Page 25 - AW SEPOCT 2019

TECHNO NEWS

Unlike the earlier effort, that used expensive materials hourly exchanges of wastewater effluent from the Palo
as the electrodes, this new MEB is cost-effective. The Alto Regional Water Quality Control Plant and seawater
electrodes in the new MEB are made with Prussian Blue, collected nearby from Half Moon Bay. Over 180 cycles,
a material widely used as a pigment and medicine, that battery materials maintained 97 percent effectiveness
costs less than $1 a kilogram, and polypyrrole, a material in capturing the salinity gradient energy.” The team also
used experimentally in batteries and other devices, which reported that every cubic meter of freshwater that mixes
sells for less than $3 a kilogram in bulk. The materials are with seawater produces about .65 kilowatt-hours of
relatively robust and a polyvinyl alcohol and sulfosuccinic energy – enough to power the average American house
acid coating protects the electrodes from corrosion when for about 30 minutes. If the 68% efficiency achieved in
in contact with seawater. a small prototype MEB can be achieved at full-scale,
Wastewater treatment is a good starting point for the energy produced would be sufficient to meet much
a practical application of the Stanford MEB study. The or even all of the electrical energy demands for a
water treatment process is energy-intensive, accounting conventional wastewater treatment plant.
for about three percent of the total US electrical load. “It is a scientifically elegant solution to a complex
If sufficient blue energy could be generated by an MEB problem,” Dubrawski said. “It needs to be tested at
system, a wastewater treatment plant could be self- scale, and it doesn’t address the challenge of tapping
sufficient and operate off the grid. blue energy at the global scale – rivers running into the
According to the Stanford news release, “The ocean – but it is a good starting point that could spur
researchers tested a prototype of the battery, monitoring these advances.” AW
its energy production while flushing it with alternating

Turning saltwater drinkable using a thin

piece of wood

team at Princeton University in New Jersey have
A developed a new kind of membrane made of natural
wood to filter salt from seawater.
Filtering the salt from seawater can take a lot of energy
or specialised engineering and a thin membrane made
of porous wood is what Jason Ren and his colleagues
used to fix that.
In membrane distillation, salty water is pumped
through a film, usually made of some sort of polymer with
very narrow pores that filter out the salt and allow only
water molecules through.
“If you think of traditional water filtration, you need
very high-pressure pumping to squeeze the water through.
This is more energy efficient and it doesn’t use fossil-fuel
based materials like many other membranes for water
filtration,’’ said Ren This method filters about 20 kilograms of water per
The membrane used is made of a thin piece of square metre of membrane per hour, which is not quite
American basswood, which undergoes a chemical as quick as polymer membranes. The researchers think
treatment to strip away extra fibres in the wood and to that may be because they did not have the equipment to
make its surface slippery to water molecules. One side of make their membrane as thin: it is 500 micrometres thick,
the membrane is heated so that when water flows over whereas the polymer membranes are generally closer to
that side it is vapourised. 130 micrometres thick.
Ren added that the water vapour then travels through Reiterating further, Ren said making the wood
the pores in the membrane toward its colder side and membranes thinner shouldn’t be too hard with the right
leaves the salt behind, condensing as fresh, cool water. equipment.
This takes far less energy than simply boiling all of the “The functional part of the membrane is a micrometre
saltwater because there’s no need to maintain a high thick,” he says. “The rest is just a supporting structure to
temperature for more than a thin layer of water at a time. make it harder to break.” AW

Asian Water SEPTEMBER/OCTOBER 2019 23