Enhanced denitrification using high-molecular-weight poly(lactic acid) blended with lactide as an effective carbon source

[Display omitted] •A biobased, biodegradable, eco-friendly PLA/LAx carbon source was prepared.•High-molecular-weight PLA biodegraded better with slight ʟ-lactide.•Phylum unclassified_f__Rhodocyclaceae played a crucial role in denitrification.•Enriched Bacteroidota and Chloroflexi endow more biodegradability for PLA.•Mechanism of incorporating slight ʟ-lactide on PLA biodegradability was revealed. The utilization of wasted Poly(lactic acid) (PLA) as low-cost carbon sources in solid-phase denitrification is hindered by its low biodegradability, which can be attributed to its high molecular weight. This study presents a new approach by blending high-molecular-weight PLA with a small amount of ʟ-lactide (PLA/LAx) to treat nitrate-contaminated wastewater. The addition of ʟ-lactide enhanced the release of carbon from high-molecular-weight PLA. An impressive denitrification efficiency of 96.7% was achieved, accompanied by extremely low levels of accumulated NO2--N (0.1 mg/L) and NH4+-N (0.4 mg/L). The quantity of ʟ-lactide used significantly impacted the bacterial community structure. A high abundance of the phyla Bacteroidota and Chloroflexi associated with polymer degradation was observed. The most dominant denitrifier was the genus unclassified_f__Rhodocyclaceae belonged to the phylum Proteobacteria. This study demonstrates that blending PLA with just 5 wt% lactide can transform it into a highly effective solid-phase carbon source to eliminate nitrates.