Aquaculture wastewater, rich in organic matter, nitrogen an phosphorus, poses a significant environmental threat when discharged untreated, leading to eutrophication and water quality degradation.

A recent study, published in Microorganisms, introduces a novel solution - the "Four Ponds and Two Dams" constructed wetland system, an innovative approach to aquaculture wastewater treatment—pioneered by a research team led by Prof. XIE Yonghong from the Institute of Subtropical Agriculture of the Chinese Academy of Sciences.

"This technology not only optimize water purification processes in intensive aquaculture systems, but also deepens our understanding of the synergistic interactions between aquatic plants and epiphytic biofilms, revealing their crucial roles in pollutant removal from bullfrog aquaculture wastewater.” said Prof. XIE, corresponding authors of the study. 

The efficient multi-stage treatment system integrates a sedimentation pond, a first filtration dam, an aeration pond, a second filtration dam, a biological filter pond, and a submerged plant stabilization pond, forming a comprehensive, eco-friendly wastewater remediation framework. A demonstration site was established at a bullfrog aquaculture facility in the Datong Lake region of Yiyang City.

Results showed that the constructed wetland system achieved a remarkable removal efficiency exceeding 60% for both total nitrogen and total phosphorus in bullfrog aquaculture wastewater, significantly reducing pollutant loads. However, treatment performance varied seasonally, with TN removal remaining stable, while TP removal fluctuated.

The aeration pond played a crucial role in enhancing dissolved oxygen levels, which provided optimal conditions for phosphorus removal in July and nitrogen removal in November. Within the aeration pond and biological filter pond, epiphytic bacterial α-diversity was significantly higher than in other treatment ponds, indicating that these zones offer an enriched microenvironment conducive to bacterial colonization and activity.

Additionally, the bacterial community composition exhibited pronounced seasonal shifts. In July, Firmicutes were the dominant phylum, whereas in November, the microbial landscape shifted to be dominated by Nitrospiraceae and Acidobacteriota, reflecting dynamic microbial succession patterns influenced by seasonal factors. 

Furthermore, functional genes associated with sulfur metabolism, nitrogen fixation, and oxidative phosphorylation displayed marked temporal variations within the aeration pond, underscoring the direct impact of seasonal growth dynamics and process optimizations on biofilm functional gene expression.

These findings highlight the vital role of aeration and aquatic plants—particularly submerged species—in enhancing aquaculture wastewater treatment efficiency.

"The synergistic interactions between aquatic vegetation and epiphytic biofilms are fundamental to improving nutrient removal and sustaining long-term system performance," said Dr. CHAO Chuanxin, first author of the study. "From an application perspective, strategic aquatic plant management and continuous aeration are key technical interventions to fully harness the potential of the 'Four Ponds and Two Dams' constructed wetland system."

"Four Ponds and Two Dams" constructed wetland purification system. (Image by CHAO Chuanxin)