Approaches to Water Treatment for Arsenic

Arsenic occurs naturally and is found in rocks and soil before being released into water supplies through erosion. Certain industrial practices, such as the production of paints and dyes, metals, soaps, drugs and wood preservatives, have the potential for releasing arsenic into the environment as a byproduct of the process. 

The types and effects of arsenic

There are two common forms of arsenic, arsenite (arsenic III) and arsenate (arsenic V). Arsenic V is the most common form and is easier to remove from drinking water. Arsenic III is more difficult to remove and is more hazardous to human health. 

Long-term exposure to arsenic is proven to have a detrimental effect on human health. Serious health issues range from skin, bladder and lung disease to prostate problems. Prolonged exposure has also been linked to cancer, cardiovascular disease, diabetes and reproductive difficulties. 

Conducting a detailed evaluation

Before evaluating an arsenic removal technology, a utility should evaluate its own system by conducting a comprehensive water quality analysis. This analysis should consist of both basic values and special analysis for species with similar chemical properties to arsenic, as these may interfere with the performance of treatment technology.

Each site with potential arsenic contamination should be evaluated as borehole waters, even on the same site, can differ significantly. Pilot tests or rapid small-scale column tests (RSSCT) should also be performed to predict the full-scale performance of certain arsenic removal treatment technologies. 

In addition to capital and operating costs, an arsenic removal technology should also be evaluated on a variety of performance criteria. These criteria should address technical requirements and history, along with commercial needs. It is also crucial for utilities to look at the ease of operation and the risk of failure of the process. Lastly, the technology supplier’s experience in the arsenic removal market should be considered. 

Treatment options for the removal of arsenic in water

Treatment options for the removal of arsenic range from ion exchange, activated alumina and reverse osmosis to coagulation/filtration and absorption. The table below shows a comparison of these technologies.

Iron oxide adsorption

Adsorption is a continuous process where water flows downward through a fixed-bed adsorber. Empty bed contact time (EBCT), linear velocity (m/h), and specific flow rate (m³ water/m³ media – hr) are the critical process parameters for proper operation. 

One of the advantages of iron adsorption is the predictability of the breakthrough. Another attractive characteristic of adsorption technology is its simplicity and relatively low cost. For example, coagulation/filtration has higher initial capital costs and is labor-intensive. Also, this technology is more complex than adsorption, a key concern for utilities without centralized treatment plants. However, coagulation or filtration can prove to be an effective solution when treating high levels of multiple contaminants. 

Reverse osmosis

The principle of osmosis is that water of a low salt content spontaneously crosses an osmotic membrane towards a higher salt content solution until it reaches osmotic equilibrium. Applying pressure to the higher-salt content solution reverses this phenomenon. Reverse osmosis removes all trace elements and requires re-mineralization for potable water use. Depending on how permeable the membrane is, maintaining the pressure gradient can generate nearly pure water. 

The risk of failure is high with this process, and the frequent removal of arsenite is more complicated. Also, as turbidity increases in the feed water, it reduces the efficiency of the process, and pre-treatment is required.

Ion exchange 

Ion exchange allows the exchange of one ion in the solids phase with one present in the water phase. Ion exchange resin usually consists of a synthetic resin, specific to the polluting agent to be removed. Arsenate forms an anion in water and is exchanged on an anionic resin. The feed water comes into contact with an exchange site, and the pollutant is exchanged.

When the resin is saturated, the bed is chemically regenerated back to its original form. The technology works in a narrow pH range and is limited by many competitive ionic species. These continue the exchange process and release arsenic from the resin if the resin has a higher affinity to these other anions. In fact, if the list of competitive ions is placed in order of affinity, arsenic would be near the bottom.

Activated alumina

Activated alumina can be used for adsorption. The system is simple, with little operator involvement and maintenance. However, the adsorption kinetics are finely balanced, and there is a significant danger of fouling. Also, the pH of the water is critical. Working best in a very narrow range, and any move away from the optimum will reduce performance.

Similar to that of ion exchange, activated alumina also shows affinities to other anions, as the bonds formed between the arsenic and the activated alumina are weak. Arsenite, with its neutral charge in water, does not absorb into the activated alumina. Activated alumina can also be chemically regenerated, but with harmful chemical regenerants and limited success as it only achieves 85-90% of original capacity after regeneration.

Precipitation coagulation

Precipitation coagulation filtration is traditionally employed in many water treatment plants for the removal of suspended solids and colloidal solids. A coagulant is added to the water, usually, an aluminum or iron-based salt, which increases the ability for colloidal particles to agglomerate in water. 

During flocculation, negatively charged ionic microparticles are attracted to the coagulant and held together. Arsenate is removed through the mechanisms of co-precipitation and some adsorption. The “flocs” that are formed can be separated from the water by filtration. Arsenite present in water forms non-ionic species; therefore, pre-oxidation must be used to achieve 90% removal of the arsenic. Systems require frequent backwashing (water losses) and a constant calibration of chemical dosing and sludge management.

De Nora’s arsenic experience

De Nora has been providing arsenic treatment solutions since the early 2000’s and has
over 100,000 gpm treatment capacity installed globally. This experience is unparalleled
in the industry, and our case history has led to the development of robust predictive models. De Nora’s inorganic treatment portfolio contains all of the technologies mentioned in this post:

• SORB 33^® arsenic removal system utilizing an iron oxide adsorbent
• Omni-SORB coagulation, oxidation, and catalytic media filtration system
• SORB 09 activated alumina for simultaneous fluoride + arsenic removal
• SORB 07 ion exchange system for simultaneous nitrate + arsenic removal
• De Nora UAT™ reverse osmosis systems

Source: WET News: Water Treatment, vol 11, Issue 12