Wastewater Treatment Issues for

While significant focus has been on coal-fired plants and discharges from wet scrubbers and coal ash ponds, combined-cycle units must also deal with increasingly stringent effluent regulations.

By Brad Buecker, Contributing Editor

At POWER-GEN International 2013 and a follow-up water treatment seminar hosted by Veolia, much information was offered regarding the status of the U.S. Environmental Protection Agency's Effluent Limitation Guidelines (ELGs) for liquid discharges from power plants.

While significant focus has been on coal-fired plants and discharges from wet scrubbers and coal ash ponds, personnel at natural-gas fired plants, particularly combined-cycle units, must also deal with stringent effluent regulations.

Compounding these difficulties, some plants are being required to use reclaimed water or other less-than-pristine sources for makeup. Meeting effluent guidelines is made more difficult when the influent is poor quality.

Throughout the latter decades of the 20th Century, power plant chemists and technical personnel primarily had to deal with the following parameters in various plant discharges:

Oil and grease (O&G)
pH
Total Residual Oxidant (chlorine, bromine, or chlorine dioxide in the cooling water discharge)
Total suspended solids (TSS)

Relatively straightforward chemical treatment and/or operational methods were sufficient to keep this chemistry within guidelines.

Now, the story is radically changing, even for discharge streams not influenced by coal combustion.

New Steam-Generated Power Production

Improved access to large reserves of shale gas coupled with what some term the EPA's "war on coal" have caused a tremendous growth in simple- and especially combined-cycle power plants for new electricity generation. Few water treatment systems at new combined-cycle projects call for once-through cooling, no doubt due to pending 316a and 316b regulations. Rather, all new proposals specify cooling towers or in some cases, air-cooled condensers. In the former situation, cooling tower blowdown typically constitutes by far the majority of spent water discharged from the plant.

The EPA has addressed cooling tower blowdown in their proposed new Effluent Limitations Guidelines, with zinc and chromium being two impurities of focus. The proposed limits are 1 part-per million (ppm) and 0.2 ppm, respectively. However, state regulators may impose limits on other cooling water discharge constituents, and regulations are now emerging that include limits on the following constituents:

Total dissolved solids
Sulfates
Zinc
Copper
Chromium
Phosphate
Ammonia
Quantity of discharge

In addition, chloride and bromide may be being examined. At the same time, regulators are pushing some plants to use alternatives to fresh water for makeup. These sources include reclaim water from municipal wastewater treatment plants and low-purity groundwater. California is a notable example regarding reclaim water use, per their Title 22 regulations.

Reasoning Behind New Guidelines

The logic behind the selection of the "original four" impurities, pH, O&G, TSS, and residual oxidants is easily understandable. Suspended solids such as coal and ash particles can fill receiving bodies of water with material. Excursions in pH, where a common regulated range has been 6.0 to 9.0, can be fatal to aquatic creatures, as can release of excessive oxidants. Oil and grease, and similar carbon-based materials, if released in excess will increase the biochemical oxygen demand (BOD) of waters, which in turn can reduce oxygen concentration and harm aquatic creatures.

Researchers now conclude that other impurities, including those just outlined, are also harmful. Many heavy metals are toxic to aquatic organisms, and in the modern power industry two of the most common that might be released in a plant discharge are zinc and copper. For many years, a small dosage of zinc served as an integral part of phosphate/phosphonate based cooling tower treatment programs. Zinc reacts with hydroxyl ions (OH-) produced at cathodic corrosion sites to generate a zinc hydroxide [Zn(OH)2] film that helps to inhibit reactions at cathodes. Tight restrictions are also being placed on copper in some locations, with discharge limits reportedly as low as 12 parts-per-billion (ppb). For new plants, copper is often not a problem as system components are almost entirely fabricated of steel. However, I have come across facilities that have cooling towers fabricated of wood, in which the wood was treated with a copper preservative. Copper leaches from the wood, and may concentrate above discharge guidelines during unit outages. Chromium is an alloy component of some steels, and thus may appear in small concentration in steam generator blowdown. Normally, chromium is of greater concern when it appears in chemical cleaning spent solutions, where much of it may be in the hexavalent (Cr+6) form, the toxic form of chromium.

With respect to the common cooling tower phosphate/phosphonate programs of the past, and still in many cases the present, concern continues to grow regarding the impact of phosphate as a nutrient in receiving bodies of water. Algae blooms have caused enormous problems at times. Zero phosphate residual is appearing in some new NPDES permits. In the same vein, ammonia discharge is coming under tighter control. Ammonia in small concentrations serves as a nutrient for aquatic plant growth, while on the other hand, accidental releases of large quantities have been known to cause substantial fish kills.

Limits are also appearing for dissolved solids and salt content of wastewater. At one plant I recently visited, the previous NPDES permit only called for monitoring of TDS. The new permit limits TDS in the wastewater discharge to slightly over 1,000 ppm and sulfates to just a few hundred ppm. This has major implications for cooling tower makeup and cycles of concentration, as we will examine shortly.

This list of "new" impurities is only expected to grow in upcoming years. For example, at coal plants with wet scrubbers, discharge of contaminants, including arsenic, mercury, selenium and boron, is also being regulated. While these impurities are not common to the combined-cycle industry, expect to see other metals show up in future discharge regulations.

Wastewater Impurities

When fresh water supplies were more plentiful, many plants took makeup water from a lake, reservoir, or river. Now, many plants are being required to use less pristine water, such as treated municipal waste or possibly high-TDS groundwater. These makeup stream chemistries can have enormous impact upon cooling tower operation and wastewater discharge. For example, I recently observed data from a proposed plant in which a snapshot analysis of the planned tertiary-treated makeup indicated 5 ppm of phosphate and 14 ppm of ammonia! In other cases, the phosphate concentration may reach 20 ppm.

Another issue of potential concern regards TDS and sulfate restrictions. A common makeup water treatment process for many years in cooling tower applications has been sulfuric acid feed to remove bicarbonate alkalinity (HCO3-), which in turn reduces the potential for calcium carbonate (CaCO3) scaling.

H2SO4 + HCO3- → HSO4- + H₂O + CO₂↑

By virtue of the stoichiometry and molecular weights, sulfuric acid and alkalinity react on a 1:1 weight basis. So, consider a cooling tower where 150 ppm of alkalinity needs to be removed, but where the limit for sulfate discharge is 400 ppm. The tower would have to operate at less than three cycles of concentration (COC) if sulfuric acid were utilized for bicarbonate removal. Consider also if the makeup was a well water with say 400 ppm TDS, but with a TDS discharge limit of 1,200 ppm. Again, the maximum COC would be limited to three.

These scenarios are potentially of great concern where the quantity of cooling tower blowdown is limited. Figure 1 outlines the discharge quantities for cooling system with very reasonable values for evaporation and circulating water flow.

As is evident, at three COC the blowdown rate is 600 gpm, but if the tower could be operated at six COC, the blowdown rate drops to approximately 250 gpm.

Dealing with All These Issues

So, how do personnel deal with these issues, either at existing plants or at facilities in the planning stage, and where air-cooled condensers are not an option? One noted expert feels that new systems should be designed with zero liquid discharge (ZLD) as the goal, with the thought that NPDES regulations will only become more stringent, so the design should meet all future contingencies. We will return to ZLD thoughts shortly.

For existing plants, process and/or equipment modifications are possible to address some of these issues. The following list outlines a number of these possibilities.

Alternative programs based on polymer feed have been developed for cooling tower treatment. Polymers are not inexpensive, but they can eliminate phosphate discharge.
Cooling tower polymer programs can also be designed to control calcium carbonate scaling, which would reduce or in some cases eliminate the need for sulfuric acid feed to reduce bicarbonate alkalinity.
Although a seemingly extreme measure at times, makeup water clarification and lime softening have been utilized to remove hardness-forming constituents from cooling tower makeup. This process can be practical for high-hardness groundwaters that may also have a high silica concentration, or for makeup that contains phosphate. The problem with full-flow makeup water treatment of any kind is that most of the water is lost to evaporation in the cooling tower. Thus, the makeup water flow rate is always much greater than is evidenced in the blowdown alone.
If ammonia discharge is of concern, and the makeup water contains significant quantities of ammonia, it may be necessary to treat the wastewater with chlorine to convert the ammonia to elemental nitrogen. Residual chlorine must then be removed by feed of a reducing agent such as sodium bisulfite to the discharge. An ammonia removal system could be rather complicated given the variable nature of the impurity. Another alternative, although not common at present for power plants, is ammonia stripping of the makeup water stream.
If the discharge quality does not have tight restrictions but the quantity is of concern, then per Figure 1 the COC can be increased to reduce blowdown.
Conversely, if the quality is restricted but quantity is not, reducing COC may put the wastewater discharge within guidelines. An important factor in this scenario is a potential restriction on makeup water quantity. If this volume is restricted, lower COCs may not be an option.

ZLD to the Rescue?

Now, let us step into a bit of discussion regarding ZLD. This process is often rather complex. Perhaps most straightforward, but with a large caveat, is the question, "Can the discharge be permitted for deep well injection without any quantity limits?" Such wells are often several thousand feet deep to avoid any discharge into shallow groundwaters used for domestic purposes. While this concept sounds simple, experience has shown that some wastewaters can generate scale within the well casings, particularly as the water warms further underground. High-pressure is generally required for this process, and if scale formation occurs capacity may decrease.

At plants in arid locations with a large land area, evaporation ponds may be sufficient to handle the wastewater discharge. However, these ponds have to be lined to prevent seepage of the wastewater with its impurities into the underlying soil and possibly shallow aquifers.

Another alternative, at sites strategically located, is to have the wastewater trucked off-site to a waste disposal company. Transportation and disposal costs for this method may be rather expensive.

If none of the above options are available, thermal evaporation of the waste stream may be the only option. Due to the energy requirements of evaporators, or to reduce costs for the other processes mentioned above, it can be very beneficial to recover most of the waste stream for reuse and reduce the discharge quantity. A rapidly emerging technology to do so is outlined in Figure 2.

One version of this process is licensed for various markets as HERO by such firms as Aquatech, GE, and U.S. Water, while Veolia supplies their Opus technologies. Keys to the process are:

Micro- or ultrafiltration (UF) to remove suspended solids in the waste stream.
Sodium bisulfite (NaHSO₃) feed to remove residual oxidizing biocides.
A sodium softener to remove calcium and magnesium.
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 to recover 90 percent of the water.

While the process appears straightforward, I can report on a number of direct lessons learned with the technology in actual application. These include,

Use of cationic polymers in the cooling water or for pre-treatment to the system can be quite deleterious. UF/MF and RO membranes often have a negative surface charge. The polymers will attach like glue, making the membranes unusable.
Subsequent to discontinuation of the polymer feed, the UF membranes were changed from an inside-out normal flow path to outside-in. This greatly improved the ability of the regular backwash sequences to keep the membranes clean.
A cooling tower of course "cycles up" the makeup stream and increases both suspended and dissolved solids concentrations. In this particular application, as with most modern systems, periodically the backwash sequence includes a chemically-enhanced backwash, in which the membranes are scrubbed with an alkaline-bleach solution to remove organics and microbes, followed by an acid stage to remove iron particulates. In this case, the alkaline CEB induced calcium silicate scaling on the membranes. The remedy has been a change to softened water for CEBs. This mechanism appears to be a difficulty at another facility where the membranes suffer from magnesium silicate fouling.

These issues aside, it is obvious that this technology can greatly reduce the volume of wastewater that must then be disposed to reach complete ZLD. Energy costs for a thermal evaporator or land area for evaporation ponds are much lower with a 90 percent reduction in wastewater volume prior to final treatment.

A Reclaim Water Scenario

Very recently, the author became involved in a project where reclaim water will be used as the makeup to a power plant cooling tower, but where the entire blowdown will be recycled to the wastewater treatment plant. Another plant in the area is already doing so.

At first glance, the natural inclination is to believe that complete blowdown recovery would cause a continual increase in solids within the system. However, if one examines this schematic using a control volume diagram, the issue becomes more clear.

The recycle of blowdown is an internal process, and after startup of the system as the blowdown solids concentration increases, at a certain point the amount of solids exiting the process balances the amount entering. The author has prepared an iterative program in Excel that demonstrates this calculation. It should be noted that a blowdown treatment system, perhaps a lime softener, may be needed to prevent excess accumulation of scale-forming solids in the WWTP. Such a system, with equipment to generate a solid sludge, can establish ZLD conditions.

Author

Brad Buecker is a process specialist with Kiewit Power Engineers in Lenexa, Kan., and a contributing editor for Power Engineering.