Page 80 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 61

(a) (b)

FIGURE 4.3 PBR designs at the University of Minnesota. (a) Multi-layer cultivation PBR
and (b) tubular PBR.

Based on a report by Argonne National Labs (2011), the carbon and nutrient
−1
−1
requirements are 1.9 kg CO kg dry algae, 0.0125 kg P kg dry algae, and 0.056 kg
2
N kg dry algae. This is based on the composition of algal cells (C : N : P = 103:10:1).
−1
The actual nutrient requirement may be higher, considering that the nutrient uptake
efficiencies are different for different algae species. When wastewater is used to grow
algae, not all the nutrients in wastewater are available to algae. For example, waste-
water from thermal conversion processes such as pyrolysis and liquefaction contains
a high level of bio-unavailable nutrients. Even though the total nutrients are high, the
algae yields are often low. In a study by Min et al. (2011), four wastewater streams
created in a wastewater treatment plant were examined for growing Chlorella sp.
The results showed that algae grown in centrate had the highest algae yield (34 g
−1
−2
m d ) and lipid yield. The COD, N, and P removal rates reached 70%, 61%, and
61%, respectively. Besides nutrients, wastewater also contains organic carbon, which
algae can take up and therefore reduce the inorganic carbon (CO ) requirement for
2
growing algae. When an injection of CO is required, CO collected from coal fire
2
2
power plants should be injected to sequestrate CO from the atmosphere. Electricity
2
is used for flue gas capture, purification, and injection, which should be accounted
for in a life cycle inventory (LCI).
Flocculation and sedimentation processes are used to separate algae from water
in the algae slurry exiting the cultivation systems. After the process, the solid content
in algae slurry can reach around 1%–2%. The supernatant collected after separa-
tion is recycled back to the cultivator, which recycles nutrients and reduces the need
for fresh water. After flocculation, the algae slurry usually goes through a dewater-
ing process such as centrifuging to further remove water and increase solid content