Page 88 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 69

TABLE 4.3 (CONTINUED)
Process Design Parameters for the Integrated Waste Reuse
System in the MWTP
Unit processes Assumptions References
Algae residual Digestion process is used in recycling energy in
treatment residual
Use A passenger car with diesel engine is modeled, GREET Model
28.08 mpg
Sludge digestion and application
−1
Digestion Digestion: CH 4 yield: 0.3 L g TSS; CH 4 in Clarens et al. (2011),
−1
biogas yield: 0.67 vol vol biogas; electricity GREET (2015)
−1
−1
use: 0.11 kWh kg TSS; heat use: 2.5 MJ kg
TSS
Biogas clean: electricity: 0.25 kWh m biogas;
−3
CH 4 emissions: 3%; electricity generation: elec.
conversion efficiency: 0.33; heat generation:
0.309
Digestate handling: CH 4 emission: 2%; N 2 O
−1
emission: 0.14 kg N 2 O kg N
Energy generation Electricity generation: 0.331 MJ MJ fuel; heat: GREET (2015)
−1
−1
0.309 MJ MJ fuel
Land use Fertilizer replaced: urea: 0.25 kg N kg N in
−1
digestate; DAP: 0.07 kg P kg P in digestate
−1
When system models were set up in LCI, the diesel produced from algae was
assumed to be used for plant vehicles; therefore, the diesel distribution was not
accounted for. Because the allocation method of waste flows has not been widely
accepted in current literature, this study still treated all waste as free or zero bur-
den. The biodiesel and bioelectricity produced from algae and sludge can replace
petroleum-based fuels, whereby their impacts are treated as credits subtracted from
the total impacts. Similarly, a wastewater treatment credit was accounted for with
algal biofuels, as some nutrients in centrate were removed during algae cultivation.
The calculation of saving wastewater treatment was based on the assumption that
the total carbon in wastewater was 0.0637 kg m (Ecoinvent 3). Consequently, every
−3
3
1 kg of carbon removed in centrate was equivalent to treating 15.7 m of regular
wastewater. In addition, the application of digestate to land created credits to replace
the use of synthetic fertilizers, including urea and Diammonium phosphate (DAP).
These credits were assigned to the digestion technology. Finally, the algae residual
was sent to sludge digestion for energy conversion; as a result, all the credits for elec-
tricity generation and fertilizer replacement were assigned to digestion.
The life cycle impact assessment (LCIA) only included major environmental
impact categories for analysis. They were fossil fuel use (MJ primary), GHG emissions
(kg CO eq.), terrestrial acidification (kg SO eq.), aquatic eutrophication (kg N eq.),
2
2
territorial ecotoxicity (kg triethyelene glycol soil), and carcinogens (kg C H Cl eq.).
3
2