Calcite and Corosex Media for pH Modification
Calcite (the technical mineral reference for natural crystal-like calcium carbonate and the essential major ingredient in limestone, chalk, and marble. For water treatment, various high-grade limestone and marble products are crushed and screened to create a loose filter-type medium. The use of calcite in correcting low-pH water is akin to the gardening practice of spreading pulverized limestone on acid soil. The hard composition and irregular granular shape of the calcite carbonate particles expose a lot of surface area as well as forming various void pockets for suspended solids for and turbidity entrapment in the bed. The calcite product in neutralization application adds some calcium hardness to the water being processed. Therefore, calcite neutralization treatment should be applied ahead of water softening.
Calcium hardness is added to the treated water through pH modification. A typical hardness increase of almost 100 percent can be expected. As a rule of thumb, an increase of three to five grains of hardness can be expected to raise the pH by one point on the scale-for example, from 6.0 pH to 7.0 pH value.
Granular calcite (either southern marble or northern limestone) medium may be used both as a filter media for silt, turbidity and ferric iron and as a sacrificial calcium carbonate pH correction bed. Generally, a calcite medium filter will remove approximately one-third of the total ferric and ferrous iron content of a water supply.
Two sources of pH neutralization medias that are widely used and available in common U.S. mesh size for neutralizing filter medium beds are Calcite and Corosex. Calcite is actually consumed in this pH modification method and, thus, the bed must be inspected periodically and calcite added to bring the bed back up to its original depth. Also, as the granules of calcite are slowly dissolved, they become finer, and the backwashing of the bed will eliminate the finest of these particles, thus preventing considerable pressure loss.
The rate of solubility of calcite is dependent on several factors. In relatively mineral-free water, the average rate is 0.001 percent (approximately one-tenth of an ounce of calcite per gallon of water) at 77°F (25°C), but solubility increases in the presence of C02. Calcite solubility is also increased by falling temperatures and rising total pressure. At the same time, an advantage of calcite is that it will not "overcorrect" for pH. This calcite technique will raise the pH evenly under proper operating conditions.
The backwash rate is very important with this highly dense mineral and should done at a minimum of 8.0 U.S. gpm per square foot of bed area for good performance. If iron is present, a minimum of 10 U.S. gpm should be used. Backwashing should be carried out for no less than 10 minutes to remove dirt, iron precipitates, and calcite fines. For iron-bearing water with low pH, a backwash time of 15 to 20 minute flow rates that are inadequate to sufficiently expand, cleanse, and regrade the calcite regular schedule. Perhaps the most common cause of neutralizer failures is improper or lack of backwash.
Corosex is mineral-based product used to raise low· pH water is magnesia (magnesium oxide, or MgO). Magnesia, which has been used for years as an antacid therapy for indigestion (Milk of Magnesia), can likewise be an effective acid-water neutralizer. Under the trade name of Corosex , the bead form is available in standard 10-40 U.S. mesh and granular in 8-16 U.S. mesh. Like Calcite, hardness will be added to the treated water. Magnesium oxide dissolves at a faster rate than natural calcite, which makes the product valuable when used with calcite. It has become very effective to use a mixture of calcite and corosex with a calcite/corosex ratio of 3: 1. This will extend the life of the neutralizing media bed to its maximum potential and reduce frequency of replacement.