Shimin Lu

Recirculating aquaculture systems (RAS) require rapid, in-loop removal of dissolved inorganic nitrogen, yet electrochemical processes optimised for seawater are difficult to translate to low-salinity operations because chloride availability is fundamentally limited and the co-generated free chlorine is acutely toxic to cultured organisms. Here we systematically characterise the inorganic-nitrogen migration pathways, the salinity- and pH-dependence of residual-chlorine generation, and UV-driven chlorine mitigation during electrochemical treatment of intensively cultured tilapia water (0.4 ‰ salinity) using a Ti/RuO2–IrO2 anode. Ammonium and nitrite were removed with apparent zero-order kinetics. The NH4+–N removal rate increased from 0.1162 to 0.1463 mg L−1 min−1 as the current density was raised from 200–230 to 300–330 A m−2. NO2−–N was removed at 0.2844 mg L−1 min−1, partitioning approximately 47.8 % to NO3−–N and the remainder, by mass balance, to gaseous nitrogen species formed via cathodic reduction. NO3−–N was unreactive, indicating that low-salinity electrochemistry must be coupled with biological denitrification for total-N control. Residual-chlorine generation displayed a sharp threshold near 0.6 ‰: below this value the rate scaled with salinity, consistent with Cl−-mass-transfer control; above it, the rate plateaued at ~1.56 mg L−1 min−1 and became governed by anode kinetics. Lowering pH from 8.0–8.5 to 5.5–6.0 raised the chlorine accumulation rate by an order of magnitude. An aeration-only dark control (in-line aerator on, UV off) showed <1 % chlorine loss in 150 min, demonstrating that the recirculation aeration is by itself essentially ineffective for dechlorination on practical timescales. By contrast, UV-C photolysis at 30 W eliminated residual chlorine at 0.416 mg L−1 min−1, with an energy efficiency of 17.56 g kWh−1 (1.18 kWh per m³ at the chlorine load tested), cutting treatment time from the hours required by conventional air stripping to under one hour. Together, these results define an operational envelope—salinity ≈ 0.6 ‰, near-neutral pH, integrated UV polishing—within which electrochemistry can be deployed safely and efficiently in low-salinity recirculating aquaculture.

Recirculating aquaculture systems (RAS) require rapid, in-loop removal of dissolved inorganic nitrogen, yet electrochemical processes optimised for seawater are difficult to translate to low-salinity operations because chloride availability is fundamentally limited and the co-generated free chlorine is acutely toxic to cultured organisms. Here we systematically characterise the inorganic-nitrogen migration pathways, the salinity- and pH-dependence of residual-chlorine generation, and UV-driven chlorine mitigation during electrochemical treatment of intensively cultured tilapia water (0.4 ‰ salinity) using a Ti/RuO2–IrO2 anode. Ammonium and nitrite were removed with apparent zero-order kinetics. The NH4+–N removal rate increased from 0.1162 to 0.1463 mg L−1 min−1 as the current density was raised from 200–230 to 300–330 A m−2. NO2−–N was removed at 0.2844 mg L−1 min−1, partitioning approximately 47.8 % to NO3−–N and the remainder, by mass balance, to gaseous nitrogen species formed via cathodic reduction. NO3−–N was unreactive, indicating that low-salinity electrochemistry must be coupled with biological denitrification for total-N control. Residual-chlorine generation displayed a sharp threshold near 0.6 ‰: below this value the rate scaled with salinity, consistent with Cl−-mass-transfer control; above it, the rate plateaued at ~1.56 mg L−1 min−1 and became governed by anode kinetics. Lowering pH from 8.0–8.5 to 5.5–6.0 raised the chlorine accumulation rate by an order of magnitude. An aeration-only dark control (in-line aerator on, UV off) showed <1 % chlorine loss in 150 min, demonstrating that the recirculation aeration is by itself essentially ineffective for dechlorination on practical timescales. By contrast, UV-C photolysis at 30 W eliminated residual chlorine at 0.416 mg L−1 min−1, with an energy efficiency of 17.56 g kWh−1 (1.18 kWh per m³ at the chlorine load tested), cutting treatment time from the hours required by conventional air stripping to under one hour. Together, these results define an operational envelope—salinity ≈ 0.6 ‰, near-neutral pH, integrated UV polishing—within which electrochemistry can be deployed safely and efficiently in low-salinity recirculating aquaculture.

Pond aquaculture is an important aquacultural model worldwide in which ammonia-oxidizing bacteria (AOB) are crucial for the removal of ammonia from water. The influence of light irradiation on AOB in an aquaculture pond was studied using artificial simulation wastewater under dark/light cycles of 24 h/0 h (L0), 12 h/12 h (L12), and 0 h/24 h (L24). The ammonia oxidation rates (AORs) in groups L0, L12, and L24 were 9.88 ± 0.19 mg h⁻¹, 6.01 ± 0.32 mg h⁻¹, and 1.85 ± 0.09 mg h⁻¹, respectively. Long-term exposure to light had a serious impact on the AOR and decreased the abundance of Nitrosomonas spp. and their ammonia monooxygenase genes. To determine the protective effect of microalgae on AOB, different doses of freeze-dried Chlorella spp. powder were added to the nitrifying bacteria community. The photoinhibition rate of chlorophyll a (Chla) in the groups with 300 and 1300 µg L⁻¹ of added Chlorella were 32.85% and 28.77%, respectively, while the Chla in the 2200 µg L⁻¹ Chlorella-added group was only 0.01%, with no significant differences (p > 0.05) in AOR between the dark/light treatment subgroups. Fluorescence in situ hybridization showed that AOB, nitrite-oxidizing bacteria, and algae coexist and grow together without free AOB in the nitrifying bacterial community. It was suggested that microalgae enhance the resistance of AOB to light irradiation in a pond through the shading effect provided by algal chlorophyll and the close symbiotic relationship between microalgae and AOB.

The use of bacterial and microalgal consortia to remove nitrogen from wastewater has garnered attention as a potential alternative to conventional systems. This approach not only reduces energy consumption but also aids in nutrient recovery. Light is essential for algae photosynthesis; however, nitrifying bacteria are also influenced by light radiation. This mini-review summarizes the current knowledge concerning photoinhibition, the light stimulation of ammonia-oxidizing bacteria (AOB), resistance to light radiation, the implementation of microalgal–bacterial systems, and the possible mechanisms involved. Nitrosomonadaceae AOB and Nitrospiraceae nitrite-oxidizing bacteria (NOB) often coexist in a microalgal–bacterial system. Studies have suggested that AOB can tolerate light radiation at 200 μmol m⁻²·s⁻¹ in microalgal–bacterial systems, whereas NOB are almost completely suppressed, which can result in partial nitrification in the bioreactor. An appropriate light level can stimulate AOB growth in microalgal–bacterial granular reactors and may improve algae metabolic activity. Granular sludges or artificial “light-shielding hydrogel” could effectively protect nitrifying bacteria from light intensities up to 1600 μmol m⁻²·s⁻¹ in wastewater treatment reactors. Microalgal–bacterial systems along with the associated “algal shading effect” have been widely used in pond aquaculture. This approach minimizes the need for costly mechanical aeration through photo-oxygenation and facilitates nutrient recovery by filter-feeding fish.

During the process of circulating aquaculture, high concentrations of nitrate will accumulate. A simultaneous denitrification and fermentation process is described to remove nitrate from a recirculating aquaculture system using endogenous carbon on a biofilm. ¹⁵N isotope technology was used to assess the nitrate removal, mainly through heterotrophic denitrification. The nitrate removal rate could be as high as 98.97%, with a final concentration of nitrate below 1 mg/L. The denitrification process obeys a Michaelis–Menten-type enzyme kinetic model, with a half saturation constant of 99.91 mg/L and a maximum nitrate removal rate of 0.39 mg L⁻¹ h⁻¹ at 28 °C. The functional genes narG and narH for nitrate removal were obtained from Nitrospirae spp. at proportions of 39.13% and 26.16%, respectively. The acetate, propionate and iso-valerate produced by anaerobic fermentation provided the principal electron donors for denitrification.

Biofloc technology (BFT) culture systems based on low salinity can meet the needs of shrimp growth and environmental protection, thus having potential application. To evaluate the effects of BFT on water quality, the microbial community, growth, digestive enzyme activity, and antioxidant and immune status of Litopenaeus vannamei under low salinity conditions, a four-week experiment was conducted in a BFT culture system (C/N ratio of 12: 1) with three salinity conditions: 5.0‰ (S5 group), 10.0‰ (S10 group), and 15.0‰. The results showed that water quality parameters were all within the range suitable for the culture of L. vannamei. There were no significant differences in growth parameters such as final weight or specific growth rate. Muscle moisture, crude protein, and crude lipid contents of L. vannamei did not differ among groups (p > 0.05). In addition, intestinal amylase and trypsin activities in the S5 group significantly increased (p < 0.05). In the S15 group, the superoxide dismutase activity and total antioxidant capacity in the serum and hepatopancreas of L. vannamei, as well as serum catalase, acid phosphatase, and alkaline phosphatase activities, markedly increased (p < 0.05). The microbial diversity (Shannon and Simpson indices) and richness (Chao1 and ACE indices) were higher in the S5 group than in the S15 group. Our findings indicated that although the shrimp in BFT systems with a salinity of 5‰ had lower antioxidant and immune levels, the digestive enzyme activity as well as the gut microbial diversity and richness improved compared to other groups, suggesting the possibility of culturing L. vannamei in low-saline areas with BFT systems.

Nitrite is harmful to aquatic animals in freshwater pond aquaculture environments. Nitrite is oxidized to nitrate mainly by nitrite-oxidizing bacteria (NOB). NOB may be promising in the control of nitrite concentration in aquaculture water. To better understand the role of NOB in the nitrogen cycling process of freshwater aquaculture ponds, we describe the characterization of NOB by using a culture derived from a freshwater aquaculture pond. Analysis by 16S rRNA sequence alignment showed that the NOB in the culture belong to the genus Nitrospira, whose generation time and nitrite–oxidizing rate were 9.87 h and 4.63 (± 0.07) pg NO₂⁻–N cell⁻¹·h⁻¹ in the exponential growth phase, respectively. The oxidation of nitrite could be inhibited completely when the concentration of dissolved oxygen was 1.0 ± 0.5 mg/L.⁻¹ The NOB could oxidize nitrite into nitrate at 29.84‰ salinity. However, the nitrification rate decreased significantly with an increase in salinity concentration ranging from 5.72‰ to 29.84‰. The optimal temperature range was 25–37 °C for the growth of NOB, and activity significantly decreased when the temperature was below 20 °C or reached 40 °C. In this study, the initial pH 6 ± 0.1–8 ± 0.1 was not observed to affect the nitrification activity of the NOB culture. Illumination of 410 ± 10 nm can completely inhibit NOB nitrification, and 520 ± 10 nm illumination partially inhibits NOB, whereas 660 ± 10 nm illumination has no effect. These findings indicate that the degree of photoinhibition in NOB increases with decreasing wavelength.

The excessive ammonia produced in pond aquaculture processes cannot be ignored. In this review, we present the distribution and diversity of ammonia-oxidizing archaea (AOA) and anaerobic ammonia-oxidizing bacteria (AnAOB) in the pond environment. Combined with environmental conditions, we analyze the advantages of AOA and AnAOB in aquaculture water treatment and discuss the current situation of pond water treatment engineering involving these microbes. AOA and AnAOB play an important role in the nitrogen removal process of aquaculture pond water, especially in seasonal low temperatures and anoxic sediment layers. Finally, we prospect the application of bioreactors to purify pond aquaculture water using AOA and AnAOB, in autotrophic nitrogen removal, which can reduce the production of greenhouse gases (such as nitrous oxide) and is conducive to the development of environmentally sustainable pond aquaculture.

Pond aquaculture undeniably offers the potential for food production worldwide. In China, 45.83% of aquatic production is currently from pond aquaculture. However, with the continuous expansion of this practice, environmental problems such as a high level of water consumption, aquaculture water deterioration, pollution from effluent and aquatic product quality decline seriously restrict the sustainable development of pond aquaculture. In this review, we summarise the (i) the impacts of pond aquaculture on the environment, (ii) research progress in pond aquaculture ecological engineering, (iii) existing technologies regarding pond aquaculture ecological engineering systems, (iv) effects of applying pond aquaculture ecological engineering and (v) summary and prospects. Moreover, we discuss the merits and drawbacks of each method and technology, and future research priorities are reviewed. With this, an understanding of the role played by ecological engineering in pond aquaculture is provided, as well as guidance for precisely managing aquaculture water and effluent, aquaculture practices, and technological developments. In summary, the pond aquaculture ecological engineering can be managed so as to improve animal welfare and the stability of water treatment systems, reducing the adverse effects on the environment and public health, and enabling the sustainable development of pond aquaculture.