What is dry Anaerobic Digestion?

 
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In anaerobic digestion there are two recognised industry processes, dry and wet. Whereas in traditional wet AD, feedstock is liquidised and pumped through a digester (with an average dry matter content of 8-11%), in dry AD the feedstocks are stacked and sealed into a digester, with leachate sprayed over the surface of the pile. 

So what actually happens inside a dry anaerobic digester?

  • The stackable organic waste (usually a ligneous feedstock suck as straw) is loaded onto a groundsheet, covered, and sealed to form a digester. 

  • The waste material is then sprayed with circulating hot leachate (process water, sometimes mixed with a high nitrogen co-feedstock such as manure), breaking down the stacked material and producing biogas.

  • The biogas is collected in the digesters and is then pumped into a bladder which can be utilised directly for cooking, converted to electricity and/or hot water using a powerQUBE CHP, or upgraded into biomethane for compression and vehicle fuel.

Dry digestion is adaptable, allowing for scalable digester sizes, and retention times, based on the feedstock quantity and properties. As a batch process, one digester can be used over and over again, emptied and re-filled.

We’ve recently installed a new rice straw project in the Philippines using our dryQUBE dry anaerobic digester. The hot climate makes it an ideal condition for developing dry AD, as less energy is spent heating up the leachate to spray onto the feedstock, in comparison to the heating requirements of other types of digesters in colder climates.

QUBE have stripped dry AD right back to its basics, and demonstrated, in partnership with Straw Innovations Ltd, the University of Manchester and University of Southampton, a new future for dry AD.

 
Alice Bayfield