Effect of Upstream Bioactivation of Plant Residues to Accelerate the Composting Process and Improve Product Quality

Abstract

Composting is considered an efficient and environmentally friendly alternative for plant waste management, resulting in compost, a high value-added product. During the process, microorganisms play a crucial role as organic matter-degrading agents. However, the highly recalcitrant nature of the lignocellulose present in plant residues sets a challenge to the microorganisms involved in the process. Therefore, the objective of this study was to evaluate the effect of a lignocellulolytic microbial consortium, previously selected in composting processes, to promote and improve the biodegradability of plant residues. For this purpose, a laboratory-scale inoculation strategy was optimized by applying different strains and doses of Bacillus spp., as well as different incubation times. Subsequently, the impact of the application of the consortium on the waste material as a pretreatment of a real composting process was evaluated. Samples from both experiments were subjected to the evaluation of parameters related to the biodegradation of the lignocellulosic fraction, in addition to those related to the stability and maturity of a compost. The inoculum proved to be effective in promoting the bioactivation of the material, favoring a higher respirometric rate and biodegradability during laboratory-scale pretreatment. In this case, inoculation with B. safensis at high doses resulted in AT4 values higher than those observed for the rest of the treatments after ten days of incubation, while B. licheniformis inoculated at lower doses was able to maintain higher AT4 values after ten days, compared to those observed in the rest of the samples analyzed. Additionally, inoculation with both strains resulted in a continuous decrease in the percentage of hemicellulose that could be detected until the end of the incubation period (thirty days), reaching biodegradation rates close to 40%. On the other hand, although the inoculation did not significantly affect the basic conditioning parameters at the beginning of the composting process (organic matter, C/N ratio, and moisture), a change in the dynamics of the lignocellulosic fractions was observed during the process, as well as in the evolution of other stability and maturity parameters, in particular the AT4 index and the lignin/holocellulose ratio, revealing an acceleration of the bio-oxidative phase during the full-scale composting process. Therefore, the treatment of plant waste with lignocellulolytic microorganisms is proposed as an effective alternative to activate the biodegradability of organic waste at the beginning of a composting process, resulting in better-quality products.