Anaerobic wastewater treatment Fourth
of a six-part series on
wastewater treatment processes
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Perry L. McCarty Daniel P. Smith Stanford University Stanford, Calif 94305 Anaerobic processes have been used for the treatment of concentrated municipal and industrial wastewaters for well over a century (7). These processes convert organic materials into methane, a fuel that can yield a net energy gain from process operations. Because of recent advances in treatment technology and knowledge of process microbiology, applications are now extensive for treatment of dilute industrial wastewaters as well. Europe has been the leader in applying the new technology, with far more installations than in the United States. The advantages of anaerobic treatment over aerobic treatment, current understanding of microbiology, and the newer processes have been described in detail by Speece (2). It is apparent that biofilm processes offer greater stability than dispersedgrowth processes do for treating dilute wastewaters. The reasons for this are not entirely obvious, and are probably multiple. One factor that makes biofilm processes more favorable is related to the role of diatomic hydrogen (H2) in the control of process dynamics.
Interspecies hydrogen transport Among the significant recent advances in the understanding of the ecol-
ogy of the anaerobic process was that prompted by the discovery of Bryant et al. (3) that the conversion of ethanol to methane requires the combined activity of three separate species of bacteria, each carrying out one of the reactions listed in Table 1. The first species oxidizes ethanol, and in the process produces acetate and H2. The second species is a methanogen that combines the hydrogen with carbon dioxide to form methane. The third species is also a methanogen, but one that splits acetic acid into carbon dioxide and methane. 1200
Environ. Sci. Technol., Vol. 20, No. 12, 1986
The value AG0' represents the standard Gibbs free energy available from the reactions when reactants and products are at unit activity and pH is 7. As indicated by the net equation in Table 1, AG” is negative; therefore, the overall reaction under such conditions is favorable thermodynamically. However, AG" for the conversion of ethanol is positive, and thus under standard conditions is not favorable. For this reaction to proceed, it is necessary for the concentrations of the reaction products (acetate and H2) to be reduced sufficiently by the methanogens to yield a negative value for the actual free-energy (AG") change as given by mk
where vik is the stoichiometric coefficient for component i in reaction k with mk components, a, is the activity of component i, R is the universal gas constant, and T is absolute temperature (5).
Because the organisms that carry out ethanol oxidation depend on methanogenic activity to reduce product concentration, and because the methanogens depend on the ethanol-oxidizing organisms for their food, a close syntrophic relationship among the three species results. Bryant and co-workers subsequently showed that this phenomenon is a general one by demonstrating that the conversion of butyrate (6) and propionate (7), which are important intermediates in the process, also requires three species for conversion to methane gas, as indicated in Table 2. There is now general agreement that methane formation of complex organic materials is a threestage process as illustrated in Figure 1 U, 9, 10). Complex organic materials are first hydrolyzed and fermented by facultative (those that live either in the presence or in the absence of oxygen) and anaerobic microorganisms into fatty acids. The fatty acids are then oxidized by /3-oxidation to produce H2 and acetate, processes termed dehydrogenation and acetogenesis, respectively. The last stage is methanogenesis. Although there are other ways in which methane can be formed, the pathways shown in Figure 1 tend to be most significant in anaerobic treatment of wastewaters. The importance of H2 in process control is illustrated perhaps better with propionate than it is with ethanol. This compound is formed through hydrolysis and fermentation of proteins and
1986 American Chemical Society
carbohydrates and through oxidation of longer chained fatty acids. In treatment of complex mixtures, such as sewage sludge, as much as 30% of the electrons associated with the methane product will flow through propionic acid, as illustrated in Figure 1 (8). Thus, Hi is an important intermediate, and the bacteria responsible for its conversion must be present in sufficient numbers for the process to operate efficiently. The amount of energy required to convert propionate to hydrogen and acetate is even less than that needed for ethanol conversion. This is illustrated by the relatively high positive AG0' for its oxidation (Table 2). The concentration of either acetate or hydrogen, or both together, could be reduced sufficiently to provide a favorable free-energy change for propionate oxidation. In fact, it is H2 that tends to be most important in the control of the process. During anaerobic treatment, its concentration is reduced to a much lower level than that of acetate. The acetate concentration in an anaerobic treatment process tends to range between 10 * M and 10 M; H2 ranges between 10"® M and Kf5 M, or about four orders of magnitude less. In addition, the H2 partial pressure can change rapidly, perhaps varying by an order of magnitude or more within a few minutes. This is related to its rapid turnover rate, which is discussed later. Figure 2 illustrates the relationship between H2 partial pressure and AG' for the three steps in conversion of ethanol or propionate to methane, as calculated in Equation 1. The relationship between H2 partial pressure (FH2) and H2 molar concentration is given by Henry’s law, which at 35 °C is as follows: 1
Conversion of ethanol to methane3
Ethanol CH3CH2OH(aq) + H20(l)
Hydrogen 2H2(g) + V2COa(g)
'/2CH4(g) + H20(l)
Acetate CH3COO (aq) + H*(aq)
CH4(g) + C02(g)
‘Thermodynamic values from Reference 4.
Conversion of propionate and butyrate to methane3 AG°' kJ
Propionate CH3CH2COO-(aq) + 2H50(I)
+ 3H2(g) +
Hydrogen 3H2(g) + 3ACOf(g)
3ACH4(g) + 3fcH?0(l)
Acetate CH3COO-(aq) Net
CH4(g) + C02(g)
CH3CH2COO (aq) + FT(aq) + VaH20(l)
Butyrate CH3CH2CH2COO-(aq) + 2HsO(l)
2CH3COO(aq) + 2H2(g)
Hydrogen 2Hs(g) + i/2C02(g)
VzCH4(g) + H20(l)
Acetate 2CH3COO"(aq) Net
2CH4(g) + 2 C02(g)
CH3CH2CH2COO (aq) + H20(l) + H+(aq)
ffcCH4(g) + 3/2COa(g)
•Thermodynamic values from Reference 4.
Three stages of methanogenesis3
«H2](g) (atm)) ([H2](aq) (mol/L))
The concentrations of acetate and ethanol, the partial pressures of carbon dioxide and methane, and the pH values assumed for the calculations are typical of those in an efficiently operating anaerobic process. Figure 1 illustrates that for the three organisms responsible for conversion of ethanol to methane, the H2 partial pressure must lie between 10"1 atm and 10~6 atm. This is a fairly large range. The energy available to the acetate-using methanogens is independent of H2 partial pressure, whereas that of the hydrogen-producing and hydrogen-consuming species is very dependent on it. Higher H2 partial pressures tend to supply more energy to the hydrogen-consuming species, whereas lower partial pressures result in a greater portion of the
Electron flow and the significance of propionate in the conversion of complex substrates are illustrated. Source: Reference 8. Adapted with permission from Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, Argonne National Laboratory, 1984.
Environ. Sci. Technol., Vol. 20, No. 12, 1986
energy going to the hydrogen-producing bacteria. With propionate use, the H2 partial pressure must lie in a relatively narrow range, between 10"4 atm and 10~6 atm, assuming that the concentrations of other reactants and products do not change greatly. Because propionate is a major intermediate in the anaerobic treatment of most complex mixtures, H2 partial pressures must remain within this range for such treatment systems to operate efficiently. This has significant implications for the way in which anaerobic treatment systems respond to perturbations, such as the sudden introduction of waste in a relatively high concentration. This will be addressed later. The low concentration of H2 within the digester also has implications for the rate of H2 turnover within the system. Loading rates to anaerobic treatment systems are frequently reported as chemical oxygen demand (COD), with typical values of 1-30 kg COD/m3 day. Theoretically, 0.064 kg COD can be converted into 1 mol CH4 (77). With most complex wastes, about one-third of the methane results from H2 consumption; the other two-thirds comes •
Because 1 mol CH* requires 4 mol H2, in a moderately loaded system that converts 10 kg COD/day per m3 of reactor volume, H2 generation would be
206 mol H2/ m3-day. For H2 partial pressure of 4 x 10 5 atm, the corresponding H2 solution concentration would be
This calculation is derived from Equation 2. For this example, the turnover rate of the H2 pool would then have to be 206/ 3.0 X 10"5, which corresponds to 6.8 X lOVday or ~80/s. H2 is therefore transferred rapidly from the H2consuming bacteria. How can this be accomplished? The producing and consuming species must be close together within the treatment reactor. For processes on this microscale, diffusion is the main mechanism for H2 transport between species. This is illustrated in Figure 3. As a simple example, consider one-dimensional flow of H2 between species separated by a dis3 X 10"5 mol/m3.