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Multi-Stage Anaerobic Digestion Biosolids Technology Fact Sheet Multi-Stage Anaerobic Digestion DESCRIPTION the organisms that perform the methane forma- tion step (the methanogenic bacteria). The Anaerobic digestion is a naturally occurring bio- acidogenic bacteria are also less sensitive than logical process in which large numbers of the methanogenic bacteria to changes in organic anaerobic bacteria convert organic matter into strength and composition in the incoming feed methane and carbon dioxide (a mixture called stream. Therefore, although many wastewater biogas) in the absence of air. It is a widely used treatment plants have traditionally performed biological process for treating wastewater solids. anaerobic digestion processes in a single tank (in This process stabilizes the organic matter in a process called single-stage anaerobic digestion) wastewater solids, reduces pathogens and odors, at a constant temperature, some facilities have and reduces the total solids/sludge quantity by separated the process into multiple stages, by converting part of the volatile solids (VS) frac- physically separating the stages or by controlling tion to biogas. Anaerobic digestion results in a the process to separate the stages in time, or product that contains stabilized solids, as well as both. This approach allows the facilities to opti- some available forms of nutrients such as am- mize the various stages of the anaerobic monia-nitrogen. digestion process to meet their needs. The process of anaerobic digestion can be divided The standard multi-stage anaerobic digestion into three separate steps, each of which is per- system is a two-stage acid/gas (AG)-phased formed by a different group of microorganisms: system, in which the acid-forming steps (hy- • Hydrolysis, during which the proteins, cellu- drolysis and volatile acid fermentation) are lose, lipids, and other complex organics are physically separated from the gas-forming step broken down into smaller molecules and be- (methane formation) by being conducted in sepa- come soluble by utilizing water to split the rate digestion tanks. The first stage, known as the chemical bonds of the substances primary or acid phase digester, consists of the hydrolysis and the first acid-production step, in • Volatile acid fermentation, during which the which acidogenic bacteria convert organic matter products of hydrolysis are converted into or- into soluble compounds and volatile fatty acids. ganic acids through the biochemical The second stage, known as the secondary or processes of acidogenesis (where monomers methane stage digester consists of further con- are converted to fatty acids) and acetogenesis version of organic matter to acetic acid through (the fatty acids are converted to acetic acid, acetogenesis, as well as the methane formation carbon dioxide, and hydrogen) step, in which methanogenic bacteria convert • Methane formation, during which the organic soluble matter into biogas (primarily methane; acids produced during the fermentation step see Figure 1). The methanogenic step also pro- are converted to methane and carbon dioxide. duces other by-product gases, including hydrogen sulfide, nitrogen gas, and several other The efficiency of each step is influenced by the gases. In a typical two-stage system, the primary temperature and the amount of time the process is digester is heated to optimize performance of the allowed to react. For example, the organisms that hydrolytic and acidogenic bacteria. The secon- perform hydrolysis and volatile acid fermentation dary digester is not normally equipped with (often called the acidogenic bacteria) are fast- mixing or heating facilities because of the exo- growing microorganisms that prefer a slightly thermic (heat-producing) nature of the methane acidic environment and higher temperatures than formation reaction. 1 Source: Wilson, et. al, 2005 Figure 1. Standard Multi-Stage Anaerobic Digestion System An alternative method for designing the system enhance the results even further. Facilities in is to separate the stages over time by adding dif- Tacoma, Washington, Inland Empire, California, ferent levels of heating at different times in the and Calgary, Alberta, Canada, have gone to process by a process called temperature-phased three-phased processes. Table 1 provides several anaerobic digestion, or TPAD. As described ear- examples of wastewater treatment facilities that lier, hydrolysis and acidogenesis can be use different types of multi-stage processes (Wil- enhanced by increasing the operating tempera- son 2003 and personal communications). ture; however, acetogenesis is adversely affected by high operating temperatures (Chang, et al. APPLICABILITY 2004). If the system is heated to enhance hy- drolysis and acidogenesis, the resulting volatile Multi-stage anaerobic digestion systems are po- acid production can overwhelm the ability of the tentially applicable for all wastewater treatment slower-reacting acetogenic and methanogenic systems, provided that the solids can be delivered bacteria to convert the volatile acids, resulting in to the system at an acceptable concentration. increased pH and inhibited acetogenesis and These can include both new installations and ret- methanogenesis (Chang, et al. 2004). Therefore, rofits. In fact, much of the current research into controlling the temperature can be critical in anaerobic digestion is directed toward retrofitting optimizing system performance. multi-stage systems into facilities where single- stage processes are already present (Cumiskey Numerous facilities use some form of TPAD. 2005; W. Parker, personal communication, 2006). For example, in 2002 the wastewater treatment facility in Waterloo, Iowa, rehabilitated its exist- The primary factor in determining whether a ing anaerobic digestion system to operate as a multi-stage anaerobic digestion process is feasi- TPAD system, in which the first digesters were ble for a system is the feed solids concentration. operated in the thermophilic range (50–60 °C Because a multi-stage process can be sensitive to [122–150 °F]) to promote pathogen destruction changes in the feed solids, it might not be feasi- with the intent of producing Class A biosolids, ble if the characteristics of the feed solids while subsequent digesters were operated in the concentrations vary significantly. The VS con- mesophilic range (30–38 °C [85–100 °F]) to re- tent in the feed should preferably be at least 50 duce VS (Iranpour and Windau 2004). This type percent, and the feed should not contain sub- of system can be abbreviated as a TPAD-TM, stances at levels that may inhibit the biological where the T represents the thermophilic first processes associated with anaerobic digestion stage, and the M represents the mesophilic sec- (see Table 2). Wastewater residuals containing ond stage. lime, alum, iron, and other substances can be successfully digested as long as the VS content Facilities can separate these stages in both space remains high enough to support the growth of and time by operating multiple digesters in se- microorganisms. ries, to increase control over the process and 2 Table 1. Example Wastewater Treatment Facilities with Multi-Stage Anaerobic Digestors Plant System Type Woodridge WWTP, DuPage County, IL Two-stage AG-MT Elmhurst, IL Two-stage AG-MM Back River, Baltimore, MD (pilot) Two-stage AG-MM Inland Empire (RP-1), Ontario, CA (farm manure) Three-stage AG-MTM Waterloo, IA Two-stage TPAD-TM Waupun, WI Two-stage TPAD-TM Rockaway, NY Two-stage TPAD-MT Pine Creek WWTP, Calgary, Alberta, Canada (pilot) Three-stage TPAD (multiple options being researched) Tacoma, WA Heated aerobic stage (71◦ C [160◦ F]) + Three-stage TPAD-TMM Table 2. Substances with Potential to Cause Biological Inhibition in Anaerobic Digestion Substance Moderately Inhibitive (mg/L) Strongly Inhibitive (mg/L) Calcium 1,500–4,500 8,000 Magnesium 1,000–1,500 3,000 Sodium 3,500–5,500 8,000 Potassium 2,500–4,500 12,000 Ammonia Nitrogen 1,500–3,000 3,000 Copper –– 50–70 (total) Chromium VI –– 200–250 (total) Chromium –– 180–420 (total) Nickel –– 30 (total) Zinc –– 1.0 (soluble) ADVANTAGES AND DISADVANTAGES single-stage systems. In addition, they can be more expensive than single-stage systems, al- The major advantages of multi-stage anaerobic though this is more of a factor when retrofitting digestion systems versus single-stage anaerobic into multi-stage systems. digestion systems is that multi-stage systems can optimize the various steps in the process by sepa- An expanded discussion of the advantages and rating them in space or time and optimizing the disadvantages of multi-stage versus single-stage specific conditions under which the various steps anaerobic digestion systems follows: take place. As described above, they can also allow a facility to adopt a specific system con- Advantages figuration to meet its goals. For example, if the Gas Recovery and Storage. Multi-stage systems facility wants to produce Class A biosolids, it can be optimized to maximize the amount of gas might require a thermophilic stage; however, they produce in the digestion phase. The gas pro- if volume reduction is its primary goal, only duced from the anaerobic digestion of biosolids is mesophilic stages may be required (W. Parker, typically composed of 55 to 70 percent methane personal communication, 2006). and approximately 25 to 30 percent carbon diox- The major disadvantage of multi-stage anaerobic ide, with the remaining fraction composed digestion systems is that they have higher opera- primarily of nitrogen, hydrogen, and hydrogen
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