Enhancing the hydrolysis process of a two-stage biogas technology for the organic fraction of municipal solid waste

The Danish company Solum A/S has developed a two-stage dry anaerobic digestion process labelled AIKAN® for the biological conversion of the organic fraction of municipal solid waste (OFMSW) into biogas and compost. In the AIKAN® process design the methanogenic (2nd) stage is separated from the hydrolytic (1st) stage, which enables pump-free feeding of the waste into the 1st stage (processing module), and eliminates the risk for blocking of pumps and pipes by pumping only the percolate from the 1st stage into the 2nd stage (biogas reactor tank). The biogas yield of the AIKAN® two-stage process, however, has shown to be only about 60% of the theoretical maximum. Previous monitoring of the hydrolytic and methanogenic activity in the two stages of the process revealed that the bottleneck of the whole degradation process is rather found in the hydrolytic first stage while the methanogenic second stage showed high efficiency.
The current project investigates how the efficiency of the hydrolytic process in the first stage can be enhanced to achieve a higher overall biogas yield by the following means:
•The influence of the process conditions in the processing module (temperature, pH, retention time, recirculation rate of percolate, ratio of admixing effluent from the anaerobic stage to the percolate, water submerge of waste) on the efficiency of the hydrolytic stage.
•The effect of addition of adapted mixed cultures and specific hydrolytic microorganisms on the hydrolysis of the waste.
•The effect of addition of hydrolytic enzymes on the hydrolysis of the waste.
The efficiency of the hydrolysis is determined for the different conditions applied both by the release of volatile fatty acids from the hydrolytic stage and by final biomethane potential (BMP) tests. The tests are performed both as lab-scale batch and pilot-scale reactor tests. A lab-scale reactor system to mimic the 2-stage set-up in large-scale is currently set-up and the results will be included in the final conference paper.
The first pilot-scale tests were performed at Solum’s pilot-scale modules by adjusting the pH of the effluent from the first stage before recirculation. In the first trials, the adjustment of the pH was tested through dilution of acids by addition of water to the leachate from the processing module before reintroducing it to this module. The results showed that the pH of the leachate became acidic (pH 4-5) within 2 to 4 days of recirculation. Replacing parts of the leachate (25 or 50 %) with an equal amount of water before recirculation into the first module did not have any significant impact on raising the pH of the leachate to the desired pH range for hydrolytic enzymes of pH 5.5-6.5.
The effect of enzyme addition was tested in batch experiments by addition of various cellulases and xylanases in different dosages to the OFMSW as received by Solum. In these set-ups the OFMSW showed a high specific methane yield of about 400 mL-CH4/g-VS and the addition of enzymes did not show significant increase of this yield.