Targeting Iron & Manganese in Drinking Water
Appearances can be deceiving when it comes to drinking water. While trace amounts of dissolved iron and manganese are invisible to the naked eye, even low concentrations of these metals can create a variety of problems for residential, commercial and industrial users.
Dissolved iron and manganese, and other common water constituents such as aluminum, chloride, total dissolved solids, zinc, and foaming agents are considered to be “nuisance” contaminants. Although they are not considered health hazards, if present above threshold concentrations established by the U.S. EPA’s non-mandatory National Secondary Drinking Water Regulations, these contaminants can create a host of aesthetic problems, such as undesirable flavors, odors, color or cloudiness, in treated water. That is why most municipal water treatment facilities take steps to comply with these voluntary standards.
The regulations call for dissolved iron in treated drinking water to be maintained at or below 0.3 mg/L, and dissolved manganese to be maintained at or below 0.05 mg/L. Above these threshold concentrations, iron and manganese can turn drinking water rusty, create stains in laundry, and cause discoloration of porcelain sinks and tubs. If left untreated, excessive dissolved iron and manganese in water can also lead to corrosion of water treatment equipment and distribution systems, and result in the deposition of insoluble metallic complexes inside piping and equipment components, which can reduce overall water flow through the system.
Water treatment toolkit
A variety of technologies are available to remove dissolved iron and manganese from source water. The most commonly used techniques are coagulation and filtration, sequestration, ion exchange, and oxidation and filtration.
In coagulation and filtration, a treated filtration media, such as manganese greensand or a newer synthetic alternative, is used to convert soluble iron and manganese in water into insoluble complexes, which are then precipitated out of the solution and removed by pressure filtration.
Sequestration involves the addition of sequestering agents such as polyphosphates followed by chlorination (or in some cases sodium silicate followed by chlorination), which keeps dissolved iron and manganese from oxidizing and precipitating out of the solution. This option, however, is only available for water with less than 1 mg/L iron and less than 0.3 mg/L manganese.
Ion exchange involves the use of a conventional ion exchange resin to selectively remove iron along with calcium and magnesium. This solution, however, is limited to applications with relatively small quantities of iron and manganese. If either of these metals is allowed to oxidize to form insoluble complexes during the process, the resulting solids can clog and foul the ion exchange resin, reducing its efficacy.
Oxidation and filtration via manganese greensand has become the most widely used method for the removal of dissolved iron and manganese in recent years because of its relative ease of operation, low maintenance, low energy requirements, and reliability compared to the other options.
Designed to both promote the oxidation and flocculation of dissolved iron and manganese—converting soluble ferrous iron (Fe2+) to insoluble ferric iron (Fe3+), and dissolved Mn2+ to the less soluble Mn4+ form—and remove the resulting flocs by filtration, manganese greensand systems rely on particles of the naturally occurring silicate mineral glauconite, which have been coated with manganese oxide (MnO) in various valence states.
As the run progresses, the greensand bed is periodically backwashed to remove the collected solids. As the oxidizing power of the bed becomes depleted over time, it can be restored using either an intermittent or continuous regeneration process that involves the use of the powerful oxidizing agent potassium permanganate (KMnO4).
Recently, a synthetic alternative to traditional manganese greensand, dubbed CalMedia GSR Plus, has demonstrated commercial-scale efficacy for the simultaneous removal of dissolved iron and manganese during drinking water treatment. This newer oxidation-filtration media consists of granules of a proprietary, inert substrate that are coated with manganese dioxide (MnO2), and offers a variety of cost and operational advantages over traditional manganese greensand.
The MnO2 coating on the CalMedia GSR Plus granules enhances the oxidation reactions that cause dissolved iron and dissolved manganese to form solid, insoluble precipitates. The MnO2 coating also acts as a buffer to reduce any excess KMnO4 (used for regeneration) in the water, ensuring that this powerful oxidation agent (and its signature purple color) does not taint the treated water or enter downstream service lines.
Similar to greensand filtration, the oxidation capacity of the CalMedia GSR Plus bed can be continuously maintained by adding a constant feed of both chlorine (fed as hypochlorite solution or in gaseous form) and KMnO4 to the water ahead of the filtration unit. Chlorine is relatively inexpensive compared to KMnO4 and does most of the work converting Fe2+ to Fe3+, so when both oxidizing agents are used simultaneously, smaller amounts of the more costly KMnO4 can be used to oxidize any remaining iron and the bulk of the dissolved manganese.
In some applications, operators may opt to regenerate the bed intermittently, using a periodic down-flow passage of a dilute potassium permanganate solution through the bed followed by a rinse cycle. (Such an approach typically uses a weak solution involving 1.5 to 2 oz of KMnO4 per cubic foot of media.)
Compared to manganese greensand, the granules that comprise the media are larger, less dense and more lightweight.
These physical attributes produce several operational and maintenance advantages. For instance, the larger particle size of the granules results in greater overall porosity and permeability of the packed beds compared to greensand beds, and this yields higher throughput rates and lower pressure drop. For non-gravity-fed systems, this can also save money by reducing pumping rates. The higher bed porosity also results in greater floc-holding capacity, which extends the filter run times between backwash cycles, and the larger particle size reduces the risk of losing smaller particles during backwashing.
An important distinction between the two competing filtration materials is that a filtration system packed with CalMedia GSR Plus requires no air-scouring step during the backwash cycle, whereas the greensand filtration system does. This not only simplifies and streamlines the backwash cycle, saving money and effort, but the elimination of the air-scour step also reduces particle attrition.
In greensand beds, the particle breakage that results from air scouring often leads to clogged pores and reduced bed permeability, which reduces throughput over time and shortens the life of the filtration media.
Case in point
South Bend Water Works in Indiana supplies drinking water to 120,000 people in St. Joseph County, including 98,000 residents of the city of South Bend. The facility treats groundwater from eight different well fields, and in 2004, its average production was 19.4 mgd, with a peak pumpage rate of more than 70 mgd.
An essential part of South Bend’s overall treatment train is its Pinhook filtration plant. The gravity filters at this plant have relied on manganese greensand to remove iron and manganese from its groundwater source since a plant upgrade in 1998.
“If you’re treating groundwater anywhere in the U.S., you’re going to have a problem with iron and manganese,” said Dave Tungate, water quality specialist at South Bend.
The filtration plant has a capacity of 12 mgd and uses eight gravity filters, each of which can filter 1.5 mgd of water. During a routine filter maintenance overhaul in the spring of 2004, operators at South Bend found that manganese greensand levels in each of the plant’s eight gravity filters were a bit low.
The utility decided to consolidate the manganese greensand in seven of its eight filtration units, and replace the oxidation-filtration media in the eighth filter with CalMedia GSR Plus to evaluate its performance against the manganese greensand, as well as against a silica sand filtration unit operating at another facility.
“Manganese greensand is hard to come by, and in our case, we were told that the lead time to replenish our filters would be as much as 18 months,” Tungate said. By contrast, the CalMedia GSR Plus filtration media was readily available. After pilot testing to demonstrate that the efficacy of the media met the operators’ expectations, in the spring of 2004, one of the eight gravity filters at Pinhook was equipped with the filtration media.
At the Pinhook facility, groundwater is first dosed with chlorine and KMnO4 and then sent through the gravity filters. The addition of KMnO4 to the water entering the filters ensures that the oxidation potential of the filtration media is continuously maintained. The finished water is then pumped to a storage reservoir, and either used for periodic backwashing of the filters or with the addition of further chlorine and fluoride, it is supplied to customers.
Each of the 20-by-20-ft greensand filters at the Pinhook facility is configured with 18 in. of manganese greensand and a 12-in. anthracite cap. By contrast, the CalMedia GSR Plus filtration unit is configured with 24 in. of the filtration media and no anthracite cap.
“While the anthracite cap provides additional filtration capabilities before the water reaches the greensand layer, the light, low-density material is always at risk of getting blown out during any vigorous backwashing, so it represents an ongoing maintenance issue,” Tungate said.
The media’s larger overall grain size compared to greensand gives the filtration vessel that is packed with it a higher overall filtration rate.
“We were initially concerned that the water would not have enough retention time to ensure proper iron and manganese oxide and removal,” Tungate said. To compensate for this, the plant operators initially closed the GSR filter effluent valve partially, to slow the bed’s water-throughput rate, but they found that the iron- and manganese-removal capability of the bed was comparable to the greensand vessels, even at faster throughput rates and lower retention times.
“We found that the bed can actually filter more water than a comparably sized greensand bed and still meet our iron- and manganese-removal standards,” Tungate said.
Meanwhile, the elimination of the air-scouring step during the backwashing of the filtration media not only reduces granule attrition, but also shortens and simplifies this part of routine filter maintenance considerably. For instance, at Pinhook, the typical manganese greensand filter bed requires about 45 minutes to complete the water and air backwash, using water backwash rates that range from 1,200 to 3,200 gpm, Tungate said.
By comparison, because the filtration vessel loaded with CalMedia GSR Plus has no anthracite cap that is susceptible to blowout, and has higher porosity and permeability, higher backwash rates (up to 5,000 gpm) can be used. This, coupled with the elimination of the air-scour step, reduces the entire backwash cycle to 22 minutes from start to finish.