Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Qiong Zhang, Ph.D.

Co-Major Professor

Sarina J. Ergas, Ph.D.

Committee Member

John Kuhn, Ph.D.

Committee Member

Kathleen Scott, Ph.D.

Committee Member

Meng Wang, Ph.D.

Committee Member

Ronald Sims, Ph.D.

Keywords

Algal-Bacterial Integration, Denitritation, High-Rate Algal Pond, Nitritation, Simulation

Abstract

Nitrogen is one of the essential elements for life; however, inefficient use of nitrogen creates major environmental problems. Nitrogen-rich streams from urban areas, farms, and wastewater treatment plants (WWTPs) lead to surface and groundwater contamination, resulting in eutrophication, hypoxia, and notable environmental, public health and economic damage. Ammonia is a form of nitrogen that can be created during industrial or biological nitrogen fixation or decomposition of biomass. Anaerobic digesters (ADs) in WWTP generate biogas from collected sludge from primary and secondary clarifiers to achieve energy recovery in the plant. However, sidestream effluent from dewatering AD sludge contains high concentrations of ammonium (up to 2000 mg NH4+-N/ L). Returning sidestream to the head of the mainstream WWTP with no treatment increases the total incoming nitrogen load by 20-30% and often exceeds facility’s treatment capacity. Conventional biological nitrogen removal (BNR) processes demand considerable energy for aeration to oxidize ammonium to nitrate during nitrification. In addition, an external electron donor, such as methanol, is often needed to reduce nitrate to nitrogen gas during denitrification. Shortcut nitrogen removal (SNR) partially overcomes these drawbacks by saving 25% of oxygen demand and 40% of organic electron donor compared with conventional BNR. The SNR process is carried out by suppressing the nitrite-oxidizing bacteria (NOB) and providing a suitable growth environment for ammonium oxidizing bacteria (AOB). Growing algae under light conditions can generate oxygen to be used by AOB. Denitritation can then be accomplished during dark conditions when algal oxygen generation ceases, and the bioreactor becomes anoxic.

This dissertation conducted a study of the use of algal-bacterial consortia for the SNR process in a photo-sequencing batch reactor (PSBR). The PSBR provides aerobic and anoxic conditions for nitritation/ denitritation by alternating light and dark periods. The overarching question that this dissertation explores is, what operating conditions and design parameters should be applied for AD sidestream effluent treatment unit using PSBR and SNR process to meet target effluent concentrations with minimum reactor volume?

Bench-scale PSBR studies were carried out to investigate feasibility, kinetics, and nitrogen removal performance. Duplicate PSBRs were set up in the Environmental Engineering laboratory at the University of South Florida and operated with fixed light and dark periods (12h/ 12h). PSBRs were operated in the following five stages: 1) feeding, 2) mixing under light (nitritation), 3) mixing under dark (denitritation) with the addition of organic carbon (sodium acetate), 4) settling, and 5) decanting. Solids retention time (SRT) and hydraulic retention time (HRT) were set at 10 and 4 days, respectively. Controlling alkalinity during nitritation promoted algae and AOB growth. It was found that excessive organic carbon addition at the beginning of the dark period negatively impacted nitritation by remaining in the PSBR during the subsequent cycle. This favored heterotrophic bacterial growth, which competed with AOB during the light period. Successful nitrogen removal (>90%) was achieved in the SNR process once inorganic and organic carbon addition were adjusted.

A PSBR model was developed with rate equations for nineteen dissolved and particulate chemical and biological species. The equations were solved using an ordinary differential equation (ODE) solver in MATLAB. Experimental results from bench-scale PSBR studies were used in model calibration and validation. The model simulated nitrogen removal, oxygen production by algae, and microbial biomass trends over 24h cycles for 150 days of reactor operation. The model also simulated biomass settling and estimated daily wasting volume to maintain the target SRT. Conversion rate diagrams are an output from the model, which allow the user to monitor oxygen, nitrogen, organic carbon, and inorganic carbon species production/ consumption in a cycle. Thus, operating conditions to control the nitrogen removal pathway can be identified and applied.

The second part of the research focused on PSBR operation under more realistic daily light conditions. Typical Tampa Bay light patterns during the summer and winter were employed for bench-scale PSBR operation using programmable LED lights. Due to lower light intensity nitrogen loading rates were reduced for winter conditions. The model was recalibrated according to varying light intensities and validated with experimental results for summer and winter mode using nitrogen species and oxygen concentrations during a complete cycle. It was found that maintaining lower HRTs during the summer and winter illumination promoted NOB growth, which deviated from an ideal SNR process. However, it is not efficient to design separate PSBRs for the summer and winter seasons. Thus, an HRT of 6 days was selected from the biomass growth simulation diagram to reach SNR in both summer and winter modes using light illumination data over a year. Effects of daily and seasonal changes were observed in simulation results for 380 days.

The last part of the research applied the model to a PSBR design for AD sidestream treatment at the South Cross Bayou (SCB) Advanced Water Reclamation Facility in St. Petersburg, FL as a real case study with sidestream effluents from ADs. Light penetration length, or reactor depth, was investigated as a key parameter in the design of the PSBR. A flow rate of 575 m3/d with NH4+-N concentration of 715 mg/ L was used as the influent characteristic for a PSBR in a raceway configuration.

The dissertation suggested different design parameter scenarios such as depths of 0.9, 1, 1.1 and 1.2 m to evaluate surface area required according to target nitrogen concentration (40 mg N/L) to be recirculated to the mainstream. Limited prior studies have included a dynamic model to monitor nitrogen removal mechanisms, long-term operation and biomass wasting calculations. Those studies with long-term simulation also incorporated constant light intensities or mainstream wastewater with low ammonia concentrations. This study provides a tool to design a full-scale PSBR incorporating algal-bacterial SNR from a high ammonia strength wastewater according to regional and seasonal light conditions.

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