MANGANESE GREENSAND FILTER

Manganese Greensand Filters are used for the removal of soluble iron, manganese and hydrogen sulfide from the well water supplies.  Manganese greensand is a purple-black filter medium processed from glauconite greensand.  Manganese greensand is a proven technology for iron, manganese and hydrogen sulfide removal.  The unique chemical and physical characteristics allow for the utilization of two different methods of operation: the continuous regeneration (CR) method and the intermittent regeneration (IR) method.

CONTINUOUS REGENERATION (CR) METHOD OF OPERATION:

Continuous Regeneration (CR) operation is recommended for well waters where the main objective is iron removal with or without the presence of manganese.  Potassium permanganate (KMnO₄) and/or chlorine (Cl₂) are fed directly into the raw water ahead of the Greensand Filter.  Chlorine should be fed upstream of the potassium permanganate by at least 10-20 seconds.  The chlorine residual should be measured in the filter effluent.  Potassium permanganate, if required, should be fed to produce a “just pink” color in the filter inlet.  This slight excess of potassium permanganate or a chlorine residual carried through the filter will maintain the greensand in a continuously regenerated state.  The manganese dioxide coating of the greensand has the ability to catalytically oxidize iron and/or manganese under certain conditions. Pre-chlorination alone without potassium permanganate, may be all that is required to carry out the oxidation process.  The Manganese Greensand CR process has been successful in removing radium and arsenic from well water supplies.  The iron and/or manganese precipitates that are formed adsorb the radium and arsenic.  Radium removal requires that soluble manganese be present in the raw water supply.  Arsenic removal has been achieved with either iron or manganese being present in the raw water supply.  Pilot testing is recommended for Radium or Arsenic removal applications.

LENGTH OF SERVICE RUN FOR CR SYSTEMS:

The length of service run is the amount of time that the filter can provide water before backwashing is required.  The equation below provides a way to calculate that length of time in minutes.  To convert to hours; divide by 60.  The Chemical Demand must be converted from mg/l to gpg by dividing by 17.1 (17.1 mg/l = 1 gpg).

CR SYSTEM OPERATING PARAMETERS:

Bed Type..................... Manganese Greensand is commonly used as a single media filter bed.  The minimum bed depth for a filter with Manganese Greensand only, is 24”.  It is recommended that an anthracite cap be placed on top of the greensand in larger filters where practical.  The bed depth on a dual media filter is 15” - 24” for the Manganese Greensand and 12” - 18” for the Anthracite cap.

Capacity...................... The removal capacity of greensand is 500 - 700 grains of oxidized iron and manganese per square foot of bed area based on potassium permanganate demand and a maximum pressure drop of 8 - 10 psi during the service run.  In some cases, ground water contains iron that filters in depth and the pressure drop may only be 4 - 6 psi before iron appears in the filter effluent, dictating that backwashing is required.

Backwash.................... Backwash is normally 10 - 12 gpm/ft², dependent on water temperature and should be performed with filtered water.  Bed expansion should be at 40%.  Air Scour can be utilized on large commercial or industrial filters to supplement backwash water if desired.  The air should flow at the rate of 0.8 - 2.0 cfm/ft² with a simultaneous backwash with treated water at the rate of 4 - 5 gpm/ft².  Backwash should last 10 to 15 minutes or until water runs clear.  Rinse is recommended at service flow rate for 3 - 5 minutes.

Flow Rate.................... Recommended service flow rates with CR operation are 2 - 5 gpm/ft².  Intermittent flow rates of 8 - 10 gpm/ft² are possible.  The higher the concentrations if iron and manganese, the lower the flow rate should be to achieve equivalent lengths of service runs.  Higher flow rates are possible with very low concentrations of iron and manganese, but test units should be run to prove that the system will perform as required.

INTERMITTENT REGENERATION (IR) METHOD OF OPERATION:

Intermittent Regeneration (IR) operation is the process by which the Manganese Greensand Filter is backwashed, then regenerated down flow with a solution of potassium permanganate (KMnO₄) in a batch process, after a predetermined volume of water has been treated.  The manganese and soluble iron are removed when it comes in contact with the surface of the greensand granules.  This process is referred to as contact oxidation.  Hydrogen Sulfide utilizes the oxidizing capacity of the greensand with the resultant precipitates removed by filtration within the bed.  Anthracite can be used in conjunction with the greensand, if iron is present in the raw water to be treated.  The batch regeneration process is done at the end of the service cycle.  The unit is backwashed and then regenerated with a solution of potassium permanganate to restore the oxidative capacity of the manganese greensand.  The regeneration level should be 2 - 4 ounces of potassium permanganate per cubic foot of media.

IR SYSTEM OPERATING PARAMETERS:

Bed Type..................... The majority of Intermittent Regeneration Manganese Greensand filters are used for domestic applications rather than commercial or industrial applications and are single media depth filtering units.  A minimum bed depth of 30” has been established for single media manganese greensand filters.  The bed should be regenerated with potassium permanganate prior to placing it in service.  An anthracite cap is not required but can be used to improve the removal of iron by acting as a filtration media for oxidized iron.

Capacity...................... Manganese greensand has the capability of removing 550 grains per cubic foot of iron alone from a water supply.  If iron and manganese exist together, then the removal capacity is 400 grains per cubic foot.  If manganese exists without iron then the capacity is only 300 grains per cubic foot.  Manganese greensand can only remove up to 175 grains of hydrogen sulfide.

Backwash.................... Backwash is normally 10 - 12 gpm/ft², dependent on water temperature and should be performed with filtered water.  Bed expansion should be at 40%.  Backwash should last 10 to 15 minutes or until water runs clear.  Rinse is recommended at service flow rate for 3 - 5 minutes.

Flow Rate.................... Recommended service flow rates with IR operation are 2 - 5 gpm/ft² or 1 - 2 gpm/ft³.  The higher the concentrations of iron and manganese, the lower the flow rate should be to achieve equivalent lengths of service runs.  Higher flow rates are possible with very low concentrations of iron and manganese, but test units should be run to prove that the system will perform as required..

Regeneration................A batch regeneration must be performed on a regular basis.  The frequency of regeneration is dictated by the capacity of the manganese greensand in relation to the amount of contaminants (iron, manganese and hydrogen sulfide) present in the water to be treated.  The regeneration level is 1 - 4 oz. of KMnO₄ per cubic foot of greensand.  A gallon of water will dissolve 2 – 4 ounces of potassium permanganate dependent on the temperature of the water.  Optimum regeneration time is 30 minutes.  Rinse volume is 40 -50 gal./cu-ft. or until all traces of potassium are gone.

Initial Conditioning..... Manganese greensand is NOT shipped in a regenerated form; therefore, it is necessary to regenerate the manganese greensand with a solution containing 1 gallon of water and 2 ounces to 4 ounces of potassium permanganate for every cubic foot of media.  The filter media should be soaked in this solution for a minimum of 1 hour, then rinsed of all traces of potassium permanganate prior to placing the unit in service.

Removing Fines.......... Prior to placing the filter into service, Manganese Greensand should be thoroughly backwashed and the top layer (approximately 1”) of fine material removed.  This is especially important if anthracite is placed on top of the greensand bed.