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Anaerobic Digestion Technologies

Several anaerobic technologies can be applied for anaerobic digestion including covered lagoons, plug flow, complete mix, upflow sludge blanket, and fixed film reactors. Technology selection is highly dependent on solids content. Of note is that swine waste is generally in the form of a slurry (>15% solids) and thus amenable to AD conventional technology while cattle waste collected from dry lots can be very high in solids content (<50%). Dairy manure collected on concrete (by scraping) generally has a total solids content between 10-16%, while flushed manure can have a solids content less than 3%, but can vary substantially depending on the amount of water used for flushing manure.

Covered Lagoons

Covered lagoons are one of the cheapest and simplest anaerobic digestion technologies available. Anaerobic digestion and subsequent production of methane takes place naturally in wastewater lagoons which contain high strength wastewater. A synthetic cover, typically plastic or rubber is used to trap and store the biogas. Because covered lagoons are difficult to heat, they are only recommended in warm climates. Too little methane is generated by covered lagoons in Colorado’s cold winter climate to justify installation of biogas capture and use equipment.

Plug Flow

Plug flow digesters are a low tech AD technology for treatment of high solids content waste (11-14% solids). The thick, high solids content waste travels down the digester in a “plug,” as a continuous mass. Plug flow digesters can be a good fit with the often high solids content waste generated at animal feeding operations in Colorado.

Complete Mix

Complete mix reactors are large, often cylindrical, tanks which have a mechanism to keep the reactor completely stirred. This mechanism can be injected biogas, or a motorized paddle. Mixing produces an ideal environment for anaerobic microorganisms by spreading the nutrients evenly throughout the reactor, while simultaneously helping to dampen shock loads of toxins which may enter the system since influent is instantaneously diluted through mixing. Complete mix reactors operate best when solids content is between 5-10%. Because solids content of waste produced at most Colorado cattle feeding operations is higher than 5-10%, complete mix reactors are often not a good fit unless an external source of water or wastewater is readily available.

Upflow Sludge Blanket

Upflow sludge blanket reactors are similar in design to a complete mix reactor, except that there is no integrated mechanism for homogenizing the waste. Instead, settling of solids is encouraged so that a sludge blanket is formed, maintaining biomass within the system, thus reducing the required holding time. These reactors are highly efficient and have been successfully up-scaled to the commercial scale. In general, waste generated at Colorado animal feeding operations is too high in solids for application of an upflow sludge blanket reactor unless it is combined with wastewater.

Fixed Film Digesters

In a fixed film digester, bacteria colonize a provided support structure within the reactor. This support structure is a high surface area material suitable for colonization, such as PVC pipe or shredded plastic. Fixed film reactors have successfully been implemented with low solids content (< 3%) dairy manure wastewaters in Florida, but are not likely to be a good fit with wastes produced in Colorado.

Last updated: October 18, 2017 at 11:36 am