Environmental Engineering Reference
In-Depth Information
2. State of the Science
Among separation techniques, mechanical filtrations are especially important in
bioprocessing. These include micro-, ultra-, and nanofiltration; deep bed filtration; static and
dynamic cross-flow filtration; electrofiltration; and centrifugation, involving both filtration
and sedimentation. In these filtrations, fouling remains a major technical obstacle. In addition,
the chemical and thermal separation techniques of ion exchange, chromatography,
electrophoresis, crystallization, and extraction are also commonly used in bioprocessing.
Research is currently underway in improving many bioseparation techniques, offering hope
that major improvements in the efficiency of bioengineered processes can be achieved by
incorporating new bioseparation technologies into process design, and several recent,
excellent compilations of modern techniques are now available [3-5].
2.1. Physical Separations
Emerging physical bioseparation techniques include aqueous two-phase extraction,
reverse micellar extraction, cloud point extraction, and magnetic and electrophoretic
separation [6, 7].
2.1.1. Two-phase partitioning bioreactors
. Among emerging techniques, the two-phase
partitioning bioreactor appears to have great potential in enhancing the productivity of many
bioprocesses. The approach integrates fermentation with a primary product recovery step by
incorporating both organic and aqueous phases simultaneously, such that microbial growth
occurs in the aqueous media and substrates and products partition into the organic phase
based on their affinities for it. This approach allows controlled substrate delivery to the
fermentation broth and effectively lowers product concentrations in the fermentation broth as
well, promoting microbial health and activity. Although it is already practiced commercially,
its effectiveness could still be improved by the discovery of low-cost solvents that are non-
toxic for microbial growth [8, 9].
2.1.2. Non-solvent-based processing
. A primary purpose of nonpolar organic solvents in
bioprocessing is the dissolution of cell membranes to release intracellular products However,
organic solvents are undesirable for several reasons: they are frequently derived from non-
renewable resources such as petroleum; they are frequently toxic and/or carcinogenic, and
consequently expensive to treat in disposal; and they frequently form non-aqueous phases that
hinder biodegradation. As a result, efforts to develop efficient, non-solvent-based approaches
to cell disruption and product purification deserve high priority.
The purification of polyhydroxyalkanoate (PHA) polymers provides an excellent
example of the benefits possible with such approaches. Following fermentation, PHA-
containing cells are separated from culture media by centrifugation, filtration, and/or
flocculation, and cells are then disrupted to recover the polymer. Subsequent recovery,
unfortunately, typically involves extraction of the polymer from biomass with large amounts
of toxic and inflammable organic solvents (e.g., chloroform, methylene chloride, propylene
carbonate, or dichloroethane). Another established method, again involving environmentally
undesirable reagents, uses sodium hypochlorite for the digestion of non-PHA cellular
materials, carrying the additional disadvantage that it partially degrades the PHA [10].
To improve the environmental friendliness of PHA processing, a non-solvent-based
process has been developed by Zeneca Agrochemicals (now part of Syngenta;
www.syngenta.com) to assist in the commercial production of poly[(R)-3-hydroxybutyrate]
