Page 735 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological
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690 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological
As noted by Parker et al.=EPA (1975, pp. 3–29), Pseudomo-
TABLE 22.6
nas, Micrococcus, Achromobacter, and Bacillus are among
Common Volatile Acid Intermediates
the bacteria than can denitrify and can use either nitrate of
oxygen as electron acceptors. They are called facultative Acid Formula
heterotrophs and can shift easily from aerobic to anoxic con- Formic HCOOH
ditions since the electron transport pathways are almost iden- Acetic CH 3 COOH
tical. They give the overall nitrate removal reaction as Propionic CH 3 CH 2 COOH
Butyric CH 3 (CH 2 ) 2 COOH
1:08CH 3 OH þ NO 3 þ 0:24H 2 CO 3
Valeric CH 3 (CH 2 ) 3 COOH
Isovaleric (CH 3 ) 2 CHCH 2 COOH
! 0:056C 5 H 7 O 2 N þ 0:47N 2 þ 1:68H 2 O þ HCO 3
(22:12) Caproic CH 3 (CH 2 ) 4 COOH
Source: Adapted from McCarty, P.L., Anaerobic waste
As seen, about 2.5 kg methanol are required per kg NO 3
as N. Methanol is a more readily available electron donor treatment fundamentals, Part one—Chemistry
and microbiology, Public Works, September
than wastewater and the denitrification rate for the latter is
1964a, p. 109.
about one-third the rate for methanol, which requires a larger
reactor volume (Parker et al.=EPA 1975, pp. 3–35).
22.3.3.4 Anaerobic From Equation 22.14 the relationships are seen by the coeffi-
As described by Stewart et al. (1959), two major metabolic cients, i.e., for each mole of glucose reacted, 0.24 mol of
groups of bacteria occur in anaerobic systems, acid-producing NH 4 þ is required, with yields of 0.24 mol of cells, 2.4
and methane-forming. The acid-producing bacteria are com- moles of CH 4 , and 2.64 mol of CO 2 .
mon facultative bacteria which exist in soil, activated-sludge Table 22.7 shows representative substrates that may be
floc, and trickling-filter slimes and have the ability to oxidize subject to the anaerobic reaction sequence, organic acid for-
organic materials to simple organic acids, alcohols, and alde- mation and methane formation, showing also the respective
hydes. The metabolic activities of methane-forming bacteria cell yields and endogenous coefficients. The Y coefficients
result in the production of methane and carbon dioxide from (the mass of cells synthesized per unit of substrate reacted)
the organic acids, alcohols, and aldehydes. were as calculated by Rittman and McCarty (2001, p. 586). In
McCarty (1964a, p. 109) depicted the general reaction, aerobic treatment, Y(aerobic) 0.5 g VSS=g BOD L , which is
on the order of 5–10 times the values shown in Table 22.7.
acid formers methane formers
complex organics ! organic acids, H 2 ! CH 4 þ CO 2 This means that, in anaerobic treatment, the cell-disposal
First-stage waste conversion Second-stage problem is much less. Also, in favor of anaerobic process,
methane gas is formed, which has an energy value (p. 570) of
waste stabilization
35.8 kJ=L (STP). A typical volumetric loading may be 5–10
(22:13) 3
kg COD=day=m reactor volume as contrasted with <1kg
3
In other words, the first stage reacts with the varied organic COD=day=m for an aerobic system (p. 571); anaerobic treat-
substance, often as found in primary sludge from a municipal ment is especially suited for high-strength wastes. On the
wastewater treatment plant such as cellulose, proteins, sugars, other hand, the minimum mean cell residence time,
lipids, cells, etc., to form organic acids such as those listed in
Table 22.6. The second stage is rate limiting since the fer-
mentation reactions in the first stage have a greater energy TABLE 22.7
yield, DG(reaction), than in the second stage, i.e., the reac- Substrate Formulae and Coefficients for Anaerobic
tions involved in methane formation (Rittman and McCarty, Process to Final Products
2001, p. 583). On the other hand, stage one may be slower for Substrate Formula Y (g VSS=g BOD L ) b (day )
1
substances such as lignocellulose, grasses, crop residues,
Carbohydrates C 5 H 10 O 5 0.20 0.05
newsprint. The topic of anaerobic treatment was reviewed
Protein C 16 H 24 O 5 N 4 0.056 0.02
comprehensively in a series of four articles by McCarty
Fatty acids C 16 H 32 O 2 0.042 0.03
(1964a,b,c,d) which explained a complex subject is a fashion
Municipal sludge C 10 H 19 O 3 N 0.077 0.05
useful for persons in practice.
Ethanol CH 3 CH 2 OH 0.077 0.05
The anaerobic reaction is called fermentation in that the Methanol CH 3 OH 0.11 0.05
electron donor and the electron acceptor are the same organic Benzoic acid C 6 H 5 COOH 0.077 0.05
molecule. Using glucose as an example Rittman and McCarty
(2001, p. 587) give the reaction stoichiometry as Source: Adapted from Rittman and McCarty, Environmental
Biotechnology: Principles and Applications McGraw-Hill, New
þ York, 2001, p. 587.
C 6 H 12 O 6 þ 0:24NH þ 0:24HCO 3
4
Notes: Y is the yield coefficient for cell synthesis as calculated from
! 0:24C 5 H 7 O 2 N þ 2:4CH 4 þ 2:64CO 2 þ 0:96H 2 O
stoichiometric equations. b is the endogenous decay coefficient.
(22:14)
