.866 0.612 0.352 0.325 0.194 0.331 0.413 0.415 0.309 0.315 0.291 0.009 0.009 0.010 0.009 0.009 0.012 0.340 0.422 0.424 0.318 0.325 0.303 1.820 2.307 2.108 1.750 1.787 1.802 I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 23 Fig. 6. TAN concentrations in the influent and effluent of the sedimentation tank and the AAFBR effluent. 3.6.3. Nitrite and nitrate removal The average nitrite values (as NO2 –N) measured on the AAFBR influent and effluent were 0.82 and 0.88 mg/L, respectively, and the average nitrate levels (as NO3 –N) calculated on the same sampling points were 4.01 and 4.56 mg/L. Despite the significant scrubbing and biomass attrition of the two-phases fluidized bed reactors (Weaver, 2006; Sandu et al., 2002), increased in this case by the high velocity circulation, the three-phase condition and the low density of particles, a raise of the nitrite and nitrate levels was measured on the AAFBR due to the nitrification processes developed inside this reactor. Because the permanent circulation of the carrier on the reactor, the zero OD conditions in its effluent or the behavior as a nitrite generator were not registered, as commented by Weaver (2006) for fine media two-phase fluidized bed reactors. 3.6.4. COD and BOD removal There was a non-uniform behavior in the obtained data especially in the BOD, probably due to the situation presented in the outlet of the culture tanks that produced sudden variations in the flow and in the concentrations of the solids, carbonaceous matter and nutrients affecting the sedimentation tank and the AAFBR hydraulic retention time. The maximum COD value registered in the sedimentation tank influent was of 113 mg/L and the average was 36 mg/L; and the maximum and average values in the AAFBR effluent were 24 and 6.3 mg/L, respectively. The maximum, average, and minimum BOD measured values in the sedimentation tank influent were 7.4 mg/L, 4.7 mg/L and 1.7 mg/L, respectively, and in the AAFBR effluent they were 8.4 mg/L, 2.7 mg/L and 0.5 mg/L. The registered values suggest that there was a trend to stabilization of the BOD amount produced in the culture tanks and the organic matter removal by the treatment system. Based on the sedimentation tank influent and the AAFBR effluent concentrations, COD and BOD removal efficiencies were calculated, and are presented in Fig. 8. The average efficiency removal values of BOD and COD were 47.44% and 77.33%, respectively. Aquaculture wastewater treatment literature mainly focused on the nitrogen compounds removal, but interesting articles as the Sindilariu (2007) and Davidson et al. (2008) reported the performance of the wastewater treatment systems in terms of the removal of the BOD. Davidson et al. (op. cit.) stated the high dilution of BOD in the RAS waters and reported removal efficiencies above the 60% in fluidized bed reactors with control of the biofilm thickness, using sand with D10 = 0.11 mm as carrier, a smaller effective size than the presented by the GAC in this research. The COD removal efficiency obtained in the AAFBR was much higher than the reported by Ng et al. (1996) who used GAC as carrier in a two-phase fluidized bed reactor. The three-phase condition and the smaller effective size of the supporting media of the AAFBR that increases the bed specific surface area contributed to improve the absorption capacity of the carrier. 3.6.5. Total phosphorous removal Total phosphorous values registered in the influent of the sedimentation tank ranged between 0.57 and 3.84 mg/L and the measured at AAFBR effluent between 0.48 and 1.53 mg/L; average calculated concentrations in those points were 1.6 and 0.9 mg/L. Fig. 7. TN concentrations in the AAFBR influent and effluent. 24 I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 Fig. 8. BOD and COD removal efficiencies registered in the treatment system. The calculated average total phosphorous removal efficiency was of 38.49%, value similar to the reported by Davidson et al. (2008) to fluidized bed biofilters in RAS, and Gebara (2006) to AAFBR in domestic wastewater treatment. The TP concentrations in the AAFBR effluent were higher than the Brazilian legislation recommended values. Before dumping the final effluents these could receive a post treatment in order to remove organic matter, nitrogen, and especially phosphorous to avoid problems related with stream damage, growth of algae and the eutrophication of lakes. Phosphorous is a macronutrient that is responsible for plants cell development and the promotion of good root growth, it is because constructed wetlands could remove an average of TP up to 35% (Von Sperling, 2005), and the three basic types of land application: irrigation (slow rate); overland flow; and infiltration-percolation (rapid rate) present expected performance for phosphorous removal of 95%; 40–80%; and 60–95%, respectively (Spellman, 2009). A source of vegetal phosphorous accumulation are mature seeds or grains because they contain high amounts of phosphorous in the form of phytic acid, typically between 50 and 85% of seed total phosphorus is found as phytic acid (Liu et al., 2005; Frank et al., 2007; Dai et al., 2007). 3.6.6. Temperature and DO concentration monitoring There was important water temperature variability during the experiment. The minimum registered value was 15.7 ◦ C and the maximum 25.1 ◦ C. Those variations produce thermal stress on the fishes reducing food consumption and promoting the fungus apparition on the epithelium. The low initial values of DO associated with high ammonium concentrations inside the tanks produced mortality in the first 2 weeks of the experiment. The lower OD concentration registered in the farming tanks was 2.0 mg/L that coincided with the minimum recommended value for culture units in RAS (Krause et al., 2006). During the experiment, it was observed that temperature influences the DO concentration; low temperatures increased the solubility and produced high concentrations (Metcalf and Eddy, 1996; Timmons et al., 2002). OD consumed by fish respiration – that increases at high temperature and during digestion process – affected the oxygen concentration too (Masser et al., 1999). Besides the mentioned aspects, air flow provided in each culture tank heterogeneity in replaced water flow, some outages, and two compressor changes influenced the DO behavior. According to Timmons and Ebeling (2010), as operational and management experience is gained, densities can be increased and systems added as needed monitoring and control, so it is desirable to use automatic control systems in future experiments. The high DO values measured contributed to the good performance of the AAFBR; the average sedimentation tank effluent OD concentration was 6.0 mg/L, recommended value by Krause et al. (2006), and average DO concentration at AAFBR effluent was 8.1 mg/L. 3.6.7. pH, oxidation reduction potential, and conductivity monitoring In the experiment, pH stayed stable with lower value oscillations from 7.0 to 8.1 that were between the recommended values for fish farming production by authors as Masser et al. (1999), and Timmons et al. (2002). These values guaranteed a stable percentage of the toxic form of the ammonium: the NH3 , and helped the growing and functions of nitrification bacteria, as stated by Metcalf and Eddy (1996), and Krause et al. (2006). Electric conductivity varied mainly due the four marine salt fish treatments executed; progressive conductivity reduction on the fish culture tanks and the other points of monitoring were produced by the weekly partial water substitution on RAS. Maximum and minimum conductivity values registered in the tanks were 440 S/cm and 174 S/cm, respectively. Only positive ORP values that vary between 265 and 510 mV were registered in the system, the average measured ORP value was 380 mV, most of the times the registered levels on RALFCC effluent were near to that value. ORP measures the system oxidation or reduction capacity, oxidant agent presence, as oxygen and chlorine represent positive values of ORP. Thus, ORP variability could be justified by DO variation concentrations and mainly by little amounts of residual chlorine in water. The redox potential measured levels were near the range defined by Lawson (1995) for healthy aquaculture systems. 3.6.8. Solids removal High variability of solids concentrations in the inflow and outflow of the sedimentation tank and the AAFBR was registered. In sedimentation tank, variability was produced by the sudden nonuniformity of outflow at the farming tanks especially in tanks 2 and 3; in the AAFBR the variability could be generated by biofilm detachment and little amounts of carrier loss. Sedimentation tank influent registered TS concentrations between 133 and 848.5 mg/L; at their effluent concentrations varied between 100 and 416 mg/L; AAFBR presented values between I.A. Sánchez O., T. Matsumoto / Aquacultural Engineering 47 (2012) 16–26 135 and 490 mg/L. Those concentrations showed that the sedimentation tank removed TS with an average efficiency of 34.01%; nevertheless, it could be necessary to improve the AAFBR sedimentation effluent system in order to reduce the carrier loss. Sedimentation tank inflow presented SS concentrations between 9 and 196 mg/L with an average of 79.2 mg/L. On AAFBR effluent SS varied between 2 and 60 mg/L and the average value was of 16.2 mg/L. Average SS removal efficiency in the sedimentation tank–AAFBR treatment system was 64.45%. Average VS concentrations at sedimentation tank influent and effluent and AAFBR effluent were 208.5, 92.3, and 171.2 mg/L, respectively; these data confirmed the biofilm (organic matter) loss at AAFBR, probably produced by carrier grains friction due to their permanent circulation into the reactor and the carrier loss previously discussed. An alternative to reduce that situation could be to increase the sedimentation zone volume on AAFBR in order to diminish upward flow and outflow liquid velocities, and to increase the outflow collecting diameter to reduce the effluent velocity. 3.6.9. Turbidity and apparent color removal Turbidity registered values at sedimentation tank influent varied between 2.9 and 131 NTU; and at AAFBR effluent between 4.71 and 1.56 NTU. The average removal efficiency of this parameter demonstrated by the sedimentation tank–AAFBR treatment system was 65.51%. The mixed three tanks effluents, that is, sedimentation tank influent had the average apparent color value of 107.9 platinum cobalt units (PCU), and the effluent average value was 35.2 PCU; the AAFBR effluent reported an average value of 32.4 PCU. Based on the 8-week values, the apparent color removal efficiency on the RAS was calculated, whose average value was of 56.37%. 3.6.10. Cumulative feed burden The cumulative feed burden (CFB, g feed/L of freshwater/day) value was calculated as: CFB = F/VFW ; where F is the daily mass of feed (g) delivered and VFW is the equivalent volume (m3 ) of freshwater replaced per day (Colt et al., 2006; Malone and Beecher, 2000). The average feed ratio during the research was 2.5% of live weight, and the maintained biomass IN the system was 30 kg/m3 , representing 450 g of feed/day; the equivalent volume of freshwater added on the RAS was 203 L/d. The calculated CFB was 2.28 g/L, 3.4 times lesser than the reported in a tilapia recirculating aquaculture system for Brazil (2006). The small value of CFB is due to the low fish density maintained on the evaluated system. The evaluated RAS showed that the investment of electricity on water pumping and air injecting were higher than the commercial value of the fish kilograms produced, however, the average stocking density maintained on the system was lesser than the reported in other experiences with tilapia as Twarowska et al.’s (1997) and Shnel et al.’s (2002) experiments in which the densities were higher than 55 kg/m3 . The AAFBR could be evaluated at different HRT’s in order to optimize its treatment performance. High stocking densities and productions are required in this recirculating aquaculture system to be able to cover investment and operational costs (Martins et al., 2010). 4. Conclusions • The average TAN removal efficiency on the evaluated wastewater treatment system was of 27.1%, and average TN removal was 24.3%. • The wastewater treatment system demonstrated a good capacity for organic matter removal at the common RAS concentrations to • • • • • • 25 intensive Tilapia farming with an average BOD removal of 47.4% and 77.3% to COD. The evaluated wastewater treatment system performance suggested it as an alternative to water reuse on RAS, capable to maintain the water quality characteristic at recommended values for fish farming in closed systems. The system maintained the water quality parameters in a range of values below the maximum permitted values defined by the Brazilian environmental legislation to final effluents disposal in water corps, except for the phosphorous concentration that was above the recommended limits. The low value of the calculated cumulative feed burden was due to the low fish density maintained on the evaluated system. 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