Birm, Pyrolusite, Greensand for Iron and Manganese Removal

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After the oxidation step (with or without a detention or settling tank), the source water is filtered through a filter media in either a pressure vessel or a gravity filter to remove the iron/manganese/arsenic solids formed in the water.  The filtration media in these systems may consist of sand, sand and coal anthracite (dual media), or propri­etary/patented products, such as Pyrolusite - Pyrolox, Filox-R, Birm, and manganese greensand. Some media such as manganese greensand, have the ability to both oxidize and filter iron and manganese effectively and at the same time. Manganese greensand,  pyrolusite, Birm, or any media coated with manganese dioxide has the capacity to oxidize iron and manganese and filter the insoluble precipitates with the filter bed. These media also have some, but limited, capacity for As(III) oxidation and arsenic adsorption.

Birm
 
Birm is an acronym that stands for the “Burgess Iron Removal Method” (Figure 1). Typical applications have been point-of-use treatment, but it has been used in municipal treatment plants. Birm has the capacity to oxidize iron, but is not very effective at oxidizing As(III) to As(V). Birm is produced by impregnating manganous salts to near saturation on aluminum silicate sand, a base material. The manganous ions then are oxidized to a solid form of manganese oxide with potassium perman­ganate. This process is similar to that used to manufac­ture manganese greensand. The   manufacturer indicates that the presence of dissolved oxygen is necessary for Birm to function as an oxidizing media for iron oxidation.
Birm is available in an effective size of 0.48 mm and a specific gravity of 2.0. To be effective, it must be used in water with a pH range of 6.8-9.0.
Alkalinity should be greater than two times the combined sulfate and chloride concentration. Injection of com­pressed air ahead of the media to maintain a dissolved oxygen content of at least 15% of the iron content may be required, especially for source water with iron at concentrations greater than 3 mg/l.­ The dissolved oxygen oxidizes iron with Birm media serving as a catalyst that enhances the reaction between dissolved oxygen and dissolved iron and manganese in the water. Further, formed ferric hydroxide attracts oxidized arsenic, which then is captured in the filter bed.
Filter loading rates should be between 9,0-12,50 m3/m2 with a bed depth of 0,75-0,90 meters. Birm is not suitable for use with water containing hydrogen sulfide or organic matter exceeding 4-5 mg/lt. Chlorination greatly reduces Birm’s effectiveness and at high concentrations can deplete the catalytic coating. Polyphosphates can coat the media, thus reducing its effectiveness for iron removal.  No chemical addition or regeneration is required for Birm.  Backwash rates should be controlled in the range of 25-30 m3/m2 in order to achieve suitable bed expan­sion of approximately 30% for cleaning. An excessively high backwashing rate and air scour should be avoided to minimize attrition loss. Underdrains may include a gravel support bed or may be of the gravel-less type.
 
Figure 1. Birm material.

Manganese Greensand
 
Another media that converts soluble forms of iron and manganese to insoluble forms that can then be filtered is manganese greensand (Figure 2). Manganese greensand has been used for several decades and is formed from processed glauconite sand. The glauconite is synthetically coated with a thin layer of manganese dioxide, which gives the dark sand a definite green color and thus its name.  Limitations for manganese greensand include a maximum limit of 5 mg/L of hydrogen sulfide removal and 15 mg/L for iron removal; also, water pH should be in the range of 6.2-8.5

The combination of a strong oxidant and manganese greensand filtration media for iron removal is commonly referred to as the “Manganese Greensand Process.” Either potassium permanganate or chlorine can be used to effectively regenerate manganese greensand filters. However, if chlorine is used alone, it may be necessary to periodically regenerate the manganese greensand using potasium permanganate by a batch process in order to maintain optimum effectiveness of the media. Prechlorination is often recommended if iron levels are significantly greater than 1 mg/L in order to reduce the need for the more expensive potassium permanganate. Manganese greensand is somewhat smaller than typical filter sand, with an effective size of 0.30-0.35 mm and a specific gravity of about 2.4.

The density of greensand at 85 lb/ft³  considerably lower than pyrolusite, but greater than Birm. A vigorous backwash by air scouring is recommended. Backwash rates typically are in the range of  25-30 m³/m² and should be preceded by an air scour of the media to attain at least 30% bed expansion. A gravel support bed with a gravel retaining screen is recommended over the underdrain system. The continuous regeneration of greensand using a strong oxi­dant has two purposes: (1) it reactivates the manga­nese dioxide on the greensand and (2) it oxidizes Fe(II) and As(III). This allows the newly formed As(V) and any residual As(V) to adsorb to the ferric hydroxide particles, which then are captured in the filter bed. Potassium permanganate should be fed in the piping far enough ahead of the filter to allow mixing and contact for several seconds before entering the filter. 

 

Figure 2. Manganese greensand material.

Pyrolusite
 
Pyrolusite is the common name for naturally occurring manganese dioxide and is available in the United States, United Kingdom, South America, and Australia. It is dis­tributed under brand names such as Pyrolox, Filox-R and MetalEase. It is a mined ore consisting of 40 to 85% manganese dioxide by weight. The various configurations of pyrolusite provide extensive surface sites available for oxidation of soluble iron and manganese. Removal rates of iron in excess of 20 mg/L are achievable.
Pyrolusite is a coarse oxidizing media with a high specific gravity of about 4.0. Like silica sand, pyrolusite is a hard media with small attrition rates of 2-3% per year. Pyrolusite may be used in the following two ways: (1) Mixing with sand, typically at 10­-50% by volume, to combine a filtering media with the oxidizing properties of pyrolusite; (2) Installing 100% pyrolusite in a suitably graded filter to provide oxidation and filtration. 
Maximum hydraulic loading rates of  9,0-12,50 m³/m² should be the basis of design for a pressure vessel. No chemical regeneration is required. Backwash is critical for proper operation. Attrition during backwash can be a benefit as it exposes more surface sites for oxidation of soluble iron and manganese. The density of pyrolusite is in the range of 120 lb/ft³, requiring a backwash rate of 50-60 m³/m² to fluidize the bed, scrub the media, and redistribute the media throughout the bed. Air scour and backwashing are recommended in simultaneous mode. If water backwash alone is used, air scour prior to backwash is recommended with a water backwash designed for 75 m³/m² in order to flu­idize the bed at least 30%. If a gravel support over the underdrain is used, a gravel retaining screen should be included in the design. The manufacturer recommends daily backwashing to maintain the effectiveness of the media for oxidizing and removing iron and manganese.

 

 

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