Clarifying the synergetic effect of magnetite nanoparticles in the methane production process

Improving the shock resistance of anaerobic digestion under demand-oriented biogas production mode by using converter steel slag powder

Introducing flexible biogas production (FB) can result in instantaneous high-shock loads for anaerobic digestion system, posing risks to the system's stable operation. Steel slag, a typical metallurgical solid waste, has been demonstrated to enhance the buffering capacity of digestion systems, thereby increasing methane production and achieving 'waste treatment using waste'. However, its efficacy under high-shock loads in FB is uncertain. Pulse feeding experiments simulating FB were conducted to analyse the system's impact resistance with steel slag addition and investigate its enhancement mechanisms. The addition of steel slag improved the methane production rate under various shock conditions, with a particularly notable enhancement under concentration shock. This scenario also saw a significant increase in the generation of soluble chemical oxygen demand and its utilization by microorganisms. This can be attributed to the enrichment of hydrolytic bacterial phyla (Firmicutes) and genera (Gelria), with functional gene analysis revealing an increase in genes associated with Fe(III) reduction and CO2-to-methane pathways. The study results indicate that the role of steel slag as an alkaline, iron-rich material enhances system alkalinity, reduces inhibition from H2 partial pressure and boosts hydrogenotrophic methanogen activity, making it suitable as an exogenous enhancer for demand-oriented anaerobic digestion.

Magnetite-based nanoparticles and nanocomposites for recovery of overloaded anaerobic digesters

The effect of magnetite nanoparticles and nanocomposites (magnetite nanoparticles impregnated into graphene oxide) supplement on the recovery of overloaded laboratory batch anaerobic reactors was assessed using two types of starting inoculum: anaerobic granular sludge (GS) and flocculent sludge (FS). Both nanomaterials recovered methane production at a dose of 0.27 g/L within 40 days in GS. Four doses of magnetite nanoparticles from 0.075 to 1 g/L recovered the process in FS systems between 30 and 50 days relaying on the dose. The presence of nanomaterials helped to reverse the effect of volatile fatty acids inhibition and enabled microbial communities to recover but also favoured the development of certain microorganisms over others. In GS reactors, the methanogenic population changed from being mostly acetoclastic (Methanothrix soehngenii) to being dominated by hydrogenotrophic species (Methanobacterium beijingense). Nanomaterial amendment may serve as a preventative measure or provide an effective remedial solution for system recovery following overloading.

Enhancement of Anaerobic Digestion with Nanomaterials: A Mini Review

In recent years, the number of articles reporting the addition of nanomaterials to enhance the process of anaerobic digestion has exponentially increased. The benefits of this addition can be observed from different aspects: an increase in biogas production, enrichment of methane in biogas, elimination of foaming problems, a more stable and robust operation, absence of inhibition problems, etc. In the literature, one of the current focuses of research on this topic is the mechanism responsible for this enhancement. In this sense, several hypotheses have been formulated, with the effect on the redox potential caused by nanoparticles probably being the most accepted, although supplementation with trace materials coming from nanomaterials and the changes in microbial populations have been also highlighted. The types of nanomaterials tested for the improvement of anaerobic digestion is today very diverse, although metallic and, especially, iron-based nanoparticles, are the most frequently used. In this paper, the abovementioned aspects are systematically reviewed. Another challenge that is treated is the lack of works reported in the continuous mode of operation, which hampers the commercial use of nanoparticles in full-scale anaerobic digesters.

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

High-solid and solid-state anaerobic digestion are technologies capable of achieving high reactor productivity. The high organic load admissible for this type of configuration makes these technologies an ideal ally in the conversion of waste into bioenergy. However, there are still several factors associated with these technologies that result in low performance. The economic model based on a linear approach is unsustainable, and changes leading to the development of a low-carbon model with a high degree of circularity are necessary. Digestion technology may represent a key driver leading these changes but it is undeniable that the profitability of these plants needs to be increased. In the present review, the digestion process under high-solid-content configurations is analyzed and the different strategies for increasing reactor productivity that have been studied in recent years are described. Percolating reactor configurations and the use of low-cost adsorbents, nanoparticles and micro-aeration seem the most suitable approaches to increase volumetric production and reduce initial capital investment costs.

New insights into enhanced anaerobic degradation of coal gasification wastewater (CGW) with the assistance of magnetite nanoparticles

Magnetite nanoparticles (Fe₃O₄ NPs) was firstly used to enhance pollutants removal during coal gasification wastewater (CGW) treatment in anaerobic digestion (AD) system. Bench-scale results revealed that 200 mg/L and 20-40 nm of Fe₃O₄ NPs addition resulted in a maximum removal capacity of total phenol (TPh) at a temperature of 36 °C and hydraulic retention time (HRT) of 36 h. Meanwhile, Fe₃O₄ NPs addition reduced the oxidation reduction potential (ORP) values and biological toxicity, and enhanced the stability of AD system. Pilot-scale results showed that the TPh and chemical oxygen demand (COD) removal efficiency (53% and 49%) were obtained with the optimal dosage of Fe₃O₄ NPs. Moreover, electron nanowires may be established with Fe₃O₄ NPs assisted to perform direct interspecies electron transfer (DIET) among Geobacter, Pseudomonas and Methanosaeta species, and finally enhanced the pollutants removal efficiency.