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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Nakhla, George

Abstract

With increasing focus on the carbon footprint of wastewater treatment and rapidly emerging paradigm shift towards resource recovery, energy consumption minimization and utilization of readily available organics for biological nutrient removal in municipal wastewater treatment plants is eliciting significant interest. The objective of this PhD work is to investigate non-traditional approach to minimize carbon and energy demand for biological nutrient removal

The feasibility of using thermal alkaline treated municipal wastewater biosolids as an alternative carbon source for biological phosphorus removal was investigated. Two sequencing batch reactors (SBRs) were operated with synthetic volatile fatty acids (acetic acid and propionic acid) and readily biodegradable organics produced from the alkaline hydrolysis of municipal wastewater biosolids (Lystek) as the carbon source, respectively. Municipal wastewaters with different strengths and COD:N:P ratios were tested. The reactors’ performances were found to be comparable with respect to nitrogen and phosphorus removal. It was observed that phosphorus removal efficiencies were between 98% to 99% and 90% to 97% and nitrogen removal efficiencies were 78% to 81%, and 67% for the SynVFA and Lystek, respectively. However, the kinetics for phosphorus release and uptake during the anaerobic and aerobic stages with Lystek were observed to be significantly lower than SynVFA due to the presence of higher order VFAs (C4 and above) and other fermentable organics in the Lystek.

A novel integrated partial nitrification-denitrifying phosphorus removal system enriched with non-conventional phosphorus accumulating organisms (PAOs) was developed for treating carbon limited synthetic wastewater. Atypical operating conditions, such as low DO (0.3±0.05 mg/L) and relatively long solid retention time (SRT) of 15 days, favored the enrichment of a wide variety of denitrifying phosphorus accumulating organisms (DPAOs), such as Rhodocyclus, Dechloromonas, and Cytophaga. In contrast to the Accumulibacter, these microorganisms can sustain in a very low DO environments and simultaneously perform denitrification and enhanced biological phosphorus removal (EBPR) using oxygen, nitrite, and nitrate as electron acceptors. Fermentative microorganisms, such as Bacteroidetes, were also observed. Low DO also favored the washout of nitrite oxidizing bacteria (NOB), leading to simultaneous partial nitrification-denitrifying phosphorus removal (PNDPR). Partial nitrification at low DO also facilitated the washout of glycogen accumulating organisms (GAOs) from the PNDPR system. When operated with synthetic wastewater, stable operating conditions were achieved within 3-4 SRT turnovers and simultaneous nitritation-denitritation (SND), nitrogen, and phosphorus removal efficiencies were maintained above 90%. Of the total P removed by EBPR, P-removal percentages via nitrite, nitrate, and oxygen were 69%, 23%, and 8%, respectively. Utilizing nitrite instead of nitrate and low DO aeration implies a significant reduction in carbon and aeration requirement for simultaneous denitrification and phosphorus removal.

Lastly, the PNDPR system was implemented for treating real municipal wastewater with low COD/N ratio. In addition to low DO (0.3±0.05) mg/L, an extended anaerobic contact time facilitated the efficient utilization of organic carbon in wastewater and nutrient removal without carbon supplementation. Low DO during the aerobic stage was favorable for anoxic P-removal rather than aerobic as evidenced by simultaneous N and P removal in the cyclic test. Most of the rapid initial P uptake during the aerobic phase was attributed to DPAOs utilizing nitrites rather than nitrates, with NOx-N accumulating after almost complete utilization of the stored PHA and associated P uptake. The ratio of COD utilized to NOx-N reduced was estimated to be 4.2, which also implies efficient utilization of carbon for nutrient removal. Due to the integration of nitrification with denitrifying phosphorus removal, more than 70% N-removal and 90% P-removal was observed even at low COD/N ratio of 5. COD removal was not impacted by low DO as effluent sCOD concentrations were consistently below 25 mg/L. Compared to the conventional EBPR process, the low DO-SNDPR process implies maximum reductions in energy and carbon consumption of 35% and 45%, respectively. This can significantly reduce the overall carbon footprint of municipal wastewater treatment plants.

Summary for Lay Audience

Nutrients in wastewater effluents, i.e. nitrogen (N) and phosphorus (P) have elicited significant interest because of eutrophication of lakes and rivers in North America and many other parts of the world. Eutrophication is generally defined as the enrichment of N & P leading to the uncontrolled growth of aquatic plants/planktons, resulting in low dissolved oxygen (DO), murky water, and destruction of the diversity of aquatic species.

In biological wastewater treatment process, nutrients are removed by bacterial microorganism consuming N and P from the wastewater for their microbial growth and maintenance.

While wastewater treatment plants (WWTPs) are vital for the safety of public health and environment, they are also one of the largest scavengers of material and energy in the community. The energy consumption by wastewater treatment plants account for 0.25%-1% of the national energy consumption in many countries. This consumption is expected to increase with increasing population, economic activity, stricter regulations, and infrastructure ageing. Furthermore, to enhance the performance of BNR, readily biodegradable carbon is generally added, if the raw wastewater does not contain enough readily biodegradable carbon. Typically, acetic acid and propionic acid are used as a carbon source, which significantly increases operational costs. Besides the economic aspects, excessive use of these chemicals also increases the carbon footprint of the WWTPs.

This PhD project aimed at developing strategies for resource recovery and minimizing carbon and energy consumption in WWTPs.

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