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DI Water Part IV: Carbon Treatment of City Water |
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October 11, 2004
Carbon Treating is actually called “Activated Carbon Adsorption.” Adsorption is a process where a liquid is electrostatically or chemically attracted to and held by the surface of a solid. Activated carbon is made from various substances including: coal, lignite, coconut shells, paper-mill waste, sawdust, etc. The raw material is “converted” by heating it in a closed retort, and oxidizing (or “activating”) it by means of air or steam, to remove the hydrocarbons, which would interfere with the adsorption of organic matter. The finished item (referred to as GAC) is very porous and has many carbon atoms with free valences. The highly porous structure and large surface area, allows the carbon to capture - adsorb - many of the impurities of water. Activated carbon is used to remove chlorine and chloramines, dissolved organics and some large inorganic complexes, such as heavy metals, from the city water. Why is there Chlorine in the City Water?Since the mid 1800’s, chlorine has been added tap waters against a wide array of life-threatening viruses, and bacteria. The part of the Chlorine that is not reduced by amines and organics etc, is called Free Active Chlorine. A typical concentration of Free Active Chlorine in tap waters is kept at 0.1-0.2 ppm, to maintain the water free from microorganisms. In the US, the Institute of Homeland Security recommends an increased control of Free Active Chlorine content for water districts. The result of this mandate has been a dramatic increase of chlorine and chloramines content in our municipal water systems since the attack on 9/11. Without this mandate, there could have been a disastrous biological attack by now. Why is the City Water treated with Active Carbon?Chlorine is a strong oxidizer and even at low concentrations, such as 0.1-0.2 ppm as Free Active Chlorine, able to react and decay organic matter, such as the RO membrane and the Ion Exchange Resin in the Softener and the Mixed Bed filters. The RO membrane is specified for a Free Active Chlorine concentration of max 0.1 ppm.Organic compounds in the City Water can act as nutrients for any eventual microorganisms, potentially causing bacterial growth in the system. Organics can also foul the softener ion exchange resin and the RO membrane. What can cause insufficient Active Carbon Treatment?Process problems in the Active Carbon filtration process are rare. Among the problems that can occur, are: - Saturation of the Active Carbon
- Channeling in the Active Carbon bed
- Backwash interval not set correctly
Saturation of the carbon canister depends on the water usage and the content of organics and Total Chlorine. Once saturated, the Active Carbon will not purify the city water sufficiently. In most Optical Disc mastering- and electroforming applications, the capacity is expected to last at least one year, hence the recommendation of annual exchange of the Active Carbon bed. Exceptional high water consumption and/or high content of organics and/or Total Chlorine can cause exhaustion of the Active Carbon before one year of use. Detection of the Free Active Chlorine by analysis of the output from either the carbon filter, or after the softener, indicates saturated Active Carbon, or other problems with the filter. An inexpensive Free Active Chlorine color comparison detection kit, distributed by many chemical distribution, is quick and simple to use and often sensitive enough to detect the limit of Free Active Chlorine of 0.1 ppm. It is recommended that daily testing of the output from either the carbon filter, or after the softener, be performed, to ensure that chlorine is not being bypassed to the RO. The purpose of the backwash is not only to remove solid impurities and debris, but also to mix the Active Carbon, preventing channels to occur, that else could cause an internal “by-pass” of the Active Carbon bed. Improper mixing of the Active Carbon can be caused by limited city water capacity. To indicate sufficient mixing is simple: During service (normal operation), the level of the Active Carbon bed can be seen as a dark shadow about 10” (25 cm) below the head valve, if a strong light source is positioned at the opposite side of the filter vessel. If this shadow is gone during “Backwash” (indicated by the program wheel at the front of the head), proper mixing can be assumed. A rinse-backwash cycle can manually be initiated by turning the knob of the program wheel clockwise about ½” (1 cm). Instructional Tip #3 – Setting the automatic rinse-backwash intervals The Active Carbon filter should be set for an automatic rinse-backwash cycle every 12:th day. This is done by extending one of the 12 skipper fingers of the skipper wheel at the front of the filter head.The “Time-of-Day” set at the program wheel should be set at least 6 hours different than the time of day set at the softener filter head. Press the red time-set button and turn the inner gear wheel of the program wheel assembly, until the time you are aiming for is displayed in the small “Time-of-Day” window. The automatic rinse-backwash cycle is initiated when the “Time-of-Day” window displays 4 AM and the arrow of the skipper wheel is pointing at an extended skipper finger.
In Part V, I will cover the RO, how it works, and what it does.
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