This article examines combined-cycle plants and their attendant heat recovery steam generators (HRSGs), where increasingly stringent wastewater regulations are forcing plant personnel to consider complex treatment methods to ensure that liquid discharge complies with regulatory guidelines.
In some cases, zero liquid discharge (ZLD) may be the only option. These issues in turn are influencing other water treatment processes, most notably cooling water treatment. At plants with cooling towers, personnel may need to look beyond what have been the mainstream treatment programs. In part, increased wastewater treatment complexity has influenced some developers to select air-cooled condensers in place of the steam surface type. Finally, by choice or often mandate, many plant designers are selecting less-than-pristine water supplies, such as treated municipal wastewater, for makeup. These supplies often contain unwelcome contaminants, including ammonia, phosphorus, organics, and suspended solids, which in turn can affect downstream processes including cooling and wastewater treatment.
While this article primarily spotlights combined-cycle power issues, a trip back in time helps to illustrate how plant discharge regulations are evolving.
When I began my utility career at City Water, Light & Power (CWLP) in the early 1980s, the plant's National Pollutant Discharge Elimination System (NPDES) guidelines consisted of the following.
The streams to which these limits applied were cooling water discharge (many plants including CWLP had once-through systems), coal pile runoff, and ash pond discharge. These were indeed simple times, but now additional constituents are appearing in coal plant effluent limitation guidelines (ELG), and especially at those plants with wet flue gas desulfurization (FGD).
But what about combined-cycle power plants, which obviously burn a much cleaner fuel and do not exhibit the environmental complexity of coal plants? First, consider changes with regard to turbine exhaust steam cooling. Whereas many large plants in the last century were equipped with once-through cooling, this is not the case for new plants. 316a (thermal discharge) and 316b (impingement and entrainment of aquatic creatures at plant intakes) regulations have generated a paradigm shift from once-through cooling to so-called closed systems. Cooling towers, and to a lesser but growing extent air-cooled condensers, have become the popular choices now. At the many plants with cooling towers, cooling tower blowdown typically represents the largest wastewater stream, but the discharge often includes some or all of the following - RO reject, evaporative cooler blowdown, quenched boiler blowdown, and plant drains.
One might be tempted to think that these seemingly benign streams would not be much more heavily regulated than the example of Table 1. Such may not be the case. First on a national level, the EPA has proposed limits of 0.2 ppm for chromium and 1.0 ppm for zinc in cooling tower blowdown per the pending ELG. This is far from the end of the story, however. Individual states are allowed to develop their own discharge guidelines, as long as the regulations are as stringent as those of the EPA. In some cases, states are promulgating tight regulations that place limits on some or all of the following additional constituents in cooling tower blowdown:
As an example of state influence, consider the new guidelines at a combined-cycle power plant with a cooling tower in one of our southern states. Prior to 2013, the plant's NPDES permit was similar to that shown in Table 1. The new discharge permit now imposes an average monthly limit of 1,200 mg/l TDS. Given that the makeup water TDS concentration sometimes reaches 400 mg/l TDS, the tower cycles of concentration (COC) may be limited to three under the new regulations, whereas previously the tower had operated at a significantly higher COC. As Figure 1 indicates, this can have significant consequences on blowdown volume.
Another impurity receiving more scrutiny is sulfate (SO4). This issue can be problematic with regard to process chemistry, as sulfuric acid feed to cooling tower makeup has been a common method to remove bicarbonate alkalinity and thus minimize calcium carbonate (CaCO3) scaling potential in the condenser and cooling system.
Tighter regulations on sulfate in the discharge stream may curtail or eliminate this straightforward method of scale control at some plants. On a related note, I have heard rumblings that chloride and bromide may be added to future regulatory lists. Limits on chloride could have the same effect as those on TDS.
As has been noted, zinc and chromium limits have been placed in the proposed national ELG. State regulations may be more restrictive. For the plant mentioned above, the expectations are that copper discharge will, in a few years, be limited to less than 30 parts-per-billion (ppb). Even tighter restrictions have already been placed on copper discharge in the Great Lakes area. At these very low limits, copper discharge can potentially be a problem from units equipped with copper-alloy condenser tubes. However, another copper source, from older wooden cooling towers, comes from copper compounds utilized as a wood preservative. These preservatives may also contain arsenic and chromium. Although wood preservative treatment methods were designed to minimize leaching of the preservatives, some accumulation of impurities may occur in the cooling water, particularly if the tower sits idle for extended periods.
Phosphorus is another constituent under scrutiny. Many bodies of water in the U.S. have been designated "phosphorus impaired," as phosphorus serves as a nutrient for plant growth and when released to open bodies of water can often initiate and propagate algae blooms. In like manner ammonia is an impurity of concern. Large ammonia releases have been known to cause massive fish kills, but in the quantities that might exist in plant discharge, will serve as a nutrient in a receiving body of water.
We will continue to explore water discharge issues during and following discussion of cooling and makeup water treatment, as control methods for impurities may need to be implemented throughout the process.
Cooling system operation is vital for plant performance and reliability. At a plant with a reclaim water makeup supply, the nutrients for explosive microbiological growth, organics, phosphorus, and ammonia, can be directly introduced to the cooling water system. But, wastewater discharge issues may also significantly affect cooling water treatment.
Adding to the complexity is that the most common cooling tower treatment program for over three decades has been based upon inorganic and organic phosphate chemistry for both scale and corrosion protection. These programs are typically supplemented with an accompanying polymer for calcium phosphate scale control and perhaps a small zinc residual for additional corrosion protection. So, at plants impacted by phosphorus discharge regulations, an alternative non-phosphorus program may be necessary.
The major water treatment companies are diligently developing non-P technologies, which are based on polymer chemistry. Co- and ter-polymers containing the active groups shown below are the outcome of these efforts.
The polymers serve as crystal modifiers and sequestering agents to inhibit scale formation. There is also evidence that the polymers form a thin coating on metal surfaces to inhibit corrosion. A common dosage concentration is 2 to 10 ppm active in the cooling water. In some cases, an all-P program may be less expensive than an equivalent phosphate/phosphonate program, although every potential application must be carefully evaluated.
At this point the reader may be wondering about those cooling systems, whose number is increasing, that take municipal wastewater plant discharge as makeup. Without any treatment, the impurities in this makeup will enormously increase the potential for microbiological fouling.
The extra phosphorus can negatively impact towers that are still on phosphate/phosphonate programs, and can be problematic for those on non-phosphorus programs, too. Also, if phosphorus, ammonia, and/or TSS are limited in the plant's NPDES guidelines, a switch to reclaim water may cause immediate difficulties in this regard.
It is now time to examine the most popular conventional and emerging treatment technologies for makeup water, cooling water, and wastewater to provide some guidance for plant personnel faced with process and regulatory issues.
At plants with or planning to accept reclaim water as makeup, we have seen that a number of impurities may be in this water, which would be negligible in fresh water supplies. Such contaminants as phosphorus and TSS can be readily removed by clarification with an iron or aluminum-based coagulant feed, but this process may be lacking for treatment of other contaminants like ammonia. A technology that is becoming increasingly popular for reclaim water treatment is biological processing of the plant intake. Bioreactors and membrane bioreactors are two technologies in this regard.
Via microbes that are seeded and allowed to grow on internal devices within the reactors, the incoming organics and nutrients are consumed. The final step which may be external or internal is filtration to minimize TSS discharge. With membrane bioreactors, where the membranes are of the micro- or ultrafilter variety, effluent turbidities may be below 0.1 NTU (nephelometric turbidity units). This is quite satisfactory for general plant makeup, and even is suitable for feed to reverse osmosis units that produce high-purity makeup for the steam generator.
At any plant with a cooling tower, and especially if extra nutrients arrive with the makeup water, microbiological control is of utmost importance. Common for many towers in the past has been bleach feed, as bleach is safer than gaseous chlorine. However, ammonia and organics in incoming makeup will consume chlorine, potentially destroying the residual that is needed to keep cooling systems clean. Also, the chemistry may form halogenated organics, which are also unwelcome. Alternative treatments may be needed. One such possibility is chlorine dioxide (ClO2), where generation methods have been greatly improved from the former sodium chlorite (NaClO2)-chlorine reaction, and in which large quantities of hazardous sodium chlorite had to be stored on site. One of the new processes utilizes a compact generator that combines sodium chlorate (NaClO3) with a pre-mixed blend of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) to induce the following reaction.
Chlorine dioxide does not react with ammonia, nor does it react with organics to form halogenated organic compounds. Also, and unlike hypochlorite, chlorine dioxide is not affected by pH. This can be an important advantage in those towers (the majority) whose chemistry programs operate in an alkaline pH range.
Some plant personnel have had good success with on-line hypochlorite-generating systems such as the MIOX process, which via brine electrolysis produces a hypochlorite solution that also contains residual hydrogen peroxide. An advantage of this technology is that the oxidant is produced on an as-needed basis rather than being stored in large tanks where it can degrade and lose strength.
Another technology that is highly recommended for cooling towers is sidestream filtration. Very often I see requests-for-proposals (RFP) that call for makeup water filtration. Developers and owner's engineers often do not recognize the fact that cooling towers are superb air scrubbers, and that many particulates are introduced to cooling water by the scrubbing action. Makeup filtration does nothing to control these particulates. A wide variety of technologies is available for sidestream filtration, ranging from conventional multi-media filters to automatic backwash systems with metal screens.
At the most recent International Water Conference, more than one presenter seemed to somewhat lightly suggest that if plant personnel are worried about cooling tower issues, air-cooled condensers (ACC) are an easy cure.
Yes, ACCs may be the only choice where water is scarce, but a number of factors must be considered before choosing ACC over a cooling tower. These factors include:
Depending upon the method of wastewater treatment allowed or available, this issue can range from straightforward to exceedingly complex. Some plants are permitted to discharge spent water to a local municipal wastewater treatment plant, provided the industrial water does not contain excessive concentrations of harmful impurities such as heavy metals. At plants in arid regions of the country, evaporation ponds may serve the purpose. However, these ponds must be permitted and installed in a proper manner. Lined ponds are de rigueur in today's strident environmental climate.
If none of the above options are available, mechanical-thermal evaporation of the waste stream may be the only choice. Accurate determination of influent water chemistry is vital for design and selection of such systems, as hardness, alkalinity, and silica can cause severe scaling problems in evaporator/crystallizers. In this regard, at one of our members' plants, the cooling tower blowdown is first treated in a softening clarifier to reduce hardness. This stream is then processed in a brine concentrator/crystallizer. The primary constituent of the solids produced by the system is sodium sulfate, which forms messy deposits but not hard scale. For other crystallizers, crystal seeding is employed. A common seed crystal is gypsum, which provides a more thermodynamically stable site for precipitation of such minerals as calcium sulfate and silicates.
Energy requirements for conventional evaporators are quite large. In large measure this is due to the fact that during evaporation the dissolved solids concentration increases, which then increases the boiling point. The boiling point rise requires additional energy. A modification to the process that has been successfully applied in the salt production industry is evaporation-crystallization with a mechanical vacuum applied to the evaporation chamber. This greatly lowers the temperature at which the liquid boils, and helps to overcome the boiling point rise that occurs in conventional units as the dissolved solids concentration increases. Considerable energy savings appear possible with the vacuum systems.
Becoming popular are treatment methods to reduce the volume of the plant waste stream before final treatment. Most notable is high-recovery reverse osmosis.
This schematic outlines the HERO process, which is licensed by such firms as Aquatech, GE, and U.S. Water, while Veolia supplies their Opus technology, which may also include high-rate softening/clarification as a unit operation. Keys to the process are:
Under proper conditions, the RO recovery rate 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 HERO system operating at a power plant near the Pacific Northwest. One of the most notable examples for any of these systems is that some standard water treatment chemicals may foul the UF membranes. Operating experience indicates that the membrane manufacturer and type greatly influence this phenomenon. Fouling is induced because membranes typically carry a negative surface charge while often cationic polymers are employed for coagulation or flocculation. Residual cationic polymer is strongly attracted to 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 each 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 particular case, conversion of the membranes from inside-out to outside-in normal flow path greatly improved the backwash efficiency.
Another interesting initial difficulty was noted with the 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. So far, so good. 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 UF was first commissioned and CEB backwashes commenced, the membranes developed a layer of calcium silicate 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.
This article hopefully illustrated many of the threads between the water treatment processes at new power plants. Complex scenarios may arise due to water chemistry issues and discharge regulations. Processes must be viewed and designed in a holistic manner and not piecemeal. Accurate and historical water quality data is an absolute must for designing reliable systems.
Brad Buecker serves as a process specialist in the Process Engineering and Permitting group of Kiewit Engineering and Design Company.
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