3.7.4 Alkalinity, anaerobic processes
The anaerobic processes influence the alkalinity. The acid step reduces the alkalinity
and the methane step increases it. The overall result is a small reduction in alkalinity.
In Example 3.13, the separate change in alkalinity for the acid step is calculated.
Example 3.13
Calculate the change in alkalinity by conversion of glucose to acetic acid, see Expression (3.45).
2.64 mol H+ per mole of glucose converted are produced corresponding to a reduction
in alkalinity of 2.64 eqv. The produced acetate increases the alkalinity as pKa for HAc
is 4.75, which means that about half of the acetate, Ac-, is converted into acetic acid,
HAc, by the titration to pH = 4.5 (by measuring the alkalinity). From /8/ we have:
_
[Ac-]
pH - PKa + log [HAc]
By substitution of pK8 = 4.75 and pH= 4.50 we find:
[Ac-] _
log [HAc] - -Q.25 or
[Ac-] _
[HAc)- 0.56
or [Ac-] = 0.56 [HAc)
Assuming that [HAc) + [Ac-) = 1, we find:
[HAc] + 0.56 [HAc] = 1 ---+ [HAc] = 0.64 and
[Acl =0.36
that is, at pH = 4.5,
[Acl = 0.36 · ([Ac-] +[HAc)) and
[HAc] = 0.64 · ([An + [HAc])
Hence the contribution to the alkalinity from Ac- is 0.64 · 2.40 = 1.5 eqv/mol glucose.
The overall change of the alkalinity, ~TAL, is:
~TAL= 1.5-2.64 = -1.14 eqv/mol glucose.
3.7.5 Kinetics, anaerobic processes
Hydrolysis, anaerobic processes
The process can be described using the same expression of kinetics as for aerobic
and anoxic processes, that is, either in a very simplified form such as:
104
(3.3a)
(3.3b)