Water monitoring
the dissolve phase is to use a process known as air stripping. Governed by ‘Henry’s Law’ of physical chemistry, which states that the volume of dissolved gas in a liquid is proportional to its partial pressure above the liquid, air stripping forces millions of tiny air bubbles of clean air through the groundwater across a high surface area, to vigorously aerate the water to a froth causing the volatile compounds to separate from where they can be removed. This simple but effective process can be used for
the removal of a wide range of VOCs and dissolved gases, including methane, carbon dioxide, and chlorinated solvents, as well as both light and heavy hydrocarbons, such as BTEX, DRO and naphthalene. The technology can even be used to remove contaminants such as MTBE and ammonia, although the latter two are particularly difficult to strip. Each contaminant can be attributed a numerical Henry’s Constant value (H), and the higher the H-value, the more easily the contaminant can be stripped. The biggest difference between air stripping and
simple aeration is use of a counter current flow. In essence, contaminated water enters the top of the air stripper, with clear air being injected from the bottom of the unit. As the contaminated water flows down through the unit, the cleanest air ultimately comes into contact with the cleanest water. Crucially, contaminants are not destroyed during this process, they are simply separated and removed. Traditionally there have been two types of air
strippers: tower and tray-based systems, although the latter are now available in both stacked and sliding trays. Sliding tray-based systems, such as the EZ-Tray Air Stripper from QED, typically allow for an increased water flow capacity which in turn creates a better air-to-water ratio. Such systems are ideally suited for harder-to-strip contaminants. While the process is, in theory, quite a simple one,
there are a number of steps that can be taken to ensure an air stripper is able to operate at its maximum level of performance. For example, the contaminated water in the well would already need to have been pumped for the removal of any free- phase organics prior to being introduced into the Air Stripper. Likewise, there must be a high surface area of contact between the air and the water, along with a high air to water ratio with sufficient contact time between the two mediums. Crucially, the water must also be free of any surfactants or other factors that could lower the Henry’s constant value, and the unit must be kept level at all times. Following the completion of the air stripping
process, the groundwater should then be in a position to meet the mandated discharge criteria for release back into the environment, without the need to be sent to a sewage treatment works. Should any final treatment be needed, the clean water could also be put through a carbon filter to complete the process. Ultimately, contaminated groundwater has the
potential to be one of the challenging elements of remediating a brownfield site. However, in many instances air stripping can help site owners both avoid hefty removal costs and ultimately expedite a major stage of the remediation process.
QED Environmental
www.qedenv.com Instrumentation Monthly November 2021 49
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