Biological Treatment Processes || Anaerobic Digestion

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<ul><li><p>14Anaerobic Digestion</p><p>Jerry R. Taricska, David A. Long, J. Paul Chen,Yung-Tse Hung, and Shuai-Wen Zou</p><p>CONTENTSINTRODUCTIONTHEORYDESIGN PRACTICEMANAGEMENT OF DIGESTIONCAPITAL AND OPERATING COSTSDESIGN EXAMPLESRECENT DEVELOPMENT IN ANAEROBIC PROCESSNOMENCLATUREREFERENCES</p><p>Abstract Anaerobic digestion results in the biological breakdown of the sludge into methane,carbon dioxide, unusable immediate organics and a small amount of cellular protoplasm,under anaerobic conditions, mainly for sludge stabilization and volume reduction. Thischapter introduces anaerobic digestion theory, biochemistry, microbiology, organic loading,temperature control, digester design, gas collection, and methane gas use, maintenance, anddesign examples.</p><p>Key Words Anaerobic digestion digester design methane production gas collection gasuse sludge pumping sludge stabilization volume reduction.</p><p>1. INTRODUCTION</p><p>Conversion of the organic material in solid wastes to methane-containing gases can beaccomplished in a number of ways, including hydrogasification, pyrolysis, and anaerobicdigestion. Hydrogasification is usually associated with the conversion of petrochemical rawmaterials. Although the process has been tried with solid wastes, it is not well defined and</p><p>From: Handbook of Environmental Engineering, Volume 8: Biological Treatment ProcessesEdited by: L. K. Wang et al. c The Humana Press, Totowa, NJ</p><p>589</p></li><li><p>590 J. R. Taricska et al.</p><p>Fig. 14.1. General anaerobic biological reactions.</p><p>therefore is not considered in this book. The production of methane from solid wastes bypyrolysis has been considered previously. The production of methane from solid wastesby anaerobic digestion, or anaerobic fermentation as it is often called, is described in thefollowing discussion.</p><p>Anaerobic digestion refers to the anaerobic decomposition of organic matter, resulting inpartial gasification, liquefaction, and mineralization. The process is generally considered tobe a two-stage biological process involving waste conversion and stabilization (Figure 14.1).The end products are principally methane (CH4), carbon dioxide (CO2), and stable organicresidues.</p><p>Anaerobic digestion of solid waste and/or wastewater sludge has long been used to stabilizeorganic wastes before final disposal of these wastes. Among the benefits to be realized fromsuch treatment are:</p><p>a. A reduction in organic content of the sludge.b. Improved sludge dewaterability.c. Destruction of most pathogens.d. Generation of a potentially valuable byproduct (methane).e. Volume reduction.</p><p>In addition to anaerobic digestion of solid waste and/or sludge, anaerobic treatment ofwastewaters (particularly certain industrial wastes) has been receiving added attention inrecent years. In addition to methane production, advantages cited for the anaerobic treatmentprocess are as follows:</p><p>a. A high degree of waste stabilization may be obtained.b. Relatively small amounts of residual organic waste are produced.c. No oxygen is required.d. Nutrient requirements are low (1).</p><p>Not only is interest in anaerobic processes being generated because of their waste treat-ment potential, but in our increasingly energy conscious society, the potential for gener-ating methane from waste materials takes on added significance. In order for the process</p></li><li><p>Anaerobic Digestion 591</p><p>potential to be fully realized, both design and operational fundamentals must be properlyaddressed.</p><p>The purpose of this chapter is to familiarize the reader with the theory of anaerobicprocesses and to present currently accepted design practices. Some attention also will begiven to operational considerations as they affect selection and design of anaerobic unitprocesses.</p><p>2. THEORY</p><p>2.1. Nature of Organic Wastes</p><p>Where solid wastes are to be digested, special preparation of the solid wastes beforedigestion is necessary. The solid wastes first should be sorted and shredded to a size thatwill not interfere with the proper functioning of digester equipment and transport systems. Itnormally is necessary to add moisture and nutrients (pH adjustment may be required) to forma slurry that can be heated before feeding the mixture into the digester. Sewage sludge oftenis used to provide the necessary moisture and nutrients.</p><p>The majority of the sludges that are of concern in the design of anaerobic digestion facilitiesare of municipal wastewater origin. These sludges result from the settling out of solids in thesedimentation processes and may or may not contain biological waste solids from secondarytreatment and chemical sludges from advanced waste treatment processes. Industrial wastesthat are tributary to publicly owned treatment works may contain inorganic and organic solidsthat can alter the characteristics of what would otherwise be typical municipal wastewatersludges. An estimate of the amount of sludge to be expected from primary settling of rawwastes can be made by use of per capita estimates of suspended solids concentration togetherwith estimates of the primary tank efficiency and the percentage total solids (TS) concentrationin the settling tank underflow. If data are available on raw wastewater and primary clarifiereffluent suspended solids concentrations together with appropriate flow data, estimates of thesludge mass and volume also can be made. Both of these techniques will be illustrated in thedesign examples presented in a later section of this chapter.</p><p>Solid waste and/or wastewater sludge to be digested normally are characterized on thebasis of the percentage total solids (TS) and volatile solids (VS) fraction expressed as apercentage of the TS, both on a dry weight basis. It would be wise for the reader to review theanalytical procedures for the laboratory determinations for residue in Standard Methods (2).Examination of organic wastes for other specific constituents, heavy metals, and so on, maybe desirable where industrial wastes are present to ensure that such materials will not interferewith the anaerobic digestion process or limit the disposal of digested solids.</p><p>2.2. Biochemistry and Microbiology of the Anaerobic Process</p><p>As was indicated earlier, the anaerobic digestion process is considered to be a minimumof a two-stage biological reaction involving at least two different groups of microorganisms,acid-forming bacteria (saprophytic) and methane-forming bacteria. Complete understandingof all of the metabolic pathways, organisms involved, and so on is still lacking, but the generalreactions involved have been identified.</p></li><li><p>592 J. R. Taricska et al.</p><p>2.2.1. Acid Phase</p><p>The first stage of the two-stage anaerobic process generally is considered to include theconversion of complex organic compounds into simpler organic compounds and finally intothe organic acids, principally acetic acid (CH3COOH) by acid-forming bacteria. Acetic andpropionic acids are the most important volatile acids frequently occurring in sick digesters.As would be expected, little actual stabilization of organic wastes occurs during the firststage because the complex organic compounds (fats, carbohydrates, and proteins) are merelychanged into simpler organic compounds.</p><p>The status of knowledge concerning the acid phase microbiology is not as far developedas is the microbiology of methane production. However, through methods similar to thoseused for ruminant bacteriology, it has been shown that most of the organisms responsiblefor acid production are obligate anaerobes (3). This fact means that it is essential to excludeoxygen from the digester environment to avoid oxygen toxicity. In order for the complexorganics to be used by the acid-forming bacteria, they first must be solubilized by enzy-matic action. The bacteria produce the necessary extracellular enzymes to accomplish thisreaction.</p><p>2.2.2. Methane Fermentation Phase</p><p>Two basic mechanisms have been identified as biochemical pathways for the production ofmethane from the end products formed during the first (acid) stage (4). Of the two reactionsshown below, Eq. (2) generally is considered to represent the more important reaction.</p><p>CO2 + 8 H+ Methane-forming bacteria CH4 + 2 H2O (1)CH3COOH Methane-forming bacteria CH4 + CO2 (2)</p><p>As is the case with all anaerobic bacteria of importance in wastewater treatment, themethane formers are very slow growing and may be subject to wash out at short hydraulicretention times unless recycling of microorganism is used. Reactor configuration will beconsidered in more detail in the next section.</p><p>The methane formation step is where the major waste stabilization occurs. Although, themethane formed is a high energy compound, its potential recovery and use makes it possibleto show waste stabilization efficiency for the anaerobic process that is quite comparable tothat expected from conventional aerobic treatment processes.</p><p>The methane formers do not constitute a single group (genus or species) of bacteria. Hence,a change in the predominance of the acid-forming bacteria present in a particular digesterbecause of a change in substrate, and so on, may cause an upset, through the productionof new first-stage end products that then require a group of methane organisms that arenot present in sufficient numbers for a balanced condition to exist. The necessary balanceshould reestablish itself unless the influent waste or sludge characteristics or other factors areconstantly changing. Figure 14.2 summarizes the major metabolic pathways involved in theanaerobic digestion process.</p></li><li><p>Anaerobic Digestion 593</p><p>Fig. 14.2. Metabolic pathways in complex waste stabilization by anaerobic processes. After (1).</p><p>2.3. Reactor Configurations</p><p>There are four basic reactor configurations that have been used for the design of anaerobicunit processes. The four are:</p><p>a. Single-stage, unmixed.b. Two-stage, mixed primary.c. Anaerobic contact process with sludge recycle.d. Anaerobic filter.</p><p>Of these four, the first two normally are used for digestion of solid wastes and/or wastewatersludge. Figure 14.3 illustrates the basic reactor types for so called conventional (standard rate)and high rate sludge digesters.</p><p>The majority of digestion tanks have circular cross sections. The conventional digesteris unmixed and normally would be designed as a single-stage unit that allows digestion,supernatant separation and withdrawal, and stabilization and withdrawal of concentratedsludge to be accomplished in a single tank.</p><p>High-rate digestion systems are designed usually as two-stage systems wherein the primarystabilization of sludge is accomplished-in the first stage using a mixed digester. Separatingsupernatant from concentrated sludge and gas storage are provided for in the unmixed second-stage digester. The term stages as used in conventional engineering practice does not referto the biochemical stages of acid production and methane formation discussed above, butrather to the physical processes of mixing and sludge-supernatant separation. Some studies(5) have suggested the design of digesters based on separation of the two biochemical stagesof digestion, but this concept has not yet been accepted as general practice.</p></li><li><p>594 J. R. Taricska et al.</p><p>Fig. 14.3. Digester reactor configuration.</p><p>Fig. 14.4. Anaerobic contact process schematic.</p><p>The advantages of mixing the reactor to optimize the biological reactions involved shouldbe readily apparent to the reader. In an effort to develop anaerobic technology to the pointwhere it could be applied to the treatment of total wastes rather than just sludges, the anaer-obic contact process (Figure 14.4) and the anaerobic filter have been developed (6, 7). Theanaerobic contact process is simply an anaerobic activated sludge process and is designed andoperated in much the same manner as complete mixed activated sludge (CMAS) process. The</p></li><li><p>Anaerobic Digestion 595</p><p>major difference in the two, other than the obvious oxygen relationships is that some means ofdegasifying the reactor mixed liquor to permit gravity separation of the solids for recycle mustbe employed in the anaerobic contact process. The low growth rate in the anaerobic systemmakes solids recovery and recycling critical considerations for the satisfactory operation ofsuch systems. The process has been used to treat certain industrial wastes that normally havehigh temperatures and high organic composition. The successful treatment of packing housewastes has demonstrated the suitability of the process for such wastes (8).</p><p>The anaerobic filter is basically an anaerobic trickling filter. The attached growth featurein this unit process may facilitate and enhance the retaining of the biomass needed to effectwaste stabilization. This process has been employed less commonly today, compared to theanaerobic contact process, but it may find application in specific instances.</p><p>2.4. Organic Loading Parameters</p><p>Since the anaerobic process is a biological treatment process, loading parameters aremost meaningful if they are expressed in terms of organic loadings. For solid wastes andorganic sludges; loadings most commonly are based on volatile solids (VS), whereas fordilute wastewaters loadings would be expressed in terms of biochemical oxygen demand(BOD) or chemical oxygen demand (COD), as is the case with aerobic unit processes. Thischapter emphasizes only anaerobic digestion of solid wastes and organic sludges. Conven-tional environmental engineering practice has been to express digester loadings on a weightto volume basis per unit time, normally, kilograms of VS per day per cubic meter of volume(kg/d/m3).</p><p>The stability of the anaerobic process and the rate of gas production are both depen-dent upon organic loading rates. At higher loadings, the process often becomes unbalancedbecause of the excessive production of volatile acids. Carbon dioxide production under theseconditions often will cause foaming of the digester and contribute to operating problems.Maintenance of uniform or near uniform loading rates based on frequent, or if possible,continuous additions of raw sludge to the digester will yield the most consistent digesteroperation.</p><p>The two basic modes of operation of sludge digesters generally are differentiated in termsof organic loading rates. The standard rate (unmixed) digester usually is loaded in the rangeof 0.48 to 1.6 kg VS/d/m3 (0.03 to 0.10 lb VS/d/ft3), whereas the high-rate digester (mixed)normally would be loaded in the range of 1.6 to 6.4 kg VS/d/m3 (0. 10 to 0.40 lb VS/d/ft3).Design loadings will be discussed in more detail in the section on Anaerobic ReactorDesign and Sizing below. The degree of stabilization of sludges by anaerobic digestion isbest expressed in terms of the volatile solids reduction that has occurred during digestion.Figure 14.5 illustrates accepted practice in determining when sludge is considered to bestabilized.</p><p>As indicated earlier, solid waste-sewage sludge mixtures have been digested successfully.Although mixtures containing from 50% to more than 90% solid wastes have been used,a 60% mixture appears to be a reasonable compromise. Loading r...</p></li></ul>