Doctor of Philosophy
Civil and Environmental Engineering
The emerging paradigm shift towards renewable resource recovery, and energy minimization in municipal wastewater treatment plants (WWTPs) coupled with increased concern over nutrients-related eutrophication accelerated the development of biosolids treatment technologies for simultaneous waste minimization, resource recovery, and carbon upgrade. Biological nutrient removal (BNR) processes, often need excess carbon source to meet stringent quality standards. Despite successful use of sludge fermentation liquid to enhance the BNR, different techniques, are applied to improve the low conversion yield of fermentation and optimize resource recovery. In this context, insights on the impact of two commonly used primary treatment techniques (i.e. conventional primary clarifier and the emerging rotating belt filtration (RBF) technology) on the single and integrated anaerobic fermentation and digestion of wastewater biosolids was investigated in this study. Techno-economic assessment and optimization to simultaneously maximize volatile fatty acids (VFAs), and biomethane recoveries, and further application of internal carbon source to enhance the BNR process using a plant-wide approach, were also among the main objectives of this project.
The fate of cellulose study revealed that roughly 80% of the raw wastewater cellulose was removed in either of the primary treatment options, while represented 35%, and 17% of the total suspended solids (TSS) in the RBF and primary clarifier sludges, respectively. Cellulose was biodegradable irrespective of the biological process configuration and tested Solids retention times (SRTs), with effluent concentrations of about 2-3 mg/L.
pH-controlled fermentation was effective in improving the VFA yields by up to 93% and 72% at pH 9, for RBF and primary sludges, respectively. Furthermore, pH 6 was proposed as optimum considering significant enhancement in VFA production, while also lowering the amount of consumed chemicals. Interrelated impact of enzyme, temperature, and SRT on the enhancement of primary and RBF sludges fermentation showed a positive impact of enzyme dose as well as temperature and SRT on the VFA and soluble COD production. Cellulase increased the VFA yields by up to 36% and 86% for primary and RBF sludges, respectively. Response surface methodology (RSM) model depicted the existence of an optimum in the high-enzyme (1%-1.5%), long-SRT (3d-4d) range. The economic viability of fermentation at full scale was confirmed by proving that VFA recovery could save up to 7.2±2.0% (RBF), and 7.6±2.7% (PS) of the overall sludge disposal costs. Integration of fermentation and anaerobic digestion negatively impacted the biogas production of the residual fermented solids by 8.4% and 12.7%, compared to fresh primary and RBF sludges due to the VFA recovery, respectively; but still economically outperformed the single stage digestion under all tested scenarios.
Both primary and RBF sludge fermented liquid (SFL) were effective in enhancing BNR. Removal efficiencies in the rectors were reached up to 57% (total nitrogen) and 92% (total phosphorus), upon supplementation with the SFL. Effluent nitrogen and phosphorus of the reactors were closely matched for the two trains in the range of 15± 6 mg N/L, and 0.5 ± 0.3 mg P/L, respectively. A case study incorporating experimental results into a plant-wide model showed a moderate (3.4%-8.5%) improvement in the effective COD:N and COD:P ratios (compared to the original feed); but a significant increase in readily biodegradable (rbCOD) and VFAs (2.5-6.1 times) in the combined feed could be achieved by utilization of fermeners.
Summary for Lay Audience
Excessive loads of nitrogen and phosphorus in the effluent stream of municipal wastewater treatment plants cause eutrophication in the surface water resources. Eutrophication is a global issue impacting many lakes and rivers in North America and many other parts of the world. Biological nutrient removal is an effective process that can effectively decrease N and P levels in the treated wastewater, by providing appropriate environment for the microorganisms to consume respective organics. This process, however requires some forms of carbon source such as synthesized or processed chemicals to enhance the removal efficiencies that not only need further investments due to maintenance costs, but also involve serious safety concerns and sustainability challenges.
On the other hand, handling wastewater sludge which is produced during treatment process, needs to be disposed safely according to the environmental standards; a costly process accounting for roughly 50% of the total operation costs of WWTPs. This waste product can alternatively be considered as a resource by further processing to generate useful chemicals such as volatile fatty acids, and biogas (methane). The former is a good resource with many industrial applications including enhancement of the aforementioned BNR processes, while the latter is used in combined heat and power (CHP) machines and can produce electricity as a source of sustainable and clean energy.
This PhD research aimed at developing strategies to maximize the recovery potentials (form both resource and energy perspectives) from wastewater sludge, and minimize the chemicals required to enhance biological nutrient removal in the wastewater treatment plants.
Bahreini, Gholamreza, "Enhanced Primary Sludge Fermentation and Anaerobic Digestion for Integrated Carbon Upgrade and Resource Recovery" (2021). Electronic Thesis and Dissertation Repository. 7651.
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Available for download on Saturday, May 01, 2021