REUSE OF POWER PLANT COOLING WATER FOR IRRIGATION

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  • VOL. 16, NO. 5 WATER RESOURCES BULLETIN

    AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1980

    REUSE OF POWER PLANT COOLING WATER FOR IRRIGATION'

    W. A. Jury, H. J. Vmx, Jr. , and L. H. Stolzyz

    ABSTRACT: Current water quality policies in California require dis- posal of saline blowdown waters from power plants in sealed evapora- tion ponds to avoid degradation of ground waters. This policy high- lights the conflict between increased energy demands, increasing scarcity of water, and environmental priorities. Saline blowdown waters can be used for the irrigation of salt tolerant crops, albeit with some reduction in yields. The results of experiments intended to specify these yield reductions are reported. If such irrigation is carefully managed, the soil profile can be used to store residual salts and ground water degradation will be avoided, provided that irrigation ceases before the salts are leached to the ground water. An analysis of discharge below a carefully managed irrigation project shows that the downward movement of salts below the root zone is no worse than with conventional methods of dis- posal. Thus, irrigation reuse with blowdown water is shown to be a viable means of saline water disposal while maintaining existing stand- ards of ground water quality protection. Further analysis demonstrates the economic feasibility of such irrigation reuse by showing that it is significantly less costly than the evaporation pond alternative. (KEY TERMS: blowdown water; saline irrigation water; evaporation pond.)

    INTRODUCTION

    Electric power demand in the United States has been con- tinually rising during the last century, and though conserva- tion measures have slowed the rate of increase, it shows no sign of leveling off (ERCDC-CAL, 1977). To meet anticipated future power demands without sharp increases in price, new electric generating facilities will have to be constructed. How- ever, the construction of new plants is currently being ham- pered by a combination of resource shortages and environ- mental restrictions, which have caused the growth of energy supply to be slower than the growth of energy demand. Nu- clear power, once thought capable of carrying the burden of increasing energy demands, is now being discouraged because of fears of reactor accidents and waste disposal problems. Oil and natural gas will become increasingly expensive as supplies grow scarcer. Coal presents environmental hazards through land damage and air pollution, which have caused delays in coal fired power plant construction.

    Along with the difficulties caused by shortages or environ- mental drawbacks associated with the alternative primary fuels

    (i.e., oil, natural gas, coal, nuclear) water supplies for cooling, particularly in the arid western states, have become an addi- tional barrier to the construction of new power plants. Until recently, the majority of power plants in California were built along the coastline and used sea water for cooling. However, when studies suggested that the thermal discharge from the cooling system was endangering certain marine life near the power plants, the California Water Resources Control Board, already concerned about air pollution discharges and earth- quake vulnerability, ruled that future plants would have to be constructed at inland sites and use a closed system for cooling (CSWRCB, 1975b). Since a large lGWe (lo9 watt output) fossil fueled plant requires about 6.1 x lo4 m3/day (18,000 acre ft/yr) of water for closed system cooling (DWR-CA, 1977) even a single plant could tax the fresh water resources of a water scarce desert region. Furthermore, a closed cooling sys- tem, such as a wet cooling tower, requires the continuous re- moval of a portion of the condenser flow to prevent excessive salt buildup in the cooling system. This residual water, called blowdown, is typically around 20 percent or 1.4 x lo4 m3/ day (3600 acre ft/yr) for a lGWe power plant, and is very high in dissolved salts (DWR-CA, 1977). The California State Water Resources Control Board has set forth a policy stipu- lating that this water must be disposed of in lined evaporation ponds to prevent its return to the ground water (CSWRCB, 1975a).

    The dilemmas posed by conflicts between energy demands, environmental priorities, and resource shortages are real and all too frequent. Often, efforts to resolve such conflicts focus almost exclusively on the relative weights or emphasis to be given to increasing energy supplies, protecting the environ- ment, and alleviating resource shortages. The result is that once the relative weights are determined, little importance is attached to achieving that mix of goals in a fashion that mini- mizes costs. In this paper, we explore a case in point by analyzing the environmental and economic impacts associated with the use of power plant cooling tower blowdown water for irrigating crops. The analysis indicates that utilization of blowdown water for crop irrigation makes productive use of

    'Paper No. 80022 of the Water Resources Bulletin. Discussions are open until June 1, 1981. 'Respectively, Associate Professor of Soil Physics, Associate Professor of Resource Economics, and Professor of Soil Physics, Department of Soil and

    Environmental Sciences, University of California, Riverside, California 92521.

    8 30

  • Jury, Vaux, and Stolzy

    water that would not otherwise be used while maintaining stipulated environmental priorities at costs which are signifi- cantly lower than those associated with the more conventional practice of disposing of blowdown water in evaporation ponds.

    COMPOSITION OF BLOWDOWN WATER

    A closed wet tower cooling system consists of a continuous cycle of cooling water that picks up waste heat at the con- denser and then removes this heat by convection and evapora- tion at the tower as the cooling water cascades down the cool- ing tower fins (Figure 1). A portion of the water (B), called blowdown, is removed during each cycle to avoid excessive salt buildup. Makeup water (M) is added at a rate sufficient to replace the evaporation and drift losses (E) as well as the blowdown (B). The ratio of makeup to blowdown water de- fines the number of cycles ( C ) of the system, and determines the concentration of the blowdown. When the makeup water is of good quality five or six cycles may be achieved before the water causes significant scaling or corrosion. The only sub- stantial alteration of the water other than by concentration arises from the addition of sulfuric acid to lower scaling poten- tial.

    Table 1 summarizes the principal ions present in blowdown samples taken from Southern California Edisori plants at Bar- stow, California, and Etiwanda, California, as well as the com- position of several synthesized treatments we have been using in our experiments (Jury, et al., 1978~) . The concentration varies with power output during the year, but all samples are characterized by high concentration of SO:- from the sulfuric acid treatment. Each of these samples is near saturation with gypsum (CaS04.2H20) and lime (CaC03).

    USE OF BLOWDOWN FOR IRRIGATION The salinity level of the blowdown water rules out its use

    for many, but by no means all, crops. Table 2, adapted from

    Ayers and Westcot (1976) summarizes the allowable Electrical Conductivity of the saturation extract (EC,) and of the irriga- tion water (ECi) for a number of salt tolerant crops under 0, 10, 25, and 50 percent yield reduction. These figures suggest that blowdown waters with salt concentrations similar to those reported in Table 1 could be concentrated several times within a root zone without seriously reducing yields of many crops.

    STACK LOSSES 265MW

    EVAPORATION

    IOOOMW

    FUEL

    2650MH -

    LOSSES I l8MW

    Figure 1. Water and Energy Balances for a 1 GWe Power Plant With Closed Wet Tower Cooling.

    Over the last three years this concept has been tested on 28 field lysimeters (1.22 m diameter) alternately growing wheat and sorghum while receiving synthesized blowdown water of 2.1, 4.2, or 7.1 mmho/cm EC (Jury, et al., 1978). Table 3 summarizes the grain yields of the three irrigation treatments normalized to a per hectare basis. This experiment was carried out with almost no drainage, so that root zone salinity was very high, and the reduction of yield in later crops was sub- stantial. The leaching fraction LF (drainage volume/irrigation

    TABLE 1. Composition of Power Plant Cooling Water Samples.

    Location and Time EC CI SOT2 HC03 Mg+2 Na+ K+ SAR TDS of Sampling (mmhoicm) PH (meq/i) (meq/l) (meq/l) (meq/l) (meq/i) (meq/l) (meq/i) (meq/# (mg/l)

    Southern California Edison Etiwanda Plant

    4/1/77 6.85 7.50 20.07 67.69 0.75 31.12 22.78 38.17 1.22 8.75 5840 1/25/78 4.73 7.50 16.08 50.39 2.49 25.38 15.00 29.19 0.56 6.50 4525

    Southern California Edison Barstow-Dagget Plant

    UC Riverside Synthesized Blowdown Waters

    2/2/76 3.70 6.50 10.50 34.50 0.50 20.50 7.00 13.60 - - 3.67 2865

    High 7.10 8.10 28.00 67.00 5.00 28.90 21.10 50.00 - - 10.46 6500 7.98 3200 Medium 4.20 8.30 11.40 32.70 4.10 14.50 7.40 26.30 - -

    LOW 2.10 8.00 6.30 13.30 3.80 8.00 4.30 11.10 - - 4.56 1560

    83 1

  • Reuse of Power Plant Cooling Water for Irrigation

    volume) ranged from 0.02 to 0.10, which is much smaller than usual field practices. As a result, the average salinity in the root zone was higher than that assumed in the guidelines in Table 2. Furthermore, many of the crops listed in Table 2 are more salt tolerant than wheat or sorghum and have the poten- tial to produce higher relative yields with blowdown water.

    TABLE 2. Salinity Tolerance of Various Crops.

    Percent Reduction in Yield

    Barley 8.0 5.3 10.0 6.7 13.0 8.7 18.0 12.0 28.0 Cotton 7.7 5.1 9.6 6.4 13.0 8

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