 |
|||||||
|
|
|||||||
| Browse our library of technical documents. Click on the appropriate link below for more information, or scroll down to view the entire library of technological terms in its entirety. Documents are organized according to their related technology: |
|||||||
Water as a Resource |
|||||||
 |
|||||||
Water as a Resource As a resource, no other compound is more important to man than water. In addition to its daily chore of sustaining all life, water provides man with a building block for social development and even possesses religious and legendary value. Water is a valuable resource ad its availability is crucial to our existence. Our vast oceans alone would lead us to believe water is an inexhaustible resource - after all, roughly two-thirds of our planet is submerged by these very oceans. For those who delight in statistics, water - as oceans, rivers, lakes, or ice-caps, total an unbelievable 324,030,000 cubic miles of water on the earth's surface. In terms of gallons this comes to a number too staggering to mention. This total has not changed since the earth was formed. Why then do we concern ourselves with the availability of this ever present gift? Facts are that only a hair over three-tenths of one percent of this supply is available fresh water that can be used for drinking and countless other domestic and industrial purposes. One begins to understand the meaning of "water, water everywhere and not a drop to drink". The average American will use 15 gallons of water per day for personal use. That and 15gpd multiplied by our population is only a fraction of the water our country uses in a day. Where does the rest go? To agriculture, to power plants and to industry. These factors increase our per capita water use more than a thousand fold. While the significance of these facts is more important than actual numbers, it is interesting to note that it takes: * 4,300 gallons of water to process enough bread and roast beef to make sandwiches for a family of four. * 10,000 gallons of water to make one automobile. All of this discussion about how much and what for - where does this water originate and how do our supplies get replenished? The Hydrologic Cycle The replenishing process is called the hydrologic cycle - what goes up must come down. Simply stated, the earth, the sun, and the atmosphere could be thought of as a huge still. The sun, acting as a heater warms the water on the earth's surface. This warm water evaporates and ascends into the atmosphere where, at higher altitudes the water is cooled, condenses and forms clouds. As the clouds become heavier they eventually release this load in the form of rain, hail, sleet or snow. Activated Carbon Activated carbon is a very mature technology that is designed to help remove taste and odor from water through adsorption of the compounds that cause problems. There are a variety of different types of carbon that are used industry-wide. They include wood, lignite, coal, and coconut as the most common sources for activated carbon. Activated carbon operates through adsorption. Adsorption is a surface phenomenon and is therefore directly related to the surface area of the media. In the case of activated carbon, the surface area is related to the pore structure of the raw materials. The cost of the media is also related to the raw materials, so there are other factors that must be taken into consideration besides the total surface area. Adsorption takes place due to intramolecular attraction between the carbon surface and the substance that is being adsorbed. The force of the attraction can be altered by increasing the density of the carbon or by reducing the distance between the carbon surface and the substance being adsorbed (typically by reducing the median pore size). As the fluid (often water) passes over and through the carbon, the attractive forces between the compounds that are the most attracted to the carbon are adsorbed onto the surface. The compounds that are the most highly attracted are typically organic compounds (which can cause taste, odor and appearance problems), volatile organic compounds (VOCs) and halocarbons such as trihalomethane (THM) compounds and other process wastes. Once all of the surface area of the carbon has been exhausted through absorption, carbon filters must be replaced because it loses its effectiveness and "dumping" occurs. This "dumping" is a release of a contaminate that has used up all of the sites that will hold its particular molecular charge. In the case of "dumping" you end up with water with a much higher concentration of contaminate than the original incoming water. Depth Filtration Filtration is the simplest of water treatment processes, but in some ways the most difficult to accomplish. If all suspended particles in water were the same size, they would be easy to filter out. Of course, the size of particles in water varies considerably, especially in surface water where debris as well as fine sand and silt are found. Single media filters such as sand can be used where particles and turbidity are fairly consistent. These require regular and frequent backwashing to avoid clogging the top portion of the filtration bed. Multi-media depth filters combine more than one size media in a single vessel. Each layer of media traps particles of a certain size thus each layer functions near its optimum efficiency. It does not matter that the finer particles pass through the upper layers because they will be trapped later. Because of this arrangement, each media layer filters throughout its bed depth, as does the entire filter, hence the term depth filter. As a result of efficient media utilization and less frequent clogging, a smaller diameter vessel can be utilized and less backwashing is required. Ion Exchange & Softening Ion exchange is a process that includes two very similar applications of the same technology. The first is water softening. This is the process of removing ions from the water and replacing them with sodium ions and chloride ions. The most common use for this is residential, where a homeowner is trying to reduce the hardness or improve the taste of the well water or the water that the municipal service provides. This also reduces deposits and scale that can be left from water with a high level of hardness. The second application is deionization. In deionization, the hardness and other ions that are initially in the water are removed and replaced with H+ and OH- ions, which can combine to form water. This is used in applications where extremely pure water is required such as in production of electronic components or pharmaceuticals. The process works like this: Ion exchange resins (little beads that are charged) are coated with the replacement ions. In the case of water softening the beads are coated with Na+ and Cl-. In the case of deionization, they are coated with H+ and OH-. Water flows over the resin. The ions in the water are attracted to the resin. The ions in the water attach themselves to the resin, and knock off the ions that are already attached. The resin is exhausted when all of the replacement ions are gone. In order to replenish the resin, also called regenerating the resin, a strong solution of the replenishment ions must be applied to the resin. This removes the ions that came from the water and regenerates the resin. The solution that is used to regenerate the water softeners is concentrated salt water called brine. There are two solutions that are used to regenerate a deionizer. One is a concentrated acid, and the other is a concentrated base. Media Filtration There are several concerns in any treatment system that must be considered before final treatment can take place. The two largest and most common concerns are iron in the water and particles that can clog or foul a system. Media filtration, also commonly known as multi-media filtration, can alleviate both of those concerns. The two media in an iron removal media filtration assembly are manganese greensand and anthracite. The manganese greensand acts as a form of chemical treatment that, when in contact with soluble iron in water, reduced the iron from the soluble form to an insoluble form that will precipitate out of solution. The anthracite then can filter both the precipitated iron out of the solution as well as other entrained particles that have entered the water source. This combination of anthracite and manganese greensand together can remove a majority of particles greater than 10 microns in size. In addition, the filter can be backwashed to remove the entrained particles and iron in order to extend bed life. The manganese will eventually be exhausted and will also need to be regenerated, something which our systems are equipped to do. Microfiltration Microfiltration is a form of filtration that has two common forms. One form is crossflow separation. In crossflow separation, a fluid stream runs parallel to a membrane. There is a pressure differential across the membrane. This causes some of the fluid to pass through the membrane, while the remainder continues across the membrane, cleaning it. The other form of filtration is called dead-end filtration or perpendicular filtration. In dead-end filtration, all of the fluid passes through the membrane, and all of the particles that cannot fit through the pores of the membrane are stopped. Crossflow microfiltration is used in a number of applications, as either a prefiltration step or as a process to separate a fluid from a process stream. (Rarely used in residential application.) Dead-end microfiltration is used commonly in stopping particles in either prefiltration or final filtration before a fluid is to be used. Cartridge filters are typically composed of microfiltration media. Nanofiltration Nanofiltration is a form of filtration that uses membranes to preferentially separate different fluids or ions. Nanofiltration is not as fine a filtration process as reverse osmosis, but it also does not require the same energy to perform the separation. Nanofiltration also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in reverse osmosis. Nanofiltration is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. An example of this is the concentration of corn syrup. The nanofiltration membrane will allow the water to pass through the membrane while holding the sugar back, concentrating the solution. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. Nanofiltration is capable of concentrating sugars, divalent salts, bacteria, proteins, particles, dyes, and other constituents that have a molecular weight greater than 1000 daltons. Nanofiltration, like reverse osmosis, is affected by the charge of the particles being rejected. Thus, particles with larger charges are more likely to be rejected than others. Nanofiltration is not effective on small molecular weight organics, such as methanol. Ozonation Ozone (O3) is found commonly in nature. Ozone is formed whenever lightning occurs, or when an electrical discharge creates a spark. The ozone layer in the upper atmosphere provides a protective screen against dangerous solar radiation. The generation of ozone is a relatively simple process. Air, dry air or oxygen is drawn into an ozone generator, at which point the air is charged with high voltage. The air is made up of diatomic oxygen (O2) and nitrogen. As the air is drawn through the ozone generator, the high voltage splits some oxygen molecules into oxygen atoms. some of these atoms then quickly react with oxygen molecules to form ozone. (o1+O2=O3) Ozone is secondary only to fluorine as the most powerful oxidant. Ozone inactivates and oxidizes organic metals and most organisms faster than chlorine. Ozone also functions as a micro flocculating agent to "polish" the water and improve clarity (clarifying iron, sulfur and manganese). Reverse Osmosis Reverse osmosis, also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the color, taste or properties of the fluid. It can be used to purify fluids such as ethanol and glycol, which will pass through the reverse osmosis membrane, while rejecting other ions and contaminants from passing. The most common use for reverse osmosis is in purifying water. It is used to produce water that meets the most demanding specifications that are currently in place. Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as crossflow which allows the membrane to continually clean itself. As some of the fluid passes through the membrane, the rest continues downstream, sweeping the rejected species away from the membrane. The process of reverse osmosis requires a driving force to push the fluid through the membrane, in residential applications municipal water pressure is used in industrial applications the most common force is pressure from a pump. The higher the pressure, the larger the driving force. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 daltons. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected. Ultrafiltration Ultrafiltration is a form of filtration that uses membranes to preferentially separate different fluids or ions. Ultrafiltration is not as fine a filtration process as nanofiltration, but it also does not require the same energy to perform the separation. Ultrafiltration also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in nanofiltration. Ultrafiltration is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. One of the uses that demonstrates the usefulness of ultrafiltration is electrode position paint recovery. In this instance the paint, composed of a resin, a pigment and water are separated into two streams that can be reused. The first stream includes the water and a small amount of the paint resin, which can be used to rinse the parts later in the process. The paint pigment is separated from that stream and can be reused in the paint bath, allowing the bath to be concentrated to a usable level. Ultrafiltration is capable of concentrating bacteria, some proteins, some dyes, and constituents that have a larger molecular weight of greater than 10,000 daltons. Ultrafiltration is only somewhat dependent upon the charge of the particle and is much more concerned with the size of the particle. Ultrafiltration is typically not effective at separating organic streams. Ultraviolet Sterilization The most familiar part of the spectrum is a narrow band of wavelength visible to the human eye. Another band with wavelengths shorter than those of visible light, and not visible to the eye is the ultraviolet part of the spectrum. Ultraviolet radiation can cause changes in living matter. The sun's ultraviolet rays can cause sunburn and ultraviolet rays between 2000 and 2950 Angstrom units poses germicidal effectiveness essentially destroying (frying) the microbe. Bacteria withstand considerably more ultraviolet irradiation in water than in dry air. The degree of microbial destruction is a function of both time and intensity of exposure to the radiation. Prefiltration may be necessary to remove turbidity to assure proper UV intensity reaches the bacteria. The unique advantage of UV sterilization over other methods such as chlorine is that nothing is added to the water. When chemical methods of treatment are used there may be new problems such as taste and odor to address.  |
|||||||
