Environmental Engineering Reference
In-Depth Information
method requires 4.9 gal of water for every kilogram of hydrogen produced (146.8 gal/
MWh) [5]. In recent work [43], it has been shown that ammonia recovery can be achieved
through an innovative, integrated waste management system including wastewater. If
such technologies are implemented, they could signiicantly affect water withdrawal
and consumption, perhaps leading to a change in the paradigm for wastewater being
now “resource water” permitting extraction of valuable industrial chemicals and poten-
tial energy sources.
28.4 Energy Use in Wastewater Management
Effective wastewater management is essential for a number of reasons such as high biolog-
ical oxygen demand, which harms aquatic systems if released untreated, odor, pathogen
health hazards, heavy metal contamination, and high nitrogen and phosphorous levels,
which can cause uncontrolled growth in aquatic systems. Population growth, limited
land and energy resources, and increased concern about emerging contaminants such as
pharmaceuticals as well as traditional pollutants such as heavy metals and pathogens are
motivators for more specialized treatment [44]. Within local government entities in the
United States, water and wastewater treatment is frequently one of the higher energy uses,
and consumes 3%-4% the electricity used annually in the nation [45,46]. Figure 28.11 [47]
displays an urban water cycle as well as energy usages of distribution and treatment. As
discharge requirements become more stringent, population pressure increases, and the
infrastructure ages, the energy cost of such treatment is likely to increase [45]. In addition,
rising electricity rates and enhanced treatment of biosolids, such as drying or pelletizing,
add additional costs to the treatment process [48]. The USEPA estimates that a 10% increase
in eficiency, which can be achieved by infrastructure upgrades and implementation of
energy eficient technologies, could save >10 billion kWh/year leading to $750 million per
year in savings [48].
In a typical wastewater treatment plant, wastewater is treated in a series of steps, gener-
ally primary, secondary, and tertiary, as shown in Figure 28.12. The primary treatment con-
sists of suspended solid removal by screens and sedimentation, which remove 50%-70% of
the suspended solids. Secondary treatment involves coagulation and biological treatment
[44]. For the solids, the inal step is either further break down in a digester and disposal at a
landill, incineration, land applied, or skipping the digester completely and proceeding to
one of the disposal routes stated above. The inal treatment for the water usually includes
disinfection and removal of residual suspended solids. Removal of excess nutrients such
as phosphorus and nitrogen, which can cause eutrophication if released in excess, occurs
in either the secondary or tertiary step, depending on the system, while disinfection is
usually a tertiary step [49]. The water is then dispersed back into the water system by
release into surface waters or percolation beds or other similar technology. The largest
energy consumers in the wastewater treatment process are typically pumping and aera-
tion. For a 10 MGD plant, pumping energy requirements range from approximately 0.6-1.2
million kWh/year based on total dynamic head of 30-60 ft (9.1-18.3 m), respectively, while
aeration treatments require approximately 0.58-2.1 million kWh/year based on secondary
treatment chosen [48].
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