Long-term effect of sediment on the performance of a pilot-scale duckweed-based waste stabilization pond

Duckweed-based waste stabilization ponds (DWPs) have been widely used in wastewater treatment. However, the effects of sediment, an essential component of DWPs, on their performance have rarely been studied. In this study, two pilot-scale DWPs (12 m²) with sediment (DPS) and without sediment (DP) were evaluated over more than 1 year to determine the effects of sediment on duckweed growth, wastewater treatment, and greenhouse gas (GHG) production and emission in DWPs. The results indicated that the annual average duckweed growth rate were comparable, but protein content, carbon (C) and nitrogen (N) recovery rates of duckweed were slightly higher in the DPS than in the DP. Meanwhile, the dissolved oxygen (DO) and oxidation reduction potential (ORP), removal efficiencies of COD, TP, TN, NH₄⁺-N, and turbidity of pond water from the DPS were significantly lower than for DP. More importantly, the DPS had considerably higher CH₄ production/emission and global warming potential (GWP) than the DP, even though more than 90% of CH₄ released from the sediment was consumed during its passage through the water column and duckweed layer. Sediment increased the recoveries of C and N by 7.94% and 8.82%, respectively. Influencing degree for COD, TP, TN, NH₄⁺-N and turbidity were -27.92%, -20.98%, -22.61%, -24.13% and -14.91%, respectively; for pond water DO and ORP, the values were - 35.68% and -44.59%, respectively; and for CO₂, CH₄ and N₂O emission and "combined GWP", they were 21.66%, 271.67%, -8.47% and 178.02%, respectively. Thus, this study indicates that sediment formed in the DWPs has a multi-faced effect on the performance of a DWP. In particular, sediment has an unfavourable effect on the wastewater treatment and the GHGs mitigation, but a favourable effect on the protein content and the C and N recoveries in duckweed.