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302 Advances in Eco-Fuels for a Sustainable Environment

existing digestate handling and the one-step digestate processing technology, respec-
tively. This annualized capital cost of the existing digestate handling process is about
nine times the annualized capital cost of the HTL-digestate processing alternative
pathway. This result is a reflection of the higher level of system complexity, as illus-
trated by the greater number of processing steps in existing digestate handling pro-
cesses, as discussed earlier.
As expected, the HTL-based one-step digestate processing pathway was shown to
have an energy cost of about 2.4 times the energy cost of the existing digestate han-
dling process. This observation is a reflection of the high conditions of temperature
and pressure typically imposed during hydrothermal liquefaction processes.
The chemical cost, specified as zero, in the HTL based one-step digestate
processing pathway is a reflection of the absence of chemical inputs during the pro-
cess, as discussed earlier in Section 10.3. Given, therefore, that the base-case scenario
estimates were based on a digestate feedstock rate for 5000tons per year, the unit
digestate processing cost for the HTL digestate processing pathway and the unit
digestate processing cost for the conventional digestate processing pathway can be
determined. It can be calculated that the unit processing cost for the existing digestate
handling process, for combined nutrient recovery, and water treatment is US$482.1
per ton of digestate processes while the unit processing cost for the one-step digestate
processing pathway is US$77.7 per ton of digestate.
The economic evaluation presented therefore suggests that the proposed HTL-
digestate processing alternative may be of interest as a cheaper and less complex
digestate processing pathway.

10.4.4 Range of possible values of the unit digestate processing
cost for the alternative pathways
Graphical output of the uncertainty simulation for a 50% to 150% variation in the deter-
minants of the unit digestate processing cost using conventional digestate processing
technology (nutrient recovery+tertiary treatment for comprehensive water purifica-
tion) and HTL processing technologies are shown in Figs. 10.5 and 10.6, respectively.
Based on the simulation output shown in Fig. 10.5, the unit digestate processing
costs using existing digestate technologies will be such that there is 95% certainty that
the unit digestate processing cost will have a mean value US$505 per ton of digestate.
Fig. 10.6 shows that the unit digestate processing costs using the proposed one-step
HTL digestate processing pathway will be such that there is 95% certainty that the
unit digestate processing cost will have a mean value US$80 per ton of digestate.
Our results therefore suggest that based on the mean values of the unit processing
cost, digestate processing utilizing existing digestate technologies for nutrient and
water recovery presents a higher unit processing cost of digestate of 6.3 times the unit
processing cost of digestate when digestate processing is achieved using the proposed
one-step HTL digestate processing. Clearly, these results present a compelling basis
for further investigations into the alternative processing of digestate via the HTL-
based one-step technology.