University of Arkansas, Fayetteville ScholarWorks@UARK Technical Reports Arkansas Water Resources Center 6-1-1986 Evaluation of Packed Towers for Removing Volatile Organics from Surface Waters James W. Moore University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/awrctr Part of the Fresh Water Studies Commons, and the Water Resource Management Commons Recommended Citation Moore, James W.. 1986. Evaluation of Packed Towers for Removing Volatile Organics from Surface Waters. Arkansas Water Resources Center, Fayetteville, AR. PUB 124. 25 This Technical Report is brought to you for free and open access by the Arkansas Water Resources Center at ScholarWorks@UARK. It has been accepted for inclusion in Technical Reports by an authorized administrator of ScholarWorks@UARK. For more information, please contact scholar@uark.edu, ccmiddle@uark.edu.
EVALUATION OF PACKED TOWERS FOR REMOVING VOLATILE ORGANICS FROM SURFACE WATERS James W, Moore Department of Civil Engineering University of Arkansas Fayetteville, Arkansas 72701 Publication No. 124 June, 1986 Technical Completion Report Research Project G-1004-04 Arkansas Water Resources Research Center University of Arkansas Fayetteville, Arkansas 72701 AW RRC Arkansas Water Resources Research Center Prepared for United States Department of the Interior
EVALUATION OF PACKED TOWERS FOR REMOVING VOLATILE ORGANICS FROM SURFACE WATERS James W. Moore Department Of C iv il Engineering U n iv e rs ity Of Arkansas F a y e tte v ille, AR 72701 Research P roje ct Technical Completion Report P roje ct G-1004-04 The research on which th is re p o rt is based was financed in p a rt by the United States Department o f the In te r io r as authorized by the Water Research and Development Act o f 1978, (P.L. 95-467). Arkansas Water Resources Research Center U n iv e rs ity o f Arkansas 223 Ozark Hall F a y e tte v ille, AR 72701 P u b lica tio n No. 124 June, 1986 Contents o f th is p u b lic a tio n do not necessarily r e fle c t the views and p o lic ie s o f the U.S. Department o f the In te r io r, nor does mention o f trade names or commercial products c o n s titu te th e ir endorsement or recommendation f o r use by the U.S. Government. The U n iv e rs ity o f Arkansas, in compliance w ith federal and sta te laws and re g u la tio n s governing a ffirm a tiv e action and no ndiscrim inatio n, does not d is c rim in a te in the re cruitm e nt, admission and employment o f students, fa c u lty and s t a f f in the operation o f it s educational programs and a c tiv itie s as defined by law. A ccordingly, nothing in th is p u b lic a tio n should be viewed as d ir e c tly or in d ir e c tly expressing any limitation, specification or d is c rim in a tio n as to race, r e lig io n, c o lo r or national o r ig in ; or to handicap, age, sex, or s ta tus as a disabled Vietnam-era veteran, except as provided by law. In q u irie s concerning th is p o lic y may be d ire cte d to the A ffirm a tiv e Action O ffic e r.
ABSTRACT EVALUATION OF PACKED TOWERS FOR REMOVING VOLATILE ORGANICS FROM SURFACE WATERS This study analyzes the p o te n tia l o f packed tower aeration as a remedial treatm ent process fo r the removal o f trace organics on e ith e r an acute or chronic basis. Both p ilo t-s c a le in s ta lla tio n s o f the packed tower process were reviewed. Included are basic modeling considerations fo r the removal o f v o la tile trace organics from raw water. Included also is an assessment o f the sta te o f the technology as ap plied to a i r s trip p in g o f w ater. James W. Moore Completion Report to the U. S. Department o f the In te r io r, Geological Survey, Reston, VA, June 1986. Keywords - - Packed Tow er/a eration/s urface Water/Water Treatment i
TABLE OF CONTENTS A b stra ct... Acknowledgements... Page i i i i In tro d u c tio n... 1 A. Purpose and O bjectives... 3 B. Related Research o r A c tiv itie s... 3 Methods and Procedures... 12 P rin c ip a l Findings and S ig n ifica n ce... 17 C o n c lu s io n s... 18 L ite ra tu re C ited... 20 i i
ACKNOWLEDGEMENTS The support o f the U.S. Department o f the In te r io r, Geological Survey, who provided the funds, and Dr. L e slie E. Mack, D ire c to r, Arkansas Water Resources Research Center, is g r a t e f u lly acknowledged. i i i
INTRODUCTION The presence o f various organic compounds in water supply sources has been a to p ic o f increasing concern fo r the past decade. Although the p rin c ip a l focus o f the a tte n tio n has been on surface water supp lie s because o f th e ir perceived gre a te r s u s c e p tib ility to contaminatio n by organic compounds, evidence has been accumulating th a t ground- waters are also su b je ct to contam ination. Recent sampling and analyse s o f groundwater sup plies have in d ic a te d th a t many o f these supp lie s contain a v a rie ty o f organic chemical compounds. Since about e ig h ty percent o f a ll p u b lic water supplies in the United States u t i liz e groundwater, the presence o f various organic compounds in both ground and surface w ater supplies is o f considerable concern. The organic compounds include a v a rie ty o f m a teria ls o rig in a tin g from a g ric u ltu ra l, in d u s tria l, and waste disposal a c tiv itie s as well as from o th e r sources. Although these compounds are o r d in a r ily present in small concentrations, th e ir presence may be s ig n ific a n t because o f to x ic it y or p o te n tia l c a rc in o g e n ic ity considerations and because conventional water treatm ent p la n t processes are unable to remove them. Because o f the re fra c to ry nature o f the tra ce organic compounds to most water treatm ent processes, one or more a d d itio n a l processes may be required to remove the compounds. C u rre n tly, the use o f a c tivated carbon in granular carbon columns is the most w idely recognized treatm ent procedure. For r e t r o f it t in g e x is tin g treatm ent p la n ts, the use o f the granular activa te d carbon columns w ill re quire repumping 1
the e n tire water flo w through the treatm ent p la n t unless the columns are used as the i n i t i a l treatm ent process. Usage as the i n i t i a l process is not p a r t ic u la r ly advantageous because o f the presence o f numerous oth er m a te ria ls, p a r tic u la r ly in surface water sources, which w ill decrease the e ffic ie n c y o f the granular a ctiva te d carbon c o l umns. Consequently, the use o f some other process more s u ita b le as an, or th e, i n i t i a l process o ffe rs several advantages. Packed towers may be s u ita b le fo r reducing the concentrations o f the v o la tile trace organics and may be useful as the i n i t i a l process. Packed towers have been used fo r many years in the chemical p ro cess and a ir p o llu tio n control in d u s trie s. In chemical process in d u stry a p p lic a tio n s, g e n e ra lly the concentrations are much greater than encountered in water treatm ent a p p lic a tio n s. A d d itio n a lly, the chemicals involved are u su a lly more v o la tile than in the a p p lica tio n s o f in te re s t fo r th is p ro je c t. Packed towers used in a ir p o llu tio n co n tro l a p p lic a tio n s are u su a lly used as absorbers to tra n s fe r mater ia ls from the gas stream in to the liq u id. Consequently, the use o f packed towers fo r reducing the concentrations o f v o la tile trace o r ganics in water treatm ent is a r e la tiv e ly new concept. There have been several recent in s ta lla tio n s fo r reducing the concentrations o f several organ ic chemicals in groundwater u s u a lly as a remedial procedure. The concept o f the packed tower is r e la tiv e ly sim ple. The tower b a s ic a lly consists o f a vessel f i l l e d w ith some type o f media which w ill provide a r e la tiv e ly large surface area yet which has a large 2
void r a t io. The la rg e surface area is needed to a llo w the mass tra n s fe r o f the chemical species from the gas to the liq u id (absorbers) or from the liq u id to the gas (s trip p e rs ). The larg e void ra tio is needed to provide a reasonably small headloss across the tow er. Gene r a lly, i t is also d e sira b le to se le ct a media which has a low weight to minimize s tru c tu ra l costs. A. Purpose and O bje ctives The purpose o f the research p ro je c t was to evaluate the p o te n tia l o f packed towers fo r reducing the concentrations o f selected trace organics from raw water. The p o te n tia l usefulness included both sole and combined packed tow er-granular activa te d carbon column a p p licatio n s. B. Related Research or A c tiv itie s The use o f absorbing and s trip p in g devices fo r potable water treatm ent a p p lic a tio n s in the form o f aerators has been practiced fo r many years. Examples include the absorption o f oxygen from a ir fo r o x id iz in g iro n and manganese, and the s trip p in g o f hydrogen s u lfid e, ammonia and methane from w ater. These a p p lica tio n s include a v a rie ty o f equipment such as w a te rfa ll a e ra to rs, spray nozzles, cascade aerato rs, m u ltip le tra y and p la te aerators, d iffu s io n or bubble aerators and mechanical aerato rs. For example, some water treatm ent plants have used spray systems in a founta in arrangement. The spray produces r e la tiv e ly small d ro p le ts o f water which y ie ld s a large d ro p le t surface area per u n it volume. For the purposes fo r which the spray device s have been employed, the surface area o f the d ro p le ts has been 3
adequate fo r tra n s fe rrin g oxygen to water. Although not used as fr e q u e n tly, the spray systems would be useful fo r s trip p in g h ig h ly vo la t i l e m a te ria ls, such as ammonia and methane from w ater. S im ila rly, the use o f bubble aerators fo r d iffu s in g a ir bubbles in water has been pra ctice d in some water treatm ent a p p lic a tio n s, although less fre q u e n tly. By u t iliz in g small bubbles, a re la tiv e ly la rg e mass tr a n s fe r surface area is obtained through which the contaminant can be reduced in concentration fo r s trip p in g a p p lic a tio n s, or the oxygen concentration increased fo r absorption a p p lic a tio n s. Plate and tra y towers operate on the p rin c ip le o f obtaining th in streams of w ater, and water d ro p le ts, to achieve the necessary surface area fo r tra n s fe r o f the chemical species desired. The spray, sparger, p la te tower and tra y tower systems have been used p r in c ip a lly fo r e ith e r the tra n s fe r o f h ig h ly v o la tile substances such as oxygen, ammonia, hydrogen s u lfid e and methane and/or in a p p lic a tio n s where high tra n s fe r e ffic ie n c ie s are not required. Their a p p lic a tio n fo r the s trip p in g o f moderate and low v o la t ilit y substances is lim ite d, p a rtic u la rly in instances where high removal e ffic ie n c ie s are re quire d. Packed Towers The basic concept o f packed towers is not new. U n til re cently, however, th e ir p rin c ip a l a p p lica tio n s have been fo r the tra n s fe r o f species present in r e la tiv e ly large concentrations and fo r the tra n s fe r o f h ig h ly v o la tile substances. The a p p lica tio n s have changed somewhat by the need f o r devices to absorb a ir p o llu ta n ts from v a r i- 4
ous gas streams. For example, packed towers have been u tiliz e d fo r the removal o f s u lfu r dio xid e from flu e gas streams. Although the s u lfu r d io xid e is h ig h ly v o la tile, the a p p lic a tio n o f the packed tower concept fo r the sm aller concentrations involved in tra ce organic chem icals is r e la t iv e ly new. Figure 1 shows a cro ss-se ctio n a l view o f three types o f packed towers. In most a p p lic a tio n s, the tower is randomly packed w ith a lig h t weight m a teria l which provides a large surface area, but which also has a la rg e void r a tio. The la rg e surface area is necessary to obtain the mass tra n s fe r e ffic ie n c ie s needed. The larg e void ra tio is needed to achieve the r e la t iv e ly low headlosses across the tow er. C o u n te r-c u rre n t C ross-flow C o-current Figure 1. Schematic C ross-s ectional Views Of Packed Towers. A v a rie ty o f packings have been developed over the years. T e lle - r i t e, Raschig rin g s, Berl saddles and In ta lo x saddles are examples o f packings which have been a v a ila b le fo r many years. Other packings have been developed in re ce n t years. 5
Packed Tower Design For s trip p in g a p p licatio n s in which i t is desired to remove one or more v o la tile contaminants from a s o lu tio n, the operating lin e is below the e q u ilib riu m lin e as shown in Figure 2. The s trip p in g o f carbon d io xid e, hydrogen s u lfid e, and v o la tile tra ce organics from water serve as examples o f the process. For absorption a p p lic a tio n s, the operating lin e is selected so th a t i t is above the e q u ilib riu m lin e as shown in Figure 3. The o v e ra ll approach used in designing packed towers is to d e te r mine the number o f tra n s fe r u n its required which, when m u ltip lie d by the height o f each tra n s fe r u n it, y ie ld s the o ve ra ll packing height. The c a lc u la tio n is as fo llo w s : Tower Height = HTU x NTU (1) where: HTU = the height o f each tra n s fe r u n it NTU = the number o f tra n s fe r u n its The height o f each tra n s fe r u n it can be defined fo r the gas and liq u id phases as: HTG = G = G = G (2) FGA ky a ( l- y ) im kgapt ( l - y ) im where: y = concentration o f the contaminant in the gas, mole fra c tio n G = s u p e rfic ia l molar mass v e lo c ity o f the gas ky = gas mass tra n s fe r c o e ffic ie n t a = s p e c ific in te rfa c e surface area/packed volume Fg = gas-phase mass tra n s fe r c o e ffic ie n t p = to ta l pressure 6
Figure 2. Operating and E q u ilib riu m Lines For A Packed Tower Used In A S trip p in g A p p lic a tio n. Figure 3. Operating and E q u ilib riu m Lines For A Packed Tower Used In An Absorption A p p lic a tio n. Ht L - L L = L (3) FLa kxa (1- x ) im kla c (1- x ) im where: L = to ta l molar liq u id ra te FL = liq u id -p h a se mass tra n s fe r c o e ffic ie n t kx = liq u id mass tra n s fe r c o e ffic ie n t a = s p e c ific in te rfa c e surface area/packed volume x = concentration in the liq u id, mole fra c tio n kl = liq u id m ass-transfer c o e ffic ie n t ( 1- x )im = lo g a rith m ic mean o f 1-x and 1-xi 7
Corresponding equations f o r the gas phase are: log y 1 NtG 2.303 log y2 y d log y + y -y i 1.152 log 1- y 2 1-y1 and: NtL x2 1 In 1-x1 2 l- x 2 (5) Two Resistance Theory. The o ve ra ll resistance to mass tra n s fe r between the two phases can be considered to be the sum o f the gas phase and liq u id phase re sistance s. In equation form, the expression would be as fo llo w s : T otal Resistance = Gas Phase Resistance + L iq u id Phase Resistance (6) I f the resistances are defined as the re cip ro ca ls o f the ra te constants fo r the liq u id and gas phases, the re s u ltin g equation would as fo llo w s (assuming phase e q u ilib riu m is governed by Henry's law at the in te rfa c e ): 1 = 1 + 1 (7) Ka KLa HcKGa where: K = the o ve ra ll c o e ffic ie n t Kl = liq u id phase c o e ffic e n t KG = gas phase c o e ffic ie n t Hc = H enry's constant a = s p e c ific in te rfa c e surface area/packed volume 8
The r a tio o f resistances can be given by: R l = KL H c K G a + Hc K g ( 8 ) L and: R l = ( 1 + K C ) - 1 ( 9 ) Rt h c k g I f the term RL/R G is very much g reate r than 1 the gas phase re sista n ce can be ignored in some circum stances. I f the resistances are approxim ately o f the same magnitude, both must be considered. Models For P re d ictin g Mass Transfer Rate Constants Several models have been developed fo r p re d ic tin g mass tra n s fe r ra te constants. Among these are the Sherwood-Holloway, Shulman and Onda models. re sista n ce. The Sherwood-Holloway model neglects the gas tra n s fe r Thus, i t would be considered a one resistance model. The Shulman and Onda models evaluate both the gas phase and liq u id phase re sistances and, consequently, are considered tw o-resistance models. Sherwood and Holloway evaluated the d e sorp tion o f oxygen, hydrogen and carbon dio xid e from water in countercurrent flo w packed c o l umns using a v a rie ty o f packing m a te ria ls over a r e la tiv e ly wide range o f liq u id and gas flo w ra te s. Since the solutes were a ll gases w ith r e la tiv e ly large Henry's constants, the liq u id resistance contr o lle d mass tra n s fe r ra th e r than gas re sistance. Consequently, the tra n s fe r ra te constant was c o rre la te d w ith the liq u id ra te and liq u id -p h a se p ro p e rtie s only as shown: 9
kla Dl 10.746«(0. 3 0 4 8 L M u L ( 10) where: D = m olecular d if f u s iv it y y = f lu id v is c o s ity p = f lu id density a & n are e m p iric a lly determined packing parameters The Shulman model involves separate estim ation o f Kl, KG and a. For the liq u id phase c o e ffic ie n t, a re la tio n s h ip between the Sherwood number, the Reynolds number and the Schmidt number was developed: m. Y* Dl V / v*a./ (11) where: = liq u id phase c o e ffic e n t DL = m olecular d if f u s iv it y ds = diameter o f a sphere having the same surface area as a u n it o f packing LM = liq u id flow ra te y. = f lu id v is c o s ity = f lu id density The gas phase c o e ffic ie n t is shown as fo llo w s : M. (d.g u Y "/ V ( 12) where: KG = gas phase c o e ffic ie n t ds = diameter o f a sphere having the same surface area as a u n it o f packing GM = gas flow rate ug = flu id v is c o s ity PG = flu id density DG = molecular d iffu s iv ity Both the gas and liq u id phase c o e ffic ie n ts involve re la tio n s h ip s between the same dim ensionless groups, the Sherwood, Reynolds and 10
Schmidt numbers. However, the liq u id phase c o e ffic ie n t is p ropo r tio n a l to the square ro o t o f the m olecular d if f u s iv it y, whereas the gas phase c o e ffic ie n t is p ro p o rtio n a l to the 2/3 power o f the molecula r d if f u s iv it y. The Onda model also e n ta ils separate estim ation o f KG, Kl and a. For th is model, the s p e c ific in te rfa c ia l area is the s p e c ific wetted packing area, aw. The s p e c ific wetted packing area is estimated as a fu n c tio n o f the liq u id flo w ra te, packing p ro p e rtie s, and liq u id p ro p e rtie s. where: at = to ta l s p e c ific surface area o f packing = c r it ic a l surface area o f packing PL = 1iq u id density HL = liq u id v is c o s ity a L = liq u id surface tension The Reynolds, Froude and Weber numbers are included in the equatio n fo r e stim a tin g the s p e c ific wetted packing area. The c o rre la tio n fo r the liq u id phase c o e ffic ie n t is shown as fo llo w s : (5 (14) where: Kl = liq u id phase c o e ffic e n t dp = nominal packing size p [ = liq u id density UL = liq u id v is c o s ity Lm = liq u id flow ra te aw = s p e c ific wetted packing area Dl = m olecular d if f u s iv it y 11
For the gas phase c o e ffic ie n t, the re la tio n s h ip is as fo llo w s : ho _ J C V Y "o Y ' \ ^,, = 6.231 ------ I I I (ojf/p)-* t^o \ av'o/ \P 0 ^0 / (15) where: KG = gas phase coefficient at = total specific surface area of packing DG = molecular d iffu sivity = gas flow rate Pq = gas viscosity Pq = gas density dp = packing diameter In a d d itio n to these models, a v a rie ty o f other s in g le resistance models has been developed. METHODS AND PROCEDURES Both lite r a tu r e review and la b o ra to ry resources were used in eval uating the p o te n tia l o f packed tower aeratio n fo r s trip p in g organic chemicals from w ater. The methods c u rre n tly a v a ila b le to lim it organ ic chemical concentrations in water supplies can be divided in to two p rin c ip a l ca te g o rie s. These are: 1) co n tro l by management o f the resource, and 2) co n tro l by reduction in the water treatm ent process. Dyksen and Hess, 1982, id e n tifie d three management techniques fo r reducing or e lim in a tin g the compounds from the water source. These are: 1) e lim in a tio n o f the source o f the compound; 2) lo c a tio n o f a new water supply source; and 3) blending o f e x is tin g water supply sources. The f i r s t technique u su a lly requires re g u la to ry a c tiv itie s above the lo c a l le v e l. The rem aining two techniques are s ite spec i f i c in th e ir a p p lic a tio n and are, thus, dependent on the a v a ila b il- 12
it y o f a new water supply source, the a v a ila b ility o f other water sources o r both. With respect to co n tro l o f organic chemical concentrations by tre atm e nt, three general approaches are a v a ila b le. These are s t r ip ping o f the organic chemicals by a e ra tio n, adsorption o f the organic chem icals, and combined systems in clu d in g both aeration and s t r ip ping. H is to r ic a lly, aeration equipment used in water treatm ent has been e ith e r w a te rfa ll or spray aerators and d iffu s e d a e rato rs. Included in the w a te rfa ll or spray aerato r category are m u ltip le tra y, cascade, spray nozzles and packed column aerato rs. These operate on the p rin c ip le o f cre a tin g d ro p le ts, th in streams or th in film s o f water surrounded by a ir. The o b je c tiv e is to develop a larg e surface area o f exposure between the a ir and water through which the contaminant can be s trip p e d. The d iffu s e d a ir systems operate on the same p r in c ip le o f developing a large surface area fo r mass tra n s fe r except th a t a ir is bubbled through the water column. The mass tra n s fe r e ffic ie n c ie s o f aeration equipment used in water treatm ent fo r both s trip p in g and absorption have not been o f great concern fo r several reasons. Among these are the re la tiv e ly low c a p ita l and operating costs o f the equipment and the h ig h ly vo la t i l e nature o f the substances which have o rd in a rily been tra n s fe rre d. For example, aeration is fre q u e n tly used fo r tra n s fe rrin g oxygen from the a ir in to water fo r o x id iz in g iro n and manganese. Since oxygen is very v o la tile and is present in large concentrations in the a ir, the 13
tra n s fe r e ffic ie n c ie s have been w ith in acceptable ranges even fo r the less e ffe c tiv e processes. S im ila rly, aeration can be used to s tr ip ammonia, hydrogen s u lfid e, carbon d io xid e, and methane from water. These are also very v o la tile substances. e ff ic ie n c ie s have also been acceptable. Consequently, the tra n s fe r Many o f the organic chemicals which may re q u ire removal have e ith e r interm ediate or low vo la t i l i t i e s. Consequently, g re a te r mass tra n s fe r e ffic ie n c ie s w ill be required fo r acceptable performance. These greate r mass tra n s fe r e ffic ie n c ie s w ill not only re q u ire optimal design o f the aeration process, but w ill e lim in a te many o f the aeration processes tr a d itio n a lly used. With respect to operating c h a ra c te ris tic s, the two extremes in aeratio n process design are the d iffu s e d aerator and the packed tower a e ra to r. Both have the c a p a b ility o f achieving the large surface to volume re la tio n s h ip s and the detention times required fo r more e f f i c ie n t mass tra n s fe r. However, the d iffu s e d aerator accomplishes the task by in je c tin g a ir bubbles in a column o f w ater. Mass tra n s fe r o f the substances occurs as the bubbles ris e to the surface. The sm aller the bubble size, the la rg e r the area to volume re la tio n s h ip and, g e n e ra lly, the more e ffic ie n t the process. is a fu n c tio n o f the depth o f the water column. o f time the a ir bubbles are in the water column. The detention time That is, the length W ithin lim its, the mass transfer efficiency is a fu n ctio n o f the detention tim e. The p rin c ip a l disadvantage o f the d iffu s e d aeration process fo r removal o f moderate and low v o l a t i l i t y substances is the pressure 14
drop across the process. That is, the a ir must be compressed s u f f i c ie n tly to overcome the head o f the column o f w ater. Thus, as the detention tim e is increased to obtain g reate r e ffic ie n c ie s, the pressure losses also increase re s u ltin g in r e la tiv e ly larg e power costs. The process does have the inherent advantage o f keeping suspended m a te ria ls in suspension because o f the turbulence created in the basin. Thus, a p p lic a tio n o f the process on r e la tiv e ly tu rb id surface w ater sources does not cre a te s e t t lin g problems. The packed tower process o ffe rs several advantages fo r s trip p in g a p p lic a tio n s. These include the r e la tiv e ly low pressure loss across the tower, larg e surface area to volume ra tio s, v a ria b le flo w rate ranges, and corrosion re s is ta n t packings. Usually the tower is operated in the countercurrent flo w mode w ith the water flo w in g down through the tower by g ra v ity and the a ir forced upward through the tower by pressure d if f e r e n t ia l. Several fu ll- s c a le packed tower systems have been co n structed and operated fo r the re d u c tio n in concentr a tio n o f organic substances in groundwater. S c h illin g, 1985, described a packed tower a p p lic a tio n fo r the reduction o f 1,1,2,2 -te tra - chloroethane, 1,2 -tra n s -d ic h lo ro e th y le n e, tric h lo ro e th y le n e and te tra ch lo ro e th yle n e in Washington. He reported removal e ffic ie n c ie s o f about n in e ty -fiv e percent fo r the 1,1,2,2 -te tra ch lo ro e th a n e and e s s e n tia lly complete removal o f the other v o la tile organic substances. The 1,1,2,2 -te tra ch lo ro e th a n e concentrations in the contaminated water were in the 17 to 300 parts per b illio n range. This a p p lic a tio n was designed to block a plume o f contaminated w ater from 15
flo w in g in to the w ell f ie ld used as a source o f supply fo r the C ity o f Tacoma. A s im ila r a p p lic a tio n fo r groundwater q u a lity co n tro l was conducted by the U.S. A ir Force at Wurtsmith A ir Force base to control a plume o f contaminated groundwater, Houel, e t a l., 1979. Several la b o ra to ry stu d ie s have been conducted using packed towers fo r s trip p in g various organic chemicals from w ater. Houel, et a l., 1979, reported the re s u lts o f s trip p in g studies o f chloroform. They reported removal rates o f 97.5 percent and g reate r w ith residual concentrations o f less than 0.2 micrograms per l i t e r. S ta llin g s, Rogers and M u llin s, 1984, reported the re s u lts o f a fie ld p ilo t-s c a le in v e s tig a tio n. They reported removal e ffic ie n c ie s o f g reate r than n in e ty percent fo r groundwater containing sixteen d iffe r e n t v o la tile organic chemicals, in clu d in g hydrocarbons, ch lo rin a te d organics and arom atics. They concluded th a t the se le ctio n o f the "best" packing m aterial would not be s o le ly be based on performance c r it e r ia, but ra th e r would invo lve a comprehensive economic analysis to compare associated system c a p ita l and operating costs. Umphres, et a l., 1983, presented the re s u lts o f two p ilo t studies which evaluated t r i - halomethane removal by packed tower a e ra tio n. Packed Tower Design Although a v a rie ty o f numerical models have been developed fo r packed tower design, th e ir development was la rg e ly based on more v o la tile m a te ria ls than the tra ce organic chemicals o f in te re s t in t h is stud y. For example, Sherwood and Holloway stu d ie d the s trip p in g 16
o f hydrogen, carbon dio xid e and oxygen in countercurrent flo w packed columns. These gases a ll have H enry's constants g re a te r than one. For substances w ith sm aller Henry's constants, however, the gas phase re sistance ra th e r than the liq u id phase resistance may c o n tro l. Gene r a lly, a p p lic a tio n s o f the two-dimensional models have been more successful than the liq u id phase re s is ta n c e one-dimensional models. At the present tim e, p ilo t-s c a le studies are o r d in a rily conducted to develop design data fo r fu ll- s c a le systems when e ith e r large removal e ffic ie n c ie s are required or when interm ediate or low v o la t ilit y substances are to be removed. A v a rie ty o f packing m a teria ls have been developed over the years fo r packed towers. These include Raschig rin g s, p a ll rin g s, Berl saddles, In ta lo x saddles, T e lle r it e, as w e ll as newer packings. Saddle packing was used in the Tacoma, Washington system, S c h illin g, 1985. P all rin g s were used in the Wurtsmith A ir Force base ground- water cleanup system. Packing se le ctio n is s t i l l a fu n ctio n o f the p ilo t-s c a le studies in clu d in g the associated economic considerations. PRINCIPAL FINDINGS AND SIGNIFICANCE The concept of using packed towers f o r removal o f selected v o la t i l e organic chemicals as p a rt o f the water treatm ent process is a v a lid one. Whether the process is used alone, or in conjunction w ith carbon (o r o th e r) a b so rp tio n, is dependent on both the organic chemicals to be removed and site specific considerations. Although not the sole determ inant o f the removal c h a ra c te ris tic s o f a s p e c ific organic chemical, the v o la t ilit y o f the substance is usu a lly an 17
important consideration. Henry's constants have not been reported fo r several of the organic substances of in te re st. Procedures fo r determining Henry's constants have been id e n tifie d. The design o f fu ll-s c a le packed towers fo r removal o f interm ediate and low v o la tility organic chemicals should be based on p ilo t- scale data at the present time. This includes packing selection as well as the other design parameters. Additional data is needed to allow fu ll-s c a le design from existing one- and two-dimensional models fo r economically e ffic ie n t design. Additional information is also needed concerning the d if f ic u lt ie s which may be encountered in applying the packed tower process as the in it a l treatment process in re la tiv e ly turbid waters. CONCLUSIONS The packed tower process has considerable potential fo r use in removing v o la tile organic chemicals in water as part of the treatment process. The application o f the process is necessarily a s ite spec if ic consideration. Depending on the specific circumstances, the process may be used either in conjunction with granular activated carbon columns or as a stand alone process fo r stripping certain substances from the water. At the present time, p ilo t scale studies are needed to determine design c r it e r ia fo r fu ll-s c a le systems when substances of intermediate and low volatility are to be stripped. When the process is applied corre ctly, high removal efficie ncies have been obtained at re la tiv e ly low cost. A major advantage o f the process is the re la- 18
tiv e ly low cap ital and operating cost fo r circumstances where volat i l e organic substances must be ro u tin e ly stripped. 19
LITERATURE CITED 1. Dyksen, J.E. and A.F. Hess, I I I. 1982. "A lte rn a tiv e s For C o n tro llin g Organics In Groundwater S upplies". Journal American Water Works A ssociation. 2. S c h illin g, R.D., 1985. " A ir S trip p in g Provides Fast S o lu tio n For P olluted Well Water". P o llu tio n Engineering. 3. Houel, N., F.H. Pearson and R.E. S elleck. 1979. "A ir S trip p in g Of Chloroform From Water". Proceedings Environmental Engineering D iv is io n, ASCE. 4. S ta llin g s, R.L., T.N. Rogers and M.E. M u llin s. 1984. "A ir S trip p in g Of V o la tile O rganics". P ro ce e d in g s-in stitu te Of Environmental Sciences. 5. Umphres, M.D., C.H. Tate, M.C. Kavanaugh and R.R. Truse l l. 1983. "Trihalomethane Removal By Packed Tower A e ra tio n ". Journal American Water Works A ssociation. 20