Biogas production

BIOGAS PRODUCTION: BIODEGRADABILITY OF EXHAUSTED FERMENTATION BROTHS

Approach

Within the project Biovino, the University of León is testing an exhausted fermentation broth as a substrate for anaerobic digestion, in order to assess its degradation ability and its biogas production. In this experiment, the exhausted broth comes from the ABE fermentation of vine shoot hydrolysate. Exhausted fermentation broths are the residue which remains after product recovery (acetone, butanol and ethanol) from the fermentation broth once that the process is finished.

Biodegradability assays are a quick method to know the ability of residues to produce a renewable resource as biogas. At the same time, it is possible to determine the biodegradability of the residue, whose organic load will be lower after the anaerobic digestion, which enables its use as an organic amendment. 

Biogas is a mixture composed mainly of methane (CH₄) and carbon dioxide (CO₂) and which can be used as a fuel in boilers, motors, etc. In addition, the residue employed for biogas production will have a lower organic load after the process.

Some factors influencing biodegradation are:

1. Substrate concentration: it must be high enough for the selected analytical methods, but low enough in the case of toxic substances.
2. Physicochemical properties of the tested substances.
3. Nutrient composition and concentration in the medium.
4. Conditions and properties of the test systems: anaerobic conditions, temperature and shaking, among others.
5. Selection of inoculum, which is the source of microorganisms.

Methodology

1. Inoculum: Sludge from the digestor of a wastewater treatment plant.
2. Characterisation of inoculum and substrate: pH, conductivity, redox potential redox, total solids (TS) and volatile solids (VS), total organic carbon (COT), chemical oxygen demand (COD), volatile fatty acids (VFA).
3. Determination of the Ratio Inoculum/Substrate (RIS): [g] Inoculum/[g] Substrate (RIS) for 1 and 2 (naming: RIS1 – RIS2).
4. Reactors: 8 flasks of 250 mL: a) 4 flasks for RIS1, 2 of them for blanks (BL1); b) 4 flasks for RIS2 with their respective blanks (BL2).
5. Determination of inoculum and substrate amounts for each ratio.
6. Connection of each flask to a gas meter. Gas measuring Will be performed by displacement of liquid.
7. Use of a thermostatic bath (37 °C) with slight shaking.
8. Comparison of biogas production between samples and blanks.
9. Test duration: 30 days.

Results

The reactor with the lowest inoculum load has a delay in its methanogenic phase in comparison to the reactor with the highest inoculum load. Even though, good biogas productions are obtained in terms of methane, reaching 217-583 L CH₄/kg VS. These results show that, in both cases, vine shoot exhausted broths are a viable substrate for anaerobic digestion.