Effect of different pretreatments on the production of bio-hydrogen and volatile fatty acids from exhausted sugar beet pulp

Resumen

The anaerobic co-digestion (co-AD) process of different types of substrates could contribute to clean energy supply and a sustainable agro-industrial wastes management. Nevertheless, more recently, attention is also being paid to the dark fermentation process (namely, acidogenic anaerobic digestion) due to the interest of the carbohydrate platform in biorefinery processes. Organic loading rate (OLR) is a key factor for the performance of the AD process and the final products that can be obtained in it. When OLR is lesser than its critical value, the methabolic rates of the different microbiological populations can be balanced and the main product of the process is amethane-rich biogas. In this case, the production of volatile fatty acids (VFAs) by the acidogenic bacteria is compensated by their transformation in acetate by the acetogenic microorganisms and the subsequent consumption of the acetate by the acetoclastic methanogenic archaea. However, when OLR reach its critical value, the rate of VFA production cannot be compensated by their transformation and removal rates. In this case, a hydrogen-rich biogas and different volatile fatty acids (mainly acetic, propionic and butyric acids) are the main products generated in the process. Therefore, the inhibitionnof methane production due to VFAs accumulation could be regarded as an opportunity to produce high value-added products in the biorefinery concept, which stands out the different strategies for organic wastes valorisation, besides the energy vectors. This study aims to determine how organic overloading affects the AD of sugar beet by-products (SBB) individually and in co-digestion with livestock manures for methane production. Moreover, a deep analysis of the behavior of the reactors operating at higher OLRs inducing VFAs accumulation has beenundertaken. Three mesophilic semi-continuous stirred tank reactors have been used. The process was successively conducted at OLRs ranged from 3.3 to 12.8 (gVS/Lr*d). The co-digestion with manures clearly enhanced the systems performance and methane production from SBB, reaching up to 70% and 31% higher rates, respectively for co-digestion with pig and cow manures, in comparison with SBB individual digestion. Volatile fatty acids were the main products generated at higher OLRs, indicating acidification of the medium, inhibition of methanogenesis, and the high performance of acidogenesis. Critical OLRs led to an increase in VFAs by 93%, 87%, and 85%, respectively for reactors containing ESBP individually, with PM, and with CM, in comparison with the optimal OLR for biomethanation in each reactor. The findings indicate that valorization of agroindustrial wastes such as SBB by AD process could be favored by co-digestion with substrates with complementary characteristics in order to obtain high methane yields. The use of semi-continuous reactors and different OLRs, together with the operation for a long period and a deep analysis of the process, make the study attractive for a possible large-scale approach at the industrial level.