By Dave Fischer
Groundwater tends to require less treatment than surface water. However, contaminants, such as volatile organic compounds (VOCs) from nearby sources like landfills, or chemicals from manufacturing sites, may leach through the ground into groundwater, thereby requiring increased treatment before use.
Some groundwater may contain high levels of dissolved carbon dioxide (CO₂), which causes increased acidity. This acidity can cause water to erode protective coatings on pipes and increase copper and lead levels.
Surface water and reused water, on the other hand, are often more susceptible to precursor contamination like algae and other biosolids. Like all drinking water, surface water and reused water require disinfection. Chlorine is the most common disinfection product because it is inexpensive. However, it can create potentially harmful disinfection byproducts (DBPs) that then require further treatment.
Air stripping, a water treatment technology proven to be effective for groundwater remediation in highly contaminated sites, is gaining traction within the drinking water treatment industry to address issues of VOCs, acidity due to dissolved CO₂, and DBPs.
Fundamentally, air stripping removes or “strips” contaminants by contacting clean air with the contaminated water, causing VOCs and other contaminants to move into the air. There are three different major types of air strippers: towers, stacked trays, and sliding trays. Each of these have benefits and drawbacks, but all utilize the same basic mass transfer process of exposing contaminated water to clean air across high surface areas. This process is governed by Henry’s Law.
The Henry’s Law constant (H) of any dissolved contaminant can be used to predict how effectively that contaminant will be driven from the water into the air. Some contaminants are of course easier to strip than others. Dissolved gases such as methane and CO₂ strip easily, light hydrocarbons less so. Methyl tertiary butyl ether (MTBE) and ammonia are relatively difficult to strip out.
Of the three different types of air strippers, sliding tray design strippers tend to have the greatest advantages and lowest costs. They are less prone to fouling, less intrusive at the site, provide a wider flow turn-down than tower strippers, and provide easier maintenance access and a smaller footprint than either towers or stacking tray designs. The main drawback is that they require a higher pressure blower than tower designs. However, this does not significantly add to the overall cost of operation when factors such as maintenance and materials are also considered.
All types of air strippers can be used for drinking water decontamination. The most clear-cut application in drinking water is in the removal of VOCs, including the trihalomethanes (THMs) that are byproducts of chlorine disinfection. Regulations and/or guidelines both limit THMs at the point of end use and require a “residual disinfection level”, or enough disinfectant to last through the entire system to the end point of use. The “catch-22” of this scenario is that residual disinfection can lead to the generation of additional DBPs even after final water treatment if precursor organics are still present.
As a result, water treatment facilities must reduce the level of THMs enough so that any generated by residual disinfection do not push the final total over the regulatory limit. Air strippers can efficiently and effectively remove THMs to far below regulatory limits, thereby leaving enough “breathing room” for the generation of more THMs caused by disinfection during the water’s path through pipes to its end use point.
Another pressing issue in drinking water treatment currently, given widespread news about lead and copper levels exceeding standards, is ensuring that the pH of water is within reasonable limits. Water, usually groundwater, with too much dissolved CO₂ can be aggressive and therefore dissolve protective coatings on pipes and also dissolve the pipes themselves, leading to excess levels of lead and copper. By stripping excess CO₂ out of the water, plants can effectively increase pH and decrease levels of lead and copper at the point of end use.
QED’s sliding tray air stripper, the E-Z Tray, was the first self-contained air stripper to achieve certification from NSF International to NSF/ANSI Standard 61: Drinking Water System Components – Health Effects. This particular design of air stripper has been employed in a variety of drinking water treatment facilities to address the issues described above at a low cost and with minimal impact to the facilities.
Air stripping technology has proven to be an effective process not just for groundwater remediation, as it has been used in the past, but also for the treatment of both groundwater and surface water sources of drinking water.
Air stripping effectively removes VOCs, including DBPs such as THMs from chlorine disinfection, and dissolved gases like CO₂ in the case of overly acidic water sources. In doing so, it reduces treatment costs over competing treatment options and ensures adherence to regulations. It is therefore a treatment option for these sources of contamination in small and medium-sized drinking water treatment plants.
Dave Fischer is with QED Environmental Systems, Inc. This article appears in ES&E Magazine’s August 2019 issue.