|Microfilter pressure vessel showing the thousands of hollow-fiber membranes. Flow is outside-in for this configuration. Photo courtesy: Pall Corp.|
By Brad Buecker, Contributing Editor, Kiewit Power Engineers
The many new combined-cycle power plants being constructed in the U.S. are being equipped with either cooling towers or air-cooled condensers for steam condensation. Once-through cooling systems are out. For plants with cooling towers, the days of relatively straightforward chemical treatment have reached an end, although in large measure that ended three decades ago when chromate treatment was banned. Plant operators and technical personnel, many of whom are new to this industry, are facing significant challenges when it comes to keeping cooling towers reliable and efficient. This article outlines a number of important details regarding modern cooling water treatment methods.
The Phosphorus Twist
Since the outlawing of chromate-based chemistry programs, the treatment method employed for many cooling towers has utilized a blend of organic and inorganic phosphates for primary scale and corrosion control along with an organic polymer for calcium phosphate scale control, and perhaps a small dosage of zinc for additional corrosion protection. These programs can be effective, but they require very close monitoring and control to prevent chemistry upsets. Accordingly, the major water treatment chemical companies developed sophisticated chemical feed and control systems for this methodology. One problem with phosphorus-based programs, which is becoming critically important, that the cooling tower blowdown obviously discharges the phosphorus, mostly, of course, in the form of phosphate (PO4). Phosphorus is a primary nutrient for aquatic organisms, including toxic algae and cyanobacteria. Many receiving bodies of water are now considered to be “phosphorus-impaired,” which means that discharges from new plants may be severely limited when it comes to phosphorus concentration (and sometimes other impurities as we will see). So, essentially two choices are available, treat the plant wastewater, perhaps even to zero liquid discharge, to remove impurities, or employ an alternative cooling tower treatment program.
Let’s discuss alternative treatment first. Researchers are working diligently to perfect non-phosphate programs, and some results have been very successful in full-scale applications. The new programs are based on polymer chemistry, where the polymer chains contain carboxyl (R-COO-), sulfonate (R-SO32-), and amide (R-CONH2) sites, among others. (“R” represents the organic substrate. Polymers of course can be manufactured in an infinite variety of chain lengths.) The polymers and their active groups sequester the scale forming cations calcium, magnesium, iron, and others to prevent them from reacting with anions such as bicarbonate alkalinity (HCO3-) and silicate (HSiO3-) to form scale. Research also shows that some of the polymers can provide effective anodic corrosion protection. One potential drawback is cost, as it appears that the polymer concentration may, depending upon the makeup water quality and cooling tower cycles of concentration, need to be maintained at several hundred parts-per-million (ppm).
Besides phosphorus, other compounds are beginning to appear on plant discharge guidelines. These regulations fall under the auspices of the National Discharge Pollutant Elimination System (NPDES), but in some cases individual states are imposing restrictions above and beyond the national guidelines. New proposed national regulations will impose restrictions on chromium and zinc (0.2 ppm and 1 ppm, respecitively), but other constituents that are appearing on new state-influenced guidelines include ammonia, copper, sulfate, and total dissolved solids (TDS). Rumors have flitted around that bromide and chloride may be added to the list, with other possibilities in the future.
These issues raise a number of concerns. For example, a common treatment method to reduce scaling potential in cooling towers is feed of sulfuric acid (H2SO4), which reacts with bicarbonate alkalinity and converts it to carbon dioxide. This reduces the potential for calcium carbonate scale formation. Obviously though, any limits on sulfate discharge can potentially affect this straightforward treatment method. TDS limits offer an interesting and at times potentially very frustrating paradox. In some states, with California being a notable example, restrictions are being placed on the quantity of plant discharge, of which cooling tower blowdown is often the major component. But, the method to reduce blowdown is to increase the cycles of concentration (COC). Now, however, the blowdown may run afoul of TDS limitations. In yet another example, a combined-cycle plant with an existing cooling tower of wood construction is facing difficulties with regard to proposed new copper discharge limits. The wood was fabricated with a preservative containing copper, and leaching of this preservative, especially during times when the unit is down, may cause violations in future discharge limits.
Sometimes the only recourse may be blowdown treatment, and the author has been involved in a number of recent projects in this regard. But, the stream can be rather complex, with the need to potentially remove phosphate, heavy metals, and other compounds such as sulfate and chloride. For these reasons some experts recommend selection of ZLD up front so that plant owners and operators do not have to worry about discharge regulations and future restrictions.
Perhaps the most straightforward ZLD technique, but with large caveats, is deep well injection. Such wells are usually several thousand feet deep to avoid any discharge into shallow groundwater used for domestic purposes. While this concept sounds simple, experience has shown that some wastewaters can generate scale within the well shaft, particularly as the water warms further underground. High-pressure is generally required for this process, and if scale formation occurs, capacity may decrease. Often, even if plant personnel would wish to incorporate this process in plant design, permission is not granted.
At plants in arid locations with a large land area, evaporation ponds may be sufficient to handle the wastewater discharge. However, these ponds must be properly lined to prevent seepage of the wastewater with its impurities into the underlying soil. Permitting may or may not be granted for evaporation ponds. Alternatively, at sites strategically located, it may be possible to have the wastewater trucked off-site to a waste disposal company.
If none of the above options are available, thermal evaporation of the waste stream may be the only choice. At a recent visit to a plant in the southwestern U.S., the author observed a brine concentrator/crystallizer system that treats the entire cooling tower discharge. While the system operates well, the inlet flow rate at full load is nearly 1000 gpm. Thus, energy requirements are quite large, as are the regular maintenance costs to remove accumulated solids from the equipment.
Per the above-mentioned reasons, becoming more popular are treatment methods to reduce the volume of the plant waste stream before final treatment. A notable example is high-recovery reverse osmosis, as outlined generically below.
Keys to the process are:
- Microfiltration (MF) or ultrafiltration (UF) to remove suspended solids in the waste stream. This is a critical process to prevent the solids from fouling reverse osmosis (RO) membranes.
- Sodium bisulfite (NaHSO3) feed to remove residual oxidizing biocides. This is also critical to remove oxidizers that would degrade softener resin and RO membranes.
- A sodium softener to remove calcium and magnesium. Otherwise the downstream equipment would suffer from calcium carbonate and magnesium/calcium silicate scaling.
- Sodium hydroxide injection to elevate the pH above 10. (The combination of hardness removal and pH elevation keeps silica in solution.)
- Two-pass reverse osmosis (RO) treatment.
Under proper conditions, the RO recovery efficiency may reach 90 percent. The RO permeate recycles to the plant high-purity makeup water system or other locations. However, while the process appears straightforward, a number of lessons-learned have emerged regarding this technology in actual application. The following lessons are taken from a system operating at a power plant near the Pacific Northwest. One of the most notable is that some standard water treatment chemicals may foul the UF membranes. Operating experience indicates that the membrane manufacturer and type influence this phenomenon. Fouling is often caused by the fact that many membranes carry a negative surface charge while cationic polymers are commonly employed for coagulation and flocculation. Residual polymer will coat the membranes. (A very similar phenomenon has been observed with MF or UF systems installed in makeup water systems downstream of a clarifier. Inexperienced designers and/or plant personnel have not always recognized that MF or UF should generally serve as a replacement for clarification, not a polishing process for the clarifier.)
A straightforward solution that has significantly improved the reliability of this particular system is conversion of the ultrafilter from an inside-out normal flow path to outside-in. Typical micro- and ultrafilter systems consist of multiple, parallel flow modules containing thousands of spaghetti-like, hollow-fiber membranes. The membranes must be regularly backwashed every 10 to 20 minutes or thereabouts to remove particulates. The backwash flow path is the reverse of the normal flow path. In this case, conversion of the membranes from inside-out to outside-in normal flow path improved the backwash efficiency.
Another interesting initial difficulty occurred during the original UF backwash process. Typically with these systems, a small portion of the permeate is collected in a separate tank at the beginning of each process cycle for use in backwash. No issues during standard backwashes. But most modern MF and UF units are now equipped with automatic chemically-enhanced backwash (CEB) systems. After a certain number of cycles, a CEB backwash kicks in where first the membranes are cleaned with a dilute caustic/bleach solution to remove organics and microbiological organisms, followed by rinsing and then a dilute citric acid wash to remove iron particulates. When this particular UF was first commissioned, the membranes developed a layer of calcium silicate scale during the CEB caustic stage. The driving force was the higher pH generated by the caustic, which in turn greatly reduced the silicate solubility. The solution to this problem was a switch to softened water for the backwash supply.
|Extreme algae growth in a cooling tower. Photo courtesy: Ray Post, ChemTreat|
The Raw Water Conundrum
Now we come to an issue that can influence all of the above processes; the quality of cooling tower makeup supply. As fresh water sources become more scarce, cooling tower makeup at an increasing number of plants, either by mandate or choice, is being taken from less-than-pristine sources. A perfect example is treated municipal wastewater effluent. While this water undergoes significant treatment to convert it from a raw sewage-laden inlet to a pathogen-free effluent, the discharge still often contains significant concentrations of ammonia, phosphorus, organic compounds, and suspended solids. When used for power plant makeup, the stream may require upfront treatment such as clarification/filtration to prevent detrimental influences in the cooling tower and on blowdown quality. Well recognized is that microbiological fouling is the issue of a greatest concern in a cooling water system. A makeup stream containing phosphorus, ammonia, and organic compounds is the perfect “bug food.” The three types of microorganisms that most affect cooling towers and cooling water systems are algae, fungi, and bacteria. Algae require sunlight to grow, and these organisms most commonly foul cooling tower spray decks and other locations exposed to the sun, as the figure from a top expert below illustrates. The growth then can degrade performance and cause unsafe working conditions.
Fungi proliferate within cooling tower internals that remain moist. The organisms are particularly troublesome in wooden cooling towers, as they can cause “rot” of wood components, which in turn greatly weakens the structure.
Bacteria are separated into the following three categories,
- Aerobic: Utilize oxygen in the metabolic process.
- Anaerobic: Live in oxygen-free environments and use other sources, i.e., sulfates,
- nitrates, or other donors for their energy supply.
- Facultative: Can live in aerobic or anaerobic environments.
A problem with microbes, particularly many bacteria, is that once they settle on a surface the organisms secrete a polysaccharide layer (slime) for protection. The slime layer is an excellent insulator, and even a small film will significantly inhibit heat transfer. The slime will also gather silt from the water, growing even thicker and further reducing heat transfer. And, even though the bacteria at the surface of a colony may be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. These bugs in turn can generate acids and other harmful compounds that directly attack the condenser and heat exchanger tubes. Also, microbial deposits establish concentration cells, where the lack of oxygen underneath the deposits causes the locations to become anodic to other areas of exposed metal. Pitting is often a result. So, poor control of microbiological fouling can lead to conditions that will cause:
- Serious efficiency loss in condensers and auxiliary heat exchangers
- Condenser tube corrosion and failure
- Restricted flow and severe loss of heat transfer in the cooling tower
- Partial cooling tower collapse due to increased weight of structural components
Established microbiological colonies induce growth of higher organisms, which in turn can lead to growth of legionella bacteria. For those unfamiliar with legionella, these organisms can cause illness (and sometimes death in weak individuals) if inhaled.
So, feed of an oxidizing biocide is still very important, as it has always been. But makeup containing significant concentrations of ammonia and organics will quickly consume the standard treatment chemical, chlorine. While bromine chemistry has been successfully used in many applications, an increasingly popular alternative is chlorine dioxide (ClO2). ClO2, which must be generated on-site, is a powerful oxidizer that does not react with ammonia or form halogenated organics. In the past, the most common technique for ClO2 generation was reaction of sodium chlorite (NaClO2) with either chlorine gas or bleach in an educator or other blend system. This required storage of large quantities of hazardous materials and was rather more expensive than either chlorine or even bromine chemistry. Now, modular systems that utilize sodium chlorate (NaClO3), activated by acid and hydrogen peroxide (H2O2) to produce ClO2, are available, and are much more safe and efficient than older processes.
Of additional benefit is use of bio-penetrant in any chemical formulation. As has been previously noted, some microbes secrete a protective layer to shield the organisms from poisons. This layer is very effective at deactivating oxidizing biocides. A bio-penetrant or bio-dispersant as it is often also called, acts as its name applies and helps the biocide to penetrate the slime layer and attack the organisms underneath.
Another important point to consider regarding biocide treatment is feed duration. A typical requirement for once-through cooling systems is a two-hour feed per unit per day. This regulation was designed to protect aquatic creatures at the cooling water discharge. However, this means that each condenser and any auxiliary heat exchangers go untreated for 22 hours per day. If microbes settle and form a slime layer, they are then protected from the next dose of biocide. Many cooling towers have been placed under the same restrictions,  but one has to ask why? Any oxidizing biocide residual in the blowdown whether it be chlorine, bromine, or chlorine dioxide, can be neutralized with a reducing agent such as sodium bisulfite (NaHSO3). Therefore, it seems very worthwhile to negotiate this point with environmental regulators to allow more frequent or perhaps even continuous biocide feed to the tower, provided the blowdown is treated to remove residual oxidant. The cost of biocide chemicals can be re-paid many times over in reliable tower performance and heat transfer. Also, continuous biocide feed, particularly at certain times of the year, can control infestations of macro-fouling organisms, including zebra mussels and Asiatic clams.
Brad Buecker is a Process Specialist in the Environmental Services group with Kiewit Power Engineers, Lenexa, Kansas.
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