Page 81 - Materials Chemistry, Second Edition

62 Life Cycle Assessment of Wastewater Treatment

to 20–30 wt.%. The algae–water separation stage is an energy-intensive stage.
Electricity use causes major environmental impact.

4.3.4.2 Algal Conversion Stage
All technologies that can covert biosolids into energy products can be used for
converting algae. These technologies are divided into two groups. One is based on
biochemical conversion, including anaerobic digestion, fermentation, and so on.
Another group is thermal chemical conversion, including pyrolysis, combustion,
gasification, hydrothermal liquefaction, and so on. Many of these technologies will
be described in Sections 4.3.1 and 4.3.2. Factors that influence the selection of the
conversion process to use include the type and quantity of algae, the desired end
products, investment and operation costs, and site conditions.
In addition, a technological pathway can use chemicals such as hexane to extract
the lipid content out of algae and then convert the lipids to biodiesel through a trans-
esterification process using methanol. Glycerin is co-produced with diesel in this
pathway. Pressure homogenization is used as a pretreatment before lipid extraction
to break the algal cell walls and facilitate lipid extraction. The technical pathway
is also called wet lipid extraction, as no intensive drying process is applied. It is a
frequently modeled technology for algal biofuel production in current literature. The
diesel yield of this technological pathway depends on the lipid content of the algae.
The residue of algae from liquid extraction could go through an anaerobic digestion
process to recycle energy remaining in the residue. The biogas collected from the
digestion is used to generate electricity for plant use. The digestate can be used as
soil nutrients.

4.3.4.3 Algal Bioproduct Use Stage
The products from the various conversion technologies include electricity; sol-
ids (char, digestate); gas (methane, syngas); and biofuels (diesel, petroleum, and
ethanol). They can be used for multiple purposes inside the plant or sold to outside
markets. Some products can be used to generate heat or electricity. Some could be
used as transportation fuels. Others can be applied to land for soil nutrients. Based
on the study by Clarens et al. (2011), the nutrients contained in the digestate are
around 40 mg N g digestate and 12 mg P g digestate. Nutrient use efficiencies
−1
−1
are approximately 25% and 7% by weight for N and P, respectively. In the use stage,
environmental impacts can be increased.
Lastly, beneficial reuse of waste streams in wastewater treatment facilities still
faces many uncertainties and challenges. For example, the composition of wastewa-
ter created in a plant varies by source, which increases the difficulty and uncertainty
of algal biomass cultivation. Furthermore, the nutrient profile of wastewater from
some sources may render it unsuitable for algae cultivation. For example, the pres-
ence of inhibitors can result in poor nutrient assimilation and significantly reduce
algal biomass productivity. In addition, the characteristics of wastewater-derived
algae are quite different from those of freshwater algae, which leads to variation in
biofuel yields, resource use, and environmental impacts from algal biomass conver-
sion. The performance of algal biofuels depends on which conversion technology