Civil Engineering Reference
In-Depth Information
mg / L) and H
2
O
2
/O
3
0.5 g / g. pH was found to have a minor effect on the degra-
dation of pesticides by the AOP, and higher hydrogen peroxide dosages showed no
improvement in degradation for this source water.
Roche and Prados
79
tested ozone alone and H
2
O
2
/O
3
for the removal of 11 pesti-
cides from water with the following characteristics: TOC
2.1 mg / L, alkalinity
240 mg / L as CaCO
3
,UV
254
0.034 / cm, pH
8.3, and ozone demand
0.5 mg /
L. Ozone alone was effective at reducing only three of the pesticides to target con-
centrations: terbutryn, isoproturon, and aldicarb. This was achieved at an ozone dose
of 1 mg / L and a 10-min contact time. H
2
O
2
/O
3
using a hydrogen peroxide dose of
H
2
O
2
/O
3
of 0.4 g / g provided effective treatment of eight pesticides, including the
following compounds, listed in descending order of reactivity: malathion, aldrin,
M. parathion, linuron, and atrazine. For three pesticides—lindane, HCB, and
-endosulfan—removals observed were low for both oxidant systems, requiring treat-
ment by activated carbon.
Comparisons of alternative AOP processes (UV / O
3
, UV/H
2
O
2
,H
2
O
2
/O
3
,O
3
at high
pH) for control of selected pesticides have produced varying results, indicating the
importance of source water characteristics.
Oxidation of pesticides may be incomplete, producing intermediate degradation
products. For example, incomplete oxidation of atrazine may yield substantial amounts
of deethylatrazine and deisopropylatrazine. Such by-products are potential candidates
for future regulatory action and must be taken into account when evaluating treatment
alternatives.
MTBE
Methyl tertiary butyl ether (MTBE) is the most common oxygenated fuel
additive used in reformulated gasoline. It has been found in an increasing number of
groundwater supplies and is difficult to remove in conventional treatment systems.
Pilot-scale investigations have demonstrated that both ozone alone and H
2
O
2
/O
3
can
remove MTBE from California groundwater sources.
80
H
2
O
2
/O
3
provided greater re-
ductions, achieving 78 percent removal of MTBE (23 g / L influent concentration) with
a 4-mg / L ozone dose and 1.3-mg / L of hydrogen peroxide; however, unacceptable
levels of bromate were produced due to high concentrations of bromide in the raw
water.
SELECTION OF AN OXIDATION PROCESS
Selecting the appropriate type(s) of oxidants and their application point(s) within the
water treatment process requires consideration of a wide range of factors, including:
•
Oxidation treatment objectives
—particularly if multiple objectives are involved.
Different oxidants exhibit varying versatility, and some objectives may require con-
flicting oxidant selection or process selection. For instance, treatment of a reservoir
water containing manganese and refractive pesticides favors the molecular ozone path-
way for one objective and the hydroxyl radical pathway for the other. Through careful
design, both mechanisms may be used through sequential addition of ozone and hy-
drogen peroxide or UV, or through use of preozonation followed by midpoint appli-
cation of an AOP.
•
Source water quality.
As previously described, water quality parameters such pH,
alkalinity, ferrous iron, and NOM concentration may promote or inhibit the molecular
