Graduation Year


Document Type




Degree Granting Department

Environmental Engineering

Major Professor

Peter G. Stroot, Ph.D

Committee Member

Daniel Yeh, Ph.D

Committee Member

Jeffrey Cunningham, Ph.D


Ammonium, Nitrate, Nitrifying bacteria, Nitrifiers, Wastewater


Wastewater treatment plants (WWTPs) are required to remove ammonium (NH4+) from wastewater due to its oxygen demand and toxicity to the aquatic organisms. Ammonium is removed in the activated sludge treatment system by nitrification and denitrification processes. Nitrification is the oxidation of NH4+ to nitrate (NO3-) by autotrophic nitrifying bacteria which use carbon dioxide (CO2) as a carbon source for growth. These bacteria grow slowly with low nitrification rates limiting WWTPs capacity. In this research it was hypothesized that supplying higher concentrations of CO2 during aeration increases nitrification rates, resulting in a reduction of the solids retention time (SRT).

This hypothesis was tested with two lab-scale sequencing batch reactors seeded with sludge from a full-scale activated sludge WWTP and fed synthetic wastewater. The control reactor was aerated with regular air (0.03% CO2) and the experimental reactor was aerated with air containing 1% CO2. Ammonium and NO3- were measured online to determine the nitrification rates. Samples for solids and chemical oxygen demand (COD) determination were collected to evaluate the system performance.

Supplying CO2 to the experimental reactor throughout the entire react cycle resulted in proliferation of filamentous bacteria, poor settling, and washout of the biomass. However, nitrate formation rates in the experimental reactor were 3 times higher than the control before washout occurred. In a subsequent experiment, CO2 was supplied to the experimental reactor only during the last 5 hours of the cycle, resulting in excellent settling and nitrification rates 6 times higher than in the control. A confirmatory experiment was conducted that lowered the SRT from 8 days to 6, 4, and 2 days. Nitrate formation rates were up to 12 times higher in the experimental reactor compared to the control, with an average of 4 times higher. Additionally, the sludge volume index (SVI) suggested a positive impact of CO2 on settling performance. No impact of CO2 on COD removal was observed.

The results obtained suggest a positive effect of CO2 on the nitrate formation and settling performance in the activated sludge system, indicating that nitrification can be achieved at low SRTs which might optimize WWTPs capacity.