M.Sc Thesis

M.Sc StudentAshkenazi Aviad
SubjectEffects of Salinity, Carbon Source and Bioreactor Type
on Community Structure of the Active Denitrifiers
DepartmentDepartment of Civil and Environmental Engineering
Supervisors PROFESSOR EMERITUS Michal Green
DR. Sharon Avrahami


This research work had been characterized the denitrifying bacteria communities developed in denitrification bioreactors. The examined bioreactors types were Fluidized Bed, Fixed Bed and Upflow Sludge Blanket (USB). USB bioreactor is characterized with high biomass concentration and that microorganisms spontaneously adhere to each other, forming dense granules. Biological granules are formed and settled in bioreactors only in specific operation conditions and the bioreactor efficiency is depended on the settling nature of the biomass.

Previous results of denitrification in USB bioreactors fed with drinking water and ethanol showed that the efficiency and stability of the process was mainly depended on the precipitation potential in the bioreactor and the resulting high mineral content of the granules. In the same bioreactor but with feed solution supplemented with 1% brine and acetic acid, the forming sludge was settled and a mineral fraction of only 10-15% was measured. When the electron donor was methanol or ethanol and the bioreactor fed with brine, non-stable granules was formed.

The hypothesis of this research was that different operations condition such as: salt concentration and electron donor might cause microbial community selection, which differing in the settling characteristics of the granules in the USB bioreactor.

This research found that bacterial community developed in USB bioreactor fed with different salt concentration (without salt addition and 1% brine) were different in all types of bioreactors tested. The difference was expressed in different patterns of the DGGE: band diversity (quantity, location in the well), and band intensity. In addition, there were differences in phylogenetic analysis.

It was also found that the electron donor had a great influence on the bacterial community structure: There was a similarity between the bacterial community structure found in all types of bioreactors fed with methanol as well as in bioreactors fed with acetic acid. On the other hand, great differences were exhibited among bioreactors from the same type fed with different electrons donor.

DGGE gel patterns based on DNA and cDNA indicated that most of the bacterial populations were active, which may suggest that these populations perform denitrification, since the condition would favor denitrifying populations' development.

The results show that in bioreactors fed with different solutions, different microbial communities were developed. In USB bioreactor fed with acetic acid and brine with stable settling sludge developed different microbial community than bioreactors fed with different solution in which the sludge was washed out of the bioreactors.