a method of using irrigation drainage water for power plant cooling

Desalination, lQ(1976) 257-268 0 Elsevier Scientific Publishing Company. Amsterdam - Printed in The Netherlands

A METHOD OF USING IRRIGATION DRAINAGE WATER FOR POWER PLANT COOLING

Hugo H. Sczphton and Get-hard Klein Uflivcr5i ty of California Seawater Conversion Laboratory

A. AdSTRkCT

The accumulation of large quJntities of saline water collected tram irrigated fields by underground ti Ic drainage systems presentr environmental problems in the South-Western U.S. and flexico. A major project conaucted in this Laboratory indicates technical and economic fcasibili ty :or using such wastewaters for power plant cooling. A novel p recess sequence is crrployed comprising (a) ion-exchange resin softening whereby nearly all the calcium is removed, followed by (b) using this softened water as feed to the power plant cooling tower, concentrating it by a factor of 5 to IS; this cooling tower blowdown is subsequently (c) further concentrated by v*-riical tube foam evaporation (VTFE) to provide a ZO-fold concentrate or the initially softened wasrewater which is then adequate to fd) serve as the sole regenerant for the ion-exchange resin used in theinitial softening step. Step (d) is carried out in an upflod fluidized bed mode which ensures favorable reaction kinetics by the formation and removal of calcium sulphate precipitate from the resin bed.

A new pi lot plant facility OF 2 ,000 gpd capacity is described including a softening and regeneralion facility, a cooling cower operating under realistic process conditions and a complete VTFE facility; test data arc presented indicating technical feasibility. Long term field tests with this facility are planned in the San Joaquin Valley to determine the viability of this Jpproach for the beneficial use and simul Caneous disposal of irrigation drainage. This project is spon5orcd by several major U.S. power utilities. the Electric Power Research Institute and the California Department of ‘..Iater Resources.

8. I NTRODUCT I ON

Fresh water for cooling large power plants ptanned for the San Joaquin Valley is anticipated co rise in price and decline in JVJiTJbiTiCy Sufficiently to justify Serious consideration of irrigation tile drainage (or agriculturai wastewater) for this service. The latter is anticipated to increase in volume lo about 500.000 acre-feet per year containing about 3,000 parts per million (ppm) of total dissolved solids (TDS; by the year 2000, Jnd to reach 700 mi 1 t ion gal Tons per dJy

at peak summer time flow (1). Utilization of this wastewater for power plant cooling appears Lo be technically feasible and economically acceptable; it wuld JISO provide a significant ecological benefit. The hock described here examine0 the application of novel technologies, developed in this Laboratory for desalination, to power plant cooling cycles such as the one shown in Figure I. The objective of this work was to obtain test data with a pilot plant, and to provide infOrtIIJtiOn

for J realistic evaluation of the possibitity of using this WJSteWJCer for power plant coolant loops of both conventional and advanced design (2).

257

258 H.H. SEPHTON AND C. KLEIN

In view of the constraints imposed by the Environmental Protection Agency (EPA) guidelines, the present tendency is toward consumptive use of the coolant in wet cooling towers, and to reduce the blowdown to a smal I vo!cme before ponding it or ta reduce it further to dry solids or a slurry. this requires some form of pretreatment of the coolant to prevent clcjosi tion of scale, and some means of final concentration, for instance by ev.3poration of the blowdown, even if fresh water is used as the coolant.

The objectives of the work discussed were to provide essential tnformation to estabish the feasibility and cost of utilizing irrigation drainage for po?Ner olant cooling, by a combination of the Following groups of Processes:

(a) Pretreatment of this wastewater by ion exchange to reduce its calcium content, and its scale-forming tendencies.

(b) Evaporation of the softened wastewater by a factor of 5 to I5 in a cooling tower.

(c) Evaporation-of the 5’ to ISi-fold concentrated coolant (blowdown from the cooling tower) in an interface-enhanced vertical tube evaporator (VTE) , Co a concentration sufficient to serve as the sole regenerant for the ion-exchange resin used under (a), wi th recycle of the distillate to the power plant coolant stream and recycle of the foaming agent to. the VTE.

(d) Regeneration of the ion-exchange resin used in (a} by means of the concentrated blowdown from the VTE derived in step (c) by a fluidized bed procedure (2) and disposal of the regenerant effluent.

(e) Ponding of the spent regenerant or VTE-blowdown, or alternatively, its final concentration in an evaporator-crystal I izec to dry solids.

This method of btowdown concentration should be more economical than others avai table. especially if it utilizes waste heat normally discharged from a power plant, for the VTE operation (3, 4).

The use of interface enhancement (5) has been shotrn to provide an approximate doubling of the usual rates ofevaporation of the coolant btowdotrn by VTE. This, and the use of the concentrated coolant blowdown for regeneration of the ion-exchange resin, provide the basis for substantial cost savings to be anticipated over the other known methods of treatment for power plant coolants.

Under EPA guide1 incs. desaicing of power plant coolant btowda+n has to beconsidered,even if good fresh water is utilized as Coolant. pretreatment or softening of even such high quality coolants is USuallY required in order to proceed to about a 20-fold concentration in a cool i ng tower. The use of irrigation drainage appeared technically and

economically feasible, with the typical. flow diagram shown here, from preliminary work done in this Laboratory; success in this endeavor should make this presently useless wastewatcr supply available for a beneficial use while also answering an envrronmentaT need. This work would provide additional useful information relevant to power plant cooling in general, for instance by evaporative cooling with fresh water softened by ion exchange.

&. PROCESS AND PI LOT PLANT USED

The proposed pouier plant coolant flotr diagram sho\+jn in figure 1 applies ion-e.xchange softening to the entire coolant aaheup stream (a),

IRRIGATION WATER FOR POWER PLANT COOLING 259

260 H.B. SEPiiTON AND G. KLEIN

rather than using the conventional side-stream softening. The softened makeup is then to be pre-concentrated to a constant, maximal permissible dissolved sol ids content consistent with an acceptable heat reject performance in the power plant condenser and cooling tower loop, with continuous addition of softened makeup and rejection of coolant biowdown from this ioop. This coolant blowdown is then to be further concentrated by VTE to provide a biowfdmm of sufficient concentration to serve as the sole regenerant Fur the tm+exchange resin. Sif3li iWfY. the simpt i fiod pilot plant flow diagram shown in Figure 2 utilizes two ion-exchange columns, for alternative cyctic use , a cool ing tower for pre-concentration. and a VTE loop for Final concentration or the resin regenerant, and for the disposal of effluents by concentration.

1. Softenino of Coolant Hakeup

Removal of calcium and magnesium from the wastewater was performed in a conventional downfior* passage through a x-foot bed depth, l-foot sqare in cross section, of polystyrene-divinyl benzene suifonatc resin in the sodium form, to the point of calcium breakthrough as in- cated by a rapid increase in calcium content of the effluent above the 10 parts per nit i ior‘ (ppm) range (2). Approximately 1,200 gallons of softened water were obtained before calcium breakthrough, wirh

irrigation drainage water having a total dissolved solids (TDS) content of 3.600 ppm. This wastewater was provided by the California Departncnr of Water Resources, from their Firebaugh site in tne San .loaquin Valley. It is subject to a seasonai variation in salt content (TDS range 2,000 to 7,OCO ppm) which depends on irrigation practice and the crops in cultivation at different times of the year. This water, as co1 lected by subterranean draintile, is generally clear.

The ion-exchange column used in this pilot plant facility is shown in Figure 3; its design was based on extensive laboratory scale develup- ment work presented elsewhere (2).

2. Coolinq Tower Desion and Operation

The 3S-foot tall cooling tower, shown in the background in Figure 3 and in sketch form in Figure 4, was designed to simulate the temperature and flow parameters appiicabie to a large power plant instai Iation, It had a packed deck section of 20 feet having a fiow cross section of 2-foot square; two types of wet deck materials were

used: standard corrugated sheets of melamine-impregnated neoprene- asbestos and self-extinguishing potyvinyi chloride, removably instaiied in a vertical orientation. spaced one-inch horizontally. Eight sections were used, four of each type of material, 16-inch deep. with i2-inch verticai spacings between wet deck sections. This coolant cascade channel was made of marine grade plywood and was supported by an angle-iron frame of 7- x 7-foot, 33-foot high. Coolant das continuously

re-circulated between this tower and a 40 KWatt- heat source, and sprayed over the top wet deck section through four stainless steel spray-cone nozzles at rates of flow variable between 2 and IO gallons per minute per square foot. Air flow upward through the wet deck sections was by induced draft, using a commercial cooling tower fan. continuously bett- driven by an electric motor. A packed section far &he elimination of drift for droplets of saline water) was instailed above the coolant spray system in the air florrt channel. The air flow-rate was variable between 4- and IO-foot per second by providing an adjustable gap in the cooi ing tower fiow channel between the fan and the drift ci iminat ion

packing.


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262 H.H. SEPHTON AND C. KLEIN

Coo!ant cascading through the twer was collected in a first basin, with gravity flow into a second basin from a low, submerged outlet. Coolant was continuously recycled from the first basin through a heat exchanger simulating the condenser of a large power plant, raising its temperature by I5 to 30°F before spraying the heated coolant into the top of the cooling tower. This heat exchanger comprised eight lengths of go-IJ copper-nickel condenser tubes of 2-J-foot length in a 4-pass arrangement this provided t. o paral lei condenser tt;bs channels of 8f-foot length. Heat impu: was by rapid re-circulation of water through the shell-side of this exchanger and four electric hot water heaters, each of 13 KUatt capacity.

Salinity control was provided by using an electrodeless conductivity sensor (Balsbaugh) actuating a feed pump to inject softened wastewater into the top of the first basin when the conductivity of the re-circu- latod coolant reachedapreset maximal limit. and to turn this pump off at a selected lower conductivity. Simultaneous overflow of concentrated coolant was thus provided for from the first basin into the second basin. The concentration of this cooling tower blowdown could thus be controlled at any desired TDS content, and it was automatically collected in the second basin for later use. A low-water shut-down probe installed in the first basin and a high-water shut-down probe in the second basin provided for the automatic tripping of all pwer to the facility, thus al lowing continuous, unattended operation.

Thermocouples installed at 12 locations in the cooIing tower loop were coupled with a stripchart recorder to provide a continuous record of all significant events in this loop. Liquid flow-rates and temperatures were accurately measured to provide periodic heat transfer performance data. The heat transfer performance of the condenser was maintained during the first 6 weeks of.operation of this pilot plant; no fouling of a biological or chemical nature was observed with the softened wastewater~ coolant iecycled at a l5-fold concentration level having a TDS of 54,000 ppm- 5iologicdl grwth was control led .by intelmi ttent dosage of the coolant with sodium hypochlori te added to the contents of the f.irst basin. Dechlorination was affected, as needed, by the addition of sodium sulfite.

3. Verticaf Tube Evaporation

The objective of this operaLion-was to further concentrate the cooling tower biowdown to about 20-fold of the original softened ‘wastewater, or sufficient to serve as the soie regenerant for the ion-exchange resin used under (I) above. A number of papers nave been published on the development of vertical tube foam evaporation VTFE (3, 6. 7) and several patents (5) have been issued on this method of increasing the rate of evaporation of liquids by .induced foarliy vapor-liquid flow. (interface enhancement). A rdtatably-mounted upflow-downflow V.TFE of 5.000 gal Ions per day (gpd) capacity was cons- tructed for this faci iity, shown in Figure 5. Fifty double-fluted aluminum-brass disti 1 lation tubes (Yorkshire-Imperial fletais ‘Ltd, England) were used, of &foot heated length, 1.5yinch diameter, rerro- vably installed with lo’-ring seals: stainless steel (type 316) was used for the-main evaporator compo:snts and pumps. tnstrumetitation provided accurate temperature measurcm&nt; on the feed (if), heating steam (I’S) and-the bri& (Tbf- rejected from the disti I lation tubes.

Tipical.opertition-of evaporators of this type, both by conventionat and the VTFE methoh. in upftw and downflott mDdes, has been published elsewhere (3. 5, 6, 7).


Heat of vaporization to this unit was supplied from a 100 horse- power boiler shown in Figure 5. This steam supply was controlled by a reducing valve (90 to 45 psi) followed by a pneumatically operated steam control valve and a desuperheater. to permit evaporation concentration by VTFE at any temperature within the range of 100 to ‘25OOi. The vapor produced was condensed ina large horizontal condenser, cooled with wafer of controlled ttlnperature to maintain a steady temperature difference (AT = Ts - Tb) _ A large evacu-ble s’cainfess steel tank ws included in the VTFE loop to provide capabi iity for the continuous concentration of 1,000 galXon batches of cooling towe- btowdown to acceptably low volumes. This blowdown was of sufficient concentration to serve as regenerant in the next step, after first removing the interface enhan&ment additive (foaming agent) and particuljtes from it by foam fractionation-flotation (8 ). The flow diagram in Figure 6 shows the process sequence applied to irrigation drainage in this piiot plant.

at ilO0 VTFE heat transfer performance

atu per hour ft2 - (U) was readily maintained

OF Lgith a AT of 8.5OF at an evaporation teliperatore of 220°F with the evaporator operated in the upflow mode with IO ppm of &zodol 25-3A (Shell Chemical Co.) added. The corres- ponding conventional persormance (U), without a foaming agent additive, was 1400 Btu per hr - ft - OF.

4. Reqeneration of the ton-Exchange Resin

Regeneration of the softening resin used in step (I) was accomplished in an upflow, fluidired bed mode using only concentrates of the previously softened waster*ater as regenerant. This step is considered the most crucial in the series of process steps employed. Successful regeneration with only concentrated blowdown of the prcsof- tened feed provides the most significant cost savings over more conventional methods of pretreatment. This regeneration step, initiated earlier in this Laboratory aati reported in.detail elsewhere (2) was successfully demonstrated in this work, first in bench-scale tests with simulated softened irrigation drainage concentrates, and finally in the pilot plant operation discussed here. The latter consisted of a plexiglass column of 8-foot by l4- inch dimensions having a stainless steel mesh screen plate near the bottom to support the resin and provide I iquid through-f low, and overflow outlets Z-foot above this screen. The cation exchange resin (Ouoiite C-20; s-cubic foot) layer was about )&inch high during the downflow, softening operation but expanded to about a 6-foot height during the fluidized-bed regeneration operation. This height was controlled by selecting a regenerant flow-rate sufficient to fluidize the resin bed but insufficient to carry resin beads overboard through the overflow outlets. This regenerant flow-rate was also sufficient to separate the relatively small calcium sulfate crystals formed interstitially in the resin bed by the regeneration reaction, and to wash them overboard into a holding tank. Precipitation of calcium sulfate in the resin bed, or close to the site of regeneration, has the beneficial effect of driving this reaction toward conpletian. In order to ensure complete regeneration and removal of CaSO4-precipitates, an approximately 400 gallon inventory of previously-used spent repenerant was accumulated for re-use as regenerant ; this was augmented by a fresh batch of concentrated soft wasteater (blowdown) for each reneneration step and reduced by discarding an equal volume from the 400 gallons of previously accumulated spent regenerant- Cyclic operation of this resin column was found to be maintainable on this basis, wiKh a sustained useful resin

264 H.H. SEPHTON AND C- KLEIN

Figure 6

IRRLCATION WATER FOR POWER PLANT COOLING 265

capacity for calcium and magnesium ions at above 40 percent of its total equi I ibrium capacity. when the regenerant concentration was at approximately a 70,000 ppm TDS level. Under these conditions, 97 to 98 percent of the calcium was removed from the wastewater having a TDS of 3,600 ppm, on a sustained, cyclic basis. An approximately ZO-fold concentration of the original wastewater after softening w3S thus needed to satisfy these conditions for regeneration of the resin.

Since the JDS of the wastewater varies with the seasons and is subject to irrigation practices. this concentration factor will varj seasonal- ly. but not necessarily the final TDS of the concentrates. The cpera- tion of this Fen-exchange softener and its regeneration is reprcsen- ted by Figure 7, and its interrelationship with the rest df the individual process steps is shown in Figure 6.

0. COHPOSlTlON OF IRRIGATION WASTEWATER DURING PROCESS SEQJENCE

The fate of the major ionic components of this wastewater is shown in Table I, responsive to the above process steps, at the tims of year when the TDS of the drainage col tected was at 3,600 ppm. Concentrations are reported in normalities. The seasonal variation in the cowosition of this water is largely due to dilution; the concentration ratios of the ionic species remain nearly constant through the year. The first column shows the concentrations of the major ions occurring in the raw wastewater. The second column shows the reductions in calcium and magnesium obtained during softening a batch of 1,200 gallons passed through the softener in step (I). The figures in the third column represent the blowdown from the cooling Lower (step 2), and the fourth column shows the further concentration by a

total factorof 20 obtained by VTFE in step (3). Thi 5 concentrate was ,Jsed for regeneration of the resin, discussed in step (4).

E. CONCLUS I ON5

Early results obtained with this recently completed pilot ptant are promising. An adequate, sustained capaci ty of the ion-exchange resin for softening irrigation drainage water was observed. USC of the softened product in a cooling tower simulating actual process condizions in a large power plant cooling loop, at a IS-fold concentration factor, presented no apparent fouling or loss in heat transfer performance during 6 weeks of continuous operation. with intermittent chlorination. Further concentration of the cooling tower blcddown (IS-fold) by VTFE to a final concentration factor of 20 was readily accorrglished at a 50 percent enhanced performance level. This VTFE concentrate was sufficient as the sole regenerant for the ion-exchange resin, after removal of the surfactant from it by foam fractionation. Regeneration of the resin in an upflow. fluidized bed mode, permits theuseof calcium crystallization near the sites of regeneration to drive this reaction towards compiction. whi le simul taneousiy providing for rwval of this precipitate by a hydrauf ic classification.

1 t is apparent, from these early tests, that the use of irrige- tion drainage water for power plant cooling is feasible, at least on a pilot plant scale. Further, tongterm tests in the field, over a futt t2-month seasonal cycle, appear justified and are planned (in collaboration with the Cat ifornia Department of Water Resources);

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IRRIGATXON WATER FOR PC!JER PLAKT COOLISC: 267

it should provide a data base for projecting the economics OF using this othcrwis2 usefcss water resource while simultaneously al lcviating its adverse cnvi ronmcntal impact.

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REFERENCES

“Focestial Use of Agricultural Uastc Water for Power Plant Cooling,” stctson c. L.. California Department of !Jzter Rcsourccs Hemorandum Report, September 1972.

“Fltridized-Ber ton Exchange with Precipitation. Principles and Bench-Scale uevelopmen~. KlLin, G., Thomas J. Jarvis and Theodore Vcrmeulcn, presented at the American Chemical Society Centennial !wcting, Snn Francisco, California, Scptcmber I, 1976.

“Recycle of Power Plant Coating Tower Slowdown by Vertical Tube Evaporation with Interface Ennancement Uti I izing Waste Heat,“ Sepnum, liuyo Y., P-oceedinas or the Second Mational Conference on Complete WatcRtuse, 1975.

“Desalination of Cooling Tower B~owdown,” Awerbuch, L. A. and A. N. Rogers, Proceedinqs of the Second National Conference on Co.mplctc WateReuse. 1975.

“Interface Enhancement 4ppl ied to Evaporation of Liquids,” Sephton, Hug0 H., U.S. Patent No. 3,846,254 and foreign counterparts, November 1374 : “Desalination 01 Lpflow Vertical Tube Evaporation with Interface Enhancrment,‘* S f-nton, Hugo H., Proceedinqs, international Oesal tinq & Envr I-xmwntal Association Conference, Ponce, Puerto Rico, Apri I 1975.

“fntcrface Enhancement for vtrtical Tube Evaporators: A Novef Way of Substantially Augmenting Heat and nass Transfer,” Sephton,

go H-9 presented at the American Society of Mechanicat Engineers neat Transfer Conference, Tulsa, OhfanCXrtJ, August 1971, ASME Publication 71-HT-38.

“Upflow Vertical Tube Evaporation of Seawater with Interface Enhancement: Process Development by Pilot Plant Testing,” Sephton, tluw H., Desalination, Vol. 16, No. 1, pp- T-13, February 1375.

“Removal of Surfactants and Particulate Matter from Seawater Desalination Blowdown Brines by Foam Fractionation.” Valdes-Krieg, Ernesto, C. Judson King and Hugo H. Sephton, Desalination, Vol. 16, No. 1, pp. 39-53. February 1375; “New Developments in Vertical Tube Evaporation of Seawater,” Sephton, Hugo H., Proceedings of the 5th lntern’tl Symposium on Fresh Water from the Sea, 2, pp. 279-287, 1976.

268 H.H. SEPHTON AND C. KLE;N y-

G. kCKNGWl_EDGEHENTS

The authors gratefully acknowledge the many helpful suggestions of the sponsors of this work, and the technical assistance of J. C. Hens1 ey and C. L. Free1 without whose dlidicated efforts this pi lot plant could not have been corrTtleted on schedule. The laboratory work on ion exchange by J. T. Jarvis, and secretarial help by Amy Edeison is acknowledged with thanks. The opinions expressed are those of the authors and not necessarily of the OUR or the participating utilities or Electric Power Research Institute.

Sponsorship for this work was obtained from the California Department of Water Resources (DWR), the Electric Power Research lnsti tute, the Southern Cal i fornia Edison Company , the Paci fit Gas and Electric Company and the Los Angeles Department of Water and Power, in a team effort coordinated by 0. 6. Brice of the OUR.

a method of using irrigation drainage water for power plant cooling

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