Kinetics of nitrogen compounds

Aquacultural Engineering 50 (2012) 20–27 Contents lists available at SciVerse ScienceDirect Aquacultural Engineering journal homepage: www.elsevier.com/locate/aqua-online Kinetics of nitrogen compounds in a commercial marine Recirculating Aquaculture System ˜ a , P Gómez b , A.M Urtiaga a , I Ortiz a,∗ V Díaz a , R Ibánez a b Dpto Ingeniería Química y QI ETSIIyT, Universidad de Cantabria, Av de los Castros s/n, 39005 Santander, Spain APRIA Systems S.L., Polígono trascueto S/N, 39600 Camargo, Spain a r t i c l e i n f o Article history: Received March 2011 Accepted March 2012 Keywords: Marine Recirculating Aquaculture System Biological treatment Trickling filter Nitrification kinetics Water quality a b s t r a c t This work reports the degradation of nitrogen compounds in a commercial marine Recirculating Aquaculture System (RAS) aimed at the culture of sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax) The annual production of fingerlings is around 18 million and the process includes a drum filter and a biological treatment in order to enhance the water quality Ammonia measurements at the inlet of the biological system showed that the concentration of this compound followed a diurnal pattern closely related to the feeding of the fingerlings; thus every day after feeding around am, the concentration of ammonia started increasing, it reached a maximum about h after feeding and then continued decreasing until the following morning With regard to nitrite concentration, no significant differences were observed between the values measured at the inlet and the outlet of the biological system during the day, with an average concentration of this compound ranging between 0.08 and 3.66 mg NO2 − N l−1 A drawback of ammonia removal by means of nitrification is the subsequent increase of nitrate as the final product of ammonia oxidation in the culture system The nitrate concentration in the biofilters inlet was found to fluctuate between 22.33 and 55.44 mg NO3 − N l−1 during the characterization period Partial water exchange was needed during the day in order to minimize the water losses during fish handling and to keep the concentration of nitrate below the maximum allowable level of 46 mg NO3 − N l−1 due to production requirements in the hatchery under study The ammonia degradation within the biological system, obtained by the ammonia measurements and comparison of the values at the inlet and outlet of the trickling filters has been fitted satisfactorily to ½order/0-order kinetic expressions in good agreement with the results found in literature for laboratory and pilot plant studies Rate constants k(1/2-order) = 0.49 g1/2 m−1/2 day−1 and k(0-order) = 0.64 g m−2 day−1 , have been obtained in this study for commercial trickling biofilters Thus, this work reports for the first time the kinetics of ammonia oxidation in trickling biofilters installed in a commercial recirculating aquaculture marine water system These results will provide useful information for the design of an appropriately sized biofilter in order to optimize the water quality and reduce the need to exchange water in this activity © 2012 Elsevier B.V All rights reserved Introduction Aquaculture is the fastest growing animal food-producing sector of the world, with an annual growth rate of almost 10% since 1970 This is coupled with the fact that there has been a sharp decline in the world’s ocean captures and an increasing human population increasing the demand for seafood In this sense, the most common fish species raised in fish farms are salmon, sea bass, sea bream and rainbow trout (Crab et al., 2007; FAO, 2009) ∗ Corresponding author Tel.: +34 942 20 15 85; fax: +34 942 20 15 91 E-mail address: ortizi@unican.es (I Ortiz) 0144-8609/$ – see front matter © 2012 Elsevier B.V All rights reserved doi:10.1016/j.aquaeng.2012.03.004 The intensive aquaculture allows a very high fish production per unit of surface but implies two important limitations On the one hand, as result of fish excretion and decomposition of uneaten feed, nitrogenous compounds (ammonia, nitrite and nitrate), organic matter and pathogens are generated Ammonia nitrogen is the most critical water quality parameter in fish culture It is mainly excreted as the unionized form NH3 , although NH3 and NH4 + are in equilibrium in water The relative proportion of the two forms depends upon pH, temperature, and to a lesser extent, salinity The sum of the two forms, NH3 -N and NH4 + -N, called Total Ammonia Nitrogen (TAN) is often used as a key limiting water quality parameter in intensive aquaculture systems design and operation (Lemarié et al., 2004; Colt, 2006; Eshchar et al., 2006) Nitrite is also found as an intermediate product in the process of nitrification of ammonia V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 to nitrate Nitrate is the end product of nitrification process and it is considered the least toxic to fish of the different inorganic nitrogen forms; nevertheless nitrate levels usually need to be controlled by daily water exchange (Singer et al., 2008; van Rijn et al., 2006; van Kessel et al., 2010) Additionally, high culture intensities require high flow rates of both recirculated and exchanged water to attain sufficiently low waste levels in the fish tanks (Sandu et al., 2008) Interest in recirculating aquaculture technology is growing worldwide for high value fish species due to limitations of existing water supplies and land availability constraints, the desire for increased systems carrying capacity, the control over the fish rearing, reduction of heat loss and reduction of waste effluent stream volumes (Losordo and Hobbs, 2000; Martins et al., 2010) Recirculating Aquaculture Systems (RAS) are emerging as the preferred technology to provide adequate culture water quality in hatchery activities RAS are typically assembled by several rearing tanks and treatment operations such as solids removal, ammonia removal/conversion and aeration/oxygenation/CO2 degassing and water exchange in order to maintain the water quality for fish rearing In such systems ammonia is mostly oxidized into nitrite and nitrate through nitrification in biological filters by means of the bacteria, Nitrosomonas and Nitrobacter (Chen et al., 2006; Itoi et al., 2007) Different types of biofilters are described in literature (Crab et al., 2007) Trickling filters, in which water flows down through a stationary filter media by gravity, are attractive biofilters for application in fish culture They present TAN removal rates ranging from 0.1 to 0.9 g m−2 day−1 (Eding et al., 2006) and several advantages like low costs of construction, operation and maintenance, robust operating meaning a greater tolerance of differences in hydraulic and organic loads, the ability to maintain high and constant oxygen levels and the removal of carbon dioxide produced by the fish Additionally, the biofilm is stripped easily from the falling water if hydraulic loading rates are adequate (Lekang and Kleppe, 2000) Nevertheless there is limited information on the impact of salinity on nitrification Several authors have pointed that average removal rate is reduced in salt water compared to freshwater Chen et al (2006) reported that many engineering companies and pilot scale long term experiments with fresh and marine water recirculation systems suggest that the average removal rate is reduced by approximately 37% in salt water compared to freshwater Rusten et al (2006) reported that data from commercial fish farms operating at a salinity of 21,000–24,000 mg l−1 , indicated that the nitrification rate was approximately 60% of what would be expected in a freshwater system for moving bed bioreactors These authors have observed that it takes significantly longer to fully acclimatize a biofilter in salt water than in freshwater Abrupt changes in salinity of greater than g l−1 , will shock nitrifying bacteria and decrease the reaction rate for both ammonia-nitrogen and nitrite-nitrogen removal Moreover, this assumption was reinforced since the amount of un-ionized ammonia increases with pH and water temperature As a result, higher levels of toxic un-ionized ammonia are found in salt water systems where the standard pH is 8.0 This means that greater attention to biological filter design and efficiency is required for saltwater systems than for freshwater systems that typically operate at pH near 7.0 Due to the limited and uncertain information in literature about the potential of nitrification in marine systems, this work is aimed at the contribution to a better understanding of commercial saline trickling filters, installed in a marine hatchery located in the north of Spain, devoted to sea bream and sea bass culture, in order to improve the water quality of the fish farm A characterization of the trickling filters system installed in the fish farm was assessed by comparing physical, chemical and microbiological properties of the seawater collected at the inlet and outlet of the biological system The nitrification kinetics and the values of the rate constants of ammonia oxidation have been obtained by means of the analysis 21 Table Technical characteristics of the biological system Technical characteristic Value Volume (m3 ) Flow rate (m3 h−1 ) Specific surface area (m2 m−3 ) 200 416–600 >160 of the conversion of ammonia nitrogen to nitrate nitrogen within the biofilters The results obtained will help to a better design and performance of the commercial trickling filters under study Materials and methods 2.1 Description of the commercial Recirculating Aquaculture System under study The Recirculating Aquaculture System under study is located in Cantabria (Northern coast of Spain) Sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax) are cultured in this hatchery The annual production of fingerlings is approximately 18 million The RAS is comprised of 40 rearing tanks of m3 each and raceways of 20 m3 each and a centralized water treatment system Each rearing unit includes an airlift pump system for water circulation in order to provide adequate rearing conditions Seawater coming from the fish tanks is filtered through a rotating drum screen filter with 40 ␮m screen mesh size (model HDF1604-1H from Hydrotech) which removes suspended solids The water flows to a pumping sump The automatic backwash of the drum filter is activated over the day every few minutes and an additional cleaning with high pressure water jets is carried out weekly to improve the system performance The process water is pumped to biological treatment, collected and then pumped again back to the tanks with a second pump Oxygen contactors add pure oxygen to the fish tanks The biological treatment consists of circular nitrifying trickling filters (NTF), with a total volume of 200 m3 (two of them with a volume of 50 m3 and the third, of 100 m3 ), filled with a crossflow plastic media of propylene, with a specific surface area of 160 m2 m−3 , spherical shape and rough surface (ADJ Serveis Tècnics, S.L.) Technical characteristics of the biological treatment are presented in Table and a basic layout of the Recirculating Aquaculture System under study is shown in Fig The total rearing tanks volume used during this study varied from 260 to 375 m3 The recirculating system provided up to complete turnovers of the water per hour, depending on the waste load Therefore, the water flow rate varied between 520 and 750 m3 h−1 and the flow rate to the biological system was 80% of the total, so the flow rate to the biofilters was between 416 and 600 m3 h−1 The water exchange rate, calculated by the differences in the meter readings during the sampling periods, ranged from 39 to 189 m3 day−1 A biomass of 5000–10,640 kg of sea bream fingerlings was grown in the fish tanks and the feed load covered a range from 140 to 505 kg per day The daily feed ratio varied from 2.1 to 3.4% at the beginning of the sampling period and from 5.5 to 5.7% during the last two months Fish were fed by means of automatic feeders, which were filled with the corresponding amount of feed between and am These devices distributed uniformly the feed into the tank every 10–15 during approximately h 2.2 Analytical procedure Water quality in the Recirculating Aquaculture System was studied during the period December 2008 to April 2009 The 22 V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 Table Water parameters at the inlet and outlet of the biological system Parameter pH Temperature Conductivity Turbidity Salinity TAN Nitrite Nitrate Phosphate Chloride COD TOC BOD5 O2 CO2 Vibrio sp Total bacteria plate count Units (◦ C) (mS cm−1 ) (NTU) (mg l−1 ) (mg N l−1 ) (mg N l−1 ) (mg N l−1 ) (mg P l−1 ) (mg l−1 ) (mg O2 l−1 ) (mg l−1 ) (mg l−1 ) (mg l−1 ) (mg l−1 ) (CFUs ml−1 ) (CFUs ml−1 ) Inlet biofilter Outlet biofilter Min Max Min Max 6.51 16.3 33.30 0.81 29,800 0.06 0.10 22.33 2.40 16,493.07 6.00 7.32 12.00 5.07 1.00 1,800 111,000 7.31 28.0 51.10 2.40 32,200 6.56 3.37 55.44 4.99 17,822.71 43.00 10.00 16.00 6.94 9.00 32,000 590,000 6.94 17.0 47.50 0.84 29,800 0.13 0.08 25.10 2.70 16,493.07 6.00 8.16 8.00 3.77 1.00 1,000 56,000 7.57 28.0 51.30 1.16 32,200 4.64 3.66 62.77 4.80 17,822.71 35.00 9.26 12.00 5.91 4.00 6,000 106,000 alkalinity, pH, salinity and the concentrations of nitrate, nitrite, Total Ammonia Nitrogen, chloride, phosphate, organic matter and dissolved oxygen were measured in samples collected at regular time intervals of 60 min, from the inlet and outlet of the biological system as indicated in Fig Table lists the physicochemical and microbiological parameters registered in the seawater samples collected every hour at the inlet and outlet of the biological treatment in a sampling protocol carried out over h periods and extended over 25 days Additionally, TAN and nitrite were measured at the inlet and outlet of the trickling filters every h over time periods of 24 h The pH was measured with a Crison pH 25 pH meter and the conductivity and the salinity were measured with a Crison CM 35 conductivity meter The turbidity was determined in a Turbiquant 3000 IR (Merck) TCOD was determined by heat of dilution COD procedure (Ruttanagosrigit and Boyd, 1989) employing mercuric sulfate to remove chloride interference Analysis of the TOC was performed using a TOC-V CHP Shimadzu analyzer For the evaluation of BOD5 the WTW OxiTop® measuring system (Weilheim, Germany) thermostated at 20 ◦ C was used The measure was done following the Standard Methods 5210D procedures (APHA, 1998) The concentration of TAN, nitrite, nitrate, chloride and phosphate in solution was measured spectrophotometrically by using a Spectroquant® Pharo 100, (Merck Company) according to Standard Methods (APHA, 1998): 4500-NH3 -D, 4500-NO2 -B, 4500-NO3 -B, 4500-Cl-E and 4500-PE, respectively Oxygen and carbon dioxide concentration was measured using a HACH Sension probe and an Oxyguard probe GO2P CO2 , respectively Sulfate was measured using ion chromatography (Dionex 120 IC, with an IonPac AS9-HC Column) Analysis of bacterial levels (Vibrio ssp and total bacteria) was also performed Counts of colony forming units (CFU) were done by the total plate count method and the number of Vibrio spp was counted using thiosulfate–citrate–bile salts–sucrose (TCBS) agar All analytical determinations were performed immediately after sampling and were done by replicate Results According to Colt et al (2006), the performance of a biofilter is difficult to analyze due to the large number of parameters that must be controlled and the number of measurements that must be carried out The most important water quality parameters in Fig Scheme of the Recirculating Aquaculture System under study (X represents the sampling points) 15 2.00 1.50 Feed Time 1.00 10 0.50 0.00 (mg NH4+-N l -1) 2.50 Ammonia concentration 20 3.00 Water intake volume (m3) 25 3.50 (mg NH4+-N l-1) Ammonia concentration 4.00 23 1.40 12000 1.20 10000 1.00 8000 0.80 6000 0.60 4000 0.40 0.20 2000 0.00 Biomass (kg of fish) V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 Sampling hour 4.00 1.20 3.50 3.00 1.00 2.50 0.80 2.00 0.60 1.50 0.40 1.00 0.20 0.50 0.00 0.00 -1 1.40 TAN concentration at the inlet (mg l ) (g NH 4+-N m-2 d-1) Fig The daily fish biomass ( ) and the ammonia concentration at the inlet of the trickling filters (᭹) over a 10-days period Samples were taken at pm everyday Ammonia removal rate Sampling hour Fig Variation of TAN removal rate ( ) through the trickling filters and the TAN concentration at the inlet (᭹) of the biofilters over a 24-h period starting at pm Organic matter in the RAS systems has been evaluated by means of BOD5 , relatively low BOD5 concentrations (8.00–16.00 mg l−1 ) were measured during the sampling periods in the RAS under study due to the relatively high new water exchange rate as will be discussed in the next section Similar values of BOD5 have been reported in the works of Krüner and Rosenthal (1983) Chemical Oxygen Demand (COD) was also measured, being the average COD concentration at the inflow and outflow of the biofilters 30.50 and 25.86 mg l−1 , respectively No statistically significant COD differences were observed between both streams Concentration of total 25 3.00 2.50 20 2.00 15 1.50 10 1.00 Feed Time 0.50 0.00 Water intake volume (m 3) aquaculture activities are temperature, salinity, pH, dissolved oxygen, ammonia (NH3 ), nitrite (NO2 − ) and nitrate (NO3 − ) In open systems, only temperature and salinity are likely to fluctuate rapidly, whereas in closed systems, the rest of parameters are more likely to vary The maintenance of water quality parameters is essential to avoid adverse conditions which could affect the growth and survival of the fish The performance of the commercial saline RAS under study has been deeply evaluated by means of the main quality parameters according to the procedures and analytical methods previously described The results of the physico-chemical characteristics of the water under study for the whole characterization period are summarized in Table where the maximum and minimum values reached both at the inlet and outlet of the biological system for each measured parameter are indicated Values of TAN concentration at the inlet and outlet of the biological treatment shown in Table indicate a concentration range from 0.06 to 6.56 mg N l−1 The TAN concentration at the inlet and outlet of the biological system over 24 h is depicted in Fig The pattern shown in this figure can be better understood taking into account that fish were fed by means of automatic feeders, which were filled with the corresponding amount of feed between 7:00 and 8:00 am These devices distributed uniformly the feed into the tanks every 10–15 during approximately h The water renewal requirement within the Recirculating Aquaculture System over a 24 h period is also depicted in Fig 2, the close relationship between TAN concentration and water renewal can be easily observed The relationship between fish biomass and the ammonia concentration measured at the inlet to the biofilters is shown in Fig 3; the concentration of TAN measured along a period of 10 days, sampling at a fixed time (4:00 pm) is represented together with the corresponding fish biomass level The values of TAN removal rate through the biological system over a period of 24 h are shown in Fig Regarding nitrite, influent concentrations in the range 0.10–3.37 were found during the sampling period Fig shows the nitrite concentration measured in the water samples collected at the inlet and outlet of the trickling filters system over 24 h The apparent conversion efficiency of NO2 -N to nitrate nitrogen, NO3 -N, in the biological system was calculated obtaining an average value of 19.5% on a single pass through the filters during the night Nitrate concentrations in the biofilters effluent varied in the range 25.10–62.77 mg l−1 The RAS under study required important water exchange in order to control the nitrate concentration, consequently, the operational costs increased Figs and show the volume of water exchange that was needed in the RAS under study in order to enhance the water quality Date Nitrite concentration (mg NO2 N l -1) Fig Ammonia concentration at the inlet (᭹) and the outlet ( ) of the trickling filters system over a 24-h period starting at pm The water renewal volume in the system is represented in bars form Sampling hour Fig Nitrite concentration at the inlet ( ) and the outlet ( ) of the trickling filters system over a 24-h period starting at pm The water renewal volume in the system is represented in bars form 24 V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 0.90 Ammonia removal rate (g NH4+-N m-2 d-1) 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00 1.00 2.00 3.00 4.00 NH4-N concentration (g 5.00 6.00 7.00 m -3) Fig Predicted and observed ammonia removal rates as a function of the ammonia influent concentration (solid circles are observed data and solid line represents predicted data) using the ½-order/0-order model described by equations: rTAN = −1 −1 0.5 − 0.24 [g NH4 + -N m−2 d ] and rTAN = 0.64 [g NH4 + -N m−2 d ] 0.49 · CTAN bacteria and Vibrio sp reported in Table guarantee the culture water quality The nitrification performance of a biofilter is usually reported in literature as surface specific TAN removal or volumetric TAN removal rate Nitrification rates in granular media are much more closely related to volume of media than surface area provided by the media In the present work, the nitrification rate has been calculated in terms of Volumetric TAN Removal (VTR), using the equation 1: VTR = ([NH4 + -N]in − [NH4 + -N]out ) · Q Vmedia (1) where VTR is the amount of TAN removed per m3 of filter media per day; [NH4 -N]in and [NH4 -N]out are the ammonia concentration measured at the inlet and the outlet of the trickling filters system (g m−3 ), respectively; Q is the flow rate through the filters (m3 d−1 ) and Vmedia is the volume of the filter media (m3 ) Fig shows the ammonia removal rate values related to the inlet ammonia concentrations to the biological system The values of Volumetric TAN Removal calculated by means of equation have been converted into surface TAN removal rate values, using the specific surface area of the media (160 m2 m−3 ) in order to compare the kinetics of the present work with values found in literature As shown in Fig the ammonia removal rate increases with inlet ammonia concentration up to a maximum inlet concentration of 3.50 g m−3 For higher inlet concentrations the ammonia removal rate is constant and independent of the inlet concentration Discussion The data reported in previous sections contain relevant information for the complete description of the behavior of a commercial saline water treatment by means of trickling biofilters In this section this information will be discussed and the most relevant conclusions aimed to the better design and performance of the biofilters will be remarked The pattern of ammonia concentration in the biofilters influent can be concluded from Figs and As shown in Fig 2, the ammonia levels in the system under study fluctuate with a factor of 4–5 over 24 h The concentration of ammonia in the system increased rapidly after the feeding began reaching a maximum value approximately h after feeding, then it decreased, defining a cyclic pattern until the following feeding Each diurnal cycle showed a unique maximum concentration as fish were fed only once a day Similar postprandial ammonia excretion patterns have been reported in literature (Dosdat et al., 1996; Robaina et al., 1999; Gómez-Requeni et al., 2003) As shown in Fig 2, changes in the ammonia concentration in the influent are closely reflected in the effluent concentration of the biofilters Additionally according to Fig 3, TAN concentration fluctuates slightly in the range 0.86–1.28 mg l−1 over the experimental period, according to the increase of fish biomass (5480–9290 kg of sea bream fingerlings), thus indicating that the higher the fish biomass cultured in the system is, the higher is the ammonia concentration Fluctuations in the assimilation of ingested feed and therefore of waste production over time could alter this relationship Fig shows that the TAN removal rate through the biological treatment increased with the TAN inlet concentration to the biofilters The calculated TAN mean removal efficiency in one pass through the biofilters was 58.3% with respect to the influent concentration The pattern of nitrite concentration in the effluent is closely related to the Ammonia presence As shown in Fig 5, the nitrite concentration in the system increased rapidly just after feeding at 8:00 am, it reached a maximum and then started decreased until the following morning This profile is identical to the ammonia pattern shown in Fig 2, as nitrite is constantly formed as an intermediate compound during the biological oxidation of ammonia to nitrate Although nitrite is usually converted to nitrate as quickly as it is produced, lack of biological oxidation of the nitrite will result in elevated nitrite levels that can be toxic to the fish However, in seawater, the toxicity due to NO2 -N is greatly reduced by the presence of the chlorine ion As shown in Fig 5, no significant differences in nitrite concentration were observed between the inlet and outlet of the biological system, although the concentration at the outlet of the biofilters was slightly higher than its concentration at the inlet in the data measured from 8:00 am to 10:00 pm due to the oxidation of ammonia within the biofilters However during the night, as the ammonia concentration decreases the nitrite produced is lower, and the nitrifying bacteria are able to oxidize the existing nitrite to nitrate Consequently, the outlet of the biofilters has a lower level of nitrite concentration than the inlet van Rijn and Rivera (1990) found that nitrite removal by a trickling filter took place when ambient ammonia concentrations were lower than 1.0 mg NH4 -N l−1 , while at higher ambient ammonia concentrations, nitrite was accumulated According to Figs and 5, the nitrate concentration was found to fluctuate during the day between 22.33 and 62.77 mg NO3 -N l−1 , being a concentration of 50 mg NO3 -N l−1 generally accepted as a safe limit for nitrate nitrogen in fish culture, but this concentration varies widely for different species and development stages (Gutierrez-Wing and Malone, 2006) Furthermore, water exchange also allowed the proper dilution of TAN and NO2 –N concentrations As shown in Figs and the water renewal was not constant over the day It varied according to the fluctuations of the concentration of nitrogen compounds over the day: at night the volume of water exchange was very low or even zero as the level of pollutants was low but higher renewal rate was required during daylight hours The trickling filters system is not able to maintain TAN and Nitrate below the required quality levels during the whole day Water renewal is required to accomplish these requirements The water exchange rate during the sampling periods ranged from 39 to 189 m3 day−1 , corresponding to a daily water renewal volume from 0.55 to 1.06 m3 kg−1 feed These values are very similar to those found in the work of Blancheton et al (2007) for commercial recirculating systems with sea bass production As shown in Figs and the water renewal was not constant over the day It varied according to the fluctuations of the concentration of nitrogen compounds over the day: at night the volume of water V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 25 Table Comparative of ½-order/0-order ammonia removal kinetics in trickling filters ½-order ammonia removal rate (g NH4 -N m−2 d−1 ) 0-order ammonia removal rate (g NH4 -N m−2 d−1 ) Transition ∗ concentration, CTAN (g NH4 -N m−3 ) Water treated Trickling filter scale Reference 0.5 0.49 · CTAN − 0.24 0.64 3.2 Seawater Commercial Present work 0.5 0.23 · CTAN − 0.11 0.28 3.0 Seawater Laboratory Nijhof and Bovendeur (1990) 0.5 0.55 · CTAN − 0.11 0.69 2.2 Freshwater Pilot-scale Nijhof (1995) 0.5 0.76 · CTAN − 0.10 0.98 2.0 Freshwater Commercial Kamstra et al (1998) 0.5 0.47 · CTAN − 0.10 0.57 2.0 Freshwater Commercial Kamstra et al (1998) 0.5 0.32 · CTAN − 0.10 0.35 2.0 Freshwater Commercial Kamstra et al (1998) exchange was very low or even zero as the level of pollutants was low but higher renewal rate was required at daylight hours This value corresponded to a daily water renewal volume of 40% in relation to the total volume of the rearing tanks This percentage indicates that the amount of water exchange needed in this system is too high and therefore the treatment system was not correctly sized for the feed rate used in this RAS Organic matter is an essential parameter to be controlled in a RAS The organics are the result of the fecal material excreted by fish and uneaten feed Several authors (Zhu and Chen, 2001; Leonard et al., 2002; Ling and Chen, 2005; Chen et al., 2006; Michaud et al., 2006) have reported the importance of organics removal from RAS as quick as possible to avoid the inhibition of the nitrification process due to the competition between autotrophic nitrifying bacteria and heterotrophic bacteria As heterotrophic bacteria have a maximum growth rate of five times and cell yields of two to three times that of autotrophic nitrifying bacteria (Ling and Chen, 2005), the ammonia removal rate will decrease as organic loading increases Values of DBO5 and COD have been reported in Section The low DBO5 values registered guarantee that the nitrification process is not inhibited in the system, as DBO5 values higher than 30 mg l−1 are needed according to Chen et al (2006) The biodegradability index, calculated as the BOD5 /COD ratio, ranged between 0.23 and 0.38 in the influent of the biological treatment Similar biodegradability indexes (BOD5 /COD = 0.24–0.29) were found in the work of Sandu et al (2008) in the inlet of the biological filter of a commercial aquaculture system Low biodegradability appears to be common in Recirculating Aquaculture System water, probably due to the fact that the bacteria in the system usually have long time to degrade the organic material and thus a relatively big amount of non-biologically degradable material remains in the system Although significant research efforts on bio-filtration in Recirculating Aquaculture Systems have been made, useful information relative to nitrification kinetics is still lacking Comparative studies (Crab et al., 2007; Guerdat et al., 2010) have shown that rotating biological contactors (RBCs), submerged, trickling, or fluidized bed filters all have different performance in terms of TAN removal Nitrification kinetics vary among filter types due to differences in design and management strategies of the biofilters (Ling and Chen, 2005) According to literature (Eding et al., 2006), the substrates removal rate in a trickling filter is determined by their diffusional rates into the biofilm Substrates first diffuse from the bulk liquid into the biofilm through a stagnant water layer and then into the biofilm Once in the biofilm, the substrate is consumed by bacteria The nitrification rate in the biofilm depends on external factors (e.g., temperature, salinity, pH or bulk phase concentrations of TAN, O2 , COD and nitrite) or internal properties (e.g., biofilm thickness, abundance of nitrifying bacteria, or hydraulic surface loading rate) In the context of commercial aquaculture saline water systems, the nitrification kinetics of seawater in trickling filters has not yet received much attention In this work experimental data from the commercial aquaculture saline tickling filters plotted in Fig have been successfully fitted to a ½-order/0order model, plotted in Fig by a solid line Consequently, the nitrification kinetics of the trickling filters system under study can be described by Eqs (2) and (3) obtaining the following values of the kinetic constants: k(1/2-order) = 0.49 g1/2 m−1/2 day−1 and k(0-order) = 0.64 g m−2 day−1 The nitrification capacity of the biological treatment will not increase for ammonia levels higher than 3.2 g m−3 , since at that level the whole filter column is operat∗ , for this ing under 0-order conditions Therefore, the value of CTAN −3 commercial system is 3.2 g m 0.5 rTAN = 0.49 · CTAN − 0.24 [g NH4 + -N m−2 d−1 ] rTAN = 0.64 [g NH4 + -N m−2 d −1 (2) ] (3) ∗ CTAN = 3.2 g NH4 + -N m−3 (4) + -N m2 day−1 ); CTAN where rTAN is the ammonia removal rate (g NH4 ∗ is the nitrogen ammonia concentration (g m−3 ) and CTAN is the transition concentration from ½-order to 0-order This value depends on the oxygen concentration and the metabolic constraints of the nitrifying bacteria and it is an important parameter in the biofil∗ ter performance, since a low CTAN value can be an indication of low oxygen levels in the biofilter or high COD loads reducing the 0-order TAN removal rate value (Eding et al., 2006) The weighted standard deviation, defined by Eq (5) was calculated as w = 0.077, thus certifying that the proposed ½order/0-order model describes satisfactorily well the kinetic data of TAN removal w = n ((Cexp i=1 − Csim )/Cexp ) N−1 (5) It should be emphasized that the ammonia removal rates shown in Fig not represent the complete nitrification rate to nitrate but only ammonia oxidation rates under the environmental conditions given in the hatchery during the sampling period: The water temperature during the study ranged from 16.3 to 28.0 ◦ C, with an average value of 21.5 ◦ C, which was within the acceptable range for sea bass and sea bream culture The nitrification kinetic model developed in the present work constitutes a useful tool in the design of biofilters for marine RAS applications Previous works reported in literature used similar ½order/0-order models for the description of laboratory or pilot plant saline biofilters (Bovendeur et al., 1987; Nijhof, 1995) Kamstra et al (1998) validated the ½-order/0-order kinetic model for a wide range of freshwater commercial biofilters The results described in this work validate this nitrification kinetic model in a saline commercial biological system Table summarizes the values of ammonia removal rates calculated with the ½-order/0-order kinetic equations in the trickling filters operating at different conditions As shown in Table 3, the maximum nitrification capacity is lower in seawater systems than in freshwater systems, this has been attributed either to the fact 26 V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 that saltwater biofilters need a much longer start-up period than freshwater systems and also to the inhibiting effect of chloride on nitrification kinetics (Nijhof and Bovendeur, 1990; Campos et al., ∗ , is 2002; Rusten et al., 2006) The transition concentration, CTAN somewhat higher in seawater biofilters than in freshwater trickling filters The maximum value of the ammonia removal rate found in the biological system under study was 0.64 g NH4 -N m−2 d−1 This value is higher than the value of 0.28 g NH4 -N m−2 d−1 reported by Nijhof and Bovendeur (1990), working both biofilters with seawa∗ ter from RAS systems The value of CTAN obtained in our study is −3 3.2 g m and it is very close to the corresponding value already reported by Nijhof and Bovendeur (1990) Conclusions This work evaluates the performance of a commercial Recirculating Aquaculture System provided with a biological treatment based on the determination and comparison of physical, chemical and microbiological properties of the seawater samples withdrawn from the inlet and outlet streams to the biofilters Additionally the kinetics of ammonia nitrification in the biological treatment have been determined The main conclusions of this work can be summarized as: • Ammonia concentration increased rapidly after feeding reaching concentration above the quality requirements in the hatchery, but decreased over the night as there was not feed in the rearing tanks • No significant differences were observed between the nitrite concentration measured at the inlet and outlet of the biofilters during the day, ranging its concentration between 0.08 and 3.66 mg NO2 N l−1 Nitrate concentration was directly controlled by daily water exchange and the water renewal volume ranged between 10.7 and 59% of the rearing tanks volume Low values of the biodegradability index, ranging from 0.23 to 0.38 were calculated in the influent of the biofilters • The kinetics of ammonia nitrification within the biological system were fitted to ½-order/0-order expressions The values of the kinetic constants were: k(1/2-order) = 0.49 g1/2 m−1/2 day−1 and k(0-order) = 0.64 g m−2 day−1 A transition concentration from ½∗ of 3.2 g NH4 + -N m−3 has been obtained for order to 0-order, CTAN the commercial trickling filters system under study • An appropriate design of the biological treatment is essential in 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within aquaculture sector ˜ Raquel Ibánez is associate professor in the Universidad de Cantabria (Spain) and she develops her R&D activity in the group “Advanced Separation Processes” Her research activity is focused on the following topics: – Electrodialysis with bipolar membranes (EDBM) in the separation and purification of milk protein; – EDBM applied to the treatment of high concentrated waters from desalination process; – Development and application of membrane bioreactors (MBR) She has authored more than 20 scientific papers and has supervised Ph.D students She has participated in the main international Congress of Membrane Technologies (Euromembrane, International Congress on Membrane and Membrane Processes, European Congress on Chemical Engineering) She was in the Membrane Technology Group of the University of Twente for six months (2002) Pedro Gómez obtained his B.Sc Degree and Ph.D in Chemical Engineering at the Universidad de Cantabria (Spain) Nowadays, he is technical manager of Apria Systems S.L., enterprise (Spain) Apria Systems provides innovative solutions in the regeneration and reuse of wastewaters and in the study of contaminated soils (specially related to hydrocarbon storage activities) His work is focused on minimization of wastes and energy consumption reduction through the development, design and optimization of advanced processes 27 Ana María Urtiaga is Professor of Chemical Engineering at Universidad de Cantabria (Spain) She is Head of the Department of Chemical Engineering and Inorganic Chemistry of that university, since 2008 The research is aimed to the development and integration of new separation technologies based on selective liquid membranes, pervaporation, ultrafiltration, reverse osmosis, gas separation membranes, and advanced oxidation process, such as electrooxidation or Fenton Applications in the fields of metals recovery, separation of organic compounds, treatment and purification of industrial effluents and landfill leachates, solvents dehydration, water reuse and hydrogen recovery from gas mixtures have been developed Mathematical models processes have also been developed She has supervised 10 Ph.D Thesis Inmaculada Ortiz is Professor of Chemical Engineering and former Department Head at Universidad de Cantabria (Spain) She obtained her B.S degree and Ph.D in Sciences (Chemistry) at the University del País Vasco (Spain) in 1980 and 1985, respectively She was Scientific Officer of the National R&D programmes on Environment, Chemical Processes and Products and Natural Resources She was proposed as coordinator of the Chemical Technology area of the Spanish ANEP She has authored more than 2000 scientific papers and has supervised 25 Ph.D students She is the leader of the research group “Advanced Separation Processes” focused on: – Membrane processes; – Advanced Oxidation Technologies; – Process intensification Applications to waste water treatment & reclamation, food processing, chemical pharmaceutical industry and environmental applications  ... properties of the seawater collected at the inlet and outlet of the biological system The nitrification kinetics and the values of the rate constants of ammonia oxidation have been obtained by means of. .. consists of circular nitrifying trickling filters (NTF), with a total volume of 200 m3 (two of them with a volume of 50 m3 and the third, of 100 m3 ), filled with a crossflow plastic media of propylene,... the fluctuations of the concentration of nitrogen compounds over the day: at night the volume of water V Díaz et al / Aquacultural Engineering 50 (2012) 20–27 25 Table Comparative of ½-order/0-order