Page 463 - Air Pollution Control Engineering

11_chap_wang.qxd 05/05/2004 5:19 pm Page 435
Gas-Phase Biofiltration 435

possible to obtain, for example, a high EC for one compound, with 99+% removal,
yet still be faced with approx 100% of another compound or metabolic intermediate.
Intermediates can often be as environmentally dangerous as the primary compounds.
Thus, to treat these other species, it may be economically (both from a capital cost and
running cost point of view) or operationally attractive to have different stages, or even
separate biofilters/bioscrubbers in the process.

4.2. Packing
Depending on the exact pollution application and bioreactor configuration (biofil-
ter vs bioscrubber, vs biotrickling filter) a choice as to the appropriate packing mate-
rial will need to be made (see Table 4). However, despite a number of the materials
listed on Table 4 having a natural biological population, it still may be advisable in
some cases for this population to be supplemented by “designed” or preacclimated
microorganisms to result in less start-up time and potentially more stable long-term
operational effectiveness.
5. CASE STUDIES

5.1. High-Concentration 2-Propanol and Acetone
It is often possible to continuously extend the range of biofiltration by use of high
3
preacclimated microorganisms and extremophiles. For example, to treat 25,000 m of
high-concentration 2-propanol (IPA) and its intermediate acetone with 95% removal,
the design of such a biofilter is as follows.
A bench-scale investigation reveals that it is possible to treat this stream with an inlet
3
3
concentration of 15 g/m of IPA (2-propanol) with a final maximum EC of 280 g/m /h.
3
Thus, C = 15 g/m and so C = 0.05 × 15 = 0.75 g/m 3
I 0
Now,
(C I × )A f
C
EC = = 280 (8)
0
V
f
and so
I (
C × ) (15 − 0 75 )25000
C A
.
V = 0 f = =1272 m 3 (9)
EC 280
If we make the bed a typical depth of 1.5 m per stage and stack the bed two stages
deep, then the cross-sectional area to treat this pollutant flow is
A bed = 1272 = 424 m 2 (10)
× 5
21.
and, in a square configuration, this leads to an approx 21-m × 21-m square bed.
For this type of operation, at a high EC and pollutant load, a preacclimated microbial
consortium would be needed (from the bench study), and an inert microbial support may,
therefore, be an option. In this case, the amount of support medium can be calculated as
follows, based on a 0.45 voidage:
3
Volume of packing = (1 − 0.45) × 1272 = 700 m of packing.