Summary
The sewage sludge generated during wastewater treatment is subsequently anaerobically stabilized. This leads to the formation of digester gas, which comprises approx. 65% methane and 35% CO2. If CO2 is removed, the methane content increases to form an energy rich gas. While physicochemical processes are already used for this purpose, biochemical processes are still at a nascent stage.
The objective of the project was to demonstrate that the biochemical separation of methane and CO2 in biogases formed during sewage sludge digestion can be achieved with algae. The CO2 fraction is converted to biomass and oxygen, thereby enhancing the methane content in the biogas. In the present process, the first conditioning step involves scrubbing CO2 from the digester gas. In the second step, the washing water that is rich in CO2 is biochemically regenerated by microalgae in a photo bioreactor. The algal biomass is then used to generate biogas, which in turn can be supplied to the gas distribution network.
Results:
- Methane enrichment did not occur since the methane content of the treated and untreated gas amounted to 60% on average.
- Reduction of the CO2 content by 75-80% on average is sufficiently low for obtaining L-gas, i.e. natural gas with a lower calorific value.
- The calorific value remains practically unchanged at 24 MJ/m³.
- The gas yield is somewhat higher for digestion of algae (0.94 l/g oDM) as compared to sewage sludge (0.87 l/g oDM).
- Methane formation relating to the degraded chemical oxygen demand (COD) is similar for digestion of algae (0.58 l/g COD) and sewage sludge. The methane content of the digester gas is about 67% for sewage sludge and 65% for algae.
- While, in principle, co-digestion of algae and sewage sludge is possible, as of now it is not cost effective.
