- c. As turbid water passes through granular filtration media, various transport mechanisms (inertial, diffusive and hydrodynamic forces) act to bring smaller suspended solids particles into close proximity with the media granules. Once particles are at the granule walls, electrostatic forces, known as van der Waals forces, will hold onto the particles and separate them from the flowing water, provided the shear forces of the water flowing by the particles are not so great as to overcome the van der Waals attraction between the media granules and the particles.
- c. Particles larger than five microns tend to be removed in multimedia water filters via gravity settling and screening. Submicron particles, because of their small size and less surface area to drag against the flowing water are better able to withstand the shear forces of flowing water, and thereby be held to the media granule surfaces by van der Waals forces of attraction. The overall performance is such that the least efficient removal tends to be of particles that are approximately one micron in size.
- b. Glauconite is an unusual iron, potassium silicate sand with a dull green color that is mined in New Jersey and is called greensand.
- a. To become an oxidizing filter medium, greensand is coated with manganese oxide groups; it is then called manganese greensand and it has the ability for the manganese oxide groups to change their valence states. The manganese greensand is utilized by charging or regenerating it to an oxidized form that will oxidize dissolved iron, manganese and hydrogen sulfide in water.
- e.
- f.
- True. Media filters that include greensand are able to remove iron, manganese and hydrogen sulfide down to concentrations as low as 0.01 milligrams per liter.
- a. One cubic foot of regenerated (oxidized) manganese greensand can treat about 12,000 gallons of water with one ppm iron, or 7,000 gallons of water with one ppm manganese or 4,000 gallons of water with one ppm hydrogen sulfide.
- True. The chlorine will oxidize any iron, manganese or hydrogen sulfide in the water, and reduce the oxidation demand placed upon the manganese greensand. The recommended concentrations for oxidation of the contaminants are: 1.3 mg/l of Cl2 for each mg/l of iron (as Fe2+), 2.6 mg/l of Cl2 for each mg/l of manganese (as Mn2+) and 5 mg/l of Cl2 for each mg/l of hydrogen sulfide (as sulfur).
- True. It is critical that the oxidation potential of the manganese greensand not be allowed to be completely depleted when treating water with hydrogen sulfide. The reduction potential of hydrogen sulfide is so great that it will strip the manganese oxide coating off the greensand if hydrogen sulfide contacts exhausted manganese greensand. It is often recommended when removing hydrogen sulfide, or even high concentrations of iron or manganese, that sodium hypochlorite or potassium permanganate be injected into the flowing water stream upstream of the manganese greensand filter.
