The principle of crossflow filtration can be seen from the diagram below.
The liquid suspension to be filtered is passed through a porous tube. As a result of a differential pressure, some of the liquid passes through the membrane tube and is collected as particulate-free filtrate. The remainder of the liquid flows through the channels of the element, back into the processing tank and is recycled through the system. Eventually enough clean liquid is extracted from the process volume that liquid containing a higher concentration of the particulate material is left in the processing tank. Frequently the liquid can even be reduced to the consistency of a light slurry. Due to the high efficiency of the ceramic filter elements, very small particles may be removed from liquid streams that cannot effectively, or economically, be removed using conventional filter cartridge technology. Generally, the ceramic filter does not remove material or salts that are in a truly dissolved state, however with proper ceramic membrane selection, many oils and grease compounds can be separated from the base liquid and collected in the retenate. Depending upon the characteristics of the liquid and of the particulate, a ceramic cross-flow filtration system may utilize “back-pulsation” during operation to clear any particulate material from the filter surface by briefly reversing the flow of liquid through the ceramic filter tube. The ceramic filter elements are not replaced regularly, as are traditional filter elements, since they actively inhibit particulate build-up or clogging by maintaining a high cross-flow velocity through the tube and by means of the back-pulsation process. Ceramic elements are constructed from pure aluminum oxide and are impervious to basic and acidic solutions, can withstand wide temperature ranges and can operate at elevated pressures.
The primary raw material used for activated carbon is any organic material with a high carbon content (coal, wood, peat, coconut shells). Granular activated carbon media is most commonly produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing to the correct size. The carbon-based material is converted to activated carbon by thermal decomposition in a furnace using a controlled atmosphere and heat. The resultant product has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place. The walls of the pores provide the surface layer molecules essential for adsorption. Amazingly, one pound of carbon (a quart container) provides a surface area equivalent to six football fields. Physical Adsorption is the primary means by which activated carbon works to remove contaminants from water. The highly porous nature of carbon provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to. The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores. The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution. Those compounds that are more readily adsorbed onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and are neutral or non-polar chemical in nature.
It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate. Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions. This reaction is important to POU treatment because this conversion of chlorine to chloride is the basis for the removal of some common objectionable tastes and odors from drinking water. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution.
HOW DOES A WATER SOFTENER WORK?
Water softeners use synthetic resin beads to remove potentially troublesome “hard” water minerals (minerals are called ions after they have been dissolved in water) from normal tap water in order to decrease equipment maintenance, increase system efficiency and lengthen the service lives of certain water purification system components by eliminating the possibility of developing hard mineral deposition on this equipment. Water heating equipment and water purification membranes are two types of equipment that are susceptible to mineral deposition and subsequent fouling. Hardness mineral free water is referred to as “soft” water. Water softener resin beads contain ion receptor sites which initially hold only sodium ions. As tap water that contains “hardness” ions (calcium and magnesium) passes through the resin, the calcium and magnesium ions are attracted to the beads, and are exchanged for sodium (“soft water ion”). This process is called ION EXCHANGE, since there is not a net removal of material from the water, simply an exchange of two TYPES of ions. This exchange continues until there are no longer any sites at which the exchange can take place. The resin is then considered “exhausted” and must be recharged with sodium ions in order for the unit to continue working to remove hardness ions from the feed water. During the recharge, or regeneration, process, a concentrated sodium brine solution is rinsed through the resin. The resin prefers sodium ions, when present in higher than normal concentrations, over calcium and magnesium ions, so calcium and magnesium ions are bumped off and go down the drain, while the sodium ions fill all the available receptor sites on the water softener resin beads. The resin is then ready for continued service.
HOW DOES A REVERSE OSMOSIS MEMBRANE WORK?
Reverse Osmosis (RO) is a modern process technology used to purify water by efficient removal of dissolved contaminants. Reverse Osmosis Systems utilize semi-permeable membranes to separate the dissolved contaminants from the processed water. Rather than retaining the separated contaminants, like a particulate filter collects sediment, the reverse osmosis system is designed to continuously flush the contaminants, still in a concentrated, solution state, to drain. “Semi-permeable” refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product. Osmosis is a natural process involving the fluid flow of across a semi-permeable membrane barrier. Consider a tank of pure water with a semi-permeable membrane dividing it into two sides. Pure water in contact with both sides of an ideal semi-permeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides. If a soluble salt is added on one side, the chemical potential of this salt solution is reduced. Osmotic flow from the pure water side across the membrane to the salt solution side will occur until the equilibrium of chemical potential is restored (Figure 1a). In scientific terms, the two sides of the tank have a difference in their “chemical potentials,” and the solution equalizes, by osmosis, its chemical potential throughout the system. Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides is equal to the osmotic pressure. The osmotic pressure is a solution property proportional to the salt concentration and independent of the membrane. With the tank in Figure 1a, the water moves to the salty side of the membrane until equilibrium is achieved. Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium (Figure 1b). Additional pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential. This phenomenon is called reverse osmosis.
The driving force of the reverse osmosis process is applied pressure. The amount of energy required for osmotic separation is directly related to the salinity of the solution. Thus, more energy is required to produce the same amount of water from solutions with higher concentrations of salt. In the real world, the “salt solution” is the source water to be purified. The HydroMax Reverse Osmosis System utilizes a pressure pump to apply pressure to this salt solution and drives purified water through the membrane while rejecting nearly all of the dissolved contaminants and flushing them to drain. The membrane barrier in a HydroMax system actually is configured into a space saving, convenient, tubular design as pictured below.
WHAT DOES ULTRAVIOLET DO FOR THE PURIFIED WATER?
Ultraviolet light is part of the light spectrum, which is classified into three wavelength ranges:
UV-C, from 100 nanometers (nm) to 280nm
UV-B, from 280nm to 315nm
UV-A, from 315nm to 400nm
UV-C light is germicidal - i.e., it deactivates the DNA of bacteria, viruses and other pathogens and thus destroys their ability to multiply and cause disease. Specifically, UV-C light causes damage to the nucleic acid of microorganisms by forming covalent bonds between certain adjacent bases in the DNA. The formation of such bonds prevent the DNA from being unzipped for replication, and the organism is unable to reproduce. In fact, when the organism tries to replicate, it dies.
Ultraviolet technology is a non-chemical approach to disinfection. In this method of disinfection, nothing is added to the water, which makes this process simple, inexpensive and requires very little maintenance. Ultraviolet purifiers utilize germicidal lamps that are designed and calculated to produce a certain dosage of ultraviolet (usually at least 16,000 microwatt seconds per square centimeter but many units actually have a much higher dosage.) The principle of design is based on a product of time and intensity - you must have a certain amount of both for a successful design. In the real world, Ultraviolet treatment technology is very useful and effictive at controlling bacteria in bulk flows of water being processed in order to help prevent active biological passage into downstream processes or storage. Ultraviolet is, however, not designed to eliminate all possibilities of downstream biological growth or contamination and other additional methods of biological control, such as chlorination, are necessary for potable water storage and distribution type water systems.
WHY DOES THE PURIFIED WATER NEED TO BE REMINERALIZED WHEN BEING SUPPLIED FOR POTABLE WATER USE?
All natural water contains a small amount of dissolved carbon dioxide. Gasses readily pass through semi-permeable membranes and when water is processed by reverse osmosis, the carbon dioxide passes through the membrane with the purified water. Since most of the alkalinity (natural pH buffering ions dissolved in tap water) are removed by the reverse osmosis process, the excess carbon dioxide causes the pH of the purified water to become acidic (low pH). Acidic water, especially water containing a low amount of dissolved ions, like reverse osmosis water, will tend to corrode and dissolve metallic piping components. When reverse osmosis water is used in typical domestic water piping systems composed of copper or iron piping materials, it is wise to temper the aggressive nature of the purified water so that the piping systems’ integrity can be maintained and so that excess dissolved metals do not end up in water being used for drinking. It is also commonly desired to replace a small amount of “good” mineral into the purified water in order to improve the palatability of the water for drinking and to aid in soap removal during washing and bathing. In order to correct the purified water chemistry, the HydroMax system uses calcite (crushed, pure limestone) filtration media to neutralize the excess carbon dioxide and raise the pH of the purified water. The calcite media dissolves very slowly in the purified water flowing through the remineralization filters, releasing a small amount of calcium. Fresh calcite media will need to be added to the remineralization filters periodically to replace that which dissolves. Potable water can also require a chemical addition in order to increase the pH and alkalinity of the purified water in order to help prevent water distribution system component and piping corrosion.
WHEN DOES CHLORINE NEED TO BE ADDED TO PURIFIED DRINKING WATER WATER?
Even though the HydroMax system uses multiple and various technologies to purify the water, it is necessary to provide a chlorine residual in the finished water in order to prevent biological recontamination during water storage and during distribution through piping systems to the locations where the water is to be used or consumed. In the United States, the Safe Drinking Water Act legislates that all public water supplies contain adequate chlorine concentrations to prevent the presence of coliform bacteria. Chlorine destroys nearly all types of biological life forms by oxidation. Direct contact with a strong oxidizer will cause the cell walls of a bacteria to rupture, effectively deactivating the organism. The HydroMax system injects sodium hypochlorite solution (bleach), a common, inexpensive source of chlorine, into the purified water in order to create legal, active bacteria free potable water.
1. Ceramic cross-flow filtration removes Particulate Material from any solution. Solution can be water or non-water based. Ceramic Micro-Filtration is not impacted by Total Dissolved Solids content of the solution so very high conductance water, or chemical solutions, can be processed using ceramic cross-flow filtration. Based upon the desired project goals, ceramic membrane tubes can be selected with which a precise range of particulate material can be removed from a solution. Specific ceramic membrane tubes are available to filter particles from as small as 0.01 microns (nano-particles) up to 5 microns in size. Average flux rates through the ceramic filters are dependent upon solids loading and viscosity of the liquid suspension. It is wise to perform pilot scale performance parameter testing on any material being considered for ceramic filtration prior to beginning system engineering design work.
2. Ceramic cross-flow filtration is especially applicable for concentrating solids in a volume of solution while simultaneously extracting a solids-free “permeate” stream. This can be useful when the solids are either valuable and it is desired to concentrate and collect these solids or when the solids loading is very high and collecting the solids on cartridge media is costly or interrupts processing frequently. Ceramic cross-flow filtration can typically be operated continuously with periodic “back-pulse” cycles to clean material from the ceramic membrane surface. Little or no back-pulsing is required for many types of suspended solids that tend not to adhere to the ceramic membrane surfaces when the system is operated in a high velocity cross-flow mode. Such operation is typical in precious metals suspension concentration, abrasive parts cleaning, chemical solution and beverage clarification, etc.
3. Ceramic cross-flow filtration can be operated in a dead-end filtration mode, (similar to how cartridge filters are operated) when there is a relatively small amount of solid material to remove in relation to the volume of solution being processed. In this operating mode, a very infrequent back-pulse cycle may be used to automatically clean the solids to waste. This type of operation eliminates the need for, and cost of, replaceable cartridge filters while providing superior particulate remove efficiency and effectiveness. Such operation is typically used for pretreatment of reverse osmosis membrane feed water and for bacteria removal in municipal water production. It is important to note that consideration needs to allow for disposal of the liquid waste stream generated in order to expel the particulate form the filtered solution stream. Typically, the waste stream can be less than 5% of the total processed flow when using a high efficiency ceramic cross-flow filter system.
4. Ceramic filter elements are extremely robust and can withstand high differential pressure operation, high fluid temperatures, acidic or basic chemicals, volatile liquids, oily, soapy, abrasive or fibrous suspensions. Consideration of the size and nature of the particulate material being filtered when selecting the flow passage capillary diameter of the ceramic filter tube in order to minimize element plugging in severe applications.
5. It is recommended to perform Ceramic Cross-Flow Filtration efficacy trials on any suspension, solution or slurry in order to develop system designs for the most advantageous removal rating and operating mode for these systems – especially when treating unique materials.