Enhancing Anaerobic Digestion Efficiency of High Strength Wastes with Different Solid Contents
Creators: Yu, Najiaowa
Publisher: name: University of Alberta Library
Year: 2023

<p>Anaerobic digestion has been considered as a sustainable technology to treat a wide range of organic wastes and to recovery value-added products such as methane and fatty acids. However, several factors such as low temperature and high solids content of substrates would lead to the imbalance of AD steps and result in process failure. This thesis aims to develop novel strategies to facilitate the economic treatment of high strength wastes, including two pre-treatment strategies treating substrates with medium and high solid contents to improve solid hydrolysis, and conductive materials amendment strategies treating low solid content wastewater to improve syntrophic and methanogenic activities. All three strategies were assessed and optimized in continuous operating reactors, with a focus on their specific roles on improving anaerobic digestion efficiency. The performances and microbial communities of the experimental bioreactors were compared to conventional anaerobic treatment systems. <br>
The thesis first employs calcium hypochlorite to the treatment of waste activated sludge generated from central wastewater treatment plants in traditional urban wastewater collection systems. Different doses of Ca(ClO)2 was applied to WAS and thickened WAS to improve AD treatment efficiency using laboratory reactors. Adding 5 – 20% Ca(ClO)2 significantly enhanced WAS anaerobic digestibility at room temperature, and led to significantly enhanced methane production rate and biomethane yield comparing to the AD of raw WAS (P &lt; 0.05), while severe inhibition was observed for 0.5 g/g TSS Ca(ClO)2 pretreated WAS. Low Ca(ClO)2 (1%) also increased the methane yield in the batch reactors treating TWAS at 35oC, by 4.8% and 8.3%, respectively, at F/I = 1 and F/I = 0.33. Then, two semi-continuous anaerobic sequencing batch reactors, one fed with Ca(ClO)2 pretreated TWAS (1%) and one with raw TWAS, were operated at mesophilic conditions (35 °C) for 145 days. The performance stability and resilience of TWAS digestion were improved with Ca(ClO)2 pretreatment by transforming the biomass to more easily digested substrates.<br>
Efforts were also made to develop an effective and cost-efficient strategy to treat blackwater with medium solids content generated in decentralized wastewater collection systems. Micro-aeration was applied to the treatment of blackwater collected from conventional toilet (9 L water/flush) in ASBRs at room temperature. The optimum micro-aeration intensity was 5 mg O2/L-reactor/cycle, which accelerated COD solubilization and enhanced methane production by 40.3% and 28.1%, respectively, when compared to no aeration. However, excessive oxygen (150 mg O2/L-reactor/cycle) had a negative effect on the AD process. VFAs accumulated in the 50 mg O2/L-reactor/cycle reactor due to the lack of downstream VFA conversion, so medium-dose micro-aeration offers an operational strategy for a two-stage anaerobic digestion system.<br>
The third aspect of this thesis is to develop a novel semi-two-phase AD reactor that separates hydrolytic/acidogenic from methanogenic in an up-flow anaerobic sludge blanket (UASB) reactor. Granular activated carbon (GAC) was packed in plastic carriers and added to the UASB reactor (GAC self-fluidized) to promote the syntrophic methanogenic reactions in the reactor column. Enhanced performance was achieved with respect to methane production and COD removal at an organic loading rate of 1500 g COD/m3/d under the temperature of 20 oC using the self-fluidized GAC configuration; with the methanation rate increased from 0.33 ± 0.08 g CH4-COD/g influent COD in the non-GAC reactor, to 0.66 ± 0.02 g CH4-COD/g influent COD in the GAC-only reactor, and further increased to 0.77 ± 0.02 g CH4-COD/g influent COD in the self-fluidized GAC reactor. The results indicate that the floated-GAC reactor combined the advantages of both reactors, performing a semi-two-phase AD process — a fermentation zone existed at the wastewater inlet, and methanogenesis was promoted throughout the reactor column with the dispersed floated-GAC.<br>
The microbial community analysis revealed that low oxidative stresses contribute to the development of fermentative and syntrophic bacteria, while high levels of oxygen reduced the activities of obligate anaerobes and leading to VFAs accumulation due to the lack of downstream conversion. Microbial community segregation was observed between the bottom and mid-top zones in the floated-GAC UASB. The non-GAC reactor and the floated-GAC reactor shared Enterobacteriaceae as the dominant bacteria at the bottom layers, while two GAC-amended reactors were more alike in the middle and top layers, with the dominant bacteria being Clostridium, Bacteroidales and Treponema. Methanosarcina were enriched throughout the settled-GAC reactor and mid-top of the SF-GAC reactor. </p>