Biomedical Engineering Reference
In-Depth Information
• Biomass concentration or areal density
• Turbulence induced by mixing (inluences light-dark cycling as cells move
in and out of the photic volume) (Grobbelaar, 2009).
The requirement for optimal light provision to all cells places unique constraints
on the geometry of reactors. As light enters a culture surface, it is absorbed and
scattered by the cells, particulate matter, colored or chemical substances, as well as
the water itself (Grobbelaar, 2009). As cells at the surface absorb light, they shade
those below them. Due to this mutual shading effect, light intensity decreases with
culture depth. Light does not penetrate more than a few centimeters into a dense
algal culture; therefore, optical depth must be minimized. Reactor scale-up is based
on reactor surface area rather than volume, as in the case of heterotrophic fermenta-
tions, and the surface-area-to-volume ratio is a critical parameter (Scott et al., 2010).
Reactor design is a trade-off between maintaining a shallow depth, or thin optical
cross-section, and the increased cost of reactor materials, decreased efficiency of
mixing, and greater land area involved.
The density of the culture determines the attenuation of light with distance from
the reactor surface. Given a certain reactor path length and light intensity, there will
be a corresponding optimal cell density. Below the optimal areal density, all cells are
exposed to excess light, and above optimal density, a significant proportion of the
culture is in the dark (Grobbelaar, 2009). At the optimal density, given sufficient mix-
ing, all cells are subject to equal light-dark fluctuations. Maximum photosynthetic
efficiency occurs in relatively dilute cultures. The increase in productivity achieved
by maintaining the optimal cell density for light provision must be balanced against
the costs of the increased reactor volume and harvesting capacity required to process
large volumes of dilute cell suspensions. In addition, a high volumetric yield does not
necessarily mean that incident light is being most efficiently used. This is measured
by areal yield, not volumetric yield, and for this there is an optimal areal cell density
as well as cell concentration (Richmond, 2000).
Microalgal cells can become acclimated to high or low light conditions. In an
effort to balance the activity of the light and dark photosynthetic reactions, cells
modulate their light-harvesting capacity (e.g., through adjusting the number of PS
II reaction centers and the pigment concentration), depending on the ambient light
intensity. The process of photo-adaptation takes 10 to 40 minutes (Pulz, 2001). Due
to the fact that a culture may become acclimated to prevailing light conditions, the
optimal biomass concentration is different at high and low irradiance. It is therefore
impossible to operate at a single optimum cell concentration when a range of irradi-
ance occurs over the course of the day (Lee, 2001).
It has been postulated that there is a phenomenon known as the flashing light
effect that leads to increased productivity at certain frequencies of light-dark cycling.
Exposing cells to very short cyclic periods of light and darkness could counterbal-
ance the two extremes of light over-saturation and inhibition. However, the effect
of flashing light is very difficult to separate experimentally from the effects of the
increased turbulence required to generate faster light-dark cycling (Grobbelaar,
2009). It is clear that enhanced mixing, up to a point at which cell damage begins to
occur, is beneficial to optimal light provision by creating an average light intensity
Search WWH ::
Custom Search