Biodegradation of Cellulose in Laboratory-Scale Bioreactors: Experimental and Numerical Studies

Abstract

Standard tests such as ISO 17556 (Plastics - determination of the ultimate aerobic biodegradability in soil by measuring the oxygen demand in a respirometer or the amount of carbon dioxide evolved. International Organization for Standardization, Geneva, Switzerland, 2019) or ASTM D5988 (Standard test method for determining aerobic biodegradation of plastic materials in soil. ASTM International, West Conshohocken, 2018) have been developed for measuring the biodegradation of polymers and assessing their biodegradability in soil. In several experiments performed according to these standard tests, the measured biodegradation of cellulose was reported unexpectedly between 80 and 85%. These results are difficult to justify since cellulose is a well-known biodegradable material. In the present study, this phenomenon is explained as the consequence of starvation occurring in a confined bioreactor. It is proposed that the favourable conditions applied to the small-size bioreactors accelerate the proliferation of the microorganisms which are responsible for the biodegradation. Such a dense microbial population may face starvation when cellulose is consumed. It is assumed that starvation causes the secretion of an inhibitory chemical signal, which can diffuse to neighbouring sites still containing food, and suppresses biodegradation. This hypothesis was supported by two experiments. First, it was shown that the final measured biodegradation increased when the microbial growth rate decreased by reducing temperature. In the second experiment, it was shown that the biodegradation proceeded slower when a new cellulose quantity was inserted into a previously used soil substrate. To confirm the hypothesis regarding the presence of an inhibitory starvation factor, which can suppress biodegradation, a Kinetic Monte Carlo (KMC) model was also developed. The KMC simulations reproduced qualitatively the experimental findings.