Valorizing Waste Biomass via Hydrodynamic Cavitation and Anaerobic Digestion

Abstract

It is essential to valorize waste biomass and promote a circular economy for realizing sustainability and decarbonization goals. In this review, we specifically focus on anaerobic digestion (AD) intensified via hydrodynamic cavitation (HC) and modeling as an effective biomass valorization strategy. AD is known to generate renewable energy (biogas) and fertilizer (digestate) and is therefore aligned with the circular economy framework and has the potential to decarbonize the transportation, energy, and heat sectors, which contribute a major share to the current global carbon emissions. The AD-based pathway, though robust, has key challenges such as poor carbon conversion and subpar biogas yields coupled with long residence times. This is mainly due to the recalcitrance offered by the feedstock. HC-based biomass pretreatment is critically reviewed here as a promising AD intensification strategy to overcome this recalcitrance. HC devices and their key aspects of design and scale-up are discussed. Vortex-based HC devices were found to be superior to conventional linear flow HC devices in terms of pretreatment performance and ease of scale up. The use of reaction engineering models of AD for overall optimization of biomass valorization is also discussed. The need for developing an intermediate modeling approach between empirical models and complex biochemical pathway-based models is highlighted. Such an intermediate approach requires simple biomethane potential (BMP) measurements for obtaining essential model parameters. With such parameters obtained from BMP data, the model was found to be useful for simulating and optimizing performance of AD. A prospective AD-based biorefinery is briefly discussed with multiple allied valorization pathways. This review will be useful to researchers and practicing engineers for realizing effective valorization of waste biomass.

Intensification of Acidogenic Fermentation for the Production of Biohydrogen and Volatile Fatty Acids—A Perspective

“…Multiple routes to biomass valorisation are currently available (Figure 2). Typically, biomass streams can be valorised either via biochemical pathways or thermochemical pathways [2]. Biochemical pathways include anaerobic digestion (AD) to produce biogas, biohydrogen, volatile fatty acids (VFAs) and fertilisers, fermentation for the production of solvents and biofuels (e.g., bioethanol, acetone, butanol), and value-added chemicals (e.g., succinic acid, citric acid, lactic acid).…”

“…Its popularity in this process is due to its ability to remediate waste streams whilst generating energy vectors in the form of biogas [9]. While the technology is mature, limitations such as long residence times (>4 weeks), leading to large reactor volumes in the order of thousands of m 3 , slower digestion kinetics, and sub-optimal carbon conversion leads to process inefficiencies and high capital expenditure [2]. In terms of revenue generation, biomethane (upgraded from biogas) is a low-value product (~EUR 0.5/kg [10]), and hence, allied products such as concentrated fertilisers from digestate are required to generate additional revenue [11].…”

“…In terms of revenue generation, biomethane (upgraded from biogas) is a low-value product (~EUR 0.5/kg [10]), and hence, allied products such as concentrated fertilisers from digestate are required to generate additional revenue [11]. Intensification strategies such as optimising operating parameters, and biomass pre-treatment to address feedstock complexity, have been proposed as effective routes to overcome these inefficiencies and maximise biomass conversion [2] and hence remain as the future perspectives for progressing the field. Thermochemical biomass conversion routes have similarly been extensively discussed in the literature [12][13][14] and the future direction for this route remains clear in maximising the techno-economics and understanding the life cycle impacts of the process.…”

“…To favour enhanced product yield by avoiding inhibitors, liquid hot water pre-treatment has been suggested as an alternative to steam explosion [44]. Another physico-chemical pre-treatment method that is gaining attention in the area of anaerobic digestion is hydrodynamic cavitation [2,45,46]. Cavitation is the phenomenon of generation, growth and implosion of vaporous cavities.…”

“…Nevertheless, due to the presence of moving parts, there are issues pertaining to clogging and maintenance at an industrial scale. Furthermore, compared to other hydrodynamic cavitation devices, rotor-stator devices are known to be relatively energy intensive [2]. Nonetheless, it is promising to see that a scalable biomass pre-treatment is being exploited for intensifying digestion yields.…”