German researchers double biogas plant productivity

Some major news just in from Germany, where biogas has become the fastest growing renewable energy segment. Researchers at the Leibniz-Institut für Agrartechnik in Potsdam-Bornim announce [*German/*.pdf] they have developed a new technology that doubles the productivity of biogas digestion.

Currently, biogas production consists of feeding biomass to a large digester, where methane-producing bacteria, under anaerobic conditions, convert it into the energy-rich gas. After the fermentation, the remaining slurry is channeled out of the fermenter, and the bacteria disappear along with the waste stream (which is often used as an organic fertiliser). The bacteria take a relatively long time to grow into concentrations high enough for the bioconversion, so removing them after all that work is not smart. This way, the retention time needed to perform the conversion of organic matter into gas, remains high.

The researchers now found a simple physical technique to keep the methanogenic micro-organisms inside the digester right before the would normally be removed and exactly at the time they have reached their optimal productivity level. By introducing a small dose of magnetic particles into the digester, the bacteria 'flocculate' around the particles, and they can be scooped off the slurry. This way, they can be reintroduced into the system to feed on a new batch of biomass. The recuperation operation is straightforward and involves a simple hard magnet to attract the micro-organisms that have flocculated around the particles:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: biogas :: digester :: anaerobic fermentation :: methane ::

To 'magnetise' the bacteria, ferrite can be used, which is widely available. The researchers found that only 0.1 grams per gram of organic matter introduced into the biogas digester suffices to trigger the flocculation process. Alternatively, less costly magnetic particles from brown coal flue gas can be used.

Trials with the technique have shown that the retention time can at least be halved, which means the productivity of the biogas plant doubles.

The researchers think the magnetic flocculation technique will find its best application first in systems that ferment water-rich substrates, such as wet distillers grains. But the technique is certainly not limited to this kind of substrates, since trials with two-phase and co-digestion of biomass from renewable energy crops has shown similarly positive results.

After the successful laboratory trials, the Leibniz-Institut für Agrartechnik is now searching for partners to test the technique on a large scale and on a continuous basis.

Image: Methanosarcina barkeri fusaro, an methanogenic Archaebacterium. Methanogens are organisms that make methane via a unique metabolic pathway with unique enzymes.

More information:
Leibniz-Institut für Agrartechnik: Mehr Leistung im Kessel - neues Verfahren macht die Biogaserzeugung effizienter [*.pdf] - March 29, 2007.