Nitrate removal efficiency and bacterial community dynamics in denitrification processes using poly (l-lactic acid) as the solid substrate

Abstract

Laboratory-scale solid-phase denitrification (SPD) reactors for nitrate removal were constructed by acclimating activated sludge with poly (L-lactic acid) (PLLA) having weight-average molecular weights (M w) of 9,900, 12,000, and 45,100 g mol -1. A good nitrate removal rate (3.5-5.3 mg NO 3--N g [dry wt] -1 h -1) was found in the reactor containing PLLA of 9,900 g mol -1, whereas the other two reactors with the higher M w PLLA showed low nitrate removal efficiency. Microbial community dynamics in the low M w PLLA-acclimated reactor were studied by 16S rRNA gene-targeted PCR-denaturing gradient gel electrophoresis and quinone profiling. Nonmetric multidimensional scaling analyses of these data sets revealed a marked population shift during acclimation of the SPD reactor with low M w PLLA. The 16S rRNA gene clone library and culture-dependent analyses showed that bacteria belonging to the family Comamonadaceae predominated and played the primary role in denitrification in the PLLA-using reactor; however, none of the bacterial isolates from the reactor degraded PLLA. These results suggest that the nitrate removal property of the PLLA-using SPD reactor is attained through the bioavailability of hydrolysates released abiotically from the solid substrate.