VOCs For Non-Scientists: Understanding, Detecting, Removing
Volatile organic compounds (VOCs) are a large class of substances that may be found in various water sources. Many water quality managers and operators at water treatment plants (WTPs) know which substances are considered VOCs but not all understand the chemistry behind the designation. What follows is a straightforward guide to understanding VOCs, specifically, what qualifies a compound as a VOC, how to detect them, and how to treat and remove them from the water system.
What Is A VOC?
As the name implies, a VOC is defined primarily by two things. The first is that the chemical is volatile, which means that it easily changes state from liquid to gas with a relatively small amount of energy. Most VOCs have a comparatively low molecular weight, which is one of the reasons for this volatility. The other key defining factor is that it is organic, meaning the molecule is composed primarily of carbon atoms.
Most VOCs are manmade products, although a few, such as acetone, are also naturally occurring. That means nearly all VOCs end up in the water supply via industrial processes, chemical spills, or other human activity.
Nearly all VOCs are manmade.
Half come from industrial processes, 45% from motor vehicles, and 5% from consumer solvents.
VOCs in water are analyzed for using the U.S. EPA method 524.2. This process involves purging potential contaminants from a water sample using inert gas then desorbing them into a capillary gas chromatography column connected to a mass spectrometer.
Federally regulated VOCs are listed under the Safe Drinking Water Act (SDWA). Of course, each individual state may have its own list of regulated VOCs beyond those in the SDWA.
For regulated VOCs, the EPA and state agencies set two levels for each chemical: a maximum contaminant level (MCL) and a maximum contaminant level goal (MCLG). The MCL is the largest permissible concentration of the chemical in water, while the MCLG is the concentration at which there is no known or expected health risk.
VOC Types And Properties
Not all VOCs behave the same. There are three sub-classifications of VOCs based on their boiling points:
- VVOCs (very volatile organic compounds). With boiling points of <0°C to 50-100°C, many of these exist solely in a gaseous state. Examples include butane, propane, and trichloromethane.
- VOCs (volatile organic compounds). Although the term VOC is often used to describe chemicals from all three subcategories, technically it applies only to those with boiling points in the 50-100°C to 240-260°C range. Some examples are ethanol, acetone, and vinyl chloride.
- SVOCs (semi-volatile organic compounds). The least volatile subclass is defined by boiling points from 240-260°C to 380-400°C. Phthalates, many pesticides (including DDT), and nitrobenzene are some such examples.
VOCs can be further categorized as either hydrophobic (repel water) or hydrophilic (attract water). Hydrophobic VOCs (e.g., benzene) usually have smaller molecular weights and do not dissolve easily in water, which makes them relatively easier to move to a gaseous state. By contrast, hydrophilic VOCs (e.g., acetone) tend to have higher molecular weights and are more easily dissolved in water, which makes them relatively harder to move into a gaseous state.
Removing VOCs From Water
There are two primary methods for removing VOCs from source water.
Air Stripping. The process of forcing air through water works well on VOCs with lower boiling points (especially VVOCs) and/or those that are hydrophobic. This includes chemicals such as vinyl chloride, methyl chloride, chlorofluorocarbons, and methane.
Activated Carbon. Higher-molecular-weight VOCs won’t be as responsive to air stripping. For these chemicals, a granular activated carbon (GAC) filter is a more effective solution. The activated carbon can adsorb most VOCs, including those that are more difficult to remove via air stripping. Because VOCs diffuse quickly through the carbon bed, however, it is important to ensure the carbon has a high iodine number. Usually, about 1,000 to 1,100 is ideal to reduce the number of changeouts.
Establishing A Buffer For VOCs
It’s rare for water treatment plants to discover new VOCs in their source water. Most water systems are well established, and the challenge is less about tackling a previously unencountered chemistry but rather struggling to meet established and new MCLs.
That said, chemical spills can happen anywhere. For example, a city that pulls from a riverway with heavy boat traffic is always susceptible to some type of spill, as anything that is on a boat can end up in the water. Even chemical spills on land, such as a tipped gas tanker, can result in VOCs in groundwater. A GAC system that is in place for everyday VOCs will act as a buffer, ready to adsorb new contaminants should they enter the source water.
Source: Calgon Carbon Corporation