Contents

3 Methane-forming Bacteria 17 7 Anaerobic Digestion Stages 51 PART II SUBSTRATES, PRODUCTS, AND BIOGAS 59 8 Substrates and Products 61 PART III OPERATIONAL CONDITIONS 77 10 Introduction to Operational Conditions 79 PART IV PROCESS CONTROL AND TROUBLESHOOTING 121 19 Upsets and Unstable Digesters 123 20 Foam and Scum Production and Accumulation 127 23 Types of Anaerobic Digesters 143 24 Anaerobic Digesters versus Aerobic Digesters 153 Chemical Compounds and Elements 167

Response To Free Molecular Oxygen

Bacteria may be divided further into three groups according to their response to free molecular oxygen (Table 2.1). These groups are 1) strict aerobes, 2) facultative anaerobes, and 3) anaerobes, including the methane-forming bacteria. Strict aerobes are active and degrade substrate only in the presence of free molecular oxygen. These organisms are present in relatively large numbers in aerobic fixed-film processes, for example, trickling filters, and aerobic suspended-growth processes, for...

Enzymatic Ability To Degrade Substrate

Bacteria degrade substrate through the use of enzymes. Enzymes are proteinaceous molecules that catalyze biochemical reactions. Two types of enzymes are involved in substrate degradation endoenzymes and exoenzymes (Figure 2.1). Endoenzymes are produced in the cell and degrade soluble substrate within the cell. Exoenzymes also are produced in the cell but are released through the slime coating the cell to the insoluble substrate attached to the slime. Once in contact with the substrate the...

Acetateforming Bacteria

Acetate-forming (acetogenic) bacteria grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria. For example, when ethanol (CH3CH2OH) is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced (Equation 2.3). CH3CH2OH + CO2 CH3COOH + 2H2 (2.3) When acetate-forming bacteria produce acetate, hydrogen also is produced. If the hydrogen accumulates and significant hydrogen pressure occurs, the pressure...

Methaneforming Bacteria

Methane-forming bacteria are known by several names (Table 3.1) and are a morphologically diverse group of organisms that have many shapes, growth patterns, and sizes. The bacteria can be found as individual rods, curved rods, spirals, and cocci (Figure 3.1) or grouped as irregular clusters of cells, chains of cells or filaments, and sarcina or cuboid arrangements (Figure 3.2). The range in diameter sizes of individual cells is 0.1-15 mm. Filaments can be up to 200 mm in length. Motile and...

Group 2 Acetotrophic Methanogens

The acetotrophic methanogens split acetate into methane and carbon dioxide (Equation 3.8). The carbon dioxide produced from acetate may be converted by hydrogenotrophic methanogens to methane (Equation 3.7). Some hydrogenotrophic methanogens use carbon monoxide to produce methane (Equation 3.9). 4CH3COOH 4C 2 + 2H2 4CO + 2H2O CH4 + 3CO The acetotrophic methanogens reproduce more slowly than the hydrogeno-trophic methanogens and are adversely affected by the accumulation of hydrogen. Therefore,...

Group 3 Methylotrophic Methanogens

The methylotrophic methanogens grow on substrates that contain the methyl group (-CH3). Examples of these substrates include methanol (CH3OH) (Equation 3.10) and methylamines (CH3)3-N (Equation 3.11). Group 1 and 2 methanogens produce methane from CO2 and H2. Group 3 methanogens produce methane directly from methyl groups and not from CO2. 3CH3OH + 6H 3CH4 + 3H2O (3.10) 4(CH3)3 - N + 6H2O 9CH4 + 3CO2 + 4NH3 (3.11) The use of different substrates by methane-forming bacteria results in different...

Respiration

Respiration is one of many cellular processes. For the purpose of this text, respiration is considered to be the degradation of substrate to obtain cellular nourishment. During respiration large compounds of high energy content are broken down to small compounds of low energy content (Figure 4.1). Much of the energy lost by the large compounds is captured by the respiring organisms. This capture results in a gain in the amount of useful energy. Two types of nourishment are obtained from the...

Anaerobic Food Chain

In natural habitats that are void of free molecular oxygen and nitrate ions, insoluble and complex organic compounds are degraded by different groups of bacteria through a variety of anaerobic or fermentative biochemical reactions. These reactions result in the production of soluble and simplistic organic compounds. These compounds do not accumulate in natural habitats. As one group of bacteria produces soluble compounds they are quickly degraded as substrate by another group of bacteria. The...

Carbohydrates

Carbohydrates are synthesized in the green leaves of plants by the conversion of carbon dioxide into glucose during photosynthesis. Carbohydrates are macromole-cules or polymers that contain numerous monomers of sugars (Figure 8.1). The range of lengths of the polymers or carbohydrates varies greatly. Within the digester all carbohydrates are degraded inside the cell of facultative anaerobes and anaerobes. Carbohydrates too large to enter the cell, that is, in an insoluble or complex soluble...

Temperaturethermophilic

Thermophilic sludge digestion and methane production occur at a high temperature (50-60 C). Thermophilic anaerobic digestion of sludge is more often used at industrial wastewater treatment plants, where process heat or steam is available to heat digesters to the thermophilic range. Because of the high operating temperature of these digesters, sludge digestion and methane production occur rapidly and significant destruction of pathogens is achieved. However, in addition to high operation costs,...

Proteins

The principal nitrogenous wastes in municipal sludges are proteins. Proteins are complex, high molecular-weight compounds. These molecules have a relatively large surface area and do not dissolve in wastewater or settle out of wastewater. Proteins are made of amino acids that are either straight-chain (aliphatic) or ring-shaped (cyclic) in structure (Figure 8.6). There are 20 different amino acids. Regardless of their structure, all amino acids contain an amino group (-NH2) and a carboxyl group...

Acetate Fermentation

Acetate is produced in several fermentative pathways. A large diversity of bacteria, collectively known as acetogenic or acetate-forming bacteria, produces Figure 6.1 There are numerous types of fermentation. The type of fermentation that occurs is classified or named after the major product(s) obtained in the fermentation process. Figure 6.1 There are numerous types of fermentation. The type of fermentation that occurs is classified or named after the major product(s) obtained in the...

Sludge Feed

Because the predominant application of anaerobic digesters is the degradation of particulate and colloidal wastes, sludge feed or organic loading rates to digesters usually are expressed in terms of volatile solids (VS). Designed and recommended loadings for anaerobic digesters that are mixed and heated are 200-450 lb VS 1000ft3 day (3.2-7.2kgVS m3 day). However, loading rates of 30-50lb VS 1000ft3 day (0.5-0.6kgVS m3 day) are typical. Higher loading rates could be treated if a more...

Configurationsingle Stage

A typical single-stage digester consists of only one tank or reactor. Digester operations consist of sludge addition and withdraw, mixing, heating, gas collecting, and supernating. These operations are possible because of stratification of the digester content. Stratification results in the following layers from top to bottom of the digester gas, scum, supernatant, active digester sludge, and digested sludge and grit (Figure 23.6). Single-stage digesters are more easily upset than two-stage...

Aerobic Digesters

Aerobic and anaerobic digesters can degrade organic compounds. The aerobic process consists of a large variety of bacteria working side by side to degrade the organic compounds, whereas the anaerobic process consists of a large variety of bacteria working in sequence, that is, one after the other (Figure 24.1). During aerobic degradation of organic compounds, aerobes and facultative anaerobes use free molecular oxygen to completely degrade organic compounds such as proteins to CO2, H2O, new...

Startup

Primary and secondary sludges that provide the substrates for an anaerobic digester also provide the bacteria needed for the hydrolysis and degradation of these compounds and the production of methane. Both facultative anaerobes and anaerobes including methane-forming bacteria are needed in an anaerobic digester. Facultative anaerobes and anaerobes are needed for 1) the hydrolysis of particulate and colloidal compounds and 2) the degradation of soluble organic compounds to volatile acids....

Stage 3methanogenesis Stage

In the methanogenic stage, methane is formed mostly from acetate and carbon dioxide and hydrogen gas. Methane is also formed from some organic compounds other than acetate (Table 7.2). Therefore, all other fermentative products must be converted to compounds that can be used directly or indirectly by methane-forming bacteria. Acids, alcohols, and organic-nitrogen compounds that are not degraded by methane-forming bacteria accumulate in the digester supernatant. The accumulation of these...

Feedback Inhibition

Fermentation often results in the production of several intermediates such as hydrogen and volatile fatty acids that are toxic. The presence of toxicity that is caused by the production of hydrogen and volatile fatty acids is referred to as feedback inhibition. Excess hydrogen production and accumulation results in increased partial hydrogen pressure. This increased pressure inhibits acetate-forming bacteria. Excess volatile fatty acid production and accumulation inhibits methane-forming...

Volatile Acids

Some organic acids are known also as volatile acids or volatile fatty acids. These acids occur as substrates and products in the anaerobic digester. Many serve as substrate for methane-forming bacteria, and they are the products of the fermentative activities of facultative anaerobes and anaerobes. Volatile acid production in an anaerobic digester results in the production of methane. Although volatile acids vary in length, most volatile acids produced in an anaerobic digester are...

Glossary

Absorb Penetration of a substance into the body of an organism aceticlastic cleavage Conversion of acetate to methane by methane-forming bacteria acetogenesis Production of acetate by acetate-forming bacteria acetotrophic Use of acetate by bacteria as a substrate activator Metal or vitamin incorporated into an enzyme that improves the efficiency of enzymatic activity acute Having a sudden onset and short course adsorb The taking up of one substance at the surface of an organism aerotolerant...

Introduction to Operational Conditions

The rate-limiting reaction in anaerobic digestion is usually the conversion of volatile acids to methane. Methane-forming bacteria obtain very little energy from the degradation of volatile acids. Most of the energy released from the volatile acids is transferred to the methane. Because of the low energy yield obtained from volatile acids by methane-forming bacteria, their growth rate is restricted, that is, the amount of substrate utilization per unit of organisms is high. Therefore, bacterial...

References

Biodegradation of organic chemicals. Env. Sci. Technol. (19). Arakaki, G., R. V. Schaaf, S. Lewis, and G. K. Himaka. Sludge pretreatment. Water Env. Tech. (12). Austin, B., ed. 1988. Methods in Aquatic Bacteriology. John Wiley & Sons, New York. Baresi, L., R. A. Mah, D.M.Ward, and I. R. Kaplan. Methanogenesis from acetate enrichment studies. App. Environ. Micro. (36). Barth, E. F., and R. L. Bunch. 1979. Biodegradation and Treatability of Specific Pollutants EPA-600...

Stage 2acidforming Stage

In the acid-forming stage, soluble compounds produced through hydrolysis or discharged to the digester are degraded by a large diversity of facultative anaerobes and anaerobes through many fermentative processes. The degradation of these compounds results in the production of carbon dioxide, hydrogen gas, alcohols, organic Figure 7.4 Although table sugar is soluble in water, table sugar is too large and complex to enter a bacterial cell. In order for bacteria to degrade table sugar, the sugar...

Volatile Acids And Longchain Fatty Acids

The presence of a relatively high concentration of short-chain (1-3 carbon units), nonionized volatile acids such as acetate, butyrate, and propionate causes a decrease in the concentration of alkalinity and a drop in pH. Propionate is perhaps the most toxic of the volatile acids and may exert toxicity at concentrations < 5 mg l. Toxicity is exerted at near-neutral pH values and occurs in populations of acid-forming bacteria and methane-forming bacteria. The presence of an excess...

Propionate And Succinate Fermentation

Anaerobic propionibacteria or propionate-forming bacteria (Table 6.4) ferment glucose and lactate (Equations 6.11 and 6.12). Lactate, the major end product of lactate fermentation, is the preferred substrate of propionate-forming bacteria. Although succinate (HOOCCH2CH2COOH) usually is an intermediate product of fermentation, some succinate is produced as an end product. Propionate is a major substrate of acid fermentation that can be converted to acetate and then used in methane production....

Mixing

Anaerobic digester content should be mixed. Mixing enhances the digestion process by distributing bacteria, substrate, and nutrients throughout the digester as well as equalizing temperature. The metabolic activities of acetate-forming bacteria and methane-forming bacteria require that they be in close spatial contact. Slow, gentle mixing ensures that contact. Also, mixing provides for efficient hydrolysis of wastes and production of organic acids and alcohols by acid-forming bacteria. For...

Bacterial Growthfixed Film

Anaerobic fixed-film (sludge blankets) systems provide a quiescent environment for the growth of an agglutinated mass of bacteria (Figure 23.2). Because bacterial growth requires relatively long periods of time to develop, the media used in fixed-film systems hold the bacteria in the digester for relatively long periods and provide for long SRTs and short HRTs. The bacteria grow as fixed films of dendritic or stringlike masses on the supportive media or as clumps of solids within the openings...

Stage 1hydrolysis Stage

In the anaerobic digester complex insoluble compounds such as particulate and colloidal wastes undergo hydrolysis. Particulate and colloidal wastes consist of carbohydrates, fats, and proteins. These wastes are polymeric substances, that is, large insoluble molecules consisting of many small molecules joined together by unique chemical bonds. The small molecules are soluble and quickly go into solu- performed by hydrolytic bacteria (facultative anaerobes and anaerobes) Acid production,...

Carbon Dioxide

Carbon dioxide (CO2) is one of several gases found in foam. With increased carbon dioxide production in an anaerobic digester, the amount of carbon dioxide within the sludge also increases. An increase in carbon dioxide within the sludge promotes foam production. Carbon dioxide content within the digester can be reduced by bubbling digester gas through a potassium hydroxide (KOH) solution or introducing natural gas into the gas system to dilute the carbon dioxide content. A decrease in carbon...

Macronutrients

Macronutrient requirements for anaerobic biological treatment processes are much lower than the requirements for aerobic biological treatment processes such as activated sludge and trickling filter processes. The reduced requirement for macro-nutrients in anaerobic processes is due to lower cell (sludge) yield compared with aerobic processes from the degradation of equal quantities of substrate. The two macronutrients of concern in any biological treatment process are nitrogen and phosphorus....

Methane Fermentation

Three types of methane-forming bacteria achieve methane production two groups of obligate chemolithotrophic methanogens and one group of methylotrophic TABLE 6.5 Genera of Sulfate-reducing Bacteria TABLE 6.5 Genera of Sulfate-reducing Bacteria methanogens. Chemolithotrophic methanogens produce methane from carbon dioxide and hydrogen (Equation 6.13) or formate (Equation 6.14). Carbon monoxide also may be used by some chemolithotrophic methanogens in the production of methane (Equation 6.15)....

Anaerobic Digestion Stages

The anaerobic digestion process and production of methane is divided into stages. Three stages often are used to illustrate the sequence of microbial events that occur during the digestion process and the production of methane (Figure 7.1). These stages are hydrolysis, acid forming, and methanogenesis. The critical biochemical reactions within these stages are presented in Figure 7.2. The anaerobic digestion process proceeds efficiently if the degradation rates of all three stages are equal. If...

Monitoring

Extensive analytical monitoring of anaerobic digesters has not been a common practice at many wastewater treatment plants. Lack of adequate and timely monitoring has resulted in numerous digester failures. Lack of monitoring often is due to the relatively large amount of time required to perform the many analytical tests. To monitor the activity of the bacteria in an anaerobic digester and evaluate and troubleshoot digester operations, several analytical tests should be performed on a periodic...

Alternate Electron Acceptors

The presence of nitrate ions (NO-) or sulfate ions (SO4-) may inhibit methane-forming bacteria. Nitrate ions and sulfate ions may be found in relatively high concentrations in industrial wastewaters or aerated and nitrified municipal wastewaters and sludges. Both ions adversely impact the activity of methane-forming bacteria by increasing the redox value within the anaerobic digester. Low redox values (less than -300 mV) are required for proper activity of methane-forming bacteria. Because SRB...

Ammonia Toxicity

Ammonical-nitrogen (NH4-N) or ammonium ions (NH+), a reduced form of nitrogen, may be transferred to an anaerobic digester or may be produced during the anaerobic degradation of organic nitrogen compounds such as amino acids and proteins. Reduced nitrogen exits in two forms, the ammonium ion and free or nonion-ized ammonia (NH3).The effects of ammonical-nitrogen ammonia in the anaerobic digester are positive and negative (Table 17.4). Ammonium ions are used by bacteria in the anaerobic digester...

Heavy Metals

Numerous heavy metals such as cobalt (Co), copper (Cu), iron (Fe), nickel (Ni), and zinc (Zn) are found in wastewaters and sludges and are transferred to anaerobic digesters. These metals are referred to as heavy because of their undesired impact on wastewater treatment processes and operational costs including their accumulation in sludges. High concentrations of metals in sludges affect sludge disposal options and costs. Figure 17.3 Heavy metals cause toxicity in the soluble form. The metals...

Settleable Solidssupernatant

The characteristics of digester supernatant vary greatly according to the type of sludge feed to the digester and the type of digester used. Solids in the supernatant that are discharged to the head of the treatment plant represent particulate organic loading and solids loading on the primary clarifier and secondary treatment process. To maintain low loadings, the settleable solids in the supernatant should be < 50ml after 4-5 hours of testing. Low levels of loading may be obtained by...

Foam and Scum Production and Accumulation

The production and accumulation of foam is a common problem experienced by many anaerobic digesters. Foam production is caused by several operational conditions (Table 20.1). Foam first appears in the annular space between the floating cover and the digester wall and may completely coat the floating cover and spill over the coping of the digester wall. Foam presents safety, housekeeping, and malodor concerns as well as maintenance and operational problems. Operational problems associated with...

Introduction

The organic content of sludges and soluble wastes can be reduced by controlled bacterial activity. If the bacterial activity is anaerobic, the reduction in organic content is achieved through sludge digestion. If the bacterial activity is aerobic, the reduction in organic content is achieved through sludge stabilization. Anaerobic digesters having suspended bacterial growth are commonly used at municipal wastewater treatment plants to degrade (digest) sludges (Figure 1.1).With the development...

Mixedacid Fermentation And Mixedacid And Butanediol Fermentation

Names Anaerobic Bacteria

A large variety of bacteria in the genera Enterobacter, Escherichia, Erwinia, Salmonella, Serratia, and Shigella are responsible for mixed acid fermentation. These organisms ferment sugars to a mixture of acids acetate, formate, lactate, and succinate. Carbon dioxide, hydrogen, and ethanol also are produced. The prevalence of acids among the products of mixed-acid fermentation account for the name of the fermentation process. Bacteria in the genera Enterobacter and Erwinia also produce...

Upsets and Unstable Digesters

Under steady-state conditions the anaerobic digester operates without difficulty. Adequate mixing and proper, uniform temperature contribute to a steady-state condition. However, interruptions of this condition do occur, resulting in upsets and unstable digesters. Seven basic conditions are responsible for upsets or unstable anaerobic digesters (Table 19.1). Many of the conditions are directly or indirectly related and include hydraulic overload, organic overload, pH changes, temperature...

Substrates and Products

In chemical reactions there are reactants and products (Equation 8.1). During chemical reactions, reactants (chemical compounds) undergo change and often release energy (heat) to the environment. The changes that occur to the reactants result in the formation of products (new chemical compounds). Often, a catalyst may be involved in a chemical reaction. The catalyst accelerates the rate of the chemical reaction and may be changed or consumed. reactants catalyst products (8.1) Chemical reactions...

Retention Times

There are two significant retention times in an anaerobic digester. These are solids retention time (SRT) and hydraulic retention time (HRT). The SRT is the average time that bacteria (solids) are in the anaerobic digester. The HRT is the time that the wastewater or sludge is in the anaerobic digester. The SRT and the HRT are the same for a suspended-growth anaerobic digester that has no recycle. If recycle of solids is incorporated in the operation of the digester, then the SRT and HRT may...

Toxicity

A variety of inorganic and organic wastes can cause toxicity in anaerobic digesters (Table 17.1). Many toxic wastes are removed in primary clarifiers and transferred directly to the anaerobic digester. Heavy metals may be precipitated as hydroxides in primary sludge, and organic compounds such as oils and chloroform are removed in primary scum and sludge, respectively. Industrial wastewaters often contain wastes that are toxic to anaerobic digesters. Although guideline values or ranges of...

Alkalinity And Fatty Acids

Alkalinity is inversely proportional to surface tension, that is, as alkalinity increases within digester sludge the surface tension of the sludge decreases. The sludge becomes more surface active and has a greater propensity to foam. Increasing alkalinity also may serve as an indicator of other operational factors that contribute to foam production. High alkalinity changes the surface tension of anaerobic digester sludge in a similar fashion as biosurfactants from dead Nocardioforms change the...

Alkalinity and pH

Sufficient alkalinity is essential for proper pH control. Alkalinity serves as a buffer that prevents rapid change in pH. Enzymatic activity or digester performance is influenced by pH. Acceptable enzymatic activity of acid-forming bacteria occurs above pH 5.0, but acceptable enzymatic activity of methane-forming bacteria does not occur below pH 6.2. Most anaerobic bacteria, including methane-forming bacteria, perform well within a pH range of 6.8 to 7.2. The pH in an anaerobic digester...

Temperature

Common recurring problems associated with anaerobic digesters are loss of heating capability and maintenance of optimum digester temperature. An acceptable and uniform temperature should be maintained throughout the digester to prevent localized pockets of depressed temperature and undesired bacterial activity. Variations in temperature of even a few degrees affect almost all biological activity including the inhibition of some anaerobic bacteria, especially methane-forming bacteria. Adequate...

Controlling Foam In Anaerobic Digesters

Digester scum consists of floating materials such as grease and vegetable matter with a specific gravity lt 1.0. Plastics, hair, and rubber products are commonly found in scum. Scum may have entrapped bubbles of gas. If scum is broken or dissipated by mixing over a very short period of time, the rapid breakdown of the grease and vegetable matter results in a buildup of volatile acids. These acids at the surface of the digester are responsible for the production of foam. TABLE 20.2 Control...

Hydrogen Sulfide

Bacterial cells need soluble sulfur as a growth nutrient and satisfy this need by using soluble sulfide HS . However, excessive concentrations of sulfides or dissolved hydrogen sulfide gas H2S cause toxicity. Hydrogen sulfide is one of the compounds most toxic to anaerobic digesters. The methane-forming bacteria are the bacteria that are most susceptible to hydrogen sulfide toxicity. Hydrogen-consuming methane-forming bacteria are more susceptible to hydrogen sulfide toxicity than acetoclastic...

Biogas

Anaerobic digestion of municipal sludges results in the production of a mixture of gases Figure 9.1 . Collectively, these gases are referred to as digester gas or biogas. The only gas of economic value that is produced in an anaerobic digester is methane. In a properly operating digester most of the gas produced from a day's feed sludge appears within 24 hours. Methane can be used as a source of fuel. It is a natural flammable gas. Methane is odorless and burns cleanly Equation 9.1 . Pure...

Micronutrients

Because methane-forming bacteria possess several unique enzyme systems, they have micronutrient requirements that are different from those of other bacteria. The need for several micronutrients, especially cobalt, iron, nickel, and sulfide, is critical. Additional trace elements on which enzymes of methane-forming bacteria are dependent include selenium and tungsten. The incorporation of micronutrients in enzyme systems is essential to ensure not only proper degradation of substrate but also...

Alkalinity

Adequate buffering capacity or alkalinity is needed in an anaerobic digester for maintenance of proper pH. Alkalinity is produced in the digester through the degradation of some wastes, for example, cationic polymers, amino acids, and proteins, and alkalinity is lost in the digester through the production of volatile acids. Acceptable alkalinity concentrations are normally 1000-2000mg l in a primary digester and 1500-3000 mg l in a secondary digester. If a deficiency in alkalinity exists, the...

Supernatant

Foam Digesters

When sludge is allowed to settle in a digester, a supernatant develops. Anaerobic digester supernatant is commonly returned to the head of wastewater treatment plants and mixed with the influent Figure 21.1 . Although the supernatant is relatively small in volume, it contains dissolved and suspended organic and inorganic materials. These materials add suspended solids, nutrients nitrogen and phosphorus , and organic compounds to the influent. The returned materials may cause a variety of...

Volatile Acidtoalkalinity Ratio

The range of acceptable volatile acid-to-alkalinity ratios is 0.1-0.2. An acceptable ratio may be obtained by adjusting volatile acid concentration, alkalinity concentration, or both concentrations. Reducing or terminating feed sludge to the digester also helps to lower the volatile acid-to-alkalinity ratio. If feed sludge cannot be reduced or terminated, the use of chemicals for alkalinity adjustment is required. An unacceptable volatile acid-to-alkalinity ratio is usually the first warning of...