The influence of the carrier material on the microbial diversity in packed-bed biofilm reactors

Microbial diversity in biofilms on two different types of carrier materials, wood chips (WD) and polyethylene carrier elements (PE) was studied in laboratory-scale packed bed bioreactors treating municipal waste water. Two packed bed bioreactors were set in series, thus enhancing also the removal of compounds more resistant to degradation. With both carrier materials the removal rate of BOD, COD and nutrients was higher in the first reactor. The maximum BOD removal rates in bioreactors R1 (WD) and R2 (PE) were 96% and 95% respectively. PCR-Denaturing Gradient Gel Electrophoresis (DGGE) analysis indicated that the bacterial species dominating the biofilm reactors differed from the diversity in influent waste water. The difference in the first and the second column was particularly obvious with the biofilm on polyethylene carrier material. Contrary to the bacterial diversity in the first bioreactor, there was only one major species detected in the second bioreactor. The bacteria detected belonged to Rhizobium genus. Acidobacteria were present in biofilms on both PE and WD, but were not detected from influent. Biofilm on WD, but not on PE, contained Verrucomicrobia and d-proteobacteria. Furthermore, only one ß-proteobacteria sequence was derived from PE biofilms.