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Revista MVZ Córdoba

Print version ISSN 0122-0268

Rev.MVZ Cordoba vol.19 no.3 Córdoba Sept./Dec. 2014

 

ORIGINAL

Evaluation of a rainbow trout (Oncorhynchus mikyss) culture water recirculating system

Evaluación de un sistema de recirculación de agua para levante de trucha arcoiris (Oncorhynchus mikyss)

Iván Sánchez O,* M.Sc, Wilmer Sanguino O, IPA, Ariel Gómez C, Esp, Roberto García C, IPA.

Universidad de Nariño, Facultad de Ciencias Pecuarias, Departamento de Recursos Hidrobiológicos, Programa de Ingeniería en Producción Acuícola. Ciudad Universitaria, Barrio Torobajo Carrera 22 # 18-109. San Juan de Pasto, Nariño, Colombia.

*Correspondence: iaso@udenar.edu.co

Received: July 2013; Accepted: February 2014.


ABSTRACT

Objective. To evaluate a water recirculation system of rainbow trout fish culture at the recirculating laboratory of the Aquaculture Engineering Production Program of the Universidad of Nariño. Materials and Methods. There were cultured 324 rainbow trout (Oncorhynchus mikyss) fries in 12 plastic tanks of 250 L capacity in a recirculation aquaculture system which treatment system was made up by a conventional sedimentation tank, a fixed bead up flow biofilter with recycled PVC tube pieces used as carrier, and a natural degassing system; the sedimentation unit effluent was pumped to a reservoir tank by a centrifugal 2 HP after passed by gravity through the biofilter and was distributed to the 12 culture units in which there were injected a constant amount of air from a blower. Results. The waste water treatment system removes 31% of the Total Suspended Solids; 9.5% of total ammonia nitrogen, and increased the dissolved oxygen to the final effluent in a 6.5%. It was calculated a biomass increase of 305% on the 75 days, the mortality percentage registered during the research period was of 4.9%. Conclusions. The wastewater treatment system maintained the physic chemical water quality parameters in the recommended values for the specie. The values of weight and size gain, food conversion, mortality and biomass production reported were between the normal values of rainbow trout fish culture in recirculating systems.

Key words: Aquaculture, cultivation, treatment, trout, water Recirculation (Source: CAB, DeCS).


RESUMEN

Objetivo. Evaluar un sistema de recirculación de agua para cultivo de trucha arcoiris en el laboratorio de recirculación del Programa Ingeniería en Producción Acuícola de la Universidad de Nariño. Materiales y métodos. Se cultivaron 324 alevinos de trucha arco íris (Oncorhynchus mikyss) en 12 tanques plásticos de 250 L de capacidad en un sistema de recirculación para acuacultura cuyo sistema de tratamiento estuvo constituido por un sedimentador convencional, un biofiltro de flujo ascendente con medio soporte fijo conformado por segmentos reciclados de tubos PVC, y un sistema de desgasificación natural; el efluente del sedimentador fue elevado a un tanque reservorio por medio de una bomba centrífuga de 2 HP para después pasar por gravedad a través del biofiltro y posteriormente ser distribuido a las 12 unidades de cultivo a las que de manera permanente se inyectó aire proveniente de un blówer. Resultados. El sistema de tratamiento del agua removió 31% de los sólidos suspendidos totales; 9.5% del nitrógeno amoniacal total, e incrementó el oxígeno disuelto al efluente final en un 6.5%. Se calculó un incremento de la biomasa del 345% en los 75 días, el porcentaje de mortalidad registrado durante todo el periodo de estudio fue del 4.9%. Conclusiones. El sistema de tratamiento mantuvo los parámetros físico-químicos de la calidad de agua dentro de los rangos requeridos por la especie. El incremento de peso y talla, la conversión alimenticia, la mortalidad y la producción de biomasa reportaron valores normales para producción de trucha en sistemas de recirculación.

Palabras clave: Acuicultura, cultivo, recirculación del agua, tratamiento, trucha (Fuente: CAB, DeCS).


INTRODUCTION

The high deterioration of productive soils caused by overexploitation processes makes possible discerning that aquaculture will be the future, since development and population growth levels increase every day, requiring nutritive and high quality food; however, the availability and quality of water has been impacted by both natural and anthropogenic activities, leading to the low quality of the liquid and reduced productivity in aquatic ecosystems, therefore water pollution has become a serious problem for the industry (1).

The aspects that limit the growth of aquaculture include the reduction of cultivable water bodies, as well as increased pollution of surface water with harmful chemicals and the eutrophication of rivers and lakes with excesses of nutrients which can lead to various problems such as toxic algal blooms, low concentrations of dissolved oxygen, dead fish and biodiversity reduction (2).

Contaminants may exert their action on aquaculture cultures in any of the following ways: by modifying the hydrobiological characteristics of water; through the direct action of bio-acid substances that may cause physiological changes or high mortality; or by the contamination of animal tissues with bio-toxins, pathogens or chemicals that render animals unusable for consumption (3).

Water is a natural resource whose location and geographic distribution is dramatically affected by anthropogenic actions, an example of this is the alteration in the availability of the liquid due to the climate change induced by human beings (4), which is expressed as the long-term variations in average weather conditions at multiple temporal and spatial scales, and may represent a natural threat, such as floods, droughts, cold or heat waves and storms (5).

Water scarcity and the increasing negative alteration of its characteristics make it necessary to investigate the use of intensive and semi-intensive production techniques for hydrobiological species, such as aquaculture recirculating systems (ARS). ARSs were developed as a technology for intensive fish production and have been mainly used when water availability is limited because they allow recycling between 90 and 99% of the liquid (6). Generally, an ARS consists of mechanical and biological filtration components, cultivation tanks, pumps, and may include additional elements for water treatment and disease control in the system (7).

ARSs are a technology that allows fish culturing at a greater intensity under a completely controlled environment where animals are bred in tanks under the safest possible conditions. In addition, in such systems the reuse of water after its physical and biological treatment is accomplished as a response to the attempt to reduce water and energy needs as well as the emission of nutrients to the environment (8).

Due to the importance of the maintenance of water quality in recirculating systems, virtually all wastewater treatment levels described by von Sperling (9) are applied, because they range from preliminary treatment devices that remove elements that may cause operation and maintenance problems, systems for the removal of suspended solids, to the removal of organic solids, nutrients and disinfection by means of physical, chemical and/or biological processes. In the ARSs, the main application of biological treatment processes is the removal of biodegradable organic substances present in wastewater in both colloidal and dissolved form. The biological treatment is based on a process in which a mixed population of microorganisms uses water pollutants as nutrients, which when in contact for sufficient time allows these microorganisms to break down and eliminate polluting solutes as appropriate.

In year 2010, diadromous fish accounted for 6% of world aquaculture production, of which the trout ranked third with about 0.7 million tons (10). Trout farming requires waters of an excellent quality, in which suspended solids do not exceed 10 mg/l, total ammoniacal nitrogen of less than 1 mg/l, ammonia below 0.02 mg/l, nitrite below 0.1 mg/l, temperatures between 10 and 18°C and dissolved oxygen concentrations between 6 and 8 mg/l (11).

The main objective of this research was to monitor a water recirculating system for rainbow trout cultivation (Oncorhynchus mikyss) in its culturing phase in terms of the efficiency in the removal of solids and ammonium, the contribution to DO concentration and the most important productive variables.

MATERIALS AND METHODS

Location. The research project was conducted in the Living Organisms Laboratory of the Aquaculture Production Engineering program of University of Nariño, located to the northeast of the city of Pasto, Department of Nariño, 01° 09' north latitude, 77° 08' east longitude and an approximate altitude of 2540 m; the multi-year monthly average temperatures of the city vary between 9.3 and 18.1°C (12).

Description of the ARS and its operation. The recirculating system evaluated consisted of 12 circular polyethylene tanks with a maximum individual capacity of 250 liters; two 2 HP centrifugal pumps; a conventional sedimentation rectangular tank; an upflow biofilter made of by a tank with a maximum capacity of 1.0 m3, inside which recycled PVC vertical segments with a diameter of ½” and a waterfall degasser tank were included.

Nine cubic meters of drinking water were taken for the operation of the system from the municipal aqueduct and stored in the underground settler, culture units, elevated tank reservoir and auxiliary tanks for the natural removal of residual chlorine. The water circulation cycle began with the opening of the valves of the elevated tanks (reservoir and its passage to the biofilter). Tanks of 1.0 m3 were arranged for the periodic partial replacement of the liquid in the system in which dechlorinated water was stored.

Water and culture storage units were cleaned, disinfected, and filled and emptied three times before being placed into operation.

The effluent of the 12 culture tanks, collected by a sanitary pipe with a diameter of 3”, passed to a sedimentation tank whose input and output devices were formed by thick-walled weirs. The dimensions of the settler made our from common and waterproof masonry with ceramic veneer and mortar were: total length of 2.3 m and effective sedimentation length of 1.45 m; 0.50 m wide and 0.90 m deep. This device with a surface area of 0.725 m2 and a sedimentation volume of 0.653 m3 was cleaned weekly for the removal of precipitated sludge.

The water, once subjected to pretreatment in the settler, passed to the output chamber which in turn served as the suction chamber of a 2 HP electric plant that subsequently discharged the liquid onto an elevated tank located at a height of 8 m in relation to the initial level of culture tanks; said tank was used as temporary storage from where the liquid was discharged towards the biological treatment unit consisting of the upflow biofilter with a maximum capacity of 1 m3, usable volume of 0.750 m3 and a water surface diameter of 0.85 m (Figure 1A). To ensure the homogenous distribution of the upflow an acrylic plate with holes of 0.5 cm in diameter was arranged, located at the base of the biofilter and which in turn provided support for the vertical tubes that served as supporting base. The biological community colonizing the surface of the tube segments used as support contributes to the transformation of the ammonium produced in the trout culture into nitrites and nitrates.

The effluent of the biofilter passed to a waterfall aeration system that at the same time served as a degassing mechanism for the removal of gases such as CO2. The water was finally transported to the 12 culture units where the input of the liquid was carried out through a perforated standpipe (Figure 1B), which generated a tangential flow and an adequate turbulence level for the needs of the species. Air from a 2 HP blower was injected through diffuser stones to improve DO conditions in tanks.

The general layout of culture and wastewater treatment units is shown in the diagram of figure 2.

Study period. The experiment was divided into two phases:

Pretest. It was carried out between July 1 and September 13, 2008, where operating adjustments were made to culturing and water treatment units, as well as to the protocols for the handling, evaluation and acclimatization of animals.

Test. Performed between September 13 and November 22, 2008, where the operation of the ARS and the variables of productive interest for the cultivation of rainbow trout during the culturing phase were evaluated.

Biological material. In each culture tank 27 rainbow trout fries were cultivated, with an average weight of 32.4 g and a length of 14.2 cm for a total of 324 and came from floating cages of the Intiyaco station of the University of Nariño, located in lake Guamues.

Adaptation of experimental units. To maintain a constant water level in culture tanks to the central drainage system, an external lateral overflow pipe was adapted using concentric pipes (Figure 3A) where the effluent from each tank underwent an upward vertical motion in the inner tube, once water reached its upper edge the edge it operated as a horizontal circular weir that caused the fluid evacuated to flow evenly through the space between the two concentric tubes to finally move towards the main pipe for the collection and transport of effluents. Culturing units were cleaned and disinfected with water and commercial chlorine. They were subsequently washed with drinking water and each tank was filled with a net water volume of 235 liters; then ventilation was provided by means of medium pore diffuser stones. Once the ARS was put into operation, the inflow of water from each tank was regulated with a flow rate of 50 ml/s, favoring a replacement (R) of 0.85 R/hour or 20 R/day during the experiment. Culture units were covered with a mesh to avoid animals jumping out from the tanks (Figure 3B).

Transfer of animals. The fish were moved from the Intiyaco station to the laboratory located at University of Nariño, to this end the animals were subject to a fast for a period of 24 hours, they were then packed in groups of 15 specimens in plastic bags with 10 liters of water and oxygen for their transport and subsequent acclimatization.

Acclimatization and farming. The bags with the animals were placed into the culture units for 15 minutes, then they were opened and the water in the bag mixed with the water of the ARS in order to gradually stabilize pH, DO, temperature and alkalinity values. As a prophylactic treatment 15 g of sea salt was added to each bag for a period of 15 minutes. The acclimated animals were released in each experimental unit for an adaptation period of 15 days, during which time they were fed with food prepared with florfenicol.

Sampling of animals. Samples were taken every 15 days, 12 animals were used which accounted for 44% of the total population of each experimental unit. To facilitate the handling of fish in order to record weight and size, the specimens were tranquilized with a solution of commercial quinaldine at a concentration of 5.0 mg/l, the activity was carried out in a plastic container with a capacity of 12 liters and the exposure time was from 2 to 3 minutes. Finally, the individuals sampled received a prophylactic wash with potassium permanganate at a concentration of 10 ppm.

Feeding. The food provided was based on a commercial concentrate with 48% of crude protein, 2.800 kcal/kg of EM in granules 3.5 mm in diameter. The amount of food to be supplied was calculated taking into account the feed conversion, condition factor, temperature and population for each experimental unit. The calculated ration was distributed in three meals supplied at 7 a.m., 11 a.m. and 6 p.m. recording the relevant data in a logbook.

Water quality monitoring. The methodologies recommended by APHA, AWWA & WEF (13) were adopted for the measurement of water quality parameters monitored. The parameters studied were measured at the entry and exit of the treatment system of the ARS, i.e. in the mixed and homogenized effluent of culture units and at the exit of the biofilter. Eight samples were taken of pH values -Electrometric method: APHA, AWWA and WEF (1998) No. 4500-H+ B-, temperature -Direct reading: APHA, AWWA and WEF (1998) No. 2550 B-, dissolved oxygen (DO) -Sodium azide modified winkler: APHA, AWWA and WEF (1998) No. 4500-O C-, total ammoniacal nitrogen (NAT) -Colorimetric method - APHA, AWWA and WEF (1998) No. 4500 D-, nitrites -Colorimetric method - APHA, AWWA and WEF (1998) No. 4500-NO2 - and nitrates - Cadmium reduction method: APHA, AWWA and WEF (1998) No. 4500-NO3 I-; conductivity values were measured three times -Electrometric method: APHA, AWWA and WEF (1998) No. 2510-, hardness - Titrimetric method: APHA, AWWA and WEF (1998) No. 2340 C-, salinity - Electrical conductivity method: APHA, AWWA and WEF (1998) No. 2520 B-, as well as the analysis of total suspended solids (TSS) at the entry and exit of the conventional sedimentation tank - Gravimetric method: APHA, AWWA and WEF (1998) No. 2540 D-. A portable pH meter EC10 model 50050 HACH, a conductivity meter HACH model CO 150 and a colorimeter HACH DR 700 were used for the measurement of parameters.

System water replacements. The water in the ARS was partially and periodically replaced as follows: every 8 days 10% of the biofilter volume and 100% of the settler volume, every fortnight 80% of each experimental unit. Such substitutions were carried out with previously dechlorinated water; in addition, siphoning was performed to the settler every week for the evacuation of the sediment solids.

Variables evaluated.

Weight increase (IP). Twenty percent of the animals of each replication were sampled at the beginning of the experiment and then every 15 days to determine the weight gain. The variable was calculated based on the equation 1:

IP = Pf - Pi [1]

Where Pf is the final weight and Pi the initial weight in each period.

Length increase (IL). Twenty percent of the animals of each replication were also sampled to determine this variable, under the same conditions for the determination of IP. The increase in length was calculated using equation 2:

IL = Lf - Li [2]

Where Lf is the final length and Li the initial length in each sample.

Mortality percentage (M%). The amount of fish that died during the study period was calculated using equation 3:

M% = (PI - PF)*100/PI [3]

Where PI and PF are the initial and final populations respectively.

Feed conversion (CA). It is estimated using equation 4:

CA = AS / IP [4]

Where AS corresponds to the food supplied and IP the weight increase.

RESULTS

Water quality parameters. In general terms, water quality parameters in culture units remained within the allowable and recommendable ranges for the cultivation of the species. Table 1 shows the most important values related to the behavior of each parameter.

Total ammoniacal nitrogen (TAN). High concentrations of TAN were recorded, with average values of 4.82 mg/L and 4.47 mg/L at the entry and exit of the biofilter, respectively.

Nitrites. Based on the values measured, the average concentration calculated at the entry of the biofilter was 0.53 mg/l and the average value calculated at the exit was 0.61%.

Nitrates. The average nitrate concentrations calculated in the influent and effluent of the biofilter were 2.77 mg/l and 2.60 mg/l, respectively.

Efficiencies of the treatment system.

Removal of solids. An inflow of 36 l/min was regulated in the conventional sedimentation tank, representing a hydraulic retention time (HRT) for the settler of 14.51 minutes and a surface application rate (SAR) of 71.5 m3/m2/d. Based on the results measured, it was determined that the efficiency in the removal of total suspended solids by the system was 31%.

Biofilter performance. The submerged and upflow biofilter with an inflow of 36 l/min, a HRT of 20.83 minutes and a surface application rate of 117.4 m3/m2/d favored the nitrification processes of the ammonia produced in the system and reported an average removal TAN percentage of 9.47%.

Degasser. The degassing for the release of CO2 and the increase in dissolved oxygen of the system reported an average percentage increase in the DO concentration in water of 6.5%.

Productive Variables.

Weight increase. On average, the initial weight in the culturing phase was 32.45±2.20 g and the final weight obtained after 75 days of study was 111.81±8.79 g; the average values obtained in six surveys are shown in Figure 4. Average weight increases calculated were 1.06 g/day and 15.90 g/fortnight.

Length increase. On average, the initial length in the culturing phase was 14.22±0.46 cm and the final size obtained after 75 days was 20.50±0.37 cm, as shown in figure 5.

Feed conversion. On average, feed conversion during the phase analyzed was 1.82:1, with a maximum value of 2.91:1 and 1.0:1.0 as minimum value.

Mortality. During the research, a total mortality of 4.9% was recorded in the study period, mainly in the final stage. This situation could occur due to higher biomass per cubic meter, which increased the DO consumption and ammonium production with the consequent deterioration in water quality.

Biomass production. The initial biomass was on average 3.49 kg/m3 for each experimental unit, increasing up to a maximum value of 12.08 kg/m3.

DISCUSSION

During the experiment, pH fluctuated within the ranges recommended for aquaculture (11), the cultivation of rainbow trout (14) and the sound performance of the fixed bed biofilter (15).

Water temperatures recorded in the ARS were within the optimal range for the species as recommended by Timmons and Ebeling (11), providing favorable conditions for growth and development.

The average DO concentration stood near the lower range recommended for the proper growth of trout (11) and above the minimum value recommended for systems with fixed bed biofilters (15).

During the experiment, hardness values were lower than 100 mg/l recommended for aquaculture (11), even though they were above the minimum acceptable value of 20 mg/l for trout (14). Alkalinity, which exerts a marked influence on the biochemical processes developed in the ARS - such as nitrification, which consumes alkalinity (16) - decreased gradually with the passage of time and recorded values lower than the optimum for the cultivation of the species (14), therefore it is recommended to maintain their levels stable and prevent possible effects on the biofiltration system in subsequent experiences.

The average value for conductivity is within the normal ranges for natural fresh water from 20 to 1000 µmho/cm (17), providing conditions similar to those for the development of the species in nature.

While TAN concentrations in the ARS exceeded the maximum value of 1.0 mg/l recommended for trout growing (11, 18), there were no deaths that could be attributed to such situation since these concentrations together with pH values close to neutrality and the low temperatures registered ensured that ammonia was practically present in ionized form (9). Differences in concentrations at the entry and exit of the treatment system showed the nitrification process.

The highest concentrations of nitrites in the effluent of the biofilter, as compared to those recorded in its affluent, ratify the development of the nitrification process in this unit, as the increase in concentration and the decrease in total ammoniacal nitrogen values indicate the transformation of ammonia into nitrites. Furthermore, the values for nitrates were within the ranges recommended for aquaculture (11) and in particular for trout (14).

In Colombia, Standard RAS (19) stipulates that for high rate trickling filters, removal efficiencies of NH3-N for domestic wastewater commonly range between 8 and 15% for surface application rates in plastic mediums between 14 and 84.2 m3/m2/d. The relatively low removal value obtained from the experiment may be due to the type of substrate used for the formation of the biofilm of nitrifying bacteria - smooth recycled PVC pipe segments with a diameter of ½” - represented a smaller contact surface to that produced by other support mediums, although the material used in this research is much cheaper and environmentally friendly due to its recycled condition. Al-Hafedh et al (20) investigated the use of various plastic support mediums, such as segments of corrugated PVC pipes in trickling biofilters for tilapia culture in ARS under a HRT of 112.5 minutes and reported a NAT removal efficiency in the order of 25.5%; moreover, Lekang and Kleppe (21) reported efficiencies exceeding 40% for trickling biofilters with surface application rates of 91 m3/m2/d with granular support mediums in expanded clay and synthetic mediums with a specific surface area much higher than that offered by the pipe segments used in this experiment.

In terms of design parameters and the performance of conventional settlers for the treatment of wastewater, Spellman (22) reported SS removal efficiencies between 40 and 60% for units with SAR between 12.2 and 48.8 m3/m2/day and HRT between 1.5 and 2.5 hours, Romero (23) recommended a SAR between 24 and 33% m/day and HRT between 1 and 2 hours for the design of primary sedimentation tanks to obtain a removal of SS between 50 and 70%, and Title E of the Standard RAS (19) defined an interval between 50% and 65% as the range of efficiencies for the removal of SS in primary sedimentation tanks with surface overflow rates of 33 m3/m2/d and a minimum HRT of one hour.

In previous cases, the SAR recommended are lower and the HRT greater than those applied in the settler used in this research, which explains the lower removal efficiency of TSS in relation to the values expected according to the literature. However, the settler was within the design parameters recommended by Lekang (24) for aquaculture systems with SAR between 24 and 120 m3/m2/d and HRT close to the lower value suggested of 15 to 40 minutes; additionally, the settler operated under the SAR range recommended by Timmons and Ebeling (11) for this type of units with a SAR from 24 to 94 m3/m2/d to obtain TSS removal efficiencies between 40 and 60%.

The removal of 31% of the TSS produced by system could obey to the low HRT of the treatment unit, the relatively high surface application rate, and the limited distribution uniformity of the flow produced by the thick wall weir used as input device to the settler. For subsequent experiences, it will be necessary to adapt an inlet system to the sedimentation unit such as for example a perforated screen that favors a homogeneous affluent flow and thus optimize the volume used for the removal of solids. The implementation of this type of devices allows the optimization of the volume allocated to the sedimentation of particles, prevents the presence of dead zones or preferential flows and improves the performance of these treatment units; while it is true that the system managed to maintain TSS levels below 80 mg/L, the maximum standard value recommended for aquaculture (11) exceeded that required for trout, where water with low turbidity is recommended for its cultivation (14).

During the research, DO levels reported upon entry to culture units - provided by the degassing device by hydraulic drop - remained above 6.0 mg/l, hence conforming to the values recommended for trout (11), (14). In order to obtain greater efficiencies in the transfer of atmospheric oxygen from free-flow hydraulic systems, it is possible to use another natural ventilation mechanism such as rectangular and especially triangular weirs, whose high efficiency has been tested by Baylar and Bagatur (25) and tried in hydraulic recirculating systems by Baylar et al (26).

The best results regarding weight increase occurred between sampling 3 and 4. The growth curve raised 83% of the time recorded in the study; the decline in values between samplings 2 and 3 can be explained by the variety of animal weights within the population sampled, which could affect the average value.

As reference data, the weight gain for trout at a temperature of 16°C is 0.92 g/day or 28.1 g/month (11), values lower than those recorded in this study were where increments of 1.13 g/day and 34.47 g/month were obtained. According to Observatorio de Agrocadenas de Colombia (27), the production cycle for trout is 9.75 months. Usually, its cultivation begins with fingerlings of 2 g and ends with animals with an average weight of 307 g, for an increase of 1.04 g/day and 31.32 g/month, values surpassed by this study in about 2%.

The daily increase of size calculated from the data reported was 0.084 cm/day for a periodic increment of 1.26 cm/fortnight. The best results in terms of increased size occurred in samplings 3 and 4 showing a steeper slope in Figure 5. The effect of the diversity in the size of animals measured in the third sampling was once again evidenced.

The longitudinal growth of trout is directly related to the temperature of the water, which in this study was on average at 19.99°C; by applying the growth formula proposed by Timmons and Ebeling (11), animals should have a minimum growth of 0.088 cm/day or 2.63 cm/month; in this research the growth calculated was 0.084 cm/day or 2.52 cm/month, 4.5% lower than those expected according to the reference quoted.

The average production cycle of trout is 9.75 months (27), starting with 5 cm fingerlings and ending with animals of 29 cm in length, for an increase in size of 0.082% cm/day or 2.46 cm/month, values exceeded in this study by 7.3%.

The more favorable values for feed conversion are similar to those reported by Arredondo et al (28) and by van Rijn (29) in ARSs. The variability observed in feed conversion can be explained by the progressive increase in biomass and the increased load per unit volume in each experimental unit throughout the study, which could cause stress in fish, increased DO consumption and the eventual deterioration of certain water quality parameters. This reduces the efficiency of animals to convert balanced food into biomass, since part of the protein supplied could be used in basal metabolism processes and not in the construction of tissues.

In conclusion, the water treatment system evaluated allowed maintaining physicochemical water quality parameters within the ranges required by the culturing phase of rainbow trout. Wastewater treatment units allowed to remove 30% of TSS (conventional sedimentation tank), allowed a 9% removal of TAN (biofiltration unit), and increased dissolved oxygen to the final effluent in 6% (degasser).

The productive variables analyzed: weight and size increase, feed conversion, mortality and production; were within the normal values for the production of the species cultivated. The recirculating system evaluated commenced with a load of 3.5 kg/m3 and ended with a load of 12.1 kg/m3, indicating an increase in biomass of 345%. The mortality percentage recorded during the entire study period was 4.9%.

Acknowledgements

To the VIPRI of University of Nariño, to Professor Roberto Salazar Cano and the students of the Aquaculture Production Engineering Program: Karen Larrañaga; Viviana Cardenas; Carlos Caicedo; Luis Enriquez; Diego Miramac; Adriana Arce; Silvia Bolaños; Felix Jojoa; Lorena Ortega; Diana Beltran and Nataly Sarasty.


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