Page 339 - Advances in Eco-Fuels for a Sustainable Environment

294 Advances in Eco-Fuels for a Sustainable Environment

_ E input ¼ _ E is + _ E it (10.22)
the energy cost per year (E c ) is estimated as follows,
_
E c ¼ 7200 u input E input (10.23)
where u input is the unit cost of electricity in US$ per kJ.
To estimate the energy required for phase separation for nutrient recovery operation, the
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unit energy cost required per m of the digestate is specified as 22.5kWh/m , according to
the work of Engeli and Edelmann [73]. The energy required for phase separation was there-
fore calculated as follows,
_
E is ¼ 3600 22:5 _ v d (10.24)
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where _ v d is the volumetric flow rate of the digestate in m /h and is given by,
_ m d
_ v d ¼ (10.25)
ρ d
Here, _ m d is the mass flow rate of the digestate in kg/h and ρ d is the average density of
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digestate specified as 1020kg/m .
Given that the average specific energy duty required in tertiary water treatment was
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reported to be 0.49kWh/m in a typical wastewater treatment plant in New Zealand [74]
and assuming complete water recovery and negligible losses in the water content of the
digestate, the energy duty value for the tertiary treatment operation was determined as
follows,

_ E it ¼ 3600 0:49 _ v w (10.26)
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where _ v w is the volumetric flow rate of water in m /h and is evaluated by

z w,d _ m d
_ v w ¼ (10.27)
ρ w
where in this relation, _ m d is the mass flow rate of the digestate in kg/h, ρ w is the water density
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specified as 1000kg/m at 25°C, and z w,d is the fractional moisture content of the digestate
specified as 0.9698.
Based on the duties determined, the electrical energy cost per kWh specified as
US$0.0681 per kWh by EIA USA was utilized in estimating the total electrical energy
cost [70].
Depreciation cost, overhead cost, repair and maintenance costs
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For consistency in the estimation approach and to avoid calculation bias, the costs due to
repairs and maintenance, depreciation cost, and overhead cost were estimated using methods
described in earlier sections.
At this juncture, it is crucial to emphasize that the methods highlighted in the comparative
economic assessment are largely approximate. The estimation methods are, however, log-
ically presented and justified using our knowledge of basic chemical engineering plant
design methods. It is therefore anticipated that the results obtained will be sufficiently robust
to achieve the objectives of this preliminary study.