1. 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).
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
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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 5 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
Table 1 Technical characteristics of the biological system. Technical characteristic
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. 2. 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 5 m3 each and 8 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 3 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 1 and a basic layout of the Recirculating Aquaculture System under study is shown in Fig. 1. The total rearing tanks volume used during this study varied from 260 to 375 m3 . The recirculating system provided up to 2 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 7 and 8 am. These devices distributed uniformly the feed into the tank every 10–15 min during approximately 8 h. 2.2. Analytical procedure Water quality in the Recirculating Aquaculture System was studied during the period December 2008 to April 2009. The
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Table 2 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
(◦ 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 )
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. 1. Table 2 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 8 h periods and extended over 25 days. Additionally, TAN and nitrite were measured at the inlet and outlet of the trickling ﬁlters every 2 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 6 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. 3. 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. 1. Scheme of the Recirculating Aquaculture System under study (X represents the sampling points).
(mg NH4+-N l -1)
Water intake volume (m3)
(mg NH4+-N l-1)
Biomass (kg of fish)
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TAN concentration at the inlet (mg l )
(g NH 4+-N m-2 d-1)
Fig. 3. The daily ﬁsh biomass ( ) and the ammonia concentration at the inlet of the trickling ﬁlters (᭹) over a 10-days period. Samples were taken at 4 pm everyday.
Ammonia removal rate
Sampling hour Fig. 4. 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 2 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
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 2 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 2 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. 2. 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 min during approximately 8 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. 4. Regarding nitrite, inﬂuent concentrations in the range 0.10–3.37 were found during the sampling period. Fig. 5 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. 2 and 5 show the volume of water exchange that was needed in the RAS under study in order to enhance the water quality.
Nitrite concentration (mg NO2--N l -1)
Fig. 2. Ammonia concentration at the inlet (᭹) and the outlet ( ) of the trickling ﬁlters system over a 24-h period starting at 2 pm. The water renewal volume in the system is represented in bars form.
Sampling hour Fig. 5. Nitrite concentration at the inlet ( ) and the outlet ( ) of the trickling ﬁlters system over a 24-h period starting at 2 pm. The water renewal volume in the system is represented in bars form.
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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
NH4-N concentration (g
Fig. 6. 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 2 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
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. 6 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 1 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. 6 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. 4. 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. 2 and 3. 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 8 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. 4 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. 2 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. 2 and 5 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. 2 and 5 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
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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 3. 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. 6 have been successfully ﬁtted to a ½-order/0order model, plotted in Fig. 6 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
∗ 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 )
It should be emphasized that the ammonia removal rates shown in Fig. 6 do 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 3 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
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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). 5. 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 order to maximize the TAN removal rate, maximize the water reuse, minimize the impact of TAN on the ﬁsh cultured and minimize the need to exchange water. The nitrifying capacity of a bioﬁlter is largely determined by the used bioﬁlter media, the volume of the ﬁlter, the ammonia loading and the hydraulic loading. Acknowledgements Financial support of projects CTQ2008-03225/PPQ, CTQ200800690/PPQ, Consolider CSD 2006-44 (Spanish Ministry of Science and Innovation (MICINN)), 080/RN08/03.2 (Spanish MARM) and 18-04-2007 (SODERCAN, Cantabria Government) are gratefully acknowledged. The collaboration of Tinamenor S.L. is also acknowledged. V. Díaz also thanks the MICINN for a FPI research grant. References APHA, 1998. Standard Methods for Examination of Water and Wastewater, twentieth ed. American Public Health Association, Washington, DC. Blancheton, J.P., Piedrahita, R., Eding, E.H., Roque d’Orbcastel, E., Lemarié, G., Bergheim, A., Fivelstad, S., 2007. Intensiﬁcation of landbased aquaculture production in single pass and reuse systems. In: Aquaculture Engineering and Environment (Chapter 2). Bovendeur, J., Eding, E.H., Henken, A.M., 1987. Design and performance of a water recirculation system for high-density culture of the african catﬁsh, clarias gariepinus (burchell 1822). Aquaculture 63, 329–353.
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V. Díaz et al. / Aquacultural Engineering 50 (2012) 20–27 van Rijn, J., Tal, Y., Schreier, H.J., 2006. Denitriﬁcation in recirculating systems: theory and applications. Aquacult. Eng. 34, 364–376. Zhu, S., Chen, S., 2001. Effects of organic carbon on nitriﬁcation rate in ﬁxed ﬁlm bioﬁlters. Aquacult. Eng. 25, 1–11. Vanesa Díaz is Ph.D. student in Chemical Engineering at Universidad de Cantabria (Spain). She currently holds a research FPI grant sponsored by the Spanish Ministry of Science and Innovation. She obtained her B.Sc Degree in Chemical Engineering and the Master on Sustainable Production and Consumption at the Universidad de Cantabria (Spain) in 2008 and 2009, respectively. She is researcher at the Department of Chemical Engineering and Inorganic Chemistry of the Universidad de Cantabria in new technologies for water reuse, treating and recovering products of food industry. Nowadays, her work is focused on water treatment 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 3 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.
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.