Conventional water treatment followed by activated carbon
adsorption effectively removes pesticides from drinking water. Using activated carbon
adsorption in water treatment is discussed in par. 5.4.11. PAC or GAC can be used for
pesticide removal. The effectiveness of carbon adsorption depends on the
concentrations of adsorbent and adsorbate, contact or residence time, and the
competition for available adsorption sites, as well as the temperature and pH of the
water. Because of these variabilities, no general rule can be given for carbon dosage
or design criteria for activated carbon treatment. Dosage and design requirements are
generally determined by laboratory methods or pilot plant operations.
b) Because pesticides and their carrier solvents have odors, water
treatment for removing these odors can somewhat reduce pesticide levels. However,
where a few milligrams per liter of PAC may be adequate for odor control, several more
milligrams per liter are generally required to remove organics. Relying on odors to
signal pesticide contamination or relying on intermittent odor control by PAC to ensure
a safe pesticide level is risky and considered poor practice. Where PAC is used,
multiple points of injection should be considered for maximum efficiency of the
adsorbant (to maximize pesticide removal). A disadvantage of PAC is that the sludge
formed after application is sometimes troublesome and difficult to manage.
c) The uncertainties involved in pesticide occurrence in water supplies
make GAC beds that are continuously online the best protection against pesticide
contamination. Organic pesticides have been demonstrated to be very strongly
adsorbed both on virgin GAC and on exhausted GAC used for odor control. The life of
a GAC bed for pesticide removal is not indefinite. However, if a GAC bed's usefulness
has been exhausted from adsorbing odors, that means it is generally time to replace it
for pesticide control, too.
Sources of VOCs. Water supplies derived from groundwaters, as well as
from surface waters, may contain VOCs. Contamination is most common in urban or
industrial areas, and is generally believed to be from improper disposal of hazardous
wastes and industrial discharges.
Many of the regulated VOCs are suspected carcinogens, and the others
may damage the kidneys, liver, or nervous system. The presence of one of these
compounds, even at a low level, is a concern since these are manufactured chemicals
(not naturally occurring in the environment), and their presence indicates the potential
for further contamination of that source water.