A project to reconfigure the East Trenches Plume Treatment System (ETPTS) at the Rocky Flats site, to improve treatment effectiveness and meet the strict water quality standards in the area, is scheduled to be completed in January 2015. The ETPTS was installed in 1999. The system was designed to intercept and treat groundwater contaminated with chlorinated solvents, chemicals commonly used in dry cleaning and to clean tools. At Rocky Flats, the solvents were used primarily as degreasers and lathe and machining coolants. During the 1950s and 1960s, a common disposal practice around the world for these chemicals was to bury them in trenches dug into the ground. Rocky Flats followed this practice at that time, which led to the groundwater contamination at the site referred to as the East Trenches Plume.

The ETPTS includes a deep trench with a drainpipe along its floor, which intercepts the plume as contaminated groundwater flows toward nearby South Walnut Creek. The collected groundwater is routed to two treatment cells, which remove the contaminants from the water. The treated water is then released back into the subsurface.

The system was originally designed to operate without any electrical or other energy assistance. Gravity would carry the contaminated waterflow into the trench, then into and through the treatment cells. The two cells were filled with zero-valent iron (iron filings created from cast iron). When the solvent molecules came in contact with the iron filings, chemical reactions stripped the chlorine atoms off and gradually transformed the chemicals into harmless byproducts.

The system was not designed to remove all traces of the contaminants, but to reduce their concentrations and lessen the impact on South Walnut Creek. Even though the original level of treatment was protective of the environment, the post-closure regulatory agreement for Rocky Flats now requires that water issuing from the system meet much stricter quality standards, which couldn’t be achieved using the old system.

One of four groundwater treatment systems at the Rocky Flats site, the ETPTS is similar to the nearby Mound Site Plume Treatment System (also located at Rocky Flats), and employs similar designs and objectives. Both systems treat chlorinated solvents using iron filings, and both include groundwater intercept trenches and treatment cells. The Mound system was also designed to reduce contamination, but like the ETPTS, it now must treat the water to meet very strict standards.

Several tractor-trailer loads of iron are used in each of the systems to treat the water. The media gradually becomes ineffective, and after a period of 3 to 5 years, it becomes a solid block of rust and other minerals. This solidified block then needs to be chipped out of the treatment cells (which are made of plastic, so they’re more fragile than concrete tanks) and hauled away for disposal. Due to the past nuclear weapons-related mission of Rocky Flats, the spent media often needs to be disposed of as low-level radioactive waste at a facility in Clive, Utah. This entire process makes using iron to treat Rocky Flats groundwater costly, even though the treatment process itself may be passive.

Working to improve treatment effectiveness and meet the strict water quality standards set by the U.S. Environmental Protection Agency (EPA), a different approach to water treatment was tested. The new method is called air stripping, and it is appropriate for chlorinated solvents.

Air stripping works by moving volatile contaminants from the groundwater into the air either by spraying the contaminated water through air or by blowing air through the water. Once in the air, the contaminants are quickly dispersed and degraded, and are at such low concentrations that air permits are not required. Tests at the Mound system used nozzles to spray water that had already been treated by the iron media (Figure 1). This approach proved to be effective at removing residual contaminants from the water.

Image
Figure 1. The first tests at the Mound system (above) used a small garden fountain pump
attached to landscaping mist nozzles, with an air monitor to detect whether the contaminants
were moving from the water to the air. The test was performed in a plastic box. The next
test (below) used a larger pump and spray nozzle. Both showed that air stripping removed
contaminants from the groundwater, and led to a larger air stripper at the Mound system
(not shown).
Image


After successful, longer-term testing at the Mound system, air stripping at the ETPTS was tested. However, in this case the air stripping process was used to treat groundwater that had not yet been treated by the iron filings (Figure 2). This was intended to evaluate whether air stripping might completely replace the iron-based treatment. Even though the extremely hard groundwater at Rocky Flats caused great accumulation of scale, air stripping still proved to be effective.

Air stripping is not a new technology. However, unlike allowing gravity to push water through a tank filled with iron filings, air stripping does require a substantial amount of energy to power the pumps, blowers, and other components. Since Rocky Flats has no electrical utilities, only small solar facilities, the air stripping approach required much more careful consideration than would be necessary if the air stripper could just be plugged into a nearby electrical source.

Image
Figure 2. Air stripping at the East Trenches Plume Treatment System focused on water that
was not yet treated using iron filings. The spray nozzles (above) are located in the gray
manhole next to the solar array shown in the photo below. The solar panels charge batteries
located inside the conex, which power the pump.
Image


Two main factors helped to suggest air stripping might be feasible as a stand-alone, complete treatment method, despite the lack of utilities at the site. The cost of solar energy components has decreased sharply over the past several years, and the amount of water intercepted and treated by the treatment system is so small that the facility could be operated on a part-time basis. A solar power facility had been installed to support the initial air stripper installed at the East Trenches system, and with very little additional power to help provide a safety margin, we could confidently power an air stripper if it could be limited to operating a few hours every day.

Air stripper manufacturers were contacted to evaluate how feasible this approach might be. Water quality data, including basic information like temperature and pH, as well as details including concentrations of the contaminants and hardness levels, were provided. Additional information included water flow rates, treatment requirements (the standards to which the discharged water would be held), location of the unit, and specifications of the existing solar power facility. One firm responded favorably, confirming that existing power would be adequate to drive a commercial air stripper for several hours of daily treatment of intercepted water, and meet EPA water quality standards. In addition, the air stripper would not need to be heated (a concern in Colorado winters), and managing the hard-water scale should not require more than normal effort.

The project to reconfigure the ETPTS for operating by air stripping rather than iron-based treatment began in late 2013. Designs were finalized in early 2014 and fieldwork began this past summer. The two plastic tanks that previously held iron filings will be repurposed. One will be used to accumulate untreated water (air stripper influent), and the other will accumulate and slowly discharge water that has been treated by the air stripper (effluent). The air stripper will operate for several hours each day (longer when flows are higher and more groundwater has accumulated in the influent tank, and shorter when conditions are drier). It will be housed in a small, insulated shed installed into the ground to benefit from geothermal heat; it will also have a small solar-thermal array on its roof to lessen the effects of winter temperatures. The system will incorporate automation to reduce the potential for undesirable events, such as overflows, dead batteries, frozen components, etc.

The reconfigured ETPTS is scheduled to be completed in January 2015.

Image
Figure 3. The new, commercial air stripper is shown in the photo above, and will be housed
in the shed shown in the photo below. The roof of the shed is angled to provide better
solar-thermal exposure. The solar/battery conex from Figure 2 is visible in the background to
the left of the shed.
Image