Water. desalination + reuse

RESEARCH | 36 | Desalination & Water Reuse | August-September 2014 A team of American and German researchers have reported an electrochemical method for desalination of seawater that uses no membranes and which they claim consumes only a small amount of energy. The system developed by Richard Crooks of the University of Texas at Austin and Professor Ulrich Tallarek of the University of Marburg uses electrolysis to create an ion imbalance between two microchannels that draws other ions through one channel leaving desalinated water in the other channel. The researchers claim the energy required for the technique is so low that it can operate with a simple battery. Sand and sediment needs to be removed from the seawater but disinfection or pre-treatments are not needed. The researchers anticipate that a parallel arrangement of many microchannels will generate the required water throughput for commercial use. The channels are about 22 µm wide. An auxiliary channel and a branched working channel each flow to the outlet and are electrically connected to each other and a power source. The channels and electrode are arranged so that chorine is released from the seawater by electrolysis to create a zone of excess positive ions. The electric field gradient then directs salts and desalinated water along separate channels. The research was supported by the American Department of Energy. Scientists report low- energy, non-membrane desalination technique New research findings have questioned the economics of power production from pressure- retarded osmosis (PRO). A team from Yale University in the USA shows that a typical constant pressure PRO would find it challenging to generate a sizeable amount of energy. PRO exploits a salinity gradient between two water sources - typically using seawater as the draw solution and river water as feed solution. The Yale paper theoretically evaluates membrane modules employing co-current and counter- current flows with different salinity source waters to determine the maximum extractable energy that will establish PRO's viability. According to the researchers, doubts over PRO's economics grow further on considering efficiency losses from reverse draw flux, external concentration polarization, membrane fouling and pre-treatment energy requirements. In December last year Norwegian state-controlled power firm Stadtkraft abandoned its development of PRO - sometimes dubbed osmotic power - after years of research and announcing plans for a large demonstration plant in Norway. Norway was seen as having an ideal combination of freshwater fjords and seawater coastlines for the exploitation PRO. PRO - sometimes dubbed osmotic power - is a membrane process that combines aspects of forward osmosis (FO) and reverse osmosis (RO) to convert seawater's natural osmotic pressure into hydrostatic pressure that can be used to drive a turbine to produce electrical energy. The concept was initially developed and patented by Sidney Loeb, the co-inventor of the RO membrane, in 1973. Researchers cast doubt on economics of osmotic power Scientists look to harness bacteria to bust membrane fouling Researchers at Rice University in Houston, Texas, are investigating how to harness the ways bacteria respond to their environment to make the bacteria perform important industrial applications including removing fouling from desalination membranes. Membrane clogging is caused by thin layers of dead cells called biofilms. And cell death is regulated by so-called two-component signalling (TCS) which the researchers at Rice have found to be the chief way a bacterium detects and responds to its environment. The researchers at Rice believe they can help reduce the build-up of biofilms in desalination equipment. "Since cell death is regulated by two-component and related signalling systems, the potential for controlling the properties of biofilms exists," said Ryan Cheng, a researcher at Rice working on the project. The researchers said freeing desalination membranes from biofilm fouling could be accomplished by introducing to existing biofilms, engineered bacteria that can mechanically weaken them through programmed cell death. Cheng and his colleagues are seeking to understand and control the way a bacterium's genes govern TCSs so they can transform the bacterium to make it perform useful tasks like reducing membrane fouling. "Our research tries to understand and potentially re-engineer two-component signaling systems," said Cheng. "The potential applications for sanitation engineers are both numerous and profound," said senior technologist and biofilm technologies practice leader at US engineering company CH2M Hill, Joshua Boltz. The researchers have a plan to modify the proteins responsible for how bacteria respond to external stimulation, to trigger the bacterium to "decide" what actions to take when confronted with given environmental conditions. Directed bacterial responses, the researchers believe, could revolutionize bacteria-based environmental cleanup, modern desalination and many other medical and industrial applications. The project is co-funded by the US National Science Foundation's Directorates for Biological Sciences and Mathematical and Physical Sciences.

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