U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

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From Ground Water Currents, September 1995, Issue No. 13

Update on Zenon Pervaporation

By Ron Turner, EPA National Risk Management Research Laboratory

The ZENON cross-flow pervaporation system employs a membrane to remove volatile organic compounds (VOCs) from aqueous matrices contaminated with liquids such as solvents, degreasers and gasoline. VOCs with water solubilities of less than 20,000 parts per million (ppm) are generally suited for removal by pervaporation. Pervaporation provides an alternative approach to treating organic-contaminated water at sites where conventional air stripping or carbon adsorption are under consideration for use. Unlike air stripping, pervaporation releases negligible amounts of VOCs to the outside air. Unlike activated carbon, the treatment medium does not require frequent replacement and disposal. Periodic cleaning of the membranes is necessary to maintain the treatment efficiency. The full-scale ZENON system can be easily transported and requires only an electrical source.

ZENON was evaluated during a SITE (Superfund Innovative Technology Evaluation) full-scale demonstration at a former waste disposal area at Naval Air Station North Island in San Diego, California in February 1995. The demonstration was conducted as a cooperative effort among EPA, ZENON, the Naval Environmental Leadership Program, Environment Canada and the Ontario Ministry of Environment and Energy. Long-time readers of GROUND WATER CURRENTS may recall that we first reported in our June 1994 issue (EPA 542-N-94-005) the good results from the Canadian government's pilot scale demonstration of Zenon's pervaporation process. At Naval Air Station North Island the ground water contained elevated concentrations of trichloroethylene (TCE), as well as other contaminants. The TCE influent concentrations were up to 250 ppm. The Zenon pervaporation system removed about 98% of the TCE at a flow rate of 10 gallons per minute or less. The average TCE concentration in the treated water was approximately 1,380 micrograms per liter. Air releases were minimal.

The ZENON membrane material is a nonporous organophilic polymer, such as silicone rubber, formed into capillary fibers measuring less than one millimeter in diameter. Silicone rubber is highly permeable to organic compounds and resistant to degradation. The capillary fibers are aligned parallel on a plane and spaced slightly apart. This arrangement of fibers forms one membrane layer. Separate membrane layers are aligned in series, with the interior of the capillary fibers exposed to a vacuum (about one pound per square inch, absolute). The number of membranes used in a particular system depends on expected flow rates, contaminant concentrations in the untreated water and target concentrations for contaminants in the treated water.

For the demonstration, contaminated water was pumped from an equalization tank through a prefilter to remove debris and silt particles and then into a heat exchanger that raised the water temperature to about 165 degrees F (75 Degrees C). The heated contaminated water then flowed into a perva-poration module that contained dense polymeric membranes. The organophilic composition of the membrane caused the organics to adsorb to the membrane (capillary fibers). The organics migrated to the interior of the capillary fibers and were then extracted from the membrane by the vacuum. Contaminated water that passed through the pervaporation module was depleted of organics and exited the ZENON system for sewer discharge. The organic vapor (called permeate) that adsorbed to the membrane and which was extracted by vacuum was condensed and vented downstream of the condenser, thus minimizing air releases.

The condensed liquid permeate, containing highly concentrated organic compounds and significantly reduced in volume, generally separates into aqueous and organic phases, rendering the organic fraction potentially recoverable. The organic phase permeate is pumped from the containment vessel to storage, while aqueous phase permeate can either be returned to the pervaporation module for further treatment or removed for disposal.

For more information, contact Ron Turner at EPA's National Risk Management Research Laboratory at 513-569-7775. Detailed information on the technology's capabilities and the results of the ZENON SITE demonstration will be discussed in the forthcoming SITE Technology Capsule (Document No. EPA/540/R-95/511a) and the Innovative Technology Evaluation Report. To get on the mailing list for these documents, send a FAX request to Ron Turner at 513-569-7787.

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