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All anaerobic digesters perform the same basic function. They hold manure in the absence of oxygen and maintain the proper conditions for methane forming microorganisms to grow. There is a wide variety of anaerobic digesters, each performing this basic function in a subtly different way. Seven of the most common digesters are described in this article. Construction and material handling techniques can vary greatly within the main categories.
For clarity, we can divide digesters into three categories:
This system takes advantage of the low maintenance requirement of a lagoon while capturing biogas under an impermeable cover (Figure 1). The first cell of a two-cell lagoon is covered, and the second cell is uncovered (Figure 2). Both cells are needed for the system to operate efficiently. A lagoon is a storage as well as a treatment system; the liquid level on the second cell must rise and fall to create storage, while the level on the first cell remains constant to promote manure breakdown.
Since they are not heated, the temperature of covered lagoons follows seasonal patterns. Methane production drops when lagoon temperatures dip below 20 degrees C. A covered lagoon located in the tropics will produce gas year-round, but gas production will drop considerably during the winter farther north. Since sludge is stored in lagoons for up to 20 years, methane-forming microorganisms also remain in the covered lagoon for up to 20 years. This means that much of the fertilizer nutrients, particularly phosphorus, also remain trapped in the covered lagoon for a long time. If lagoon effluent is recycled to remove manure from buildings, liquid retention time is generally 30 to 60 days – depending on the size and age of the lagoon.
A complete mix digester (Figure 3) is basically a tank in which manure is heated and mixed with an active mass of microorganisms (Figure 4.). Incoming liquid displaces volume in the digester, and an equal amount of liquid flows out. Methane forming microorganisms flow out of the digester with the displaced liquid. Biogas production is maintained by adjusting volume so that liquids remain in the digester for 20 to 30 days. Retention times can be shorter for thermophyllic systems. The digester can be continuously or intermittently mixed. Intermittent mixing means the tank is stirred during feeding and only occasionally between feedings. Sometimes the process takes place in more than one tank. For instance, acid formers can break down manure in one tank, and then methane formers convert organic acids to biogas in a second tank. Complete mix digesters work best when manure contains 3 percent to 6 percent solids. Digester size can be an issue at lower solids concentrations. Lower solids mean greater volume, which means you need a larger digester to retain the microbes in the digester for 20 to 30 days.
The idea behind a plug flow digester (Figure 5) is the same as a complete mix digester – manure flowing into the digester displaces digester volume, and an equal amount of material flows out (Figure 6). However, the contents of a plug flow digester manure are thick enough to keep particles from settling to the bottom. Very little mixing occurs, so manure moves through the digester as a plug – hence the name “plug flow." Plug flow digesters do not require mechanical mixing. Total solids (TS) content of manure should be at least 10 or 15 percent, and some operators recommend feeding manure with solids as high as 20 percent. This means you may need to add extra material to manure to use a plug flow digester. This is not always a bad thing if you consider the added material may also be biodegradable. More degradable material means more biogas. Plug flow digesters are usually five times longer than they are wide. Recommended retention time is 15 to 20 days.
Returning some of the active organisms to the digester decreases digestion time. This is done in plug flow systems by pumping some of the effluent leaving the digester to the front of the digester. In complete mix systems, solids are settled in an external clarifier, and the microbe-rich slurry is recycled back to the digester. The systems are called contact stabilization digesters or anaerobic contact digesters (Figure 7).
A fixed film digester (Figure 8) is essentially a column packed with media, such as wood chips or small plastic rings. Methane-forming microorganisms grow on the media. Manure liquids pass through the media (Figure 9). These digesters are also called attached growth digesters or anaerobic filters. The slimy growth coating the media is called a biofilm. Retention times of fixed film digesters can be less than five days, making for relatively small digesters. Usually, effluent is recycled to maintain a constant upward flow. One drawback to fixed film digesters is that manure solids can plug the media. A solid separator is needed to remove particles from the manure before feeding the digester. Efficiency of the system depends on the efficiency of the solid separator; therefore, influent manure concentration should be adjusted to maximize separator performance, usually 1 percent to 5 percent total solids). Some potential biogas is lost due to removing manure solids.
In these types of digesters, microbes are suspended in a constant upward flow of liquid. Flow is adjusted to allow smaller particles to wash out, while allowing larger ones to remain in the digester. Microorganisms form biofilms around the larger particles, and methane formers stay in the digester. Effluent is sometimes recycled to provide steady upward flow. Some designs incorporate an artificial media such as sand for microbes to form a biofilm; these are called fluidized bed digesters.
Suspended media digesters that rely on manure particles to provide attachment surfaces come in many variations. Two common types of suspended media digesters are the upflow anaerobic sludge blanket digester, or UASB digester (Figure 10), and the induced blanket reactor, or IBR digester (Figures 11 and 12). The main difference between these two systems is that UASB digesters are better suited for dilute waste streams (<3-percent total suspended solids); whereas, the IBR digester works best with highly concentrated wastes (6 percent to 12 percent TS).
An anaerobic sequencing batch reactor (Figure 13), or ASBR digester, is a variation on an intermittently mixed digester. Methane forming microorganisms are kept in the digester by settling solids and decanting liquid. An ASBR operates in a cycle of four phases (Figure 14). The digester is fed during the fill stage, manure and microbes are mixed during the react phase, solids are settled during the settle stage, and effluent is drawn off during the decant stage. The cycle is repeated up to four times a day for nearly constant gas production. Liquid retention times can be as short as five days. Although ASBR digesters work well with manure in a wide range of solids concentrations, they are particularly well suited for very dilute manures (< 1 percent TS), and if filled with active microbes during start-up, can even produce biogas with completely soluble organic liquids. Sludge must be removed from the ASBR digester periodically. Concentrated nutrients are harvested during sludge removal.