Hydraulic mixing and free chlorine residual in reservoirs

DISTRIBUTION SYSTEMS

Hydraulic mixing and free chlorine residual in reservoirs Interior sampling of a reservoir illuminates its mixing characteristics and supports the assumption of stratification or partitioning.

C

Paul F. Boulos, Walter M. Grayman, Robert W. Bowcock, Jonathan W. Clapp, Lewis A. Rossman, Robert M. Clark, Rolf A. Deininger, and Ashok K. Dhingra

oncern is widespread regarding the integrity and viability of the nation’s drinking water systems. Of principal concern is the maintenance of treated water quality in drinking water distribution systems. Historically, these systems have been designed for efficient water delivery, hydraulic reliability, and fire protection, whereas most regulatory An extensive sampling study of reservoir water quality was mandates focus on enforcconducted in Azusa, Calif. Primary emphasis was placed on ing treatment concentraproviding a better understanding of the dynamics of hydraulic tions at the plant. Howmixing and free chlorine residual concentration distribution in ever, modern purveyors the reservoir. The reservoir approached completely mixed have an additional manbehavior with two exceptions: a degree of short-circuiting date: to ensure that the between the inlet and outlet (which significantly affected the T10 distributed water is safe time) and the presence of a stagnant zone in the center core of the and conforms to current reservoir where there was less mixing and thus older water. These and emerging standards. results support the assumption of stratification or partitioning in Many of these standards reservoirs. Regular field sampling is recommended to facilitate must be met at the conthe effective management of distribution system water quality. In sumer’s tap, forcing incluparticular, interior sampling of reservoirs can provide useful sion of the entire distribuinformation that could not be inferred otherwise. tion system in compliance decisions.

Copyright (C) 1996 American Water Works Association 48

JOURNAL AWWA

The Ed Heck Reservoir near the city of Azusa, Calif., was built to provide operational and emergency storage and to maintain chlorine contact time for water leaving the filtration plant.

SWTR specifies the use of a disinfectant to minimize risk from microbiological contamination. However, chlorine or other disinfectants interact with natural organic matter in treated water to form DBPs. Raising the pH of treated water can assist in controlling corrosion but may increase the formation of THMs.

Water storage is integral part of distribution system Storage facilities, integral components of distribution systems, have historically been considered to be neutral in terms of their effects on water quality. They have been located, designed, and operated primarily on the basis of structural safety and hydraulic integrity and reliability. The latter objective pertains to maintenance of system pressure, equalization of demands on supply sources,

The Total Coliform Rule, the Lead and Copper Rule, the trihalomethane (THM) regulation, the pending Disinfectants/Disinfection By-products (D/DBP) Rule, and the Surface Water Treatment Rule (SWTR) are oriented toward water quality and monitoring in the distribution system. The lack of a disinfectant residual, presence of coliform bacteria, and high levels of THMs or haloacetic acids in distributed water can result in seriater storage reservoirs are generally ous violations of regulations and subsequent public notidesigned to meet specific hydraulic fications. Storage tanks and reservoirs can influence all of criteria that may conflict with criteria these regulated parameters. associated with good water quality. The SWTR requires that a detectable disinfectant residual be maintained at representative locations in the distribution system to reduction in sizes or capacities of existing mains, proprotect against microbial contamination. The Total vision of water for normal demand, excess storage Coliform Rule regulates coliform bacteria, which are for peak demand and fire suppression, and supply of used as surrogate organisms to indicate whether treatuninterrupted water service during power outages.1–7 This policy means that water may remain in the ment has been adequate or the system is subject to contamination. Monitoring for compliance with the system for long periods of time. Long residence times can adversely affect the quality of finished water in Lead and Copper Rule is based entirely on samples taken at the consumer’s tap. The current standard for terms of reduced disinfectant residuals, increase in DBPs, occurrence of nitrification, proliferation of THMs is 0.10 mg/L for systems serving more than 10,000 people, but the anticipated D/DBP Rule will bacteria, and aesthetic degradation (taste, odor, and impose a reduced THM standard for haloacetic acids appearance). Long residence times may have been justified in the past because of the absence of techon all systems. This regulation also requires monitornology for evaluating such effects and the level of ing at selected locations in the distribution system. regulatory control. The issue of distribution storage Some of these regulations may provide potentially contradictory requirements. For example, the is a national one that affects utilities of all sizes,

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Copyright (C) 1996 American Water Works Association JULY 1996 49

from small one-well systems to large regional water suppliers. To ensure against contamination many utilities will have to take corrective actions, including improving tank operations and maintenance programs, exercising storage tanks more frequently, routinely monitoring water quality, evaluating the structural integrity of tanks, and contriving design changes such as structural retrofitting to improve mixing. The results of structural or operational changes may be improved water quality or reduced risk of detrimental, potentially serious, or fatal episodes of degraded water quality. To ensure that water quality meets regulations and does not degrade in the distribution system, it is necessary to identify how storage facilities affect the quality of finished water and to implement strategies that minimize adverse effects. In addition, it is necessary to enhance utilities’ understanding and characterization of the dynamics of water quality behavior in distribution and storage facilities. Only when these inner mixing and transport mechanisms are carefully examined can water quality be accurately predicted. Field sampling studies are an effective, useful way of quantifying hydraulic and water quality

FIGURE 1

changes in storage facilities. Exterior sampling, which includes influent and effluent water, can produce information useful for estimating water quality distribution within the storage facility. Interior sampling, which is invasive, provides the tool to effectively measure and verify water quality variability across the facility. Insights may be gained into the complex physical processes that occur during mixing, allowing predictions of system behavior under differing conditions. The two types of sampling may be combined to give a more realistic representation of the spatial and temporal interaction of water quality and hydraulic behavior. This identifies existing water quality conditions in a storage facility and enables the collection of information that may be used to formulate and evaluate alternative operational policies and design changes to improve effluent water quality.

Ed Heck Reservoir Construction of the Ed Heck Reservoir in the vicinity and downstream of the Canyon Filtration Plant in the city of Azusa, Calif., was recently completed. The reservoir was built to provide operational and emergency storage for the distribution system

Sampling locations at Ed Heck Reservoir

Inside diameter = 154 ft (47 m) Height = 30 ft (9 m) Influent and effluent sampling taps in shed Center vent

24-in. (61-cm) inlet pipe

Pumphouse

30-in. (76-cm) outlet pipe Hoses Approximate distance from tank to shed = 190 ft (58 m) Approximate distance from pumphouse to shed = 1,200 ft (366 m) Inlet and outlet lines are 5.25 ft (1.6 m) apart (centerline to centerline)

Hatch Hoses Temporary receptacles and drain lines

Schematic detail of in-tank sampling configuration at hatch and center vent:

Seven motors

Flexible 0.75-in. (1.9-cm) garden hose

Bottom of sampling pipes at 0, 4, 8, 12, 16, 20, and 24 ft (0, 1.2, 2.4, 3.6, 4.9, 6.1, and 7.3 m) above floor of tank; 1-in. (2.5-cm) rigid pipe


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Concentration—mg/L


and to maintain chlorine conFIGURE 2 Tank fluoride concentration tact time for water leaving the filtration plant. Because the filtration plant Influent Effluent is the single largest producer 2.0 of Azusa’s drinking water, it 1.8 was important to understand any effects the reservoir would 1.6 have on water quality. Toward 1.4 this end, a comprehensive sampling study of the facility 1.2 was carried out in a coop1.0 erative agreement between Azusa, a consulting engineer0.8 ing firm,* and the US Envi0.6 ronmental Protection Agency (USEPA). The results of this 0.4 study are presented here to 0.2 show how mixing characteristics within a specific reservoir 0.0 can be effectively determined. 10 20 30 40 50 0 The city of Azusa purveys Time After Fluoride Injection—h water to parts of the San Gabriel Valley in Southern California, including sections of Irwindale, Covina, West FIGURE 3 Influent fluoride probe measurements Covina, and Glendora and unincorporated areas of Los 10 Angeles County. The total service area comprises 15.9 sq mi (4,118 ha). The city of Azusa serves water to approximately 25,000 connections serving about 81,000 people. Azusa represents 56.3 percent of the 1 service area; Irwindale, Covina, West Covina, Glendora, and the unincorporated county represent 3.6, 9.6, 10.1, 0.3, and 20.1 percent, respectively. In 1993, Azusa supplied 6.4 bil gal (0.024 ˘ 109 m3) of water to its cus0.1 tomers from a combination of 70 75 80 85 90 95 100 105 110 treated surface water (San Electric Potential—mV Gabriel River) and groundwater from wells. Treated surface water from the Canyon Filtration Plant is discharged by pump station to the recently completed capacity of 4 mil gal (15 ˘ 109 m3). It is fed by a 24Ed Heck Reservoir. The filtration plant is capable of in. (0.6-m) inlet pipe from the plant pump house. producing 10 mgd (0.0379 ˘ 106 m3/d). This treatThis pipeline enters near the base and has a rightment facility consists of a raw water intake structure, angle elbow at its end that directs water counterflash mix basin, flocculation chamber, automatic back- clockwise along the reservoir’s perimeter. wash filter, and sludge lagoons. The energy of the inflowing water is sufficient to The Ed Heck Reservoir is an aboveground circu- cause a visible rotation of the reservoir water in the lar tank made of poured-in-place concrete corewall same counterclockwise direction. Water flows into with wrapped prestressed strand over the corewall. the distribution system from the reservoir through a The prestressed strand is covered with shotcrete. The 30-in. (0.8-m) transmission main. This pipeline also inside diameter of the reservoir is 154 ft (46.9 m), and the height is 30 ft (9.1 m). The reservoir has a *Montgomery Watson, Pasadena, Calif.


Tank inflows and outflows during study (part A); tank levels during study (part B)

Flow—gpm

Tank influent

Tank effluent

9,000

585

8,000

520

7,000

455

6,000

390

5,000

325

4,000

260

3,000

195

2,000

130

1,000

Flow—L/s

FIGURE 4

65 0

10

20

30

40

50

60

27.0

8.3

26.5

8.1

26.0

7.9

25.5

7.7

25.0

7.5

24.5

7.3

24.0

7.1 0

10

20

30

40

Time After Fluoride Injection—h


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50

60

Water Level in Tank—m

Water Level in Tank—ft


A shed was constructed to aid in the collection of exterior samples at the reservoir. Sampling taps for the influent and effluent lines were installed and housed in the shed.

sets of seven centrifugal pumps* with 0.5-hp (0.37-kW) motors was constructed for spatial sampling at selected depths from the reservoir hatch and the reservoir center vent. The pumps were mounted on gratings located over the hatch and vent openings on the roof of the reservoir. Rigid pipes were installed with inlets at seven depths (4-ft [1.2m] intervals starting at the bottom of the reservoir). A check valve was installed at the end of each rigid pipe. For the pipes in the center vent, a short elbow was also located to help ensure that water was drawn laterally.

lies at the reservoir base 5.25 ft (1.6 m) clockwise from the inlet pipe. Normal operating level for the reservoir is anticipated to vary between 15 and 27 ft (4.6 and 8.2 m) above the reservoir bottom. The normal mode of operation for the reservoir is simultaneous inflow and outflow. The istribution storage is a national issue reservoir has two openings that affects utilities of all sizes, on the roof: a periphery hatch (located near the from small one-well systems to large outer wall about three quarters of the way around regional water supply organizations. the reservoir from the inlet) and a center vent. These access areas permit sampling in both longitudinal and The outlets from the pumps were connected to vertical directions. A schematic of the reservoir is hoses leading to the ground, where sampling shown in Figure 1. occurred. Each hose was clearly marked to identify the sampled depth. The pumps were only needed to Experimental procedures start flow in each hose, which then continued to flow An extensive field sampling study was under- by siphon as long as the tap inlet remained subtaken, with emphasis on identifying the general free merged. This allowed for continuous sampling at varchlorine dynamics in the reservoir. The study con- ious depths within the facility. A schematic of the sisted of both interior and exterior sampling. Inte- sampling apparatus is shown in Figure 2. rior sampling was carried out using a specially Sampling equipment. During the study, fluodesigned sampling apparatus. Exterior sampling was ride, free chlorine residual, and temperature were performed using standard water quality analysis pro- measured. A direct-read spectrophotometer† was cedures. The direct interaction between hydraulics used with fluoride indicator solution‡ as a means of and water quality was quantified using a nontoxic analyzing samples to determine fluoride concentraconservative tracer (fluoride). The experimental pro- tion. Two voltage meters§ and two fluoride-specific cedures used are described in subsequent paragraphs. combination electrodes** were also used. The elecSampling apparatus. To aid in the exterior sam- trode–meter measured electrical potential in millipling of the reservoir, a shed was constructed 190 ft volts, from which fluoride concentration could be (57.9 m) from the reservoir and 1,010 ft (307.8 m) calculated using a calibration curve developed from from the filtration plant. The shed protected the sam- simultaneous meter readings and samples analyzed pling equipment from vandalism and the elements. Sampling taps for both influent and effluent lines *Model 1 ACE, Covert, Galion, Ohio †DR-2000, Hach Co., Loveland, Colo. were installed and housed in the shed. ‡SPADNS Invasive interior sampling of the reservoir was §Fisher, Pittsburgh, Pa. carried out as follows. An apparatus consisting of two **Orion, Boston, Mass.

D


FIGURE 5

Tank effluent fluoride concentration (probe)

Electrode data

1.6

CSTR results

1.4


1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

10

20

30

40

50

60


FIGURE 6

Effluent fluoride probe measurements


10.0

1.0

0.1 65

70

75

80

85

Electric Potential—mV

90

95

100

pump sampling apparatus and sampling equipment were tested. Fluoride from hydrofluosilicic acid was used as a tracer to determine the mixing behavior and residence time distribution in the reservoir. Fluoride was injected at a constant mass rate over a 26h period into the wet well at the pumphouse using an electronic pump.† A 24 percent solution by weight of hydrofluosilicic acid was used with a concentration of 226,000 mg of fluoride per litre of acid. The approximate injection rate of acid was 107.5 mL/min. At a flow rate of 5,800 gpm (365.9 L/s) leaving the pump house (the actual flow rate varied above and below this value), the resulting fluoride concentration was 1.19 mg/L above the background value of about 0.5 mg/L. The fluoride injection was started at 9:10 a.m. May 18 and continued until about 11:00 a.m. May 19. The fluoride concentration at the reservoir influent is depicted in Figure 3 based on samples taken at the shed near the reservoir and analyzed using the direct-read spectrophotometer. Additionally, the electrical potential of the influent was measured using a meter equipped with a fluoride-specific electrode. A graph of potential in millivolts versus spectrophotometric concentration readings along with a best-fit curve is presented in Figure 4. The mathematical relationship for the mathematical curve is: C = 10(–0.01502 V + 1.165)

in the laboratory. For samples collected from within the reservoir, a digital colorimeter* and the directread spectrophotometer were used to measure free chlorine residual. Temperature measurements were taken using digital thermometers and a temperature probe. All sampling equipment was carefully tested and calibrated. Sampling program. The sampling program was held May 16–20, 1994. During the first two days, the

in which C is the fluoride concentration in milligrams per litre and V is the electrical potential reading in millivolts. Effluent samples from the reservoir analyzed with the direct-read spectrophotometer were taken over a period of 53 h. The results are shown in Figure 3. Influent and effluent sampling taps were continuously purged to ensure that each sample obtained *Hach Co., Loveland, Colo. †Model 45-050/KIM, Wallace & Tiernan, Belleville, N.J.


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(1)



was representative of current FIGURE 7 Chlorine concentration at hatch conditions. During the study, effluent flows from the plant were 0 ft (0 m) 12 ft (3.6 m) 24 ft (7.3 m) 1.35 recorded by the city’s supervisory control and data acquisition (SCADA) system at 121.30 min intervals. A graph of plant effluent flows into the reser1.25 voir is shown in Figure 5, part A. Reservoir water levels were 1.20 also recorded at 12-min intervals, and the inflow and water 1.15 levels were used to compute reservoir outflows. Reservoir 1.10 outflows and water levels are shown in parts A and B, 1.05 respectively, of Figure 5. The reservoir outflow concentration of fluoride analyzed 1.00 0 10 20 30 40 50 60 with the electrode is shown in Time After Fluoride Injection—h Figure 6. During the study, the system was operated to keep the water level in the reservoir within the range of 24.5 to FIGURE 8 Chlorine concentration at vent 26.5 ft (7.5 to 8.1 m). For the first half of the study, the flow rate into the reservoir was 0 ft (0 m) 12 ft (3.6 m) 24 ft (7.3 m) maintained at about 5,800 1.20 gpm (365.9 L/s). After the fluoride feed was stopped, the 1.15 influent flow rate was lowered to about 4,300 gpm (271.2 L/s) to study the effect of the lower 1.10 flow rate on mixing. During the first part of the 1.05 study (when fluoride was being fed), the primary method of effluent fluoride con1.00 centration analysis was the direct-read spectrophotometer. During this period, elec0.95 trical potential readings (using the specific combination elec0.90 trode connected to a voltage 0 10 20 30 40 50 60 meter) were taken at selected Time After Fluoride Injection—h times to develop a relationship between the voltage readings and the fluoride concentration. During the second part of the study (when fluoride feed was shut down), fewer samples were analyzed This relationship was used in conjunction with elecwith the spectrophotometer, and the majority of trode readings to determine fluoride concentration samples were analyzed with the electrode. A graph during the second part of the study, whereas the labof electrical potential in millivolts versus concentra- oratory results were used to determine fluoride contion for all periods when both samples and readings centration during the first part of the study. During the study, free chlorine concentrations were taken, along with a best-fit curve, is shown in Figure 7. The mathematical relationship for the cal- were taken at the plant effluent using a permanently mounted continuous chlorine meter* and at the reseribration curve is: C = 10 (–0.01437 V + 1.2097)

(2)

*CL-17, Hach Co., Loveland, Colo.


FIGURE 9

of free chlorine bulk decay in the water leaving the plant. In these tests, 10 samples were taken at the same time from the plant effluent and kept in sealed, headspace-free dark bottles at ambient water temperature (14.2oC). At intervals, a bottle was opened and the free chlorine concentration was measured using the direct-read spectrophotometer. The results of this analysis are summarized in Figure 10. As shown in the figure, there is variation from the classical semilog leastsquares linear regression fit. By fitting different portions of the decay curve, decay rate coefficients from –0.301/day to –0.82/day can be inferred. The former value represents the average decay over the full decay test study, and the latter value represents the decay over the first 4 h.

Chlorine bottle decay tests k = – 0.301/day

Field data

k = – 0.82/day

Concentration/Initial Concentration

1

0.1 0

5

10

15

20

25

30

35

Time—h

FIGURE 10

Fluoride at hatch—May 18, 1994

Analysis of results 9:25 a.m.

20

Height Above Tank Bottom—m

Height Above Tank Bottom—ft

24

Several analyses were performed on the sampling results, including • a comparison of effluent 7.5 fluoride concentrations with the theoretical fluoride con6.0 centrations from a completely mixed reservoir, • an examination of the 4.5 temporal variation of the fluoride depth profile concentra3.0 tions at the reservoir hatch and vent, 1.5 • an examination of free chlorine residual in the reservoir effluent and temporal and 0 vertical variation of free chlo1.4 1.6 rine residual in the reservoir hatch and vent, and • an estimation of the T10 value for the reservoir. Analysis of fluoride in the reservoir effluent. The temporal pattern of fluoride concentration in the reservoir effluent can give some insights into the mixing characteristics of the reservoir. If the reservoir was acting as an instantaneously completely mixed tank (or continuously stirred tank reactor [CSTR]), the theoretical concentration of fluoride in the reservoir

9:50 a.m. 10:20 a.m. 11:00 a.m. 12:00 p.m. 3:00 p.m. 7:10 p.m. 11:02 p.m. 9.0

16

12

8

4

0 0.4

0.6

0.8

1.0

1.2


voir effluent at the shed using a second continuous meter.* In addition, selected samples were taken from the pump apparatus using a digital colorimeter† or direct-read spectrophotometer for analysis. Figures 8 and 9 give the free chlorine sampling results taken at three elevations at the reservoir hatch and vent, respectively. Additionally, free chlorine bottle decay tests were conducted in the laboratory to determine the rate

*CL-17, Hach Co., Loveland, Colo. †Hach Co., Loveland, Colo.


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The sampling apparatus, which consisted of two sets of centrifugal pumps, was mounted on gratings located over the hatch and vent openings on the roof of the reservoir.

can be mathematically calculated. The resulting curve is shown in Figure 6 along with measured fluoride concentrations. As illustrated in the figure, the CSTR-predicted concentrations are in general agreement with actual outflow concentrations, although several periods of significant deviations are apparent. For example, the observed fluoride concentrations during the start of the fluoride feed period are higher than the theoretical CSTR concentrations, suggesting some level of “short-circuiting” between the reservoir inlet and outlet. Analysis of fluoride in the reservoir. An extensive database was collected using the sampling apparatus at the reservoir hatch and vent. These data can be used to deduce the general mixing pattern that was present during the study. The sampling results from the interior of the reservoir suggest some degree of incomplete mixing in the reservoir. Figure 11 depicts the vertical distribution of fluoride concentration at the reservoir hatch at irregular intervals following the start of fluoride injection. As illustrated, FIGURE 11

9:25 a.m.

Fluoride at vent—May 18–19, 1994

9:50 a.m. 10:20 a.m. 11:00 a.m. 12:00 p.m.

20

Height Above Tank Bottom—m

Height Above Tank Bottom—ft

24

fluoride concentration increases with time as expected, but there are no apparent trends and relatively little vertical variation. This suggests that the part of the reservoir represented by the hatch (i.e., the annular ring extending to the reservoir wall) is well mixed at all times. Dissimilar results are evident in Figure 12, which shows fluoride concentrations at the vent. Most notably, concentrations are significantly different at different depths, and the concentrations at the vent at any time during the fluoride addition phase of the study are significantly lower than those observed at the hatch. At the start of the study, fluoride concentrations were at the background value, about 0.5 mg/L. Six hours into the study, the concentration at the reservoir floor at the vent was still at the back3:00 p.m. 7:10 p.m. 11:02 p.m. ground level, whereas the 9.0 concentrations near the top of the water column were at 7.5 0.76 mg/L. This compares with the concentrations at 6.0 the hatch at the same time, which varied from 0.98 to 1.1 mg/L. 4.5 As time passed, the concentrations at the vent con3.0 tinued to lag behind the concentrations at the hatch. 1.5 Slowly, though, the vertical differences decreased. About 24 h after fluoride injection 0 was started, the fluoride con1.4 1.6 centrations at the vent varied from 1.24 mg/L at the reservoir floor to 1.41 mg/L near the water surface. This com-

16

12

8

4

0 0.4

0.6

0.8

1.0

1.2



FIGURE 12

Inferred zones in reservoir (part A); inferred mixing patterns in reservoir and resulting water age (part B)

A

Increasing water age

shed effluent tap varied from 1.16 to 1.03 mg/l (1.10 mg/L average). The difference reflects a loss of 0.04 mg/L between the inlet and outlet. As illustrated in Figure 8, there was relatively small variation in free chlorine residual vertically at the hatch. Typically, the residual was slightly higher near the water surface than at the bottom, but the difference was generally on the order of only 0.05 mg/L. At the vent (Figure 9), the free chlorine residual was higher at the water surface than at the bottom in almost all cases, and generally the difference was about 0.1 mg/L. These results are consistent with the mixing characteristics observed with the fluoride tracer. T10 estimation. The fluoride tracer was used to estimate the T10 value for the reservoir (i.e., 90 percent of the water leaving the reservoir has a residence time >T10). This is the value specified by the USEPA for use in C ˘ T calculations for disinfection credits under the SWTR. The calculated T10 value on the basis of the sampling data was 26 min. Under the operating conditions used, the theoretical mean residence time for a completely mixed tank reactor is about 9.7 h; the theoretical T10 value is 61 min. Because the residence time in a CSTR is exponentially distributed, the theoretical T10 value can be computed from

B

E

T10

0

1 }} e–t /tw dt = 0.1 wt

(3)

which simplifies to the following: T10 = 0.105 tw

(4)

in which tw is the mean residence time of the reservoir. The observed T10 value, which was significantly shorter than the theoretical value, indicates the presence of short-circuiting between the inlet and outlet. This is in agreement with the previous observations.

Discussion

pares with a representative concentration of 1.5 mg/L at the hatch. This indicates that under the flow rate and water level conditions of the study, it takes longer than 24 h before the volume of water in the center of the reservoir is replaced. Analysis of free chlorine residual. Analysis of free chlorine residual in the plant effluent and reservoir effluent indicates a relatively narrow range of observed concentrations. For the plant, free chlorine residual varied between 1.27 and 1.04 mg/L (1.14 mg/L average). For the reservoir, the residual at the

Mixing patterns in a storage reservoir exhibit dynamic characteristics that can be affected by the inlet and outlet configurations and by the design and operation of the facility. Sampling data indicate that the Ed Heck Reservoir may be viewed as having two zones (Figure 13, part A): (1) an annular cylinder extending from the reservoir walls toward the center and (2) a core cylinder. Flow enters the reservoir and, because of the direction of the inlet line, produces a counterclockwise flow pattern. Sampling results from the reservoir hatch indicate that the annular ring is vertically well mixed. Flow is transferred from the annular ring near the top of the reservoir and then gradually moves down the inner core cylinder (Figure 13, part B). A mass–balance analysis suggests that the volume in the annular ring is significantly greater than the volume in the central core. The sampling data also indicate that at the flow


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rates and operation of the reservoir during the study, longer than 24 h is required to achieve complete mixing in the reservoir. The predominant flow pattern in the reservoir is a counterclockwise flow around the reservoir. Travel time around the reservoir varies depending on inlet flow. For flow conditions observed during the sampling study, travel time around the reservoir was about 15 min, resulting in a degree of short-circuiting with a period of 15 min. Additional short-circuiting could also have resulted from direct flow between the inlet and outlet.

Conclusion The following conclusions can be drawn from the field study conducted at the Ed Heck Reservoir: (1) The roof-mounted sampling apparatus used in this study is a feasible means of collecting continuous interior samples at predetermined depths up to the theoretical limitation of the pumps (30 ft [9.1 m]). (2) Fluoride tracer studies are useful for determining both residence time distribution and internal mixing dynamics in storage reservoirs. (3) The flow pattern in the Ed Heck Reservoir consists of a relatively fast rotational flow around a large outer annular ring coupled with a slower downward flow within a smaller central core. (4) For flow rates used in this study, the reservoir behaves on average as a completely mixed reactor with a residence time of 9.7 h and a negligible loss of free chlorine residual. (5) The short-circuiting between the inlet and outlet lines causes the T10 for this reservoir to be less than half that of a true CSTR. Although water storage reservoirs are frequently the most visible components in a water distribution system, they are usually the least understood in terms of their effects on water quality. These facilities are generally designed to meet specific hydraulic criteria that in many cases may conflict with criteria associated with good water quality. A careful trade-off based on sound engineering decisions may be required to reach an acceptable balance between the potentially competing hydraulic and water quality objectives. Field sampling studies are useful tools for understanding and quantifying hydraulic and water quality behavior of a storage reservoir. This will lead to better operation and management of water storage reservoirs. The authors recommend that field sampling be conducted regularly to facilitate the effective management of distribution system water quality. In particular, interior sampling of reservoirs can provide useful information that could not be inferred otherwise. The methodology described may be useful to any utility attempting to comply with impending regulations.

Acknowledgment The authors thank Dan Ryan, Steve Seffer, Sonny Gowan, Ken Vetting, Robert Field, Harry

Wright, Kim Redmile, and Joe Coetezar of the city of Azusa for their assistance in this study. Their cooperation and enthusiasm were invaluable. Acknowledgments are also due to Joe Hsu of Azusa and Rudy Tekippe and Rhodes Trussell of Montgomery Watson for their useful comments. The interior sampling apparatus used in this study was designed and built by Dan Ryan of the city of Azusa. The Azusa Light and Water Department provided financial support for this study.

References 1. KENNEDY, M.S. ET AL. Mixing Characteristics in Distribution System Storage Reservoirs. Proc. 1991 AWWA Ann. Conf., Philadelphia, Pa. 2. CLARK, R.M. & GRAYMAN, W.M. Distribution System Quality: A Trade-Off Between Public Health and Public Safety. Jour. AWWA, 84:7:18 (July 1992). 3. GRAYMAN, W.M. & CLARK, R.M. Using Computer Models to Determine the Effect of Storage on Water Quality. Jour. AWWA, 85:7:67 (July 1993). 4. KENNEDY, M.S. ET AL. Assessing the Effects of Storage Tank Design on Water Quality. Jour. AWWA, 85:7:78 (July 1993). 5. MAU, R.E. ET AL. Explicit Mathematical Models of Distribution Storage Water Quality. Jour. Hydraulic Engrg.—ASCE, 121:10:699 (Oct. 1995). 6. MAU, R.E.; BOULOS, P.F.; & CLARK, R.M. Multicompartment Models of Distribution Storage Water Quality. Proc. 1995 AWWA Computer Conf., Norfolk, Va. 7. MAU, R.E.; BOULOS, P.F.; & BOWCOCK, R.W. Modeling Distribution Storage Water Quality: An Analytical Approach. Jour. Applied Mathematical Modeling, 20:4:329 (Apr. 1996). About the authors: Paul F. Boulos is director of Water Distribution Technology, Montgomery Watson, 300 N. Lake Ave., Suite 1200, Pasadena, CA 91101. Walter M. Grayman is a consulting engineer at 730 Avon Fields Lane, Cincinnati, OH 45229. Robert W. Bowcock is water utilities manager for the Light and Water Department, 777 N. Alameda Ave., Azusa, CA 91702. Jonathan W. Clapp is an associate engineer in the Water Distribution Technology Department, and Ashok K. Dhingra is vice-president and senior client manager, Montgomery Watson, 301 N. Lake Ave., Suite 600, Pasadena, CA 91101. Lewis A. Rossman is chief of the Engineering and Cost Section, and Robert M. Clark is director of the Drinking Water Research Division, US Environmental Protection Agency, 26 W. Martin Luther King Dr., Cincinnati, OH 45268. Rolf A. Deininger is professor in the School of Public Health, University of Michigan, Ann Arbor, MI 4810
