Observation of Self Regulation of Biofilm Thickness in Denitrifying MBBR

Fixed film technologies are successful biological processes, but not much data is available on the regulation of biofilm thickness and process mechanisms. To better understand process robustness of denitrification MBBR and seasonal biofilm thicknesses development and self regulation, a pilot study was implemented at the Blue Plains AWTP between December 2007 and July 2008. Biomass density testing was performed regularly throughout this study to evaluate the self-regulationof the biofilm on the carrier. The hypothesis of this study was that for a constant (well mixed) external mixing shear applied, the bacterial population supported on the carrier media to a large extent ‘self-regulated’ its biofilm thickness to address either changes in external loadings or internal growth rates. During this study, it was observed that significantly less biomass was growing on the outside of the carriers than on the inside, probably caused by scraping action due to carrier collision, limiting accumulation. As the MBBR pilot trains each consisted of two anoxic reactors in series, the first reactor would consistently have more biomass accumulation due to a higher influent loading, as compared to the biomass in the second reactor downstream. Once the biomass was established it remained fairly constant (12-14 g/m2) at 16.5°C until the temperature dropped to 13.5°C, at which point the biomass density started to increase quite rapidly reaching a density of 17-19 g/m2. Once the temperature increased again to 22.1°C, the biomass density decreased again down to ∼12 g/m2. This implied that the bacterial growth was greater during the colder months and the demand for nutrients required for growth (nitrogen and phosphorous) was transiently greater during periods of biomass accumulation onto the media. In conclusion, the results of this pilot study implies that the biomass on biofilms self-regulate to adjust to changes in external loading and temperature.