High-Temperature Air Stripping of Groundwater VOCs

A high-temperature air-stripping and liquid-phase carbon-adsorption treatment system was installed at McClellan Air Force Base (Sacramento, CA) to remove volatile and semi-volatile organics from the ground water contaminated by spills and tank leaks.

As diagrammed in the figure, the contaminated groundwater is pumped to a storage tank, which provides flow and waste-strength equalization. The storage tank water is then fed to a series of heat exchangers to increase VOC air-stripping efficiency. The first in the series is a water-to-water plate-and-frame, single-pass heat exchanger; the second is a single-pass fin-tube air-to-water heat exchanger. Thereby the water temperature rises--initially to approximately 95 °F from about 65 °F, then an additional 7 to 10 °F.

From the heat exchanger, the water is pumped, at approximately 270 GPM and an air-to-water ratio of 30:1, to the stripping tower packed with 2-in. plastic-balls. (The original 1-in. tower packing material was particularly prone to calcium and magnesium carbonate fouling.) The liquid effluent then enters a wet well, where it subsequently is pumped to two in-series GAC units to remove the trace quantities of other organic pollutants not amenable to air stripping. The GAC's effluent discharges to a nearby creek.

The stripper off-gas is preheated in two air-to-air heat exchangers in series, where its temperature is brought to approximately 1200 °F before being incinerated at 1815 °F. The incinerator gases recycle to preheat both the stripper off-gas and the groundwater stream to the stripper. Following heating-value recovery of the waste gases, they flow to a caustic scrubber to neutralize hydrochloric acid before being discharged into the atmosphere.

The extreme off-gas temperature combined with the presence of hydrochloric acid create a corrosion problem within the incinerator, resulting in frequent mechanical failures. As parts wear out, they are replaced with components of special metals and alloys.

The facility is continually undergoing modifications. An early correction was to equalize plant flows in an attempt to eliminate downtime when the influent-flow control valve and the stripper level control valve fail. Each valve works independently, but each one senses changes in plant flow and makes the changes necessary to maintain its preset operating level, either by opening or closing the valve.

As for polishing the stripper effluent, the efficiency and economics of the GAC may need to be re-evaluated against an alternative process, such as chemical oxidation.

The foregoing case study is one of several that appears in "Ground-Water and Leachate Treatment Systems," EPA/625/R-94/005.