Denitrification performance and biofilm characteristics using biodegradable polymers PCL as carriers and carbon source

• A fixed bed reactor packed with PCL was developed to remove nitrate.
• TN removal efficiency was more than 95% at HRT of 3–6 h.
• PCLs are favorable for biofilm attachment.
• Genus Diaphorobacter belonging to family Comamonadaceae accounted for most of the sequences.
• The chain scission was proved due to hydrolytic degradation of PCL.

In this study, a fixed bed bioreactor filled with biopolymer polycaprolactone (PCL) was developed to remove nitrate from groundwater through denitrification. PCL serves both as carbon source and biofilm carriers. During a long-term operation of 561 d, the performance of the reactor in nitrogen removal, characteristics of biofilm attached to biodegradable carriers, and the hydrolysis and utilization of solid carbon source were studied. With temperatures exceeding 24 °C, the average nitrate concentration in the effluent was lower than 3.7 mg N L−1, and more than 95% of TN was removed at hydraulic retention time of 3–6 h. Nitrite and ammonium remained at low levels (less than 0.32 and 0.78 mg N L−1, respectively). The calculated PCL amount consumed ranged between 1.6 and 3.7 g PCL g−1 NO3–N. The FT-IR spectrum of the used PCL indicated the chain scission by hydrolytic degradation. A pyrosequencing analysis of the biofilm showed that genus Diaphorobacter belonging to family Comamonadaceae accounted for most of the sequences. Bacteria of genera Hydrogenophaga, Rhodocyclaceae uncultured and Desulfovibrio were highly enriched in the PCL biofilm. Microelectrode data indicated that the biofilm had an average thickness of around 800 μm and the intensive denitrification activities occurred in the area of 300–500 μm of biofilm with values up to 400 μmol cm−3 h−1.