The manganese greensand process has been used effectively to remove iron, manganese, and hydrogen sulfide since the 1950s in the U.S. Greensand is an organic fertilizer that contains the deposits that were once part of the ocean floor. It is a bluish-green color, and is also called “glauconite”. The glauconite is first stabilized then coated with manganese oxide. This coating provides the glauconite with its special chemical oxidation-reduction properties for the removal of iron and manganese, as well as small quantities of hydrogen sulfide. The greensand process for removing iron and manganese is usually accomplished by using one of the following two procedures.
Continuous regeneration (CR) or intermittent regeneration (IR)
Continuous Regeneration is primarily used when iron removal is the main objective. The CR method continuously feeds an oxidizer, such as chlorine, potassium, permanganate, or a combination of the two, into the raw water prior to the filter. This process can remove 15 milligrams per liter or more of soluble iron.
Intermittent Regeneration is normally used when manganese removal is the main objective. In this process, manganese oxidation occurs directly using the properties of the freshly regenerated manganese greensand. After treating a specific amount of water, the oxidation capacity of the media will be consumed and regeneration is required. One drawback in the IR process is the extra time the filter out of service for backwashing. In comparison, the IR process takes 75 minutes and the CR process can be as fast as 15 minutes. If there are high levels of both iron and manganese then both methods can be used in combination.
All new greensand must be fully regenerated prior to placing it in a manufactured unit, homemade unit, or other filter container for it to work. The procedure is printed on each bag of greensand that you purchase and consists of soaking the media for several hours in a 2 to 3 percent solution of potassium permanganate (KMnO4). This can be done in a bucket or the filter unit. An easy procedure to follow is to mix one pound of KMnO4 per five gallons of water. Apply the solution to the filter media until it is covered, then allow it to rest overnight. The minimum depth of the greensand bed is 24-30 inches when using the IR method and 18-20 inches when using the CR method. If you are removing large amounts of pre-oxidized iron, reducing the depth of the greensand will allow for a deeper cap of anthracite.
One thing to consider is if manganese and hydrogen sulfide are the primary contaminants the filter may remain effective for several weeks before regeneration is needed. However, with levels of iron ranging from 0.5 up to 10 or more mg/L, regeneration may be needed from every two days down to every 4 hours, depending on whether there is continuous or intermittent flow through the filter. If chlorine and/or KMnO4 are used for pre-filter oxidation of iron, the run time can be extended for higher iron levels with the addition of an anthracite layer on top of the greensand media. A primary concern is to not trap too much ferric hydroxide within the greensand because you can have precipitated iron coming through and a much greater pressure will be needed for adequate backwashing and regeneration. NOTE: Keep in mind that time to let the water run for a filter unit on a private water system is based on the time water is actually flowing through the filter.
For example, your water system may have a pump capacity to provide a flow rate of over 10 gallons per minute and a 20-gallon storage tank. If your filter recommends using a 36-hour backwash and regeneration cycle for the iron level in your water for its 1 square foot surface area, this is based on continuous flow time. The 36 hours of flow time on the filter may take more than a month depending on your water use. If your household uses about 600 gallons of water per day, your pump would run about 60 minutes per day and your backwash cycle would be every 36 days. Most of the manufactured units have timers and are set up for automated backwash and regeneration, just like water softeners.
The following morning the marinated media is backwashed until no permanganate (pink color) remains in the wash water. If chlorine is being used as a pre-oxidant followed by KMnO4 for continuous regeneration (CR) of the greensand, maintaining a chlorine residual at the filter effluent of greater than 0.5 mg/L will ensure that the greensand retains its manganese oxide coating on the sand particles. Before changing the configuration of any filter bed, contact your supplier or the manufacturer. Maintain safety first while performing any of these steps.
NOTE: If residual chlorine level is too high, it should be reduced to a safe human consumption level by carbon filtration.