Oxidation - Public Water Systems - page 545

The ability of an oxidant to remove a specific micropollutant is largely dependent
on the rate constant. For conventional preozonation following the molecular pathway,
SOCs with rate constants of greater than 10 5 or 10 6 M 1 s 1 will be degraded if a
measurable ozone residual is maintained. In the case where the rate constant is below
10 2 M 1 s 1 , use of an AOP should be considered; however, some natural waters pro-
mote decomposition of ozone to the point where relatively refractory SOCs will de-
grade. Conversely, water quality conditions that favor scavenging of hydroxyl radicals
may impair the ability of AOPs to degrade SOCs. For intermediate values, it is nec-
essary to conduct pilot-plant tests under actual source water conditions to assess per-
formance capabilities. 27 Rate constants for reactions of ozone with a range of inorganic
and organic compounds have been developed by Hoigne, Bader and others. 67-69
In waters with relatively high levels of NOM, the oxidant demand of the bulk
organic material must be overcome before the micropollutant can be effectively de-
graded. In addition, high-NOM waters tend to impair the effectiveness of AOPs by
promoting radical scavenging.
The following sections present empirical results for the use of oxidants to degrade
pesticides and MTBE—two SOC issues currently of high concern to water purveyors.
Pesticides Removal of pesticides and herbicides is of increasing interest as greater
numbers of these toxins become regulated, often to very low concentrations. U.S.
standards for regulated pesticides are listed in Chapter 1. European standards limit the
concentration of an individual pesticide or herbicide to no more than 100 ng / L and
limit the total concentration of all pesticides to 500 ng / L. Particular focus has been
placed on the removal of atrazine, one of the most heavily used herbicides in the
United States and Europe, and a compound that results in early breakthrough when
using activated carbon treatment. However, there is a wide range of pesticides and
herbicides employed in the agricultural industry, as discussed in Chapter 3. As controls
on one type of compound become more restrictive, use of alternative chemicals in-
creases to meet agricultural production requirements. This proliferation of compounds,
each with differing properties that impact treatability, pose a serious challenge to the
water purveyor.
A number of studies have been conducted on the ability of oxidation processes to
reduce pesticide concentrations in water. 70-77 Two studies that appear representative of
this work are described below. Both efforts evaluated the ability of ozone alone and
an AOP (H 2 O 2 /O 3 ) to treat a range of pesticides under typical water treatment con-
ditions.
Meijers and coworkers 78 evaluated oxidation processes for removal of 23 pesticides
spiked into source water from the River Meuse. At neutral pH, using typical ozone
doses required for disinfection (O 3 / DOC 0.55 g / g), ozone alone was found to be
a poor barrier against pesticides, providing effective degradation of only six com-
pounds: dimethoate, chlortoluron, diuron, isoproturon, metoxuron, and vinclozolin.
Increasing the ozone dose (O 3 / DOC 1.0 g / g) resulted in an effective barrier for 50
percent of the pesticides, expanding the list of impacted compounds to include di-
azinon, parathion-methyl, linuron, methabenzthiazuron, metobromuron, MCPA, and
MCPP. With ozone alone, pesticides were degraded more effectively at high pH and
temperature, reflecting the impacts of the radical oxidation and improved kinetics.
With advanced oxidation, 21 of the 23 pesticides were effectively degraded, in-
cluding atrazine, propazine, simazine, chlorfenvinphos, tetrachlorvinphos, 2,4-D, 2,4-
DP and 2,4,5-T. Only dicamba and didikegulac were resistant to AOP treatment.
Moderate oxidant dosages were used for the AOP process: O 3 / DOC 1.4 g / g (3.0