Design of Biofilm Using Fine Particle Technology

Abstract

The biological denitrification process is widely used in waste water treatment due to the low operational cost compared with that in physicochemical treatment. However, two different reactors are required in this process because ammonia nitrogen is converted into harmless nitrogen gas through two different biological steps （nitrifying and denitrifying steps） required for different environment （aerobic and anaerobic conditions). The purpose of our research is to design a composite biofilm of denitrifying bacteria and nitrifying bacteria that imitate a biofilm in the nature from the viewpoint of fine particle technology. Five different types of denitrifying bacteria and two different types of nitrifying bacteria were used as model bacteria. Surface physicochemical properties can be treated as an indicator of the adhesive properties of microbial cells. The electrophoretic mobility was measured as a function of ionic strength using the laser Doppler method and the surface potential was estimated using the soft particle theory. The microbial cells used in this study were charged negatively. The contact angle between a microbial lawn and a droplet of a specified fluid was measured using the sessile drop technique and the change in the free energy of interaction, ΔG, between one species of microbial cells, between different species of microbial cells. This result indicated that denitrifying bacteria and nitrifying bacteria could not be aggregated thermodynamically because ΔG was positive in all combinations. To promote the microbial adhesion, disrupted microbial cell suspension, chitosan, polyethyleneimine and dopamine were added to the mixed cell suspension, respectively. It was found that polydopamine was effective to aggregate denitrifying bacteria and nitrifying bacteria artificially. Finally, the formation of the composite biofilm of denitrifying bacteria and nitrifying bacteria were succeeded.