There are various options available for Arsenic Removal ranging from ion exchange, activated alumina, reverse osmosis, coagulation/filtration, and, of course, adsorption. Today, we are going to walk you through the details of one of our favorite adsorption products, the SORB 33®.
How does the SORB 33® process work? What types of media are involved?
The De Nora SORB 33® arsenic removal system is a fixed-bed adsorption system that uses a granular ferric oxide media, Bayoxide® E33, for the adsorption of dissolved arsenic. The system employs a simple “pump and treat” process that flows pressurized well or spring water through a fixed-bed pressure vessel containing the media where the arsenic removal occurs. Both arsenite (arsenic III) and arsenate (arsenic V) oxyanions are removed from water via a combination of oxidation, adsorption, occlusion (adhesion), or solid-solution formation by reaction with ferric oxide ions.
What sizes and configurations are available?
De Nora SORB™ arsenic removal solutions are available in a range of sizes and configurations, from a 4-feet to 14-feet in diameter per single vessel (multiple vessels can be used together). With the capability of taking various process configurations — including parallel flow, with bypass and blending, or series flow — these systems can treat a wide range of flow rates based on water quality considerations.
How is the optimal size and configuration determined?
Configuration of a water utility’s arsenic treatment system can best be answered by first prioritizing that utility’s water treatment design philosophy, its water quality (arsenic level), and the state’s design criteria for water treatment systems. The key criteria are:
- Capital available for arsenic treatment
- Available footprint, or space, for installation
- The level of redundancy required if an absorber is taken out of service
- State rules regarding treatment bypass and blending
- The trade-off between capital and operating costs for treating high arsenic waters
- Optional centralized treatment
What are the considerations and options for configurations that are available for adsorption systems?
The most straightforward configuration for adsorption processes is parallel flow between two or more adsorbers. The most economical configuration is parallel flow with bypass and blending. Since Bayoxide® E33 media will adsorb arsenic to levels of < 3 μg/L throughout most of the adsorption cycle, some water can be bypassed via flow control and blended with treated water to an average arsenic level of 1 to 2 μg/L below the 10 μg/L MCL. This process extends media life and reduces operating costs. It also allows for smaller adsorber vessel designs due to reduced loading rates. Bypass and blending are recommended for waters with arsenic levels of < 25 μg/L.
Treatment through two adsorbers in series flow configurations (“lead/lag”) is recommended for waters with high arsenic levels (> 30 μg/L). The media’s arsenic adsorption capacity can be optimized by allowing the first “lead” bed of media to continue to adsorb arsenic past the 10 μg/L MCL while the second bed acts as a polishing, or “lag,” adsorber. This is possible because the arsenic breakthrough curve is gradual, even when the treated water exceeds 10 μg/L of arsenic. When media is changed out in the first adsorber, the second one becomes the lead adsorber while the first one is placed in the lag position. The media can adsorb up to 40 percent additional arsenic in this configuration when compared with proportional operating costs for parallel flow. These savings can offset the more expensive capital costs, which can be up to 70 percent greater than that of the parallel flow design.
“(N + 1) redundancy” parallel flow configuration uses three or more adsorbers designed so that one adsorber can be taken out of service for backwash, etc., while the other adsorbers continue to treat the full well capacity. The adsorbers in service treat water above the minimum EBCT contact time and below the maximum loading rates. Some state regulations require this level of treatment system redundancy.
Optimization of the series flow and (N + 1) redundant designs is the sequencing configuration. Three or more adsorbers are designed with a valve and piping manifold that allows for parallel, N + 1, simultaneous parallel, series flow and parallel flow with simultaneous backwash. This configuration optimizes media capacity while minimizing capital costs and area requirements.
How can the media life be optimized?
Media life typically ranges from six months to six years, depending on water quality levels. Media usage can be optimized in multiple parallel flow adsorber systems via staging. The media lifecycle for each adsorber is staggered such that one is operating in the latter stages of its life while others are operating at earlier stages of media life. The adsorber with the oldest media can operate to 12 to 14 ppb arsenic breakthrough because its water is blended with other adsorbers discharging water with arsenic in the 2 to 6 ppb range. The treated water from the adsorbers contains an arsenic level of about 8 ppb.
Although systems are usually simple to operate, owners may consider the additional backup of service agreements to support the most effective operation of the system to extend media life and undertake media change-out when required.
What do you see in the future of adsorption systems for arsenic removal?
The innovation team at De Nora is continually seeking process improvements and are in a constant state of development to offer our customers even greater efficiencies.
Source: Water Online, How To Weigh Your Options for Arsenic Removal.
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