Environment International, Vol. 9, pp. 363-368, 1983
0160-4120/83 $3.00 + .00 Copyright © 1984 Pergamon Press Ltd.
Printed in the USA. All rights reserved.
REMOVAL OF TRIHALOMETHANE PRECURSORS BY REVERSE OSMOSIS Talbert N. Eisenberg Department of Civil Engineering, Tennessee Technological University, Cookeville, Tennessee 38505, USA
E. Joe Middlebrooks Provost and Vice President for Academic Affairs, Tennessee Technological University, Cookeville, Tennessee 38505, USA (Received 1 June 1983; Accepted 12 October 1983) The effect of pre- and postchlorination on reducing trihalomethane (THM) precursors by reverse osmosis (RO) was investigated. Prechlorination of the feedwater, in contrast to postchlorination of the permeate, resulted in improved reduction of maximum total THM potential (MTP) due to adsorption. No significant difference in permeate MTP concentration occurred between the prechlorinated feed and the postchlorinated permeate. Recirculating permeate water through the RO unit was successful in restoring flux rates and resuspending the precipitate that accumulated on the RO membranes. The small loss in free chlorine residual indicated prechlorination may be economically attractive in controlling biological fouling of cellulose acetate membranes. Reverse osmosis was not reliable in removing THM precursors and failed to produce a permeate with MTP concentration less than the maximum contaminant level of 0.10 mg/L.
1979), little work has been done on investigating the effect of reducing THM precursors by reverse osmosis. Three experiments were conducted at a chlorine dose of 16 rag/L, one with a 30-min contact time and two with a 1-week contact time. Two experiments were performed with chlorine dosages of 2.4 mg/L with 1-h contact time, and three experiments were conducted with a chlorine dose of 1.3 mg/L and a 1-h contact time. The pH value of the feedwater was lowered to between 5.4 and 5.9 with reagent ACS grade concentrated sulfuric acid. Feed and permeate water were chlorinated with a 5.25°7o sodium hypochlorite solution (Clorox) Samples were collected from the feed, permeate, and concentrate streams. A mass balance of the electroconductivity of the feed, permeate, and concentrate streams was conducted to check for steady-state conditions before sampling was started. The passage of 151-189 L (40-50 gal) of water through the unit (or about 40 min of operation) was sufficient to allow adequate purging of the system. Samples were tested for pH value, free and total chlorine residuals, electroconductivity (EC), total organic carbon (TOC), and maximum total trihalo-
Since the promulgation of regulations by the U.S. Environmental Protection Agency (U.S. EPA) for the control of trihalomethanes (THMs) in drinking water (U.S. EPA, 1979) different treatment alternatives have been considered to produce a water that will meet the maximum contaminant level (MCL) of 0.10 mg/L for total trihalomethanes (TTHMs). Basic approaches for reducing THMs have been removing THM precursors before chlorination, reducing chlorine dosage, changing the point of chlorination, the use of disinfectants other than chlorine, and removing THMs after they are formed. Methods suggested for reducing precursors have been adsorption on powdered (Anderson et aL, 1981) or granulated activated carbon (Symons, 1978), chlorine dioxide, ozonation (Siemak et aL, 1979), sand filtration (Glaser and Edzwald, 1978), aeration, and coagulation (Babcock and Singer, 1979; Kavanaugh, 1978). One approach which has not received much attention is reverse osmosis. While halogenated aliphatics and THMs have been shown to be ineffectively removed by reverse osmosis (Hrubek et al., 1979; Wojcik et al., 363
methane potential (MTP). Pump displacement pressure, system feed pressure, concentrate pressure, permeate flow rate, and concentrate flow rate were monitored.
RO unit The reverse osmosis unit contained six pressure vessels arranged in series, each vessel containing three membrane modules. The membrane modules were 5.08-cm (2-in.) diam. spiral wound cellulose acetate with a 90% NaC1 rejection and a flux rate of 0.37 m3/m2.d (9 gfd) at 2758 kPa (400 psig). New water entering the RO unit was filtered through a 25-#m cartridge filter. The objective of this study was to investigate the effect of pre- and postchlorination on T H M formation and the ability of reverse osmosis to remove precursors and THMs. Maximum total T H M potential (MTP) was used as the precursor measurement. It was hypothesized that chlorination of the permeate would result in lower MTPs, because the precursors would be rejected by the reverse osmosis membrane due to molecular weight. In contrast, the chlorination of the feedwater was expected to form THMs which would pass through the reverse osmosis membrane into the permeate.
Equipment and Procedures Source o f water Groundwater from an artesian well with a depth o f 107 m (350 ft) was the source o f water for the laboratory studies. The water is anaerobic, containing methane and hydrogen sulfide. It is unlikely that the well would be used for drinking water in its present state. However, the water was expected to be a good source for experimental testing, due to the expected stability and reproducibility o f the groundwater with respect to concentration of organics and inorganics over time. T H M concentrations in some surface waters have been shown to vary over a 24-h sampling period from day-to-day (Smith et al., 1980).
Experimental procedure Experimentation ran from October 12 to November 10, 1981, during which time fourteen batch experiments were performed. For eight of the experiments, the feedwater was prechlorinated. For six of the experiments, the feedwater was unchlorinated, and the permeate or product stream was chlorinated.
System performance New membranes were installed prior to starting the laboratory studies. The reverse osmosis unit ran without problems from October 12 to November 3; after that date, the system feed pressure would not rise above 345 kPa (50 psig). The manufacturer (Fisher, 1981) suggested that a blown membrane or damaged O-ring was responsible for the pressure loss. The permeate collector manifold was removed and the unit was started. Prod-
T.N. Eisenberg and E. J. Middlebrooks
uct flow was uniform in all six pressure vessels. Nevertheless, the modules were then removed and inspected for damaged O-rings. One O-ring was suspect and was replaced. The 18 modules were reinstalled, the pressure relief valve was taken apart and reassembled, the unit was reassembled, and normal operation returned. Membrane flux began to decline on November 8, with permeate flow declining from 151 to 132 L / h (40 to 35 gph). Organic fouling was suspected; a 0.25% by weight solution of "Biz" laundry detergent was circulated through the unit at a system feed pressure of 345 kPa (50 psig) for l h. No improvement in flux was observed. The end caps were removed, and a white precipitate was found on the inside of the end plates and on the O-rings on the last two pressure vessels. Calcium sulfate precipitation was suspected because gypsum (CaSO4 + 2H20) was present in the groundwater and because o f the use of sulfuric acid to lower the feed pH value. Cleaning solutions were considered to remove the calcium sulfate fouling; however, the manufacturer (Fisher, 1981) felt that cleaning solutions were generally ineffective in removing calcium sulfate and suggested that product water be recirculated through the system. Upon recirculating product water, the precipitate was dissolved, and normal operation returned. Sodium hexametaphosphate was considered as an additive to the feedwater to inhibit scaling, but was rejected to avoid altering the experimental procedure near the end of the study. Instead the unit was back flushed between tests with product water until the reject EC approached the product water EC. This method was successful, and no further problems were encountered. System feed pressure ranged from 2620 to 2826 kPa (380-410 psig), and, excluding the precipitate problem, product flow rate was kept at 151 L / h (40 gph) with 73 percent recovery. The pH value of the product water averaged 0.40 pH units less than that observed in the feedwater, due mainly to diffusion of carbon dioxide through the membrane. Chlorination of the feedwater resulted in little residual lost in the product water. An average of 75°70 of the total available chlorine and 76% of the free available chlorine in the feedwater diffused through the membrane into the product water. The small loss in chlorine residual makes prechlorination attractive as a treatment procedure. Feedwater EC averaged 1300 + 75 ~mhos/cm, with a range from 1200 to 1450 ~mhos/cm. EC rejection remained between 80°70 and 84%. Chlorination had no effect on EC rejection. A material balance indicated that a low percentage of inorganics, possibly calcium sulfate, was adsorbed on the membrane. If an RO system was operated continuously rather than periodically, as done in these experiments, the need for a scale inhibitor becomes apparent. The consistent EC rejections over time were the immediate assurance of proper system performance for removal of inorganics.
Removal of trihalomethane precursors
500" 500 ._.d 733
Fig. 1. M T P vs. T O C for chlorinated feedwater.
Fig. 3. M T P vs. T O C for permeate o f chlorinated feedwater.
TOC and M T P removals Results of TOC and MTP removals are shown in Figs. 1-12. TOC concentrations in the unchlorinated feed averaged 3.8 4- 0.5 m g / L , with a range from 3.3 to 4.7 mg/L. TOC concentrations in the chlorinated feed averaged 3.7 4- 0.2 m g / L , with a range from 3.4 to 4.2 m g / L . TOC concentrations in the permeate from the unchlorinated feed averaged 2.0 4- 0.4 m g / L with a range from 1.2 to 2.3 mg/L. TOC concentrations in the permeate from the chlorinated feed averaged 1.3 4-
0.9 m g / L , with a range from 0.6 to 3.0 mg/L. TOC removals from the unchlorinated feed averaged 47% 410°70, with a range from 37°70 to 65°70. TOC removals from the chlorinated feed averaged 65°70 4- 22°70, with a range from 20% to 86%. TOC adsorbed from the unchlorinated feed averaged - 6 % 4- 15%, with a range from - 2 4 % to 14%. TOC adsorbed from the chlorinated feed averaged 8% 4170/0, percent with a range from - 2 2 % to 27°70. Chlorination improved the removal of TOC and the adsorption
TOC mg/L Fig. 2. M T P vs. T O C for unchlorinated feedwater.
Fig. 4. M T P vs. T O C for permeate of unchlorinated feedwater.
T.N. Eisenbergand E. J. Middlebrooks
2000 45 o'1
TO C mg/L
Fig. 5. MTP vs. TOC for concentrate of chlorinated feedwater.
Fig. 7. MTP removal vs. MTP adsorption for chlorinated feedwater. of TOC onto the membrane. A linear regression of TOC removal vs. TOC adsorption (Fig. 9) for the chlorinated feed produced a linear relationship with a correlation coefficient r of 0.90; an F test showed that the regression was significant at the 95% confidence level. TOC removals appeared to be caused by adsorption onto the membrane rather than rejection into the concentrate stream. For both chlorinated and unchlorinated feeds, TOC was desorbed and released into the permeate. MTP (the T H M precursor measurement) concentrations in the unchlorinated feed averaged 237 ± 129
t~g/L, with a range from 85 to 420 t~g/L. MTP concentrations in the chlorinated feed averaged 394 ± 97 t~g/L, with a range from 227 to 550 tzg/L. MTP concentrations in the permeate from the unchlorinated feed averaged 293 ± 193 #g/L, with a range from 62 to 482 #g/L. M T P concentrations in the permeate from the chlorinated feed averaged 283 ± 141 t~g/L, with a range from 103 to 575 /zg/L. The results were the opposite of what was expected. There was no significant difference in permeate MTP concentrations in the pre-
s_ I 0 0 0
n F- 100
Fig. 6. MTP vs. TOC for concentrate of unchlorinated feedwater.
Fig. 8. MTP removal vs. MTP adsorption for unchlorinated feedwater.
Removal of trihalomethane precursors
Fig. 9. TOC removal vs, TOC adsorption for chlorinated feedwater.
and postchlorinated treatments. Fulvic and humic acids were probably not the T H M precursors, because they would have been rejected by the RO membranes due to molecular weight, and the permeate from the unchlorinated feedwater would be lower in M T P concentration. M T P removal f r o m the unchlorinated feed averaged - 3 8 % ± 104%, with a range from - 2 0 0 % to 57%. M T P removal for the chlorinated feed averaged 26% + 40%, with a range f r o m - 6 4 % to 56%. Prechlorination improved M T P removal; however, in one instance, enrichment of M T P in the permeate occurred. M T P adsorbed from the unchlorinated feed averaged - 142% +
% TOC Removed Fig. 11. MTP removal vs. TOC removal for chlorinated feedwater.
209°70, with a range f r o m - 4 9 0 % to 58%. M T P adsorbed from the chlorinated feed averaged 9°70 + 14%, with a range from - 13% to 30%. Prechlorination increased the adsorption of M T P onto the m e m b r a n e and lessened the likelihood of M T P being desorbed from the m e m b r a n e and released into the permeate. A linear regression of M T P removal vs. M T P adsorption (Fig. 8)
..~ Q) o
F Ca) n~
(~ 0 I--
55 45 -I00 35
% TOC Adsorbed Fig. 10. TOC removal vs. TOC adsorption for unchlorinated feedwater,
% TOC Removed Fig. 12. MTP removal vs. TOC removal for unchlorinated feedwater.
368 for the u n c h l o r i n a t e d feed p r o d u c e d a c o r r e l a t i o n coefficient r o f 0.97, a n d a n F test s h o w e d that the regression was significant at the 95070 c o n f i d e n c e level. R e l a t i o n s h i p s between T O C a n d M T P were e x a m i n e d for b o t h c h l o r i n a t e d a n d u n c h l o r i n a t e d feeds. F o r the p e r m e a t e o f the c h l o r i n a t e d feed, a linear regression o f M T P vs. T O C (Fig. 3) s h o w e d a direct r e l a t i o n s h i p (r = 0.85), a n d a n F test s h o w e d t h a t the regression was significant at the 95°70 c o n f i d e n c e level. F o r the chlor i n a t e d feed, a linear regression o f M T P r e m o v a l vs. T O C r e m o v a l (Fig. 11) s h o w e d a direct r e l a t i o n s h i p (r = 0.82), a n d an F t e s t s h o w e d t h a t the regression was significant at the 95°70 c o n f i d e n c e level. T h e q u e s t i o n r e m a i n s as to w h y p r e c h l o r i n a t i o n imp r o v e d M T P r e d u c t i o n . O n e e x p l a n a t i o n lies in the m a t e r i a l b a l a n c e . A d s o r p t i o n o f T H M p r e c u r s o r s was i m p r o v e d when N a O C L was i n t r o d u c e d into the feed. C h l o r i n a t i o n m a y increase t h e p o l a r i t y o f the p r e c u r s o r s a n d aid in a d s o r p t i o n . A s u r f a c e p h e n o m e n o n on the m e m b r a n e m a y be r e d u c i n g the p o r e size o p e n i n g o f the m e m b r a n e . T h e m o s t i m p o r t a n t p o i n t is t h a t in n o instance d i d the p e r m e a t e satisfy the M C L o f 100 # g / L .
Conclusions l. R e c i r c u l a t i n g p e r m e a t e w a t e r t h r o u g h the reverse o s m o s i s unit at low p r e s s u r e was successful in r e s t o r i n g flux rates a n d r e s u s p e n d i n g the p r e c i p i t a t e t h a t acc u m u l a t e d in the R O unit. 2. W h e n p r e c h l o r i n a t i o n was p r a c t i c e d , 75°70 o f t h e free c h l o r i n e residual in the feed d i f f u s e d t h r o u g h the m e m b r a n e into the p e r m e a t e . T h e small loss in c h l o r i n e residual m a k e s p r e c h l o r i n a t i o n a n e c o n o m i c a l l y a t t r a c tive m e a n s o f c o n t r o l l i n g b i o l o g i c a l fouling o f cellulose acetate m e m b r a n e s . 3. E l e c t r o c o n d u c t i v i t y c a n n o t be used to m e a s u r e system p e r f o r m a n c e f o r r e m o v a l o f organics. 4. P r e c h l o r i n a t i o n , in c o n t r a s t to p o s t c h l o r i n a t i o n , i m p r o v e d the r e m o v a l o f T O C (65°70) a n d the a d s o r p t i o n o f T O C o n t o the m e m b r a n e . 5. T O C r e m o v a l was d i r e c t l y r e l a t e d to T O C a d s o r p tion for the c h l o r i n a t e d feed, a n d the regression was significant at the 95°70 c o n f i d e n c e level (r = 0.90).
T.N. Eisenberg and E. J. Middlebrooks 6. N o significant d i f f e r e n c e in p e r m e a t e M T P conc e n t r a t i o n (283 vs. 293 # g / L ) o c c u r r e d b e t w e e n the c h l o r i n a t e d a n d u n c h l o r i n a t e d feed. 7. P r e c h l o r i n a t i o n i m p r o v e d M T P r e m o v a l a n d M T P a d s o r p t i o n ; h o w e v e r , p e r m e a t e e n r i c h m e n t did occur in one instance. 8. Reverse o s m o s i s was not reliable in r e m o v i n g T H M p r e c u r s o r s a n d failed to p r o d u c e a p e r m e a t e with an M T P c o n c e n t r a t i o n less t h a n the M C L .
Acknowledgements--The work described herein was completed while
both authors were employed at Utah State University, Logan, UT.
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