This is a physical-chemical treatment that alters the properties of suspended or colloidal particles, helping them bind together into larger particles for easier filtration. In addition to particle removal, addition of chemicals causes soluble elements to precipitate, generally as hydroxides, which then adsorb other dissolved elements, including arsenic.

Equipment required for this application includes chemical dosers, mixing systems, flocculators, decanters (if particle concentration is high), filters, and sludge systems.

If arsenic (III) is present, it must be oxidized to arsenic (V) to improve removal, and a pH correction may be necessary for the coagulant to work in its optimum range.

This method can remove 90% of the arsenic, but the investment in equipment and the space required for the plant are limiting factors.

This type of treatment is very similar to coagulation-filtration. The main difference is that it's generally used for well water with low concentrations of suspended solids and colloids. In water with high levels of iron and manganese, it's efficient since the arsenic is removed by adsorption on the surface of iron and manganese hydroxides formed in the oxidation stage. It removes as much as 80% of arsenic from water.

Because of the absence of oxygen underground, well water usually contains higher concentration of arsenic (III) than surface water, which usually contains 100% arsenic (V).

Different technologies can be used for the filtration stage, but ultrafiltration is the most effective. It's usually combined with coagulation for enhanced filtration. Addition of a ferrous coagulant is followed by filtration through a low-pressure polymer membrane (ultrafiltration).

Generally, the water's pH value must be reduced before it reaches the membrane system in order to minimize residual arsenic concentration in the treated water. Addition of both the coagulant and the acid are performed in line, regardless of reaction tanks and pumping systems.

This type of process not only allows coagulant dosage to be 80% less than in conventional processes like sedimentation-flocculation, but the space required is significantly lower, which reduces both initial investment and operating expenses.

In addition to particle removal, this method offers the following benefits:

• Removal of viruses and bacteria
• Removal of small flocs, reducing the required coagulant dose
• Excellent quality of treated water no matter the quality of the feed water

The adsorption of arsenic on media coated with iron hydroxide is very effective because of the affinity between the two elements. In addition, low operating costs make this process the best alternative when treating high arsenic concentration for the production of both drinking water and mineral water. (More information about the media can be found under VSN-33.)

It is important to consider that other specific media, such as those based on titanium oxides, perform poorly with minimal variations in raw water quality. With even minimal variations in pH, which often occur seasonally in well water, the media can collapse and cease to operate correctly.

Removal of arsenic by ion exchange is one of the least-used methods since the selectivity of resins with arsenic is low. Anions that compete with the exchange sites, such as sulfates and nitrates, greatly reduce the resins' capacity to remove arsenic.

In addition, regeneration requires large amounts of chemicals, which generates waste streams high in arsenic and regenerant.

Resins used in this process are expensive and must be replaced more often than commodity resins, so this is usually not a cost-effective option.

Membrane separation can achieve up to 90-95% arsenic removal, but it has several significant drawbacks, including a higher investment cost, and the high arsenic concentrate produced by the process must be treated.

Different species of arsenic have different requirements: Rejection of arsenic (V) is higher due to its higher ionic charge, and arsenic (III) must be oxidized in order to maximize total rejection. A pH reduction is required to improve the effectiveness of removal.

VSN-33 is a convenient and cost-effective solution for removing arsenic from drinking water. The iron hydroxide-based media was designed to help reach the newest World Health Organization (WHO) requirement for arsenic: 10 parts per billion.

VSN-33's high capacity makes it an ideal non-regenerable, single-use product. It offers an affordable, easy-to-operate approach, especially for small or mid-sized systems. Eliminating regeneration means avoiding the hassles and cost of chemical storage and usage, and eliminating waste stream disposal issues.

VSN-33’s strong affinity for arsenic allows it to maintain a strong bond to the removed arsenic, allowing easier and safer disposal. It also features a longer useful life than traditional adsorbent media, for more productive, cost-advantaged operation.

VSN-33 removes not only arsenic (III) and arsenic (V), but also antimony, molybdenum, copper, phosphates, lead, uranium, selenium, and vanadium.