Key Takeaways & Executive Findings
- •• • Capsule shell pore size (0.25 ± 0.063 μm) enables rapid mass transfer; nitrate removal reached 83.61% by day 21, demonstrating fast activity recovery critical for industrial RAS integration. • • Biomass increased to (21.96 ± 0.28) mg·(g-pellet)−1 by day 30, while effective diffusion coefficient declined to 0.232 × 10−9 m²·s−1, indicating a trade-off between biomass accumulation and substrate transport that must be managed for sustained performance. • • Dominant genera after acclimation: Methylobacterium (22.8%), Brevibacillus (18.1%), Azospirillum (17.3%); nirS/nirK denitrifiers >99% Proteobacteria, with increases in Bosea, Bradyrhizobium, Rhizobacter, Alicycliphilus, and Zoogloea, enhancing metabolic versatility and biofilm formation. • • Effective diffusion coefficient stabilized at ~0.233 × 10−9 m²·s−1 from day 21–30, correlating with biomass stabilization, providing a design parameter for predicting long-term capsule performance.
Abstract
To address nitrate nitrogen accumulation in aquaculture tailwater, a core-shell encapsulated immobilized denitrifying bacterial capsule containing lychee seed powder and denitrifying activated sludge was developed. The capsule's micro-morphology, bacterial activity recovery, growth, community structure changes, and diffusion coefficient dynamics during acclimation were investigated. The capsule shell exhibited a honeycomb porous structure with an average pore size of (0.25 ± 0.063) μm. Denitrifying bacterial activity recovered rapidly, with nitrate nitrogen removal efficiency stabilizing at 83.61% by day 21. Biomass within the capsules increased progressively, reaching (21.96 ± 0.28) mg·(g-pellet)−1 on day 30. The effective diffusion coefficient decreased with biomass growth, dropping to 0.232 × 10−9 m²·s−1 by day 30. Organic carbon source and encapsulation acclimation environment altered the microbial community structure; after acclimation, dominant genera were Methylobacterium (22.8%), Brevibacillus (18.1%), and Azospirillum (17.3%). Denitrifying bacteria containing nirS- and nirK- genes predominantly belonged to Proteobacteria (>99%). Genera involved in organic carbon metabolism and denitrification, including Bosea, Bradyrhizobium, Rhizobacter, and Alicycliphilus, increased in abundance. The encapsulated denitrifying bacteria exhibited short activity recovery time and excellent denitrification performance, making them suitable for denitrification of aquaculture tailwater or other low C/N ratio wastewater.
1. Introduction
Recirculating aquaculture systems (RAS) suffer from nitrate nitrogen accumulation due to high dissolved oxygen and insufficient organic carbon, which inhibits denitrifying bacteria. Conventional denitrification technologies are ill-suited for the high water recirculation rates and low C/N ratios typical of RAS, necessitating frequent water exchange that raises operational costs and environmental burden. Immobilized microbial technology offers advantages such as preventing biomass washout and maintaining high cell densities, but traditional entrapment methods suffer from poor mass transfer and cell damage during crosslinking. This study introduces a core-shell encapsulated denitrifying capsule fabricated via spray coating with a nanoporous membrane, which minimizes microbial injury and accelerates activity recovery.
The key bottleneck addressed is the quantitative understanding of how biomass growth during acclimation alters the effective diffusion coefficient and microbial community structure, which ultimately governs denitrification efficiency. By systematically tracking capsule morphology, biomass accumulation, diffusion coefficient changes, and community shifts, this research provides critical data for engineering design and process optimization of encapsulated denitrifiers for aquaculture tailwater treatment.
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LI Hua, ZHAI Luoluo, DUAN Yafei, DONG Hongbiao, MA Yanwu, ZHANG Jiasong, JU (2026). Diffusion Coefficient and Microbial Community Structure Dynamics During Acclimation of Encapsulated Immobilized Denitrifying Bacteria. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511004
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Frequently Asked Questions
What is the operational stability of the encapsulated denitrifying capsules under varying nitrate loads and dissolved oxygen levels?
The study reports stable nitrate removal efficiency of 83.61% by day 21 under defined conditions (DO 5.01 mg/L, nitrate 50 mg/L). However, data on performance under fluctuating loads or higher DO levels are not provided. Further stress testing is required to assess robustness for industrial RAS.
How does the effective diffusion coefficient change with biomass growth, and what are the implications for scale-up?
The effective diffusion coefficient decreased from initial values to 0.232 × 10−9 m²·s−1 by day 30 as biomass increased to (21.96 ± 0.28) mg/g-pellet. This indicates increased mass transfer resistance, which could limit substrate supply to inner cells. For scale-up, capsule size and biomass loading must be optimized to maintain adequate diffusion.
What is the cost comparison of this encapsulation technology against conventional denitrification methods?
The paper does not provide cost analysis. However, the use of low-cost materials like lychee seed powder and PLA suggests potential economic viability. A detailed techno-economic assessment is needed to compare with conventional suspended-growth systems.
How does the microbial community structure evolve during acclimation, and what functional roles do the dominant genera play?
After acclimation, Methylobacterium (22.8%), Brevibacillus (18.1%), and Azospirillum (17.3%) dominated. Denitrifiers with nirS/nirK genes were >99% Proteobacteria. Genera like Bosea, Bradyrhizobium, Rhizobacter, and Alicycliphilus increased, indicating enhanced organic carbon metabolism and denitrification. Zoogloea increased, promoting biofilm formation. This community shift likely contributes to stable performance under encapsulated conditions.
What is the long-term durability of the capsule shell under continuous operation and mechanical stress?
The study does not report long-term durability beyond 30 days. The shell's porous structure (pore size 0.25 μm) may be susceptible to fouling or degradation over extended periods. Mechanical strength and longevity need further investigation for practical applications.
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