Graduation Year

2024

Document Type

Thesis

Degree

M.S.E.V.

Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina J. Ergas, Ph.D.

Co-Major Professor

Jeffrey Cunningham, Ph.D.

Committee Member

Erica Dasi, Ph.D.

Keywords

Acetate, Methanol, MicroC, Soluble Microbial Products, disinfection by-products

Abstract

Nitrogen removal from wastewater is critical to preserve ecosystems, as well as human health and welfare. Excess nutrients in receiving waters can cause many problems to the environment, such as algal blooms, drinking water contamination, and economic losses. Nitrogen removal in wastewater occurs mainly through the oxidation and reduction processes of nitrification and denitrification, which occur in engineered systems at wastewater treatment plants or water reclamation facilities.

The site for this research was the South Cross Bayou (SCB) Water Reclamation Facility, located in Pinellas County, FL, USA. At SCB, nitrogen is removed through a Modified Ludzak-Ettinger (MLE) process with the downstream addition of denitrification filters. SCB utilizes methanol as an electron donor and external carbon source in the denitrification filters to achieve low Total Nitrogen (TN) effluent levels. The problem is that methanol is a flammable and toxic compound, and the facility is in a residential area. Therefore, the motivation for this research was based on this safety concern brought by Pinellas County. The objective of this study was to determine the efficacy of alternative electron donors, meaning their ability to remove TN to low levels, as well as determining potential impacts regarding the formation of trihalomethanes, since the wastewater is chlorinated prior to being discharged at Joe’s Creek.

To determine the effectiveness of different electron donors, a previously designed and constructed pilot-scale deep bed denitrification filter at the SCB facility was used to test the performance of methanol, MicroC™, and acetic acid. The pilot-scale system was operated at the same hydraulic loading rates as the full-scale filter at the facility. Once the system was running continuously, samples from the influent and effluent were collected four times for each electron donor and were analyzed for different water quality parameters. Average NOx (the sum of nitrite and nitrate) and TN removal rates were also calculated to assess pilot system performance. Alternative electron donors were also tested through bench-scale denitrification reactors in the lab. The electron donors tested in the lab were: methanol, MicroC™, MicroC™ incubated at 30°C, acetic acid, sulfur, and an inoculum-only control (i.e. no electron donor addition). Once denitrification was complete, the supernatant was collected and chlorinated in the lab to investigate if different electron donors lead to different levels of disinfection byproduct formation.

Results from the pilot-scale system suggest that MicroC™ is a promising alternative electron donor for denitrification, being able to achieve similar removal rates to that of methanol if it is used in its pure form rather than diluting it. The average NOx removal rate under methanol was 405 mg/(L*day) versus 402 mg/(L*day) for MicroC™. When it comes to TN removal, methanol performed better at 476 mg/(L*day) versus 423 mg/(L*day) for MicroC™. However, the use of MicroC™ in the pilot-scale system led to operational challenges, such as significant biofilm growth in the chemical feed tank due to fermentation occurring and increased need for backwash. Currently, pilot studies with acetic acid are still ongoing at SCB.

Trihalomethane (THM) formation potential studies showed that the THM formation across the different electron donors is similar, except for MicroC™, which showed significantly less formation, which was most likely caused by a diverse microbial community that can consume different SMPs generated during denitrification. In addition, a correlation between dissolved organic carbon (DOC) concentration and THM formation was not found in this study, and it is hypothesized that the organic carbon matrix and the presence and structure of the soluble microbial products (SMPs) can be used to explain THM formation patterns based on the literature available. SMPs formed under acetic acid and methanol are similar, which explains their similar THM formation levels, as well as speciation. SMPs are also produced under sulfur oxidizing denitrification and even without any carbon addition, which explains high THM formation levels across these microcosms. Lastly, an increase in temperature led to an increase in the formation of brominated THMs but not chloroform, and the impact of reaction time varied across the different microcosms.

To sum up, MicroC™ is a promising alternative to methanol for denitrification at the SCB facility’s denitrification filters, as it can achieve high NOX and TN removal rates and leads to a lower formation of disinfection byproducts that are regulated by FDEP and are toxic to the ecosystems and human health. MicroC™, however, is more expensive and requires more frequent backwashing, increasing the county’s operational costs. These costs are offset by the non-hazardous nature of MicroC™, as methanol poses a significant safety hazard to the neighboring community.

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