Invasive macrophytes impair domestic wastewater treatment in stabilization ponds: evidence from litter decomposition in field-deployed microcosms

ABSTRACT Wastewater stabilization ponds (WSPs) provide critical services to society, yet knowledge gaps remain regarding heterotrophic bacteria's role in decomposing invasive macrophyte litter, the dynamics of decomposition within and across WSP treatment stages, and the effects on water quality and WSP performance. Using standard methods, heterotrophic bacteria and decomposition of Pistia stratiotes and Ludwigia adscendens litter were simultaneously monitored in wastewater from anaerobic, facultative, and maturation ponds. Total nitrogen (TN), total phosphorus (TP), total organic carbon (TC), spectral absorption coefficient at 254 nm (SAC254), and BOD5 were measured in wastewater with and without macrophyte litter following decomposition. Compliance of effluent from litter-impacted WSPs with East African standards was also assessed. Decomposition removed 52.5–91.5% of initial dry mass, with loss rates varying substantially within and across treatment stages but remaining statistically similar between species. Mass loss was strongly correlated with heterotrophic bacterial levels (r = 0.53–0.94, P < 0.05). Litter-amended wastewater exhibited higher concentrations of TN, TP, TC, SAC254, and BOD5, with fluctuating trends across treatment stages, whereas unamended wastewater showed lower concentrations that declined progressively. Effluent from litter-impacted WSPs failed to comply with discharge limits for TP, BOD5, and turbidity. Macrophyte litter is thus a significant pollutant source that may impair WSP performance.