Optimal cultivation concentration of duckweed for pollutant removal from biogas slurry

Duckweed is an important plant for wastewater remediation and a promising alternative protein source for animal feed. This study aims to evaluate the biomass, protein accumulation, and wastewater purification capacity of duckweed under controlled cultivation conditions. Using pig farm biogas slurry as a nutrient source, this study analyzes the purification ability of duckweed at different slurry concentrations (2%, 4%, 6%, 8%, 10%) on chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH₃-N) in biogas slurry. The study also describes changes in duckweed weight, chlorophyll content, and protein accumulation. Our results showed that a 4% biogas slurry concentration was the optimum condition for the cultivation of duckweed, which corresponded to an NH₃-N concentration of 29.56 mg/L. The dry matter production capacity of duckweed under these conditions was 1.78 g/(d·m²), the relative growth rate (RGR) was 0.29 g/(g·d), the doubling time was 2.42 days, and the protein content was 36.25% by dry weight. In terms of pollutant removal, duckweed efficiently removed 54.69% of COD, 86.89% of TN, 97.25% of NH₃-N, and 85.22% of TP. The results of this study provide an important reference for the operation of the duckweed production system, which is crucial for the design and operation of pilot-scale and large-scale duckweed production systems moving forward.