Dry Digestion vs. Wet Digestion
Biogas production processes are commonly divided into two main categories, according to the dry matter content in the process: Dry Anaerobig Digestion, and Wet Anaerobig Digestion. These processes can be applied as continuous or batch processes.
Conventional Wet Digestion Plants uses bio-slurries, which can be pumped and transferred via pipelines. Dry matter content of the bio-sulrries are usually less than 15%. Biomaterial is usually diluted with a wast quantity of water to make it flow through pumps and prevent blockages in the pipelines. These wet digestion processes have a good history of usage, and the degree of process control is high in these plants. The common challenges in the wet processes are foam formation, high consumption of energy and water, and blockages in the pipelines. Sometimes,there are problems with mixing the digested slurry with feed-in slurry.
Dry matter content in a Dry Digestion Plant is greater than 20%, usually 22-40%. Suitable biomaterials are grain straws, manure, and other organic wastes. The water content in the process is the main factor for process energy consumption. With lower water content, the dry process has lower energy consumption compared to wet process. Need for heating energy in wet processes is around 30% of total produced energy while dry process needs around 10% produced energy for heating. The water volume in the process has an effect also on the required reactor size, thus dry fermentation requires a smaller reactor.
Additional water to dilute biomaterial is not needed in dry processes, since pumps and pipelines are not used to transfer the biomaterial. Benefits of dry digestion are higher retention of biomass, better control of feed of the reactor, and simple pre-treatment of biomaterial. Wet digestion processes are usually continuous, but the most common type of dry digestion process is a batch process. These are cheap to build, but to gain a constant production rate of biogas, there are need for several batch reactors, which increases the cost of the plant.
Continuous dry digestion process has a constant rate of biogas production with sufficient mixing and homogeneous feed-in biomaterial. Reactors used with continuous dry digestion processes are usually vertical or horizontal pipe reactors. Pipe reactors ensure a plug-flow in the reactor, thus there are no by-bass or return flows inside the reactor, which prevents digestate to mix with feed-in biomaterial.
A low water content in the process allows also easier digestate treatment, for utilizing it as fertilizer. Pre-treatment and post-treatment of biomaterial is energy efficient due to low water volume. High water and energy economy in the process enables to build economically viable small scale dry digestation plants.
- Reasons to select a continuous Dry Digestation process:
- low cost of investment due to smaller reactor size
- scalability to smaller units
- energy efficiency
- low water consumption
- no waste water: residue water in digestate can be evaporated or bind in the final production
- digestate can be utilized as fertilizer as it is or refined to granules
- steady biogas production rate
- low labor cost and low work load due to robust process automation and remote monitoring
Sources and links:
- Book: Modern Solid State Fermentation: Theory and Practice
- http://www.sciencedirect.com/science/article/pii/S1364032114004638 (registration and password)
- Artikkeli: Weiland, P. Biogas production: current state and perspectives 2009
DRY DIGESTION THERMOPHILIC FAQS