Page 85 - Materials Chemistry, Second Edition

66 Life Cycle Assessment of Wastewater Treatment

4.5 AN LCA CASE STUDY OF BENEFICIAL REUSE
OF WASTE STREAMS WITHIN A MUNICIPAL
WASTEWATER TREATMENT PLANT

4.5.1 a MeTropoliTan WasTeWaTer TreaTMenT planT
(MWTp) anD iTs WasTe sTreaMs
The Metropolitan Wastewater Treatment Plant (MWTP) is located in St Paul, MN.
It treats 250 million gallons of municipal wastewater per day with 98.9% pollutant
removal efficiency (Metropolitan Council Environmental Services, 2014). The LCA
study was assumed to be conducted within this real-life plant. The major solid waste
flow at this plant is the vast amount of sludge, 265 dry tons daily, collected from the
bottom of primary and secondary tanks. The currently practiced waste management
of the sludge is anaerobic digestion and then landfilling. Besides sludge, the plant
creates approximately 1 million gallons of centrate per day in the sludge dewatering.
Currently, centrate is sent back to the aeration tank and combined into municipal
wastewater. The characteristics of municipal wastewater treated at the plant, cen-
trate, and sludge can be found in Tables 4.1 and 4.2.
As mentioned in Section 4.3.3, the centrate is a good nutrient source for oil-rich
microalgae, which can be converted to algal biodiesel to replace petroleum-based fuels.
The algal biofuel production can benefit not only the environment but also the wastewa-
ter treatment plant by saving energy or compensation by selling biofuels to the market.
Therefore, this study proposed to integrate centrate algal biofuel production into the
existing sludge digestion to produce more bioenergy for the MWTP. In addition, the
study proposed to use digestate as land nutrients instead of dumping it in landfills.
The process integration of the waste treatment system is shown in Figure 4.5. The
sludge collected at the bottom of the sludge-thickening tank was assumed to be con-
verted to biogas through conventional anaerobic digestion. The biogas created was
cleaned and used to generate electricity and heat in turbines. The centrate collected
from the biosolids dewatering was assumed to grow algae in the PBRs. In addition,
the supernatant from digestion and sludge thickening was also assumed to be sent
to the PBRs to grow algae. Algae harvest from the PBRs was assumed to be dewa-
tered in centrifuges and converted to algal biodiesel and glycerol with the wet lipid
extraction technology. The non-lipid algae residual was assumed to be combined
with sludge and digested to recover energy. Finally, the digested sludge and algae
residual was dewatered in centrifuges and transported out for land application.

TABLE 4.2
Characteristics of Sludge from MWTP in St Paul, MN
Higher Lower
Heating Value Heating
Proximate analysis (wt.%) Elemental analysis (wt.%)
(HHV) (MJ Value (LHV)
−1
−1
Moisture Ash Volatile Fix C C H N O kg ) (MJ kg )
4.53 15.01 68.57 16.42 53.24 7.39 6.12 33.25 24.42 21.77