Transformation of dissolved organic matter during aquaculture wastewater treatment: Insights into the biological toxicity, spectral indices and molecular signatures

Introduction

Aquaculture is a globally significant source of food production, but intensive farming generates wastewater laden with organic pollutants, nutrients, and potentially harmful compounds. To protect aquatic ecosystems and optimize treatment processes, it is critical to understand how dissolved organic matter (DOM) changes during Wastewater Treatment and which components contribute most to toxicity. This study focuses on a widely used treatment configuration known as‘three ponds, two dams, one wetland’ (3P-2D-1W), evaluating how DOM composition evolves through each stage of treatment.

Methods and Analytical Approach

The researchers collected wastewater samples from multiple stages of the 3P-2D-1W system, starting from the aquaculture pond inflow and continuing through sedimentation, aeration, purification ponds, and constructed wetlands. Analytical techniques included:

• Spectroscopy (UV-Vis and fluorescence)for optical property assessment
• Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS)for precise molecular characterization
• Biotoxicity assaysto directly measure biological inhibition reflecting ecological risk

By correlating chemical indices, molecular signatures, and toxicity measurements, the study aimed to pinpoint the molecular features associated with the highest environmental risk.

DOM Composition and Toxic Contributors

Results showed that DOM in untreated aquaculture effluent exhibits complex molecular structures, with many compounds linked to respiration, feed residues, and microbial metabolism. Notably:

• Aliphatic compoundsand highly unsaturated phenolic substances showed positive correlations with biological toxicity.
• The study used correlation analysis between spectral indices such as humification index (HIX) and aromaticity and toxicity measures, indicating that specific molecular signatures can serve as early indicators of ecological harm.

These findings reveal that not all organic components pose equal risk — identifying which chemical classes drive toxicity is essential for targeted process improvement.

Treatment Stage Performance

The 3P-2D system alone achieved moderate pollutant reduction, removing approximately 59.4% of dissolved organic carbon (DOC) and reducing toxicity by 55.4%. After including the constructed wetland (3P-2D-1W), effluent quality improved significantly, with DOC reduced to 12.3 mg/L and biological inhibition (used as a proxy for toxicity) lowered to 7.16%. Interestingly, this treated effluent was cleaner than the original river source water used for aquaculture, which had a DOC of 15.7 mg/L and a 9.21% inhibition rate.

However, the study also highlighted limitations:

• The system showed limited selectivity for removing specific toxic compounds.
• The aeration pond stage contributed to increased effluent toxicity, possibly by releasing intermediate breakdown products that were not fully degraded.

Molecular Reaction Dynamics

Using FT-ICR-MS data, the researchers conducted reaction pair analysis — comparing loss and gain of molecular fragments throughout treatment — and found:

• The constructed wetland exhibited the greatest diversity of molecular reactions, especially in removing CHO group compounds.
• Organic nitrogen transformation (±CHON) processes increased significantly in the wetland stage, suggesting more complete breakdown of nitrogenaceous organic matter.

These insights at the molecular level underscore how different treatment modules contribute distinct biochemical transformations, offering clues for optimization.

Discussion

Understanding DOM transformation mechanisms is more informative than traditional measures like COD or DOC alone. Molecular profiling and toxicity correlation:

• Provide granular indicators for pollutant removal beyond quantity reduction
• Enable identification of specific compound classes responsible for ecological harm
• Support development of rapid assessment tools using spectroscopic indices to predict pollution levels

The redundant aeration pond, for example, may not be necessary if downstream wetland processing is robust enough, suggesting possible process redesign for efficiency gains.

Conclusions

This study advances our understanding of how aquaculture wastewater treatment influences the molecular composition and toxicity of dissolved organic matter. By combining high-resolution molecular analysis with bioassays, researchers can now propose more rational strategies to optimize treatment processes, reduce ecological risks, and inform best management practices for aquaculture wastewater globally.