Wastewater–phosphorus coupling accelerates biofilm–mineral–particulate interactions in irrigation pipelines

Biogas slurry, a liquid by-product of anaerobic digestion, is increasingly reused in agriculture, but its low phosphorus content often requires external supplementation. When phosphorus is introduced into slurry fertigation, it interacts with Ca²⁺, Mg²⁺, bicarbonates, and organic matter, creating conditions that can accelerate pipeline clogging. In this study, we combined a 60-day accelerated pipeline experiment with mineralogical analysis, scanning electron microscopy, and high-throughput sequencing to examine how wastewater–phosphorus coupling influences fouling. Slurry alone caused little short-term clogging, but the addition of phosphorus led to sharp increases in fouling mass (up to 130%) and flow reduction (up to 90%). Mineralogical analyses identified secondary phosphate precipitates such as brushite, baricite, and apatite, while microbial community profiling showed greater diversity, persistence, and biofilm-forming capacity under phosphorus conditions. Correlation analysis and structural equation modeling demonstrated that precipitates provided scaffolds for biofilm growth, particulates enhanced microbial attachment, and biofilms linked physical and chemical processes to hydraulic decline. These results show that pipeline fouling under wastewater–phosphorus coupling arises from the synergy of biofilms, minerals, and particulates, and they offer mechanistic guidance for designing fouling control systems that support sustainable reuse of livestock wastewater.