Page 76 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 57

terrestrial toxicity. Cao and Pawlowski (2013) conducted an LCA of pyrolysis to treat
digested sludge versus nondigested sludge, and found that the environmental perfor-
mance of treating digested sludge was better than that of treating nondigested sludge.
Mills et al. (2014) conducted an LCA comparing sludge treatment technologies and
concluded that adding pyrolysis to further recycle energy in digested sludge improved
the environmental performance of regular AD technology. Buonocore et al. (2016)
analyzed the environmental impacts of gasification of digested sludge with the LCA
tool. The results showed that adding gasification to treat digested sludge improved
the environmental performance on all impact categories examined, particularly on
eutrophication and human health potential.

4.3.3 scuM To bioDiesel
Scum has a high oil content, and there is scope for a technology to convert the lipids
into bio-oil. Researchers from the University of Minnesota proposed and conducted
an LCA of a technology that converts scum to American Society for Testing and
Materials-grade diesel. Figure 4.1 shows the dried scum and biodiesel produced
from scum. Figure 4.2 shows the pilot refinery machine of Scum-to-Biodiesel. The
technology proposes a six-step energy conversion pathway: Filtering, acid washing,
acid-catalyzed esterification, base-catalyzed esterification, and finally, refining (dis-
tillation). These steps are described in the following list.

1. Filtering: In this process, larger particles are filtered out from the raw scum
flow. The scum is heated to 60°C and filtered for 1 h. In this way, 95 wt.%
of scum oil can be filtered out. The water separated from this process can
be used to dilute acid in the following acid wash processes. The solid waste
is filtered out and sent to the combustion or digestion process to recover
additional energy. Energy is consumed for heating scum.
2. Acid wash: The scum flow is then washed by 5% H SO solution to convert
2
4
soap present in scum into FFA. The process aims to maximize the biodiesel

(a) (b)

FIGURE 4.1 Scum-to-Biodiesel technology. (a) Scum collected from Metropolitan
Wastewater Treatment Plant at St Paul, MN. (b) Final product (biodiesel).