CFD Analysis for Irvine Ranch Water District Zone 3 No. 2 ... · IRWD Zone 3 No.2 Reservoir CFD Study FSI V106082 September 19, 2011 SUMMARY The water recirculation patterns in the

Flow Science Incorporated 420 Neff Avenue, Suite 230, Harrisonburg, VA 22801 (540) 442-8433 FAX (540) 442-8863

© 2 0 1 1 F l o w S c i e n c e I n c o r p o r a t e d

H a r r i s o n b u r g , V A • P h i l a d e l p h i a , P A • P a s a d e n a , C A w w w . f l o w s c i e n c e . c o m

C F D A n a ly s i s f o r I r v i n e R a n c h Wa t e r D i s t r i c t Z o n e 3 N o . 2 R e s e r v o i r

Prepared for Irvine Ranch Water District

3512 Michelson Drive Irvine, California

FSI V106082 September 19, 2011

Prepared By Li Ding, Ph.D., P.E. (VA)

Reviewed By E. John List, Ph.D., P.E. (CA)

Reviewed By Imad A. Hannoun, Ph.D., P.E.(PA)

IRWD Zone 3 No.2 Reservoir CFD Study FSI V106082 September 19, 2011

SUMMARY

The water recirculation patterns in the Irvine Ranch Water District (IRWD) East Irvine Zone 3 No.2 reservoir, which is located in the City of Irvine, California, have been evaluated using Computational Fluid Dynamics (CFD) modeling. The goal of the CFD modeling was to evaluate the mixing performance of two mixing systems implemented by IRWD, and to determine the general flow patterns and mixing characteristics in the reservoir. This report includes a general discussion of water circulation issues in the reservoir. Various methods for determining the reservoir performance are then discussed. This work has been performed by Flow Science Incorporated of Pasadena, California, under contract with IRWD of Irvine, CA.

The IRWD East Irvine Zone 3 No. 2 reservoir is circular in shape with an internal diameter of 194 feet (ft). The reservoir features a single inlet/outlet that is 30 inches in diameter and located near the reservoir side wall approximately 4 ft above the reservoir floor. The opening of the inlet/outlet pipe points horizontally toward the center of the tank (the jet inflow discharges horizontally).

Two mixing system, the Severn Trent Systems (STS) Rover system and the Superior Water Technologies (SWT) Vortex system, which were installed by IRWD in the same reservoir at different times, were evaluated in this study. Furthermore, the reservoir without any mixing elements was also evaluated. The STS Rover system pumps water at a rate of 22 gallons per minute (gpm) with an inlet near the bottom of the tank and discharges it vertically upward towards the surface through a single 2-inch nozzle. The SWT Vortex system pumps water at a rate of 50 gpm and discharges it through three 11/32-inch nozzles at 45 degrees above the horizontal. Both of the mixing systems are combined with the chloramine analyzer/injection equipment that monitors the water entering the pump and doses the mixing zone with chloramine if the concentration of total chlorine residual falls below 2.5 milligrams per liter (mg/L). All information used in the modeling of these systems was provided by IRWD.

Before the completion of the CFD analysis, an analysis of reservoir field data was conducted. This analysis shows high water levels (i.e., high water volume) and low flow rates occurred during the period between 1/26/10 and 2/8/10, which leads to high water ages. Therefore, because high water age generally correlates to lower chemical concentrations in the reservoir water, this period was chosen as the most conservative one to model the fill/draw cycle in the reservoir. The flow rates during this period were simplified/normalized to achieve the a pseudo-steady state in flow patterns and water age as follows: (1) the reservoir is assumed to have a fill cycle followed by a draw cycle; (2) the reservoir fills at the rate of 646 gpm (0.93 million gallon per day or MGD) for a period of 12 hours followed by a draw cycle at a rate of 646 gpm (0.93 MGD) for a

IRWD Zone 3 No. 2 Reservoir CFD Study FSI V106082 September 19, 2011

2

period of 12 hours; (3) this cycle is repeated for the duration of the simulation. During the modeling period, the average reservoir volume was approximately 5.1 million gallon (MG) with an average water depth of 23.0 ft. Based on the reservoir volume and fill/draw flow rate, the average residence time was 11.0 days (computed as the volume of 5.1 MG divided by the daily average flow rate of 323 gpm [=646 gpm × (12 hr/24 hr)]).

Based on the analysis of the reported total chlorine residual data, a decay coefficient range of 0.1/day to 0.2/day (reductions of 9.5% per day and 18% per day, respectively) was observed. The 0.2/day decay rate was chosen for modeling purposes because it represented a more conservative value (i.e., faster decay).

After completing the data analysis, the flow patterns and total chlorine residual distribution for the reservoir were evaluated for three scenarios using CFD: 1) Base Case (current reservoir geometrical configuration without any mixing system), 2) the STS Rover mixing system (current reservoir geometrical configuration with the STS Rover mixing system, and 3) the SWT Vortex mixing system (current reservoir geometrical configuration with the SWT Vortex mixing system).

Based on the CFD study results, mixing in the reservoir without a mixer or external dosing (i.e., Base Case) is deemed as moderate efficiency mixing (based on arbitrary criteria discussed in the text) and the predicted chlorine residual can be as low as 0.2 mg/L in the areas of limited mixing. Overall, total chlorine residual in the reservoir was below 0.8 mg/L. This was mainly due to long average residence time and lack of external dosing. In the case where the STS Rover mixing system was used, mixing efficiency improved from moderate to good and the lowest predicted chlorine residual was approximately 1.8 mg/L. The mixing efficiency was further improved to excellent with the SWT Vortex mixing system in use, which produced a near-fully mixed reservoir with a minimum predicted chlorine residual of about 2.4 mg/L. This indicates that injected chloramines are effectively distributed throughout the reservoir by this mixing system.

It should be noted that in some instances the field data showed episodes of low observed chlorine residual in the reservoir that cannot be duplicated in the CFD analysis (when either of the two mixing systems was in operation). The field data also showed that total chlorine residual was spatially uniform within the reservoir when either of the two mixing systems was in operation. This is consistent with the findings of the CFD study. Thus, the cause of low total chlorine residual events in the reservoir was likely not related to mixing, but could be caused by one or more of the following factors:

• Inflow water quality may vary (e.g., variable concentration of total chlorine and organic carbon) causing variable chlorine inflow loading or in-reservoir chlorine demand.


3

• Sediment scouring and resuspension that may affect the chlorine decay rate. It was observed that there is a layer of sediment on the reservoir’s bottom (personal communication with Mark Malmquist of SWT). Scouring of the sediment may result in periods of high chlorine demand.

• The injection equipment was not performing according to the specification.

Some limited analysis was performed to determine whether the low levels of total chlorine residual were correlated to the large reservoir fill events. The results were inconclusive as some of the low levels of total chlorine residual also occurred during the reservoir draw periods. If sediment resuspension was the cause of rapid chlorine decay within the tank, then the exact determination of the periods of sediment resuspension cannot be identified from the existing data. This is a result of lack of information about the sediment content of the inflow (variability, size distribution, etc.) and the sediment distribution within the tank at various times. Furthermore, it should be noted that if sediment was being resuspended, the period of resettling would be strongly dependent on the particle size distribution and the reservoir flow rates, as well as the reservoir mixing rates. Field studies are the proper tools to understand the exact causes of low level of the reservoir total chlorine residual. In such field studies, the sediment content of the inflow, its total chlorine residual and total organic carbon content, along with its distribution patterns within the reservoir (at the reservoir bottom and at the area of the sediment suspension) should be performed on a regular basis.


4

INTRODUCTION

The water recirculation patterns in the IRWD East Irvine Zone 3 No.2 reservoir located in the City of Irvine, California, have been evaluated using CFD modeling. The goal of the CFD modeling was to evaluate the mixing performance of the two mixing systems installed in the reservoir by IRWD at different times, and to determine the general flow patterns and the mixing characteristics of the reservoir. This report includes a general discussion of the water circulation issues in this reservoir. Various methods for determining the reservoir performance are then discussed. This work has been performed by Flow Science Incorporated of Pasadena, California, under contract with IRWD of Irvine, CA.

BACKGROUND

Treated water storage reservoirs are of vital importance in water treatment and distribution systems. These facilities have traditionally been constructed to secure system hydraulic integrity and reliability, and water quality issues have generally been considered secondary maintenance items. To meet emergency supply goals, large volumes of reserve storage are usually incorporated into system operation and design. This can result in long average detention times (stored water volume divided by the average inflow per day) and significant consequences for distribution water quality. Indeed, redundant storage and excessive residence times result in aged water that can significantly reduce disinfectant residual content through reactions with oxidizable material in the water. Lack of adequate disinfectant levels can result in the risk of microbial contamination and serious regulatory problems.

Water quantity and quality requirements are frequently in conflict. While water quantity objectives promote maximizing storage, water quality objectives are geared toward minimizing residence times, eliminating recirculation zones, and frequent filling and draining of treated water facilities to maximize the stored water disinfectant residual. Appropriate balancing is therefore required to assure disinfection effectiveness and sufficient level of service.

PREDICTING TRANSPORT AND MIXING PROCESSES IN RESERVOIRS

In recent years, much effort has been devoted to developing, understanding, and improving the predictive capability for the transport and mixing processes in storage facilities and their impact on the quality of water in the distribution system. Regulatory requirements, customer expectations, and the desire to minimize water quality deterioration, and the ability to provide more reliable and safe operation, have motivated this effort. The most commonly used technique involves simulating the flow in a reservoir using the CFD tools.


5

CFD MODELING

Until recently the procedures reported and applied to treated water storage facilities have traditionally focused on the use of the conceptual compartmentalization approach. A more current alternative to multi-compartment models is provided by CFD. CFD is a generalized modeling method that is more robust and accurate than earlier approaches to numerical modeling.

Flow Science solves the reservoir-mixing problem directly by using CFD. CFD provides a rigorous two or three-dimensional hydrodynamic model for simulating chemical mixing and internal patterns of the flow within distribution storage reservoirs. A CFD model explicitly considers the generally accepted basic equations governing the turbulent motion of water. The resulting model is reasonably accurate, robust, and can be readily applied to all types of storage reservoir configurations, characteristics, and hydraulic conditions. The model provides a valuable tool for managing distribution system water quality.

The basic concepts underlying CFD, as applied to a water storage reservoir, consist of a set of conservation equations (mass, momentum, and energy) that are solved using the numerical method of finite differences. The computational domain, i.e., the reservoir, is subdivided into small computational elements over which the conservation equations are solved. Typically, 25,000 to 300,000 computational elements are used. In general, a smaller grid size provides more accurate results, but with commensurately longer execution times for the simulation. Inputs to the model consist of a geometrical description of the reservoir and a set of initial and boundary conditions (e.g., inlet/outlet velocities and geometry). The model output produces spatial and temporal solutions for the variables (pressure, velocity, and temperature) that can be presented in graphical and tabular form.

METHOD OF ANALYSIS

In this report, the flow within the reservoir is modeled using Flow Science's CFD program, FLOWMOD. FLOWMOD has been validated in various applications that include flow modeling within reservoirs and distribution system reservoirs. A discussion of the model’s underlying theory and method of solution is provided in Hannoun and Boulos (1997). A sample model application to simulate flow in clearwells is included in Hannoun et al (1998).

In the present application a steady-state solution for the velocity field under isothermal conditions (no temperature effects) is obtained through iteration for both fill and draw scenario. The velocity field is then used to predict the fate of a non-buoyant, conservative (non-decaying) tracer injected into the inflow, as well as for


6

determining the water age. During the fill phase, the computed fill flow velocities are used; the draw velocities are used during the draw phase. Various retention time parameters are then computed from the tracer advection. The water age is also computed.

Water age is a measure of the amount of time when a water particle at a certain location (computational cell) has been residing in the reservoir. For example, the age of inflow water is considered to be zero. As this inflow water travels through the reservoir, its water age increases by one day for each day it remains in the reservoir. As time passes, water already present in the reservoir continues to “age” as it mixes with incoming new water. Therefore, the age of a water particle can be determined by following its path line and calculating how many days it takes for the water to travel along this path from the inflow to the outflow location, while taking into account the mixing that occurs within a given computational cell. Due to the internal reservoir mixing not all the particles follow the same path through the reservoir, thus water age may vary spatially in the reservoir.

Water age therefore represents the length of time water particles spend in the reservoir. As an example, if a reservoir is filled at time t = 0, and all inflows and outflows are halted, the water age in the reservoir would be uniform and increase at the rate of one day per day.

After the computations are completed, the reservoir performance is classified based on its maximum water age near steady-state conditions. The ratio, R, of maximum water age to average residence time is then evaluated. Average residence time, tave, is defined as the ratio of reservoir volume to flow rate. R is an indicator of the mixing characteristics of a reservoir and is a useful dimensionless parameter to compare reservoir performance under different flow conditions. A completely mixed reservoir has an R-value of 1; whereas a reservoir with poor mixing has an R-value of 3.5 or larger (this is somewhat subjective). Intermediate qualitative classifications of Excellent (<1.1), Good (R=1.1 to 1.4), Moderate (R= 1.4 to 3.5), and Poor mixing (R> 3.5) are made based on experience with other reservoirs. It is noted here that two reservoirs with identical geometry and flow patterns would have an identical value of R. However, the maximum water age in a reservoir is also dependent on tave. For a specific reservoir geometry (including the inlet and the outlet), reductions in the maximum water age can be achieved by reducing tave. On the other hand, for a specific reservoir tave, the maximum water age may be achieved by reducing R. Improving the circulation patterns in a reservoir may reduce R, whereas tave could be lowered by increasing the reservoir flow rate, reducing the reservoir storage volume, or both.

In general, water quality criteria are determined by the maximum water age. Acceptable values of maximum water age are based on maintaining an adequate disinfectant residual and preventing the formation of unwanted disinfection by-products. Values for acceptable water age vary drastically from place to place, depending on the


7

type of the disinfectant used (e.g., free chlorine versus chloramines) and the presence of various constituents in the water (such as organics).

ANALYSIS RESULTS

RESERVOIR GEOMETRY/MIXERS

The IRWD Zone 3 No. 2 reservoir is circular in shape with the internal diameter of 194 ft, as shown in Figure 1. The reservoir features a single inlet/outlet that is 30 inches in diameter located near the reservoir side wall and approximately 4 ft above the floor. The opening of the inlet/outlet pipe points horizontally toward the center of the tank (the jet inflow discharges horizontally).

Prior to 2011 the reservoir was originally equipped with a Reservoir Management System (RMS), consisting of a STS Rover mixer and STS chloramine analyzer/injection system. The STS Rover mixer incorporates a submersible pump and a 2-inch discharge nozzle (Figure 2). The mixer continuously pumps water at a rate of 22 gpm with an inlet near the bottom of the reservoir and discharges it vertically upwards through the discharge nozzle. The chloramine analyzer/injection system monitors the water entering the pump and doses the water if the concentration of total chlorine residual falls below 2.5 mg/L. The detailed dosing process is as follows:

• When 1 mg/L < total chlorine residual < 2.5 mg/L, 12.5% sodium hypochlorite is injected at a rate of 111 ml/min.

• When total chlorine residual < 1 mg/L, 12.5% sodium hypochlorite is injected at a rate of 185 ml/min.

• When total chlorine residual > 2.5 mg/L, no sodium hypochlorite is injected.

A new mixer system, a SWT Vortex System, was installed in the reservoir replacing the STS Rover mixer during the first week of 2011 for demonstration testing. Rather than a single nozzle discharging vertically upward, the SWT Vortex system is designed with three discharge ports located on either side of a triangular plate (Figure 3). The nozzles are located 120 degrees apart and discharge the pumped water at 45 degrees above the horizontal plane. Each nozzle is 11/32-inch in diameter. The new mixer pumps water at a rate of 50 gpm. The same chlorine dosing process described above is used for the new system. All the information about the mixers was obtained from IRWD.


8

DATA ANALYSIS

The inflow rate and water level (i.e., volume of water stored) in the reservoir are highly variable, as shown for the year 2010 in Figures 4 - 10 (the referenced data was obtained from IRWD). A high water level (i.e., high water volume) and low flow rates that occurred during the period between 1/26/10 and 2/8/10 resulted in high water age in the reservoir. Consequently, this period was chosen to model the fill/draw cycle in the reservoir since a high water age generally produces lower chemical concentrations in the reservoir water and hence is considered to be a conservative length of time for modeling. The flow rates during this period were simplified/normalized to achieve a pseudo-steady state in flow patterns and water age as follows: (1) the reservoir is assumed to have a fill cycle followed by a draw cycle; (2) the reservoir fills at the rate of 646 gpm (0.93 million gallon per day or MGD) for a period of 12 hours followed by a draw cycle at a rate of 646 gpm (0.93 MGD) for a period of 12 hours; (3) this cycle is repeated for the duration of the simulation.

During the modeling period, the average reservoir volume was approximately 5.1 million gallon (MG) with an average water depth of 23.0 ft. Based on the reservoir volume and flow rate, the average residence time was 11.0 days (computed as the volume of 5.1 MG divided by the daily average flow rate of 323 gpm [=646 gpm × (12 hr/24 hr)]).

An analysis to determine chlorine decay rate based on the field data was conducted in order to simulate total chlorine residual in the reservoir. Total chlorine residuals were measured throughout 2010 and early 2011 at two locations: at the mixer and at the periphery of the reservoir (roughly 8 -10 ft from the bottom of the reservoir, according to Arseny Kalinsky of IRWD). Figures 11-17 show the total chlorine residuals measured at the mixer (labeled as “Online Cl2 Residual” in the figures), and total chlorine residuals measured at the periphery of the reservoir (labeled as “Cl2 Residual Field Data” in the figures). There are no total chlorine residual data available for the inflow or the outflow. Given this, and the chloramine injection by the RMS system, periods of time in 2010 were searched for conditions of no inflows or chloramine injection, so the chlorine decay rate could be estimated using an exponential decay formula that incorporated in-reservoir measured chlorine residual data for these periods. Based on the analysis (see Figure 18), an exponential decay coefficient range of 0.1/day to 0.2/day (9.5% and 18% per day respectively) was observed. The 0.2/day decay rate was chosen for the modeling purposes because it was a more conservative value (i.e., faster decay).

It should be noted that the available data points during the period of no inflow and no chloramine injection is very limited. The instantaneous decay rates were calculated for all the periods to provide a lower bound for the decay rate. Figure 19 shows that the


9

instantaneous decay rate varies widely and can be as high as 4/day (a reduction of 98% in a day). Approximately a dozen episodes were identified with an instantaneous decay rate greater than 1/day. This indicates that the adopted decay rate of 0.2/day may be too small during some periods. The underlying reason for these high decay rates is not known. For instance, it could be high levels of total organic carbon in the inlet flow.

MODELING SCENARIOS

The flow patterns for IRWD Zone 3 No.2 reservoir were evaluated for three scenarios. The scenarios considered are as follows:

Base Case: Represents the current reservoir geometrical configuration without any mixing system.

STS Rover System: Represents the current reservoir geometrical configuration with a STS Rover mixing system.

SWT Vortex System: Represents the current reservoir geometrical configuration with a SWT Vortex mixing system.

The finite-difference grid used to simulate the reservoir consists of 101 x 101 grid points in the horizontal x- and z-directions (2-ft resolution, see Figure 20) and 11 points in the vertical y-direction (2.1 ft for a 23 ft depth). This provides a three dimensional representation of the flow in the reservoir.

BASE CASE

Particle Paths and Velocity Vectors

Figure 21 shows the computed streamlines at the surface, the bottom, and along a vertical plane at the center of the reservoir for the fill cycle. These streamlines represent trajectories of massless particles injected into the reservoir at various points. These particle paths (i.e., streamlines) show only the horizontal velocities in the plane of the paper; vertical velocities (perpendicular to the plane of the paper) are not shown. As a result, some of these particle paths may appear to hit the sidewalls. In reality, however, the particles approaching the walls will be transported vertically alongside the boundary. As shown in Figure 21, the flow originates from the inlet near the wall and spreads out towards the edge of the tank. Most of the flow particles travel around the perimeter of the reservoir and then eventually are drawn back toward the inlet. However, some of the flow is caught in twin vortices, or recirculation zones, on either side of the tank. There is


10

another vortex with a horizontal axis shown in the vertical plane (cross-sectional view in Figure 21). This vortex is generated by the inflow jet that reaches the opposite side wall, rises to the surface and falls back after traveling along the surface.

Figure 22 shows the computed particle paths at the surface, the bottom and along the vertical plane at the center of the reservoir for the draw cycle. It shows that water is drawn back into the inlet/outlet from all directions during the draw cycle.

Figures 23 and 24 show the computed velocity vectors for the fill and draw cycles. The velocity vectors show a similar geometric trend to the streamlines depicted in Figures 21 and 22.

Tracer Advection

The enclosed Attachment A (compact disc) presents a tracer concentration “movie” for the Base Case. The animation movie shows the time sequence of the evolution of concentration contours, presented in false color, resulting from the injection of a slug of tracer mixed with the inflow at time t = 0. In the movie, the tracer concentration is presented in arbitrary units. Note that the red color indicates high tracer concentrations, and the blue color indicates lower concentrations, as depicted at the accompanying color legend in linear scale. Instructions for viewing the movies are included in Attachment B. Figure 25 shows a total of six surface snapshots taken from the animation movie. The figure, as well as the animation, shows that tracer travels along the streamlines shown in Figure 21. At the end of 96 hours, the tracer is mixed in most of regions, except for areas near the side wall.

Water Age

The modeling was also used to predict the water age in the reservoir. Figure 26 presents the computed water age at the reservoir surface and the reservoir bottom and along the vertical plane at the center of the reservoir under near steady-state conditions. The center of the twin vortices at the surface exhibits the highest levels of water age; the maximum water age is about 14.9 days. The calculated R value, i.e., the ratio of maximum water age to average residence time (11.0 days), is 1.4, indicating moderate mixing within the reservoir.

Chlorine Residual

Total chlorine residual contours are presented in Figure 27. Contours’ units are mg/L in linear scale. Concentrations are shown at horizontal planes at the surface and the bottom of the reservoir and also along the vertical plane extending through the center of the reservoir.


11

The contours of total chlorine residual resemble the water age contours; however, there is an inverse relationship between the two. Chlorine decays in the water with time; therefore, older water age produces lower chlorine residual concentrations. The highest concentrations were found near the inlet since this is the newest water. The lowest concentrations, 0.2 mg/L, were located near the center of the vortices at the surface, since water at that location had the highest age. Overall, total chlorine residual in the reservoir was below 0.8 mg/L. This is mainly due to long average residence time and lack of external dosing.

STS ROVER SYSTEM

The STS Rover mixing system was located approximately 30 ft from the center of the reservoir; the schematic of the installation is shown in Figure 28.


The streamlines near steady-state conditions during the fill and draw cycles (the surface and the bottom plan views and the cross-sectional view) are presented in Figures 29 and 30, respectively. During the fill cycle, the flow patterns were influenced by both the inlet jet and the jet discharged by the mixer. During the draw cycle, the flow patterns were heavily influenced by the mixer.

Figures 31 and 32 show the computed velocity vectors during the fill and draw cycles respectively these figures mimic what is depicted in Figures 29 and 30.

Tracer Advection

The enclosed Attachment A presents a tracer concentration animation “movie” for the STS Rover System scenario. The movie shows the time sequence of the evolution of concentration contours, presented in false color, resulting from the injection of a slug of tracer mixed with the inflow at time t=0. Figure 33 shows a total of six surface snapshots from the animation movie. At the end of 96 hours, the tracer is well mixed throughout the reservoir and this indicates that the mixer provides good mixing in the reservoir.

Water Age

Figure 34 presents the computed water age near steady-state conditions. The predicted water age is fairly uniformly distributed, indicating a well-mixed reservoir.


12

The predicted maximum water age over the whole reservoir is about 13.0 days. This yields to the R value of 1.2, indicating good mixing.

Chlorine Residual

Figure 35 presents the computed total chlorine residual at the reservoir surface and the reservoir bottom and at the central vertical plan under near steady-state conditions. The minimum chlorine residual in the whole reservoir is over 1.8 mg/L.

SWT VORTEX SYSTEM

The SWT Vortex mixing system is located approximately 20 ft from the center of the reservoir: a schematic of the installation is shown in Figure 36.


Figures 37 and 38 show the computed streamlines during both the fill and the draw cycles, respectively. It is clear that the flow patterns were heavily influenced by the jets discharged by the mixer during the fill and the draw cycles. Figures 39 and 40 show the corresponding velocity vectors resembling what is depicted in Figures 37 and 38.

Tracer Advection

A tracer concentration animation “movie” for the SWT Vortex mixing system scenario is enclosed in the Attachment A. The movie shows the time sequence of the evolution of concentration contours, presented in false color, resulting from the injection of a slug of tracer mixed with the inflow at time t=0. Figure 41 shows a total of six surface snapshots from the animation movie. The tracer is well mixed at approximately 12.0 hours and this indicates that this mixer provides sufficient mixing to fully mix the reservoir during the fill cycle.

Water Age

Figure 42 presents the computed water under near steady-state conditions. The results of the simulation show a near-fully mixed reservoir and the maximum water age in the reservoir of 11.4 days. This yields to the R value of 1.0 indicating excellent mixing.

Chlorine Residual

Figure 43 presents the computed total chlorine residual under near steady-state conditions. Total chlorine residual in the whole reservoir is above 2.4 mg/L, indicating


13

that chloramines injected by the RMS are effectively distributed throughout the reservoir by the mixer.

CONCLUSIONS AND DISCUSSION

Based on the analysis, the mixing in the reservoir without the mixer or the external dosing (i.e., Base Case) is deemed as moderate mixing and the predicted total chlorine residual can be as low as 0.2 mg/L in the areas of limited mixing. Overall, total chlorine residual in the reservoir is below 0.8 mg/L. This is mainly due to a long average residence time and lack of the external dosing of the chloramines. With use of the STS Rover mixing system, the mixing efficiency is improved from moderate to good and the lowest predicted total chlorine residual is approximately 1.8 mg/L. The quality of mixing is further improved to excellent with the use of the SWT Vortex mixing system which produces a near fully mixed reservoir with a minimum predicted total chlorine residual of approximately 2.4 mg/L. A summary of the simulation results is presented in Table 1.

Table 1: Summary of Three Scenarios Evaluated Using CFD

Scenario

Average Residence

Time (days)

Minimum Total

Chlorine Residual (mg/L)

Maximum Water Age

(days) R Mixing

Quality

Base Case 11.0 0.2 14.9 1.4 Moderate STS Rover System 11.0 1.8 13.0 1.2 Good

SWT Vortex System 11.0 2.4 11.4 1.0 Excellent Note that the field data show episodes of low observed chlorine residual in the

reservoir (the typical low level total chlorine residual episode lasted about a few days) that cannot be duplicated in the CFD analysis (when either mixing system was in operation). Figure 44 shows the comparison of the total chlorine residual measured at the mixer (near the center of the reservoir) and at the periphery of the reservoir. The above mentioned total chlorine residual values follow each other very closely, implying that the chlorine residual was spatially uniform within the reservoir when either mixing system was in operation. This is consistent with the findings in this CFD study. Thus, the cause of low total chlorine residual events in the reservoir is likely not related to mixing, but rather is caused by one or more of the following factors:

• Variable inflow water quality (e.g., variable concentration of total chlorine residual and total organic carbon) causing variable chlorine inflow loading and in-reservoir chlorine demand.


14

• Sediment scouring and resuspension, which may affect the chlorine decay rate. It was observed that there is a layer of sediment on the reservoir’s bottom (personal communication with Mark Malmquist of SWT). Scouring pf the sediments may result in periods of high chlorine demand.

• The injection equipment was not performing according to the specification.

Some limited analysis was performed to determine whether the low levels of total chlorine residual were correlated to the large fill events. The results were inconclusive as some of the low levels of total chlorine residual also occurred during the reservoir draw periods. If sediment resuspension was the cause of rapid chlorine decay within the tank, then the exact determination of the periods of sediment resuspension cannot be identified from the existing data. This is a result of insufficient information about the sediment content of the inflow (variability, size distribution, etc.) and the sediment distribution within the tank at various times. Furthermore, it should be noted that if sediment was being resuspended, the period of resettling would be strongly dependent on the sediment particle size distribution and the reservoir flow rates, as well as the reservoir mixing rates. Field studies are the proper tools to identify the exact causes of low level of the reservoir total chlorine residual. In such field studies, the sediment content of the inflow, its total chlorine residual and total organic carbon content, along with its distribution patterns within the reservoir (at the reservoir bottom of the reservoir and at the area of the sediment suspension area) should be performed on a regular basis.

REFERENCES

1. “Using hydraulic modeling to optimize contact time”, I.A. Hannoun, P.F. Boulos, and E.J. List. Journal AWWA 90, August 1998.

2. “Optimizing distribution storage water quality: a hydrodynamic approach”, I.A. Hannoun and P.F. Boulos, Journal of Applied Mathematical Modeling 21, pp.495-502, 1997.


15

FIGURES

FSI V106082September 19, 2011 Figure 1

X

Y Z

30 inchInlet/Outlet19

4ft

Cell Dimension = 2 ft x 2 ft

X

Y

Z

23ft

IRWD Zone 3 No.2 Reservoir Geometry

Cell Dimension = 2 ft x 2.1 ft30 inch

Inlet/Outlet


Schematic of STS Rover System


Schematic of SWT Vortex System


IIC East Zone 3 #2 Reservoir Level and Inflow/Outflow Rate (1/1/2010 - 2/28/2010)

10

100

1000

10000

1/1/

2010

1/3/

2010

1/5/

2010

1/7/

2010

1/9/

2010

1/11

/201

01/

13/2

010

1/15

/201

01/

17/2

010

1/19

/201

01/

21/2

010

1/23

/201

01/

25/2

010

1/27

/201

01/

29/2

010

1/31

/201

02/

2/20

102/

4/20

102/

6/20

102/

8/20

102/

10/2

010

2/12

/201

02/

14/2

010

2/16

/201

02/

18/2

010

2/20

/201

02/

22/2

010

2/24

/201

02/

26/2

010

2/28

/201

0

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35

Inflow Rate Outflow Rate Reservoir Level

Critical Simulation Period

Measured Inflow/Outflow Rates and Reservoir Surface Elevation(1/1/2010 – 2/28/2010)

Water Level (ft)



10

100

1000

10000

3/1/

2010

3/3/

2010

3/5/

2010

3/7/

2010

3/9/

2010

3/11

/201

03/

13/2

010

3/15

/201

03/

17/2

010

3/19

/201

03/

21/2

010

3/23

/201

03/

25/2

010

3/27

/201

03/

29/2

010

3/31

/201

04/

2/20

104/

4/20

104/

6/20

104/

8/20

104/

10/2

010

4/12

/201

04/

14/2

010

4/16

/201

04/

18/2

010

4/20

/201

04/

22/2

010

4/24

/201

04/

26/2

010

4/28

/201

04/

30/2

010

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)



10

100

1000

10000

5/1/

2010

5/3/

2010

5/5/

2010

5/7/

2010

5/9/

2010

5/11

/201

05/

13/2

010

5/15

/201

05/

17/2

010

5/19

/201

05/

21/2

010

5/23

/201

05/

25/2

010

5/27

/201

05/

29/2

010

5/31

/201

06/

2/20

106/

4/20

106/

6/20

106/

8/20

106/

10/2

010

6/12

/201

06/

14/2

010

6/16

/201

06/

18/2

010

6/20

/201

06/

22/2

010

6/24

/201

06/

26/2

010

6/28

/201

06/

30/2

010

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)



10

100

1000

10000

7/1/

2010

7/3/

2010

7/5/

2010

7/7/

2010

7/9/

2010

7/11

/201

07/

13/2

010

7/15

/201

07/

17/2

010

7/19

/201

07/

21/2

010

7/23

/201

07/

25/2

010

7/27

/201

07/

29/2

010

7/31

/201

08/

2/20

108/

4/20

108/

6/20

108/

8/20

108/

10/2

010

8/12

/201

08/

14/2

010

8/16

/201

08/

18/2

010

8/20

/201

08/

22/2

010

8/24

/201

08/

26/2

010

8/28

/201

08/

30/2

010

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)



10

100

1000

10000

9/1/

2010

9/3/

2010

9/5/

2010

9/7/

2010

9/9/

2010

9/11

/201

09/

13/2

010

9/15

/201

09/

17/2

010

9/19

/201

09/

21/2

010

9/23

/201

09/

25/2

010

9/27

/201

09/

29/2

010

10/1

/201

010

/3/2

010

10/5

/201

010

/7/2

010

10/9

/201

010

/11/

2010

10/1

3/20

1010

/15/

2010

10/1

7/20

1010

/19/

2010

10/2

1/20

1010

/23/

2010

10/2

5/20

1010

/27/

2010

10/2

9/20

1010

/31/

2010

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)



10

100

1000

10000

11/1

/201

011

/3/2

010

11/5

/201

011

/7/2

010

11/9

/201

011

/11/

2010

11/1

3/20

1011

/15/

2010

11/1

7/20

1011

/19/

2010

11/2

1/20

1011

/23/

2010

11/2

5/20

1011

/27/

2010

11/2

9/20

1012

/1/2

010

12/3

/201

012

/5/2

010

12/7

/201

012

/9/2

010

12/1

1/20

1012

/13/

2010

12/1

5/20

1012

/17/

2010

12/1

9/20

1012

/21/

2010

12/2

3/20

1012

/25/

2010

12/2

7/20

1012

/29/

2010

12/3

1/20

10

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)



10

100

1000

10000

1/1/

2011

1/3/

2011

1/5/

2011

1/7/

2011

1/9/

2011

1/11

/201

1

1/13

/201

1

1/15

/201

1

1/17

/201

1

1/19

/201

1

1/21

/201

1

1/23

/201

1

1/25

/201

1

1/27

/201

1

1/29

/201

1

1/31

/201

1

Date

Flow

Rat

e (g

pm)

0

5

10

15

20

25

30

35



Water Level (ft)


IIC East Zone 3 #2 Reservoir Level and Chlorine Residual (1/1/2010-2/28/2010)

0

1

2

3

4

5

1/1/

2010

1/3/

2010

1/5/

2010

1/7/

2010

1/9/

2010

1/11

/201

01/

13/2

010

1/15

/201

01/

17/2

010

1/19

/201

01/

21/2

010

1/23

/201

01/

25/2

010

1/27

/201

01/

29/2

010

1/31

/201

02/

2/20

102/

4/20

102/

6/20

102/

8/20

102/

10/2

010

2/12

/201

02/

14/2

010

2/16

/201

02/

18/2

010

2/20

/201

02/

22/2

010

2/24

/201

02/

26/2

010

2/28

/201

0

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35

Online Cl2 Residual (mg/L) Cl2 Residual Field Data (mg/L)Reservoir Level

Measured Chlorine Residual and Reservoir Surface Elevation(1/1/2010 – 2/28/2010; Online Cl2 data were

measured at the mixer; field data were measured at the side wall)

Water Level (ft)



0

1

2

3

4

5

3/1/

2010

3/3/

2010

3/5/

2010

3/7/

2010

3/9/

2010

3/11

/201

03/

13/2

010

3/15

/201

03/

17/2

010

3/19

/201

03/

21/2

010

3/23

/201

03/

25/2

010

3/27

/201

03/

29/2

010

3/31

/201

04/

2/20

104/

4/20

104/

6/20

104/

8/20

104/

10/2

010

4/12

/201

04/

14/2

010

4/16

/201

04/

18/2

010

4/20

/201

04/

22/2

010

4/24

/201

04/

26/2

010

4/28

/201

04/

30/2

010

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)



0

1

2

3

4

5

5/1/

2010

5/3/

2010

5/5/

2010

5/7/

2010

5/9/

2010

5/11

/201

05/

13/2

010

5/15

/201

05/

17/2

010

5/19

/201

05/

21/2

010

5/23

/201

05/

25/2

010

5/27

/201

05/

29/2

010

5/31

/201

06/

2/20

106/

4/20

106/

6/20

106/

8/20

106/

10/2

010

6/12

/201

06/

14/2

010

6/16

/201

06/

18/2

010

6/20

/201

06/

22/2

010

6/24

/201

06/

26/2

010

6/28

/201

06/

30/2

010

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)



0

1

2

3

4

5

7/1/

2010

7/3/

2010

7/5/

2010

7/7/

2010

7/9/

2010

7/11

/201

07/

13/2

010

7/15

/201

07/

17/2

010

7/19

/201

07/

21/2

010

7/23

/201

07/

25/2

010

7/27

/201

07/

29/2

010

7/31

/201

08/

2/20

108/

4/20

108/

6/20

108/

8/20

108/

10/2

010

8/12

/201

08/

14/2

010

8/16

/201

08/

18/2

010

8/20

/201

08/

22/2

010

8/24

/201

08/

26/2

010

8/28

/201

08/

30/2

010

9/1/

2010

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)



0

1

2

3

4

5

9/1/

2010

9/3/

2010

9/5/

2010

9/7/

2010

9/9/

2010

9/11

/201

09/

13/2

010

9/15

/201

09/

17/2

010

9/19

/201

09/

21/2

010

9/23

/201

09/

25/2

010

9/27

/201

09/

29/2

010

10/1

/201

010

/3/2

010

10/5

/201

010

/7/2

010

10/9

/201

010

/11/

2010

10/1

3/20

1010

/15/

2010

10/1

7/20

1010

/19/

2010

10/2

1/20

1010

/23/

2010

10/2

5/20

1010

/27/

2010

10/2

9/20

1010

/31/

2010

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)



0

1

2

3

4

5

11/1

/201

011

/3/2

010

11/5

/201

011

/7/2

010

11/9

/201

011

/11/

2010

11/1

3/20

1011

/15/

2010

11/1

7/20

1011

/19/

2010

11/2

1/20

1011

/23/

2010

11/2

5/20

1011

/27/

2010

11/2

9/20

1012

/1/2

010

12/3

/201

012

/5/2

010

12/7

/201

012

/9/2

010

12/1

1/20

1012

/13/

2010

12/1

5/20

1012

/17/

2010

12/1

9/20

1012

/21/

2010

12/2

3/20

1012

/25/

2010

12/2

7/20

1012

/29/

2010

12/3

1/20

10

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)



0

1

2

3

4

5

1/1/

2011

1/3/

2011

1/5/

2011

1/7/

2011

1/9/

2011

1/11

/201

1

1/13

/201

1

1/15

/201

1

1/17

/201

1

1/19

/201

1

1/21

/201

1

1/23

/201

1

1/25

/201

1

1/27

/201

1

1/29

/201

1

1/31

/201

1

Date

Cl 2

Res

idua

l (m

g/L)

0

5

10

15

20

25

30

35




Water Level (ft)


y = 3.1955e-0.1078x

R2 = 0.9273

y = 3.8118e-0.2012x

R2 = 0.8815

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 0.5 1 1.5 2 2.5 3Days

Chl

orin

e R

esid

ual (

mg/

L)

2/12/10 - 2/14/106/7/10-6/9/10Exponential Decay Approximation (2/12/10-2/14/10)Exponential Decay Approximation (6/7/10-6/9/10)

Chlorine Residual versus Time


Instantaneous Chlorine Decay Rate (1/1/2010-3/28/2011)

0

1

2

3

4

5

6

1/1/

2010

2/1/

2010

3/4/

2010

4/4/

2010

5/5/

2010

6/5/

2010

7/6/

2010

8/6/

2010

9/6/

2010

10/7

/201

0

11/7

/201

0

12/8

/201

0

1/8/

2011

2/8/

2011

3/11

/201

1

Date

Cl 2

Dec

ay R

ate

(1/d

ay)

-12000

-8000

-4000

0

4000

8000

12000

Instantaneous Decay Rate 3-Day Rolling-averaged Decay RateInflow Rate

SWT Vortex Mixer

Inflow R

ate (gpm)

STS Rover Mixer

Modeling Period


X

Y Z


4ft


X

Y

Z

23ft

IRWD Zone 3 No.2 ReservoirBase Case - Computational Grid

Cell Dimension = 2 ft x 2.1 ft30 inch

Inlet/Outlet


X

Y Z

Plan View - Near Bottom

A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirBase Case - Fill Cycle Streamlines

Cross-Sectional View - A - A'

X

Y Z

Plan View - Near Surface


X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirBase Case - Draw Cycle Streamlines


X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirBase Case - Fill Cycle Velocity Vectors


0.1 ft/s

X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirBase Case - Draw Cycle Velocity Vectors


0.01 ft/s

X

Y Z



Time = 0.96 hours Time = 2.40 hours Time = 12.00 hours

Time = 24.00 hours

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time = 48.00 hours

TraceConc.

(%)

Time = 96.00 hours

IRWD Zone 3 No.2 ReservoirBase Case - Tracer Concentration at Surface


X

Y Z


A A'

X

Y

Z14121086420

IRWD Zone 3 No.2 ReservoirBase Case - Water Age


Water Age(days)

X

Y Z



X

Y Z


A A'

X

Y

Z

54.543.532.521.510.50

IRWD Zone 3 No.2 ReservoirBase Case - Chlorine Residual


ChlorineResidual(mg/L)

X

Y Z



X

Y Z

Center

194

ftCell Dimension = 2 ft x 2 ft

STS RoverSystem

A'A

30 inchInlet/Outlet

30 ft

X

Y

Z

23ft

IRWD Zone 3 No.2 ReservoirSTS Rover System - Schematic

30 inchInlet/Outlet

STS RoverSystem

A - A'


X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSTS Rover System - Fill Cycle Streamlines


STS RoverSystem

X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSTS Rover System - Draw Cycle Streamlines


X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSTS Rover System - Fill Cycle Velocity Vectors


0.5 ft/s

X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSTS Rover System - Draw Cycle Velocity Vectors


0.5 ft/s

X

Y Z




Time = 24.00 hours

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time = 48.00 hours

TraceConc.

(%)

Time = 96.00 hours

IRWD Zone 3 No.2 ReservoirSTS Rover System - Tracer Concentration at Surface


X

Y Z


A A'

X

Y

Z14121086420

IRWD Zone 3 No.2 ReservoirSTS Rover System - Water Age


Water Age(days)

X

Y Z



X

Y Z


A A'

X

Y

Z

54.543.532.521.510.50

IRWD Zone 3 No.2 ReservoirSTS Rover System - Chlorine Residual (K = 0.2/day)



X

Y Z



X

Y Z


4ft


SWT VortexSystem

A'A

NozzleOrientation

Center20 ft

X

Y

Z

23ft

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Schematic

30 inchInlet/Outlet

SWT VortexSystem

A - A'


X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Fill Cycle Streamlines


SWT VortexSystem

X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Draw Cycle Streamlines


X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Fill Cycle Velocity Vectors


0.5 ft/s

X

Y Z



X

Y Z


A A'

X

Y

Z

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Draw Cycle Velocity Vectors


0.5 ft/s

X

Y Z




Time = 24.00 hours

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time = 48.00 hours

TraceConc.

(%)

Time = 96.00 hours

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Tracer Concentration at Surface


X

Y Z


A A'

X

Y

Z14121086420

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Water Age


Water Age(days)

X

Y Z



X

Y Z


A A'

X

Y

Z

54.543.532.521.510.50

IRWD Zone 3 No.2 ReservoirSWT Vortex System - Chlorine Residual (K = 0.2/day)



X

Y Z



Comparison of Online Chlorine Data and Lab Field Data (1/1/2010-3/28/2011)

0

2

4

6

8

10

1/1/

2010

2/1/

2010

3/4/

2010

4/4/

2010

5/5/

2010

6/5/

2010

7/6/

2010

8/6/

2010

9/6/

2010

10/7

/201

0

11/7

/201

0

12/8

/201

0

1/8/

2011

2/8/

2011

3/11

/201

1

Date

Cl 2

Dec

ay R

ate

(1/d

ay)

0

8

16

24

32

40

Chlorine Residual Concentrations at the Mixer Chlorine Lab Data at the Periphery of the ReservoirReservoir Level

SWT Vortex Mixer

Water Leve (ft)

STS Rover Mixer

Modeling Period


A-1

ATTACHMENT A

List of Animations


A-2

LIST OF ANIMATIONS

1. Base_Case.rm 2. STS_Rover.rm 3. SWT_Vortex.rm


B-1

ATTACHMENT B

Instructions on How to Use Framer


B-2

INSTRUCTIONS FOR INSTALLING AND USING FRAMER TO VIEW MOVIE FILES

INSTALLATION OF FRAMER Copy the files from the CD to a directory on your computer. Running Framer 1) In the Start Menu, choose “run.” In this window, type “framer.exe.” This should

open a “Framer Open File” window, in which you find the proper directory and choose the file that you wish to view.

2) Commands for running the movie files are in the toolbar in the upper left corner of

the framer window.

Documents

CFD Analysis for Irvine Ranch Water District Zone 3 No. 2 ... · IRWD Zone 3 No.2 Reservoir CFD Study FSI V106082 September 19, 2011 SUMMARY The water recirculation patterns in the