Environmental Engineering Reference
In-Depth Information
Commercially available systems using water turbines can transfer the excess pressure to
electricity, and current systems require a low rate of at least 1-1.5 MGD with a pressure
differential of 30-35 psi. This system delivers approximately 40 kW/gal/min [54].
Another opportunity to generate electricity while treating water can be realized
through the use of multichamber microbial fuel cells to desalinate water, treat waste-
water, and generate electricity. These systems work similarly to conventional microbial
fuel cells; bacteria in the anode chamber is used to oxidize substrates while generating
electron low that causes reduction in the cathode chamber; the saline water in the middle
is desalinated as ions are drawn out from each of the adjoining chambers [55]. Still under
development, this scheme has been shown to achieve salt removals of up to 94% for 20 g/l
stock solutions or create a power density of up to 931 mW/m
2
[56,57]. Nanotechnology
plays a key role here in the use of high-eficiency electrode materials and development
of proton exchange membranes (PEMs) for rapid transport faster than conventional sys-
tems comprising PEMs like Naion. In a recent study [58], functionalized porous silicon
membranes were used as the membrane to demonstrate robust H
2
fuel cell operation.
Extension of such technologies to microbial fuel cells can signiicantly assist in increasing
energy density for these systems.
One drawback of microbial fuel cell desalination is that both the anode and cathode
solutions gain salinity during the process. An alternative, the microbial capacitive desal-
ination cell, uses three-chamber microbial fuel cells relying on capacitive desalination.
Here, the cell sections are separated by both ion-selective membranes and active carbon
capture cloth; the irst allows only the correctly charged ions to pass and the ion adsorbs
to the second. These cells, with high-surface-area electrodes, can enhance desalination
eficiency by 7-25× over conventional capacitive deionization methods [41,55,59].
28.7 Alternate Energy Sources
Alternate energy sources such as solar, wind, and geothermal are commonly assumed to
use less water than traditional thermoelectric power generation; however, water needs are
very process speciic and a wide range of consumptions are realized with these power
sources. Solar thermoelectric power uses parabolic troughs to concentrate and transfer
heat to a luid that creates steam in an unired boiler. Much like fossil fuel thermoelec-
tric power plants, water is used to cool these systems at a consumption rate of 770-920
gal/MWh. Solar power towers, which operate at higher temperatures but similar method,
consume approximately 750 gal/MWh [5]. Hydropower, the largest section of renewable
energy produced in the United States at approximately 7%, has a fast response time to
demand, making it a valuable resource for the grid. However, the storage dams for these
systems create evaporation on the order of 4500 gal/MWh from reservoirs [5].
Technologies such as solar photovoltaics and wind do not require water during opera-
tion; thus, water consumption is limited to production and maintenance such as washing.
The long life cycles of these devices make water consumption over the lifetime of the
device relatively small. The main drawback to these devices is the intermittent nature
of the power harvested, low eficiencies, and large land requirements (72 km
2
/TWh for
wind and 28-64 km
2
/TWh for solar; non-dual-purpose land use). Land requirements can
be somewhat mitigated by installation of photovoltaics on building roofs or use of wind
power sites for agriculture [60].
