CWQA Position Statement

Greensand

Greensand, or glauconite is a naturally occurring mineral that is granular in appearance and, as its name implies, green in colour when dry. When treated with manganese salt solution such as MnCl2 and potassium permanganate (KMn04), greensand can remove Mn(II), Fe(II), hydrogen sulphide and phenols. The mechanism by which removal of these substances occurs is a combination of sorption and oxidation, whereby the oxidized greensand adsorbs ions such as those of Fe(II) and Mn(II), which are oxidized with permanganate. Reduction in removal efficiency not due to interferences is due to depletion of the supply of electrons available for exchange. The greensand can be regenerated (i.e., electrons replaced) with the addition of an oxidizing agent such as potassium permanganate.

In addition to its oxidation properties, manganese greensand has catalytic properties that permit maximum utilization of oxidizing agents such as  permanganate, chlorine or dissolved oxygen, resulting in increased rate and completeness of reaction.

Manganese greensand particles are rounded with nodules. This, along with favourable particle size distribution, gives greensand excellent filtration properties by creating flow patterns offering maximum contact area.

Manganese greensand beds are typically operated in the down-flow mode and are fairly durable, with some beds operating continuously in excess of 10 years not showing any signs of deterioration. It is recommended, however, that pressure differential across the bed be limited to a maximum of 83 kPa (12 psi)

There are two forms of the process that are differentiated based on regeneration techniques used. When iron or sulphide removal (with or without manganese present) is the primary objective, a continuous treatment is usually applied. This treatment involves the continuous addition of potassium permanganate and other oxidizing agents directly into the water to be treated prior to manganese greensand filtration. Occasional backwashing and rinsing are required, which typically take about 20 minutes. If manganese is to be removed, intermittent regeneration treatment is employed. This technique involves removing the bed from service for about one hour to wash the bed.

Regenerate with a dilute solution of potassium permanganate, rinse and return it to service. One disadvantage of utilizing this process for manganese removal is bed growth due to oxidation of manganese onto the manganese oxide coating of the greensand bed. In waters containing 2 to 3 mg/L Mn2+, bed expansion could exceed 40% per year reducing bed life to two to three years.

The continuous process can easily treat water containing 18 mg/L Fe2+, and experiments have demonstrated that levels up to 35 mg/L could be removed; this would not be feasible at full scale, however, because almost half of the water produced would be required for wash water. A properly maintained greensand process will normally produce treated water with iron concentrations not exceeding 0.1 mg/L and manganese concentrations not exceeding 0.01 mg/L.

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Water Talk - Water Treatment Devices - For the Removal of Taste, Odour and Chemicals