TTHMs and Bacteriological Water Quality in Desalinated Seawater ...

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TTHMs and Bacteriological Water Quality in Desalinated Seawater: Experience And Ongoing Research in the US Virgin Islands H A Minnigh1, Annelise Knudsen2, Greg Rhymer2, Remy-Martín Ramírez2, Werner Wernicke2, Mike Quetel2 and Harold Mark3 1- RCAP Soutions, Inc, PO Box 48, Lajas, PR 00667, [email protected], 2-USVI Water and Power Authority, 3-Division of Environmental Protection of the Department of Planning and Resources of the US Virgin Islands ABSTRACT Islands and coastal regions throughout the world are turning increasingly to desalination of seawater for supply. For example, Tampa, FL has a plant just entering production and San Diego, CA and Texas others under consideration. While much of this desalination relies on membrane processes, sites where electrical power is steam-generated are uniquely suited to distillation processes. The US Virgin Islands has relied for desalination for much or most of their water needs for about 50 years. In the past 5 years total trihalomethanes approached MCL levels on one of the islands and the Water and Power Authority of the USVI (WAPA) initiated studies to address the problem, elucidate the sources and causes and reduce concentrations of these compounds in the distribution system. KEYWORDS TTHMs, Desalinated Water, TTHMs and Bacteriological Water Quality INTRODUCTION WAPA has operated hypobaric distillation desalination plants on the three major islands for over 15 years.[1] The two major plants provide potable water for populations of about 50,000 per sons. In the last two years one of the islands experienced a sudden increase in TTHMs at the long-residence-time sites (LRT – see). This increase was roughly concurrent with a

Copyright ©2006 Water Environment Foundation. All Rights Reserved

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Figure 1. St. Thomas, with approximate area with most LRT sites. number of other changes, including several in disinfection practices driven in part by concerns over the maintenance of bacteriostatic disinfectant residuals throughout the systems. The increase would have violated limits imposed by the Stage 1 Disinfectant-Disinfectant By-Product Rule. Further, and unlike TTHMs in most water systems, in the WAPA system average concentrations were between 30 and 50% bromoform, with up to 90% brominated species.[2] Illustrative results are presented at Table 1, with comparative results at Table 2. A strategy developed to identify the source or cause of the unusual TTHM distribution and to address the concomitant problems of maintaining bacteriostatic residuals and reducing TTHMs to levels compliant with current and upcoming MCLs. The possible factors contributing to the problem are presented and results to date reviewed. Those factors included increased residence times due to a successful leak reduction program, reduced area usage, a recent change to corrosion control applied and higher send-out residual concentrations to ensure bacteriostatic disinfectant residuals through the system. These problems parallel the concerns that moved US EPA to manage bacteriological water quality and disinfection by-products concurrently.


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Table 1. Bromoform and chloroform in WAPA TTHMs before 2004 (typical). time (days)

Bromoform (μg/L)

% TTHM

Chloroform (μg/L)

% TTHM

Total THM (μg/L)

0.2 1 2 2 5 7 12

0.63 1.2 1.8 1.1 9.6 82 120

16% 12% 13% 10% 16% 79% 67%

2.1 1.8 3.1 1.1 6.8 3.2 1.2

53% 17% 22% 10% 12% 3% 1%

4 10 14 11 58 104 179

Table 2. New Jersey, all at T >10ºC[3] time (days)

Bromoform (μg/L)

% TTHM

Chloroform (μg/L)

% TTHM

Total THM (μg/L)

0 2 >3

8 8.8 9.2

24% 23% 22%

0.27 0.16 0.28

1% 0% 1%

33 38 42

STRATEGY THMs had been at 20-40 μg/L levels (well under the MCL) for over 10 years at the highestresidence-time (LRT) sites on St Thomas (STT) and THMs in excess of the MCL were first detected about June, 2001. The earliest cause suspected was the long residence time in the Donoe tank and East End distribution system, a recent development following the closure of a major user on the East End, accounting for about 35-50% of daily usage. Over the next year it became apparent that THM levels were elevated throughout the system, but not in a manner that suggested that residence time was a primary cause, at least as residence time in the the system was understood at that time. In distribution systems the causes of elevated THM concentrations are generally, more or less in descending order of import: 1. Chlorine concentration. 2. Temperature (essentially constant for WAPA, not a factor with the possible exception of extended storage in Donoe Tank). 3. Presence and concentration of precursors - Organic carbon (TOC, DOC, AOC or other measurements) these may be a factor but expected to be mediated only by aftergrowth in the STT distribution system. Copyright ©2006 Water Environment Foundation. All Rights Reserved

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4. Residence time 5. Bromide (for bromoform and brominated species formation) The strategy developed consisted of the following stages or subjects for study and correction: • Identify the source of the bromoform and brominated species and correct or adapt operations to reduce these, if possible. • Identify the reason or reasons for the increase in TTHMs and correct or adapt operations, while maintaining the large reserve supplies required by climatologic and geologic threats to the Islands. I.e., reducing residence time cannot be the sole solution. • Maintain bacteriological water quality. • Identify and correct high disinfectant demand or excessive residence times throughout the system in order to maintain bacteriostatic residuals. RESULTS TO DATE Bromoform In studies conducted by a contractor and by the Authority it became apparent much or most of the bromine was associated with the use of electrolytically-generated mixed oxidants (MOs) for

Mean mg/L in combined effluents

.5

.4

.3

.2

.1 Cl 0.0

Br ON

OFF

Figure 2. Chlorine and Bromine in effluents from all desalination units in operation. disinfection in process water. Comparative concentrations for periods with the generator on and off are presented in Figure 2 and - . In - we can see the bromine concentration when the electrolytic oxidant generator was turned off at 0800 and back on at 1200; the concentration of about 0.9 mg/L was typical of this unit with the desalination equipment in use at that time; i.e., a single 2 MGD unit.


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Figure 3. Bromine concentrations immediately after generator turned-off and on. 1.0

Mean Br, mg/L

.8

.6

.4

.2

0.0 8:10

9:00

9:30

10:00

10:30

11:00

11:30

12:00

13:30

Accordingly, the use of MO for all process water was suspended. Within a few days the bromoform contribution dropped dramatically, and by the end of the year was at a level more representative of other systems (Table 2). However, the concentration of bromine is not the only mediator of brominated species; numerous studies have noted that TTHMs form more rapidly and result in higher concentrations when bromine is present.[3-7] However, for the units in use simply leaving the MO generators off has worked; process water disinfection, when needed, can be supplied by tablets or short-term MO use. This occurred in June 2002 and the reduction in bromoform (and brominated species) may be seen in Figure 4. 60

Bromoform (µg/L)

50

40

30

20

10

0 28Mar02

24Jul02 26Jun02

12Dec02 17Sep02

10Jun03 03Apr03

09Sep03

Figure 4. Bromoform in STT through and beyond end of MO use.


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Other Factors Residence Time There are a number of factors that caused or require relatively long residence times in parts of the STT system. To wit: 1. Unanticipated causes of increased residence time a. An aggressive program to reduce water losses in the system reduced unaccountedfor water from about 40% to about 20%, mostly due to leak repair and reduced unauthorized use. This effectively raised the residence time by up to 4 days in certain areas of the distribution system. b. Loss of a major user due to hurricane damage to user facilities with no replacement in the (essentially) closed service area. c. Operation of storage at the main pump station where piping layouts minimize exposure to catastrophic failure in the event of geologic activity; operated in the easiest fashion, without regard to residence time, degraded water quality. 2. Intentional causes of increased residence time a. Vulnerability to tropical storms and difficulty of replacing with alternative supply – islands cannot easily connect to adjacent suppliers. b. Vulnerability to geologic activity and difficulty of replacing with alternative supply. Loss reduction. A series of flushing sites was selected and automatic flushing devices are being placed. In addition, a flushing program has been implemented and will be expanded as better estimates of residence time become available. East End Use Reduction. The loss of the East End user was partially compensated for by changing the manner in which the storage for the area – Donoe Tank – was operated. Ordinarily the tank was maintained between 90 and 100% full, with pumping beginning at 90% capacity. East end use = 500,000 gal/day (0.5 MGD) Donoe Tank = 5,000,000 gal (5.0 MG) East end residence time > 10 days 10%

East end residence time < 6 days

50%

Donoe Tank Approximate Residence time with differing pump regimes Jan20030031

5

none

Figure 5. Donoe Tank Operation.


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The change was to operate the tank between 50 and 100% capacity, i.e., beginning to pump when the tank reached 50% capacity and pumping until capacity was reached. This normally occupied about 20 hours and reduced residence time in the tank to about 6 days from >10 days. Operation is shown schematically in Figure 5. Main Pump Station. As built, there was little flexibility in the manner in which tanks at the main pump station were operated, i.e., filled and drawn. A routine was developed to operate valves in a manner to minimize residence time, switching from tank to tank on a weekly basis, rather than the 30 or 90 day routine earlier practiced. As tanks are rehabilitated WAPA capital plans now call for revisions in the pipe layout to add to this flexibility and are expected to be fully implemented by 2015. The effects due, in part, to these changes may be seen in Figure 9 and Figure 10. Chlorine concentration The Authority had been gradually increasing the chlorine residual at send-out and this was about 3.5 mg/L at the time due to difficulties in maintaining bacteriostatic residuals at the ends of the system. Problems with send-out disinfectant concentrations were found to contribute to the problem; in addition to the operation of storage facilities these included automatic disinfectant rate control and instantaneous water supply demand changes.

Figure 6. Chlorine residual chart. Automated disinfectant application. The system utilized automated residual measurement. A typical residual chart is shown in Figure 6; the excursions that are clearly visible occurred at all hours. Both the application site and the sampling site were moved to reduce the effects of a junction of two major distribution lines located near those points. Pumps and solution concentration were changed to allow use of the pumps at approximately 60% of their rated capacity; these had been used at 10-15% of capacity for some time. In addition, the logic of the controller was


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adjusted to respond gradually to changes in residual measurements. These changes served to ease the problem. In conjunction with automated and manual flushing, these changes have served to allow maintenance of a disinfectant residual around the desired 2 mg/L; results for June 2002 and June 2005 are presented at Figure 7 and Figure 8.

26Jun02

Total THM (µg/L)

200

100

0 .1

.2

1.0

2.0

5.0

7.0

12.0

Days

Figure 7. TTHMs by residence time for June, 2002. 18 June 2005 30

Mean Total THM, ug/L

25

20

15

10

5

0 .1

1.0

2.0

5.0

7.0

12.0

residence time, days

Figure 8. TTHMs by residence time for June, 2005; note change of scale.


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DISCUSSION Responses to date have allowed the Authority to comply with newer disinfection by-product standards; see Figure 9 on page 9. We show only mean system concentrations in that figure, but individual running averages also comply. In Figure 10 on page 9 we show the history of TTHMs in this system by residence time over three years, showing the reduction due to the efforts detailed here. TTHMs in St. Thomas through Dec05 200.0

Total THM, ug/L

150.0

43 100.0

35

50.0

130 123

0.0 28Mar02

24Jul02 26Jun02

12Dec02 17Sep02

10Jun03 03Apr03

09Dec03 09Sep03

14SEP04

08JUN04

28Mar05

15Jun05

20Dec05 15Sep05

Date

Figure 9. Mean TTHMs for STT Date 28Mar02 26Jun02 24Jul02 17Sep02 12Dec02 03Apr03 10Jun03 09Sep03 09Dec03 08JUN04 14SEP04 28Mar05 15Jun05 15Sep05 20Dec05

200.0


150.0

100.0

50.0

0.0 .1

.2

1.0

2.0

5.0

7.0

12.0


Figure 10. TTHMs for STT by Residence Time


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Date 08JUN04 14SEP04 28Mar05 15Jun05 15Sep05 20Dec05

40.0


30.0

20.0

10.0

0.0 .1

.2

1.0

2.0

5.0

7.0

12.0


Figure 11. TTHMs for STT by Residence Time since Jun03; notice change in scale. Figure 11 shows these results in better detail, albeit only for the last 18 months. At this point, only a single, under-utilized part of the system does not comply with DBPR Stage 2 requirements and corrective action for this area includes manual and automatic flushing.[8] A number of issues remain to be addressed, including regrowth in the distribution system. On several occasions in 2005 and 2006 heterotrophic plate counts and Pseudomonas counts have triggered manual flushing for specific areas of the distribution system and the Authority intends to include these areas in additional studies detailed below. This concurrence – lowered disinfectant residual and rising microbial densities – is neither surprising nor unanticipated; US EPA in the several treatment regulations projected just such interaction and methods to control these will almost certainly include stabilization of the product water in some manner. Additional studies underway and intended 1. Revised residence time studies. 1.1. These are required to verify the HAAC sampling sites and to verify flows through the system, especially following some changes in the distribution system undertaken in 2004. 2. Estimates of THM Formation Potential projected onto system residence times.[7] One of these is presented below. 3. A comprehensive flushing program to include both large-scale systemic flushing and use of automated flushing devices. 4. Physical changes to storage tanks to improve water quality and maintain disinfectant residuals and water quality in storage.


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4.1.

For example, improved mixing at entry and withdrawal will tend to minimize residence time to near-hydraulic and might improve intrinsic water quality if designed appropriately. 5. Stability tests to suggest treatment changes or additions to control TTHM formation, minimize regrowth in the distribution system and provide corrosion protection for system units. Tentative THM Formation Potentials Estimates of THM formation potentials for disinfectant concentrations of 1 mg/L are shown at Figure 12. These are still tentative and do not take into account the changes in operations and treatment undertaken to date. These will be verified during stability testing for this system. It appears, however, that for 2 or 3 mg/L that the Authority can continue to comply with DBP standards for the foreseeable future with the corrective measures already implemented. Fit, 1 mg/L Chlorine 120 r2=0.576

100

Total THMs (µg/L)

80 60 40 20 0 -20 -40 0

2

4

6

8

10

12

Residence time (days)

Figure 12. THM Formation Potential, 1 mg/L Cl. While WAPA has been unlucky in the concatenation of effects of location, topography and climate in terms of THM formation, those same characteristics make St. Thomas what it is; one of the most desirable locations to live in the world. Stability studies should allow treatment of product water to ensure that the estimates of THM formation presented here will continue at this level or lower. Not surprisingly, this depends, in large part on the will and continued operational skill and attention of the Authority.[9]


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REFERENCES 1. 2.

3. 4. 5. 6. 7. 8. 9.

Bruno-Vega, A. and K.S. Thomas. 1994. The Virgin Islands Desalination Experience. Desalination, 1994. 98: p. 443 - 450. Minnigh, H.A., Annelise Knudsen, Remy-Martín, Ramírez Jiminian, Harold Mark, Michael Díaz, TTHMs in Desalinated Seawater: Experience in the US Virgin Islands., in Desalination of Seawater and Brackish Water, W.C. Lauer, Editor. 2006, AWWA, Denver, CO. p. 138-151. Chen, W.J. and C.P. Weisel. 1998. Halogenated {DPB} Concentrations in a Distribution System. JAWWA, 1998. 90(4): p. 151-163. Symons, J.M., Disinfection By-Products: A Historical Perspective, chap. 1, in Formation and Control of Disinfection By-Products in Drinking Water, P.C. Singer, Editor. 1999, AWWA: Denver, CO 80235. p. 1 - 25. Sung, W., et al. October, 2001. Chlorine Decay Kinetics of a Reservoir Water. JAWWA, October, 2001. 93(10): p. 101 - 110. Kirmeyer, G.J., et al. July, 2001. Practical Guidelines for Maintaining Distribution System Water Quality. JAWWA, July, 2001. 93(7): p. 62 - 73. Chowdhury, Z.K. and G.L. Amy, Modeling Disinfection By-Product Formation, chap. 3, in Formation and Control of Disinfection By-Products in Drinking Water, P.C. Singer, Editor. 1999, AWWA: Denver, CO 80235. p. 53 - 64. Pontius, F.W., Regulation of Disinfection By-Products, chap. 7, in Formation and Control of Disinfection By-Products in Drinking Water, P.C. Singer, Editor. 1999, AWWA: Denver, CO 80235. p. 139 - 159. Minnigh, H.A. and G.I. Ramírez-toro. 1999. The Viability of Small Water Systems: Making Sense Does Not Mean It Will Work. in AIDIS Regional Congress, 1999. 1999. Kingston, Jamaica.


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