Aquifer Storage Recovery has a Significant Role in Florida s Reuse Future. Introduction

Transcription

1 Aquifer Storage Recovery has a Significant Role in Florida s Reuse Future Mark B. McNeal, P.G. CH2M HILL Tampa, FL Introduction Aquifer Storage Recovery (ASR) is a proven, viable method of beneficial water storage. ASR is becoming increasingly popular worldwide as a method of managing various water, wastewater, and stormwater systems. During the 1980 s and 1990 s, the advancement of ASR for storing treated drinking water grew tremendously. Utilities, regulatory agencies, and the public now generally embrace ASR as a method of maximizing the use of existing drinking water treatment facilities. Because of the seasonal imbalance between supply and demand faced by Florida s reuse systems, ASR is rapidly under development for large volumes of seasonal reclaimed water storage in Florida. Many states now have rules in place to guide the development and operation of potable ASR systems. ASR wells are regulated under the Underground Injection Control (UIC) division of the US Environmental Protection Agency (EPA) unless the UIC program has been delegated to the state. The state of Florida, which has primacy, recently adopted rules governing the development and operation of ASR systems designed to store and recover highly treated reclaimed water in brackish groundwater aquifers. This has helped facilitate awareness of the need for seasonal storage of reclaimed water and has provided a more concise regulatory framework for reuse utilities to follow. More than one dozen reclaimed water ASR systems are under various stages of development in west central Florida. Other reuse utilities in central Florida and along Florida s east coast are also investigating ASR s potential role in their reuse future. Some of these programs are in the early planning stages. Others on the west coast have taken their programs to the next level of obtaining construction permits from the Florida Department of Environmental Protection (FDEP) to construct a Class V, Group 3 ASR well system. Four facilities have constructed their ASR pilot wells and are close to operational testing. Finally, as of June 2002, two utilities are currently performing operational testing of their completed reclaimed water ASR pilot wells. These first two systems in operation are the Hillsborough County Water Department and the Englewood Water District. This paper discusses many of the technical and regulatory challenges that have been successfully resolved to bring these initial reclaimed water ASR programs online and allow operational testing to begin. Reclaimed water quality characterizations presented challenges associated with disinfection byproducts, total coliform, phthalates, and select secondary drinking water standards prior to recharge operations. Ambient groundwater characterization and future uses of the groundwater systems are also important considerations in siting and permitting these programs. As many of these barriers are eliminated, ASR programs will become a common element to many reclaimed water systems in Florida. Regulatory Framework Regulations governing non-potable ASR systems are relatively extensive. The primary agencies responsible for permitting ASR systems in Florida include the Florida Department of Environmental Protection (FDEP) and the local Water Management District (WMD). Initially, a Class V injection well construction permit must be obtained from FDEP prior to constructing the ASR well. One to two requests

2 for additional information can be anticipated for these permit applications, in addition to two mandatory public notice periods and a public meeting, resulting in the entire process taking approximately 1 year to complete. This is not unusual, as potable water ASR systems in Florida typically require approximately the same amount of time to permit due to the stringent process that has been adopted. The FDEP typically has a Technical Advisory Committee (TAC) to assist with the Class V construction permitting process. The TAC typically includes members from the local FDEP office, FDEP Tallahassee, the local WMD, the U.S. Environmental Protection Agency (USEPA), and the US Geological Survey (USGS). Input may also be solicited from the local Department of Health (DOH). In essence, a demonstration must be made to FDEP that all drinking water standards (DWSs) are met in the water targeted for storage in the reclaimed water ASR well; otherwise a variance is needed to allow the program to move forward. Under the current regulatory framework, it is important that all federal primary DWSs are met in the proposed recharge water. If one of these standards cannot be met (for example, if nitrate concentration exceeds 10 mg/l or Total Trihalomethanes [TTHMs] are above 0.80 mg/l) an Aquifer Exemption (AE) would be required. These are extremely difficult, if not impossible, to obtain within the existing regulatory framework. Determination of compliance for the primary DWSs is considered on a single-sample basis, with no averaging allowed to mitigate unusual variations in recharge water quality. The need to meet secondary drinking water standards is less stringent. First, groundwater discharge standards allow averaging of the secondary DWSs, which provides considerable relief relative to the single-sample criteria for compliance with the primary DWSs. Second, regulated levels are not necessarily the DWS; rather, they are the higher of the DWS and the native water quality. For example, if chloride concentration in the recharge fluid averages 300 mg/l, but the chloride concentration in the native groundwater is 900 mg/l, the chloride standard that must be met is 900 mg/l (rather than the DWS of 250 mg/l for chlorides). Finally, in the event that a secondary DWS cannot be met reliably, a fairly straightforward variance called a Water Quality Criteria Exemption (WQCE) can be obtained. This allows a higher standard to be set for the secondary DWSs, which are aesthetic-based standards rather than health-based standards. The Englewood Water District (EWD) applied for, and obtained, a WQCE for color as part of its reclaimed water ASR program. The compliance level for color was increased from the DWS of 15 color units to 75 color units through this process. EWD s reclaimed water averages approximately 35 color units during normal operations. The cost of the application to FDEP for a WQCE is currently $6,000 per parameter requested. The exemption is good for up to 5 years and generally coincides with the FDEP Class V construction or operating permit expiration date where practical. Following construction of the ASR well, cycle testing must be completed in support of a future FDEP Class V Operating Permit and a WMD WUP. The cycle test program must be approved by FDEP prior to starting recharge. It is important to work closely with FDEP and the other agencies during the cycle testing to ensure that the appropriate data is collected in support of the future operational permitting. Typically, water recovered from the ASR well during the initial cycle testing is sent to waste. This is often a groundwater to surface water discharge, and may require obtaining a generic permit from FDEP for this discharge if not specifically authorized in the FDEP Class V construction permit. Once a demonstration is made to FDEP that the water recovered from the ASR well is of suitable quality to place into a public access reuse system, the recovered water during cycle testing can be placed into the reuse system for beneficial use. This requires, at a minimum, a demonstration that the Total Suspended Solids (TSS) is less than 5 mg/l, fecal coliform is absent, and Carbonaceous Biochemical Oxygen Demand (CBOD) is less than 5 mg/l (per Chapter , Florida Administrative Code [FAC]).

3 Finally, to recover water from the ASR well, a WUP must be obtained from the local WMD for the Class V ASR well. This permit is necessary to address potential environmental impacts or impacts to existing users. This typically occurs following several cycle tests once impacts from the ASR well, if any, are better defined. Input on the cycle test program should be solicited from the WMD to ensure that the District s objectives for testing are met as well. This will help facilitate future WUP activities. The County DOH also has a significant role in permitting potable water ASR wells. Since the water recovered from the reclaimed water ASR wells would not be intended for public drinking water supply, DOH s involvement may not be significant. They may play a more prominent role if competing groundwater use is prevalent in the project area and the aquifer targeted for storage or overlying aquifers are used for public supply. Regardless, DOH should be copied on regulatory correspondence to provide input into the permitting process if they desire to do so. FDEP s rules governing ASR permitting are covered in several chapters of the FAC. The most pertinent regulation is Chapter , FAC, which governs underground injection in Florida. ASR wells are regulated as injection wells in Florida and nationwide. Chapter , FAC, Florida s reuse rule, has substantial language recently adopted which sets guidelines for reclaimed water ASR development. Groundwater standards and exemptions are set forth in Chapter , FAC, and become important particularly in the event that secondary DWSs may need relief. Finally, drinking water standards are provided in Chapter , FAC. As DWSs change in the future, the ASR wells would be required to meet the newer and (likely) more stringent standards imposed in updated federal drinking water regulations. Florida s First Reclaimed Water ASR Systems In July 2001, a significant milestone was achieved for the future of reclaimed water optimization in Florida. Reclaimed water ASR systems were brought on-line for both Hillsborough County and the Englewood Water District (EWD). Hillsborough County is recharging tertiary treated reclaimed water into a slightly brackish aquifer containing total dissolved solids (TDS) concentrations of approximately 1,000 mg/l. This zone is located between approximately 300 and 400 feet in depth. The EWD s ASR well injects advanced secondary treated (filtration followed by high level disinfection) reclaimed water into a saline aquifer (approximately 20,000 mg/l TDS) at a depth of between 500 and 700 feet. Hillsborough County successfully recharged and recovered 30 million gallons (mg) of water during its initial cycle test. Another major milestone was achieved in January 2002 when reclaimed water was recovered from the ASR well and placed back into the County s reuse system for beneficial use. This water was significant during the 2002 dry season, providing an important supply to help the County meet its irrigation demands. The County is currently preparing to begin the recharge phase of its fourth cycle test with a target storage volume of 90 mg during this cycle prior to recovery. The EWD system successfully recovered 2 mg of irrigation-quality water initially, and has stored 90 mg at this time. Recovery activities for EWD s initial major cycle test began in June 2002, with a goal of recovering 10 to 30 mg during this initial event. These programs have successfully removed institutional barriers that previously hindered use of the ASR technology for storing non-potable water for later beneficial use. Many other non-potable ASR applications are in various stages of development in southwest Florida. Manatee County has constructed a well at its Southwest WWTP and will begin its cycle testing program following resolution of some outstanding issues with its reclaimed water quality. Manatee County s

4 reclaimed water ASR well is the first of what should be many ASR wells associated with a large, regional agricultural reuse system (MARS). The City of St. Petersburg has constructed its initial reclaimed water ASR well, is currently constructing the above ground facilities, and expects to begin cycle testing in late summer, St. Petersburg implemented a unique approach to expedite permitting by utilizing a zone that has freshened following years of deep well injection of excess reclaimed water in a deeper zone. Preinjection ambient groundwater quality of this zone was well in excess of 10,000 mg/l therefore primary DWSs are not mandatory for water recharged into this zone (CH2M HILL, January 2002). Other reclaimed water ASR systems in various stages of development include systems in southern Hillsborough County, Sarasota County, the City of Sarasota, Lehigh Acres (Florida Water Services), and Collier County. Feasibility studies are underway at numerous other locations to evaluate the potential benefits and viability of seasonal storage at these locations. Figure 1 shows EWD s pilot reclaimed water ASR system. An ASR well has been installed in addition to a storage zone monitoring well (SZMW), a shallow monitoring well (SMW), and an offsite monitoring well. The purpose of the SZMW is to monitor water quality and water level changes in the storage zone during recharge and recovery events. The purpose of the SMW is similar, except that a shallower zone is monitored to evaluate possible vertical movement of the recharge water. A shallow monitoring well is typically required to be located within 150 feet of the ASR well for non-potable ASR applications in Florida. The location of the storage zone monitoring well varies from site-to-site, but typically targets a radius from the ASR well that will monitor the arrival of reclaimed water once the design volume is stored. Other wells, such as the offsite monitoring well in the case of EWD, may be required by FDEP to ensure that groundwater discharge standards are maintained at distance from the ASR well. Water table monitoring wells, typically 20 to 30 feet in depth, are often required to monitor shallow groundwater quality during well installation, but are typically plugged following demonstration that no groundwater contamination occurred during well construction activities. FIGURE 1 EWD RECLAIMED WATER ASR SYSTEM

5 Technical and Regulatory Challenges There have been many technical and regulatory challenges that have surfaced during the implementation of the initial reclaimed water ASR systems in Florida. This was not unexpected, as many other states are still struggling with how to regulate and operate potable water ASR systems. Presented below are examples of some of the challenges that have been successfully addressed on Florida s reclaimed water ASR projects to date. Reclaimed Water Quality The ability to meet the vast majority of DWSs in reclaimed water from reuse facilities in Florida is welldocumented. For example, Hillsborough County s reclaimed water produced at its River Oaks AWTP and Northwest Regional WRF met all DWSs with the exception of TTHMs. An ammoniation system was added that allowed the facilities to reliably meet TTHM standards through the application of approximately 1 mg/l of ammonia to form a combined residual (chloramines) during recharge events. This is a very common water treatment practice to control TTHM formation in drinking water distribution systems. The EWD in Charlotte County reliably meets all DWSs with the exception of color, for which an exemption was granted. St. Petersburg s reclaimed water meets the vast majority of drinking water standards with an occasional excursion for nitrate and nitrite. Nitrate and nitrite routinely exceeded DWSs in the past, but operational protocols were implemented to routinely produce reclaimed water well below these standards. Manatee County s reclaimed water also reliably meets all primary DWSs, with occasional excursions for some secondary DWSs such as manganese, color, iron, and odor. The apparent presence of Di(2-ethylhexyl)phthalate (DEHP) may also provide a regulatory challenge in obtaining clearance for operational testing of the ASR well. DEHP is a common indicator of plastics, and has a very low primary DWS of 6 μg/l (ppb). It is not unusual to get interference either in the laboratory or in the field due to PVC well casing, PVC sample tubing, or other plastics such as possibly squeeze bottles or gloves. Hillsborough County had a few samples exceed the DWS of 6 ppb during the initial reclaimed water characterization and undertook a detailed weekly sampling program for 6-months to demonstrate that DEHP was not present in typical reclaimed water (CH2M HILL, May 2000). All samples came back below the MCL of 6 ppb, which is attributed primarily to the new sample station that was installed in the vicinity of the ASR well. The new sample station did not contain any plastic piping or valves. Total coliform concentrations must also be controlled below the groundwater discharge standard of 4 colony forming units per 100 milliliters (CFU/100mL). Since reuse systems typically are not treated with the same care as drinking water systems (disinfection of pipelines, etc.) during construction or operation, it is not unusual to have total coliform in the distribution system. Maintaining a relatively high total chlorine residual in the reuse system may control total coliform below these levels, but it may be necessary to install supplemental disinfection, such as an in-line UV system, at the ASR wellheads if the ASR well is located at the extreme reaches of the reuse system. This would be one advantage of locating a reclaimed water ASR well at the WRF to be able to better control total coliform at the ASR wellhead. Fecal coliform, which must be absent in the reclaimed water to meet reuse standards, is not expected to be an issue because this is an existing water quality standard that the reuse system is designed to reliably meet.

6 Color is routinely exceeded in most reclaimed water produced in Florida. This parameter is easily exempted through the WQCE process, and several exemptions have been issued for reclaimed water ASR programs along Florida s west coast (CH2M HILL, February 2001). Odor is another common excursion, but is also often much better in the irrigation water than in the native water, thereby not requiring an exemption. This was the case at the two operational reclaimed water ASR facilities (EWD and Hillsborough County) which did not need to seek relief for the elevated odor values because the groundwater was much higher in odor values due primarily to the presence of high levels of hydrogen sulfide in the native groundwater. Iron is another commonly exceeded secondary DWS, although this too can be effectively dealt with through the WQCE process if necessary. Although TDS concentrations are commonly above the secondary DWS of 500 mg/l for many reuse systems, TDS concentrations in the native groundwater should be well in excess of concentrations in the reclaimed water, and therefore should not require further consideration. FDEP Minimum Criteria for Sewage Effluent must also be monitored in the reclaimed water associated with a reclaimed water ASR system. Table 1 lists the current minimum criteria requiring monitoring. These are not considered regulatory limits, such as the drinking water Maximum Contaminant Levels (MCLs), but rather guidance concentrations or only constituents that warrant monitoring in the reclaimed water. TABLE 1 FDEP MINIMUM CRITERIA FOR SEWAGE EFFLUENT Toluene 1,2 Dichlorobenzene Chloroform 1,2 Dichloroethylene Chloroethane Aldrin Dieldrin Diethlyphthalate Dimethylphthalate Butylbenzylphthalate Napthalene Anthracene Phenanthrene Phenol 2,4,6-Trichlorophenol 2-Chlorophenol Ammonia Organic Nitrogen Total Kjeldahl Nitrogen Nitrite Soluble Orthophosphate Total Phosphorus Antimony Ambient Groundwater Quality Native water quality characterization can also provide a regulatory challenge. Proper characterization is important for reclaimed water ASR systems because regulatory requirements change if the aquifer is greater than 1,000 mg/l TDS, greater than 3,000 mg/l TDS, or greater than 10,000 mg/l TDS. For example, stringent Total Organic Carbon and Total Organic Halogen requirements make reclaimed water ASR project development in an aquifer containing less than 1,000 mg/l TDS nearly impractical under today s regulations. Procedures for defining ambient groundwater is not specifically addressed in the regulations and it is open to varying professional opinions as to how best to quantify pre-existing conditions. Native water quality is often disturbed during the drilling process and it may take an extended pumping test to better characterize ambient conditions. Similarly, water quality will often change during the initial pumping activities from a well until stabilization of the water quality occurs. This could take an extended period of time to accomplish. The pumping rate can also be an important variable in determining native conditions, with higher pumping rates often resulting in higher TDS concentrations from the well in these coastal locations. The regulators currently generally maintain that the background water quality in a well is the first water removed from the well. A more reasonable and better defined basis should be established to take into account some of the issues identified above. A reasonable withdrawal rate for a reasonable duration of

7 time for a future reasonable potential use from the aquifer under consideration is proposed to establish ambient conditions in a reclaimed water ASR storage zone. Recovered Water Reuse Compatibility Water recovered to a reuse system must not only be of an acceptable quality from a regulatory standpoint, but must also meet the water quality standards required of the reclaimed water customers. For residential reuse, it is generally accepted that chloride concentrations should be maintained below a concentration of approximately 600 mg/l or certain ornamental plants, such as azaleas, may be damaged (City of St. Petersburg, September 1988). For golf course irrigation, most turfs can sustain higher concentrations of chlorides, particularly over a short period of time. It may be possible to utilize recovered water containing elevated chloride concentration and other salts for a short time to meet peak dry season irrigation demands, realizing that a much fresher irrigation supply will be available once system demands return to normal levels. A knowledge of the reuse customer plant and sod varieties is important in understanding the quality of reuse water that can be tolerated in the reuse system during the initial stages of ASR implementation. If a reclaimed water ASR system is constructed at the wastewater treatment facility, blending opportunities should be considered in the design of the system. This is important to allow a consistent water quality to all reuse customers if that is an objective of the reuse utility. The benefit of this blending concept is shown in Figure 2. As this figure shows, blending 2 mgd of ASR recovered water with a TDS concentration of 2,000 mg/l with 8 mgd of 600 mg/l TDS reclaimed water would result in 10 mgd of reclaimed water with a TDS concentration of approximately 880 mg/l (chloride concentration estimated at 350 mg/l). In this manner, the total available supply has been increased by 25 percent, even though a FIGURE 2 CONCEPTUAL BLENDING OPPORTUNITY FOR A RECLAIMED WATER ASR SYSTEM

8 slightly poorer quality water is provided to the reuse system for a short period of time. This scenario could represent early recovery results from an ASR well completed into a brackish water aquifer, or possibly even longer term results for an ASR well completed into a more saline aquifer. The native water quality of the EWD reclaimed water ASR storage zone is very saline, with TDS concentrations of approximately 20,000 mg/l. Blending opportunities will be an important aspect of EWD s reclaimed water ASR system early on to allow a higher TDS threshold before terminating a recovery event. Blending water prior to placement in the reclaimed water system may not always be possible. Many times it is necessary to locate the ASR system away from the water reclamation facility due to native water quality, competing well users, system demands, or other extenuating circumstances. Above ground storage facilities may provide some opportunities to blend the recovered water with the reuse water prior to distribution to customers. There may also be customers such as golf courses that can accept a lower quality water for a short period of time, or could supplement the reclaimed water with groundwater and use surface storage (e.g., golf course ponds) to blend the water to an acceptable quality. These types of situations should be evaluated during the feasibility investigation as a means of providing a backup in the event recovered water quality becomes a concern during the initial cycle testing activities. Many potable ASR systems are successfully utilizing brackish water aquifers to return a satisfactory recovered water meeting the more stringent drinking water standards of 500 mg/l TDS. Providing for less than optimum recoverability, however, is good practice and provides a safety factor into the design of these systems. A utility must demonstrate that water recovered from an ASR well designed to supplement irrigation systems meets public access reuse standards. This requires a demonstration that CBOD and TSS are less than 20 mg/l and 5 mg/l, respectively, and that fecal coliform is absent. Hillsborough County has made this demonstration and is recovering water from its reclaimed water ASR well without any additional treatment (disinfection) prior to placing the water back in the reuse system. Other utilities, such as the City of St. Petersburg, have elected to re-disinfect recovered water prior to reuse. Either mode of operation is acceptable, and can be implemented at the owner s preference. Other geochemical reactions can be expected to occur with the reclaimed water and host rock interaction. Certain metals such as iron and manganese can be leached from the host rock increasing concentrations of these constituents in the water recovered from the well. Other constituents, such as TTHMs and chlorine residual typically experience significant reductions following residence time in the aquifer. A monitoring program will be required by FDEP to monitor changes that occur not only in the recovered water quality but also in monitoring wells constructed to monitor changes in groundwater quality over time. Results from this testing will be used to demonstrate that reuse standards are maintained in the recovered water and that customer satisfaction is maintained throughout the recovery process. Conclusions Reuse utilities are finding themselves in an increasingly difficult situation in providing a sustainable supply of water during extended dry periods. Most reuse systems do not have the luxury of augmenting the systems with potable drinking water or fresh groundwater. Drought resistant water supplies must be developed to provide supplemental water supplies to further promote and encourage the use of reclaimed water. Reuse customers have typically made a substantial investment to acquire this source, and must have it year-round or be faced with losing their investments such as lawns, shrubs, or crops. Reuse utilities also make a significant investment in their programs, often supplementing their funding through

9 other water and wastewater programs, and would therefore be able to more effectively optimize their investment by expanding their systems to accommodate all available and willing reuse customers. Without seasonal storage of reclaimed water, Florida reuse utilities are often built-out while only utilizing roughly 50 percent of their available resource. This is due to the seasonal imbalance of supply and demand. The highly treated reclaimed water is too precious of a resource to allow one-half of this water to go to waste. The ASR technology is being implemented in many locations along Florida s west coast to better utilize this valuable resource. Many regulatory hurdles have been overcome in the past few years to move these programs forward. Methods of controlling TTHMs and total coliform bacteria at acceptable levels in the reclaimed water have been implemented to allow these programs to advance. Methods of sampling the reclaimed water have been implemented to demonstrate the absence of DEHP in the reclaimed water as well. Initial discussions have been held to better define ambient groundwater conditions to ensure that a suitable level of treatment is required for the targeted storage zone for each program. Resolution of these and other issues has allowed operational testing to begin at two sites in west central Florida. Significant advancements have been made in effectively managing reclaimed water systems. Seasonal storage of reclaimed water using the ASR technology will better enable reuse utilities to save excess reclaimed water during periods when demands are down, for beneficial use during peak demand periods such as dry weather periods or severe droughts. While steps have been taken in Florida to facilitate development of these ASR programs, further regulatory strides will be needed before the full benefits of reclaimed water ASR will be realized. Other states besides Florida will soon realize the benefits that seasonal storage of highly treated reclaimed water can have to extend their limited water resources, and will therefore have a better regulatory model to use once the regulatory framework in Florida has been more fully developed. References Cited CH2M HILL, January Well Completion Report for the Aquifer Storage Recovery Test Well System at the Southwest Water Reclamation Facility. Prepared for the City of St. Petersburg, Florida. CH2M HILL, May Technical Memorandum RWASR-2 Hillsborough County Northwest ASR Aquifer Performance Testing and TTHM Control at NWRWRF. Prepared for the Hillsborough County Water Department, Tampa Florida. CH2M HILL, February 19, Florida Department of Environmental Protection Application for a Water Quality Criteria Exemption for the Englewood Water District Class V Group 3 Injection System. Prepared for the Englewood Water District, Englewood Florida. City of St. Petersburg, September Project Greenleaf. FDEP, 1992b. Chapter , Florida Administrative Code. Underground Injection Control. FDEP, August 8, Chapter , Florida Administrative Code. Reuse of Reclaimed Water and Land Application.