Effects of Methyl, Ester, and Amine Surface Groups on Microbial Activity and Communities in Nitrifying Biofilms

The objective of this study was to determine how different attachment surface chemistries affected the initial and long-term performance and microbial populations of nitrifying biofilms under well-controlled hydrodynamic mixing conditions. While much previous research has focused on the effects of surface properties such as hydrophobicity on bacterial attachment in pure cultures, this study evaluated the effects of specific functional groups on mixed culture composition and functional behavior. Three surfaces with varying hydrophobicity and charge were evaluated for biofilm community development and performance: unmodified poly(dimethylsiloxane) (PDMS), which included terminal methyl groups and was relatively hydrophobic (P-Methyl), PDMS silanized with ester groups (P-Ester), which was uncharged and relatively hydrophilic, and PDMS modified with amine groups (P-Amine), which possessed a positive charge and was the most hydrophilic. The surface chemistries of the three attachment surfaces were characterized by contact angle goniometry, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). These surfaces were inoculated with dilute activated sludge, and biofilms were grown in rotating annular bioreactors for 80 days, with experimental triplicates. Nitrification rates increased most rapidly in P-Amine biofilm reactors, and their biofilm communities contained significantly more Nitrosomonas (p < 0.05) than those on the other surfaces in early growth stages (days 40-50). From days 50-60, the P-Amine surface biofilm had significantly higher nitrate production rates than the P-Methyl and P-Ester biofilms. The biofilms grown on the P-Amine and P-Methyl surfaces were significantly (p < 0.05) more diverse than the P-Ester biofilms, containing higher relative abundances of the order Rhizobiales, including a significantly higher abundance of the nitrifying genus Nitrobacter (p < 0.05), which coincided with higher rates of nitrate generation. Conversely, biofilms grown on the uncharged hydrophilic P-Ester surface were consistently less productive and had lower diversity than biofilms on the other surfaces. These results indicate that surface chemistry may be a useful design parameter to improve the performance of nitrifying biofilm systems for wastewater treatment and that surface chemistry affects mixed biofilm community composition.