Explaining ASR Technology
Too little or too much? With an average of 50-plus inches of rainfall per year, Florida is one of the wettest in the nation. It’s all about timing, however. Ironically, more than 60 percent of the annual precipitation falls between June and November — the very time human demand is at its lowest. In addition, the high evaporation rate that occurs during this time means that only a small portion of that rainfall soaks into the ground to help recharge the aquifer. On the other hand, when it’s not raining and water becomes scarce, human demand reaches its peak.
Although fluctuating weather patterns are a way of life in west-central Florida, population growth remains constant. The area’s challenge is to meet the growing needs people have for water while protecting those resources that define and enrich the state’s quality of life. The District is meeting that challenge through good planning and innovative thinking.
One vital piece of the water supply puzzle is finding a place to store excess water during the rainy season to be used later when it is needed. Reservoirs are one method currently used. However, the state’s flat topography and ecologically sensitive river systems pose challenges to the traditional approach of damming rivers to store water. Off-stream reservoirs, another option, avoid the flooding of these sensitive ecosystems but they typically have storage capacity limited to a few weeks at a time without replenishment.
Looking for other storage methods, water managers throughout the state are turning to another solution — the aquifer. Specifically designed wells are being used to inject excess treated water during the rainy season into the underground aquifer. These same wells are used to withdraw that water during the dry season. This process, coined Aquifer Storage and Recovery or simply ASR, is being relied upon more and more to meet water supply demands.
ASR — GROWING ALTERNATIVE SUPPLY
The first ASR studies in Florida were conducted in 1983 by the Southwest Florida Water Management District at a county facility at Lake Manatee. The study demonstrated that ASR is a feasible alternative for storage of water. Since this time, over 35 sites throughout Florida are being developed. Along the way, a few unanticipated technical and permitting issues have been encountered that could have impacts to any ASR program in Florida. These range from from the interaction of the injected water with the aquifer formation, increasing the recoverable amount of injected water, the necessary level of water treatment prior to injection, construction and operation of ASR systems within urban settings and the need for coordinated planning of ASR facilities by the different developers of these systems.
HOW IT WORKS
Through the ASR process, water injected into a well forms a storage bubble in the aquifer that typically stays within 1,000 feet of the well. Depending on the aquifer characteristics, the storage bubble can take on various shapes and sizes. The success of the project will depend on being able to recover the injected water; this makes the selection of the site very important. A feasible site is one where the injected water will not move or migrate away from the well too quickly or mix too much with the naturally occurring salty water typically present in the aquifer. Selecting a favorable location for the ASR system is based on extensive subsurface testing consisting of core samples of the aquifer, borehole geophysics, pumping tests and cycle tests.
Additional testing to determine the percentage of injected water that is recoverable is referred to as “cycle testing.” A cycle encompasses a period of injection, followed by a storage period and then a recovery period. Because ASR systems are typically located in areas where the aquifer contains brackish (salty) water, some mixing occurs which degrades a portion of the injected water and makes it unusable.
Recovery efficiency is the percentage of the injected water that can be recovered and used. Recovery efficiency may initially start out low, but improve with each successive cycle as the injected water that is left behind freshens up the storage zone. In some instances, high quality water is injected as an investment to freshen up the storage zone and improve the recovery efficiency.
There are three types of water used for ASR: potable water, reclaimed water and partially treated surface water. The permitting requirements depend on the quality of the water source and the quality of the water in the aquifer. An ASR system that stores potable water is fairly straightforward to develop since the water meets all the drinking water standards. Attention should still be given to the water quality resulting from the interaction between the injected water and the aquifer.
There are several ASR systems that currently inject excess potable water. These systems usually have an excess capacity ranging between 2 to 12 million gallons per day (mgd) during the wet season. The excess water is available during this period because irrigation use is low due to the wet conditions. The height of the dry season normally occurs over a 90-day period between the months of March and May.
Permitting requires reclaimed water ASR projects to be located where the natural aquifer water quality is salty and unfavorable for potable use. Reclaimed water generally consists of water from advanced secondary wastewater treatment facilities. The treatment process is sufficient to produce water that meets all the primary drinking water standards but may have a few secondary water quality parameters that are exceeded. The secondary list includes nuisance parameters, such as odor and color.
Use of reclaimed water for irrigation is a common practice, yet even with this use, it is common for a wastewater facility to dispose of up to 12 mgd of quality water during the wet season when demand for irrigation water is low. The ability to store this water in an ASR system for later use would provide a significant benefit to the water resources.
There are currently 13 reclaimed water ASR projects under development in our District. One project has performed two cycles of testing and there are three projects that will initiate cycle testing within the next six months.
Partially treated surface water is a new type of ASR that is in the early stages of study. Partially treated systems would divert and store a small fraction (typically 10% or less above a certain minimum flow) of river flow during the wet season. This water would most likely undergo filtration to remove particulate matter that may clog the well.
Without further treatment the water may still contain low concentrations of microorganismsmicroorganismsmicroorganisms such as coliform, giardia, and cryptosporidium that can be present in surface water bodies at times. Under current rules and regulations, microorganisms such as these would have to be treated prior to injection. However, the raw river water quantities available during the wet season would exceed the remaining treatment capacity of virtually any existing potable water treatment system dictating that additional treatment capacity be constructed at considerable expense. There is, however, some existing information that suggests these microorganisms may die off naturally in the aquifer during the storage period. This raised the question of what level of treatment is necessary prior to injection.
Studies are under way to determine the potential impacts of this type of ASR system. There are no partially treated surface water ASR systems currently operating in the state.