Abstract
Carbon degradation indicates the efficiency of anaerobic digestion processes. Common carbon degradation determination methods define gross carbon degradation (C deggross) of substrate and inoculum inseparably. The aim of this study was to test an isotope-based method defining solely substrate-based net carbon degradation (C degnet) on maize silage. As the natural abundance of stable isotopes in agricultural substrates vary, the method’s applicability was tested on (i) different maize silages sampled from agricultural farms, (ii) maize silage in fresh (MSfresh) and impaired storage (MSimpaired) conditions.Experiments included six maize silages digested in a total of 19 lab-scale batch reactors, analyzed for digestion parameters, stable isotopes, gross and net carbon degradation. MSimpaired showed significantly different stable carbon isotope composition at the start of the experiments, compared to MSfresh. Both methods indicated quality losses in MSimpaired. Results showed significantly higher C degnet values, ranging from 58.4% to 86.5%, compared to deggross values, ranging from 23.1% to 48.7%. This indicated the applicability of an isotope-based method C degnet to assess net carbon degradation of maize silages more detailed by excluding the masking effect of the inoculum. The isotope-based net carbon degradation method was found applicable on maize silages from (i) different farms and (ii) in different storage qualities.
Highlights
Maize silage is one of the most important substrates in German biogas production (FNR 2017)
Degradation determination methods are based on volatile solids (VS), which serve as basis for specific biogas
Our study focused on the implementation of a simple isotope-based method to identify maize silage-bound net carbon degradation (C degnet) in agricultural biogas plant inoculum
Summary
Maize silage is one of the most important substrates in German biogas production (FNR 2017). An efficient transformation of substrate-bound carbon (C) to biogas during the anaerobic digestion process is one key aim for biogas plant operators. Many scientifically conducted experiments in lab-scale biogas reactors focused on maize silage degradations in order to analyze degradation efficiency. The carbon masses are used to determine gross carbon degradation of the substrate in anaerobic digestions (C deggross) (Bisutti et al 2004). Anaerobic digestion experiments are often performed in batch processes which, in the simplest case, are set up with one inoculum and one substrate (e.g. maize silage). Gross carbon degradation determination methods, offer no possibility to identify solely substrate-bound carbon degradation without taking carbon of the applied inoculum into account. To achieve a better understanding of the net degradation process, it is required to gain additional information about substrate degradation independently of the contributing inoculum
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