Water Well Journal

May 2016

Water Well Journal

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Precipitation, or more appropriately chemical precipitation, is the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the pre- cipitate. The chemical causing the solid to form is called the precipitant. Without sufficient force of gravity (settling) to bring the solid particles together, the precipitate remains in suspension. After sedimentation, especially when using a cen- trifuge to press it into a compact mass, the precipitate may be referred to as a pellet. The precipitate-free liquid remaining above the solid is called the supernate or supernatant. Oxidation refers to a process where an oxidizing substance (air, chlorine, ozone, potassium permanganate) is introduced into a liquid, commonly water, to effect a change in dissolved substances in the liquid. Redox is a contraction of the name for a chemical reduction -oxidation reaction. A reduction reaction always occurs with an oxidation reaction. Redox reactions include all chemical reactions in which atoms have their oxidation state changed. In general, redox reactions involve the transfer of electrons between chemical species. The chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. Oxygen is not necessarily included in such re- actions, as other chemical types can serve the same function. A common redox reaction is shown in Figure 2 where the introduction of potassium permanganate, a powerful oxidant, is introduced into a flow stream to reduce dissolved or ferrous iron (Fe ++ ) into a filterable substance, ferric iron (Fe +++ ). Again, the term redox comes from two basic concepts in- volved with electron transfer, reduction and oxidation. It can be explained in simpler terms: (1) Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion. (2) Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Finally, the last basic process in a chemical alteration process is ion exchange. Generally known by the term of water softening, this process usually involves the exchange of ions in a water treatment scenario. Typically, conventional water-softening appliances depend on an ion-exchange resin in which hardness ions are exchanged for a different type of ion. Ion exchange devices reduce the water's hardness by re- placing the elements causing water hardness—magnesium and calcium (Mg 2+ and Ca 2+ ) ions—with sodium or potassium ions (Na + and K + ), thus the term of ion exchange. Sodium or potas- sium used for the exchange process is generally derived from a brine solution created from water and sodium chloride (salt) or potassium chloride. What Is a Micron? I have repeatedly used the term micron when referring to the size of the particles we want to remove. Obviously, in order to understand the concept of solids separation it is important to also understand the terms used to describe what we want taken out. One micron is a physical dimension of .001 millimeter, or .00003937 inch. Generally, it is easier to equate the larger value of 1000 microns = 1 millimeter (.03937 inch, or slightly smaller than 1/25 of an inch). A convenient way of relating microns to inches and screen mesh is shown in Figure 4. Although Figure 1 is helpful when discussing the relative sizes of varying particles, it doesn't il- lustrate the sizes of various types of materials often found in well water that may need removal. The information contained in Figure 5 provides the typical size range (in microns) for various constituents such as coarse sand, medium sand, fine sand, silt, and clay. ENGINEERING from page 35 Figure 2. Contaminant removal methods Figure 3. Barrier method of contaminant removal waterwelljournal.com 36 May 2016 WWJ

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