Biogas systems utilize various methods for digesting organic materials. One of the innovative approaches is the dry digester, which permits substrates to remain in a stackable state throughout the digestion process. In this configuration, food waste is blended with green waste—such as yard debris—to enhance its structure and porosity before it is loaded into a long, rectangular vessel. This vessel is securely sealed and heated, with warm water or percolate sprayed over the waste stack. The percolate, rich in biological activity, facilitates the digestion process and is collected to be recycled. Additionally, this percolate is directed to a distinct methanization digester tank where biogas is produced, and the percolate is reused. There are variations in design, including vertical down-flow reactors where waste is introduced from the top and exits from the bottom after several days of digestion.

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Anaerobic Processing of Livestock Waste: Types of Digesters

This document focuses on anaerobic digesters utilized in agricultural applications involving liquid, slurry, or semi-solid manure handling. For those interested in solid-state digestion techniques, consult Extension Fact Sheet BAE for insights on digesters that process stackable, high solids materials, such as cotton gin waste, yard debris, food leftovers, feedlot remnants, and municipal solid wastes.

Digesters Classification

All anaerobic digesters serve a fundamental role: they maintain manure in an oxygen-free environment, fostering conditions suitable for the growth of methane-producing microorganisms (methanogens). Though their basic function is consistent, the specificity of each digester’s design varies slightly. The following are the three primary categories of digesters:

1. Passive Systems:

   These systems are integrated with existing treatment components, exerting minimal control over the reactor environment.

2. Low Rate Systems:

   Here, the manure added primarily contributes to methanogen presence. The solids retention time (SRT) corresponds to the hydraulic retention time (HRT), indicating the period solid materials remain in the digester.

3. High Rate Systems:

   These systems trap methane-forming microorganisms within the digester to enhance efficiency, with SRT surpassing HRT.

Passive Digesters

Covered Lagoon: This design capitalizes on the low maintenance of lagoons while effectively capturing biogas beneath an impermeable cover. A two-cell lagoon system requires one cell to be covered and another uncovered for optimal operation, supporting both storage and treatment functions. The liquid level must fluctuate in the uncovered cell to facilitate storage, whereas a constant level is maintained in the covered cell to promote efficient manure breakdown. If lagoon effluent is recycled to remove manure from buildings, the HRT typically ranges from 30 to 60 days. Sludge can remain stored in lagoons for up to 20 years, leading to extended SRT within a covered lagoon, effectively retaining nutrients like phosphorus.

Heating covered lagoons for optimum biogas production involves significant expense, as these lagoons follow seasonal temperature patterns—hence the term "ambient temperature digesters." Methane production significantly decreases once lagoon temperatures drop below 20°C, resulting in higher methane yields in tropical locations compared to cooler temperate climates.

Figure 1: Initial covered cell of a lagoon located at the Oklahoma State University Swine Research and Education Center.

Figure 2: Schematic representation of the covered lagoon digestion system.

Low Rate Digesters

Complete Mix Digester: This design features a tank where manure is heated and combined with microorganisms. Incoming liquids displace existing volume, resulting in an equal flow out. Methanogens exit the digester with this displaced liquid, while biogas production is sustained by controlling volume to ensure liquids stay within the digester for 20 to 30 days, with thermophilic systems allowing for shorter retention times. Mixing may occur continuously or intermittently, depending on the process.

Figure 3: Three Complete Mix Reactors forming the Dane County Community Digester in Madison, Wisconsin.

Figure 4: Schematic illustration of a Complete Mix Digester.

Plug Flow Digester: Borrowing the same principle as the complete mix digester, this system maintains sufficiently dense contents to prevent settling. Manure flows through the digester in a “plug,” hence the name. A solid content of 15% is typically required, with some configurations favoring even greater concentrations. This digester design is usually five times longer than it is wide, with recommended retention times ranging from 15 to 20 days.

Figure 5: Plug Flow Digester operational on a dairy farm.

Mixed Plug Flow Digester: As a patented variation of the plug flow design, this digester includes manure flow along a hairpin raceway with contents heated along a central divider, inducing a corkscrew mixing pattern.

Figure 6: Schematic overview of a Mixed Plug Flow Digester (from US Patent 8,202,721).

High Rate Digesters

Solids Recycling: This method permits the return of active microorganisms to the digester, thereby enhancing SRT and decreasing digestion duration. In plug flow systems, some of the effluent is pumped back to the digester's front; in complete mixes, solids settle in an external clarifier, and the microorganism-rich slurry is reintroduced to the digester. Such systems are referred to as Contact Stabilization Digesters or Anaerobic Contact Digesters.

Figure 7: Schematic illustration of a Contact Stabilization Digester.

Fixed Film Digester: In this type, methane-producing microorganisms develop on supportive media like wood chips or small plastic rings located within a digestion column. Hydraulic retention times can be less than five days, leading to smaller digesters.

Figure 8: Fixed Film Digester situated at the University of Florida Dairy Research Farm.

Figure 9: Schematic drawing of a Fixed Film Digestion System.

Suspended Media Digesters: In these systems, microorganisms are suspended within an upward liquid flow, allowing smaller particles to wash out while larger ones stay in the digester. These designs may use artificial media like sand for support, known as Fluidized Bed Digesters, or they can be based on manure particles. Two common variations include the Upflow Anaerobic Sludge Blanket Digester (UASB) and the Induced Blanket Reactor (IBR).

Figure 10: Schematic representation of an Upflow Anaerobic Sludge Blanket (UASB) digestor.

Figure 11: Schematic of an Induced Bed Reactor (IBR) Digester (courtesy of Conly Hansen, Utah State University).

Sequencing Batch Reactor: The Anaerobic Sequencing Batch Reactor (ASBR) is an intermittently mixed digester, maintaining methanogens through settling solids and liquid decanting. This method operates in cycles encompassing four phases: feeding, mixing, settling, and effluent removal, typically recurring multiple times throughout the day.

Figure 12: ASBR digester as seen at the Oklahoma State University Swine Research and Education Center.

Figure 13: Four phases of the ASBR Cycle.

Conclusion

Producers of livestock face a myriad of options when selecting an anaerobic digester for biogas production on their farms. These systems can be categorized into passive (such as covered lagoons), low rate (including complete mix, plug flow, and mixed plug flow), and high rate varieties (contact stabilization, fixed film, suspended media, and sequencing batch systems). Despite every reactor executing core functions, operational efficiency can significantly vary based on the manure’s consistency. Typically, more efficient reactors are characterized by increased complexity and a higher cost relative to their size.

Douglas W. Hamilton

Waste Management Specialist

Biosystems and Agricultural Engineering

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