A sulfur-based cyclic denitrification filter for marine recirculating aquaculture systems

Nitrification characteristics and microbial community changes during conversion of freshwater to seawater in down-flow hanging sponge reactor

In recirculating aquaculture systems (RAS), maintaining water quality in aquaculture tanks is a paramount factor for effective fish production. A down-flow hanging sponge (DHS) reactor, a trickling filter system used for water treatment of RAS that employs sponges to retain biomass, has high nitrification activity. However, nitrification in seawater RAS requires a long start-up time owing to the high salinity stress. Therefore, this study aimed to evaluate the nitrification characteristics and changes in the microbial community during the conversion of freshwater to seawater in a DHSreactor fed with ammonia-based artificial seawater. The total ammonia nitrogen concentration reached 1.0 mg-N·L-1 (initial concentration 10 mg-N·L-1) within 11 days of operation, and nitrate production was observed. The 16 S rRNA gene sequence of the DHS-retained sludge indicated that the detection rate of the ammonia-oxidizing archaeon Candidatus Nitrosocosmicus decreased from 23.9 % to 14.0 % and 25.8-17.6 % in the upper and lower parts of the DHS reactor, respectively, after the introduction of seawater. In contrast, the nitrite-oxidizing bacteria Nitrospira spp. increased from 0.1 % to 9.5 % and from 0.5 % to 10.5 %, respectively. The ammonia oxidation rates of 0.12 ± 0.064 and 0.051 ± 0.0043 mg-N·g-MLVSS-1·h-1 on the 37th day in the upper and bottom layers, respectively. Thus, nitrification in the DHS reactor performed well, even under high-salinity conditions with short operational days. This finding makes the transition from freshwater to saltwater fish in the RAS system simple and economical, and has the potential for early start-up of the RAS.

Low-carbon wastewater treatment and resource recovery of recirculating aquaculture system by immobilized chlorella vulgaris based on machine learning optimization

Immobilized microalgae biotechnologies can conserve water and space by low-carbon wastewater treatment and resource recovery in a recirculating aquaculture system (RAS). However, technical process parameters have been unoptimized considering the mutual interaction between factors. In this study, machine learning optimized the parameters of alginate-immobilized Chlorella vulgaris (C. vulgaris), that is, 474 μmol/(m2·s) of light intensity, 23 × 106 cells/mL for initial cell number, and 2.07 mm particle size. Importantly, under continuous illumination, the immobilized C. vulgaris and microalgal-bacterial consortium improved water purification and biomass reutilization. Transcriptomics of C. vulgaris showed enhanced nitrogen removal by increasing pyridine nucleotide and lipid accumulation via enhanced triacylglycerol synthesis. Symbiotic bacteria upregulated genes for nitrate reduction and organic matter degradation, which stimulated biomass accumulation through CO2 fixation and starch synthesis. The recoverable microalgae (1.94 g/L biomass, 47 % protein, 26.23 % lipids), struvite (64.79 % phosphorus), and alginate (79.52 %) every two weeks demonstrates a low-carbon resource recovery in RAS.

Water flush boosts performance of elemental sulfur-based denitrification packed-bed systems: Optimization and mechanisms

Despite the promising potential of elemental sulfur-based denitrification (ESDeN) packed-bed progresses, challenges such as excessive biofilm growth and gas entrapment persist, leading to denitrification deterioration. Water flush (WF) is recognized as an effective strategy, yet its effects remain underexplored. To address this knowledge gap, this study systematically investigated WF effects on ESDeN packed-bed denitrification. Results demonstrated that controlling WF effectively regulated denitrification, achieving superior and stable rates. Compared to no WF (0.45 kgN·m-3·d-1), rates improved by 1.20 ∼ 1.56 times under low-frequency (weekly WF, 0.54 kgN·m-3·d-1) and low-intensity WF (0.54 ∼ 0.70 kgN·m-3·d-1). High-frequency (hours WF) and high-intensity WF (30 & 50 m/h) further amplified denitrification rates by 1.73 ∼ 2.29 times. The enhanced denitrifications under low-frequency/intensity WF were mainly attributed to prolonged actual hydraulic retention time (AHRT), while high-frequency/intensity WF improved both AHRT prolonging and biofilm thinning, facilitating mass transfer. This study offers a promising avenue for fine-tuning denitrification rates via strategic WF adjustments.

Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

In the field of wastewater treatment, nitrate nitrogen (NO3--N) is one of the significant contaminants of concern. Sulfur autotrophic denitrification technology, which uses a variety of sulfur-based electron donors to reduce NO3--N to nitrogen (N2) through sulfur autotrophic denitrification bacteria, has emerged as a novel nitrogen removal technology to replace heterotrophic denitrification in the field of wastewater treatment due to its low cost, environmental friendliness, and high nitrogen removal efficiency. This paper reviews the advance of reduced sulfur compounds (such as elemental sulfur, sulfide, and thiosulfate) and iron sulfides (such as ferrous sulfide, pyrrhotite, and pyrite) electron donors for treating NO3--N in wastewater by sulfur autotrophic denitrification technology, including the dominant bacteria types and the sulfur autotrophic denitrification process based on various electron donors are introduced in detail, and their operating costs, nitrogen removal performance and impacts on the ecological environment are analyzed and compared. Moreover, the engineering applications of sulfur-based electron donor autotrophic denitrification technology were comprehensively summarized. According to the literature review, the focus of future industry research were discussed from several aspects as well, which would provide ideas for the application and optimization of the sulfur autotrophic denitrification process for deep and efficient removal of NO3--N in wastewater.

Elemental sulfur-driven autotrophic denitrification process for effective removal of nitrate in mariculture wastewater: Performance, kinetics and microbial community

To date, there is a lack of systematic investigation on the elemental sulfur-driven autotrophic denitrification (SDAD) process for removing nitrate (NO3--N) from mariculture wastewater deficient in organic carbon sources. Therefore, a packed-bed reactor was established and continuously operated for 230 days to investigate the operation performance, kinetic characteristics and microbial community of SDAD biofilm process. Results indicate that the NO3--N removal efficiencies and rates varied with the operational conditions including HRT (1-4 h), influent concentrations of NO3--N (25-100 mg L-1) and DO (0.2-7.0 mg L-1), and temperature (10oC-30 °C), in the ranges of 51.4%-98.6% and 0.054-0.546 g L-1 d-1, respectively. Limestone could partially neutralize the produced acidity. Small portions of NO3--N were converted to nitrite (<4.5%) and ammonia (<2.8%) in the reactor. Operational conditions also influenced the production of acidity, nitrite and ammonia as well as sulfate. Shortening HRT and increasing influent NO3--N concentration turned the optimal fitting model depicting the NO3--N removal along the reactor from half-order to zero-order. Furthermore, the NO3--N removal was accelerated by a higher temperature and influent NO3--N concentration and a lower HRT and influent DO concentration. Microbial richness, evenness and diversity gradually decreased during the autotrophic denitrifier enrichment cultivation and the reactor start-up and operation. Sulfurimonas constituted the predominate genus and the primary functional bacteria in the reactor. This study highlights the SDAD as a promising way to control the coastal eutrophication associated with mariculture wastewater discharge.

Determination of the Acute and Chronic Toxicity of Sulfate from the Sulfur Autotrophic Denitrification Process to Juvenile Zebrafish (Danio rerio)

Sulfur-based materials are widely used as electron donors for denitrification to enhance nitrogen removal from water. This leads to an increased sulfate concentration in the effluent or sulfate accumulation in recirculating aquaculture systems. This study explored acute and chronic toxicity of sulfate to juvenile zebrafish (Danio rerio) and investigated the histopathological changes in the gills of juvenile zebrafish exposed to sulfate. Results show that zebrafish had a high tolerance to sulfate, with no acute toxicity at sulfate concentrations from 250 to 3200 mg/L. For the chronic toxicity study, it was found that zebrafish mortality decreased with the increase in sulfate concentrations ranging from 250 to 1500 mg/L. In contrast, when the sulfate concentration was 1500-3000 mg/L, zebrafish mortality increased with the increasing sulfate concentration. In addition, in the ion balance test, KCl was added to balance the effects of Na⁺ from the Na₂SO₄ used to obtain the desired sulfate concentrations, showing that fish mortality correspondingly increased with increasing KCl addition. Furthermore, when living in an environment with elevated sulfate concentrations for a long period, changes were observed in the morphology, behavior, and gill tissue of the zebrafish, including slow and lateral swimming; bottom settling; and large opening and closing, lamellar fusion, and necrosis of gills. This research reveals the toxicity of sulfate to aquatic organisms, providing a scientific basis for the promotion and application of sulfur or sulfur-based materials in autotrophic reduction processes for wastewater treatment.

Enhanced biological antimony removal from water by combining elemental sulfur autotrophic reduction and disproportionation

Antimony (Sb), a toxic metalloid, has serious negative effects on human health and its pollution has become a global environmental problem. Bio-reduction of Sb(V) is an effective Sb-removal approach. This work, for the first time, demonstrates the feasibility of autotrophic Sb(V) bio-reduction and removal coupled to anaerobic oxidation of elemental sulfur (S⁰). In the S⁰-based biological system, Sb(V) was reduced to Sb(III) via autotrophic bacteria by using S⁰ as electron donor. Meanwhile, S⁰ disproportionation reaction occurred under anaerobic condition, generating sulfide and SO₄^2- in the bio-systems. Subsequently, Sb(III) reacted with sulfide and formed Sb(III)-S precipitate, achieving an effective total Sb removal. The precipitate was identified as Sb₂S₃ by SEM-EDS, XPS, XRD and Raman spectrum analyses. In addition, it was found that co-existing nitrate inhibited the Sb removal, as nitrate is the favored electron acceptor over Sb(V). In contrast, the bio-reduction of co-existing SO₄^2- enhanced sulfide generation, followed by promoting Sb(V) reduction and precipitation. Illumina high-throughput sequencing analysis revealed that Metallibacterium, Citrobacter and Thiobacillus might be responsible for Sb(V) reduction and S⁰ disproportionation. This study provides a promising approach for the remediation of Sb(V)-contaminated water.

Aquaculture waste nutrients removal using microalgae with floating permeable nutrient uptake system (FPNUS)

Large area requirements and huge energy consumption restrict the applications of microalgae in wastewater treatment. In this study, in-situ nutrient removal was tested using a floating permeable nutrients uptake system with pore sizes of 1, 5, 10, and 40 µm, and Chlorella sorokiniana and Scenedesmus acuminatus. Results showed that N transfer rate across FPNUS varied with membrane pore size and N-type. Average transfer rate of NH₄⁺-N, NO₃^--N, and NO₂^--N across 1 µm membrane was 2.6, 14.6, and 2.3 mg m^-2h^-1, respectively, sufficient to support microalgal growth. The NH₄⁺-N and NO₃^--N removal rate in shrimp wastewater reached 1.32 and 1.88 mg L^-1d^-1, comparable to some BNR processes used in RAS. According to the developed area ratio prediction model, FPNUS to pond area ratio of 21% is sufficient to balance N loading of 0.05 mg L^-1d^-1. These results indicate extraordinary potential of in-situ nutrient removal from wastewaters using FPNUS.

Transforming heterotrophic to autotrophic denitrification process: Insights into microbial community, interspecific interaction and nitrogen metabolism

For investigating the microbial community, interspecific interaction and nitrogen metabolism during the transform process from heterotrophic to synergistic and autotrophic denitrification, a filter was built, and carbon source and sulfur concentration were changed to release the transformation process. The results demonstrated that the transformation process was feasible to keep nitrate nitrogen (NO₃^--N) discharge concentration lower than 15 mg L^-1, however, nitrite nitrogen (NO₂^--N) accumulation and its rate reached 7.85% at initial stages. The dominant denitrification gunes were Methylophilaceae, Thiovulaceae and Hydrogenophilaceae for three processes, respectively, and the microbial interspecific interaction of heterotrophic denitrification was more complex than others. NO₂^--N accumulation was confirmed by the low abundance of EC1.7.7.1 and EC1.7.2.1, and the dominance degree of dark oxidation of sulfur compounds and dark sulfide oxidation improved in synthesis and autotrophic denitrifications.

Wood and sulfur-based cyclic denitrification filters for treatment of saline wastewaters

This study investigated the performance and microbiome of cyclic denitrification filters (CDFs) for wood and sulfur heterotrophic-autotrophic denitrification (WSHAD) of saline wastewater. Wood-sulfur CDFs integrated into two pilot-scale marine recirculating aquaculture systems achieved high denitrification rates (103 ± 8.5 g N/(m³·d)). The combined use of pine wood and sulfur resulted in lower SO₄^2- accumulation compared with prior saline wastewater denitrification studies with sulfur alone. Although fish tank water quality parameters, including ammonia, nitrite, nitrate and sulfide, were below the inhibitory levels for marine fish production, lower survival rates of Poecilia sphenops were observed compared with prior studies. Heterotrophic denitrification was the dominant removal mechanism during the early operational stages, while sulfur autotrophic denitrification increased as readily biodegradable organic carbon released from wood chips decreased over time. 16S rRNA-based analysis of the CDF microbiome revealed that Sulfurimonas, Thioalbus, Defluviimonas, and Ornatilinea as notable genera that contributed to denitrification performance.