A ENVE 436 Course Project
Presented to Dr. Sczechowski of
California Polytechnic State University
San Luis Obispo
Ever since the industrial evolution, the number factories and industries have been growing like mushrooms to produce merchandise to improve our quality of life. However, during the manufacturing process, many chemicals and other toxic wastes have been produced as part of the by-product. Most of the toxic wastes and contaminants have been disposed of improperly into the earth or a body of water. Today, the contaminants from the past has ended up in the groundwater we drink and in the soil we plant our crops. Several processes of degradation of contaminants have been discovered to alleviate the problem. One process used to degrade several kinds of organic contaminants in groundwater and soil is hydrogen peroxide and UV treatment.
Both hydrogen peroxide and UV can be used alone to facilitate the degradation
of certain contaminants. Hydrogen peroxide is a very strong oxidizing agent
that is capable of destroying some halogenated compounds and most nonhalogenated
compounds in aqueous media (Anderson, 3.21). UV light itself is also very
capable of degradation by initiating bond cleavage. However, the range
of contaminants UV can degrade by itself is limited. The time for the degradation
to occur can also be very slow. The combination of the two treatments,
hydrogen peroxide and UV can create a very fast and efficient process for
Hydrogen peroxide and UV together, form two free hydroxyl radicals (OH· ) which are potent oxidizing agents.
H2O2 + hv ® 2OH·
The free hydroxyl radicals are an excited state species since they are characterized by a one-electron deficiency and are therefore extremely unstable. Because of their instability, they tend to react with the first chemical it comes in contact with (Watts, 358). Hydroxyl radicals (OH· ) also tend to completely oxidize dissolved organic contaminants in aqueous media and produce carbon dioxide, water and salts as by products (Heeks, Smith, and Perry 1991). Hydrogen peroxide is catalyzed with UV irradiation and ozone to create highly reactive radicals which react and cleave a wide variety of organics (Anderson, 3.58).
The efficiency is dependent on several factors. These factors consists
of the optical path length of medium (turbidity of the water sample), molar
extinction coefficient (dependent on wavelength which is most commonly
220 nm), concentration of the substrate (contaminant) and the intensity
and wavelength of the light source used (Anderson, 3.57). The formation
of the free radicals is favored by: UV light, high pH and a low bicarbonate
concentration (Stowell and Jensen 1991).
EQUIPMENT AND PROCESS
The hydrogen peroxide and UV process can be implemented by using a stainless
steel reactor called a Ultrox System (Figure 1), that can range from 1,100L
to 14,800L. The reactor "contains vertical, low temperature mercury lamps
inside quartz tubes. Four to eight reactors can be staged in series, the
number depending on treatment conditions" (Anderson, 3.26). The process
can be operated as a batch, continuous, or intermittent and may be fully
automated. The hydrogen peroxide is injected directly into the influent
wastewater stream. The contaminated water is then passed through several
columns of UV lamps before it is discharged. Ozone can also be added to
facilitate the reaction.
The entire process is shown in Figure 2 on the following page. Generally
the treatment process is divided into three steps that can be identified
as 1) the sulfide oxidation phase, 2) the VOC treatment phase, and 3) the
discharge phase. In the sulfide oxidation phase, sulfide in the groundwater
is oxidized to sulfate by the addition of hydrogen peroxide at a pH of
9.2. The pH is lowered to prepare for the VOC treatment phase. In the VOC
treatment phase, VOCs are either destroyed in an UV oxidation system or
removed by carbon adsorption. In the discharge phase the pH is raised so
that the treated water is suitable for discharge.
POSSIBLE CONTAMINANTS THAT WILL DEGRADE
The following are possible tested contaminants that have been successfully degraded by using the Ultrox system:
Any hindrance of the transmission of UV light to the water sample can decrease the efficiency of the process. In other words, a dirty lamp with rust or other interferences cannot emit as much light as a clean one, therefore the efficiency will decrease. It is necessary to clean the lamps periodically to avoid this problem. With cleaning in consideration, the maintenance, labor and downtime should be included into the operating costs (Anderson, 6.4).
If the pH needs to be adjusted to become basic, sodium hydroxide (NaOH)
should be used instead of dissolved carbonate (CO3-2)
or bicarbonate (HCO3-). Both carbonate and bicarbonate
ions react with the free hydroxyl radicals as scavengers. With the increased
competition of the organic contaminants with the scavengers, the rate of
degradation will be drastically effected for the worse.
The reaction of hydrogen peroxide with UV light is a very effective treatment. Although UV can be very inexpensive (one can use sunlight), the cost of hydrogen peroxide can be very expensive. Costs for the peroxide range from $0.03 to $3.00 per 1,000 liters. For a treatment facility operating at 200,000 gpd operation costs can reach as high as $200,000 per day or around $1.00 for every gallon of treated wastewater. Other factors that influence the costs to implementing the UV/oxidation. These include 1) types and concentration of contaminants, 2) degree of contaminant destruction required, and 3) requirements for pre-treatment and/or post-treatments.
One of the big advantages of this process is the resonance time. For
example, an initial concentration of 75 mg/L of 2,4-DNT in water with a
mole ratio of 13:1 of H2O2:2,4-DNT was degraded to
a concentration of 1 mg/L in 45 minutes or less (Ho 1986). With such a
rapid process, maybe there is hope for a fast cleanup for many of the highly
contaminated superfund sites!
[PROCESS FLOW DIAGRAM]
LIST OF REFERENCES