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Thesis Format

Integrated Article


Doctor of Philosophy


Civil and Environmental Engineering

Collaborative Specialization

Environment and Sustainability


Nakhla, George


Cellulose from toilet paper is a significant fraction of particulate organics, which is recoverable. For the first time, comprehensive mapping and tracking the fate of cellulose across various unit processes at full-scale in two water resource recovery facilities located in North America and Europe was undertaken. The influent cellulose content accounted for approximately one-third of the total suspended solids (TSS). More than 80% of the raw wastewater cellulose was captured in primary treatment. The high cellulose content of the primary sludge accounting for 17%-35% of the TSS facilitates cellulose recovery. Cellulose biodegradation efficiency varied between 70%-90% of the primary effluent, confirming that cellulose recovery from primary treatment is beneficial to reduce oxygen demand.

Aeration is a major contributor to the high energy demand in municipal wastewater treatment plants. Thus, it is important to understand the dynamic impact of wastewater characteristics on oxygen transfer efficiency to develop suitable control strategies for minimizing energy consumption since aeration efficiency is influenced by the biodegradation of pollutants in the influent. The real-time impact of acetate as a readily biodegradable substrate and cellulose as a slowly biodegradable substrate were studied at different operational conditions. At an ambient DO of 2 mg l-1 and air flow of 1.02 m3 h-1 (0.6 SCFM), the α-factor was more sensitive to readily biodegradable substrates than to cellulose. On average, α-factor decreased by 48% and 19% due to the addition of acetate and cellulose, respectively. At a DO of 4 mg l-1 and air flow of 1.7 m3 h-1 (1 SCFM), α-factor remained constant irrespective of cellulose and acetate concentrations. An inverse correlation between the α-factor and reactor sCOD was defined and incorporated into a dynamic model to estimate the real-time airflow rates associated with the improvement of the oxygen transfer efficiency due to biodegradation.

The effect of bioreactor configurations on the dynamics of oxygen demand and aeration performance was assessed by conducting an advanced calibration study of a newly developed aeration model against experimental data during a pilot SBR study, and by utilizing the validated aeration model to assess different bioreactor configurations. Three different correlations to estimate α-factor were applied in the study. The first correlation which estimated the α-factor based on the operating reactor sCOD was able to predict the temporal measured air flow rate change in the SBRs pilot. The second correlation which estimated the α-factor based on the influent COD overestimated the air flow rates as it considered the impact of the influent loading rates on the α-factor and overlooked the improvement in α-factor due to biodegradation. The third correlation which estimated the α-factor based on an MLSS underestimated the air flow rates as it overlooked the impact of the influent loading rates on the α-factor. Results indicated that a completely mixed stirred reactor (CSTR) showed an aeration energy reduction of 56%-67% when compared to the plug flow model. The model-based analysis showed that the step-feed plug-flow reactor achieved a 15 % reduction in aeration energy relative to the plug-flow reactor. However, both systems had equivalent aeration energy when denitrification was considered. In a plug flow reactor and CSTR, denitrification reduced the aeration energy by 30% and 11%, respectively.

Cellulose hydrolysis rate constants under both anoxic and aerobic conditions were estimated using a calibrated batch model based on experimental measurements. The aerobic cellulose hydrolysis rate constant was 3.74±0.33 d-1, and the anoxic hydrolysis rate was 0.7±0.31 d-1. The estimated hydrolysis rate constants were then incorporated into a calibrated SBR model to estimate cellulose fraction in the influent wastewater. Influent cellulose accounted for 21% of influent total COD and 35% of influent TSS.

The addition of the fermented primary sludge at different SRTs to the SBR increased the efficiency of nitrogen and phosphorus removal by up to 92% and 98% when compared to the feed with RBF effluent only. The fermented primary sludge, however, had a marginal impact on α-factor, αSOTE, and OUR. The addition of the fermented primary sludge increased aeration energy by 25%-36% compared to the case of RBF effluent.

Summary for Lay Audience

Cellulose in municipal wastewater originates from the use of toilet paper. It represents a large fraction of the influent organic contaminants in wastewater. Biological wastewater treatment is one of the most economical approaches to the treatment of municipal wastewater using active bacteria (biomass) that are present in the influent wastewater. Active biomass biodegrades organic contaminants, including cellulose. To keep biomass functioning, oxygen is needed for respiration, and for this reason, there is a major step in any biological wastewater treatment plant known as aeration where the air is supplied to help the growth of bacteria. Aeration is designed to provide oxygen to active biomass so that it can decompose and biodegrade organic contaminants such as cellulose. According to the literature, aeration consumes more than half of the total energy of the treatment plant.

Cellulose is a particulate matter that can be removed by any solid separation technique, such as primary clarification, where the particles are removed by settling or rotating belt filters, where the particles are removed by sieving. Removing cellulose from influent wastewater reduces aeration energy and can be further treated with other discarded solids to produce energy or soluble organic compounds that can be used to improve the biological treatment process.

In this Ph.D. thesis, the fate of cellulose in the municipal wastewater treatment plant using data from two different full-scale treatment plants in two different regions; Europe and North America has been tracked and shown that more than 80% of the influent cellulose can be removed through primary solid separation. In addition, a pilot study was conducted to thoroughly investigate the impact of cellulose and other organic contaminants on aeration efficiency. Modeling was also used to understand the theoretical impact of organic biodegradation on aeration efficiency. The results of the study showed that the removal of cellulose through the primary solid separation step reduced the aeration energy by 25%. It also showed that aeration efficiency improved with the time of reaction due to biodegradation and therefore a mathematical relationship between aeration efficiency and organic biodegradation was developed to be used as a new design tool for aeration tanks.

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.