The goal of this three-part series is to highlight ASR well technology advancements and to demonstrate how these advancements can improve the efficiency of ASR well systems. In our introduction, we discussed the evolution of Aquifer Storage and Recovery (ASR) technologies and the current well recharge methods utilized within the industry; each with its advantages and disadvantages. Part One described the alternative recharge method we call “reverse siphon”, in which the hydraulic controls to prevent air entrainment and to initiate recharge are located at the surface, instead of connected to the pump assembly. In Part Two, we discuss the use of epoxy coating on the column pipe and tube assembly to prevent iron oxide particulates from clogging the well screen and filter pack interface. Though epoxy coating is not a new technology, the application and longevity of the coating has vastly improved.
The first question one might ask is why epoxy coat the column pipe and tube assembly? Figure 1 shows the specific capacity for recharge and pumping of an ASR well. Specific capacity is defined as the amount of water that is pumped or injected under a measured unit (feet) of lowering or raising of the water level in a well. The higher the specific capacity, the greater the capacity and efficiency of the well to recharge and withdraw water. In Figure 1, the overall downward trend (from 0 to 360 cycles on the x-axis) in specific capacity for both pumping and recharge indicates this ASR well was progressively clogged for 4.5 years. After 4.5 years of operation, the pump assembly was removed for inspection and the following was discovered:
- The mild steel column pipe and tube assembly pitted, corroded, and oxidized resulting in the formation of iron oxides.
- Eventually, the iron oxide particulates were entrained during injection and clogged the well screen and filter pack area. Iron oxide particulates may also have been injected into the aquifer.
- Over time, the accumulation of iron oxide particulates overwhelmed the well screen/filter pack area resulting in reduced specific capacity in both recharge and pumping modes (Figure 1).
The remedy for mitigating iron oxide formation was to apply an epoxy coat onto the mild steel column pipe and tube assembly inside and outside. Applied properly, this epoxy coating minimizes clogging agents in the well screen and reduces the potential for chemical and biological reactions to occur on the column pipe and tube assembly. Recently, the City of Phoenix installed the new epoxy-coated column pipe and tube/shaft assembly in an ASR well, foregoing any well rehabilitation activities (e.g., well screen cleaning). After 5 months of recharge and backwashing activities, this ASR well has improved 20% in specific capacity for both recharge and pumping operations. This specific capacity trend continues to be in a positive direction and shows no indication of declining efficiency.
As a result of this improvement, Phoenix plans to expand the use of epoxy-coating to all their water supply and ASR wells to extend the life of column pipes and inner tube assemblies, thereby reducing the frequency of maintenance or replacement, which is currently every 5 years. This preventive action would potentially save the City over $150,000 per maintenance event (costs include rig time, labor, equipment costs, and loss opportunity costs).
In the third installment of this blog series, we will talk about glass beads as a filter pack media. With all the discussion about naturally occurring sands/gravels versus the use of man-made glass beads as a filter pack media, we will attempt to shed some light on this issue and offer an alternative solution for recharge operations. Till next time….
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