Civil Engineering ETDs


Yunjie Tu

Publication Date



Enhanced biological phosphorus removal (EBPR) is a popular modification of the activated sludge process, and has been studied and applied for decades. It was hypothesized that the bacteria responsible for EBPR, polyphosphate accumulating organisms (PAOs), have an advantage over their competitors, the glycogen accumulating organisms (GAOs). Because research EBPR systems have been primarily sequencing batch reactors (SBRs), which include rapid feed addition and relatively high substrate concentrations, while full scale systems often include complete mix reactors, which yield generally lower substrate concentrations due to instantaneous dilution of influent, evaluating this hypothesis has both fundamental and applied implications. The primary objectives of this research were to (1) evaluate the effects of carbon substrate (acetate) concentrations on PAO/GAO competition in conjunction with variable pH and temperature, and (2) evaluate whether GAOs tend to be causes of PAO failure or opportunists arising after PAO failure laboratory-scale sequencing batch reactor (SBR) EBPR systems were operated with synthetic feed, and acetate as the main carbon source for more than 650 days. Experimental variables with respect to reactor operation include pH, temperature, and rate of acetate addition during each cycle feeding phase. Consistent with previous research, PAOs were dominant at higher pH values (7.4-8.4) when acetate was added rapidly (Stage 1), and when pH was decreased, EBPR failed (Stage 2). When the rate of acetate addition was then slowed to maintain low concentrations in the reactors (Stage 3), PAOs were able to once again dominate the system, supporting the hypothesis that PAOs may be favored by low acetate concentrations. Returning the system to an increased rate of acetate addition again led to PAO failure (Stage 4). PAO failure in Stages 2 and 4 did not appear to be caused by GAOs, based on measurements of acetate concentrations, and it appeared that high acetate concentrations at low pH were themselves inhibitory to PAO metabolism. Batch tests with chemical inhibitors indicated that PAOs and GAOs likely use different transport systems to take up acetate, which provide PAOs the ability to scavenge low acetate concentrations through their particular transport mechanism, and inhibition was increased at lower acetate concentrations. Lower acetate concentrations did not provide an advantage of PAOs over GAOs when the temperature was increased from 22°C to 29°C. Lower acetate concentrations may be advantageous to EBPR process performance, as they increased the biomass polyphosphate content by 50% relative to higher acetate concentration conditions (22°C, pH = 7.4-8.4). All the performance data were consistent with the Neisser staining, FISH and pyrosequencing analyses results.


Sewage--Purification--Phosphate removal, Sewage--Purification--Activated sludge process, Acetates.


National Science Foundation, grant #0852469

Document Type




Degree Name

Civil Engineering

Level of Degree


Department Name

Civil Engineering

First Advisor

Schuler, Andrew

First Committee Member (Chair)

Thomson, Bruce

Second Committee Member

Howe, Kerry

Third Committee Member

Takacs-Vesbach, Cristina