Environmental Engineering Reference
In-Depth Information
Installation of a centralized treatment system with high-cost equipment and hazardous chemicals
creates a logistical challenge. It is not practical or safe to install many treatment centers throughout
the wide area of the plumes, yet it is difi cult to pipe extracted groundwater back to a centralized
treatment center. Residents in developed neighborhoods overwhelmingly oppose putting extraction
wells and pipelines conveying contaminated water in their residential streets. In order to overcome
property owner and local government objections, P/GSI was forced to install a 4479-ft-long hori-
zontal ini ltration well to extract and divert groundwater to the treatment system (Brode et al., 2005).
This well may be the longest trenchless horizontal well yet installed for environmental remediation
purposes anywhere in the world.
The UV
H
2
O
2
remediation treated 1,4-dioxane-contaminated water produced at up to 1300 gpm
from 18 extraction wells. Inl uent 1,4-dioxane concentrations were 5000-10,000 ppb, and efl uent
concentration was 5 ppb or less (Fotouhi et al., 2006). Treated water was discharged to Honey
Creek, a tributary to the Huron River that enters the river upstream of the City of Ann Arbor's water
supply intake. Prior to treatment, extracted water entered a lined pretreatment pond where it was
mixed with a 93% solution of sulfuric acid to lower the pH to 3.8, which P/GSI determined to be
optimal for UV
+
H
2
O
2
reactions (Brode et al., 2005). Following pH reduction, a 50% H
2
O
2
solution
was injected into the treatment line and mixed into groundwater by a static mixer. The mixed solu-
tion was then exposed to UV when it passed through multiple chambers containing 22 UV lamps.
Following UV exposure, the pH was raised back to 6.9 with a 40% solution of sodium hydroxide.
Sodium bisuli te was added in order to comply with surface discharge requirements and to remove
excess H
2
O
2
(Brode et al., 2005). Although the areal extent of the plume did not decrease signii -
cantly as a result of this treatment, 1,4-dioxane concentrations in the plume core decreased 100-fold
to below 10,000 ppb.
P/GSI is permitted to discharge up to 1300 gpm to Honey Creek. The system was the largest
known 1,4-dioxane treatment operation anywhere in the world, operating 24/7/365, excluding brief
maintenance periods (Fotouhi et al., 2006). P/GSI's UV
+
H
2
O
2
treatment system operated from
October 1997 through March 2005, prior to changing the treatment technology to ozone-peroxide.
The successful operation of the UV
+
H
2
O
2
treatment system required large volumes of reactive
chemicals (hydrogen peroxide, sulfuric acid, sodium hydroxide, and sodium bisuli te), and the UV
bulbs consumed a lot of electricity, averaging 600 kW h/day. The overall cost of the UV
+
H
2
O
2
treatment system was $3.50/1000 gallons. Following careful engineering analysis, P/GSI deter-
mined that an ozone-peroxide system would use considerably less energy and would result in a
signii cant reduction in chemical use, making the technology safer, more environmentally friendly,
and less expensive to operate (Brode et al., 2005).
The ozone-peroxide system requires 50% less H
2
O
2
consumption and eliminates the need for
sulfuric acid and sodium hydroxide (Brode et al., 2005). The new system transfers water from
extraction wells to a pretreatment pond where insoluble iron settles out. Peroxide is introduced via
injector quills and then ozone is separately administered through a series of Venturi injectors.
Following the addition of post-treatment chemicals, the water again enters a settling pond prior to
surface discharge. The l ow rate for the ozone-peroxide system is 1200 gpm. Inl uent 1,4-dioxane
concentrations range from 2000 to 7000 ppb, whereas efl uent 1,4-dioxane concentrations are below
5 ppb. Treatment costs using the ozone-peroxide technology are approximately $1.50/1000 gallons,
yielding a savings of approximately $450K compared to the UV
+
H
2
O
2
costs of approximately
$1.4 M/year for a 1200 gpm l ow (Brode et al., 2005; Fotouhi et al., 2006). In addition to the reduction
in chemical use, the electrical demand for the ozone-peroxide treatment system is approximately
one-third the demand of the UV
+
H
2
O
2
treatment system. P/GSI also deployed a remotely operated
mobile ozone-peroxide treatment unit to treat the Unit E plume (Fotouhi et al., 2006).
An important consideration for operating ozone-peroxide treatment is the formation of bromate,
as described in Chapter 7. Bromate, a probable human carcinogen, forms as a by-product of ozone
treatment of groundwater containing bromide. The MCL for bromate is 10 ppb; a 5 ppb MCL was
considered but not adopted. Michigan allows discharge to surface water of bromate in concentrations
+
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