|
|
Water PurificationGeneral information about water filtration ChlorinationChlorine is an effective biocide, and the residual chlorine in an automatic water system helps keep microbial contamination under control. Hyperchlorinated water is not as corrosive as acidified water, but the piping must be brass-wetted. One disadvantage of chlorine is that it may react with contaminants in the water supply, creating toxic or carcinogenic compounds. Chlorine dissipates rapidly when exposed to heat, light and organic matter. Free chlorine in drinking water should be 5-12ppm. Another reference {4537} states that 12-16ppm is appropriate. A proportioner is used to add chlorine or acid to water.{4099} FiltrationThe vast majority of purification systems involve filtration of some sort. All filtration works on the same principle; only the pore sizes of the filters are different. The best filtration that is commercially available is filtration by Reverse Osmosis (RO). Pore sizes here are .0006 microns. For large impurities, other filtration media ranging from 1-100 µ, commonly known as sediment filters, are used. Carbon filters are filtration devices which use carbon to attract undesirables out of the water and onto the carbon. A common technique in filtration is attaching a chemical feeder before the filtering media to inject chemicals in water to change the chemical properties of the undesirables in water so they can be filtered out by the media. Water softeners are popular conditioning devices which serve this function for Reverse Osmosis filters. Without some sort of pre-conditioner, hard water tends to destroy Reverse Osmosis membranes. The most popular water conditioners use salt to exchange sodium ions with calcium ions in the bicarbonate molecule. The positive of water conditioners is their ability to almost completely remove calcium, the main hardness culprit. The negative involves their installation and the necessity to use salt in the conditioning process. Atmospheric storage tanks enable the system to meet peak water demand when demand exceeds the rate at which the RO can produce water. For example, a typical 800 gallon per day system produces 2/3 gallon per minute. To store water for use, RO permeate is collected in an atmospheric storage tank. The most common tanks store between 200 and 500 gallons of water. A liquid level switch in the tank turns the RO pump "on" when the water level in the tank drops and "off" when the tank is full. Because the tank is vented to the atmosphere, contaminants may enter, causing bacterial growth that's controlled with ozone or chlorine. Otherwise, water can be sterilized with an ultraviolet sterilizer after it leaves the tank. Post-treatment can incorporate many processes. Permeate (the good water coming out of the RO system) can be passed through a granular activated carbon tank to improve the taste and odor of the water if the application requires. Carbon can also remove trace amounts of hydrogen sulfide gas in water. Depending on the pH of the feed water, RO can decrease the pH leaving it slightly acidic. Acidic water can corrode piping and fixtures. A calcium carbonate tank can be used to increase the pH. Gases, such as carbon dioxide and hydrogen sulfide, will pass through the RO system. These can be removed by aeration. Reverse OsmosisR.O. is a membrane process that acts as a molecular filter to remove up to 99% of all dissolved minerals. The pores in the membrane are about .0006µ. The smallest known bacteria are .02µ. Water passes through the membrane while the dissolved and particulate materials are left behind. As pressure is applied to the concentrated solution, the flow is reversed and water is forced through the membrane from the concentrated side to the dilute side. Water molecules penetrate the thin cellulose acetate layer of membrane and diffuse through it molecule by molecule. Dissolved salt ions would also diffuse through this layer, except that the solubility of the salt ions in the acetate is much less than that of the water. Thus, the water moves through more rapidly with the result that a separation occurs. The driving force is furnished by both the pressure and the concentration differentials across the thin layer. For water, the pressure effect is the most important. Therefore, increases in pressure increase the water flux without a corresponding increase in salt flux. This process removes most of the dissolved mineral salts, almost all of the particulate matter, and most of the dissolved organic compounds. With reverse osmosis systems, water pressure must be maintained at 40-70 pounds per square inch (psi) to keep a driving force across the membrane to produce a high clarity, low mineral content water. Most RO systems operate in the 25-50% conversion range. This means that at 50% conversion, 100 gallons of feed water will produce 50 gallons of pure water and 50 gallons of brine. Other methods of water treatment such as water softening and anti-scalant injection are also important and it is a combination of these along with RO that will normally produce the highest quality water. The central part of the RO system is the module which is a pressurized container housing the semi-permeable membrane. Cellulose triacetate (CTA) membranes are used for chlorinated supplies. Here the feed water will be separated into usable product called permeate and waste product called concentrate. On either side of the module may be two carbon filters, one which pre-treats the feed water, the other which post-treats the permeate. Reasonable care must be taken to prevent damage to the RO membrane. Factors which can shorten the life of the membrane include scaling, excessive pressure or temperature and bacteria and chlorine degradation. By exercising care, avoiding extremes in feed water impurities, and occasionally cleaning and monitoring the system, these problems may be prevented. However, periodically changing the membrane will be necessary. To properly size an RO system, determine the water temperature and total dissolved solids (TDS) level and multiply the % of loss by the gallon rating of the unit. A sample of water is a must when sizing the RO system. The main water tests that must be considered are: Top |
|
©1999, Janet Becker Rodgers, DVM, MS, DipACLAM, MRCVS All rights reserved. Comments? Send an email to janet.rodgers@vet.ox.ac.uk |