Arsenic contamination of drinking water is a global concern. The World Health Organization (WHO) recommended in 1993 that the arsenic standard of 50 µg/l be reduced to 10 µg/l. As a result, countries began implementing the 10 µg/l standard on varying time tables. Germany adopted the revised standard in 1996, a European directive was set in 2000 with compliance dates running through 2009, the United Kingdom adopted the standard in 2003 and the United States adopted the standard in 2006.
Facing compliance in December 2003, Severn Trent Water in the United Kingdom started searching in 1994 for a method of arsenic removal that would have high arsenic removal capacity; use a dry medium which was easy to handle, store and ship; and would be tough and reliable both in performance and results, all at an economic cost. Adsorption rapidly appeared as the most effective technology, and the choice of media was then considered.
Earlier research showed that promising results had been achieved on a small scale in a granular form of ferric hydroxide as an adsorption media. A follow-up resulted in a cooperative agreement with German chemical giant, LANXESS (formerly Bayer AG), which had developed a totally new granular ferric oxy hydroxide for Severn Trent Water.
Combining LANXESS’s experience with chemicals and Severn Trent's expertise in water treatment resulted in the combination of an adsorption system and ferric oxide media specifically designed for arsenic removal. Following intensive laboratory tests, pilot schemes and full-scale field trials, adsorption systems were installed at 16 affected sites (59 vessels) in the United Kingdom where they have been operating successfully, in some cases since 1999, with arsenic levels consistently lowered to less than 3 µg/l.
The Test
In the course of their investigations, the Severn Trent Water team gathered an immense amount of data on such matters as optimum size and density of the media, adsorption performance, effective hydraulic pressures, backwash intervals and more, enabling them to optimize performance and operating costs.
In an effort to address global demand for a viable arsenic removal treatment technology, Severn Trent Water transferred its knowledge base on adsorption systems and ferric oxide media to its U.S. sister company Severn Trent Services, which then commercialized the SORB 33® arsenic removal system and Bayoxide® E33 adsorptive media. To date, Bayoxide media is the most widely accepted and employed arsenic removal adsorption media in the United States, permitted and operating in more than 26 states.
Tailoring a Proven Product to a New Market
Water quality in the affected areas containing arsenic contamination across the United States varies significantly from the water quality found in the United Kingdom. In the United Sates, a prevalence of interfering ions such as silica and vanadium and high pH can be found in hot spot areas such as in the West. Further, the co-occurrence of elevated arsenic levels with iron and manganese levels is experienced in areas such as the Northeast and Midwest.
As a result of varying water quality and the potential effect on SORB 33 system and Bayoxide media performance, Severn Trent Services established a lab-based research program on U.S. waters, focusing on levels of pH, arsenic, silica, phosphate, vanadium and more across a broad array of waters. Once completed, an extensive series of pilot tests were then undertaken to further predict full-scale operational performance of SORB 33 systems and Bayoxide media on U.S. waters.
The combination of practical experience transferred from Severn Trent Water and investigative back-up conducted by Severn Trent Services to address the U.S. market ensured the introduction of a commercial arsenic removal system with a proven track record, supported by a history of laboratory investigations and actual operational information.
System Optimization
Since the introduction of the SORB process and Bayoxide media in 1998, Severn Trent Water and Severn Trent Services personnel in the United Kingdom and the United States have worked together to develop a number of methods to optimize the performance of the system and media. The issues for which solutions were developed include:
- the “water hammer effect,” which causes media attrition
- interfering ions
- pretreatment to remove particulate matter
- vessel sequencing to optimize media life
- backwash media expansion with temperature
Hammer Effect
The mechanical properties of Bayoxide E33 media can be adversely affected if well water is rapidly brought into contact with the media. This phenomenon is referred to as the “water hammer effect,” whereby the media becomes friable and breaks down to form smaller particles or “fines,” primarily at the interface of the media and the water. The resulting fines cause a high differential pressure across the media bed resulting in a higher frequency of backwashing along with a loss of media.
Over a period of several years, the media depth readings from 64 vessels at U.K. SORB 33 plants were analyzed. The results are summarized below:
- 66% of the vessels in the plants showed no media attrition
- 25% of the vessels showed media attrition between 1% and 10%
- 8% of the vessels showed media attrition between 11% and 20%
- 2% of the vessels showed media attrition between 21% and 30%
The operational experience in the original U.K. plants and newer plants in the United States demonstrated that it is important to minimize the risk of the hammer effect by slowly introducing water when a vessel is brought into normal operation. There are numerous means of controlling the water velocity at start-up. Variable speed pumps with a slow start and motorized valves on the combined vessel inlet have both been used successfully.
It also is important to maintain approximately 24” to 36” of water above the media bed at start-up. The water attenuates the velocity of the incoming water and minimizes the risk of damaging the media.
Interfering Ions
Bayoxide E33 media will remove other cations and anions in addition to arsenic. These ions generally compete for the same adsorption sites as arsenic, and the resulting effect is to reduce the media’s capacity to adsorb arsenic. Experience in the United States has shown that antimony, phosphate, silica, and vanadium reduce the capacity for arsenic adsorption.
In 2006, Severn Trent Services received a U.S. patent for a technology related to a method for removing silica from water treatment adsorption media, including Bayoxide E33. The technology comprises a scrub solution composed of NSF-approved products that can significantly increase the adsorptive capacity of the bed. The arsenic removal media bed soaks in the scrub solution until silica is removed from the media particles, usually 20 minutes at the beginning of a routine backwash cycle. The scrub solution containing the silica is then removed and the media bed flushed during a normal backwash.
Pretreatment to Remove Particulate Matter
In early tests of the SORB system and Bayoxide media, it was discovered that the media will filter out finely divided particulate matter (e.g., precipitated iron, manganese and sand). However, this results in a need for increased frequency of backwashing. The SORB 33 system is designed for limited backwashing, from once every one to four months. As solids are removed by the Bayoxide media, the need to backwash will increase.
If chlorine is used upstream for chlorination or to oxidize arsenite +3 to arsenate +5, other soluble matter (such as iron and manganese) will oxidize to form insoluble solids that get filtered by the media and increase the need to backwash. If wells are prone to have sand, sand separation is a beneficial pretreatment step to limit backwashing requirements on the SORB 33 system and Bayoxide media. When trapped within the Bayoxide media, the abrasive nature of sand can be harmful and cause the media to break down.
In 2005, Severn Trent Services introduced the pretreatment Omni-SORB™ filter media, which is specifically designed to provide removal of iron and manganese compounds from water and wastewater supplies. This pretreatment media enhances the use of Bayoxide, which follows for arsenic removal. Unlike other iron and manganese removal media, Omni-SORB is not a processed mineral. It is an engineered product using refined manganese that has high catalytic activity for oxidation and adsorption of these metals.
Vessel Staggering to Optimize Media Life
Multiple vessels in a SORB 33 treatment plant can be arranged to provide parallel flow or series flow. Depending upon manpower resources or the level of plant automation, vessels can be managed to optimize media life. Provided that one or more vessels contain relatively new media (i.e., treated water from the vessel <5 µg/l arsenic), the vessel containing the oldest media can be kept in operation beyond the statutory arsenic limit of 10 µg/l. Vessel staggering is easier in SORB 33 treatment plants having three or more vessels.
Example:
- Vessel 1 outlet arsenic 1 µg/l
- Vessel 2 outlet arsenic 5 µg/l
- Vessel 3 outlet arsenic 10 µg/l
- Vessel 4 outlet arsenic 16 µg/l
- Combined outlet arsenic (1 + 5 + 10 + 16) / 4 = 8 µg/l
Backwash Media Expansion with Temperature
As with most media, Bayoxide E33 media expands at different rates depending upon backwash water temperatures. This physical property is an important criterion when designing new SORB 33 treatment plants because efficient backwashing of Bayoxide E33 media improves its overall performance. The fines generated in production, transportation, delivery and normal vessel operation are completely removed during a backwash.
Summary of SORB 33 treatment plant benefits
In their use at plants in the United Kingdom and United States, the SORB 33 arsenic removal process and the Bayoxide E33 ferric oxide media have demonstrated a number of operational benefits:
- Plants can be switched off and on to meet water demands
- Treatment plant has a small footprint
- Arsenic removal treatment time is only three minutes
- The wastewater generated is minimal and non-hazardous
- Some of the Severn Trent Water SORB 33 plants have had zero process water loss
- SORB 33 plants are designed with little or no automation, reducing operating complexity by limiting the number of interfacing systems
- If required, SORB 33 plants can be fully automated
- Exhausted Bayoxide E33 and OmniSORB media is generally disposed to landfill but it can be regenerated if deemed necessary.
For more information, e-mail info@severntrentservices.com.
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