Though drinking water tends to be relatively clean compared with water that
is being considered for other uses, it sometimes can contain contaminants.
Air stripping, a proven water treatment technology primarily used in the past
for groundwater remediation in highly
contaminated sites, is gaining traction
within the drinking water treatment industry. Air stripping has been proven to be
highly effective for the removal of Volatile
Organic Compounds (VOCs) and disinfectant byproducts (DBPs), as well as for pH
adjustment through removal of dissolved
CO2. It is gaining in popularity for the
treatment of drinking water thanks to its
efficiency, simplicity, and low cost per
gallon in treatment, as well as its efficacy
in helping to achieve finished water quality
DRINKING WATER TREATMENT
Drinking water sources have been designated as such because they have been deemed
clean enough for human consumption.
Of course, this does not mean that these water sources are clean enough for
consumption without treatment; indeed, strict standards are in place across the
U.S. to ensure proper treatment of water before it reaches consumers. The most
important treatment targets for drinking water are biohazards, and so drinking
water must be disinfected. Other important concerns addressed by drinking
water standards are the presence of Volatile Organic Compounds (VOCs), disinfectant byproducts (DBPs) such as the carcinogenic trihalomethanes (THMs)
left behind by the oxidation of organic materials during chlorine disinfection,
and water pH. These concerns often result in violations of standards put forth
by governmental regulations, particularly in small and medium-sized treatment
Drinking water may come from either ground water or surface water sources,
and each of these classes of water present their own challenges for treatment.
In addition, a decade of near drought conditions in California and Texas have
driven the implementation of water re-use approaches. Highly treated wastewater is now used to supplement and recharge some source waters for drinking
use. If this water is used to recharge a groundwater source, it will have precursor organic levels closer to a surface water, leading to treatment needs that are
somewhat of a hybrid of both types.
Groundwater tends to require less treatment than surface water; however,
contamination (of VOCs in particular) from nearby sources like landfills or
chemicals from manufacturing sites may leach through the ground into groundwater, thereby requiring increased treatment. Some groundwater may contain
high levels of dissolved CO2, which causes increased acidity. This acidity can
cause water to erode away protective coatings on pipes and increase copper
and lead violations.
Surface water and reused water, on the other hand, are often more susceptible
to precursor contamination like algae and other bio-solids. Like all drinking
water, surface water and reused water require disinfection. Chlorination is
the most common disinfection technique because it is inexpensive; however,
chlorination can create potentially harmful DBPs that then require further
Air stripping, a water treatment technology proven to be effective for ground-
water 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.
AIR STRIPPER TECHNOLOGY AL-
LEVIATES THESE ISSUES
Fundamentally, air stripping removes
or “strips” contaminants from water by
contacting clean air with the contami-
nated water, causing the VOCs and other
contaminants to move from the water
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 con-
taminant will be driven from the water
into the air. Some contaminants are of course easier to strip than others; for
instance, dissolved gases such as methane and carbon dioxide strip easily, light
hydrocarbons less so, and MTBE and ammonia are relatively difficult to strip.
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 THMs that are byproducts of chlorine disinfection.
Chlorine is most frequently used to disinfect drinking water, and existing
regulations 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 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 nationwide
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 the excess CO2 out of the water, plants can effectively
increase pH and decrease levels of lead and copper at the point of end use.
Air Stripping Technology Effectively Removes VOCs, THMs, & CO2
For Improved Adherence To Water Quality Regulations