Date of Award


Document Type


Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Earth Science


Yang, Yanru

Committee Member

Freedman , David L.

Committee Member

Karanfil , Tanju


Microbial aggregation was investigated in this study using the activated sludge process as a model system, the performance of which is highly dependent on effective bioaggregation. This research includes two experiments. First, four laboratory-scale activated sludge sequencing batch reactors (SBRs) were constructed in order to evaluate the effect of solids retention time (SRT) on microbial aggregation. These SBRs were operated at SRT of 4, 8, 16, and 32 days, respectively, fed with a protein-based synthetic wastewater, and monitored over a period of 268 days for performance and sludge settling characteristics including soluble COD in the effluent (sCOD), sludge volume index (SVI), mixed liquor suspended solids (MLSS), effluent total suspended solids (eTSS). Additionally, microbial community structure was determined using polymerase chain reaction (PCR) (or nested PCR) and denaturing gradient gel electrophoresis (DGGE). All four reactors reached steady state after five times the SRTs. In agreement with previous research, a positive correlation was demonstrated between SRT and sludge settling capability at steady state, indicating an impact of SRT on bioaggregation. While this is previously thought due to the involvement of protozoa and extracellular polymeric substances (EPS) in aggregation, this study provides an alternative explanation, i.e., selection of a microbial community that has better aggregation capability. This is supported by two lines of evidence: 1) significantly different microbial community structures between the reactors at steady state; and 2) less diverse bacterial community with an increase in SRT, which might be resulted from disaggregation of growth-rate dependent aggregating bacteria in response to the increase in SRT and subsequent selection due to predation and/or mechanical washout. These results provide important insights into the complex relationship between SRT, microbial diversity, and settling characteristics of activated sludge.
Second, three full-scale WWTPs were surveyed for microbial communities in the aeration tank, clarifier effluent and return activated sludge (RAS) using PCR-DGGE for comparing the community structure of these different components of activated sludge. It was found that the planktonic and floc communities of one origin share significant similarity, mostly more than 50% for the general and specific bacterial groups and eukarya examined. This result is in agreement with other research findings in our lab derived from cultivation and isolation approach, and provides complementary evidence. It is shown that a significant fraction of bacteria in activated sludge are present in both floc and planktonic communities.