Environmental Sustainability of Nile Tilapia Reared in Biofloc Technology (BFT) System: Evaluation of Carbon, Nitrogen, and Phosphorus Dynamics and Indicators of Sustainability

Abstract

This study aimed to evaluate the dynamics of total carbon (TC), total nitrogen (TN), total phosphorus (TP), and some indicators of environmental sustainability of Nile tilapia reared in a biofloc technology (BFT) system. Nile tilapia fingerlings were cultured in three BFT units of production (4.2 m³ each) at a stocking density of 395 fish/m³. After 70 days, the survival rate was 98.05%, with a final average weight of 20.43 g, and apparent feed conversion of 1.05. Nutrient inputs were from supply water, initial fish biomass, and feed; outputs were measured from the final fish biomass and effluent. TC, TN, and TP concentrations in the water increased linearly over time (p < 0.001) by 1.54, 1.66, and 0.44 mg/L, respectively. Feed contributed over 88% of nutrient inputs, while fish final biomass accounted for over 50% of output. Nutrient retention in fish final biomass was 29.74% (TC), 45.38% (TN), and 46.34% (TP). The system had low eutrophication potential, estimated at 57.39 kg TC, 20.02 kg TN, and 5.70 kg TP per ton of fish. Water use was minimal (0.0074 m³/ton), but energy demand was high (114.59 MJ/kg). The closed system reduces biodiversity risks by preventing fish escape. In conclusion, BFT supports high fish productivity with efficient nutrient use, minimal water use, and limited environmental impact, contributing to environmentally sustainable aquaculture.

1. Introduction

The increasing global population and demand for high-quality food have led to the growth of aquaculture [1]. To meet this rising demand, aquaculture systems have become more intensive, which consequently generates higher levels of nitrogen, phosphorus, and organic matter (feces and uneaten food) that compromise water quality [2], and negatively impact the aquatic environment.

A common management practice in traditional aquaculture is to use a high rate of water renewal to remove excess nutrients and organic matter, ensuring adequate water quality and animal welfare during the production cycle [3]. However, this also results in the continuous release of nutrient-rich effluent into the environment, which can increase the negative impact of aquaculture [4].

Sustainable aquaculture focuses on environmentally friendly production to meet current demands for food production without compromising the environment and future generations. The biofloc technology (BFT) system fulfills some of these requirements, as it allows minimal or zero water renewal by promoting the activity of microorganisms that assimilate and metabolize residues generated in the system, thereby maintaining adequate water quality [5,6], making water reuse possible with biosafety in consecutive production cycles [7]. In addition, protein flakes from BFT can be consumed by fish as a complementary food source [8,9,10].

The methods for evaluating environmental sustainability are essential tools in decision-making and the improvement of production systems. These methods include life cycle analysis, which estimates resource utilization and potential impacts throughout the production cycle [11]; emergy analysis, which quantifies the energy required in some production processes [12,13]; and carbon footprint, which estimates the total greenhouse gases emitted directly or indirectly by a system [14]. However, these methods require a substantial amount of data for assessment [15]. Additionally, these methods do not fully integrate the various aspects that encompass aquaculture sustainability, such as water use, nutrient and organic matter utilization and release, the impacts of feed supply, and disease introduction. Consequently, it is essential to explore more holistic sustainability assessment methods to facilitate decision-making regarding the improvement of the production process [16].

The dynamics of nutrients is a straightforward method for determining the transformation processes of various compounds in aquatic systems [17]. The mass balance stands out as a quantitative framework for estimating the input, output, accumulation, and loss of nutrients within a production system [18]. The fundamental principle of the mass balance approach is the law of mass conservation, which states that the total amount of a nutrient entering a system must equal the sum of the amounts retained, transformed, or lost through various outputs [19]. Therefore, the mass balance approach provides comprehensive insight into resource utilization and system sustainability. Despite its significance, the application of mass balance techniques to quantify key nutrients, such as carbon, nitrogen, and phosphorus, in BFT systems remains limited, underscoring the need for further research in this area.

Currently, a set of specific indicators has been developed to assess the environmental sustainability of aquaculture systems. These indicators typically encompass resource use efficiency (e.g., water, energy, and feed), nutrient recycling, pollutant discharge potential, spatial occupation, and biodiversity conservation. They are intended to provide an integrated and practical evaluation of multiple environmental impacts, helping to identify strengths and weaknesses and to guide improvements [15]. Such frameworks are especially relevant for evaluating intensive systems like biofloc technology, which aim to optimize nutrient utilization and minimize environmental discharge. To the best of our knowledge, no studies to date have applied these environmental sustainability indicators specifically to evaluate the production of Nile tilapia juveniles in BFT systems.

Thus, the objective of this study was to evaluate the dynamics of total carbon (TC), total nitrogen (TN), total phosphorus (TP), and some indicators of environmental sustainability of Nile tilapia juvenile reared in a BFT system.

2. Materials and Methods

2.1. Ethical Statement

The study was conducted at Itaipu Binacional (Foz do Iguaçu, Paraná, Brazil). All experimental procedures were approved by the Ethics Committee for the Use of Animals of the State University of Mato Grosso do Sul (protocol no 006/2020), in accordance with the Brazilian Council for the Control of Animal Experimentation (CONCEA).

2.2. Fish, Experimental Conditions, and Feeding

Male Nile tilapia fingerlings (n = 4977; initial average weight 3.42 ± 0.07 g) were obtained from a commercial fish farm, classified, and them randomly distributed in equal numbers among three outdoor units of production with a biofloc technology (BFT) system. Each unit of production comprised a tank with a useful volume of 4.2 m³ and was placed inside a greenhouse (Figure 1A). The average stocking density was maintained at 395 fish/m³ (n = 1659 per unit). Within each production unit, one-third of the total volume consisted of matured inoculum sourced from a previously stabilized BFT system, while the remaining two-thirds were filled with clean water from an artesian well. To promote biofloc development, sugar was added as a supplemental carbon source, maintaining a carbon–nitrogen (C:N) ratio of 12:1. Continuous aeration was provided by a 0.7 kW air compressor connected to a microporous hose system to ensure adequate oxygenation and the suspension of solids.

During the experimental period of 70 days, fish were fed a 2 mm extruded experimental diet containing 28% crude protein (25.6% digestible protein) and 3200 kcal/kg digestible energy [10]. The feeding rate varied between 3 and 5% of the biomass, which was provided four times (8:00 A.M., 11:00 A.M., 2:00 P.M., and 4:00 P.M.) daily during the experiment. Biweekly, fish samples were collected from each production unit for biometric evaluation. The fish were anesthetized by immersion using benzocaine at a concentration of 100 mg/L [10] and individually weighed to estimate biomass and adjust the feeding rate.

2.3. Growth Performance Variables

At the end of the experimental period, growth performance was evaluated based on the following variables:

• Survival rate (SR; %) = (final number of fish/starting number of fish) × 100;
• Weight gain (WG; g) = final weight (g) − initial weight (g);
• Apparent feed conversion (AFC) = feed consumption (g)/weight gain (g);
• Specific growth rate (SGR; %/day) = 100 − [ln final weigh (g) − ln initial weight (g)/trial period].

2.4. Water Quality Monitoring

The temperature (°C), dissolved oxygen (DO), and water salinity were measured twice daily (8:00 am and 2:00 pm) using a multiprobe device (YSI^® ProDSS model, Yellow Springs, OH, USA). Ammonia (${\mathrm{N}\mathrm{H}}_3$), nitrite–nitrogen (${\mathrm{N}\mathrm{O}}_2^{-}$-N), nitrate–nitrogen (${\mathrm{N}\mathrm{O}}_3^{-}$-N), and phosphate (${\mathrm{P}\mathrm{O}}_4^{3-}$) were measured weekly using a colorimetric kit (Hanna Instruments^®_, Woonsocket, RI, USA). Alkalinity (as ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{o}}_3$) was determined weekly following the methodology proposed by Golterman et al. [20], and the pH using Digimed^® equipment (DM-22 model, São Paulo, SP, Brazil). Chlorophyll-α was determined every fifteen days in accordance with the methodology proposed by Lorenzen [21].

Sedimentable solids (SS) and total suspended solids (TSS) were analyzed weekly following the methodology of Avnimelech [8] and Strickland and Parsons [22], respectively. When SS levels exceeded 40 mL L⁻¹, the tank water was pumped into 200 L conical settling tanks to remove excess solids. The quantity of solids removed was quantified and analyzed through sampling to control nutrient output. After decanting the solids, the supernatant water was returned to the tanks, and the removed volume was replaced with supply water. No change of water occurred during the study, except for the replacement of the amount lost by evaporation and the removal of solids.

2.5. Nutrient Quantification in the System

To calculate the mass balance of total carbon (TC), total nitrogen (TN), and total phosphorus (TP) within the production unit, the input of nutrients was considered from the supply water, the initial biomass of fish, and the feed-diet (Figure 1B) during the experimental period. The output of nutrients was determined based on the amounts retained within the final fish biomass and those present in the effluent (Figure 1B). The effluent comprised the removal of both settled solids and cultivation water from the production unit, including the suspended microbial biomass (bioflocs) naturally formed during the culture period.

Water samples from the water supply and each unit of production were collected weekly for TC, TN, and TP concentration analysis. All samples were placed in plastic containers, identified, sealed, and refrigerated (±4 °C) until analysis. TC, TN, and TP content in the water samples were analyzed according to the methodology proposed by Baird and Bridgewater [23].

The nutrient input in the form of feed was calculated based on the total amount of feed provided during the study period. A sample of the diet was analyzed to determine the levels of TC, TN, and TP, as well as dry matter (DM), ether extract (EE), crude protein (CP), crude fiber (CF), and mineral matter (MM), in accordance with the methodology specified by AOAC [24].

To determine the concentration of nutrients retained in the fish, samples of approximately 100 g were collected from each production unit at the beginning and end of the experimental period. The fish in the samples were descaled and eviscerated. These fish samples were subsequently dried in an oven at 55 °C for a duration of 72 h, or until a constant weight was achieved, in order to determine their dry mass and analyze their chemical composition. The analysis of TC, TN, and TP content in the fish samples was conducted in accordance with the methodology specified by AOAC [24].

The removal of excess solids carried out during the experimental period and the final water were considered as effluent. The solid samples were subjected to drying in an oven at 55 °C for a period of 72 h until a constant weight was reached. The determination of TC, TN, and TP contents in the dried solids samples was carried out using the AOAC [24].

The final water samples were analyzed for TC, TN, and TP contents using the method of Baird and Bridgewater [23]. Additionally, to evaluate the nutritional composition of the bioflocs in the BFT system, the suspended solids from these water samples were separated by evaporation in an oven at 55 °C. The dried biofloc biomass was then analyzed for DM, EE, CP, CF, MM, TC, TN, and TP contents [24].

2.6. Nutrient Mass Balance

Nutrient mass balance was calculated based on the methodology proposed by Silva et al. [25] and Sahu et al. [26]. The following equations were used for nutrient input:

• Initial water (IW) = TC, TN, or TP concentration analyzed in the initial water (mg L⁻¹) × tank water volume (L);
• Water replacement (WR) = TC, TN, or TP concentration analyzed in the water supply (mg L⁻¹) × volume of water used (L);
• Initial biomass of fish (IB) = TC, TN, or TP concentration analyzed in the initial carcass (g kg⁻¹) × initial biomass of fish (kg);
• Feed (F) = TC, TN, or TP concentration analyzed in the feed (g kg⁻¹ based on dry matter) × total amount of feed provided (kg);
• Total nutrient input (TNI) = IW + WR + IB + F.

For nutrient output, the following were considered:

• Final water (FW) = TC, TN, or TP concentration analyzed in final water (mg L⁻¹) × tank water volume (L);
• Solids removed (SOR) = TC, TN, or TP concentration analyzed in the solids removed during the test (g kg⁻¹) × volume of solids discarded (L);
• Final biomass of fish (FB) = TC, TN, or TP concentration analyzed in the final carcass (g kg⁻¹) × final biomass of fish (kg);
• Total nutrient output (TNO) = FW + SOR + FB.

The unaccounted nutrient fraction (UNF) was determined using the following equation: UNF = TNI (TC, TN, or TP) − TNO (TC, TN, or TP).

The percentage of nutrient retention in fish biomass was calculated according to the respective equation provided by the NRC [27]: Retention rate (%) = 100 × [(final weight × final nutrient (TC, TN or TP)) − (initial weight × initial nutrient (TC, TN or TP)]/nutrient intake in the feed.

2.7. Indicators of Sustainability

To assess the environmental sustainability of production in a biofloc system, a set of indicators proposed by Boyd et al. [28] and Valenti et al. [15] were employed. These indicators were categorized into different categories, including use of natural resources, efficiency of use, release of pollutants, and conservation of species biodiversity, as described in

Table 1. Indicators of environmental sustainability applied to evaluate Nile tilapia juveniles produced in BFT system.

Category	Indicator	Formula
Use of the resources	Use of space 1	E = area used (hectare)/production (tons)
	Use of water 2	A = volume used (m3)/production (tons)
	Use of energy 2	EN = pump power (hp) × aeration time (h.) × 0.745 kW/hp/production (kg) × 0.9 (MJ kg −1)
	Use of nutrients 1	U = nutrients applied (kg)/production (tons)
Efficiency in using resources	Production actually used 1	PEU (%) = (production − not used) × 100
	Efficiency in the use of nutrients 1	EU (%) = (nutrient mass in fish/nutrient mass applied) × 100
Release of pollutants	Eutrophication potential—carbon, nitrogen, and phosphorus released into water 1	PE = mass of nutrient released in the effluent (kg)/total production (tons)
Risk of production for the conservation of genetics and biodiversity	Produced species risk—increasing levels of impact according to the organism produced 1	REC = 1, 2, 3, 4, 5, 6 or 8

¹ Valenti et al. [15]; ² Boyd et al. [28].

. The conservation of local biodiversity was evaluated across different categories, with assigned values ranging from 1 to 8 based on the classification proposed by Valenti et al. [15].

2.8. Statistical Analysis

Descriptive statistics were conducted, considering the mean and standard deviation of the three units of production in the BFT system, to evaluate the growth performance, chemical composition of the flakes, mass balance of nutrients, and sustainability indicators. In addition, regression analysis (using first and second-degree linear models) was performed to examine the behavior of water quality variables and the accumulation of carbon, nitrogen, and phosphorus over the days of cultivation. The Least Squares Method was employed to estimate the regression equation, aiming to minimize the distances between the observed data points and the corresponding points on the fitted regression line [29]. Moreover, the F-test was applied to assess the significance of the regression coefficient estimates at the 1% probability level for committing a Type I error, using the three experimental units as replicates for each day of cultivation. The best regression model was selected based on the significance of the regression coefficients, the magnitude of the coefficients of determination (R²), and the behavior of the variables under study. All analyses were conducted using R software version 4.4.1 [30].

3. Results

3.1. Water Quality

During the experimental period, the mean values ± standard deviation for the physicochemical water quality parameters were as follows: temperature at 22.52 ± 3.18 °C, dissolved oxygen at 7.99 ± 0.68 mg/L, pH at 7.47 ± 0.32, alkalinity at 71.76 ± 19.42 mg CaCO₃/L, and settleable solids at 35.53 ± 21.96 mL/L. The concentrations of ${\mathrm{N}\mathrm{H}}_3$, ${\mathrm{N}\mathrm{O}}_3^{-}$-N, TSS, and phosphate (${\mathrm{P}\mathrm{O}}_4^{3-}$) increased linearly over time (p < 0.001), reaching their highest levels at the end of the cultivation period (Figure 2A,C–E). In contrast, ${\mathrm{N}\mathrm{O}}_2^{-}$-N exhibited a quadratic effect (p < 0.001), reaching a maximum value at 37 days, followed by a subsequent decrease (Figure 2B). The concentration of chlorophyll-a decreased linearly (p < 0.001) with the biofloc maturation (Figure 2F).

3.2. Nutrient Quantification

TN, TP, and TC concentrations in the culture water increased linearly over time (Figure 3A–C), with daily increases of 1.66 mg/L (p < 0.001), 0.44 mg/L (p < 0.001), and 1.54 mg/L (p < 0.001), respectively.

The chemical composition of the feed provided to the fish during the experimental period, as well as the composition of the dried biofloc biomass collected at the study’s conclusion, is detailed in

Table 2. Chemical analysis of feed used and biofloc from the units of production.

Variables 1	Feed	Biofloc 2
Carbon (%)	42.9	33.97 ± 1.16
Nitrogen (%)	4.55	4.64 ± 0.23
Phosphorus (%)	1.01	2.62 ± 0.17
Crude Protein (%)	28.44	29.98 ± 1.40
Crude Fiber (%)	4.18	6.17 ± 0.68
Ether extract (%)	5.54	0.81 ± 0.13
Mineral Matter (%)	6.94	27.67 ± 1.66

¹ Values based on dry matter. ² Proximate compositions (mean values ± standard deviation) of dried biofloc biomass obtained by evaporating water samples from the three units of production.

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3.3. Mass Balance

The mass balance, detailing the input and output flows of TC, TN, and TP, is presented in

Table 3. Nutrient mass balance in the production of Nile tilapia juveniles in a biofloc system for 70 days.

	Total Carbon (TC)		Total Nitrogen (TN)		Total Phosphorus (TP)
	Mean ± SD (kg) 1	% 2	Mean ± SD (kg) 1	% 2	Mean ± SD (g) 1	% 2
Initial water + replacement	0.36 ± 0.06	2.30 ± 0.23	0.09 ± 0.02	4.93 ± 0.76	0.12 ± 0.03	0.03 ± 0.01
Feed	14.75 ± 0.85	94.35 ± 0.18	1.56 ± 0.09	88.36 ± 0.54	399.84 ± 23.06	89.26 ± 0.27
Fish initial biomass	0.52 ± 0.02	3.35 ± 0.09	0.12 ± 0.00	6.71 ± 0.23	47.96 ± 1.79	10.72 ± 0.27
Total input	15.63 ± 0.92	100.00	1.77 ± 0.11	100.00	447.92 ± 24.68	100.00
Effluent (final water + solids)	2.58 ± 0.38	34.32 ± 3.07	0.67 ± 0.07	44.84 ± 3.00	194.92 ± 11.35	45.53 ± 1.33
Fish final biomass	4.91 ± 0.35	65.68 ± 3.07	0.83 ± 0.02	55.16 ± 3.00	233.49 ± 18.95	54.47 ± 1.33
Total output	7.49 ± 0.62	100.00	1.50 ± 0.06	100.00	428.42 ± 30.30	100.00
Unaccounted portion	8.15 ± 0.31		0.27 ± 0.17		19.50 ± 9.85
Retention rate (%)		29.74 ± 1.42		45.38 ± 2.76		46.34 ± 1.72

¹ Mean values ± standard deviation (SD) of inputs and outputs of carbon (in kg), nitrogen (in kg), and phosphorus (in g) considering the three units of production; ² percentage based on average values of the three units of production.

. For clarity, each compartment’s percentage fluxes for nutrient inputs and outputs were mapped based on the mean values calculated from the three units of production.

Throughout the experimental period, the primary nutrient input was from the feed, constituting 94.35% TC, 88.36% TN, and 89.26% TP of the total nutrient input (Table 3). The initial biomass of juveniles contributed 3.35% TC, 6.71% TN, and 10.72% TP of the total nutrient input (Table 3). Water replacements represented 2.30% TC, 4.93% TN, and 0.03% TP of the total nutrient input (Table 3).

At the end of the cultivation period, the majority of nutrients were retained in the fish biomass, accounting for 65.68% TC, 55.16% TN, and 54.47% TP of the total nutrient output (Table 3). The effluent, which includes the sum of nutrients retained in the final water and solids removed during the 70 days, constituted 34.32% TC, 44.84% TN, and 45.53% TP of the total nutrient output (Table 3). Within the total nutrients retained in the effluent, 59.69% TC, 90% TN, and 99% TP were found in the final water of the system, while the remainder was effectively removed through solids management.

The BFT system showed efficient nutrient use, with retention rates of 29.74% of TC, 45.38% of TN, and 46.34% of TP (Table 3). The unaccounted amount of nutrients was estimated at approximately 8.15 kg of TC, 0.27 kg of TN, and 19.50 g of TP (Table 3), representing the difference between inputs and outputs.

3.4. Sustainability Indicators

The environmental sustainability indicators of the BFT system are detailed in

Table 4. Indicators of sustainability used to evaluate the production of tilapia juveniles in the BFT system.

Category	Indicators	Cages 1	BFT 2
Use of resources	Use of space (ha ton−1)	0.0014	0.0150 ± 0.0008
	Use of water (m3 ton−1)	0.7540	0.0074 ± 0.0005
	Carbon use (kg of TC ton−1)	700.00	442.47 ± 12.77
	Nitrogen use (kg of TN ton−1)	77.50	46.63 ± 1.36
	Phosphorus use (kg of TP ton−1)	18.25	10.24 ± 0.05
	Use of energy (MJ kg−1)	0.028	114.59 ± 6.95
Efficiency in using resources	Efficiency in the use of TC (%)	-	29.74 ± 1.41
	Efficiency in the use of TN (%)	25.82	45.56 ± 2.86
	Efficiency in the use of TP (%)	16.87	46.56 ± 1.62
	Production actually used (%)	100.00	100.00 ± 0.00
Release of pollutants	TC eutrophication potential (kg ton−1)	-	57.39 ± 7.64
	TN eutrophication potential (kg ton−1)	59.50	20.02 ± 2.74
	TP eutrophication potential (kg ton−1)	22.00	5.70 ± 0.50
Risk of production for the conservation of genetics and biodiversity	Produced species risk	5	4

¹ Mean values obtained from tilapia production in net cages [31]; ² mean values ± standard deviation considering the three units of production.

. Additionally, reference values for Nile tilapia production in cages, as established by Fialho et al. [31], have been incorporated into Table 4.

In the ‘resource use’ category, BFT demonstrates negligible water consumption compared to the production of tilapia in cages; however, it requires more space and energy (Table 4). Concerning ‘efficiency in using resources’, BFT has greater efficiency in the use of carbon, nitrogen, and phosphorus, resulting in lower nutrient usage per ton of tilapia compared to the cage production system (Table 4). In the ‘release of pollutants’ category, BFT shows a lower potential for eutrophication than production in cages (Table 4). Assessing the ‘Risk of production for the conservation of genetics and biodiversity’, BFT carries a risk level of 4 due to its closed system, irrespective of the species produced (Table 4). Conversely, production in cages holds a risk level of 5 due to the potential escape of tilapia into the environment (Table 4).

3.5. Productive Performance

After 70 days of culture in the BFT system, the growth performance of Nile tilapia juveniles from each unit of production is presented in

Table 5. Growth performance of Nile tilapia juveniles after 70 days of cultivation in BFT system.

Variables	Unit of Production			Mean ± SD 1
	BFT 1	BFT 2	BFT 3
Survival rate (%)	98.19	95.96	100.00	98.05 ± 2.02
Initial weight (g)	3.46	3.34	3.45	3.42 ± 0.07
Final weight (g)	21.80	19.77	19.73	20.43 ± 1.19
Weight gain (g/fish)	18.34	16.43	16.27	17.01 ± 1.15
Apparent feed conversion	1.03	1.03	1.08	1.05 ± 0.03
Specific growth rate (%/day)	3.06	2.74	2.71	2.84 ± 0.19

¹ Mean values ± standard deviation (SD) considering the three units of production.

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4. Discussion

4.1. Water Quality

The BFT system effectively maintained the physical–chemical parameters of the water in the present study, with all variables within the favorable range for tilapia juvenile production [32]. The continuous supply of feed and the accumulation of feces and other excreta can lead to increased concentrations of nitrogen, phosphorus, and organic matter, rich in carbon [33]. Excess nutrients in production systems can have a negative impact on cultured fish [32]. In a BFT system, the addition of carbon sources can help to recycle nutrients through the action of heterotrophic microorganisms [26,34]. A high carbon (C)–nitrogen (N) ratio stimulates the growth of heterotrophic bacteria in the water of the system, which assimilate and mobilize nutrients into microbial biomass [7]. In the present study, the C–N ratio was maintained between 10 and 12:1 [35], suggesting that the nutrients from the units of production were mobilized into microbial protein.

Nitrogen compounds are also removed in BFT through the nitrification process, which is carried out by chemoautotrophic bacteria through the oxidation of nitrogen compounds [36]. In this study, nitrogen compound concentrations oscillated over time but remained within the recommended limit for tilapia production [32]. The concentration of ${\mathrm{N}\mathrm{H}}_3$ increased linearly over time, followed by an increase in ${\mathrm{N}\mathrm{O}}_2^{-}$-N concentrations, with peaks at 37 days and a subsequent decrease, and a gradual increase in ${\mathrm{N}\mathrm{O}}_3^{-}$-N concentration. These fluctuations in ${\mathrm{N}\mathrm{O}}_2^{-}$-N and the accumulation of ${\mathrm{N}\mathrm{O}}_3^{-}$-N over the experimental period indicate the activity of nitrifying bacteria and the stabilization of compounds [17,26].

The protein content in the diet is a factor that influences the accumulation of nutrients in production systems, as the deamination of amino acids in feed releases ammonia, the main product excreted in the water of the system. Therefore, a low-protein diet (28% CP) was used, as recommended by Hisano et al. [10]. This resulted in low concentrations of nitrogenous compounds, as there was no excess of protein to potentiate nitrogen excretion by the animals. According to Hisano et al. [10], the use of flakes can reduce the protein content of the diet, which consequently decreases the input of nutrients into the system and feed costs and improves the sustainability of the production system.

The application of carbonates may be necessary for the BFT to maintain ideal conditions for the nitrification process and promote the buffering effect of the system [37]. According to Ebeling et al. [35], for each gram of ${\mathrm{N}\mathrm{H}}_3$ converted into ${\mathrm{N}\mathrm{O}}_3^{-}$-N, 7.05 g of calcium carbonate (${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{o}}_3$) is consumed, and hydrogen ions are released. Throughout the experimental period, a decline in ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{o}}_3$ values were observed. However, pH values remained constant, demonstrating the buffering capacity of the system, without the need for supplemental carbon sources.

Chlorophyll concentration values declined over time. Nile tilapia has a filtering habit and, therefore, can efficiently consume phytoplankton in production systems [38]. Thus, the decrease in chlorophyll concentrations may be related to phytoplankton consumption by fish [39,40]. Additionally, during the stabilization of the BFT system, a shift occurs from high algal concentrations to increased bacterial dominance [41]. This may also be related to the decrease in chlorophyll, as the increase in particulate matter in suspension (TSS) reduces the incidence of solar radiation. However, the amounts of solids in the system were controlled to avoid harmful effects on the species. The volume of SS ranged from 40 to 70 mL/L, as recommended by Schryver et al. [5], and TSS concentrations remained below the maximum recommended limit of 500 mg/L [42].

4.2. Nutrient Balance

The nutrient mass balance analysis showed the efficient utilization of nutrients by fish biomass in a BFT system. Over a period of 70 days, it became evident that the tilapia juveniles reared in this system displayed a remarkable capacity for nutrient retention. In comparison to previous findings, the nutrient retention rates in this study surpass those observed in water recirculation systems, where tilapia juveniles retained approximately 28.42% of nitrogen and 27.05% of phosphorus [43]. Similarly, laboratory tests investigating BFT mass balance revealed that tilapia juveniles retained between 44% and 54% of nitrogen and 30% to 33% of phosphorus in their biomass [44].

Nitrogen and phosphorus exhibited the highest retention rates, with values of 45.38% and 46.34%, respectively. Carbon, conversely, exhibited a lower retention rate at 29.74%. This difference might be attributed to the fact that carbohydrates, known to be excellent sources of carbon, primarily serve as energy sources for vital processes in specific fish tissues [45]. In Nile tilapia, the apparent digestibility of carbohydrates is generally lower than that of protein when plant-based ingredients are used [46], which may partially explain the lower carbon retention observed in comparison to nitrogen and phosphorus. Nevertheless, the observed carbon retention rate of 29.74% is considered efficient, as it roughly doubles the value reported by David et al. [12], who observed a retention of around 12% in tilapia biomass. Furthermore, it exceeds the retention reported by Flickinger et al. [47], who observed retention between 3% and 8% in tambaqui, Colossoma macropomum.

Tilapia in the early stages of production tend to use natural food available in the BFT system [10]. In our study, the consumption of high-nutrition flakes had a positive impact on retention rates. This observation corroborates the findings of Oliveira et al. [48], who found that tilapia in a control system, consuming only BFT flakes, experienced a threefold increase in their average initial weight over 70 days. Additionally, Gallardo-Collí et al. [9] evaluated periods of food restriction in tilapia juveniles produced in BFT and pointed out that 36 days of normal feeding and 12 days of restriction (without exogenous food) induce a complete compensation of body growth and restoration of reserve energy, lost during the restriction period. This is attributed to the fact that during such periods, animals can make better use of nutrients from the protein flakes available in the system.

The load of nutrients eliminated through effluents in ponds is capable of generating negative environmental impacts, given the high volume [3]. However, the BFT system minimizes this impact by allowing wastewater reuse across multiple production cycles [49] and the recycling of solids as valuable by-products [50]. The management of solids in BFT is carried out because the excess of solids can cause damage to the animals, such as obstruction of the gills and mortality in more extreme cases [37]. The inappropriate disposal of waste can exacerbate the environmental impact of the activity. Therefore, studies should evaluate the feasibility of using materials resulting from this management, such as feed for fish or other animals, biogas production, and fertilizer for vegetables, as a strategy to mitigate environmental pollution stemming from the activity.

The unaccounted nutrients in the mass balance can likely be attributed to solid sedimentation within the system and their utilization by microorganisms [51]. In ponds, the unaccounted carbon in the mass balance is attributed to sedimented organic carbon, CO₂ from the respiration of heterotrophic organisms in the system, and CO₂ mineralized by decomposing microorganisms [47]. These elements percolate through bottom sediments with infiltrated water or are absorbed by phytoplankton. According to Flickinger et al. [47], the bottom of production tanks may contain more nutrients than the water column. In BFT, regardless of the agitation of water molecules through constant aeration, a thicker layer of sedimentable solids can accumulate at the tank’s bottom, explaining the unaccounted nutrients in our study. The direct quantification of these settled solids could enhance the accuracy of nutrient mass balance assessments in future research. Moreover, a portion of nitrogen is lost during the production cycle through volatilization and denitrification [26]. In contrast, phosphorus lacks a removal pathway like nitrogen and tends to accumulate in closed systems [25].

Nutrient losses through volatilization were not quantified in this study but may represent a significant component of the overall nutrient balance. Techniques involving floating gas chambers have been developed for aquaculture systems to estimate greenhouse gas emissions such as carbon dioxide, methane, and nitrous oxide [52]. However, the implementation of such methods requires specialized infrastructure, equipment, and protocols, which were beyond the scope and logistical capacity of the present study. Therefore, future research incorporating direct measurements of gaseous nutrient losses, along with strategies for sediment management, is essential for a more comprehensive assessment of the environmental sustainability of biofloc technology systems.

4.3. Sustainability Metrics

Due to the absence of sustainability indicators specific to BFT, comparative information from other aquaculture systems was utilized for analysis and benchmarking purposes. In the present study, it was estimated that juvenile tilapia reared in BFT require an average volume of 0.0074 m³ per ton. In contrast, in cage systems, this indicator ranges from 0.75 m³ per ton [31] to 4.69 m³ per ton [53]. Another significant factor contributing to the sustainability of the BFT system is the high stocking density practiced. While juvenile tilapia are typically produced in recirculation systems with stocking densities between 200 and 250 fish/m³, BFT systems employ higher stocking densities of 350 fish/m³ without compromising the growth [54]. This approach enables BFT to achieve high productivity within a smaller space and with minimal water usage.

The continuous aeration required to maintain suspended solids and meet the high oxygen demand in BFT systems results in considerable energy consumption. A recent study reported an annual electricity use of approximately 114,000 kWh in BFT operations [12], highlighting the system’s high energy dependency. This energy demand can be attributed to operational inefficiencies in aeration equipment, such as low oxygen transfer efficiency and the continuous operation of blowers. Therefore, a comprehensive assessment of the environmental sustainability of BFT should also consider these technical factors, and also the environmental impacts associated with energy use. To increase sustainability, future efforts should focus on identifying and mitigating operational inefficiencies, with the adoption of high efficiency aerators, optimization of aeration schedules, and integration of real-time monitoring systems to better match oxygen supply with biological demand. Moreover, incorporating renewable energy sources (e.g., solar photovoltaic systems) could significantly offset electricity use and reduce the carbon footprint of BFT operations. Implementing such technological improvements and best management practices is essential to improving the overall energy efficiency and environmental performance of BFT systems [12,55].

Intensive aquaculture systems depend on the supply of artificial feed, which increases nutrient input into the production environment. In this context, the BFT system demonstrated lower inputs of carbon, nitrogen, and phosphorus when compared to Nile tilapia production in cage systems in southeastern Brazil, as reported by Fialho et al. [31]. It is important to highlight that the cited study considered a complete production cycle (8–805 g), with durations ranging from 189 to 263 days and water temperatures between 25 °C and 27 °C. This reference was selected due to its representativeness of commercial cage farming practices in tropical regions and its comprehensive assessment of nutrient dynamics, making it a suitable benchmark for comparison. On average, cage systems require approximately 700 kg of carbon, 77.5 kg of nitrogen, and 18.25 kg of phosphorus per ton of tilapia produced [31]. In contrast, the BFT system used in the present study required an estimated 257.54 kg of carbon, 30.86 kg of nitrogen, and 7.77 kg of phosphorus per ton of fish produced, indicating greater nutrient use efficiency and lower potential for environmental impact.

The BFT system shows high nutrient use efficiency, resulting in a lower eutrophication potential compared to conventional systems. Although waste is inevitably generated in aquaculture systems, the estimated eutrophication potential in this study is considerably lower compared to the net cage system, which discharges approximately 57.39 kg of carbon, 20.01 kg of nitrogen, and 5.70 kg of phosphorus per ton of fish produced [53]. This reduction is primarily attributed to the unique microbial community dynamics in BFT systems, where heterotrophic bacteria assimilate dissolved nitrogenous compounds into microbial biomass. This microbial activity not only minimizes nutrient loss but also promotes internal recycling, as bioflocs serve as an additional nutrient source for fish. Moreover, the closed-loop nature of BFT facilitates nutrient retention in the water column, allowing for their reuse in subsequent production cycles. These biological and operational characteristics enhance nutrient recovery and significantly reduce the release of pollutants into the environment.

The risk of the BFT system to local biodiversity conservation is considered medium [15]. Closed systems like BFT prevent the spread of exotic species and microorganisms due to the physical barrier they present [53]. This configuration ensures safety against leaks and reflects a high environmental safety standard [31].

Considering these environmental indicators, BFT stands out as a high-productivity system with minimal water usage and production area. Additionally, the BFT system’s efficient techniques in retaining nutrients in animal biomass, reducing eutrophication potential, and conserving local biodiversity highlight its overall sustainability.

4.4. Growth Efficiency

The survival rate of Nile tilapia juveniles, after the experimental period, varied between 95.96 and 100.00% in the different units of production. High survival rates are essential for profitable fish culture, as they reduce losses and increase the overall yield of production. These results are consistent with previous studies on tilapia production in BFT [56,57,58]. High survival rates were observed in BFT and can be related to the ingestion of microbial flakes, which promote a probiotic effect in fish reared in this system, which enables increased immunity and reduced mortality [56,59,60,61].

Although the average temperature during the trial remained below the range of 28–30 °C, which is recommended to optimize tilapia performance [38], this factor did not limit fish growth. The final weight, weight gain, and specific growth rate corroborate this information. Similar results were observed by Hisano et al. [58], who evaluated different feeding frequencies for tilapia in BFT, in a culture condition similar to those of the present study (outdoor tanks), and Oliveira et al. [48], who evaluated the growth performance of Nile tilapia juveniles fed different amounts of feed in a BFT. These findings suggest that Nile tilapia are adaptable to a range of temperatures and can grow well in BFT systems even when temperatures are slightly below the recommended range.

The AFC of fish in the units of production was on average 1.05, which demonstrates high feed efficiency. This tendency is often observed in fish culture in the BFT since the microbial flakes from the system can be used as a natural complementary food [8,59]. Thus, an experimental diet with a crude protein content below the requirement for the phase was used to maximize the use of flakes in the system for possible protein compensation. In water recirculation systems, where animals depend exclusively on the artificial diet, the AFC is higher than in BFT and varies between 1.48 [62] and 1.76 [63]. Additionally, the improvement of the AFC provides better utilization of the nutrients in the diets and consequently generates less waste to the aquatic environment [64]. Overall, the results of growth performance demonstrate that the BFT system is a viable option for the production of Nile tilapia juveniles. The fish showed high survival rates, good growth performance, and high feed efficiency.

5. Conclusions

The mass balance analysis revealed a high level of nutrient retention within the fish biomass, with minimal nutrient losses to the aquatic environment. This efficiency minimizes the release of potentially eutrophying effluents, directly contributing to the environmental sustainability of the production system. Sustainability indicators confirm that the BFT system supports high productivity while significantly reducing water consumption and space requirements. These characteristics make BFT a strategic solution for regions facing water scarcity, environmental restrictions, or the need to intensify production sustainably. However, the long-term stability of the BFT system is crucial to maintaining these benefits, as unstable conditions can compromise productivity and system efficiency. Additionally, it is important to highlight the system’s high dependency on electrical energy, which must be carefully considered when evaluating its overall economic and environmental viability. Future research should focus on optimizing energy use and enhancing system stability to improve the overall viability of BFT as a sustainable aquaculture technology.