Some Q&A about ZLD Program - WCTI
Zero Liquid Discharge
Technology eliminates scale and corrosion in ZLD operation of cooling towers. Silica chemistry maintains heat transfer, prevents galvanized white rust, controls bio-fouling without chemicals.
US 6,929,749 / Silica Scale Control US 6,949,193 / Silica Scale Control US 6,998,092 / Corrosion Inhibition/Silica US 7,122,148 / Corrosion Inhibition/Silica
Is it true that the make-up water stream for WCTI technology has to be almost entirely softened (Ca and Mg removed)?
Correct, the process requires sufficient softening of makeup water to maintain total hardness levels at less than 30 mg/L as CaCO3 in the tower water to maximize water conservation and performance. Higher hardness levels can be supported at higher TDS due to common ion effect (example, higher hardness in seawater effect). Lower COC with some blow down from low hardness makeup can be maintained with results comparable to traditional chemical treatment.
Hardness excursions are however well tolerated at high COC by this chemistry due to residual alkalinity stored in the tower water (similar to boiler chemistry) that will control hardness precipitation to prevent scaling for several days or weeks. Simple corrective actions are hardness reduction in the tower by blow down and restoration of soft makeup.
If we have high hardness, high silica and high alkalinity water, will we need a mixed bed demineralizer along with a softener?
The WCTI process does not need or require a mixed bed demineralizer (IX) to remove hardness from the makeup water. We welcome high levels of alkalinity - in fact we prefer it. Instead, a high efficiency softener (HES) is used, which regenerates using saturated sodium chloride solution to remove only hardness from the water.
Example: If we assume that 180 GPM of makeup water is required to operate at three cycles of concentration, with 60 GPM of blow down. Based on the 180 GPM makeup figure and an estimated 4,700 ton evaporative load, the WCTI process would reduce makeup to 120 GPM required to replace evaporative loss by eliminating 60 gpm of blow down (172,800 gallons per day of makeup). The wastewater generated by the softening regeneration process would total 1,460 gallons per day or 1.03 GPM. These figures are based on treating 172,800 GPD of make up at a peak total hardness of 320 PPM and an average total hardness of 280 PPM. The WCTI tower makeup requirement is based on operation at 50 cycles of concentration.
We utilize a propriety high efficiency softener (HES) that uses sodium chloride for regeneration. Excellent exchange efficiency and polished hardness removal quality are provided with use of 1/3 the salt and regenerate wastewater volume using the WCTI design. For the WCTI process to succeed as designed, we cannot guarantee success with a standard softening approach and use of WCTI systems is required under our license agreement.
Since blow down with the WCTI process will stop or decrease to very small amount but a very concentrated stream, how does the WCTI technology address the buildup of TDS and the ways to remove the buildup from the tower water?
There is no particular need to remove TDS from the tower water, as it presents no scale or deposition risk with this technology. The TDS are all highly soluble ions.
The site can choose to operate at lower tower concentrations (10X COC, 10% BD & drift wastage), higher COC (50X COC, 2% BD & drift wastage) or zero tower discharge (drift waste only). Please note that high TDS water from the WCTI process has never been an issue in all of the installations operating with little or no blow down. You can verify this by contacting/visiting end users.
Solids precipitate continuously in the tower water as described by the WCTI technology. How is TSS removed?
Tower TSS accumulates primarily as a result of particles scrubbed from the air, not precipitation from the water. Some of the polymerized silica will be adsorbed and precipitated with these air scrubbed solids, but the accumulation rate is no greater than with conventional chemical treatment, which also have calcium and chemical precipitates in the tower water. Most of our customer applications clean such scrubbed and settled TSS from the tower basin once or twice a year per normal maintenance, and some already have side stream filtration systems that reduce TSS accumulations. Side stream filtration is not required unless dictated by significant TSS in local air quality. Silica in this process is also a natural polymeric settling aid that keeps most of the TSS in tower basins, and tower water is usually much clearer and cleaner than conventional chemical treated tower water.
Since Monovalent ions (Cl and Na and other) continuously build up in the tower water, how are they removed?
The softening process replaces calcium and magnesium ions with sodium ions in the tower makeup water, and chloride is discharged with the spent regenerate along with the hardness ions removed. As noted previously, there is no need to remove such soluble ions since they have no scale impact because the scale-forming ions have been removed, no corrosive impact with the silica inhibition of metals because they are protected with a highly effective silica film, and the added TDS aid the biostatic process as well.
The TDS could be built up to and above 44,000 mg/L. How will the monitoring instruments continue to function at this level of TDS is unclear? Wouldn’t the maintenance on different probes (TDS/Conductivity, pH and other) be high?
We have customer systems operating from 10,000 TDS to 200,000 TDS without deposition, corrosion or biological issues. The final TDS level depends on customer choice and how effectively their systems are designed to control system leaks and drift. Use of pH control or monitoring becomes obsolete, as more highly concentrated tower water, with no acid or chemical feed, reaches natural pH equilibrium at given discharge or drift losses. There is no scaling potential with soft water, and traditional scale control indexes become obsolete. TDS control systems also become optional with zero tower discharge or limited discharge, but work very well at high TDS with minimal maintenance, particularly since scale and bio-fouling are minimized by tower chemistry. Most of our customers choose to eliminate TDS / Conductivity controllers. Again, this can be verified through site visits and communication with end users.
At very high operating TDS, the drift mist will be depositing a lot of salts outside the tower and may create an issue with soil and groundwater contamination. High salt drift from cooling towers became an issue in the past in several instances where public was exposed to it. It appears that WCTI relies on losing the concentrated TDS through the cooling tower drift. Most towers have an air permit requirements and drift TDS is limited by such a permit. For this case high TDS in the drift; will it cause all the nearby equipment, buildings and other installations to experience severe exposure to TDS and potential damage?
First, the WCTI process does not rely on drift as a means of losing tower TDS. The goal of the program is to build TDS in the system and not reduce it through drift losses. All modern cooling tower designs minimize drift to very low levels. As noted previously, TDS level can be easily controlled at the optimum level for the site with respect to tradeoffs in water conservation, aesthetic impact from higher TDS (salt) concentration, and permitting. Most cooling towers will not fall under drift regulation requirements as modern drift eliminator designs minimize drift impact to levels well below the quantity impact limits that are regulated. Since the use of softened makeup provides highly soluble sodium salts versus insoluble calcium salts, drift solids buildup is easily maintained by water wash down if build up occurs.
The conclusion that “high TDS in the drift will cause all the nearby equipment, buildings and other installations to experience severe exposure to TDS and potential damage” is not accurate and can be readily verified by contacting end users or through site visits. Preventative maintenance with water wash down and use of TDS resistant coatings for immediate adjacent piping is beneficial, even with traditional low TDS treatments.
See Anderson report on cooling tower drift and permitting posted in reports and papers section on WCTI website (water-cti.com).
Since the technical aspects of the WCTI technology are sound as far as water savings. However, there are also increased waste streams from regeneration of softening resins. If regeneration requires use of NaCl only, then aren’t there increased chlorides of sodium in the tower?
Both capital and operating costs, as well as discharge volume and costs, based on previous projects we have evaluated using de-ionization, reverse osmosis, HERO, and precipitation softening used with either side stream or pretreatment approaches to recover water and reduce discharge were approximately 3X to 10X greater than pretreatment softening with salt regenerated ion exchange. The approximate cost for softening makeup with WCTI design equipment for 5,000 tons is $0.10 per 1000 gallons makeup. Regeneration waste from the HES is minimized to very low levels (1-2% of TV or treated water volume) as compared to demineralization / deionization (6 - 10% of TV) or RO (10-30% of TV) systems. There are increased chlorides in the regenerate waste, rather than the tower makeup, but they pose no problems to being discharged to a sewer. Most plants that operate boilers use softeners, and softener regeneration waste is typically discharged to the sewer. The Department of Ecology has verified that this is not an issue. This process does increase sodium in the tower, which is highly beneficial to the WCTI process. Sodium ions are almost infinitely soluble and produce a beneficial “common ion effect” in the tower water chemistry as well.
The claim of the WCTI technology that the blow downs can be discharged to the sewer would have to be checked with the local sewer authorities?
We encourage you to check with the local sewer authorities or your regional Department of Ecology to ask that very question. In fact, the total quantity of TDS that will be discharged from the tower using the WCTI process will be less than the quantity of TDS discharged with conventional tower chemical treatment and blow down wastage. This is due to the increased volume of makeup water used, with the TDS contained in the source water being concentrated and discharged by tower blow down, thus increasing total TDS poundage per day of TDS discharged. We have evaluated the increased TDS impact from the salt used and discharged during regeneration, plus the tower TDS discharged at increased tower concentrations (from 10 to 50X) and found the TDS contribution to municipal discharge is no more or less than traditional chemical treatment water wastage at lower COC operation. Use of de-ionization will not provide more efficient ion exchange nor reduce TDS contribution, but more likely will increase total TDS contribution to the sewer.
The heat balance for the cooling tower can be affected by the significant blow down reduction or elimination. One key point is that the boiling point elevation due to high TDS means that the lower temperature on the Cooling Tower will be higher?
If the cooling tower is not adequately designed to meet cooling water heat load requirements without using excess makeup to cool the tower return water, then that is certainly a problem. But that would not be a good excuse to avoid water conservation. Cooling tower manufacturers provide data on the impact of higher TDS on heat rejection efficiency from cooling towers, as related to operation with seawater cooling sources. This data shows a probable impact and recommended increase in tower heat rejection capacity ranging from 1% to 2%. Most cooling towers are designed to provide more capacity than peak loads conditions, and would not notice this potential impact if the tower is operated at higher range of TDS. Full scale installation experience verifies this point, as no end users that converted their systems to the WCTI process have experienced a reduction in cooling capacity. In fact, cooling capacity has been increased at many locations due to the elimination of scale and bio-fouling effects on heat transfer efficiency, which has a much higher impact on cooling capacity. It is also interesting to note that many tower systems have to operate at higher water flow and at capacities to compensate for poor heat transfer rates that result from scale, deposition and bio-fouling that occurs with traditional chemical treatment approaches. WCTI systems operate without scale or bio-fouling.
Is it fair to claim ZLD (zero liquid discharge) as the softener regeneration waste is a substantial liquid waste and that too at high TDS and Chlorides?
The term zero liquid discharge ZLD is the terminology used in the water treatment industry when the cooling tower discharge is eliminated. This cannot be done without some form of pretreatment or side stream treatment that removes some of the insoluble ions. The WCTI process is just a new and more cost effective process to accomplish ZLD from the tower. In fact, the WCTI (HES) process substantially reduces the total volume of regenerate waste as compared to conventional softener, demineralizer and RO processes.
If one calculates the LSI (Langelier Solubility Index) of the circulating water, it will become evident that water will be on highly scaling side for even low levels of influent hardness?
The LSI and other indexes are irrelevant with the WCTI process, since calcium and scale forming ions are removed, and no longer provide the basis for the calculations and their predictions. Hardness is maintained below 30 mg/L in the tower water with the polished quality of WCTI softening equipment, even at higher COC ranges. A visit to any WCTI installations will verify that scale is not an issue. Corrosion and bio-fouling are non-issues as well.
Should we conduct Piloting of the WCTI before we try this process?
Please be aware that this process has been already been piloted and has been operating on all types of cooling water applications commercially for over five years, in addition to being professionally peer reviewed in major conference reports and papers. The WCTI process has enjoyed a 100% success in mitigating scale, corrosion and biological concerns in all applications without exception. We highly recommend that any interested customer investigate this technology through site visits and talking to end users to learn about this technology through their direct experience with it.
This technology is very new and it would be risky to implement it w/o having more information on its performance in the industry?
There are over 50 installations in the U.S. using this technology and more are coming on board every day. The technology may not be main stream yet, but WCTI and the licensees applying it are also very experienced with traditional chemical treatment approaches, performance evaluations and water conservation alternatives. Given the constraints with water and wastewater issues becoming more and more important, this technology offers the best means to address water conservation, wastewater discharge and in using final effluent as a makeup source. You will find that communicating with WCTI or its licensees to learn more about the information will put this to rest. If you feel it is risky to implement WCTI without having more information on its performance, we encourage you to do just that - get more information. We will be happy to provide relevant application references, contact information and answer any questions you have, technical or otherwise.
What size softeners do we need for this process?
This would depend on your peak evaporation rate flow for your systems as we size the (HES) softeners based on peak load through put. WCTI can provide softeners from as little as 7 gpm, and greater as needed.
How much does it costs to run the program?
Our process, once installed, requires service on our part to verify the softening process, maintenance on the units as needed, and verify control of tower water chemistry with specifications. WCTI licensees charge a fixed monthly fee to use the technology that includes testing, reporting and maintenance services associated with the program that are comparable to traditional water treatment program costs. The fee charged is dependant on the size of the cooling tower system and generally falls within close proximity as that of a traditional chemical program. HES units begin at $1400 (polisher) for 7 gpm (not including shipping and installation)
What if our water has very little alkalinity or silica available?
Both silica and alkalinity may be supplemented to the makeup to the tower to establish method control residuals. Once a pH of 9+ has been achieved and the silica levels are achieved, no more products should be needed as constant evaporation and “no bleed” will keep the process in range.
How is the program implemented?
If the system is on a traditional chemical program, there is no need for anything other than the start of the softener and keep close tabs on the tower total hardness. Depending on prior treatment program deposition, with introduction of the new soft water make-up, some hardness “leaching” can take place for a period of time. This is essentially the re-solubilizing of the hardness ions from the pre-existing scale deposits. In time, the tower water should stabilize relative to hardness which will be maintained at or less than 30ppm. The pH in most cases will increase in time; however, some alkalinity may be added initially by the licensee to attain the pH needed.
Most inhibitors left over from the traditional program can continue to be pumped into the system to help deplete inventory but is not necessary.
New system start-ups are essentially easier depending on the incoming make-up. Low pH and soft water sources may require the addition of sodium silicate and NaCO3 to get to the desired range.
Is there any other equipment needed along with the softener?
Some source waters may contain TTS or iron that require a simple low micron filter prior to the HES to filter out any undesirable suspended solids that could interfere with the softener operation. A hardness test kit is required along with a reliable supply of salt for regeneration.
How do control bacteria with WCTI technology?
Until the pH reaches around 9.2 and above, supplemental non-oxidizing biocides specified by the licensee (such as Isothiazoline) or oxidizing biocides may be used by the licensee to control bio-growth. Natural biostatic chemistry starts to build with increasing pH and TDS (see Anderson report on WCTI website). The licensee may also increase pH by caustic addition initially to promote biostatic chemistry.
What should be done if we have high levels of ammonia in our supply water or if we have ammonia leaks from or cooling process?
The WCTI process naturally removes ammonia (striped when circulated over the tower) contained in either the makeup or introduced into the tower water to very low residual (<> 9.7). This will also eliminate potential odor issues as the ammonia is stripped incrementally as it is introduced by makeup to the larger circulating volume of the tower water. No further treatment is required unless copper metals are contained in the system, in which case the licensee will add a small quantity of copper inhibitor each month during service visits to further increase protection of copper metals (very little inhibitor is consumed if there is no blowdown wastage). Excessive sustained losses of ammonia from a leaking condenser may cause a volatile organics discharge issue.
What if the softener fails and hard water starts coming into the tower?
The system should begin bleeding down; however, since the system will be at a highly buffered condition, this should not be a problem for at least a couple of days until the softener is repaired. Just as with any traditional program, efforts are made to maintain the working tools, whether they are chemical inventory, stuck bleed solenoids, broken pumps, or the case of the WCTI program a HES (High Efficiency Softener).
Can you explain the patent briefly so we can understand what it is we are getting into?
Essentially, any process used (Softening, DI, RO, etc.) to limit makeup hardness introduced to evaporative cooling tower applications is covered in this process if the makeup and tower water are operated within the method control ranges. Pertinent control ranges are tower water pH (> 9.0) and silica content (> 10 mg/L). Further patents of this process are also pending. Most softened or reduced hardness makeup blends that are highly cycled in the cooling tower water will likely operate within these control ranges and infringe on the patents. Supplemental use of other chemicals does not provide circumvention. Optimum performance in corrosion inhibition and biostatic control are attained at higher concentration control ranges, but at no added expense, as silica and alkalinity are typically provided in the source water unless removed by processing (RO, DI, etc.).
Since we are cycling up so high to levels in some cases >100COC’s, aren’t you creating precipitants?
Precipitants do not form in the WCTI process to any greater extent than traditional chemical treatment, and side stream filtration is only needed to remove suspended solids scrubbed from the air by the tower, and only to the same extent it would be needed in conventional systems. This was presented in the patents and pilot process results. This should not be a concern.
What if any are the negative sides to the WCTI Technology?
No. In fact, even deposits that might form on the drift eliminators (see movie below) are now chemically different due to the WCTI process. These deposits are now non-adherent and highly soluble since they no longer contain the hard scale forming ions of calcium and magnesium. Deposits can now be easily washed down with HES conditioned water at garden hose pressures saving man-hours and damage to the fill caused by high pressure cleaning.
Customers who have experienced severe biological issues in the past with chemical and NCD treatments may wish to operate at higher pH. This is made possible by WCTI’s ability to develop “sustained natural chemistry” and pH will begin to naturally buffer out at about a pH of 10. In this case, the owner may look at choosing seals and belts more tolerant to high pH as a part of their scheduled maintenance programs.
As with any cooling tower and water treatment program, drift under certain conditions, in areas sensitive to vehicles or building aesthetics can be addressed on a case by case basis to reduce the problem. Your WCTI professional will be happy to work closely with the owner’s staff and mechanical contractor to provide potential solutions.