Acetone, butanol. ethanol
discussed in Section 2.3.2, however, the production of H allows more oxidized products like acetate to be produced. As a result, more ATP can be produced by bacteria when they generate H_, allowing them to have a higher biomass yield per unit of substrate processed.
Methanogens are obligate anaerobes that have very restricted nutritional requirements, with the oxidation of acetate and hydrogen being their main sources of energy. Even though methane is produced from the reduction of carbon dioxide during the oxidation of H:, methanogens lack the components of a standard electron transport chain, and thus carbon dioxide does not function as a terminal electron acceptor in a manner analogous to nitrate or oxygen.2" Rather, reduction of carbon dioxide to methane involves a complex sequence of events requiring a number of unique coenzymes.1,1 However, there is a sufficient free energy change during methane formation for the theoretical production of two molecules of ATP and it appears that a normal chemiosmotic mechanism is involved,'" although it involves a sodium motive force as well as a proton motive force.M Regardless of the exact mechanisms involved, it is important to recognize that ATP generation in Archaea is different from that associated with both respiration and fermentation in Bacteria and Eucarya. Furthermore, like bacteria growing in anaerobic environments, methanogens have low yields.
Factors Influencing Energy for Synthesis. Energy for synthesis represents the energy required by microorganisms to synthesize new cell material. In the absence of any other energy requirements, the energy required for synthesis is the difference between the energy available in the original substrate and the energy associated with the cell material formed, or in the common units of the environmental engineer, the difference between the COD of the original substrate and the COD of the biomass formed. Consequently, the energy for synthesis and the yield are intimately linked. If the efficiency of ATP generation were the same for all bacteria, it would be possible to theoretically predict the energy for synthesis, and hence the yield, from thermodynamic considerations."" However, as we saw above, the amount of ATP generated per electron transferred differs from microorganism to microorganism, which means that the efficiency of energy generation differs. This, coupled with the fact that the pathways of synthesis and degradation are not the same in all microorganisms, makes it difficult to use exactly the thermodynamic approaches for predicting yields that have been presented in the environmental engineering literature. Nevertheless, there are many instances in which it would be advantageous to have a theoretical prediction of the energy for synthesis or the yield prior to experimental work and a technique based on the Gibbs energy dissipation per unit of biomass produced appears to be best.21 Regardless, thermodynamic concepts are most useful for understanding why different substrates and different terminal electron acceptors have different energies of synthesis and yields associated with them.
During biomass growth, energy is required to synthesize the monomers needed to make the macromolecules that form the structural and functional components of the cell. This suggests that more energy would be required for a culture to grow in a minimal medium containing only a single organic compound as the carbon and energy source than in a complex medium in which all required monomers were supplied. Actually, such a conclusion is false."'' For example, the energy needed to synthesize all of the amino acids needed by a cell amounts to only about 10% of the total energy needed to synthesize new cell material. This is because macromolecules are too large to be transported into the cell and must be formed inside even when all of the needed monomers are provided in the medium. Consequently, although the complexity of the growth medium has some effect on the energy required for synthesis, it is not large.
Of more importance are the oxidation state and size of the carbon source.2' The oxidation state of carbon in biomass is roughly the same as that of carbon in carbohydrate."'' If the carbon source is more oxidized than that, reducing power must be expended to reduce it to the proper level. If the carbon source is more reduced, it will be oxidized to the proper level during normal biodégradation and no extra energy will be required. Therefore, as a general rule, a carbon source at an oxidation state higher than that of carbohydrate will require more energy to be converted into biomass than will one at a lower oxidation state. Pyruvic acid occupies a unique position in metabolism because it lies at the end of many catabolic pathways and the beginning of many anabolic and amphibolic ones. As such, it provides carbon atoms in a form that can be easily incorporated into other molecules. Indeed, three-carbon fragments play an important role in the synthesis of many compounds. If the carbon source contains more than three carbon atoms it will be broken down to size without the expenditure of large amounts of energy. If it contains less than three carbon atoms, however, energy must be expended to form three-carbon fragments for incorporation. Consequently, substrates containing few carbon atoms require more energy for synthesis than do large ones.
Carbon dioxide, which is used by autotrophic organisms as their chief carbon source, is an extreme example of the factors just discussed, being a single-carbon compound in which the carbon is in the highest oxidation state. Consequently, the energy for synthesis for autotrophic growth is much higher than for heterotrophic growth. As a result, the amount of biomass that can be formed per unit of available electrons in the energy source is quite low.
True Growth Yield. The true growth yield (Y) is defined as the amount of biomass formed per unit of substrate removed when all energy expenditure is for synthesis. In this context, the substrate is usually taken to be the electron donor, although it can be defined differently. If the electron donor is an organic compound, it is common in environmental engineering practice to express Y in terms of the amount of soluble COD removed from the wastewater. This is because wastewaters contain undefined, heterogenous mixtures of organic compounds and the COD is an easily determined measurement of their quantity. In addition, the COD is fundamentally related to available electrons, having an electron equivalent of eight grams of oxygen. Thus, a Y value expressed per gram of COD removed can be converted to a Y value per available electron by multiplying by eight. If the electron donor is an inorganic compound, such as ammonia or nitrite nitrogen, it is common to express Y in terms of the mass of the element donating the electrons. Furthermore, regardless of the nature of the electron donor, it has been common practice to express the amount of biomass formed on a dry weight basis, i.e., mass of suspended solids (SS), or on the basis of the dry weight of ash-free organic matter, i.e., mass of volatile suspended solids (VSS). When grown on a soluble substrate, microorganisms have an ash content of about 15%, and thus the value of Y when expressed as VSS will be slightly less than the value of Y when expressed as suspended solids. As will be discussed later, there are advantages to expressing biomass concentrations on a COD basis rather than on a SS or VSS basis, and thus yields are sometimes expressed as the amount of biomass COD formed per unit of substrate COD removed from the medium. This convention will be used throughout this book. If we assume an empirical formula for the organic, i.e., ash-free, portion of biomass of GH-O.N, the COD of that organic portion can be calculated to be 1.42 g COD/g VSS. " Furthermore. if we assume the ash content of biomass to be 15%, the theoretical COD of biomass is 1.20 g COD/g SS. These values can be used to convert between the various ways of expressing the yield.
The nature of the substrate influences the yield. Hadjipetrou et al. " summarized data from one species, Aerobucter acrogcnes, which was grown in unrestricted batch growth in minimal media on a number of substrates, and found Y to vary from 0.40 to 0.56 mg biomass COD formed per mg substrate COD removed. (The values were not reported on a COD basis, but were converted to it for this book.) Recognizing that the yield expressed on the basis of cell COD formed per unit of substrate COD removed is a measure of the amount of energy available in the substrate that was conserved through cell synthesis, it can be seen that 40 to 56% of the available energy was conserved while 44 to 60%' was expended.
The species of organism will also affect Y, although the effect will not be as great as the effect of substrate. Payne " collected Y values for eight bacterial species growing aerobically on glucose in minimal media and found them to vary from 0 43 to 0.59 mg biomass COD formed per mg substrate COD removed. The data were from a number of different published reports and thus some of the variation may be due to differences in experimental conditions, rather than to species. Nevertheless, they clearly show that the microbial species has an impact. (As above, the values were not originally reported on a COD basis, but were converted to it for this book.)
The growth environment, including media complexity, type of terminal electron acceptor, pH, and temperature will all affect Y. 1 As explained above, biomass grown in complex media will have only slightly higher Y values than biomass grown in minimal media, whereas biomass grown with oxygen as the terminal electron acceptor will exhibit significantly higher yields than biomass grown with nitrate as the acceptor. The yield from fermentations will depend on the reduced end products and the method of expressing the yield. If Y is expressed on the basis of the amount of the original substrate removed, ignoring the COD returned to the medium as reduced end products, the value will be very small, on the order of 0.03 to 0.04 mg biomass COD formed per mg substrate COD removed. However, when expressed on the basis of the COD actually utilized (accounting for the COD remaining as reduced end products), the Y value is not much different from that obtained with aerobic cultures.' On the other hand, when methane is produced, so that most of the reduced end product is lost from the system as a gas, then the COD removed from solution is actually much higher than the COD utilized by the microorganisms, making the yield per unit of COD removed about an order of magnitude lower than for aerobic growth. The pH of the medium has long been known to affect microbial growth, but the quantitative effects are unclear. The yield is likely, however, to have a maximum around pH 7 because that is optimal for so many physiological functions. Temperature also affects Y, as shown in Figure 2.4.J" Although the significance of temperature is apparent, no generalizations can be made, and most engineers assume that Y is constant over the normal physiological temperature range. A final factor that
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.