Mixed Plug Flow Digesters

Plug flow digesters are tanks where manure and organic waste flows in one end and out the other in a “plug” manner, minimizing back-mixing. These can be in horizontal or vertical orientations.  Manure and organic waste with high solids content is fed into the digester, where it moves slowly through the tank. The residence time can vary but is typically 15-20 days, allowing for complete digestion of the feedstock and production of biogas. 

The liquid digestate, or effluent, is pumped to a treatment facility where polymers (flocculants) are typically added to help speed up the solid-liquid separation process. After polymer addition, the solids are separated using specialty equipment such as centrifuges, belt presses, or screw presses. The dry sludge can then be disposed of by means of trucking to landfill or a compost facility, used as a solid fertilizer, or provided to the barns for bedding. The liquid fraction can be further treated for reuse, or used directly for irrigation. The correct polymer is essential in digestate treatment as it reduces the moisture content of the sludge, resulting in a denser, more manageable solid fraction. Efficient dewatering minimizes the volume of sludge, thereby lowering transportation and disposal costs. Additionally, the improved separation reduces the load on downstream processing equipment, further cutting operational expenses and enhancing overall cost-effectiveness in waste management. 

Advantages

Effective for high solids content waste: Mixed plug flow digesters are particularly effective for processing waste with high solids content. The design ensures thorough digestion as the waste moves slowly through the tank, maximizing biogas production. This makes them ideal for farms with large amounts of high-solids manure and organic waste.

Disadvantages

Requires consistent feedstock with high solids content: These digesters require a consistent feedstock with a high solids content to operate efficiently. Variations in the feedstock can affect the digestion process and biogas production. This need for consistent, high-solids input can be a challenge for some farms, particularly those with variable waste streams.