Anaerobic Digestion of Animal Manures: Types of Digesters

Anaerobic Digestion of Animal Manures: Types of Digesters

Categories of Digesters

Anaerobic digesters are essential for managing manure in the absence of oxygen, providing the ideal environment for methane-producing microorganisms. There are several types of anaerobic digesters, each with specific characteristics and uses. These digesters can be broadly categorized into three types:

1. Passive Systems: Biogas recovery is added to an existing treatment component. The reactor environment is not tightly controlled.
2. Low Rate Systems: In these systems, manure itself is the main source of methanogens. Solids retention time (SRT) equals hydraulic retention time (HRT).
3. High Rate Systems: These systems aim to trap methane-producing microorganisms in the digester, making SRT greater than HRT for increased efficiency.

Passive Systems

Covered Lagoon: This system involves capturing biogas under an impermeable cover in the first cell of a two-cell lagoon. The second cell is uncovered and used for storage and treatment. Though heating covered lagoons for optimal biogas production is costly, they follow seasonal temperature patterns and effectively store sludge for up to 20 years.

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Low Rate Systems

Complete Mix Digester: This type of digester heats and mixes manure with active microorganisms. Liquid flows in and out of the digester, maintaining biogas production by keeping liquids inside for 20 to 30 days. Mixing can be continuous or intermittent. This type of digester works best with manure containing 3% to 6% solids.

Plug Flow Digester: Manure in a plug flow digester moves through the system as a plug, preventing particles from settling. These digesters work best with manure containing at least 15% solids, but up to 20% is recommended. They do not require mechanical mixing and usually have a retention time of 15 to 20 days.

Mixed Plug Flow Digester: This variant of a plug flow digester involves manure flowing down a hairpin raceway, mixing in a corkscrew pattern due to central divider heating.

High Rate Systems

Solids Recycling: Returning active organisms to the digester increases SRT and efficiency. This is achieved by recycling some of the effluent back to the digester front, thus called Contact Stabilization Digesters or Anaerobic Contact Digesters.

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Fixed Film Digester: Methane-producing microorganisms grow on supporting media like wood chips or plastic rings. This method allows for short hydraulic retention times, making small digesters feasible, although it requires solid separators to avoid clogging.

Suspended Media Digesters: These digesters keep microbes suspended in an upward flow of liquid. Microorganisms form biofilms around larger particles. Examples include Upflow Anaerobic Sludge Blanket (UASB) and Induced Blanket Reactor (IBR) digesters. UASB systems work best with low solids influent, while IBR systems require high solids manure.

Sequencing Batch Reactor: Also known as an Anaerobic Sequencing Batch Reactor (ASBR), this type treats manure in four distinct phases. Solids are settled, and liquid is decanted to maintain methanogens in the digester, with retention times as short as five days.

Summary

Livestock producers have various anaerobic digesters to choose from for on-farm biogas production. Options include passive systems (covered lagoons), low rate systems (complete mix, plug flow, mixed plug flow), and high rate systems (contact stabilization, fixed film, suspended media, and sequencing batch). While all digesters serve the same basic function, they operate optimally with specific manure consistencies and vary in operational complexity and cost.

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