Integrated field-scale natural composite oxidation pond system for livestock wastewater treatment: Microbial insights and nutrient removal dynamics

Livestock wastewater rich in organic matter, nitrogen, and phosphorus can cause eutrophication and degrade aquatic ecosystems. Current water management practices in rural areas are insufficient to meet environmental protection requirements, highlighting the urgent need for low-cost, low-energy, ecologically sustainable approaches to livestock wastewater management across diverse settings. However, current hybrid systems are often limited to lab-scale or pilot studies. Therefore, we developed a natural composite oxidation pond system, integrating biological contact, oxidation ponds, and artificial aeration, to treat piggery wastewater, while simultaneously supporting aquaculture. High-resolution in-situ monitoring, metagenomic sequencing, and partial least squares modelling were employed to assess the system's pollutant removal performance and to elucidate the underlying mechanisms. The natural composite oxidation pond systems achieved average removal efficiencies of 76.6 ± 9.5 %, 60.8 ± 23.1 %, 70.8 ± 10.4 %, 74.2 ± 11.6 %, 81.9 ± 13.0 %, and 78.9 ± 39.5 % for total phosphorus, soluble reactive phosphorus, total nitrogen, NH₄⁺-N, NO₃⁻-N, and NO₂⁻-N, respectively (n = 72), improving water quality from inferior Class V (pond 1) to Class IV or V (pond 4 or pond 5). The removal efficiencies of soluble reactive phosphorus, NO₃⁻-N, and NO₂⁻-N showed significant variation with temperature (p ≤ 0.01). Proteobacteria dominated nitrogen removal through dissimilatory nitrate reduction and denitrification, with sediment microbes playing a more prominent role than water-column communities. Despite seasonal variability challenges, this closed-loop system provides a sustainable, dual-purpose approach to rural wastewater treatment and resource recovery.