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MANGANESE DIOXIDE MEDIA FOR IRON, MANGANESE, AND HYDROGEN SULFIDE REMOVAL
FluksAqua Community, Water and wastewater community,
Iron (Fe), Manganese (Mn), and Hydrogen Sulfide (H2S) can present a very complex challenge to drinking water treatment. The combination of these contaminants is sometimes referred to as the “troublesome trio.” In Canada and the United States the maximum permitted concentration in drinking water is 0.3 ppm for iron, 0.05 ppm for manganese and hydrogen sulfide being a gas is limited to 3 threshold odor numbers and still not regulated by Environmental Protection Agency (EPA) [1 ppm= 1 mg/L]. The same treatment method used for reduction of one species can often be applied to all three1. However, due to the complexity of water chemistry, there can be variations in the treatment scheme. There are many treatment methods, which can be used for the reduction of all three contaminants1.
- -Ion exchange- Cationic and Anionic resin
- -Aeration plus filtration- Using atmospheric pressurized aeration followed my multimedia or filter media like sand.
- -Chemical oxidation plus filtration- Using oxidants such as Chlorine (Cl), Potassium Permanganate (KMnO4), Ozone (O3),Hydrogen Peroxide (H2O2)
- -Catalytic oxidation filtration- Manganese Dioxide (MnO2) based media
For the purpose of this article, we will be specifically discussing MnO2 based media. MnO2 based medias for Iron, Manganese and Hydrogen Sulfide reduction have been in use from around World War II. Greensand (more recently named Greensand Plus) was the most common media used for the reduction of the three contaminants. However, with recent advancement in technology, Greensand Plus has been losing its charm, primarily because of the high regeneration cost associated with the media and diminishing amount of support core material (example: gluaconite) needed for coating the oxidizing MnO2 layer. The media uses KMnO4 for regeneration. This chemical is expensive, hazardous and has a high maintenance cost.
The MnO2 based medias are of two kinds:
- -Manganese Dioxide coated media with a support base- Birm, Greensand Plus, Katalox light etc.
- -Manganese oxide solid mined ore- Filox, Pyrolox, MangOX etc.
For the purpose of this article, we will be doing a comparison between Birm, Greensand Plus, and Filox* only.
*Birm is a trademark product of Clack Corporation; Manganese Greensand is a trademark product of Inversand Company; Filox is a trademark product of Watts Water Technologies, Inc.
HOW DO MANGANESE DIOXIDE BASED MEDIAS WORK?
MnO2 based media are capable of doing two important operations. First, they oxidize the iron into particulate form. Second, the particulates are trapped in the media bed. Upon backwash, the particulates are reduced from the bed1.
If the water also has hardness ions present, it is recommended to reduce iron before you reduce water hardness5. Iron in dissolved form ferrous iron (Fe2+) is usually associated with carbonates. When the iron passes through the bed of iron reduction media, the iron gets oxidized to Ferric (Fe3+) and forms insoluble particulates of rust (ferric hydroxide, Fe(OH)3). The MnO2 based media acts like a catalyst in the process7. A catalyst is a substance, which increases the reaction rate without itself changing in any chemical form. Similar to iron, manganese in dissolved state is manganous ion (Mn2+) and is converted to insoluble manganese (Mn4+) ion. Hydrogen sulfide gets reduced to yellow sulfur particulate1, 2. The conversion reaction is dependent on the pH of raw water.
MANGANESE DIOXIDE MEDIA OPERATING PARAMETERS
The table below will show some of the key parameters for Birm, Manganese GreenSand and Filox:
|Operating Parameters (per cu.ft)
||Greensand Plus#, 4
|Service Flow rate (gpm/sq.ft)*
|Backwash Rate (gpm/sq.ft)*
| Bulk Density (lbs)
|% MnO2 conc.
#Service and backwash rates are based on different water temperatures (4-27 degree Celsius). When water is cold, it is more viscous and had higher density. Hence, a lower gallon per minute (gpm) amount of water is required per square foot (sq. ft) for backwash and vice versa for higher water temperature.
Birm– This is a light media and hence requires low backwash water. However, presence of dissolved oxygen (DO) is very important for the operation of this media. DO content of at least 15% and 29% is needed for iron and manganese reduction respectively 3. Hydrogen sulfide cannot be reduced using Birm media. Free chlorine in the water should be less than 0.5 ppm in concentration, as this can affect the media layer coating. No chemicals are needed for the regeneration of this media3.
Manganese Greensand– Because of the coating, this is a light media also. In this case, KMnO4 is needed to regenerate the media, so that the original oxidizing power of media layer is at full strength. The media can be used in two methods of operation: continuous regeneration (CR) and intermittent regeneration (IR) 4. The CR method is recommended for predominantly iron bearing waters with or without the presence of manganese, and the IR method is recommended for use on ground waters in which manganese predominates4. The capacity of reduction of this media depends on the KMnO4 dosage.
Filox– ORP (oxidation reduction potential) plays a very important role in operation. It is recommended that the ORP of the water to be treated is at least -170 mV 5; otherwise more oxidants need to be added. Chlorine is the most common oxidant. However, it is recommended that not more than 4 ppm of free Chlorine be added into the water as it may destroy the media6. Being a very heavy media, backwash is extremely important in the effective operation of this media. Optimum backwashing makes sure that the mined ore granules rub against each other to create attrition so that a fresh layer of MnO2 is available for operation 6, 7.
Reduction capacity of the three medias for individual contaminant presence per cu.ft is given in a table below:
||Greensand PlusX, 4
||1ppm using 1 ppm KMnO4 regent
||1ppm using 2 ppm KMnO4 regent
||1ppm using 5 ppm KMnO4 regent
XUsing continuous regeneration (CR) method.
Note: For Birm and Filox pilot studies need to be conducted if more than one contaminant is present in water.
From the above literature it is clear that each media has its own pros and cons depending on the water chemistry of the raw water. However, following points can be noted from the above discussion:
- In terms of bulk density Filox is the heaviest media and hence requires more water for backwashing compared to Birm and Greensand.
- Filox has the best contaminant reduction efficiency primarily because of its very high purity of manganese dioxide.
- Birm cannot be used in presence of any oxidants as this can hamper its performance, which is not the case with Greensand or Filox (but in limited quantity).
- The pH range for operation of all medias is almost identical.
Residential Water Processing. A Reference Handbook by Wes McGowan, third edition. Water Quality Association (WQA)
Clack Corporation. http://www.clackcorp.com/
Inversand Company. http://www.inversand.com/
Reverse Osmosis, Industrial Applications and Processes. Author: Jane Kucera. Wiley Publication
Watts Water Technologies, Inc. http://www.watts.com/pages/_products_details.asp?pid=5558
The Magic of Manganese Dioxide, What It Is and Why You Should Care. Author: Matthew Wirth. Water Conditioning & Purification, March 2013
The Nalco Water Handbook. Second Edition
Abundant Flow water. http://store.afwfilters.com/media-resin/iron-reduction/birm-media-1-0-cu-ft/
LINKS FOR FURTHER READING