Denitrification Performance and Microbial Community Characterization in a PHBV-Based Reactive Aquifer Considering In Situ Remediation Situations

Groundwater contaminated by nitrate has become a pronounced issue in some areas worldwide, yielding health concerns and a paucity of high-quality drinking water. Biodegradable polymer-based (BDP) denitrification has been widely studied for the remediation of nitrate-contaminated groundwater due to its stable high-rate denitrification and minimal effluent pollution. However, few studies have comprehensively considered real-world factors such as onsite plume flow velocity, the appropriate ratio of BDPs to sand, and extremely high nitrate concentrations in aquifers when investigating in situ nitrate reduction. This study addresses these gaps by simulating denitrification under controlled yet realistic aquifer conditions using a cuboid apparatus packed with poly-3-hydroxybutyrate-co-hydroxyvalerate (PHBV) and sand (volume ratio 1∶189), with a fixed flow rate of 8.33 cm·d−1. Over 81 days, synthetic wastewater with nitrate concentrations ranging from 18 to 117 mg NO3−─N·L−1 was introduced, and water quality was assessed through collecting effluent water samples, along with microbial community composition analysis. Results showed that during Stage 1 (influent: 18–56 mg NO3−─N·L−1), effluent nitrate concentrations ranged from 0.013 to 15 mg·L−1, achieving up to 99.9% removal efficiency with denitrification rates between 1.2 and 5.3 mg NO3−─N·L−1·d−1. Effluent dissolved organic carbon (DOC) concentrations ranged from 76 to 108 mg·L−1, and pH ranged from 7.2 to 7.4. In Stage 2 (influent: 59–117 mg NO3−─N·L−1), effluent nitrate concentrations ranged from 0.3 to 3.7 mg·L−1, with a peak removal efficiency of 96.2%, while the denitrification rate increased to 5.3–11.6 mg NO3−─N·L−1·d−1. Effluent DOC concentrations ranged from 6.3 to 27.2 mg·L−1, and pH ranged from 7.1 to 7.8. This implies that the system has good nitrogen removal capacity and can effectively purify water quality. The microbial community was predominantly composed of chemoheterotrophic bacteria, with Proteobacteria as the most abundant phylum (52.29%) and Acinetobacter as the dominant genus (33.65%). Functional predictions using FAPROTAX identified chemoheterotrophy and denitrification as the primary ecological roles. Our results indicated that PHBV could provide sufficient carbon sources, fostering the growth of denitrifying microorganisms under real-world aquifer conditions and achieving stable nitrate removal. The study highlights the interplay between pH, nitrogen content, and microbial community structure in influencing denitrification efficiency. These findings provide practical insights into optimizing BDP-based denitrification for large-scale groundwater remediation, offering a viable alternative for managing nitrate pollution in contaminated aquifers.