Optimization of the process parameters controlling dry anaerobic digestion of spent animal bedding in leach-bed reactors
Anaerobic Digestion (AD) is a process which allows the treatment of organic waste and the production of renewable energy. In particular, dry AD allows the treatment of solid organic substrates, offering several possibilities to the enhancement of agricultural waste such as spent livestock bedding (a mixture of straw, faeces and urine). Among the available biotechnologies in AD, leach-bed reactor (LBRs) is a promising but yet poorly known process both at scientific and industrial level.
In order to develop this process, several issues have been studied: (i) the bio-physico-chemical characterization of spent animal bedding and its digestion potential in LBRs; (ii) the optimization of the start-up and the operating temperature of the digesters; (iii) the co-digestion of spent animal bedding with an easily-degradable substrate and the issues connected to the management of the volatile fatty acids (VFAs) produced.
The results showed that spent animal bedding is a slowly-degradable substrate which needs a long digestion time. However, it is a substrate suitable to be treated through AD displaying high degradation and methane production rates when processed in LBRs. This substrate is, therefore, a valuable organic resource in the agricultural context.
Spent animal bedding was shown to contain an active methanogenic population able to start the process efficiently, both in thermophilic and mesophilic temperature, without requiring a specific external inoculation. An economic study at industrial scale proved that this peculiarity can be used to diminish the investment costs and then promote the development of this process. Moreover, thermophilic temperature was proved to be less advantageous over mesophilic condition. In fact, despite the very close methane yield reached in both temperature range, the different biogas production rates in thermophilic conditions would lead to a reduction of the final electric energy production in this condition. Mesophilic temperature was then shown to be the best operating condition for this process.
Finally, the role played by the leachate recirculation in the mobilization of the VFAs accumulating in the solid bulk was highlighted in the case of a reactor co-digesting slowly- (spent livestock bedding) and easily-degradable substrates. A strategy was even proposed to efficiently face such a problem by optimizing both the VFA extraction and consumption with the objectives of increasing the overall process efficiency.
In the end, this work allowed to optimize some important parameters for the correct management of the LBRs. This technology was proved to be efficient in the treatment of spent livestock bedding, both as a sole substrate or in co-digestion with an easily-degradable substrate. This research study demonstrates that LBRs is an adapted process for the agricultural context and this technology can easily answer to the full scale issues usually encountered. This work represents a significant advance towards the comprehension and development of LBRs to treat agricultural waste and, more generally, to the development of renewable energies based on biomass.