What Causes the Black or Brown Staining from Water?
Manganese will cause a black stain and is often accompanied by iron staining and hydrogen sulfide (egg) odor. In combination with iron, manganese staining will sometimes be chocolate colored or brown.
Evidence of manganese staining is usually most prominent in the dishwasher. Detergents raise the pH of the water high enough (>8) to allow the manganese to precipitate easily. In the toilet reservoir tank, it will show up as a film floating on the surface of the water. Shining a flashlight across the surface will make the film more obvious. In severe cases, this film is sometimes mistaken for oil. Touching the surface of this water breaks the film into flakes with jagged edges. At high concentrations manganese can be a health risk. The State of Maine has set a maximum exposure guideline (MEG) of 0.5 mg/L for manganese.
Manganese Generally Exists in Two Forms
Manganous manganese – This form of manganese will dissolve in water and like dissolved sugar it is invisible. Just like sugar when dissolved in water, the manganese can not be mechanically filtered from the water. This is the most common form found in our local Maine water.
Manganic manganesee – This form has precipitated (formed a solid and is no longer in solution) and gives the water a cloudy black appearance. This form of manganese can be mechanically filtered. It is rare to find manganic manganese in Maine ground water because waters in our area rarely have a pH above the 8 required to oxidize the manganous form into the manganic form.
TREATMENT OF MANGANESE IN WATER OPTIONS
Manganic manganese can be removed by any of the backwashing filters commonly used. Since the manganese is already oxidized the water only needs to be filtered.
Manganous manganese concentrations can be reduced by either of two methods. Ion exchange in a water conditioner(water softener) is the most common method used to reduce manganese. This method can be used on almost any concentration of manganese. This method will only be successful by itself if all the manganese is in the manganous form, the TDS is relatively low (generally <500ppm), the pH is low (generally <8) and there is very little oxygen in the water. The TDS has to be low to assure that there is no bleed through due to the manganese being removed from the resin once attached. High TDS indicates there are other minerals in the water competing for the sites on the resin (media) the manganese has attached to. The low pH and low oxygen assure that the manganese will not oxidize while attached to the media (resin). If the manganese oxidizes once attached to the media, it can not be removed during regeneration.
A second method to remove manganese, is a two step process called oxidation-filtration. The manganese is first oxidized by the use of oxygen, chlorine or potassium permanganate. The oxidation causes the manganous manganese to form manganic manganese. The manganic manganese is then removed by filtration. This method is not typically used on very high concentrations (> 8 or 10ppm) of manganese because the filter beds will require more frequent backwashing (automatic cleaning) than is reasonably possible. This method will usually require the use of some kind of pH correction because manganese will not oxidize below a pH of 8. Some water treatment companies will try to convince their customers that the pH does not have to be above 8 to guarantee the removal of manganese. THIS IS NOT TRUE. There are conditions that the pH does not have to be above 8 but since these conditions will not always be true the results will be inconsistent
Oxidation Filtration Methods
Oxygen with Air Injection
There are number of techniques used to introduce air into water to provide oxygen for the the oxidation of manganese. The most common method is called air injection and will be the focus of the following discussion. Air injection uses a venturi to inject air into the water. There are many brand names for this type of system but they all require a minimum flow (usually at least 5 gpm) from the well pump to allow the venturi work. This flow must be tested before this type of system can be used. Good systems will consist of 3 parts; venturi, air release tank, and filter tank. The air release tank removes any undissolved air. If the air is not released, severe spitting will result at the faucet. The filter tank uses a catalytic that contains manganese to speed the oxidation process. Once oxidized, the media removes the precipitated (manganic) manganese that was formed by the oxidation process. We generally avoid using any type of oxidation system that uses air because they are easily clogged and rendered ineffective by the oxidized mineral. These system can also bleed manganese into the water. The manganese that bleeds into the water can get to levels high enough to be toxic.
Chlorine is introduced into the water by one of two methods; it is pumped in with a solution feed pump or dropped in tablet form directly into the well. The water is then either sent to a retention tank and then to a filter or directly to a filter. The choice of method will be determined by the severity of the manganese problem and the type of media chosen for the filter tank. If chlorine is used, it may be desirable to remove it with carbon. If the manganese levels are low (<2 ppm), the carbon can also be used as the filter that removes the manganic manganese that is formed. If the manganese is greater than this (2 ppm) then a separate filter with filter aggregate media (AG) should be used to remove the manganic manganese so the carbon will not be fouled. Some people will opt to use filter AG as the filter and only use carbon at a sink as a point of use treatment to remove the chlorine from the drinking water. If this is done, there will still be chlorine in the water at all other faucets and showers. The chlorine should be able to be regulated so that it is of no higher a concentration than that found in city water. CARBON SHOULD NOT BE USED UNTIL THE WATER IS CHECKED FOR RADON BECAUSE THE CARBON CAN BECOME A RADIOACTIVE SOURCE.
This method uses a catalytic media (called greensand) coated with manganese that is treated periodically (like the regeneration of a softener) with potassium permanganate. The potassium permanganate acts as an oxidant (like the chlorine or air). When the manganese in the water comes in contact with the surface of media it oxidizes and the manganic manganese is then filtered out by sticking to the media before it finds its way through the filter. We do not recommend this method because these systems can easily bleed manganese into the water, the potassium permanganate is poisonous and the potassium permanganate stains badly if spilled. The manganese that bleeds into the water can get to levels high enough to be toxic. You will hear these filters referred to as manganese filters, greensand filters or potassium permanganate filters.
The catalytic method uses a media similar to greensand. The media has many different trade names but is usually a naturally occurring mineral called pyrolucite (manganese dioxide). This method of treatment counts on there being enough air in the water to provide the oxygen necessary to turn the manganous manganese into manganic manganese. This media can also bleed manganese into the water. It doesn’t work to remove manganese in the water we find locally because there is not enough naturally dissolved oxygen. A second major draw back is that the media is so heavy it is very difficult to back wash properly with the flows available from most residential pumps.