A U.S. energy company was looking at ways to reduce its water demand throughout its operations in order to reduce its environmental impact and improve its sustainability. One of their sites specifically wanted to find ways to reduce intake of freshwater on the local aquifer.
The owner 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 by over 10%.
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 65-75% recovery, meaning 25-35% 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 was established to ensure a reliable operation of the RO 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. 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 vet 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.
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, 50% 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.
Recovering 50% of the wasted RO concentrate amounted to a projected reduction of ~13.9 MMUSG per year of freshwater intake. This represented a reduction on demand to the aquifer of over 10%, exceeding the target set for this evaluation.
Based on the results, two long terms options proved viable. First, was that the permanent addition of a separate recovery RO. The second was minor modification of the existing RO system to achieve the same net system Recovery.