Experimental investigation and mathematical modelling of batch and semi-continuous anaerobic digestion of cellulose at high concentrations and long residence times

Abstract

In the context of the anaerobic digestion of slowly biodegradable substrates for energy and chemicals production, this study investigated the anaerobic digestion of cellulose without any chemical pre-treatments using open (undefined) mixed microbial cultures. The anaerobic conversion of cellulose was investigated in extended-length (run length in the range 518–734 days) batch and semi-continuous runs (residence time 20–80 days), at high cellulose concentration (20–40 g L−1), at temperatures of 25 and 35 °C. The maximum cellulose removal was 77% in batch (after 412 days) and 60% (at 80 days residence time) in semi-continuous experiments. In semi-continuous experiments, cellulose removal increased as the residence time increased however the cellulose removal rate showed a maximum (0.17 g L−1 day−1) at residence time 40–60 days. Both cellulose removal and removal rate decreased when cellulose concentration in the feed was increased from 20 to 40 g L−1. Liquid-phase products (ethanol and short chain organic acids) were only observed under transient conditions but not at the steady state of semi-continuous runs. Most of the observed results were well described by a mathematical model which included cellulose hydrolysis and growth on the produced glucose. The model provided insight into the physical phenomena behind the observed results.

Degradation of Cellulose and Hemicellulose by Ruminal Microorganisms

“…Volumetric rates of cellulose degradation during mesophilic AD are surprisingly sparse in the literature. Bolanji and Dionisi [89] reported a maximum cellulose degradation rate of 0.17 g L −1 d −1 in a lab-scale anaerobic digester fed Sigmacell 20 MCC at 20 g L −1 concentration, at residence times of 20-80 d. Donaldson and Lee [93] achieved much faster rates, up to 1.34 g L −1 d −1 in a 55-L digester fed mixed paper (shredded to 1 mm) at 5-50 g L −1 concentration and operated at a hydraulic retention time of 16-50 d, but they also added an equivalent amount of carbon as methanol to stimulate methanogenic activity. These rates on an hourly basis (0.007 and 0.060 g L −1 h −1 , respectively) are 170 and 20 times slower than the ruminal value calculated above.…”

“…This suggests that microbial contact with and adherence to fiber -the initial steps in initiating fiber degradation-is dependent on the chemistry or structure that fiber, and that the microbes preferentially recognize and adhere to a non-cellulose component of the fiber, perhaps one or another hemicellulose. Sludge digester ND b 0.04-0.13 [84] Sludge digester "Cellulose powder" 5 0.94 [85] Sludge digester Filter paper 2 0.247 [86] Sludge digester ND b 0.1 [87] Sludge digester ND b 0.066 [88] Sludge digester Sigmacell 20 MCC 20 0.252 [89] a MCC, microcrystalline cellulose. b ND, not described.…”

“…An essential feature of biomass utilization by the ruminant is its high productivity, underpinned by a rapid rate of cellulose degradation. The superior degradative capability of the rumen microbial community on cellulosic biomass (compared another familiar degradative process, anaerobic digestion (AD)) can be inferred from comparison of its first-order rate constants for cellulose degradation, which are typically 10-20× those in a mesophilic (35-40 • C) anaerobic digester [74,75,77,79,[84][85][86][87][88][89] (Table 3). A notable exception was found in the AD study of Noike [85], but in this latter study cellulose removal rates were calculated by difference, and at the shortest solids retention time only ~2% of the original 5 g cellulose L −1 were removed, a fraction subject to large measurement errors.…”

Characterization of bovine ruminal content focusing on energetic potential use and valorization opportunities

“…Analysis of these results could be the best approach to establish the level of degradation of cellulose, hemicellulose, and lignin ingested in animal feeding. Mathematical modeling was established to understand the reduction of cellulose content in rumen digestion [ 8 , 9 ]. Lignin content was determined as an overriding factor in the degradation of complex hemicellulose-lignin during digestion [ 10 ].…”