Considering sizing, function & reliability for commercial water treatment applications
During my early involvement with Water Quality Assn. committee work, I had the opportunity to be a member, and later chair, of the Commercial & Industrial (C&I) Section. One of our major accomplishments was the simple task of defining C&I and how it differed from residential in terms of design. One school of thought was simply to define C&I by the size of the equipment or diameter of the pipe. Using this definition, C&I was simply a larger version of residential, with all other design parameters held the same. This was simply not a good description of C&I. The final breakthrough came with the philosophical realization that size does not really matter—it is the reliability of function and the consequences of failure that separate the two categories.
When It Absolutely Must Work
The motel down the street has a 36-in. softener to supply softened city water to the heating systems for the guests and the laundry facilities. The requirements for this type of application are no more stringent than those of a residential softener—the unit is simply a larger version of a residential softener. At 36 in. in diameter, it can easily handle 75- to 100-gal-per-minute (gpm) flow, and with 22 cu ft of resin, it has a nominal capacity to soften more than 25,000 gal of 20-grain-per-gal (gpg) feedwater. If the motel owner forgets to put salt in the brine tank, it is likely nothing will happen. He might hear a complaint, but probably not. The consequences of failure are minimal. This unit is not really a C&I design.
Next door is a car wash. The owner installed a water softener a few years ago to reduce hard water spotting on those hot summer days when the water might dry before the detail crew can hand-wipe the vehicles. He bought a twin alternating 14-in. softening system to provide a continuous supply of treated water with less than 1 ppm of residual hardness at 10 gpm on a 24/7 basis.
This scenario is quite different from the motel application. First, it is a twin alternating softener designed for continuous operation. Its 3 cu ft of resin have a capacity of about 90,000 grains, because it is regenerated at 12 lb of salt per cubic foot to guarantee the less-than-1-ppm leakage. At 10 gpm, it can run 20-grain feedwater for 450 minutes, or 7.5 hours. That is plenty of time for the offline unit to regenerate.
This design was done with a higher level of reliability and performance in mind. If the unit breaks down in the middle of the work day, the twin setup provides a standby unit that is ready to go and can deliver soft water for one shift. Hopefully, a service technician can come out to fix the unit the same day. Otherwise, a lot of hand labor and elbow grease will be needed. It will be inconvenient and involve extra costs, but it is not the end of the world. The consequences of a failure have an economic impact, but will do little financial damage otherwise. This is a great example of a commercial system. It has ample capacity, some duplicity and better-than-average reliability, and it is set up to produce higher-quality soft water than the motel neighbor.
Across the street is a small pharmaceutical lab that performs medical testing. To avoid introducing analytical errors, it uses high-purity deionized water produced from a small multiple-cartridge system. The lab must take into account many more considerations than the motel or car wash. What quality must the product water be? How does the lab ensure that quality? What does it use for backup?
The lab system feeds tap water and product water with a minimum quality of 10 megohm, or 0.1 micro-Siemens. That is about 0.05 ppm total dissolved solids (TDS). Pretreatment is built in with cartridges: a 5-µ sediment filter, 1-µ carbon block, two high-purity virgin mixed-bed resin cartridges in series, a high-intensity ultraviolet (UV) light and a 0.1-µ sterilizing filter. There is a pressure drop measurement on the prefilters, a conductivity/resistivity monitor with alarm, and a UV intensity meter with alarm, plus an automatic shutdown in case something is amiss.
Despite these protections, a lot can go wrong, but the lab staff will know about any problems long before they affect any test samples. The only way to improve on the design would be to have two systems. The lab hedges that bet by keeping a supply of new cartridges on hand. This small system has the performance and quality assurances of the pre-feed for a nuclear power plant. It is a good example of a tabletop industrial filter system.
Water Treatment System Expectations
Residential water filters can improve both the aesthetics and safety of water for home use. The aesthetic aspects can fall short without causing dire results—a little odor and some water spots will not hurt anyone. If the filters are intended to remedy a truly unsafe condition—such as high levels of heavy metals, arsenic or nitrates—they should at least be installed with a lead/lag series system with automatic salt delivery and periodic monitoring and lab testing. Every residential treatment device sold with a health claim should undergo periodic maintenance whether it is required or not. Remember the consequences of failure: If it has to work, it has to work well.
C&I water treatment generally is provided to protect a piece of equipment downstream or deliver tangible cost savings, such as equipment maintenance, like pre-softening for a reverse osmosis unit. Whether it is a high-pressure boiler, a cooling tower, a nuclear reactor or a paint booth, a failure in the quality or quantity of water produced can cause extreme financial damages to process equipment or result in downtime and production losses and rejects. In addition, C&I equipment is sized for a specific operation cycle between regenerations or cleanings. Unlike residential designs that tend to be one size fits all, C&I systems may be sized so regeneration times fall during a certain shift or day of the week.
A residential softener with 1.5 cu ft of resin might be rated at 36,000 grains and 10 gpm. However, it is only required to produce 350 gal per day. At 10 gpm, that is only 35 minutes of full flow. On 20-gpg feed, it will treat 1,800 gal of water and regenerate every five days. To size a C&I system to treat 10 gpm on a 16-hour cycle, assuming 20-gpg feed, the systems would have to produce 9,600 gal (10 x 60 x 16) and remove 192,000 grains of hardness (9,600 x 20). It can regenerate at 8 lb per cu ft (see Figure 1 for leakage and capacity) with a resulting capacity of 24,000 grains per cu ft, so the burn rate is 8 cu ft per cycle (192,000/24,000).
Three or four years down the road, the demand for water may increase. The feed hardness may jump, the resin may oxidize and lose capacity, and some slight fouling may occur that will result in a shorter run length. To compensate, water professionals employ an “engineering” factor that effectively oversizes the system up front to allow for adequate capacity under those anticipated, yet unknown, conditions. Typically, this might be 10 to 15% for a cation system, but for a softener with a long resin life expectancy of eight to 10 years, it might be 20%. The aforementioned design jumps up to 10 cu ft per cycle. This might suggest a 24x72-in. vessel.
The next factor to consider is the hydraulics. Ten gpm on a 24-in., 10-cu-ft system is on the ragged edge of too large. The rule of thumb is 1 to 3 gpm per cu ft and 4 to 10 gpm per sq ft of surface area. With the original design of a 24-in. system, it is at 1 gpm per cu ft and 3 gpm per sq ft. Channeling and excess leakage is the possible result. The consumer may prefer to save money and space and work with a smaller unit that could deliver an eight-hour cycle.
We have a 4-cu-ft-per-cycle burn rate and a 5-cu-ft design, with the engineering factor. Instead of a 24x72-in. pressure vessel with 10 cu ft of resin, we can use an 18x65-in. vessel with 5 cu ft of resin. This is still a twin alternating design because of the potential for continuous operation. The slight oversizing allows this unit to operate at up to 15 gpm and run up to 10 hours as delivered.
Figure 1 is a generalization of an operating capacity curve for a softener. It represents a set of conditions that vary by flow rate, feed hardness, hardness/sodium ratio and TDS. The operating capacity will be different for each installation and, unfortunately, there is no book of charts to cover all the bases.
For this example, the green curve represents the theoretical capacity at 100% salt efficiency (6,000 grains per lb). The blue curve is the actual or “operating” capacity realized to a 1-gpg break with a 20-gpg feed run at 2 gpm per cu ft. The hardness/sodium ratio is 1:1, and the feedwater TDS is about 650 ppm. The red curve represents the leakage for the given salt dosage, and the vertical red lines represent four salt dosages of 6, 8, 10 and 14 lb per cu ft delivered in co-flow mode. Where those red lines cross the blue capacity curve represents the capacity at that particular salt dosage, and the dotted black lines that run from the leakage (red) curve to the right-hand axis give leakage values.
For this particular chart, a regeneration level of 6 lb per cu ft gives a capacity of about 20,500 grains per cu ft and a leakage of 9 ppm, or about half a grain. To improve the leakage (quality) of the treated water, move to the right and increase the salt dose. In a typical C&I installation where leakage of less than 1 ppm may be required, move all the way to 14 lb per cu ft. With each increase in the salt dose, there is a corresponding increase in the capacity and decrease in the leakage. C&I equipment is designed for a specific leakage requirement first, with the resulting capacity then used to properly size the equipment. Salt efficiency is not the first objective but can be improved upon though countercurrent regeneration, recycling brine and resin selection.
C&I water treatment equipment differs from residential equipment primarily in its inherent reliability of function and mindfulness of the consequences of failure. When it must work, it must work well. This philosophy also extends to residential treatment equipment designed to protect health.