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A COST-EFFECTIVE TREATMENT SYSTEM FOR THE(6,ABSTRACTf t h e r i s i n g costs f o r f oa t i o n t h a t such f u e l s are exhaustible,increasing a t t e n t i o n has been focused on theearth's heat as a source f energy. I t has beenestimated t h a t geothermal resources i n the UnitedStates alone could produc 140,000 megawatts o fpower over a l i f e expectancy o f 30 years,. This i sthe equivalent o f 140 nuclear generHowever, there are unsolved problew i t h u t i l i z i n g geothermal brines tRecent t e s t work conducted a t t h e Imperial .Valley,C a l i f o r n i a , provides a s o l u t i o n t o one o f thesemajor problems.A treatment system haswhich spent geothermal brines from a flash-tanheat-extraction p l a n t are s t a b i l i z e d t o permitr e i n j e c t i o n of t h e t r e a t e d b r i n e while maintainingthe i n t e g r i t y o f the w e l l s over an extended period.This treatment system i s both c o s t - e f f e c t i v e andenvironmentally sound. I t incorpolowing unit-process operat0Reconstituting the m idissolved from the;geothermal s t r a t a format i o n s i n a Reactor-Clarifitact precipitation reactioPolishing o f the c l a r i f i eg r a v i t y dual -media f i1t e rBrine from Magmamax #1 now feedsa four-stage f l a s h heat-extraction p l a n t a t theGeothermal Loop Experimental F a c i l i ty, operated bythe San Diego Gas & E l e c t r i c Company and theDepartment o f Energy. Spent b r i n e from the f l a s hsystem i s transported by pipe1i n e approximately5,000 f e e t t o Magmamax #3 well, and i n j e c t e d backi n t o the formation.t o f injection i sThe spent b r i n esupersaturated w i t h respect t o heavy metals ands i l i c a (Table 1), whose p r e c i p i t a t i o n , along w i t hplugglng o f the s t r a t a w i t h d i s c r e t e suspendeds o l i d s i n the brine, has adversely a f f e c t e d t h ei n t e g r i t y o f the i n j e c t i o n well. The v i a b i l i t y ofthe complete p r o j e c t i s i n jeopardy unless coste f f e c t i v e methods can be devised t o s t a b i l i z e thespent b r i n e before i n j e c t i o nTable 1.Chemical analysis o f spent geothermalb r i n e treated i n p i l o t p l a n t reactor-.Sodi umPotass iumCal c i umChlorideIrondisposal.INTRODUCTION'46Salton Sea Known Geothermal Resource Area i nImperial Valley, C a l i f o r n i a , t o demonstrate thev i a b i 1ity o f converting the underlying geothermalbrines t o energy. The p r o j e c t includes f o u r wellsd r i l l e d by Imperial Magma. Two o f the wells,Magmamax #1 and Woolsey #1, are production wells;Total Dissolved SolidspH - ,Temperature range53,27610,25922,414134,48327268520753 1129293-177'228,4485.618O0F-2OO0F82OC- 93OC

I n December 1977, personnel o f ImperialMagma and Envirotech Corporation met t o discussmethods o f t r e a t i n g the spent b r i n e t o produce anon-scaling l i q u i d s u i t a b l e f o r w e l l i n j e c t i o n .A t t h a t time, i t was conceived t h a t autop r e c i p i t a t i o n o f minerals i n the spent b r i n e couldbe achieved i n a s o l i d s contact c l a r i f i e r , producing an e f f l u e n t o f stable saturated l e v e l s o fs i l i c a and heavy metals.Accordingly, a p i l o t p l a n t Reactor-Clarifier,furnished by Envirotech Corporation t o ImperialMagma, was i n s t a l l e d adjacent t o Magmamax ,#3i n j e c t i o n w e l l and operated continuously 24 hours,per day, 7 days per week from February 28, 1978t o A p r i l 12, 1978, c o n s t i t u t i n g the f i r s t phase o ft e s t work.0- Upflow r a t e- Reaction-well r e t e n t i o n time- Solids concentration i n reaction well.0Measure the s i l i c a and t u r b i d i t y concentrat i o n s o f the e f f l u e n t from various designso f Reactor-Clarifiers.0Measure the q u a n t i t i e s o f s o l i d s p r e c i p i t a t e di n and discharged from t h e Reactor-Clarifier.0Determine the thickening and dewatering chara c t e r i s t i c s o f sludge produced, and the bestdewatering equipment t o use.0Correlate a l l data c o l l e c t e d to:Determine design c r i t e r i a f o r the ReactorC l a r i f i e r t h a t would most e f f i c i e n t l y produce stable saturated l e v e l s o f s i l i c a i nthe e f f l u e n t .Establish c r i t e r i a f o r p o l i s h i n g the ReactorC1a r i f i e r e f f l u e n t .REACTOR-CLARIFIERThe fundamental component o f the spent geothermal b r i n e treatment system i s the EnvirotechReactor-Clarifier, which i s a tank o f 8'0" diameterx 10'6" sidewater depth. A l l external surfaces areinsulated t o minimize heat l o s s o f the spent brine.Area o f the c l a r i f i c a t i o n zone i s 37.7 square feet.The u n i t operates on the p r i n c i p l e o f s o l i d s contact c l a r i f i c a t i o n . Large volumes o f sludge(10 t o 15 times the volume o f feed) are r e c i r c u l a t e d i n t e r n a l l y through a d r a f t tubeJimpellerarrangement t o a r e a c t i o n well. Previously prec i p i t a t e d sludge r e c i r c u l a t e d i n t h i s manner comesi n intimate contact w i t h the feedstream i n thereaction well. DrOVidinQ seed n u c l e i on which d i s solved s o l i d s - i n the feedstream p r e c i p i t a t e underc o n t r o l l e d conditions o f temperature, r e a c t i o ntime, and s o l i d s concentration.The r e s u l t a n t l i q u i d / s o l i d mixture flows fromthe reaction w e l l i n t o the c l a r i f i c a t i o n compartment, where l i q u i d and s o l i d s are separated bygravity. Treated l i q u i d overflows i n t o c o l l e c t i o nlaunders a t t h e upper surface o f the u n i t . Rakearms move the s e t t l e d sludge t o a thickening conea t the bottom center o f the tank, where thethickened sludge i s discharged by g r a v i t y o rpumps.The r e s u l t s o f t h i s t e s t work have l e d t o acontract f o r a 55'0"-diameter Envirotech ReactorC l a r i f i e r having 2,375 f e e t o f t o t a l surface area,able t o t r e a t the t o t a l flow o f spent b r i n e from a10-MW, two-stage, f l a s h heat-extraction demonstrat i o n plant.Determine design c r i t e r i a o f the ReactorC l a r i f i e r f o r s i z i n g on a commercial scale,including:--- quirements.Determine sludge handling and disposal r e - Estimate order o f magnitude costsf o r spentb r i n e treatment and sludge handling anddisposal f o r a 55 MW geothermal energy plant.PILOT PLANT TEST RESULTSSummaryAverage t e s t data from the p i l o t plant,presentedi n F i g 1ol,;ws: are sumarized f o r odesign criteriais0No chemicals are required i n the treatmento f spent geothermal brines.0Upflow Reactor-Clarifier Rate 0.72 G P M / f t 2 .0Solids Concentration i n Reaction Well 2.5%.0Reaction Time i n Reaction Well 17 minutes.0E f f l u e n t Q u a l i t y a t Design C r i t e r i a- S i l i c a - 172 ppm- T u r b i d i t y - 15 NTU (44 ppm s.s.)DESIGN EXPERIMENT0The f i r s t - s t a g e t e s t program was designed t odevelop a v i a b l e and c o s t - e f f e c t i v e system f o rt r e a t i n g spent geothermal b r i n e and f o r handlingand disposing o f sludges produced i n the treatment.The objectives o f the design experiment weret o:0Test t h e o r i g i n a l concept o f auto-precipitationo f minerals i n the spent brine.Sludge Produced:1.7 1b/day per GPM-0 Sludge Characteristics:- From Reactor-Clarifiers- From Thickener: 4.5% s o l i d sLj

In Ppm15 NTU Turbidity2? GPM,Tzszis o l i d s concentration held i n the reaction well.As shown i n Fig. 2, a stable s o l u b i l i t y l e v e l o f172 ppm S i O n i s achieved when the s o l i d s concentrat i o n i n the reaction w e l l approaches 2.5% s o l i d sby weight. Further increases above t h e designconcentration range do not markedly improve s i l i c ao f a Reactor-Clarifier i s depenn the s e t t l i n g rates o f p r e c i p i t a t e d s o l i d si n the u n i t . It cannot be operated a t an upflowt e r than the s e t t l i n g r a t e o f the p r e c i p i ids; otherwise, c l a r i f i c a t i o n would n o tn accordance w i t h the Kynch theory, s o l i d si n hindered s e t t l i n g e x h i b i t s e t t l i n g v e l o c i t i e sinversely proportional t o the sol i d s concentrationi n the zone through which they s e t t l e . This i sv e r i f i d i n Fig. 2. Upflow r a t e s as h i g h as 1.33G P M / f t can be maintained i n t h e Reactor-Clarifierwhen r e l a t i v e l y low sol i d s concentrations i n thereaction w e l l are h e l d a t approximately 1.2% s o l i d sby weight. As these concentrations are increased,upflow r a t e s must be reduced accordingly; down, f o rexample, t o a measured low o f 0.62 GPM/ft2 a t as o l i d s concentration o f 2.63%.5I n achieving a c o s t - e f f e c t i v e process, theres t s a compromise between the continuous product i o n o f stable saturated e f f l u e n t from the ReactorC l a r i f i e r and the maximum permissible upflow r a t ei n the u n i t . Test data show t h a t an optimums o l i d s concentration o f 2.5% by weight can bemaintained i n the reaction w e l l while p e r m i t t i n ga maximum design upflow r a t e o f 0.72 GPM/ft2 t oproduce the desired stable saturated e f f l u e n t of172 ppm Si02.1.6