Microalgal and duckweed based constructed wetlands for swine wastewater treatment: A review

[Display omitted] •Microalgae based constructed wetlands (CWs) are effective for swine wastewater treatment.•Duckweeds based CWs have advantages in swine wastewater (SW) treatment.•Characteristics of microalgae and duckweeds based CWs in SW treatment were reviewed.•Mechanisms of and factors affecting microalgae and duckweeds based CWs were discussed.•Challenges and future perspectives of microalgae and duckweeds based CWs were proposed. Constructed wetlands for swine wastewater treatment have been one of the most exciting research topics. Usually hydrophytes based constructed wetlands could not adapt well to high concentration of ammonia nitrogen in swine wastewater, while microalgal and duckweed based constructed wetlands are promising for the nutrient removal. In this critical review, the important roles of microalgae and duckweeds played in wastewater treatment in constructed wetlands were first summarized. Performances including biomass growth, nutrient removal capacities and mechanisms of microalgal and duckweed based constructed wetlands were reviewed for swine wastewater treatment. Challenges for the applications of constructed wetlands including microalgal and duckweed based ones were discussed which includes a better understanding and utilization of synergistic effects among microalgae and duckweeds, difficulty and costs in harvesting biomass, applications in various field conditions including low temperatures, and selections of various types of microalgal and duckweed species. Future research needs were also proposed accordingly.