To comply with the International Maritime Organizations Marine Pollution Convention (MARPOL) Annex V and other environmental regulations, U.S. naval vessels require compact, energy efficient water purification technology that will allow most of the wastewaters produced onboard (bilge, gray, black, etc.) to be discharged overboard following purification. Membrane filtration does not achieve the degree of purification required, and a final “polishing” process is needed prior to discharging wastewaters overboard.
The overall objective of this project is to advance the development of an electrochemical advanced oxidation process (AOP) which will be used as a final polishing step following membrane filtration of shipboard wastewater. The specific objectives of this project include producing AOP electrodes that demonstrate improved service life and improved performance at low substrate concentrations, developing methods for reprocessing the electrodes, and identifying optimal operating conditions for the AOP.
Existing equipment for producing small test electrodes in the laboratory has been upgraded. An apparatus permitting longterm testing of the electrodes has been built, and a correlation of service life versus current density will be determined. Tests will be developed to evaluate the kinetics of different oxidation mechanisms for several substrates. X-ray diffraction, scanning electron microscopy, and specialized surface analyses will be used to characterize the crystal structure, surface morphology, and surface composition of the electrodes. Improvements in the electrode coating process have decreased the brittleness of the porous anodes produced and, thereby, allow reprocessing of spent electrodes at a fraction of the replacement cost.
Two small baking chambers have been constructed, allowing complex electrode production procedures to be simulated and perfected in the laboratory. The first long-term test of electrode life has been completed. Kinetic data has been collected for several test substrates that react with hydroxyl by different reaction mechanisms, and the necessary analytical methods have been demonstrated. A series of titanium-fiber samples representing different stages of the electrode coating process have been provided to a specialized outside laboratory for X-ray diffraction and surface analytical studies. These data revealed that the precoating process affects different fibers differently. The electronics needed for improved measurements of the electrical properties of the oxide coat have been assembled and are now operational. A device for measuring the brittleness of the coated fibers by repeated bending until failure has been built and is in use. A coating procedure has been developed successfully that provides good current yield values. At this time the attainable values of current yield are no longer limited by the quality of the oxide coat, and process operating conditions that favor good current yield have been identified.
In combination with improved membrane filtration technology, electrochemical AOP will allow existing ships to be retrofitted for compliance with MARPOL Annex V and other regulations. Cost savings are estimated at $1.49 billion (an estimate of the cost to off-load untreated wastewaters) over 20 years.