PALM BEACH COUNTY WATER UTILITIES DEPARTMENT –

WATER TREATMENT PLANT 2 MAGNETIC ION EXCHANGE By: Monica Pazahanick and Chris Reinbold, P.E. - Carollo Engineers, Inc.

Co-Authors: Stephen McGrew, P.E. and Patti Brock - Palm Beach County Water Utilities Department,

Vincent Hart, P.E. and Lyle Munce, P.E. - Carollo Engineers, Inc.

INTRODUCTION

Many utilities are faced with the challenge of treating raw water with high organic matter concentration, complying with more stringent disinfection byproduct regulations, preventing nitrification in chloraminated water distribution systems, and providing an aesthetic water quality that is color and turbidity free. Anion exchange resin has been an option for color removal (organics removal) but in the past, system capital and operation and maintenance costs have limited its applications.

Advances in the technology and modifications to the ion exchange system configuration have resulted in lower capital costs. In addition, this change is a direct result of increased loading rates from 2-3 gpm/ft2 to 8 gpm/ft2 for the resin contactors and settling equipment, as well as, the combination of mixing and settling basins into one unit. The design for Palm Beach County Water Utilities Department (PBCWUD) resulted in reduced operation and maintenance (O&M) costs as well as more operator friendly operations.

BACKGROUND

Palm Beach County Water Utilities Department - Water Treatment Plant No. 2

Water Treatment Plant No. 2 (WTP 2) is a conventional groundwater softening treatment plant owned and operated by PBCWUD and located in West Palm Beach, Florida. The plant was originally constructed in 1973 and expanded in multiple projects to achieve the current rated capacity of 16.4 million gallons per day (mgd).

The initial water treatment processes at WTP 2 consisted of lime softening, granular media filtration, and ozonation for color control. Because the existing ozone system was approaching the end of its useful life, PBCWUD performed several studies to identify the lowest life cycle cost alternative to replace or upgrade the ozone system. These studies showed that the magnetic ion exchange (MIEX®) treatment system had the lowest costs when compared to upgrading the ozone system or fixed bed ion exchange. As a result, a high rate MIEX® treatment system was recommended and Carollo Engineers, Inc. was retained to provide services for design, permitting, construction, start-up and commissioning for the new anion exchange treatment system for WTP 2 for organics removal.

The MIEX® system was integrated into the conventional treatment system upstream of the lime softening process since the resin is minimally affected by suspended solids in the raw water. In addition to ion exchange, Carollo added a softening bypass line post ion exchange, which increased the rated plant capacity by 13% without modifications (or additions) to the downstream processes (softening basins or filters).This allowed WTP 2 to produce a more stable finished water by allowing some of the hardness and alkalinity in the raw water to remain.

These improvements increased WTP 2’s treatment capacity to 16.4 mgd. The system is currently in service and has been successfully operating since early 2012. Shown in Figure 1 is the constructed facility.

Figure 1. View of the Magnetic Ion Exchange System (MIEX®) at WTP 2

Technology Overview

Ion exchange technology has been in use for decades. The principle of ion exchange is adsorption of a cation or an anion to the surface of a functional group in exchange for a cation or anion that is more weakly held by the functional group. The adsorption of the cation or anion is dependent upon the factors such as function of ionic charge, hydrated radius, functional group interaction, and concentration level. Most dissolved organics present in the water are negatively charged particles and the use of anionic ion exchange is an effective mechanism for the removal of these compounds. One of the challenges with ion exchange is that in a drinking water matrix, there are numerous other anions competing for “space” on the function group of the anion exchange resin. The selectivity or the preference of the anion exchange media to remove specific anions is critical to the performance of the media. Figure 2 represents the selectivity chart for selected ion exchange resin.

Figure 2. MIEX® Resin Anion Selectivity

The MIEX® treatment process is a patented process developed by Orica Watercare, Inc (Orica). The resin is a strong base anion exchange developed with improvements resulting in high affinity for the removal of dissolved organic carbon (DOC). The MIEX® resin particles are smaller than resins used in conventional ion exchange, with mean particle size of 200-250 micrometers (μm). This size distribution increases the available surface area by 4-5 times over conventional resins. In addition, these particles have magnetic properties that facilitate agglomeration and settling. The magnetic particles are embedded within the resin matrix so that as the resin breaks down, it still retains its magnetic properties.

The resin’s ability to be regenerated, once its exchange capabilities are expended, is one of the key parameters to the long-term performance of the process. The regeneration process utilizes high concentrations of a much weaker anion (chloride) to remove the original target anion (DOC). The selectivity of an anion can be overcome by increasing the concentration of the weaker anion. In this particular case, the resin is regenerated with a sodium chloride solution. Once the resin has been regenerated, the resin can be placed back in the system and reused. There are other regenerants that can be used for resin regeneration, such as potassium chloride and sodium bicarbonate; however, sodium chloride has proved to be the most cost effective if waste disposal of the used brine is not an issue.

Traditionally, larger diameter ion exchange resins, were installed in a pressure vessel configuration. This type of configuration presented restrictions in size, scale-up limitations, and potential problems with solids accumulation (typically required downstream of the filtration process). Orica, with the goal of reducing operating costs, minimizing the footprint of the treatment system, and improving the system performance, developed the high rate MIEX® treatment system. This process uses the same basic fundamentals as the previous version of the anion exchange system, but with a significantly higher resin concentration of up to 250 mL/L and reduced reaction or contact times from 10-30 minutes down to 5-10 minutes. In addition,

the resin is in contact with the raw water in a fluidized bed configuration and the contact and settling processes are combined into a single process (contact in the lower portion and settling in the upper portion of the contactor). These process modifications allow for loading rates of up to 8 gallons per minute per square foot (gpm/ft2) in the contactor and eliminate the constraints of the vessel configuration (footprint, ability to scale-up, and solids accumulation). Other sidestream processes associated with the system include fresh resin storage and reuse, resin regeneration, regenerant storage, and virgin resin storage.

Water Quality Goal Summary

A feasibility study was performed to determine the design parameters of the ion exchange treatment system and quantify the water quality goals. This study compared the regeneration system operating parameters (referred to as “bed volumes”. Bed volume is the volume of water treated divided by the volume of resin used for treatment) to the target water quality parameters (in this case DOC). A summary of the raw water characteristics, ion exchange treatment results, and percentage removal are presented below in Table 1.

Table 1. Summary of Raw Water Characteristics and MIEX® Treatment Results (at 1,000 BV)

Parameter Units Raw

Water MIEX® Effluent

Minimum Percent Removal

Requirement (%)

DOC mg/L 13.1 3.95 60

True Color cu 59 9 80 Source: MIEX® Resin Treatability Study for Dissolved Organic Carbon (DOC), Color Removal and Disinfection By-Products Control From Palm Beach County WTP 2 Raw Water, 1,000 BV for Final Design.

The feasibility study indicated that any bed volumes below 600 did not significantly reduce the DOC/color further. Therefore, it was determined that 600 - 1,000 BV was needed to meet the performance criteria of the process. The more conservative 600 BV was selected for design to provide PBCWUD the ability to increase regeneration frequency if high organics was desired. Based on regulations and laboratory testing performed by Orica and PBCWUD’s desire to reduce color and organics, a finished water color goal of <5 CU and DOC removal goal of 60% were selected.

DESIGN CONSIDERATIONS

The most costly aspect associated with the operation of the MIEX® system is the cost of the resin. Due to this, Carollo’s design approach for WTP 2 ion exchange system was focused on minimizing resin attrition and reducing resin loss. The WTP 2 design included process and hydraulic considerations, and careful evaluation and selection of piping systems and equipment to achieve minimum resin loss. These design considerations are discussed further in this section.

The resin loss rate for the high rate MIEX® treatment system for WTP 2 was expected to be in the range of 1-2 gallons of resin /million gallons of treated water. In operation, the variability in resin loss may be attributed to improvements in the resin handling system, frequency of resin regeneration, fluctuations in loading rate (raw water flow changes), and advances in the resin pumping and piping systems design. A process flow schematic of WTP 2’s MIEX® system is shown in Figure 3.

Figure 3. WTP 2 MIEX® System Process Flow Schematic

Ion Exchange Equipment Requirements

The high rate MIEX® treatment system for WTP 2 was incorporated into the treatment system such that re-pumping of the ion exchange effluent was not required. Raw water from the well pumps enters the distribution structure (splitter box); the flow splits into the two contactors and continues through the ion exchange process. Therefore, the only pumping systems designed as part of the project are for resin and brine handling. The resin handling refers to the resin transfer to and from the contactor, after and before the regeneration process. Once the resin inside the contactor looses its exchange capabilities, the resin is pumped from the contactors to the resin transfer tanks. From the transfer tanks the resin flows by gravity to the regeneration system where the resin is put in contact with saturated brine. Once the resin is regenerated, the fresh resin is pumped to the fresh resin tank and finally introduced back into the system.

Air Lift Pumps for Resin Handling

The resin handling system includes resin transfer tank (where resin is accumulated from the process while awaiting regeneration), virgin resin added to the system, and fresh (regenerated) resin stored in the fresh resin tank. From the fresh resin tank, the resin is metered at a controlled rate back into the process. To minimize resin attrition in the system, the selection of low shear pumps was critical for these applications. Air-lift pumps were selected for the resin transfer service due to experience with similar applications, as well as the low shear nature of this type of pump. Air-lift pumps have limited horizontal suction and discharge pumping capabilities, which was accounted for in the layout and design of the resin transfer tank with respect to the contactors. The resin transfer tank was located centrally to both contactors so that the discharge of each air-lift pump was essentially vertical.

Centrifugal Screw Type Pumps for Resin Regeneration System

Centrifugal screw type impeller pumps were selected for the regeneration system (Figure 4) to convey fresh (regenerated) resin to the fresh resin tank. Fresh resin is metered out of the fresh resin tank by gravity into the influent distribution structure and back into the process. Screw type impeller pumps were selected because they have a relatively steep curve, which allows control throughout a wide range of head and flow conditions. A specific velocity range was targeted to minimize shear and keep the resin slurry in suspension.

Figure 4. WTP 2 MIEX® Regeneration System

Contactor Mixers

The MIEX® treatment system was designed with a fluidized bed, or mixing zone, in the base of the contactor and the settling zone (above the mixing zone) with tube settlers. The raw water enters the contactor and it is distributed uniformly across the bottom of the mixing zone. As the fluidized resin and water mixture rises into the settling zone, the resin particles are attracted to each other and agglomerate and then settle back down into the mixing zone.

In addition to resin pumping, the mixing provided in the contactors is also one of the leading causes of resin attrition. For this reason, it was critical to design the mixers to provide sufficient input energy for keeping the resin in suspension while not providing excess energy, which may cause resin shear or cause high-localized velocities into the tubes settlers, which could lead to increased resin carryover. Scale model and computational fluid dynamics (CFD) modeling was performed for this system in the Philadelphia Mixing Solutions laboratory. The results of this testing verified the mixer layout (Figure 5), specific pumping capacity, and impeller type for these mixers. The design included nine mixers per contactor (typical design for a unit this size is 4 mixers), based on desired mixing intensity, basin dimensions, inlet piping configuration, and process recommendations from the resin manufacturer. The speed of each mixer is controlled with a variable frequency drive (VFD). The use of VFDs to control the mixer speed is key to maintain the resin bed within a desired height and resin concentrations.

Figure 5. WTP 2 MIEX® System Section Rendering Showing Mixing Configuration Inside the Contactors

Increasing Capacity with Softening Bypass

Originally, WTP 2 used the softening process for the removal of calcium, and some color reduction. With the incorporation of the ion exchange treatment system, the reliance of the precipitators for color removal was eliminated, allowing PBCWUD the flexibility to focus only on softening. The softening process is inherently an “all or nothing process” with regard to calcium removal so the effluent from the precipitator will typically be the minimum calcium solubility (40-50 mg/L as CaCO3) plus the remaining magnesium (about 10 mg/L as CaCO3). This mode of operation produces a finished water hardness of approximately 65 mg/L. One method of

optimizing the calcium and alkalinity content of the finished water is to bypass a portion of the flow around the precipitators. This results in a more stable finished water.

For this reason, a softening process bypass line was installed as part of the project allowing ion exchange treated water to bypass softening for blending downstream with the existing softeners effluent. This regime allows PBCWUD to “dial-in” the desired finished water alkalinity and hardness. The bypass line was designed for a maximum bypass capacity of 1.9 mgd, increasing the total plant capacity from 14.5 mgd to 16.4 mgd. In addition, the lower pH ion exchange treated water blends with softener effluent in the filter influent, reducing the need for costly carbon dioxide (CO2) for pH control.

WATER QUALITY RESULTS

Dissolved Organic Carbon Removal

Figure 6 shows the DOC concentration results for the raw water source, ion exchange (IX) effluent, and finished water before and after the installation of the ion exchange treatment system. The ion exchange at WTP 2 removes an average of 75% of the dissolved organics. The results of the full-scale ion exchange system exceed the water quality goal for DOC removal (60% minimum) and the TOC removal requirements associated with Stage 1 Disinfectants/Disinfection Byproducts Rule.

Figure 6. Dissolved Organic Carbon Concentration Results Before and After the Installation of Ion Exchange

Disinfection By-Products (DBPs) Reduction

Figure 7 shows the formation results for trihalomethanes (THMs) and haloacetic acids (HAAs) for both before and after the installation of the ion exchange treatment system. The addition of ion exchange reduced the THMs by 45% and HAAs by 57%. The THM and HAA averages on the finished water were approximately 11 and 13 g/L, respectively. These results are well

below the current regulatory requirements of 80 g/L and 60 g/L for THMs and HAAs respectively.

Figure 7. Disinfection By-Products Concentration Results Before and After the Installation of Ion Exchange

Color Reduction

The average color of the raw water at WTP 2 is approximately 50 CU. The addition of ion exchange reduced the finished water average effluent color to 4 CU, which represents a color reduction of approximately 92%. Figure 8 shows the color results before and after the installation of the ion exchange treatment system. Figure 9 shows the difference in color before ion exchange and after bringing the ion exchange system online, this picture was taken hours apart from each other.

Figure 8. Color Results Before and After the Installation of Ion Exchange

Figure 9. Precipitator No. 1 (downstream Ion Exchange process) After and Before the Installation of Ion Exchange

Alkalinity Impacts

The anion exchange resin has the capability to remove some bicarbonate (HCO3-); however, the selectivity of bicarbonate is very low and the impact on alkalinity is not significant. The average raw water alkalinity at WTP 2 is approximately 250 mg/L as CaCO3 . The water quality results for WTP 2 indicate that bicarbonate removal occurred slightly due to ion exchange; however, the addition of the softening bypass gives WTP 2 the flexibility to adjust the alkalinity concentration (Figure 9).

Figure 9. Alkalinity Concentration Results Before and After the Installation of Ion Exchange

OPERATIONAL COST SAVINGS

This section summarizes the operational cost savings and additional costs identified as a result of the incorporation of the ion exchange treatment system and the softening bypass line at WTP 2. These savings are associated with the reduction in chemical usage, power consumption, and distribution system flushing (reduction in nitrification in the distribution system). In addition to these savings, the installation of ion exchange brings additional operational expenses for resin replacement, salt for the regeneration system, and waste brine disposal. Table 2 summarizes the projected operational costs and the total capital cost of the ion exchange project for WTP 2.

Table 2. Summary of Operational Costs

Item Description Costs

Total Capital Cost $ 7,300,000

Annual Additional Operational Cost

Resin Replacement - High Resin Loss (0.8 gal/mgd) $ 145,000 - Low Resin Loss (0.4 gal/mgd) $ 75,000 Salt for Regeneration $ 99,000 MIEX® Power Cost $ 22,000 Annual Operational Cost Savings

Chlorine Dose Reduction $ 59,000 Lime Dose Reduction $ 82,000 Residuals Handling & Disposal $ 45,000 Ammonia Usage Reduction $ 5,000 Carbon Dioxide Usage Reduction $ 5,000 Distribution System Flushing $ 100,000

Energy Consumption Reduction $ 211,000

Net Operational Cost Savings at High Resin Loss

$ 241,000

Net Operational Cost Savings at Low Resin Loss

$ 311,000

Note. 1. Calculations performed for the average annual flow of 10 mgd.

Chlorine Dose Reduction

Before the installation of ion exchange, WTP 2 had a chlorine dose of 11 mg/L and following the installation of ion exchange, the dose was reduced to 8 mg/L. The dose reduction is attributed to the reduction of chlorine demand due to the DOC removal.

Lime Dose Reduction

The addition of the softening bypass line allows WTP 2 to reduce the lime usage, since a portion of the ion exchange effluent flow is not treated in the softening basins. The net lime reduction at WTP 2 is approximately 3,300 lb/day (at an average flow of 10 mgd). In addition, as a result of the reduction in lime feed to the softening basins, the reduction in lime residuals is approximately 3,800 dry lb/day.

Ammonia Usage Reduction

Chloramines are used as the disinfection method in the distribution system. The WTP 2 staff reported that ammonia dose was reduced from 0.8 to 0.5 mg/L to achieve the targeted chloramines residual for the distribution system. The dose reduction is attributed to the reduction of DOC concentration in the finished water, increasing the stability of the chloramines in the distribution system, which reduces the required chloramines residual. The reduction in the chloramine demand in the distribution system also reduces the potential for nitrification due to the reduction in ammonia release.

Carbon Dioxide (CO2) Usage Reduction

WTP 2 utilizes CO2 for pH control and re-carbonation. The addition of the softening bypass line allows blending the lower pH and higher alkalinity and hardness ion exchange effluent with the higher pH and lower alkalinity and hardness softening process effluent, allowing the reduction of CO2 and producing finished water with a more stable water quality.

Distribution System Flushing

Flushing is used to manage the potable water quality in the distribution system. Flushing is used as an aid in the control of nitrification and disinfection residual management. Some of the parameters that promote nitrification within the distribution system are high chloramines residual and decay, high DOC concentration in the finished water, high temperatures, low alkalinity and pH, and high free ammonia. The addition of the ion exchange system and the softening bypass line, allow WTP 2 to reduce the chloramines residual (and rate of decay) sent to the distribution system, reduce the DOC concentration in the finished water, and produce a more stable finished water. As a result of these improvements, PBCWUD has reported that the amount of water used for flushing the distribution system was reduced by approximately 53%.

Energy Consumption Reduction

As described in a previous section, one of the purposes of the installation of ion exchange at WTP 2 was to replace the aging ozone system. This system represented one of the largest energy consuming processes of the plant’s operations. The elimination of the power demand by the ozone generation system saves PBCWUD approximately $220,000 per year. In comparison, the energy consumption associated with the ion exchange treatment system is approximately $22,000 per year.

Resin Replacement

Resin replacement is the largest component of the operating cost associated with the ion exchange treatment system. For this reason, the WTP 2 ion exchange system design was focused on reducing resin attrition and consequently resin loss. The major design considerations were low shear pumping, gravity flow where feasible, mixer impeller design/layout, and ideal resin feed locations.

As a result of the design enhancements included in WTP 2 ion exchange system, the actual resin loss ranged from 0.40 to 0.80 gallons of resin per million gallons of water treated (gal resin /MG water treated). This measured loss is significantly below the manufacturer’s typical range of 1 – 2 gal resin/MG water treated, which significantly reduces operational costs.

CONCLUSION

The implementation of a high-rate ion exchange treatment system at WTP 2, to replace the existing ozone system, allows PBCWUD to save between $241,000 and $310,000 per year in operational costs. Other features, such as the softening bypass, allowed an increase in the rated treatment capacity of the plant to 16.4 mgd, without any modifications to the existing downstream processes. In addition, the softening bypass allows PBCWUD to “dial-in” the finished water hardness and alkalinity to obtain specific treatment goals and produce a more stable finished water. Moreover, water quality results also indicated that the ion exchange treatment system exceeded the water quality goals specified, reducing the DOC concentration and color by approximately 75% and 92% in the finished water. Finally, the innovative design of the ion exchange system at WTP 2 focused on reducing resin shear and resin loss, which demonstrated PBCWUD’s ability to save in resin replacement cost and therefore reducing the operational costs of the system.