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278 Advances in Eco-Fuels for a Sustainable Environment

however, leads to the generation of associated digestate byproducts that must be man-
aged. This is because the digestate byproducts are characterized by both high moisture
and low dry matter content, which may lead to high transportation and storage costs.
Crucially, the current management of the traditionally high moisture content digestate
streams requires a complicated and costly system that emphasizes digestate treatment
prior to its disposal to the environment rather than value extraction from the digestate
[3]. Digestate treatment is necessary to minimize any exposure of livestock and
humans to zoonotic agents [4] and unwanted antibiotic residues [5] that may be
retained in the digestate after the anaerobic process is completed.
A literature review shows that the existing digestate handling pathway incorporates
processes that facilitate nutrient recovery via nutrient retention in the solid fraction of
the typically high moisture digestate [6]. The nutrient-rich solid fraction of the
digestate that is recovered is utilized as fertilizer [6]. In countries with extensive envi-
ronmental pollution mitigation laws, the treatment of the liquid phase of the digestate
is considered mandatory prior to the discharge of the liquid phase to the surrounding
water bodies, thus further complicating the digestate management process [6].
Clearly, the process utilized in existing digestate processing systems is rather complex
compared with the proposed alternative hydrothermal liquefaction (HTL)-based one-
step digestate processing operation, which we have suggested as a viable intensifica-
tion technology that will simplify existing digestate management methods. Digestate
processing using the proposed HTL-based, one-step digestate technology has the
capacity of being economically cheaper because the HTL of digestate will facilitate
the sterilization of the digestate stream while also enabling the production of useful
product streams via a one-step process. A simplified illustration of the proposed HTL-
based, one-step digestate processing technology compared to the existing digestate
processing technology is presented in Fig. 10.1.
In Fig. 10.1, the existing digestate processing pathway for nutrient recovery incor-
porates an initial solid-liquid phase separation operation for solid and liquid recovery
from an acidified digestate residue. The acidification of the digestate is typically
undertaken to facilitate a reduction in pH to avoid the possible unwanted release of
NH 3 gas during additional downstream processing steps [7]. Solid-liquid phase sep-
aration operations may require the utilization of some form of membrane technology
[8] to enable a highly efficient phase separation. After separating the liquid and the
solid phases, the recovered solid fraction, may be utilized safely on agricultural lands
as fertilizer, after suitable stabilization operations such as drying to produce pellets or
aerobic digestion to produce compost have been undertaken [9]. Additionally, in
regions of the world with strict environmental pollution regulations, the separated liq-
uid phase may be transported to a wastewater treatment plant for further processing to
reduce the possible high concentration of mineral elements such as phosphorous and
sulfur and also to completely eradicate zoonotic microbes [10]. The treatment of the
separated liquid phase may incorporate complex processes such as ultrafiltration and
advanced oxidation [10]. After the liquid treatment operation, the treated water can be
utilized safely on agricultural lands or released to the surrounding water bodies.
On the other hand, the proposed HTL of digestate will incorporate a single trans-
formation step with product separation achieved simply by exploring the