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.
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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
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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 tion and maintenance (O&M) requirements than sulfide (USEPA 1979). Typical digester gas 3 exhibits a heat content between 18,630 and Other advantages of multi-stage anaerobic diges- 26,080 thousand Joules per cubic meter (kJ/m3) tion versus single-stage anaerobic digestion or between 500 and 700 BTU/ft3, which is ap- processes include: proximately two-thirds the heat content of the natural gas delivered by gas utilities. Therefore, • Multi-stage systems require less digester digester gas can be an economical energy source volume to handle the same amount of input for plant operations. It can be temporarily stored volume because they have lower retention and/or mixed with natural gas through the pipe- times and allow higher loading rates than line system for in-plant use as a source for heat, single-stage systems. electricity, or steam. It is ideal as fuel to fire hot • Multi-stage systems have achieved VS re- water boilers, internal combustion engines, heat duction, which provides better odor control. drying equipment, and incinerators. Some plants • A multi-stage system can be configured to scrub their digester gas to reduce the levels of reduce foaming problems. (See discussion of carbon dioxide, hydrogen sulfide, siloxane, and foaming in the “Operation and Maintenance” other gases and in several cases have marketed the section below.) gas as a high-value natural gas source to their local gas utility systems. • Multi-stage systems reduce the short circuit- ing of solids by separating the stages and Biosolids Quality. Multi-stage anaerobic diges- optimizing the retention time in each stage. tion systems that use a thermophilic stage can produce biosolids that meet Class A pathogen Disadvantages reduction requirements. Much of the current re- search into anaerobic digestion is devoted to • The piping requirements for a multi-stage pathogen control through temperature phasing system, operation, and maintenance are more and pretreatment of waste through processes like complex than those for a single-stage system. enzyme hydrolysis prior to its anaerobic diges- tion. For example, recent research by the City of DESIGN CRITERIA Los Angeles indicates that their product resulting from systems operated at thermophilic tempera- Location in the Solids Processing Train tures achieved Class A status and had lower odor Multi-stage anaerobic digestion is typically lo- than the product produced by mesophilic proc- cated in the solids processing train after esses. In addition, their results indicated that the thickening but before dewatering. Thickening of odor concentrations in solids digested using the solids prior to digestion is beneficial because mesophilic temperatures continued to increase as it reduces the biomass volume, digester size re- the biosolids went through the digestion process quirement, supernatant volume, and heating and even after they were applied on farmland. requirements (WEF 1998). (Material produced by digestion at mesophilic temperatures and received at their land applica- Solids Feed Rate tion site had odor concentrations 10 times higher The solids feed rate is typically 5 to 6 percent of than the material being introduced into the cen- the mixed solids retention range. trifuges for dewatering.) In contrast, the odor content of material subjected to thermophilic Organic Loading digestion temperatures decreased by about 70 percent by the time it reached the land applica- Typical VS loading rates for both mesophilic and thermophilic multi-stage systems are in the 482– tion site (Haug et al. 2002). Enzyme hydrolysis 3 3 is being heavily researched in Europe. Additional 642 kg/m /day (30–40 lb/ft /day) range, which is significantly higher than the average of 2.57 discussion of pretreatment through enzyme hy- 3 3 drolysis is presented later in the “Design” kg/m /day (0.16 lb/ft /day) for single-stage an- section. aerobic digester systems (Sieger 2001).
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Solids Retention Time A summary of typical SRTs and VS loading As discussed earlier in the “Description” section, rates is provided in Table 3. solids retention time (SRT) is a critical factor in the design of a multi-stage anaerobic digestion Heat Exchangers system. High SRTs increase the digestion but Temperature is important in determining the rate reduce the rate of throughput for the system. of digestion. The design operating temperature Therefore, each facility must determine the op- establishes the minimum SRT required to timum SRT to achieve the required amount of achieve a given amount of VS reduction. As de- digestion while also maximizing the facility scribed above, most anaerobic digesters currently throughput. in operation are designed to operate in the meso- philic temperature range, although many current Because the stages are optimized to maximize designs for multi-stage systems include phases digestion, the SRTs of multi-stage systems are operated at thermophilic temperatures––through typically shorter than those of single-stage sys- TPAD systems with thermophilic processes. tems. For example, Sieger (2001) reported an average SRT of approximately 20 days for Typical auxiliary heating methods include steam mesophilic single-stage systems, while the SRTs injection, internal heat exchangers, and external for multi-staged systems typically ranged be- heat exchangers. External heat exchangers are tween 14 and 18 days. the most common because of their flexibility and the ease of maintaining their heating surfaces. In general, the SRT for a multi-stage system is Internal coils and heat-jacketed draft tube mixers determined by the required end-product and the can become caked and effectively blocked, ne- sequence of the phasing. For example, if the fa- cessitating removing them or taking the digester cility is producing Class B biosolids, it might use out of service to empty and clean the system. a lower SRT than a facility producing Class A Steam injection results in dilution of the digester biosolids using a similar configuration. contents and can be energy-inefficient.
Table 3. Comparison of Anaerobic Digestion Processes
VS Loading SRT per Tank Total SRT at Operating Digestion Rate at Max Pathogen Level at Max Month Max Month Temperature Process Month Produced (days) (days) Regime 3 (lb/ft /day) Single-Stage Meso- 20 20 M 0.16 Class B philic Staged or Extended 15/1.5/1.5 18 T 0.30 Class Aa Thermophilic TPAD 5/10 15 T/M 0.30 Class Aa, b ATPc 1.5/15 16.5 T/M 0.30 Class A Two-Phase 2/12 14 M/T; T/M; T/T; 0.40 Class Ad or M/M Pre-Pasteurization 30 min./15 15.02 ~70 C/M 0.40 Class A
Source: Adapted from Sieger 2001. Notes: a Believed to meet Class A requirements, but formal pathogen equivalency has not been approved by EPA. b One process has been approved as a site-specific process by EPA, but the technology has not been approved for national equivalency for Class A. c Aerobic Thermophilic Pretreatment. d Testing may proceed on variations of feed and temperature of each phase.
5 Mixing bient air out, the covers prevent digester gas from Auxiliary mixing of the digester contents is being released and also reduce the amount of heat beneficial for reducing thermal stratification, loss to the atmosphere. Anaerobic digester covers dispersing the biosolids for better contact with can be fixed or floating. Fixed covers are flat, the microorganisms, reducing scum buildup, conical, or dome-shaped and are constructed of diluting levels of any inhibitory substances or reinforced concrete or steel. Floating covers can adverse feed characteristics, and retaining inor- rest directly on the liquid surface or float on the ganic material (grit) in suspension (WEF 1995). gas and be supported by side skirts at the side of Without adequate mixing, the digestion process the tank. can be short-circuited and solids that have not The appropriate type of cover for any given ap- been sufficiently digested might be prematurely plication depends on the design and size of the discharged. Such solids will not be properly sta- digester. Both fixed and floating covers have bilized and might not be suitable for the intended advantages and disadvantages. For example, end use. floating covers rise and fall with the liquid level The three mixing methods that have typically in the digester and therefore prevent formation of been used are mechanical mixing, hydraulic mix- a vacuum, which could damage the vessel or the ing, and gas recirculation. cover. Floating covers also prevent air from be- ing drawn into the digester during solids Mechanical mixing includes the use of impellers, removal. In contrast, a fixed cover is often easier propellers, and turbine wheels to mix the digester to design, requires less maintenance, and is less contents. prone to develop gas leaks. Hydraulic mixing is accomplished by recirculat- ing digester content through use of an external Enzyme Hydrolysis Pretreatment pump network. The hydraulic mixing can pump In January 2002 legislation was enacted in the the digester contents from the lower half of the United Kingdom (UK) that required pathogen digester to the top of the digester to potentially reduction in municipal wastewater sludge for the stop the formation of a significant scum layer, first time. This new requirement led many utili- which can be a nuisance or detrimental to di- ties to search for methods to optimize their gester operation. existing anaerobic digestion systems (Cumiskey 2005), particularly mesophilic digesters, which Gas recirculation systems use the digester gas included the majority of operating systems in the produced by the anaerobic digestion process to UK at that time. Investigations by United Utili- mix the digester contents. The gas is compressed ties (UU) in the UK indicated that the major and recirculated through the tank to promote pathway for killing pathogens in mesophilic an- mixing. The gas can be introduced into the tank aerobic digesters was solubilization or hydrolysis through one of several methods, including: (Mayhew et al. 2004). In anaerobic digestion, • Lances mounted on the inside of the tank hydrolysis occurs before the conversion of or- cover so they project down into the tank ganic particulate matter to organic acids. UU found that pathogen reduction could be im- • Diffusers mounted on the floor of the tank proved, and could be achieved at much lower • Draft tubes in the tank temperatures (mesophilic temperatures instead of thermophilic temperatures) by separating the • Bubble guns mounted inside the tank hydrolysis stage from the mesophilic anaerobic The type of mixing device suitable for any di- digestion stage (Mayhew et al. 2004). Therefore, gester depends on the design (vessel and cover) UU developed a specialized plug flow enzymic and size of the digesters. hydrolysis process to pretreat the sludge before anaerobic digestion. The enzyme hydrolysis step Types of Covers breaks down cell wall lipoprotein structures (Kelly 2003), enhancing the digestion process. It is necessary to cover the digesters to maintain This process results in a better energy balance anaerobic conditions. In addition to keeping am- 6 and the enhanced digestion increased biogas pro- mesophilic acid-stage digester was added to the duction relative to other processes. UU uses a existing digestion facility, which was converted plug-flow configuration that operates at 42 °C to a thermophilic gas-phase digester. The new (108 °F) with a 2-day hydraulic retention time. mesophilic acid-stage digester receives a feed of UU began installing the enzyme hydrolysis 46,000 GPD at a 4–5 percent solids content, with method in its facilities, including facilities in approximately 11,325 kg/day (25,000 lb/day) of Macclesfield, Bromborough, Crewe, and Black- suspended solids and 9,060 kg/day (20,000 burn. Initial tests at the Macclesfield facility lb/day) of volatile suspended solids. This stage show that the enzymic hydrolysis step results in has a retention time of approximately 1 day. Af- a 104 reduction in E. coli. The enzyme hydrolysis ter passing through this stage, the biosolids flow process in Bromborough enables the plant to to the methane-phase digester, which operates at operate at 4.0 kg VS/m3/day (250 lb VS/ft3/day) a thermophilic temperature of approximately 52 while also producing a high-quality product that °C (126 °F) and produces approximately 190,000 meets the new standards. The plant has also in- standard cubic feet (SCF) of gas per day with an creased its gas production from 4,500 m3/day to average methane content of 64 percent. 5,500 m3/day (158,916 ft3/day to 194,231 ft3/day) (Monsal 2004). The overall VS reduction averages approxi- mately 65 percent. During the first 4 months of 2000, fecal coliforms were reduced by an aver- PERFORMANCE age of 99.996 percent. The facility experiences Multi-stage anaerobic digestion can achieve su- no foaming, and the digested sludge is highly perior performance relative to single-stage desirable as a soil enhancer for agricultural pur- conventional digestion for most wastewater sol- poses. The digester gas is recirculated to power ids and for all loading rates. In addition, this the digesters, and excess gas is used to produce increased performance can be achieved with electricity. smaller digester volumes because of the higher loading rates that can be achieved with multi- Inland Empire Regional Water Recycling stage digesters. Compared to single-stage sys- Plant 1 (RP-1), Ontario, California tems, the multi-stage process achieves higher VS The anaerobic digestion system at the Inland reduction with shorter residence times. Typical Empire Utility Agency’s (IEUA) RP-1 was up- VS reduction for a first-stage digester ranges graded in 2000 and went online as a three-stage from 40 to 60 percent, and up to 5 percent addi- AG-MTM process in 2001. Before 2001 the fa- tional reduction can occur in subsequent stages. cility had operated as a thermophilic single-stage Multi-stage systems also produce more biogas of system. The system had experienced odor prob- a higher quality (as measured by its methane lems, however, and thus it had already gone to content) than that produced by single-stage proc- separate acid and gas phases using both a semi- esses. Finally, these systems reduce, and batch and a continuous approach. After spending potentially eliminate, the foaming problem that 2.5–3.5 days in a 32–40 °C (90–104 °F) meso- often occurs in single-stage systems. philic acid digester, the biosolids can be diverted to a semi-batch 56–58 °C (133–136 °F) thermo- Case studies highlighting the performance of philic gas-phase digester, where they are retained several multi-stage anaerobic digestion facilities for 18–20 days, or can go to a 50–52 °C (122– follow. 126 °F) thermophilic gas-phase digester, where they are retained for 14–16 days. After the ther- Woodridge WWTP, DuPage County, Illinois mophilic gas-phase digester, the biosolids are The Woodridge wastewater treatment plant sent to a mesophilic gas-phase digester. Flow (WWTP) was converted from its original single- from the semi-batch process goes to a 42–48 °C stage process to a two-stage AG-MT anaerobic (108–118 °F) system for 13–17 days, while flow digestion system in the late 1980s in an attempt from the continuous system goes to a 46–49 °C to control foaming problems in the old system. (115–120 °F) system for 5–6 days. To convert the facility to a two-stage process, a 7 Overall, VS reduction improved for the facility, Tacoma, Washington from approximately 55 percent to 60–65 percent The City of Tacoma, Washington, has operated with the AG-MTM process. Both processes an anaerobic digestion system for many years, showed non-detects for helminth ova, enteric but it has had a history of odor problems. In viruses, and Salmonella. The semi-batch process 1993 Tacoma transitioned from a single-stage qualified through time and temperature as Class thermophilic system to a two-stage AG-MM A biosolids under alternative 1 of 40 CFR Part system, thereby improving the odor of its 503, while the continuous process received site- TAGRO end-product so that it was more accept- specific EPA approval as Class A was granted able to customers. Although the odor of the end- under alternative 3 of 40 CFR Part 503 (Wilson product was acceptable, the hydrogen sulfide et al. 2005). odors in the plant’s belt-filter press room were extremely unpleasant to the workers and close to Waterloo, Iowa dangerous levels. Therefore, the plant began ex- The City of Waterloo wanted to increase its bio- perimenting with various temperature-phasing solids treatment capacity and improve its VS approaches to try to reduce odors. Eventually, destruction and gas production. In 2002 the city the plant determined that a thermophilic- meso- upgraded its anaerobic digestion process from a philic-low mesophilic approach of 55-38-32 °C single-stage mesophilic process to a TPAD-TM (131-100-90 °F) with a total retention time of 21 system by converting two of its six digesters into days was ideal. By lowering the middle digester thermophilic digesters. The city began the pro- from 46 °C to 38 °C (115 °F to 100 °F), the plant ject by taking each of its six digesters out of significantly reduced its odor problems. In addi- service one at a time and retrofitting them with tion, lowering the temperature from 38 °C to 32 the necessary piping, heating equipment, and °C (100 °F to 90 °F) in the final digester seems mixers for the new system. This approach al- to have improved dewatering. (Recent data show lowed the plant to continue to operate while the that dewatering has improved from 22 percent to facility was upgraded. Once all the new equip- 24 percent). The facility uses the biogas gener- ment was in place, two of the digesters were ated by the digestion process to run its boilers. sequentially transitioned to thermophilic tem- The plant has been operating with this system peratures. First, the feed rate into the digester since 2004 (D. Thompson, City of Tacoma, per- was slowed, and then the temperature was raised sonal communication, 2006). from 35 °C (95 °F) to 53 °C (131 °F) over a pe- riod of 3 days, allowing the organisms to Three-Stage TPAD (bench-scale) stabilize until they were achieving good VS de- Salasali et al. (2005) performed bench-scale tests struction. Once the first thermophilic digester of several three-stage TPAD configurations to was stabilized, the second was transitioned the evaluate the level of VS reduction and biogas same way. This quick transition from mesophilic production in these configurations. These re- to thermophilic was important because it limited searchers undertook these experiments to the number of mesophilic organisms that might determine whether modifying the operating prac- survive in the thermophilic digester. During this tices for standard mesophilic digesters could transition, it was also important to limit the load- achieve high performance VS reduction and ing rate so that the digester would not be Class A pathogen reduction so that facilities op- overloaded as the thermophilic organisms grew. erating mesophilic digesters could achieve high- The city’s new system achieved its goals. VS re- quality biosolids without going through the sub- duction improved from approximately 47 percent stantial costs of adding new digesters or in the old system to approximately 60–64 percent reconfiguring existing digesters. The authors in the new system; gas production increased to evaluated two three-stage configurations (35-35- 0.18–0.21 m3 per kg of VS destroyed (14–16 35 °C [95-95-95 °F] and 42-35-35 °C [108-95-95 ft3/lb) (Wilson et al. undated). °F]), as well as a two-stage system (35-35 °C [95-95 °F]). The authors used 20l samples of a mixture of primary and thickened waste- activated sludge with a concentration of between 8
4.0 and 5.2 percent solids from the City of Ot- control and to control the potential for metals tawa, Canada, WWTP. The trials used a and other chemicals to inhibit the process (see hydraulic retention time of 15 days (5 days in Table 2) (WEF 1995). Sodium bicarbonate, so- each stage for the three-stage systems, and 5 dium carbonate, and lime can be used to provide days in the first digester and 10 days in the sec- alkalinity. Ferrous chloride, ferrous sulfate, and ond digester for the two-stage system) and alum can be added to precipitate or coagulate measured conventional parameters (total solids, inhibitive chemicals or to control digester gas VS, pH) as well as pathogen indicators (fecal hydrogen sulfide content. coliform bacteria, Escherichia coli, fecal Strep- tococci, Salmonella spp., Cryptosporidium A common operational problem with any an- perfringens). Each of the configurations was able aerobic digestion system is foaming, which is the to achieve the 38 percent VS reduction required trapping of fine bubbles of gas in the semi-liquid for vector attraction reduction, although both of digestion contents. Foam forms primarily when the three-stage configurations achieved better VS the carbon dioxide-to-methane ratio is higher reduction and biogas production than did the than normal. This usually occurs during start-up two-stage configuration. Bacterial results showed operations, but it can occur whenever a fresh that, with the exception of Salmonella spp., food supply suddenly contacts active microor- pathogens were reduced to the greatest extent in ganisms. This is one reason continuous slow feed the 42-35-35 °C (108-95-95 °F) configuration. of solids is preferred to batch feeding of digest- ers. In addition, a common bacterium, Nocardia, has a filamentous structure that traps gas, leading OPERATION AND MAINTENANCE to foaming. These bacteria should be eliminated Because multi-stage anaerobic digestion systems in aeration basins before the solids are fed to the involve multiple stages, each having its own spe- digesters. Two-stage AG anaerobic digestion cific O&M requirements, these systems have naturally overcomes this problem because the higher overall O&M requirements than do sin- first stage (acid phase) digester has low gas pro- gle-stage anaerobic digestion systems. duction and low pH, along with higher volatile acid concentrations, which together are detri- Maintaining a stable operating temperature and mental to foam-causing microorganisms. pH within the digesters is critical, particularly for the methane formers, which are sensitive to Another important operational concern is odor changes in temperature and pH (Dague 1968). control at the plant during the anaerobic diges- Changes in digester operating temperature tion process. As discussed previously, hydrogen greater than ~1.0 °C (~2 °F) per day can result in sulfide and ammonia are produced during an- process upset due to heat shock of microorgan- aerobic digestion. The most common way to isms. The optimum pH range for anaerobic control odors from a digestion system is to use digestion is 6.8–7.2. A reduction in pH, which covers, as discussed earlier. can be caused by overloading the digester, inhib- Periodic clean-out of the digesters is necessary for its methane formation. Methane formation is all digestion systems. The frequency of cleaning further inhibited as the acid fermentation stage of is based on several factors, including the accumu- digestion continues, possibly leading to digester lation of grit and scum (which can reduce the upset and failure. Temperature control is also effective volume of the tank); the condition of important to ensure satisfactory operation of the internal mixing or heating equipment; the avail- digestion system. Fluctuations in temperature ability of backup solids handling equipment; and can result in the die-off of microorganisms and tank structure (WEF 1998). Typically systems process inefficiency. As discussed earlier, heat require cleaning approximately every 5 years. exchangers are commonly employed to control Because digesters are confined spaces, safety is a temperatures in the digester. primary consideration. Before personnel enter a Chemical addition to anaerobic digesters might digester, the air composition inside the tank must occasionally become necessary for pH/alkalinity be monitored for oxygen levels and the presence of hazardous gases. 9 COSTS tons of fertilizer per year, providing local farm- ers with an estimated $47,000 in no-cost The construction and operation and maintenance fertilizer annually. In addition, the facility directs costs of multi-stage anaerobic digestion depend the biogas to a dedicated boiler, which provides largely on the quantity and quality of the solids the heat for the digesters, as well as for the solids to be stabilized, the size of the digesters, and the processing building. By using the biogas from type of mixing and heating equipment. Capital the anaerobic digestion process to power the items include digester tanks, piping and pumps, boiler, the facility has reduced its peak electrical digester heating and mixing systems, digester demand by 706 kilowatts per month, a 14 per- gas-handling equipment, and chemical feed cent decrease (Western Lake Superior Sanitary equipment. Design and construction costs from District 2001). several example facilities follow. In addition to constructing new anaerobic diges- The City of Grand Island, Nebraska, is in the tion systems, many facilities are upgrading design stages for the construction of a $10.7 mil- existing anaerobic digesters to multi-stage sys- lion two-stage AG anaerobic digestion system. tems to produce high-quality biosolids, reduce The city’s 12-MGD WWTP receives approxi- odor problems, or produce biogas to power plant mately 40 percent of its flow from large operations or sell. Depending on the configura- industrial agricultural operations (including a tion of the current system (number of digesters, meat packing plant) and has had odor problems. piping configuration, capacity and location of The city determined that replacing its open aero- heating and mixing equipment, feed capabilities), bic digesters with an anaerobic system will the costs of retrofitting existing anaerobic diges- reduce these problems and generate a usable end- tion systems to multi-stage systems are typically product for land application as fertilizer minimal and usually include only the cost of (Overstreet 2006). installing new piping or reconfiguring existing Western Lake Superior Sanitary District piping. For example, the IUEA RP-1 in Ontario, (WLSSD) in Duluth, Minnesota, began operating California, was able to reconfigure its existing a new two-stage TPAD-TM anaerobic digestion system and add new variable speed pumps and system at its 43-MGD regional WWTP in 2001. controls for $2.5 million (P. Cambiaso, IEUA, The new system was the result of a multiyear personal communication 2006). Similarly, al- planning process that evaluated options for more though the exact cost figures were not readily environmentally and fiscally responsible alterna- available, the city of Tacoma, Washington, was tives to the existing sludge incineration process. able to transition from a single-stage thermo- The committee recommended anaerobic diges- philic system to a two-stage AG-MM system at tion, which would produce a usable end-product “a very low cost” by re-plumbing its existing as well as biogas. system (D. Thompson, City of Tacoma, personal communication, 2006). The new anaerobic digestion facility had a con- struction cost of $32.6 million and consists of Operation and maintenance costs include costs four 1-MG digesters. The solids are digested in associated with operating and maintaining mix- the first digester for 5 days at a temperature of 55 ing, heating, and pumping equipment; operating °C (131 °F). The thermophilically digested solids and maintaining gas-handling equipment; clean- are then transferred to one of the three meso- ing of digesters; and the purchase of chemicals. philic digesters for an additional 15 days. The Table 4 summarizes typical O&M costs in dol- facility markets the end-product as “Field Green” lars per dry ton of solids through the anaerobic and expects to produce approximately 8,000 dry digesters.
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Table 4. Typical Operation and Maintenance Costs for Digesters
Range per Dry Ton Average per Dry Ton of of Biosolids Biosolids Operation $5.30 – $41.03 $17.47 Maintenance $4.09 – $10.48 $7.44 Total $9.39 – $51.51 $24.91
Source: Multi-Agency Benchmarking Study 1999.
It should be noted that anaerobic digestion sys- Chang, F., L. Otten, E. LePaige, and B. van tems often pay for themselves through the Opstal. 2004. Is 100% Diversion from a combination of reduced costs for biosolids dis- Landfill an Achievable Goal? A Report to posal (owing to a reduction in biosolids volume the Toronto, Canada, New and Emerging through the digestion process), the potential Technologies, Policies and Practices marketing of a Class A biosolids product, and Advisory Group. the recovery of usable biogas. For example, the City of Tacoma markets the end-product from its City of Tacoma. 2006. Web site. anaerobic digestion process, TAGRO, for $6.00–
11 Mayhew, M. M.S. Le, D. Harrison, and C.E. Tchobanoglous, G., and F.L. Burton. 1991. Brade. 2004. Plug Flow Digestion––a Case Wastewater Engineering: Treatment, Disposal, Study. Presented at the 9th European Reuse. 3rd ed. Metcalf and Eddy, Inc. Biosolids and Biowastes Conference, Thompson, D., City of Tacoma, Washington. Wakefield, UK. 2006. Personal communication. Monsal. 2004. News release from Monsal. USEPA (U.S. Environmental Protection http://www.processingtalk.com/news/meq/ Agency). 1979. Process Design Manual meq100.html. Accessed June 2006. Sludge Treatment and Disposal. EPA Multi-Agency Benchmarking Study. December 625/625/1-79-011. Washington, DC: U.S. 1999.
Woods, C.E., and V.E. Melina. 1965. Stage Digestion of Wastewater Sludge. Journal Water Pollution Control Federation 37: 1495.
ADDITIONAL INFORMATION Earth Tech, Inc. Tom Wilson 675 North Washington Street, Suite 300 Alexandria, VA 22314 Inland Empire Utility Agency Patrick Shields 6075 Kimball Avenue Chino, CA 91710 City of Tacoma Dan Thompson 747 Market Street Tacoma, WA 98402 The mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Environmental Protection Agency. Office of Water EPA 832-F-06-031 September 2006
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