Environmental Engineering Reference
In-Depth Information
[poly(3HB)] and poly(3HB-co-3V) by Alcaligenes eutrophus. In this process, cells are
brought to 80°C and treated with a mixture of hydrolytic enzymes, including lysozyme,
phospholipase, and lecithinase that hydrolyze cellular components without degrading the
polymer. The polymer can then be recovered as a white powder after washing and drying,
demonstrating the technical feasibility of performing novel separations utilizing advanced
bioengineering techniques. Cost of necessary reagents, such as the enzymes involved in this
case, remains an important consideration, however, and must be addressed to enable such
techniques to compete with existing routes to synthesis of plastics based on petrochemical
sources [11]. Additional non-solvent based methods of cell lysis, including modern
developments in enzymatic, chemical, and mechanical cell disruption, are considered in detail
in the recent topic, Bioseparations Science and Engineering [5].
2.2. Mechanical Separations
A great variety of membrane-based techniques are not only under development, but are
already central to commercial integrated bioprocesses, enabling further avoidance of
environmentally deleterious reagents in bioproces sing. These approaches separate products
from culture media based on hydrophobicity, volatility, and/or affinity for membrane
components, and offer the potential for great selectivity as well as low-energy and waste-
disposal requirements in biorefining [12].
2.2.1. Pervaporation
. Pervaporation is the separation of various liquid mixtures by partial
vaporization through a non-porous membrane. The membrane acts as a selective barrier
between the two phases: the liquid feed and the vapor permeate phases. It allows the desired
component(s) of the liquid feed to dissolve within it and then to transfer through it by
vaporization, resulting in a separation based primarily on differences in polarity rather than
volatility. Pervaporation has shown great promise in separating alcohols such as ethanol and
butanol from bioreactor media, as in the production of ethanol from rice straw by Pichia
stipitis [13] as well as other azeotropic and close-boiling mixtures, including isomers [14].
A challenge associated with the use of pervaporation is the accumulation of less volatile
components (higher alcohols) in fermentation media that can cause microbial growth
inhibition [9]. Nevertheless, industrial uses for this method are widespread, and membrane
technologies as well as integration of pervaporation with other unit operations are still
showing promising improvement [14].
2.2.2. Size-based separations
. Membranes have traditionally been used for size- based
separations that require high throughput but relatively low resolution. Examples include
microfiltration (MF) for clarification and sterilization as well as ultrafiltration (UF) for
product concentration and buffer exchange Microfiltration is competitive with depth
filtration, centrifugation, and expanded-bed chromatography for the initial harvest of products
from bacterial, yeast, and mammalian cell cultures [15, 16, 5]. Use of 0.2 µm-rated MF
membranes can yield a particle-free harvest solution requiring no additional clarification prior
to purification, while larger pore-size membranes can be used to improve product yield and
throughput when the filtrate is subsequently treated with a normal flow filter for final
clarification. MF systems are generally operated at constant flux instead of constant
transmembrane pressure to improve product yield and throughput, although this practice tends
to exacerbate fouling problems [17]. An emerging solution to this problem is high-frequency,
back-pulsing air scrubbing to clean the surface of the membrane continuously [18, 5].
