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...

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 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...

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...

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....

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...

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...

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...

Volatile Acids

An increase in volatile acid concentration without an increase in alkalinity is an indicator of an adverse operational condition within an anaerobic digester. Acetate is a precursor for most of the methane produced in an anaerobic digester. Butyrate and propionate are important intermediates or precursors of methane production. The accumulation of these acids or an increase in volatile acid concentration can be associated with digester instability or stress. Acceptable volatile acid...

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...