Patent Application
20140001124
This invention relates generally to the treatment of potable water and more specifically, to a system and method for oxidizing arsenic(III) in groundwaters or other source water having high iron and manganese concentrations.
Arsenic is a common, naturally occurring contaminant in groundwater. When arsenic (III) is the predominant arsenic form in source water, oxidation of arsenic (III) to arsenic (V) is required for more effective removal. The presence of iron and manganese in source water has also proven to be problematic.
As required by the Safe Drinking Water Act of 1973, the U.S. Environmental Protection Agency (EPA) established a Maximum Contaminant Level (MCL) for arsenic in drinking water at 0.05 mg/L in 1975. On Jan. 18, 2001, the EPA revised the arsenic MCL from 0.05 mg/L to 0.01 mg/L and then on Mar. 23, 2003 the EPA clarified the MCL rule to be 0.010 mg/L (or 10 μg/L). The final revised rule required all community and non-transient, non-community water systems to comply with the new MCL by Jan. 23, 2006. With the lower arsenic MCL, the EPA estimated that approximately 5,000 water systems would be out of compliance and that the vast majority of these water systems would need to install some type of arsenic removal system. Furthermore, most of these water systems were small systems serving less 10,000 consumers with many providing water to less than 1,000 people. With limited resources, these small/very small systems have a critical need for low cost arsenic treatment systems, especially when iron and manganese are also present in source water.
In accordance with one embodiment of the present invention, a mixed-bed oxidizing media vessel for oxidizing arsenic(III) in source water having concentrations of iron and/or manganese is disclosed. The mixed-bed media vessel comprises: a housing; a first oxidizing media for oxidizing the dissolved iron and/or the manganese present in the source water; and a second oxidizing media for oxidizing arsenic(III) to arsenic(V) present in the source water, wherein the second oxidizing media is positioned below the first oxidizing media within the housing.
In accordance with another embodiment of the present invention, a system for oxidizing arsenic(III) in source water having concentrations iron and/or manganese is disclosed. The system comprises: at least one mixed-bed media vessel for oxidizing dissolved iron and/or manganese and for oxidizing arsenic(III) to arsenic(V); and a backwash system coupled to the at least one mixed-bed media vessel.
In accordance with another embodiment of the present invention, a method for treating source water having concentrations of arsenic and at least one of iron and manganese is disclosed. The method comprises the steps of: providing a pre-oxidation system, wherein the pre-oxidation system comprises: at least one mixed-bed media vessel comprising: a housing; a manganese dioxide-coated media; and a manganese dioxide-based media, wherein the manganese dioxide-based media is positioned below the manganese dioxide-coated media within the housing; providing an adsorption vessel containing arsenic adsorption media coupled to the at least one mixed-bed media vessel; providing a backwash system coupled to the at least one mixed-bed media vessel and to the adsorption vessel; running the source water through the at least one mixed-bed media vessel; oxidizing ferrous iron present in the source water to ferric iron with the manganese dioxide-coated media; oxidizing reduced Mn2+ present in the source water to Mn4+ with the manganese dioxide-coated media; oxidizing arsenic(III) present in the source water to arsenic(V) with the manganese dioxide-based media; adsorbing arsenic(V) present in effluent from the at least one mixed-bed media vessel with the arsenic adsorption media in the adsorption vessel; and backwashing at least one of the mixed-bed media vessel and the adsorption vessel.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
a is a schematic of a test system used to evaluate and compare the oxidizing capability of three oxidizing media products: Filox®, Pyrolox®, and Birm®.
b is a schematic of the test system
a is a schematic of another embodiment of an arsenic treatment system having two parallel pre-oxidation/filtration vessels and an adsorption vessel.
b is a schematic of the arsenic treatment system of
a is a line graph showing a comparison of manganese concentration in the raw water used during the performance evaluation study versus the manganese concentration in the same source water after having been treated by a Birm®/Filox® oxidation system.
b is a line graph showing a comparison of manganese concentration in the raw water used during the performance evaluation study versus the manganese concentration in the same source water after having been pre-oxidized by a Birm®/Filox® oxidation system and by an Adsorbsia® GTO® system.
Although it can exist in both organic and inorganic forms, only the inorganic arsenic is significant in potable water supplies. Inorganic arsenic has two oxidation states: arsenic (+III) (arsenite) and arsenic (+V) (arsenate). Both As(III) and As(V) exist in the pH range of 6 to 9. The primary arsenate species are monovalent H2AsO4− and divalent HAsO42−. These anions result from the dissociation of arsenic acid (H3AsO4), which exhibits pKa values of 2.2, 7.0, and 11.5. The predominant As(III) species is uncharged arsenious acid (H3AsO3). Only at pH values above its pKa of 9.2 does the monovalent arsenite anion (H2AsO3−) predominate.
As(V) is effectively removed by most arsenic treatment processes whereas uncharged As(III) is poorly removed. For example, anion exchange (AIX) resins can remove nearly 100% of As(V), but no As(III). (See Ficklin, W. H.1983. Separation of Arsenic (III) and Arsenic (V) in Groundwaters by Ion Exchange. Talanta, 30(5):371; see also Clifford, D. A., Sorg, T. T. and Ghurye, G. L. 2011. Ion Exchange and Adsorption of Inorganic Contaminants. Chapter 12, Water Quality and Treatment, Sixth Edition, McGraw Hill). The AIX process, therefore, requires As(III) to be oxidized to As(V) to achieve satisfactory removal.
Referring to
In one study, seven different oxidants—chlorine, permanganate, ozone, monochloramine, chlorine dioxide, Filox® (a manganese dioxide [MnO2] solid oxidizing media manufactured by Watts Water Quality and Conditioning Products, Inc.), and ultraviolet light (UV) at 254-nm—were tested for their ability to convert As(III) to As(V). (See Ghurye, G. L. and Clifford, D. A. 2001. Laboratory Study on the Oxidation of Arsenic III to Arsenic V. EPA/600/R-01/021. U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Cincinnati, Ohio). The study evaluated the effects of pH (6.3 to 8.3), temperature (5 to 25° C.), and interfering reductants such as Mn2+, Fe2+, S2−, and total organic carbon (TOC). Of the seven oxidants, only chlorine and permanganate were shown to provide complete oxidation in less than 1 min under all conditions. Ozone was fast and effective, but significantly impacted by TOC.
One of the seven tested oxidants was Filox®, which is a brand name for pyrolusite, a naturally-occurring MnO2 product supplied in granular forms. Filox® is slightly different from other pyrolusite products such as, Pyrolox®, because of its very high MnO2 content (75 to 85%). One of the conclusions drawn from the tests conducted with Filox® was that the media was very effective for As(III) oxidation under most conditions tested. More than 95% of As(III) was oxidized with both low and high dissolved oxygen (DO) levels and at contact times as short as 1.5 min when the test water contained no interfering reductants such as dissolved iron, manganese, sulfide, and total organic carbon (TOC). All interfering reductants studied showed some detrimental effects particularly with test waters containing low DO and with a low empty bed contact time (EBCT) of 1.5 minutes. With high DO and an EBCT of 6 minutes, the effects of the interfering reductants were attenuated.
In another study of four solid oxidizing media (Filox®, Pyrolox®, Birm®, and manganese greensand) using a NSF International (NSF) challenge water, preliminary screening tests on Filox®, Pyrolox®, and Birm® showed only Filox® and Pyrolox® to be very effective for As(III) oxidation (>95%). (See Lowry, J., Clifford, D., Ghurye, G, Karori, S., Narasimhan, R. and Thomson, B. 2005. Arsenic Oxidation by Solid-Phase Media or a UV-Sulfite Process. Awwa Research Foundation, Denver, Colo.). The least effective media was Birm®, achieving only <60% oxidation under optimum conditions of high DO and 1 to 3 minutes EBCT. Based upon these short-term screening tests, Birm® was dropped from further testing and replaced by manganese greensand.
In continuing tests with DO at low levels, only Filox® and Pyrolox® were found to effectively oxidize As(III) whereas manganese greensand only achieved 33% As(III) oxidation. Further testing found that ferrous iron at 2.0 mg/L had a negative impact on As(III) oxidation of both Filox® and Pyrolox® within a short period of time and As(III) oxidation continued to decrease with time. The decrease in oxidation effectiveness was attributed to the oxidation of ferrous iron that competes with As(III) and to the precipitated ferric hydroxide that coats the media and reduces accessibility of As(III) for oxidation sites. Because Filox® is less fragile than Pyrolox®, Filox® may be the preferred media when a solid oxidizing media is used.
A pilot study was conducted to evaluate the effectiveness of six different commercially available adsorptive media for As(III) and As(V) removal. (See Chen, A. S. C. 2011. Pilot-Scale Evaluations of Arsenic Removal Adsorptive Media at Licking Valley High School in Newark, Ohio. Letter Report (Unpublished), Contract No. EP-C-05-057, Task Order No. 0019. U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Cincinnati, Ohio). The pilot study was conducted with a water supply extracted from a well. The groundwater contained high levels of total arsenic (around 74 μg/L), total iron (around 2.2 mg/L), and total manganese (around 0.165 mg/L). Speciation tests indicated that arsenic existed mainly as As(III).
In order to conduct the media tests for As(V) removal, oxidation of As(III) was required. To select a pre-oxidation system, a pilot study was conducted to evaluate and compare the oxidizing capability of three oxidizing media products: Filox®, Pyrolox®, and Birm®. The test system used during the pilot study consisted of three 2-in×4 ft glass columns, each containing 12.5 in of a media. A schematic of the test system is shown in
During the 48-day (1,172 hr) pilot study, the groundwater had an average total arsenic concentration of 74 μg/L, an average total iron concentration of 2.2 mg/L, and an average total manganese concentration of 0.165 mg/L (12 water samples). Of the soluble arsenic fraction, speciation tests indicated that 90 to 96% was As(III).
The tests began using an EBCT of 2.1 minutes and no backwashing of the media. After one week of operation, none of the three media were found to be effective for As(III) oxidation (12 to 22%), which was attributed to the lack of backwashing of the media. The media were backwashed and after another week of operation, very poor oxidation of As(III) was observed again. After backwashing and increasing the EBCT to 5 minutes with no improvement in As(III) oxidation, it was concluded that the media were fouled (coated) with iron because of the lack of backwashing. All of these media have been used successfully for iron and manganese removal and were found to remove iron during the tests. Because precipitated iron will adsorb some As(III) and As(V), all three media tested reduced total arsenic concentrations by 37 to 43%.
After replacing the media in all three columns, new tests were conducted using an EBCT of 5 minutes and all columns were backwashed every two days. Although the oxidation of arsenic improved, the results were still considered unsatisfactory. The best results occurred with Filox®, which is a manganese dioxide-based media (see
After three weeks of testing, the test system was modified to include a fourth test column that was loaded with fresh Filox® media, and which received the effluent from the Birm® column (
The results of the pilot oxidizing media study also showed that all three oxidizing media could remove a fraction of arsenic from raw water. The amount of arsenic removed from an average raw water level of 65 μg/L was 33.7 μg/L (48.2%) for Pyrolox®, 36.1 μg/L(44.5%) for Birm®, and 27.8 μg/L (57%) for Filox®. The removal by Filox® was especially significant at the beginning of the run, with arsenic concentrations reduced to as low as 8.1 μg/L two days into the run. Removal rates leveled off to those of Pyrolox® and Birm® approximately three weeks into the run. When the Birm®-treated water was directed to a fresh Filox® column, the Filox® further reduced arsenic concentrations by approximately 18%.
Another advantage to the two-step Birm®/Filox® system was reduced backwashing flow requirements. Birm®, because of its low density, can be easily backwashed (see Table 1). Filox®, on the other hand, being a much heavier media, requires a higher pressure pump to achieve the same bed expansion as Birm® (see Table 1). With the two-step Birm®/Filox® system, only Birm® needs to be backwashed. Because Birm® removes iron and manganese, Filox™, does not require backwashing.
A major advantage of using a solid oxidizing media over a chemical oxidant is simplicity (no chemical addition). Another advantage of solid oxidizing media, mainly over the use of chlorine, is that it does not have the potential problem of disinfection by-products (DBPs) and it eliminates the requirement (and the cost) for DBP monitoring. For these reasons and others, Filox® may be used as a pre-oxidation step with small arsenic removal systems (using mainly ion exchange and adsorptive media processes) and has been found to be very effective in oxidizing As(III) to As(V).
As discussed above, studies on Filox® have found that it can be fouled (coated) by iron and even manganese and once it is fouled, it loses its ability to oxidize As(III) and must be replaced with virgin (fresh) media. Because of its high cost, therefore, it is not practical to use Filox® on groundwaters having moderate to high levels of iron that can quickly foul the media. For this reason, the use of Filox® has been limited to waters containing very small amounts of iron (<0.3 mg/L) and or manganese (<0.05 mg/L).
For groundwaters or other source waters containing iron and/or manganese, Filox® requires a pre-treatment step to the removal iron. With pre-treatment, Filox® is very effective for As(III) oxidation. However, the two-step process can be costly because of the need for two columns; the first column with Birm® to remove iron and manganese and the second column with Filox® to oxidize As(III). Water softeners have also been used to protect Filox®, but they require salt regeneration and can be more costly than using Birm®.
According to one embodiment of the present invention, an As(III) oxidation system may comprise a single column of Birm® on top of Filox® (see
A study was conducted wherein a 30-gpm Adsorbsia® GTO® adsorptive media arsenic removal system was to be used at the selected site. Adsorbsia® GTO® is a granular titanium oxide media manufactured by the Dow Chemical Company for arsenic removal.
The source water at the selected site contained 13 to 15 μg/L of total arsenic with approximately 50% existing as As(III). The water also had an iron concentration of around 0.3 mg/L and a manganese concentration of 0.116 mg/L. Referring to
Operation of the complete As(III) treatment system lasted for 676 days, during which time the system treated approximately 5,198,000 gallons of water. As shown in
As seen in
The Birm®/Filox® system was also effective in removing iron and manganese, reducing their concentrations to <25 and 4 μg/L (on average), respectively (see
Water samples may also be taken at several sample locations during treatment so that pH, temperature, DO/ORP (dissolved oxygen/oxidation reduction potential), and speciation test's may be conducted as well as tests for Fe levels, Mn levels, Ti levels, Ca levels, Mg levels, F levels, No3 levels, SO4 levels, SiO2 levels, P levels, turbidity, alkalinity, and any other applicable tests that may be conducted to evaluate the system's performance. The testing may occur on a weekly or monthly basis or at any other suitable time interval.
While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.
