By C.F. ‘Chubb’ Michaud, C.E., CWS-VI
Brine efficiency is defined by the number of grains of recoverable hardness removal capacity achieved by a softener per pound of salt used in regeneration. A perfect score is around 6,000 grains/lb. Softeners rated as ‘high efficiency’ by WQA run at 4,000 grains/lb., but most operate in the 2,500-grain range due to the inherent lower efficiency from the use of more salt than is needed for good results (see Figure 1). Counterflow regeneration of a softener saves salt and saves water. But that’s not the only thing green about it. It also saves customers money while actually improving performance. From the viewpoint of the manufacturer/dealer, there’s a lot of sizzle with the higher-tech approach to softening that will certainly offer numerous advantages to help improve sales.
Co-flow versus counterflow
Ion exchange softeners generally operate downflow in service and downflow in regeneration as well. This is called co- flow operation. Counterflow regeneration is usually upflow using conventional softener valves. Although counterflow offers many efficiency advantages, which will be pointed out in this article, it is not simply a plug-and-play technology. Care must be taken in setting up the system to have the regenerant flow sufficiently slow in order not to mix the bed and disturb the highly regenerated polishing zone that develops at the bottom. Bed expansion during regeneration must also be avoided, as it will result in lower capacity, since brine passes through the expanded bed more readily and makes less contact with the resin. In reference to the capacity versus regenerant-level curve offered in Figure 1, note a book capacity of about 24,000 grains of recoverable capacity for a cubic foot of resin regenerated at the eight-lb./cu. ft. level (3.000 grains/lb.). Also of note is the red leakage curve, which shows an average leakage of about five ppm that can be expected. The capacity curve for counterflow regeneration would only show about five- to eight-percent capacity improvement, but the corresponding leakage would be less than one ppm.
Figure 1. Co-current capacity versus brine dose
Limitations in reduced brine dosage
Now look at the response expected from a three-pound dose. We can expect 13,500 grains capacity and efficiency jumps to 4,500 grains/lb. Leakage, however, will approach 14 ppm, and in problematic waters with moderate TDS will exceed one grain (technically, this could no longer be called a softener). In addition, funny stuff happens with brine efficiency at low dosage. Brine gets highly diluted in the freeboard headspace of conventional downflow systems and these capacity levels are never achieved. Leakages are even higher. Additionally, total brine contact time is reduced to less than eight minutes, which compounds the whole regeneration situation. From experience, the lowest practical salt level for co-flow is around 4.5 lbs. before the curve takes a detour.
In downflow service, resin beads at the top of the column do most of the work. By the time capacity is reached, the upper third of the bed is totally exhausted, while the bottom third is still in a moderately regenerated form. This is represented in Figure 2.
With co-flow regeneration, brine enters from the top of the column and flows down through the bed. Here is where the ‘column effect’ enters into play and explains the dynamics of the equilibrium that makes ion exchange work. The top of the bed sees a continuous flow of regenerant brine that scrubs and polishes the hardness off the resin and allows it to pass down the column and eventually out the drain line. This way, the top of the bed becomes highly polished for the start of the next cycle. But where does all that hardness go?
This will explain the drawbacks of co-flow regeneration. The hardness stripped from the top of the bed now has to pass through the unexhausted zone at the bottom of the bed (See Figure 3). By the time the brine gets to the bottom, it is fairly well spent and loaded with hard- ness. Hardness deposits on the bottom of the bed and unless there is sufficient brine to continue the cleaning process all the way through the bed, the bottom resin can remain in a moderate state of exhaustion (see Figure 4). When the next cycle starts, incoming hardness is stripped by the highly polished resin at the top of the bed, converting the feed water to a dilute brine solution (as soft water), which continues down through the bed. When this low-hardness water containing sodium reaches the bottom portion of the bed, it equilibrates with the exhausted resin and slowly regenerates it, producing hardness leakage. It is easy to understand why leakage goes up with a decreasing salt dose, and down with an increasing dose. With low salt doses, one will note that the highest leakage occurs immediately after regeneration and continues to improve throughout the run (as hardness rinses off the bottom). As effluent hardness begins to climb towards the end of the cycle, it is an indication of exhaustion (not really an increase in leakage) (see Figure 5).
Drawbacks of co-flow regeneration
Decreasing the brine dose in a co-flow system presents three major issues. One is the dilution of the brine in the headspace mentioned earlier. The average freeboard in a one-cubic-foot (28.3 liter) softener is about four gallons (15.14 liters). Using a four-pound (1.81 kg) salt dose, we have incoming brine at 10 percent (about one lb./gal). At that dose, we have about four gallons of 10-percent brine. The initial brine entering the top gets diluted to one percent or less, and the last of the brine gets diluted to about seven percent. Regeneration drops dramatically below about eight-percent brine, leading to shorter runs and higher leakage. The second issue is that unless the control valve was set up with a smaller injector to slow the brine flow, the unit will try to regenerate at about 0.75 gpm (2.83 L/m). To draw four gallons will take only six minutes. This short contact time does not allow full utility of the weak brine solution, and hardness is not driven off the bed (shorter capacity with higher leakage). The third major issue (presented earlier) is that the bottom of the resin bed is overwhelmed with hardness that is driven down from the top. This produces a very long rinsing process that adds considerable background leakage. Breakthrough comes early and again, results in shorter capacity with higher leakage.
Lab curves versus real world
Capacity curves such as those in Figure 1 are generated from lab data using a one-inch column with perfect plug flow of 10-percent brine and long contact times. Several years ago, I produced a curve utilizing a 10 x 44-inch resin tank with one cubic foot of resin. I did this by running 20 grain-per-gallon feed at four gpm for five cycles at each of the following salting levels: 15, 12, 9, 7.5, 5.0, 3.75 and 2.5 lbs./cu. ft. (6.80, 5.44, 4.08, 3.40, 2.26, 1.70 and 1.13 kg). The average capacity curve followed the norm and continued to grow closer to the theoretical efficiency line (green line in Figure 1) until about the 3.75-lb. data point, where capacity took an unanticipated plunge (due to the three items mentioned above).
The beauty of counterflow
In a counterflow system, brine is introduced to the bottom of the bed (see Figure 6), where it encounters the cleanest resin. It passes through the bottom without further dilution, creating a highly polished zone. As brine encounters exhausted resin mid- range through the bed, it pushes off hardness into an upper zone that is even more exhausted. This continues until the hardness is pushed all the way to the top and then rinsed out of the bed (see Figure 7). Since less brine was used sweeping the hardness from the bottom, more brine becomes available to sweep it from the top. As a result, more of the bed is regenerated, producing higher capacity. The bottom of the bed is fully regenerated at most any brine level and becomes the polishing zone, producing very low leakage. Brine entering from the bottom is not diluted by freeboard; the rest of the bed not only has a higher percentage of regenerated resin, but that resin is also more highly regenerated. With proper under-bedding, incoming brine can be more evenly distributed at slow flowrates and the regenerant solution travels almost plug-flow up (uniform flow in a filter column where water moves like a ‘plug’) through the bed with perfect distribution. Slowing the flow of four gallons of brine to 0.25 gpm (0.94 L/m) lengthens the contact to 16 minutes, which does a much better job than six minutes of regeneration. The net result is higher capacity with extremely low leakage. This allows for fewer regeneration cycles per year resulting in less salt, water and cost for users. Everybody wins. This is particularly applicable to commercial and industrial systems, which tend to use excessive brine levels to achieve sufficiently low leakages (often less than 0.25 ppm).
Water savings with counterflow
It is not necessary to use a backwash step (save water) with counterflow, and fast rinse can be accomplished by simply extending the displacement rinse to flush the bed (save more water). If backwashing is a must due to turbidity, it should be done prior to regeneration rather than after, unless a secondary soft water source is available for the backwash (as with twin alternating systems).
Lowering the brine dose on a conventional softener will produce higher brine efficiency (more bang for the sodium chloride buck). Hardness leakage, however, may increase above the desired target. Counterflow regeneration eliminates many of the inherent design problems for low-dose, co-flow units, resulting in lower brine discharge levels, less water for regeneration and lower costs of operation. By trimming the needs for backwash and fast rinse volumes, total waste can be reduced to less than 25 gallons per cubic foot in well-designed systems, and brine levels of three pounds per cubic foot become practical.
About the author
Chubb Michaud, C.E., CWS-VI, is CEO and Technical Director of Systematix Company, which he founded in California in 1982. He has served as chair of several sections, committees and task forces with WQA, is a past director and governor and currently serves on the PWQA Board, chairing the Technical and Education Committees. Michaud is one of the original WC&P Technical Review Com- mittee members. He was a past recipient of the WQA Award of Merit and PWQA Roberts Gan Award as well as the PWQA Hall of Fame. Michaud can be reached at (714) 522-5453 or via email at [email protected]