Groundwater is of particular concern in that these waters move very
slowly and do not have a rapid natural cleansing mechanism. Thus, once
groundwaters are contaminated, they will generally remain so for many years or
Treatment. Methods for removing VOCs include aeration and GAC. PAC
treatment or conventional drinking water treatment (coagulation, sedimentation, and
filtration) have not proven effective. Methods such as reverse osmosis and
macromolecular resins may eventually prove useful in removing VOCs. Before
implementing a VOC control strategy, check local environmental regulations and permit
requirements. Aeration could cause violation of air quality standards. Spent carbon
from GAC adsorbers could be considered a hazardous waste.
Sources of DBPs. Chlorine, when used for bacterial and viral disinfection
of water supplies, interacts with organic precursors present in natural waters to form a
variety of chlorinated organic compounds collectively called disinfection byproducts.
DBPs are associated with a number of chronic health problems, including cancer.
Because the natural organic precursors are more commonly found in surface waters,
water taken from a surface source is more likely than groundwater (with some
exceptions) to have high DBP levels after chlorination. A number of DBPs have been
targeted for regulation, including THMs and haloacetic acids (HAAs). Other oxidants
used for disinfection, i.e., ozone and chlorine dioxide, can also form DBPs (although
not to the same extent as chlorine).
Treatment. Treatment options available to meet DBP standards are to
substitute new disinfectants for chlorine that do not generate DBPs or that produce
fewer DBPs; to reduce organic precursor concentrations before chlorination; and to
remove DBPs after formation.
Ozone, chlorine dioxide, and chloramine are possible alternate
disinfectants. Use of these chemicals for disinfection purposes is discussed in
par. 5.4.5. It is good practice to monitor carefully the microbiological quality of the
treated and distributed water during the transition period to an alternate disinfectant.
Treatment processes to reduce or control precursor levels include
offline raw water storage, aeration, improved coagulation, ion exchange resins,
adsorption on PAC and GAC, ozone-enhanced biological activated carbon (BAC), and
adjustment of the chlorine application point. Bench- and pilot-plant studies should be
performed to determine which treatment process will most effectively reduce precursor