Biomedical Engineering Reference
In-Depth Information
Gravity sedimentation is relatively less energy intensive as fewer motors,
pumps, and settling tanks are needed for its operation, thus resulting in low capital
and operational cost and high expected life span. However, for a commercial-scale
(>4 hectares) algae cultivation process and considering the slow sedimentation rates
of algae, multiple tanks of large volumes (~100,000 L each) may be required.
Centrifugation is a very efficient technique, but the large energy requirement for
the process clearly eliminates it as an option for harvesting a low-value energy crop.
Therefore, this process can be ruled out for harvesting algae biomass for biofuel
production, at least for first-stage dewatering (increasing solids content from algae
culture on the order of 1%), on both cost and energy grounds.
Membrane filtration would be the next most efficient harvesting option; however,
field experience on algae farms would be required to verify the lifetime and mainte-
nance costs of the filter elements. Membrane filtration may be a competitive option
if the back-flush function could be carried out with air knives in place of water jets,
in order to achieve the desired consistency of 1% to 5% solids. Dissolved air flotation
would be the expensive option in terms of cost and energy burden.
Of the flocculation-based processes, polymer flocculation is not only the most
efficient, but also the most energy-intensive technique for dewatering. On the other
hand, electro-flocculation techniques are cost-effective with low energy burden, but
these techniques are still in their infancy and large-scale field testing is required
to verify the overall process efficiency. Auto-flocculation is the lowest cost, low-
est energy dewatering process by far, at one-tenth those of membrane filtration and
polymer-based dewatering. Moreover, the chemicals required are pond nutrients,
which can be recovered from the biomass for re-use either via anaerobic digestion, as
would be the nitrogen, phosphorous, and potassium nutrients, or via an inexpensive
carbonic acid extraction process if necessary, thereby avoiding the production-scale
limitations imposed by synthetic polymer flocculants. As with the growth ponds
and the anaerobic digesters, auto-flocculation employs managed natural processes to
achieve its ends, at considerable savings in cost and energy.
Although the data presented in Table 6.1 appear straightforward and it would
be easy for anyone to compare the harvesting and energy efficiencies of various
processes, there are a variety of fundamental operational issues associated with
each process. Therefore, it is important to carefully analyze several parameters,
such as cell morphology, ionic strength of the media, pH, culture density, and final
downstream processing of harvested biomass, when selecting a suitable harvesting
technique. For example, very small sized algae could hinder the harvesting effi-
ciency and would have a negative impact on the economics of biomass production
if subjected to gravity sedimentation and filtration. However, if such algae could
be made to float via the DAF (dissolved air flotation) flocculation process, this
may facilitate harvesting. Furthermore, downstream processing of the harvested
biomass to get final products will also be an important factor in selecting the har-
vesting process. If the biomass will be subjected to anaerobic digestion (AD) for
biogas production, a solids content up to 5% (w/v) would suffice; whereas, if lipid
extraction followed by biodiesel production is the goal, the biomass needs to be
dewatered to lower moisture contents. There is considerable interest in efficient but
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