Water management at power plants The goal is zero pollution discharge, but
Gerald Westbrook and Louis Wirth, Jr.
TDS is a big problem. Here is one way to handle it, and save some liquid cash
Dow Chemical U.S.A.
Like it or not, the zero discharge concept is becoming a way of life in the steam electric industry. Now zero discharge does not necessarily mean an operating entity cannot produce a waste. But it may mean that a production facility must digest or manage its wastes within the property limits of the facility. With the zero discharge concept, sharp limits are placed on wastes of all kinds. Regulatory bodies, and the effects of their regulations, are felt in every phase of design, construction, and operation of new facilities. They will ultimately impact heavily on the eventual fate of existing facilities. The result of all this will be higher costs to the consumer. One very important facet of waste management is that of total dissolved solids (TDS). The many thousands of gallons of water, containing varying amounts of salt, which are generated by washing and regenerating processes, such as blowdown from cooling towers, sluicing of salt-bearing substances, to name a few, present significant problems when zero discharge is mandated. The problem is one of too much water carrying too little salt, but still having a TDS content too great for discharge to a receiving stream. The electric power industry faces this problem today, especially in the water-short, high-TDS areas of the western U.S. The same problem will eventually extend to the midwest and other areas. Several processes for concentrating salts or desalting water can address this problem. For example, a relatively new system of brine concentration, using electrodialysis (ED), can reduce the magnitude of the brine disposal problem. ED brine concentration has significant cost advantages when compared to other means of attacking this problem. Also, cooling water salinity, properly analyzed and optimized, will provide for efficient management of the salt disposal problem, while maintaining best possible efficiency of the power generation system under zero discharge conditions.
Of primary importance is the impact of high-sodium content condenser water leakage at high TDS levels on ion-exchange condensate polisher operation, regeneration schedules, and quality of condensate that can be effectively maintained. In addition to problems that these operating abnormalities cause for turbine and once-through steam generators, additional wastes are generated because of more frequent regeneration of the ion-exchange polishers. As mentioned previously, the side stream softening process is a very effective method of reducing scaling substances such as calcium and silica, thus permitting operation at high cycles of concentration. The one major drawback to this approach, however, it the very high cooling-water salinities that result, and the associated problems that operating at such salinities brings. Nevertheless, the increase in cooling-water salinity that results with use of the side stream softening process can be controlled by adding an appropriate desalting unit or water recoverylbrine concentration system. At the same time, valuable water will be recovered, and waste brine would be reduced. The most important aspect of incorporating a side stream desalting process into a cooling tower circuit is the process’ ability to allow the salinity of this water to become an independent variable. A key question then becomes: “What is the optimum salinity of this recirculating cooling water?” An answer to that question, which considers capital requirements, would be in order. Although higher cooling-circuit salinities will lead to significantly higher capital and operating costs for several key steam and power systems, the opposite is true for water recovery/brine concentration (WR/BC) systems. The reason is that in zero discharge plants, the higher the initial feed concentration to the WR/BC system, the lower the ultimate volume of water that must be finally evaporated, treated, or processed. Thus, with an evaporative cooling tower in the system, it is desirable to use this evaporator to preconcentrate the cooling system’s recirculating water to a sufficiently high level to make economic sense. The general nature of the system proposed is illustrated in Figure 1. The total system would consist of: chemical softening filtration water recovery/brine concentration desalting brine disposal
Side stream processing Treating cooling tower blowdown for zero discharge requirements is expensive; hence, reducing the volume of blowdown should become a primary objective. One way to accomplish this is through the concept of side stream processing. For example, side stream treatment by lime and soda ash is an effective means of reducing scaling substances, so cooling towers can operate at higher cycles of Concentration. However, high levels of TDS in the cooling water will result. This can add to capital and operating cost requirements for several of the basic steam and power generation systems. Systems that will be affected as cooling water salinity increases include: cooling water pumping system cooling towers, fans, and drift eliminators condensers condensate polisher steam boiler turbine
Midland, MI 48640
FIGURE 1
Side stream process
,
-
Condensate
softening
I
Sludge
disposal 140
D
Filtration
Desalting
Environmental Science & Technology Brine disposal
FIGURE 2
Alternative water reuse/brine concentration processes
Key R / C = Reactoriclarifier F =Filter E = Evaporation loss D = Drift loss R O = Reverse osmosis ED = Electrodialysis VC = Vapor compression evaporation
Evaporation pond 1215 gpm
4
Make-up treatment
820 acres E
-
softening
F
RIC
AL
20 290 gpm
-4500 gpm
500 ppm
230 gpm
153 acres E
20000
Case 3.1, and 3.3 RO-based process
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5460.
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20 745; 20 700 gprn
RIC
2000,
625, '5,580gpm
-
8000 ppm
5460; ( 1270 gpm
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ED-based process
430, 1 '' 400 acres I
AL
4750; 4700 gpm 115,
50gpm
83, 40acres 8, 16%
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Three types of desalting processes are considered for the WRIBC system: reverse osmosis (RO), electrodialysis (ED), and vapor compression evaporation (VC). These three processes would operate on a side stream from the main cooling circuit. Chemical softening Almost all schemes for zero discharge include the use of side stream processing with warm lime or lime-sodatreatment. With either RO or ED it is first necessary to treat the blowdown water by lime soda or other suitable treatment. This treatment will provide for calcium, silica, and suspended solids control in the water returned to the circulating water. In fact in many cases it will eliminate the need for lime-soda treatment of the cooling tower makeup water. A much smaller stream of more concentrated, warm water taken from the circulating water return line will be softened by a high-density sludge unit. Side stream filtration by itself can be of major utility on cooling tower circuits. At an Electric Power Research Institute (EPRI) seminar, D. E. Noll said, “Dirt is the greatest enemy of recirculating cooling systems . . . the presence of dirt makes corrosion control very difficult.” The same can undoubtedly be said about scale control. And at the 34th International Water Conference (October 1973), G. J. Crits noted, “As the cycles increase above five concentrations, the dirt injected by the air becomes an important limitation.” Data from his work show that although 80% of these solids would settle out in the tower basin, additional removal would be desirable. Side stream desalting For side stream desalting, here is what RO-based process can do: As TQS mounts, RO gradually becomes a less effective brine concentrator in the sense of its ability to concentrate the recirculating water further. With today’s RO technology, the maximum feed water salinity likely is below 8000 ppm, and maximum brine concentration would be roughly double that level. As RO technology is developed for desalting seawater, however, its range of application should increase. FIGURE 3
Preliminary estimate-optimum Capital (millionsof $1 30
salinity
Recirculatingcooling water 2000-MW plant Case 1
Evaporation pond
25
20
Case 3 RO-based process
Case 5 VC evaporation
15
10
5 Incremental capitalbasic steam & power systems
0
10 000
20 000
Recirculatingwater salinity ppm
142
Environmental Science & Technology
31
IO
Cooling tower operation side stream desalting Water
t
Evaporation
Cooling tower
I With the ED-based process, the pretreated blowdown water can be concentrated 10 times or more within the ED system; desalted water is recovered and recycled back to the tower at one-half the original TDS concentration. To accomplish this, a volume of water is pretreated, filtered and passed through the ED system. Because of recirculation, it is possible to concentrate the salts into a small stream, while allowing for recovery back to the tower of about 9 0 % of the water removed at one-half concentration of TDS. In the case of the VC evaporation-basedprocess, an evaporator is the most expensive WRIBC system to buy and operate, but is a highly effective means of handling the problem of brine concentration. In large installations, one evaporator of selected size may accomplish much of the work normally done by the makeup demineralizer. At the same time, the salts present in cooling tower blowdown are concentrated to high levels. But beyond this single unit, other means of concentratingbrine would be more economical. Detailed design studies and cost estimates for several alternative water reuselbrine concentration processes have also been completed. Five different technologies have been studied, as indicated in Figure 2. The side stream softeners are sized to provide the necessary calcium and silica bleed. The desalters are sized to provide the necessary salt bleed for the salinities indicated. The estimated capital costs are plotted in Figure 3 as a function of recirculating water salinity. The side stream process design utilizing electrodialysisresults in the lowest capital costs for the conditions specified. The cost of solar evaporation ponds, where they can be used, is the determining factor. While of necessity of a preliminary nature, and clearly not incorporating operating costs, the optimization presented in Figure 3 is believed to be of sufficient accuracy to indicatethe optimum salinity range to be 10 000-16 000 ppm. Water management considerations With regard to management of wastewater from zero discharge plants, it is a natural and historical practice to utilize the inherent evaporator capability of the cooling tower to take the cooling waters to as high a salinity as can be achieved, in order to minimize the costs of any further concentration required. However, as the salinity of the cooling water increases, the capital, operating, and maintenance costs for several key steam-electric plant subsystems also increase. While the individual increases are not big, in total, they can add up to a substantial amount. Hence, an optimum level for cooling water salinity exists. This
ROlED high-recovery plant, Roswell, New Mexico Ion exchange (1x1 regenerant Partially desalted recycle
6
1
8
2
5
3
IX
I
ED
RO
Brine
Raw feed Product water
9
Recovery-91.7% overall
Service water
Operating data 1
3
4
5
36.3 2060 102 35 2310 1500 98
36.3 2206 22 7 2310 1500 98
14.5 184 2 248 51 6
21.8 3550 35 10 3680 2465 160
21.4 2690 25 10 2580 2095 160
0.4 49 500 600 151 62 500 22 200 40
0.24 49 500 600 151 62 500 22 200 40
6105
6143
491
9900
7560
134 991
134991
Flow
14.9 1035 Na 217 Ca 63 Mg +t 1560 CI so,-- 548 HCOs- 217
mgi L +
+ +
TDS
rngi L 3640
7
2
-
optimum will vary between types of plants, plant configuration, feed water salinity, and ultimate brine disposal method. The results from an initial evaluation of one situation indicates the optimum salinity region to be in the range of 10 000-16 000 PPm. The process combining side stream softening plus electrodialysis had the lowest overall capital requirements for the cases studied. If the comparison were made on overall annual operating costs, it is expected that the same results would occur. In addition to lowest capital requirements, this design has attractive process features including: a smaller, more stable, and more easily operated chemical softening unit, with the use of warm, concentrated, circulating water rather than cold raw makeup water far better suspended solids removal, not only for precipitated solids but also for solids introduced to the system from the air less severe corrosion environment, owing to salinity and suspended solids reduction removal of the maximum point of gypsum scaling tendency from the condenser to the brine side of the electrodialysis unit reduction in capital, operating, and maintenance expenses for several key steam and power plant subsystems that would be adversely affected by operation at high salt levels; pounds/day of salt carried out in the drift would also be reduced. While unlikely to pay for the total capital outlay for the WR/BC system, these cost reductions could recover a significant percentage. Further, improved system reliability could lead to a potential payoff for this investment. Electrodialysis with side stream softening is thus an excellent combination to operate in the proposed salinity region to achieve the necessary calcium, silica, and overall salt bleed from a cooling system.
6
8
9
0.9
Additional reading Jordan, D. R., Mcllhenny, W. F., and Westbrook, G. T., "Cooling Tower Effluent Reduction by Electrodialysis," American Power Conference, 1976. Rice, J. K., "Evaluating Cooling Systems with Zero Aqueous Discharge," 1976 Generation Planbook, pp 81-86. Frazer, H. W., "Side Stream Treatment of Recirculating Cooling Water," Cooling Towers 2, (Prepared by the editors of Chemical Engineering Progress), pp 76-8 1, (1975). NOH, D. E., "Chemistry of Recirculating Cooling Water," EPRI-ASMEOSU Condenser Seminar, 6/2/75, Columbus, OH. Crits, G. J., and Glover, G., "Zero Discharge From Cooling TowersProblems and Some Answers," 34th International Water Conference, October 30, 1973. Warner, M. E., and LeFevre, M. R., "Salt Water Natural Draft Cooling Tower Design Considerations," Proceedings of the American Power Conference, Volume 36, 1974.
Gerald Westbrook is project manager of the Electrodialysis business venture of The Dow Chemical Co. Functional Products & Systems Dept. For the past 70 years, his principal work has been in water purification technology and energy industries.
Louis Wirth, Jr., is marketing manager for Ion Transfer Products, The Dow Chemical Co. He has been an active leader in development and marketing of water treatment and ion exchange processes for more than 40 years with Dow and with Nalco Chemical Co. Coordinated by JJ Volume 11, Number 2, February 1977
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