Pre-aeration promotes nutrient removal in a pilot-scale duckweed-based pond by influencing the duckweed growth and bacterial community

Abstract

Duckweed-based ponds (DPs) are widely used to treat wastewater due to the rapid growth and high-value biomass of duckweed. However, their performance at nutrient removal is restricted by low dissolved oxygen (DO) concentrations in pond water, and direct aeration of a DP is impracticable because floating duckweed is vulnerable to interference by bubbles. Thus, we developed an influent pre-aeration treatment process for a DP and investigated its effect on the DO concentration and performance of the pilot-scale (12 m2) DP, as well as the underlying mechanism over a 9-month period. Pre-aeration increased the DO concentration (by 478 %) and oxidation reduction potential (by 55 %) in pond water, and promoted duckweed growth (by 33 %) and total phosphorous, total nitrogen, and NH4+-N removal (by 84 %, 80 %, and 91 %, respectively). It also increased the contributions of microorganisms to nitrogen and phosphorous removal (13.37 % and 35.52 % greater, respectively), indicating that it had greater beneficial effects on microorganisms than on duckweed. In the bacterial community, pre-aeration likely promoted nutrient removal by improving the growth and activity of bacterial assemblages involved in nutrient cycling (e.g., Rhodopseudomonas, Cloacibacterium, Candidatus_Limnoluna, and Novosphingobium in pond water), and by improving duckweed growth by altering the relative abundances of bacterial assemblages attached to duckweed. These findings help clarify the beneficial effects and mechanisms of influent pre-aeration, providing a new insight into the improvement and application of DPs.