Power Magazine recognized Dominion Energy and its Surry Power Station with the 2023 Water Award for their Reverse Osmosis Concentrate Recovery project, which enabled the company to meet and exceed its goal of saving 21 million gallons of water over a 5-year period.
Dominion Energy is always seeking innovative solutions to help them achieve and exceed their environmental stewardship goals. Power generation is a water intensive operation, whether from coal, gas or nuclear, and finding new ways to reduce water demand across its fleet is a high priority for its employees and partners.
The Dominion Surry Nuclear Plant is not any different, and water management and optimization are on everybody's mind. The Surry operating team approached Veolia to investigate what could be done to reduce the feedwater consumption associated with the outsourced demineralized water system. Through test scenarios and pilot testing, it was demonstrated that water consumption could be reduced significantly.
Reverse osmosis (RO) concentrate was identified as the target area for reducing freshwater consumption.
RO is a membrane-based technology that is used as a preliminary demineralizer, removing up to 98% of the total dissolved solids. It is a continuous process that splits a feed water source into two effluent streams: a purified water stream called “permeate”, and a waste stream called “concentrate” that contains the rejected dissolved solids. RO operation is described in terms of %Recovery, meaning the percentage of the feed water recovered as permeate. RO systems are typically operated in the range of 55-75% recovery, meaning 25-45% of the feed water flow becomes the waste stream. The solution to the plant’s challenge was to maximize the %Recovery, hence reducing the volume of feed water required for a given production of permeate.
Several factors are considered when establishing an ideal % Recovery for an RO system, including permeate quality desired, feedwater fouling potential, and expected concentration of sparingly soluble salts in the waste stream.
Historically, the Recovery rate of the Surry pretreatment RO system was established to ensure a reliable operation of the membranes without requiring the use of antiscalant chemical. This particular feedwater contains high levels of silica, a contaminant that can cause severe membrane fouling and can inhibit functionality if it reaches high enough concentrations (Figure 1). Increasing the RO recovery to a higher level, as was suggested, would require the use of a specific antiscalant to prevent precipitation of silica in the back end of the RO system. The plant requested that Veolia provide a “proof of concept” temporary system to evaluate the increase in overall system % Recovery. This was accomplished by installing a small, separate RO skid to directly process the main RO system concentrate. During this trial period, the permeate from the recovery RO was recycled back to the inlet of the main RO system.
Result
Throughout the trial period, data was collected on the new concentrated waste stream and on the permeate produced by the recovery RO (Figure 2). Operational parameters of the recovery RO were monitored to determine long term viability of the new process.
The trial demonstrated that with appropriate antiscalant addition, roughly half of the concentrate waste stream could be treated to acceptable quality for it to be reused, without the system suffering from fouling by precipitated silica.
Based on the results, two long term options proved viable. First, was that the permanent addition of a separate recovery RO. The second was a minor modification of the existing RO system to achieve the same net system recovery with the use of specialty antiscalant.
The recovery of the wasted RO concentrate amounted to a reduction of ~13.9 MMUSG per year of freshwater intake. This represented a double-digit reduction on water demand, exceeding the target set initially for the project. This equates to the amount of water used by more than 126 American households annually, or enough water to fill three football stadiums up to the 10 feet goal post!