By C.F. ‘Chubb’ Michaud, CWS-VI
Water softener efficiency is a term generally used to rate a water softener by the relationship to its recovered capacity per pound of salt (NaCl) used in regeneration. Softeners of old might be rated at 3,000 grains per pound (gr/lb). This, of course, is borne out in the typical rating of 24,000 grains on an eight pound per cubic foot setting. Those familiar with the workings of a salt regenerated ion exchanger will recognize that the brine efficiency is very much related to the regeneration salt dose. The higher the salt dose, the higher the capacity, but the lower the efficiency in terms of capacity gain per pound of salt. That same softener, with a five-pound setting, might deliver 20,000 grains or 4,000 g/ lb and at 12 pounds, only 2,500 g/lb. This is shown in Figure 1.
The push by regulators for better brine efficiency (to keep the unwanted higher brine levels out of the sewer system) have led to a general reduction in factory brine settings to achieve the high-efficiency rating definition of 4,000 gr/lb (1).
Currently, many of the western states are caught in a severe drought condition. At the same TDS in their treated sewage wastewater is rising, making it harder to resell (for non-potable uses)
or discharge (limitations on chloride discharge levels to water ways). This is bringing more pressure on salt use that has led to several recent automatic softener bans and restrictions. So, how much TDS does a water softener really add to the sewer line? The answer may surprise you.
How much TDS does a home water softener add to household waste water?
If a typical household with a family of four uses 75 to 85 gallons of treated (softened) water per day (gpd) per person, they will use 320 to 350 gpd or 10,000 gal/month. This is about 120,000 gal/year and that amount of water weighs one million pounds. If that water is 20 gpg hard, we have to remove 2,400,000 grains of hardness per year. Assuming we have an efficient softener that can deliver 4,000 g/lb, we divide the total hardness to be removed (2.4 MM grains) by the salt efficiency (4,000 g/lb) and we see that we will use 600 pounds of NaCl salt. If we put 600 pounds of salt in 1 MM pounds of water, we have an added TDS of 600 ppm. In most cases, this more than doubles the feed level. That’s without consideration for the actual efficiency or the reserve capacity. Older time clock systems may actually be double that figure. An added 600 pounds of NaCl adds 235 pounds (or ppm) of sodium and 365 pounds (or ppm) of chloride.
What is water efficiency?
There is another side to the softener efficiency ratings that often goes unnoticed. That has to do with the amount of water used per regeneration cycle. Some municipalities have stated limits that bounce around five gallons of water used in genera- tion per 100 gallons treated or 95-percent water efficiency. In the above calculation, and if we assume a capacity of 20 kilograins (Kgr) capacity/cu ft of resin at a five lb/cu ft setting, we will regenerate (2.4 MM gr/20 Kgr) = 120 cycles. At 95-percent water efficiency, we are allowed (120,000 gal x 5%) = 6,000 gallons of water for 120 cycles. That’s 50 gallons per cycle. Does your water softener do that? Again, there is no reserve and we are assuming high efficiency.
Obviously, the water efficiency of a softener will be influenced by the hardness of the feed. We will use the same number of gallons per year but the number of cycles will vary with the hardness. Let’s look at 15, 20 and 25 gpg TH in the feed and keep our salt setting and salt efficiency constant. 120 K gal at 15 gpg = 1.8 MM gr/yr. 20 gpg is 2.4 MM gr/yr and 25 gpg is 3.0 MM gr/yr. Dividing by 20 Kgr/cu ft/cycle, we have 90, 120 and 150 regeneration cycles respectively. Again, with 95-percent water efficiency, we have an allowance of 6,000 gallons, which calculates to 67, 50 and 40 gallons per cycle, respectively. Not many systems can even be programmed to regenerate on 40 gal/ cu ft. What steps can we take to get there?
Where am I?
If you check most owners’ or OEM manuals, you will find that they are quite generous in the amount of water used in regenerating their systems. This is especially true of the rinse volumes. A good backwash is needed to clean and reclassify the resin bed (to reduce clogging and pressure drop). An adequate backwash can be minimized but not eliminated. Brine and rinse can be calculated as well as fast rinses to minimize the total water used. Let’s compare.
Older mechanical systems(2) used a pre-rinse to purge the bed, a backwash, brine and slow rinse, fast rinse and a settling rinse to set the bed. The standard injector drew brine at 0.38 gpm and had a slow rinse of 0.45 gpm. All other rinses were at 2 gpm. The total water used per cycle was:
Pre-rinse: 2 gpm @ 5 minutes = 10 gal
Backwash: 2 gpm @ 10 minutes = 20 gal
Brine draw: (8 lb setting = 3 gal brine) 0.38 gpm @ 8 minutes = 3 gal
(24,000 gr/cu ft capacity)
Slow Rinse: (controlled by eductor) 0.45 gpm @ 60 minutes = 27 gal Fast Rinse: 2 gpm @ 10 minutes = 20 gal
Settling Rinse: 2 gpm @ 5 minutes = 10 gal
Total = 90 gallons/cycle
Newer electronic systems allow you to skip steps and run any cycle time desired.
Backwash: 2 gpm @ 7.5 minutes = 15 gal
Brine draw: (5.0 lb setting = 2 gal brine) 0.2 gpm @ 10 minutes = 2 gal
(20,000 gr/cu ft capacity)
Slow Rinse: (controlled by eductor) 0.3 gpm @ 50 min = 7.5 gal Fast Rinse: 2 gpm @ 5 min = 10 gal
Total = 34.5 gallons/cycle
Based on our 95-percent water efficiency, the older mechani- cal system using 90 gallons to regenerate has to produce 1,800 gallons of softened water to comply. With 24 Kgr capacity, it would, therefore, be limited to treating feeds at 13.3 gpg hard- ness or less (13.3 x 1,800 gal = 24,000 grains). Our electronic unit using a water saving 35 gal/cycle only has to produce 700 gallons of treated water to comply and with a capacity of 20 Kgr/cu ft, it can treat feedwater up to 28.6 gpg hardness (28.6 x 700 = 20,000 grains).
In Figure 2, the green box represents the hardness range that can be treated while maintaining water efficiency at 95 percent. The red line representing a 5 lb/cu ft regeneration level (see Figure 1), however, is probably the only option for both brine and water efficiency.
Where do I want to go?
The modern electronic softener controls are perfectly suited to help improve both salt and water efficiency. By adjusting the salt dose down to 5 lbs/ cu ft, one can achieve 4,000 gr/lb efficiency. If one would like to be able to claim efficient operation up to 25 gpg, you would be limited to using only 40 gallons of water to regenerate. A 20,000 gr softener would produce (20,000/25) = 800 gal of soft water per cycle. If five percent is allowed to regenerate, you are limited to (800 x 5%) = 40 gallons. 4,000 gr/ lb and 40 gal/cycle is a respectable goal.
How do I get there?
For most system configurations, an adequate backwash can be achieved with a 7- to 8-minute cycle. Backwash flow should be about 5 gpm/sq ft of tank surface area and will vary with water temperature. Using as small an injector as is practical will lengthen the draw time for brine, which will help increase brine efficiency. You will need at least 12 to 15 minutes of actual brine contact. Your slow rinse can be reduced to about one empty tank volume (a 9 x 44 will contain 12.5 gallons). Fast rinse can be reduced to five minutes. This should keep you at about 40 gal/cycle.
Designing home water systems for better brine efficiency by reducing the brining level will reduce overall capacity per regeneration cycle. Reducing the capacity means that there will more regeneration cycles required. This reduces water efficiency. Therefore, adjustments to the regeneration time cycle must be made to avoid water waste. While brine efficiency and water efficiency do not go hand in hand, they can be made compatible with a little planning.