Unit 3.5: High Solids Anaerobic Digestion

 

Background

High solids digestion is an emerging field. Every day new processes are being created to anaerobically digest stackable, high solids content organic matter. These processes are similar to anaerobic composters; however a high solids anaerobic digester (HSAD) is specifically designed to capture methane and energy during the process.

Guiding Question

  • What are the most common types of HSAD?

 

High Solids Digesters

High Solids Anaerobic Digestion (also known as “dry fermentation”) is used to treat stackable materials such as food scraps, yard debris, and municipal solid wastes. As the name implies, high solids digesters work with materials that have a solids content of 25 – 50 percent.   Manure may be mixed with the solids to add fermentative microbes.

High solids substrates are loaded into digestion chambers. Chambers can either be vertical (silo) (Figure 14) or horizontal (tunnel) (Figure 15). Many patented systems have chambers that are operated in either anaerobic or aerobic phases. The chamber is kept airtight during the anaerobic phase. Air is forced into the chamber during the aerobic phase.

Leachate is collected at the bottom of chamber during both phases. During the anaerobic phase leachate is high in organic acids. In some systems, leachate is sprayed on the top of the solids pile within the digestion chamber where the organic acids are converted to methane by microbes in biofilm on the solids. In other systems, the leachate is pumped to a separate liquid digestion reactor where methane production takes place.

 

Advantages

  • Can use solid materials
  • Treats under both anaerobic and aerobic conditions

 

Disadvantages

  • Needs stackable material with at least 25% solids
  • Complicated process
  • Expensive
Figure 14: Schematic drawing of silo type high solids digestion system with external leachate digester. Source: DW Hamilton

Figure 14: Schematic drawing of silo type high solids digestion system with external leachate digester. Source: DW Hamilton

 

Figure 15: Tunnel style high solids anaerobic digestion chamber. Source: www.hotrot.co.uk

Figure 15: Tunnel style high solids anaerobic digestion chamber. Source: www.hotrot.co.uk

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