Environmental Engineering Reference
In-Depth Information
extremophile may be able to survive in an extreme condition, that doesn't mean it
can thrive in such conditions.
Many research groups have therefore turned to using enclosed photobioreac-
tors of various designs as a means of preventing culture collapse or takeover by
low oil strains, as well as decreasing the vulnerability to temperature fluctuations.
The significant downside is the much higher capital cost of current photobioreactor
designs. While such high costs are not prohibitive when growing algae for pro-
ducing high value products (specialty food supplements, colorants, pharmaceutical
products, etc.), it is a significant challenge when attempting to produce a low value
product such as fuel. Therefore, substantial focus must be placed on designing much
lower cost photobioreactors and tying algae oil production to other products (animal
feed or fertilizer from the protein) and services (growing the algae on waste stream
effluent to remove eutrophying nutrients, or growing nitrogen fixing algae on power
plant emissions to remove NO
x
emissions).
An additional challenge, when trying to maximize oil production with algae, is
the unfortunate fact that higher oil concentrations are achieved only when the algae
are stressed - in particular due to nutrient restrictions. Those nutrient restrictions
also limit growth (thus limiting net photosynthetic efficiency, where maximizing
that is a prime reason for using algae as a fuel feedstock). How to balance the desire
for high growth and high oil production to the total amount of oil produced is no
small task. One of the goals of DOE's well-known Aquatic Species Program was to
maximize oil production through nutrient restriction; however their study showed
that while the oil concentration went up, there was a proportionally greater drop in
reproduction rate, resulting in a lower overall oil yield.
One approach to balancing these issues has been successfully tested on a small
commercial scale (2 ha) by Huntley and Redalje [25], using a combination of
photobioreactors and open ponds. The general approach involves using large photo-
bioreactors for a “growth stage”, in which an algal strain capable of high oil content
(when nutrient restricted) is grown in an environment that promotes cell division
(plentiful nutrients, etc.) - but which is enclosed to keep out other strains. After
the growth stage, the algae enter an open raceway pond with nutrient limitations
and other stressors, aimed at promoting biosynthesis of oil. The nutrient limitations
discourage other strains from moving in and taking over (since they also require
nutrients for cell division).
Waste oils, such as restaurant grease and spent frialator oil, can also be used in
the production of biodiesel. This eliminates the “food or fuel” debate that affects
virgin edible oil sources. These waste oils normally cost money for restaurants and
other establishments to dispose off. This can have a negative feedstock cost which
reduces the overall cost of production. However, like virgin oils, traditional pro-
cesses of converting waste oils to biodiesel can result in soap formation due to the
presence of water and free fatty acids. The waste oils usually contain particulates
that require filtration or separation prior to processing. Demand for waste oil as a
biodiesel feedstock has already resulted in companies now paying restaurants for
their waste vegetable oil (WVO). Quantities of WVO are limited (it is estimated to
be about 1.1 billion gallons per year in the US), but it is certainly a good option for
producing biodiesel.
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