الفهرس 
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Abstract
The anaerobic digestion technology has been in existence for centuries and its underlying theory established for decades. It was considered a useful technology for the generation of renewable energy, and provides means to alleviate problems associated with GHG emission. Therefore, a great deal of current research is targeted towards the optimization of this technology under diverse digestion process conditions. This study discussed how metal oxides additives could be used to optimize AD performance through identification of methods that could accelerate syntrophic interactions of different microorganisms for the improvement of methanogenic reactions. Recent advances in term of iron and titanium oxides NPs in addition to iron oxide powder and natural soil-derived MnS additives were discussed in order to offer a general but comprehensive synopsis of accumulated knowledge in the theory of AD, as well as an overview of future directions and opportunities of the AD technology. Achieving a sustainable energy system requires comprehensive reforms not only for economic, social and policy aspects, but also in all technical aspects, which represents one of the most crucial future investments for AD systems.
Several assumptions were obtained from this study:
A. Impacts of Fe2O3 NPs, TiO2 NPs, and their mixtures on the AD process
1. The effect of metal oxide nanoparticles (MONPs) at four different concentrations in two different combinations, 20 (R1) and 100 (R2) mg/L for Fe2O3, 100 (R3) and 500 (R4) mg/L for TiO2, and a mixture of Fe2O3 and TiO2 at rates of 20, 500 (R5) and 100, and 500 (R6), on hydrogen sulfide (H2S) mitigation using the batch AD of DCM was 2.13, 2.38, 2.37, 2.51, 2.64, and 2.17 times lower than that of the control (R0), respectively. Additionally, biogas and CH4 production were 1.09 and 1.105, 1.15 and 1.191, 1.07 and 1.097, 1.17 and 1.213, 1.10 and 1.133, and 1.13 and 1.15 times higher than those of R0 when the DCM was treated by the aforementioned MONPs, respectively. The highest specific production of biogas and CH4 was 336.25 and 192.31 mL/gVS, respectively, which was achieved by R4 supplemented with 500 mg/L TiO2 NPs, while the corresponding values in the case of R0 were 286.38 and 158.55 mL/gVS.
2. TiO2 NPs were effective for enhancing methane generation in AD through EET, which could occur when anaerobic methanogenic archaea were in contact with the nano-TiO2. Furthermore, the TiO2 effectively mitigates H2S by absorbing SO2/SO3, forming irreversible Ti-S bonds.
3. The supplementation of biodigesters with Fe2O3 NPs improved AD, and consequently resulted in higher methane production and organic matter degradation through the release of Fe+2/+3, which likely promoted the production of metabolic intermediates and activity of key enzymes in the methanogenic archaea. Additionally, Fe2O3 NPs had reduced the amount of H2S in the digestate by forming a ferrous sulfide deposit (FeS).
4. The effectiveness of the MONPs for enhancing methane and biogas and mitigating the emission of hydrogen sulfide was dose-dependent.
5. The combination of 100 and 500 mg/L of Fe2O3 and TiO2 NPs, respectively, might increase the rate of the aggregation and agglomeration, so their interactions with sulfur would decrease, allowing SRB to utilize more sulfate and release H2S. In addition, the combination might result in a competing electron interaction between both metals that reduced their action on the sulfates.
6. TVFA and pH were two major components in AD and did not change significantly in this experiment.
7. The higher acetic acid contents with higher methane emissions indicated that metal oxide NPs had promoted the emission of CH4 through the reduction of H2/CO2 by hydrogen-utilizing methanogens.
B. Feasibility of adding of Fe2O3 NPs and TiO2 NPs to the semi-continuous AD of cattle manure
In the present experiment, iron oxide NPs (Fe2O3 NPs) and titanium oxide NPs (TiO2 NPs) were mixed daily with manure from dairy cattle and added to the biodigesters daily at concentrations of 100 mg/L (D1) and 500 mg/L (D2), respectively. In addition, a control with no NP supplementation was studied (D0).
1. The results revealed that the generation rates of biogas and CH4 increased by 25.43% and 62.43%, respectively, for D1 during the first 7 days retention time, as compared with the D0. Conversely, TiO2 NPs addition had reduced the generation rates by 28.76% and 56.92%, respectively, during the same period.
2. Overall, for D1, the biogas and CH4 yields over the study period increased by 5.48% and 12.35%, respectively, whereas for D2, the rates were reduced by 11.36% and 18.58%, respectively.
3. Therefore, the presence of Fe2O3 NPs had a stimulatory influence while that of the TiO2 NPs had an inhibitory effect.
4. TiO2 NPs were assumed to either change the dominance of the archaea in the AD system or induce a toxic impact on methanogens.
5. The toxicity of NPs towards microbial consortium had been rarely studied under an anaerobic continuous pattern and was thus recommended for future research. In addition, further research is still needed on the variations in mechanisms following the Fe2O3 NPs enhancement in dark AD performance and the converse feature induced by TiO2 NPs application.
C. Comparison between the influences of microscale WIP and Fe2O3 NPs on the AD
This experiment examined the effects of different types of iron additives (WIP and Fe2O3 NPs) on the performance of AD of DCM and the H2S profile.
1. The results showed that the addition of iron in the form of microscale WIP (generated from the laser cutting of iron and steel) at concentrations of 100 mg/L, 500 mg/L, and 1,000 mg/L improved methane yields by 36.99%, 39.36%, and 56.89%, respectively. In comparison, the equivalent dosages of Fe2O3 NPs improved yields by 19.74%, 18.14%, and 21.11%, respectively.
2. The highest WIP dose (1,000 mg/L) achieved the maximum improvement in the rate of hydrolysis (k), which was 1.25 times higher than in control reactions, and a maximum biomethane production rate (Rmax) of 0.045 L/gVS/d according to kinetic analysis models (i.e., first-order and the Gompertz kinetic models).
3. The use of WIP in AD could gain a net profit of 42.04 USD, at 1000 mg/L.
4. The rate of H2S production was also significantly reduced (by 45.20%, 58.16%, and 77.24%) using the three WIP concentrations in comparison with Fe2O3 NPs (which achieved reductions of 33.59%, 46.30%, and 53.52%, respectively).
5. The supplementation of AD processes with iron in the form of WIP could overcome some of the particular physicochemical properties of Fe2O3 NPs in the enhancement of manure digestion. Additionally, WIP could potentially reuse in AD systems as an alternative to its improper disposal and the associated environmental impacts. Therefore, considering the benefits of WIP recycling, the significant increases in CH4 production, and the reduction of H2S gas production and its associated negative effects, the use of WIP in AD opens up new economic and practical horizons.
6. In the future, to make AD processes supplemented with WIP more sustainable, the post-treatment removal of the WIP-containing digestate requires further investigation.
D. Potential of biogas production from manure of dairy cattle fed on natural soil supplement rich in iron under batch and semi-continuous AD
The presented study had provided a novel method for improving the AD of Holstein dairy manure (HDM) by the direct addition of Mineraso (MnS), a natural soil-derived supplement, to the feed of Holstein dairy cattle (HDC). MnS was chiefly composed of approximately 69.08% Fe3O4 and was supplemented at rates of 0 (F1), 25 (F2), and 50 (F3) g/head of HDC/d for two months. The HDM was then examined for non-absorbed iron prior to the batch and semi-continuous bench AD experiments.
1. The results revealed that MnS enhanced CH4 generation in F2 and F3 by 25% and 42%, respectively, in the batch experiments compared to that of F1.
2. The gas yield improved in F2 and F3 by 45% and 66%, respectively, over the control after 7 d in the bench experiments.
3. MnS use in the semi-continuous bench experiments was preferred for the first week to make a profit of 56.16 USD for M3.
4. MnS, a cheap source of Fe, effectively improves the CH4 yield and did not negatively influence the AD process. MnS was anticipated to be a sustainable alternative to diminish the operational and running costs of a biogas facility.