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DI (Deionized) Water – Part III: Softener Function vs. RO Fouling |
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October 4, 2004
In TIP #32, I mentioned the chemical reactions that take place prior to the RO filter. In this TIP I will describe the reasons the RO filter clogs so easily if the softener process is neglected. The quick failure of an RO membrane due to input water with high hardness is probably best described as follows:
One (1) 4” diameter X 40” length TFC membrane has approximately 82 square feet (7.6 m2) of active surface area configured in a very tight envelope with a nonwoven web support lining. This lining acts as a water carrier to disperse the incoming water throughout the membrane surface, while promoting turbulence. The lining also catches particles that would otherwise clog the membrane.
2000 gallons of typical city water (less than one weeks consumption in most cases) with a hardness of 10 GPG (9.6odH or 172 ppm as CaCO3) contains almost 3 lbs (almost 1.5 kg) hardness salts. This is quite enough to foul the RO membrane severely if the softener filter is not maintained properly, or if the RO setup is wrong.
The softener is typically set for a salt consumption of 8-12 lbs (3.6-5.4 kg) per regeneration. Therefore the capacity per regeneration is typically considered to be in the range of 1000-2000 gallons (3800-7500 l), depending on the local city water hardness. Most RO systems are setup so that the Permeate- and the Concentrate flows are set to the ratio max 2:1. This means the Permeate flow (what goes to the Supply tank) is usually set at 1.0-1.5 gpm (gallons of water per minute, = 3.8-4.7 l/min), and the Concentrate flow (what goes to the drain) to 0.5-0.75 gpm (1.9-2.9 l/min), at an inlet pressure of max 200 psi. The Backflow (recycling a part of the concentrate) should be set so that the Permeate flow, Concentrate flow and Backflow altogether equal a minimum of 4 gpm (15 l/min). This helps prevent the filter from fouling. As discussed earlier, the composition of city water supplies can vary greatly. Local water treatment consultants can often supply valuable information on how to optimize the system for your specific water, particularly if the water differs from typical city waters. Maintaining the softener filter includes daily check of brine tank level and salt content. Unusual brine level is often the first indication of a softener problem. Frequent hardness analysis is recommended, particularly where the city water is very hard. Logging the daily water usage is also recommended, especially when timer controlled regeneration is utilized. Instructional Tip #2 – Setting the Capacity of an Ion Exchange Resin SoftenerOver the years, the PW/1000 has been supplied with 3 different versions of softener filters; Dual (2 x ¾ ft3 resin) Volume Controlled; Single (1 ft3 resin) Timer Controlled; and Single (1 ft3 resin) Volume Controlled. Setting the capacity is different for the different versions. The more detailed instructions below covers the latest softener version, the Single (1 ft3 resin) Volume Controlled Softener. For the other versions, see notes below. The parameters used to set the capacity of a softener filter are: - The volume of ion exchange resin (1 ft2)
- The hardness of the water to be softened
- The salt setting (amount of salt used during each regeneration)
Generally, a salt setting of 12 lbs would be used for each regeneration for a 1 ft2 resin softener. However, for soft- and medium hard waters (here; up to 13.3 grain/gal. = 12.7odH), the capacity of the resin would exceed the maximum capacity that can be set on the softener head (2 100 gal.). For an optimized economy, it’s better to use a lower salt setting. See table below:
Tap Water Hardness Salt setting Hardness capacity Brine vol.
Less than 12.8 GPG (12.3odH) 8 lbs (3.6 kg) 24 000 grains 14.3 l
12.8-13.3 GPG. (12.3-12.7odH) 10 lbs (4.5 kg) 27 000 grains 17.8 l
More than 13.3 GPG (12.7odH) 12 lbs (5.4 kg) 28 000 grains 21.4 l
Calculate the hardness of your water to GPG (grains per gallon):
1odH (deg German H) = 0.958 GPG Example 1: 12 odH = 12.5 GPG.
or 1 ppm as CaCO3 = 0.058 GPG. Example 2: 200 ppm as CaCO3 = 11.6 GPG In the table above, find the salt setting for the hardness of your water
(12 odH = 12.5 GPG à 8 lbs.) The salt setting is the amount of salt used per regeneration (8 lbs = 3.6 kg, corresponding to 14.3 l of brine solution {saturated NaCl = 250 g/l}). Set the salt setting (triangular, white plastic), located at the backside of the softener head at the same side as the brine inlet, to the value selected in the table above (8 lbs). Calculate the capacity of the filter in gallons by dividing the hardness capacity for your salt setting (8 lbs à 24 000 gal.) with the hardness of your water in GPG. (24 000 grains/12.5 GPG. = 1916 gallons)
- Set the capacity wheel at the front of the softener head by pulling the center knob and turning it until the “white dot mark” marks a value just below your calculated value. Note that capacity wheel is marked in x100 gallons (1900 gal = 19 x100 gal) and note that the arrow outside the wheel marks the remaining capacity to the next regeneration.
Note: 1 gallon = 3.78 l (1900 gal = 7180 l).
The set-up procedure for Single (1 ft3 resin) Timer Controlled Softeners is similar to the Single (1 ft3 resin) Volume Controlled Softeners, however, instead of setting the gallon capacity, the number of days between the regenerations is set. Set the skipper wheel to number of days you can run between the regeneration according to the maximum water consumption per day and calculated capacity of your softener.
Note that the reject from the RO also is included in your water consumption. Avoid set-up regeneration of the softener the same day and time as the active carbon filter is set to backwash.
(If the RO pump is switched off and make-up tank of the PW/1000 is reduced by max 200 gallons per 24 hours, the maximum amount of water passing the softener is 200 +100 = 300 gallons per 24 hours, when the RO is on and the set-up permeate-to-reject ratio is 2:1. If the calculated capacity of the softener is 1916 gallons, the 12-day skipper wheel of the softener can be set at every 6:th day; 6 x 300 gallons = 1800 gallons; <1916 gallons: - two skippers, in counter position to each other, extended).
The Dual (2 x ¾ ft3 resin) Volume Controlled Softeners set-up is different from the single 1 ft3 softeners. The salt setting is set by “programming” the time to fill up the brine tank with a proper volume of water to be used at the next regeneration. Under the cover of the filter head, at the backside, you will find the program wheel with groups of pins at the edge. Check that there are 3 groups of pins and that the gap between the second- and third gap is 3 pin positions. 3 pin positions represents 6 minutes at 0.5 gpm (2 l/min) = 3 gallons (11.5 l). This volume will dissolve 6 lbs (2.8 kg) salt. Knowing the hardness of your city water, the set capacity can be calculated, using the table below: Set Capacity Dual Volume Controlled Softener, Set gallons =307 800 / hardness in ppm as CaCO3 17 240 / hardness in odH (German degrees) 18 000 / hardness in GPG (grains per gallons) Example: Total city water hardness: 12.5 GPG: 18 000 / 12.5 = 1 440 gallons. Set the Capacity Wheel at 14 (x100) gallons. Set the capacity wheel in the same way as for the Single Volume Controlled Softener.
Next Tip: Carbon filtration, what it does, and why it is important. |
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