Page 79 - Materials Chemistry, Second Edition

60 Life Cycle Assessment of Wastewater Treatment

to the variety of their outstanding properties. Algae offer relatively high area oil
productivity compared with terrestrial bioenergy feedstocks, and they grow with
short cycles that facilitate frequent harvesting (National Algal Biofuels Technology
Roadmap, 2010). Algae can also be cultivated in marginal land with harsh conditions
and can grow in saline conditions and wastewater (Ruiz et al., 2013; Soh et al., 2014).
In addition, algae have the ability to grow in flue gas derived from coal power plants
and thus sequestrate CO from the atmosphere (Singh and Olsen, 2013). Much exist-
2
ing literature supports the argument that algal biofuels would be a potential alterna-
tive to petroleum-derived fuels in the transportation sector in the United States.
Numerous studies have suggested that using nutrient-laden wastewater to cultivate
algae can improve the environmental performance of algal biofuels, because it reuses
waste N, P, and water, which reduces the energy use and emissions from acquir-
ing these inputs, as well as reducing overall biofuel production costs. Additionally,
algae remove nutrients from the wastewater streams, which reduces loading in treat-
ment facilities and in turn, decreases electricity consumption. Algae cultivation has
been examined using various wastewaters, such as municipal wastewater, industrial
wastewater, and animal manures. The liquid wastes created in the wastewater treat-
ment facilities contain high levels of nutrients, which are a good source for growing
algae. A study by Min et al. (2011) tested algae growing in various wastewaters and
found that the centrate from municipal wastewater yielded the highest algae growth.
Algal biofuel production can be divided into three major stages: algae cultivation,
algae conversions, and algal bioproduct use.

4.3.4.1 Algae Cultivation Stage
This stage includes the algae cultivation facility and algae–water separation and dewa-
tering processes. There are two types of algae cultivation facility. One is the raceway
open pond structure with wide surface area (100–1000 m ) and shallow tank (~0.3 m).
2
The algae yield ranges from 13.6 to 24.7 g m d in various locations across the United
−2
−1
States. Another type is photobioreactors (PBRs) with various designs and structures.
Figure 4.3 shows some PBR designs. These designs facilitate obtaining higher algae
yields (up to 50 g m d ), use fewer resources, and offer easier operation conditions.
−1
−2
The open pond cultivation has a lower initial investment cost and is fit for large-scale
production, but it requires a relatively flat area and more easily becomes contaminated.
In contrast, PBRs cost more for installation and operation, but they produce more algae
per unit reactor and have lower requirements for the installation site.
The algae species are critical to algae production, because different species have
different growth rates and lipid content. Several species, such as Chlorella sp. and
Spirulina sp., have been extensively examined as energy feedstocks. A lot of research
has focused on the cultivation of algae species that have high yield and energy content
or that are capable of growing in harsh conditions, because the productivity of algal
biofuel depends on the biomass yield and lipid/heat content of the algae. However,
algae with higher lipid/heat content normally have lower yields, and improving both
lipid content and yield tends to require greater inputs of energy, nutrients, or other
resources into the system, which increases the environmental burden. Therefore,
algae productivity and electricity use are actually often a tradeoff in algae cultivation
and need to be better understood to produce more biofuel.