Biomedical Engineering Reference
In-Depth Information
It is increasingly recognized that the provision of nutrients, especially
combined nitrogen, to bioprocesses affects their life-cycle impact (Harding 2009;
Harding et al., 2012). Lardon et al. (2009) illustrated that the provision of fertil-
izer accounted for 15% to 25% of the energy requirements per unit biodiesel. This
was substantially reduced under nitrogen-limited conditions (6% to 9%). Clarens
et al. (2010) illustrated that 50% of the energy requirement and GWP for biomass
production is attributable to nutrient provision in their raceway system. The poten-
tial exists to replace these with wastewaters such as effluent from the conventional
activated sludge process or source-separate urine. The former has the potential
to provide a water supply simultaneously. Similar benefits can be achieved by
maximizing the nutrient recycle (Stephenson et al., 2010; Richardson, 2011) and
minimizing the nitrogen input required, either by optimizing the nitrogen limitation
or selecting an algal species of low nitrogen content, for example,
Phaeodactylum
tricornutm
at 0.8% N over algal species with a typical nitrogen content of 6% by
mass (Richardson et al., 2012b).
9.3.4 b
ioMass
r
eCovery
Dewatering of the dilute algal suspensions required to minimize light limitation in
the bioreactor is recognized as a key challenge in large-scale algal processes. This
is aggravated by small algal cell size and a density only slightly greater than that
of water. An initial dewatering step is required to achieve a solids concentration of
1% to 2.5% by mass prior to concentration using an energy-intensive operation such
as centrifugation to a solids concentration of 5% to 20% (Benemann and Oswald,
1996). Ideally, flocculation and sedimentation are used for the primary dewatering,
facilitated by the algal species selected (Lardon 2009; Stephenson et al., 2010). Other
options include natural settling (Collet et al., 2010), filtration (Yang et al., 2011), and
dissolved air flotation (Campbell et al., 2010). The decanter centrifuge, spiral-plate
centrifuge, and rotary press are most typically proposed for the second dewatering
step (Lardon et al., 2009; Campbell et al., 2010; Clarens et al., 2010; Collet et al.,
2010; Stephenson et al., 2010). Stephenson et al. (2010) report a relative electricity
consumption for flocculation of 1 unit versus 4 units for centrifugation and 14.4 units
for the cultivation. On using a more dilute algal culture, as typically found with the
raceway, both pumping energy and the need to increase flocculant supplied to main-
tain its volumetric concentration impact the process.
9.3.5 b
ioMass
d
ryinG
and
C
onversion
In a number of studies, it has been assumed that the biodiesel production from algal
oil should be conducted using the same approach as for vegetable oil. This requires
the drying of the algal biomass to 90% solids by mass (Sazdanoff 2006; Lardon
et al., 2009; Batan et al., 2010; Yang et al., 2011). Typically, belt or drum drying
is used, with energy provided by natural gas. This may represent the major or a
significant portion of the energy demand of the process. Sander and Murthy (2010)
estimated that 89% of the energy requirement of their “well-to-pump,” raceway-
based algal biodiesel system was required for drying the intermediate product prior
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