Environmental Engineering Reference
In-Depth Information
Pump the wastewater through the plants.
●
Create enough aeration for the aerobic bacteria to develop and degrade the organic matter.
●
Separate the solids created during bacterial fermentation by sedimentation.
●
Remove scum (i.e., oils, grease, soap, and plastics) by flotation.
●
In the end, before discharged into rivers or the ocean, the treated water needs chlorination
followed by neutralization of the chlorine. Solids from the sedimentation step are further
treated in heated digesters and sent to landfills or used as fertilizer.
Electricity consumed in activated sludge plants depends on the volume of wastewater as
well as the solid content that is rated through the biological oxygen demand. Larger volumes
require more pumping and higher solid content needs more oxygen that is translated into
intensification of the aeration process. In activated sludge systems, about two-thirds of the
electricity is used to provide aeration. Energy used per unit of biological oxygen demand is
variable. Just as a reference, municipal wastewater treatment plants in the state of New York
have a consumption of electricity that varies between 1.2 to 3.8 kWh per lb of biological
oxygen demand removed (Yonkin et al., 2008). Reports for the San Francisco area indicate
uses between 0.4 and 2.6 kWh per lb of biological oxygen demand removed and a total
consumption of electricity in the range of 508 to 2,428 kWh per million gallons (Pacific Gas &
Electric [PG&E], 2003).
Emissions from aerobic wastewater treatment
Aerobic wastewater treatment systems produce direct and indirect emissions of GHGs and
environmental pollutants. Indirect emissions, mainly in the form of carbon dioxide, come
from purchased electricity. The magnitude of the indirect emission will depend on the
treatment plant and the method to produce the electricity (see Chapter 8). The indirect
emission of carbon dioxide per pound of biological oxygen demand removed can be calcu-
lated by multiplying the consumption of electricity times the emissions to generate the
electricity in that particular region. As an illustration, take an average consumption of elec-
tricity of 2.5 kWh per pound of biological oxygen demand removed and an emission of
0.466 kg/kWh (from Table 8.7); then the indirect GHG emissions expressed as units of
carbon dioxide equivalent per pound of biological oxygen demand removed (CO
2
-eq/lb BOD)
is calculated as:
kWh
lb of BOD
kgCO eq
.
kgCO eq
.
CO -eq/lb BOD
=
2.5
×
0.466
2
=
1.165
2
2
kWh
lb of BOD
Indirect emission of pollutants during electricity production includes nitrogen oxides, sul-
fur dioxide, particles, among others.
Direct emissions of GHGs (carbon dioxide, methane, and nitrous oxide) come from the
treatment process. Even in aerobic systems, methane is formed as a result of the uncontrolled
anaerobic fermentations in the system. It is estimated that methane produced at wastewater
treatment plants accounts for 5 percent of the total methane emissions (El-Fadel and Massoud,
2001). Nitrous oxide is formed during the nitrification (aerobic) and denitrification (anaero-
bic) processes of the nitrogen present in the wastewater. And the formation of nitrous oxide
depends on the amount of protein available in the wastewater. In the case of municipal waste-
water, the formation of nitrous oxide is contingent to the diet of the population. In places with
high consumption of protein, emissions of nitrous oxides will be higher. Non-GHG direct
