Disposal of Concentrates From Brackish Water

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Desalination 78 (1990) 71-76 71Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

Disposal of Concentrates from Brackish WaterDesalting Plants by Means of Evaporation TechnologyLEON AWERBUCH AND MALCOLM C. WEEKJ SSBechtel National, Inc., 1620 Eye Street, Washington.D.C. 20240 (USA)Tel. 202-3934747SUMMARY

The use of reverse osmosis (RO) plants to desalt brackish water in SouthFlorida has increased rapidly in the last decade. The concentration of scaleforming species such as calcium sulfate and silica usually limits RO recovery inthese plants to about 75% of the total feedwater flow. The remaining 25% of theflow is reject brine which presents a serious disposal problem for plants ininland locations. This paper describes the use of brine concentrators to reducethe RO reject brine to 2% of the overall flow and so significantly reduce thedisposal problem. The hybrid concept of RO followed by evaporative brineconcentration has been applied to similar problemslNTRODUCIION

Reverse osmosis plants discharge a permeate stream with low dissolvedsolids (IDS), and a reject stream with high IDS. In the reject stream, theconcentration of scale f&g species such as calcium sulfate and silica must becontrolled to prevent damage to the RO membranes. As a result, in desaltingbrackish water, the permeate stream is usually limited to about 75% of the feedstream. The disposal of the remaining 25%, the brine stream, presents technical,regulatory, and financial problems for plants in inland locations.Evaporative brine concentrators may be used to treat the RO reject to recoveran additional 23% of the feedwater as a stream containing less than 20 ppm totaldissolved solids. This reduces the volume of the stream requiring disposal tojust 2% of the f&water volume.Evaporation is a commercially available process that has a successful trackrecord in concentrating RO brines and waters of similar quality in power plants,water reclamation facilities, and industrial plants.Ooll-9164/90/$03.50 Q 1990 Elsevier Science Publishers B.V.


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The hybrid brackish water desalting plant consisting of RO followed byevaporative brine concentration gives the following advantages:- Recovers 98% of the feedwater as potable water- Reduces the size of a new RO plant by 23%- Reduces the RO pretreatment operating cost (chemicals, power, etc.)- Reduces the size and cost of the pretreatment equipment- Reduces the limits imposed on the RO plant because it can now produce lesspure water which may be mixed with the distillate from the brine concentratorto produce potable water- Reduces the quantity of brine requiring disposal from 25% of the raw waterto 2%, resulting in a significant reduction in disposal costs.The concentrate stream from the evaporation process step may be placed in aholding pond for temporary storage pending final disposal to the ocean.

PROCESSDESCRIPTIONFigs. 1 and 2 are process flowsheets of two versions of the evaporative

brine concentration process known as vapor compression evaporation (VCE).Fig. 1 represents the process developed by Resources ConservationCompany (RCC) of Bellevue, Washington. The feed to the concentrator ispumped from a holding tank and treated, if necessary, with a small dose of scaleinhibitor. The feed then passes through a heat exchanger where it recovers heatfrom the condensate stream as that stream leaves the concentrator. The preheatedfeed next passes through a deaerator where non-condensible gases are removedand vented to the atmosphere. After leaving the deaerator, the feed, at its boilingtemperature, enters the evaporator sump. The feed mixes with concentrate slurryand is continuously recirculated to the flood box of a vertical tube evaporator.From the flood box, the slurry is distributed to the inside walls of the heattransfer tubes as a thin film. As the thin film runs down the inside of the tubes,water is evaporated and the resulting steam passes through a mist eliminatorbefore entering the suction line of an electrically driven compressor.Compression raises the condensation temperature of the steam above the boilingpoint of the recirculating brine. As this steam releases its heat of condensation, itcondenses on the shell side of the tubes and is collected in the product tank.With the release of the heat of condensation, more water is evaporated from thebrine film on the inside of the tubes. Consequently, the compressor supplies theenergy which drives the system. To provide the initial charge of vapor to thecompressor, a small auxiliary boiler supplies steam to the evaporator for a shorttime at start-up (Anderson, 1976).


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Fig. 1. The RCC vapor compression evaporation process flow diagram.

Fig. 2 represents the process supplied by Ambient Technologies of NewYork. As in the RCC process, the feed is treated with scale inhibitor, if required,then preheated and deaerated. The feed then goes to a condenser where avacuum pump connected to the condenser continuously removes non-condensable gases. This vacuum pump is also used to reduce the pressure in theevaporator at start-up. The feed then mixes with recirculating brine, enters theevaporation chamber in the main vessel and is sprayed on the external surface ofa nest of horizontal heat transfer tubes at a rate just sufficient to create a thincontinuous film. Through its suction, a radial blade centrifugal compressorprovides a pressure lower than the equilibrium pressure of the water sprayed onthe tubes. As a result, part of the water flashes into vapor.After passing through a separator to remove droplet carryover, the vapor iscompressed and discharged to the inside of the tubes. There it condenses, givingup its latent heat which evaporates a portion of the brine on the outside of thetubes. The condensate is pumped out by the product pumps. The unflashedbrine accumulates as concentrate at the bottom of the evaporation chamber.Unlike reverse osmosis, the vapor compression evaporation process cantolerate the presence of scale former-s. Deposition on the plant components isprevented by maintaining a comparatively high concentration of calcium sulfatecrystals in the recycled brine. These crystals provide nuclei on which the scaledeposits, in preference to the equipment and piping. Any scale deposited on theevaporator surfaces is scoured off by the recirculating slurry.


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I INHIWORI_______________-__JFig. 2. Process flow diagram of the vapor compressionevaporator supplied by AmbientTechnologies.The VCE process supplied by Ambient Technologies was tested at theCalifornia Department of Water Resources Demonstration Desalting Facility atLos Banos. The average operating conditions obtained during this test aresummarized below:

Feedfhgpd 56,680productflow,gpd 53,980FeedTDS,ppm 9,084ProductTDS,ppm 10Concentrate TDS, ppm 206,676Recycled brine temperature,OC 48.9Fmducttem~, c 48.6Several commercial VCEs of the type supplied by Ambient Technologies

are in successful operation worldwide.The RCC process is operating successfully at 40 locations throughout theworld. Typical operating conditions for an RCC unit are shown below:Feed flow, gpd 504,oooRoclllct flow, gpd 478,800Feed TDS , ppm 12,000Product TDS, ppm cl0Recycled brine TDS, ppm 210,000operating temperature, C 105.6

The equipment supplied by RCC uses titanium tubes, while the equipmentsupplied by Ambient Technologies uses aluminum alloy tubes. Jn addition to thetwo equipment suppliers mentioned in this paper, there are several other vendorsof vapor compression evaporators worldwide.


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PROJECTEDHYBRIDPLANT PERFORMANCE

For the purpose of this presentation, it is assumed that the feed to the VCEhas a TDS concentration of 10,000 ppm and is saturated with calcium sulfate.Vapor compression evaporation will recover an additional 23% of the feedwater,for a combined RO and VCE recovery of 98%. This leaves only 2% of thevolume of the feed stream for final disposal.Plant sizes of 1,s and 10 mgd of product water, were considered for thisevaluation. The projected performance of these plants is given below:

RO product RO reject0 VCE product Brine toultimatedisposal1mgd 765,ooO 255,100 234,700 20,4005 mgd 3,826,500 1,275,500 1,173,500 102,00010 mgd 7,653,OOO 2,551,OOO 2,347,OOO 204,100

COSTS

There are several features inherent to the VCE that affect its economics. Allother things being equal, the smaller the temperature rise and, hence, thecompression ratio, the higher the overall efficiency of the process. On the otherhand, a small temperature rise requires a large heat transfer area. Consequently,the choice of operating conditions is a trade-off between the cost of energy andplant investment. A second consideration concerns the operating temperature ofthe VCE. The choice of low operating temperature will greatly decreasecorrosion but will provide vapor of high specific volume, thereby increasing thesize of the compressor and vapor lines. The price of the VCE system depends onthe application. Small sizes may be installed for about $7.50 per gpd. As thesizes increase, the installed price may be reduced to approximately $6.00 pergpd.The vapor compression cycle is very attractive because of its high energyeffectiveness under commercially attainable operating conditions. For dual effectsystems, the energy consumption can be reduced to the range of 60 to 70kWh/lOOO/gal of condensate produced by the evaporator.The energy consumption for a typical brackish water RO plant isapproximately 6 kWh/lOOO gal.If waste heat is available as the energy source for evaporation, multi-effectdistillation may be substituted for VCE as the evaporative portion of the hybrid


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If waste heat is available as the energy source for evaporation, multi-effectdistillation may be substituted for VCE as the evaporative portion of the hybriddesalting plant. The use of waste heat would cut the energy cost significantly.Bechtel is currently evaluating the optimization of the components of a hybriddesalting plant to reclaim agricultural subsurface drainage water in the SanJoaquin Valley of California. This project is being funded by the CaliforniaDepartment of Water Resources. The selection of the pretreatment, RO, andevaporation components of this system is being made so as to minimize theoverall cost of the facility. The final report for this project will be available inearly 1989.REFERENCES

P.J. De Moel, et al., Design of 50,000 m3/d of brackish water at 99 percent recovery:Proceedings, Second IDA World Congress on Desalination and Water Reuse, Bermuda,1985.H. Kohli, Hybrid desalting technology maximum recovery: Proceedings, Second IDAWorld Congress on Desalination and Water Reuse, Bermuda, 1985.E.D Home, et al., Assessment of three technologies for the treatment of cooling towerblowdown: U.S. EPA-600/7-79-220, September 1979.J.H. Anderson, Development history of the RCC brine concentrator for concentratingcooling-tower blowdown: Presented at the SPE Winter Annual Meeting, New York,December 5,1976.

Disposal of Concentrates From Brackish Water

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