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Water Treatment



An integrated solution for water treatment

By Michael Smith - 18/05/2018

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There are many treatment options to remove hardness and Fe/Mn from drinking water, but finding a single solution to address them both can be a challenge for a municipality. We spoke to Tonka Water, a U.S. Water brand, about taking an integrated approach to this problem, demonstrated with a case study applied in Ohio, USA.
 
The challenge
Iron and manganese (Fe/Mn) are common in groundwater supplies. Iron is the more common contaminant and is known to be a major cause of membrane contamination and product failures, but the two often occur together. Although iron and manganese do not pose any known adverse health risks for most people, high levels of these minerals can cause discoloured water and stained plumbing fixtures, and they might contribute to an unpleasant metallic taste to drinking water.
 
Therefore, the US Environmental Protection Agency (EPA) has established a recommended maximum contaminant level of 0.3mg/L Fe and 0.05mg/L Mn. These limits are set for aesthetic reasons and they are not currently enforceable by the EPA, but they are intended as guides, which individual states can adopt as guidelines or enforce them as contaminants.  Many membrane manufacturers ask that iron be reduced to less than 0.1 mg/l to avoid fouling of their component product.
 
Treatment Options
Iron and manganese can be found in water either in a dissolved or particulate state. While water tests generally report the overall level of Fe/Mn, they don’t usually indicate the form, which is important to know when trying to select a treatment option.  In addition to the type of Fe and/or Mn that is present, the effectiveness of treatment system options can also be affected by the water’s pH and hardness content as well as the presence of microorganisms such as iron bacteria. The most appropriate and cost-effective options can depend on the concentration and form of Fe/Mn, water chemistry, and downstream equipment protection requirements.


Sequestering:  Rather than removing Fe/Mn from the water, sequestration binds Fe/Mn in soluble form, preventing oxidization. This is only an option if the Fe/Mn are in the form of ferrous iron (Fe2+) and manganous (Mn2+), and if the combined concentration is below 3 mg/L. Polyphosphates followed by chlorination can be an inexpensive method for sequestering Fe/Mn in a pH range of 5 to 8. However, because phosphate compounds are nutrients that contribute to the eutrophication of surface waters, appropriate methods of treating the waste water must be used. Sodium silicate and chlorine – which breaks down less readily than phosphate compounds in hot water systems – is an alternative for Fe, but is less effective for Mn.

Ion exchange: Salt-based softeners that contain a cation exchange resin can usually help reduce small amounts of dissolved iron and/or manganese from water (less than 1 mg/L iron). Water softeners are usually only considered if water hardness is also a problem, and when the combined Fe/Mn is below 5mg/L. Ion exchange will not work if the iron has oxidized, if the iron has combined with organic material or humic acid, or if iron bacteria is present.

Oxidizing filters: Oxidizing filters are generally used to treat higher levels of Fe/Mn. Examples of products that fall into this category include greensand, anthrasand (anthracite sand) and zeolite filters. Potassium permanganate (KMnO4) is used to coat greensand and anthrasand with manganese oxide, giving it a catalytic effect; natural and synthetic zeolite filter media have a catalytic effect. While these systems can generally be used to treat both dissolved and particulate iron, the pH of the water must be at least 7, and greensand and anthrasand systems require regular maintenance/backwashing to remove the Fe/Mn captured by the system.

Chlorination/filtration: This treatment option is recommended for situations involving iron bacteria in the water supply. The chlorine will not only control iron bacterial growth, but also oxidize the iron, bringing it out of solution so that it can then be filtered through a sediment or backwashing filter.

Reverse Osmosis: For water supplies containing trace levels of iron that are adversely affecting taste, a reverse osmosis system can be used. This water purification technology uses a semipermeable membrane to remove ions, molecules and larger particles from water. Reverse osmosis can remove many types of dissolved and suspended species from water, including bacteria, and is used in both industrial processes and the production of drinking water.
 
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Integrated approach
Therefore, each approach has its strengths and limitations, and it can be challenging to find one solution to address the requirements of a municipal area. An obvious alternative is to provide an integrated approach, using different options simultaneously, chosen to suit the area in question. We spoke to Tonka Water about a recent case study that exemplifies this approach.
 
The Village of Mount Sterling is located in the central region of Ohio, USA, where the residents needed an integrated solution to achieve treatment of their water for Fe/Mn and hardness. They selected Tonka Water, a U.S. Water Brand‘s customized water treatment solution including the Dualator III and the Flux RO/NF reverse osmosis membrane system.
 
To address Fe/Mn concerns, Tonka Water’s Dualator III was selected for membrane pretreatment. The Dualator III’s unique design includes aeration, detention and filtration in a single compact unit. It is equipped with a low-profile aerator for oxidation, followed by auxiliary oxidant addition, 30 minute detention with baffled settling compartment, and a four-celled gravity filter.
 
The Dualator III features Tonka Water’s Simul-Wash backwash system, saving millions of gallons of washwater over the lifetime of the equipment. This design can be used where the effluent from the filter is collected by gravity and routed to a clearwell, or is pumped directly to the distribution system. It is relatively easy to integrate with ion exchange softening or other post treatment.
 
An additional feature of the Mount Sterling Dualator III system is an isolated cell design, providing an efficient footprint while offering operational flexibility. Operators have the ability to isolate individual cells within each vessel, so they can take a cell off-line for maintenance while continuing to treat water in the other cells.
 
During the second stage of treatment, filter effluent is directed into two streams: one, is a blendstream bypassing the Flux RO/NF membrane system, and the second stream feeds into the Flux RO/NF system for hardness reduction. The membrane bypass stream is then blended back with the more chemically aggressive membrane effluent stream to form stable product water for consumption.
 
Conclusion
An integrated approach to water treatment can help to ensure that the various requirements of an area can been addressed and that a single responsible manufacturer is engaged. In the example shared here, Fe/Mn was managed in drinking water through the use of oxidation, detention, filtration and reverse osmosis. Combined, these approaches remove Fe/Mn in dissolved or particulate state, across a range of pH and hardness content, as well as the presence of microorganisms such as iron bacteria.
 
The system described, which has been developed by Tonka Water, provided an efficient footprint and operational flexibility, while minimizing washwater and reducing the environmental impact of the process.
 
The future of municipal water management may involve more systems such as these, which avoid fragmentation of system responsibility and are breaking new ground in water treatment technology, providing more drinkable water for people across the world and working toward greater sustainability.

 

Contact:
Mary Sitko, Director of Municipal Equipment Sales
E: msitko@uswaterservices.com
W: www.tonkawater.com