Upgrading a Legacy HERO RO System for High-Salinity, High-Silica ZLD Operation
Combined cycle power plants operating in arid regions face a dual challenge: maintaining grid reliability while operating under increasingly tight water constraints. At a large gas-fired combined cycle power plant in the U.S. Southwest, these pressures were amplified by Zero Liquid Discharge (ZLD) requirements, challenging cooling tower blowdown chemistry, and an aging high-efficiency RO (HERO) system that was no longer delivering consistent performance.
To address these constraints, the facility partnered with ROTEC to execute a full-scale retrofit using patented Flow Reversal Reverse Osmosis (FR-RO) technology, delivered as a turnkey EPC project and installed during a tight maintenance window. The result was a significant operational upgrade that improved recovery, reduced chemical and cleaning demands, and stabilized long-term ZLD performance
The Operational Challenge: High-Salinity Blowdown in a ZLD Environment
The plant treats cooling tower blowdown characterized by high salinity, elevated silica (~86 mg/L), iron (0.1–0.8 ppm), and consistently high Silt Density Index (SDI) (>5. While HERO systems are designed for high recovery, the legacy system at this site relies heavily on elevated pH operation (~10.6), complex pretreatment, and frequent clean-in-place (CIP) events to manage scaling.
In practice, the system struggled to sustain its design recovery of 82–85%. Actual recovery often dropped to 64–65%, driven by persistent silica and iron fouling. CIP events were required every 2–4 weeks, and operators spent significant time rinsing the Weak Acid Cation (WAC) unit to maintain hardness targets ahead of the RO trains.
For a ZLD facility, these limitations had cascading effects. Lower RO recovery increased concentrate volumes, placing additional strain on downstream brine management and threatening the long-term capacity of the site’s evaporation ponds. Rising OPEX, operational complexity, and reduced flexibility made it clear that a different approach was required
The Retrofit Strategy: Integrating FR-RO into an Existing HERO Framework
ROTEC retrofitted the plant by replacing both first-pass RO trains with FR-RO, while retaining upstream lime softening and WAC units, as well as downstream polishing processes. This approach minimized disruption to the existing process flow while fundamentally changing how scaling and fouling were managed within the RO system.
The new FR-RO trains were engineered to tolerate:
- Feed conductivity ranging from 9,500 to 19,000 µS/cm
- Flow variability between ~255 and 369 GPM
- High SDI (>5) and high silica concentrations (>86 mg/L)
Achieving higher recovery under these conditions required operation at elevated pressures. The FR-RO system was designed to operate at feed pressures exceeding 900 psi, using high-rejection seawater RO membranes selected for durability in high-salinity, high-pressure environments. The entire retrofit was delivered as a turnkey EPC project, completed within a 2.5-week maintenance shutdown, including engineering, demolition, mechanical installation, piping, and electrical work.
Additionally, FR-RO excels in handling harsh feedwater conditions, including feedwater with severe scaling and fouling limitations. By optimizing flow patterns and mitigating scaling, the system achieves higher recovery rates and greater operational stability compared to traditional RO systems.
FR-RO is tailored to meet the unique needs of industries grappling with challenging wastewater streams. Like power, mining, oil & gas. With its flexibility and efficiency, FR-RO seamlessly supports industries in achieving ZLD and MLD objectives, making it a vital tool in sustainable water management.
How FR-RO Enabled Stable High Recovery and ZLD
Unlike conventional RO or HERO configurations that concentrate fouling at fixed locations, FR-RO’s patented process combines periodic flow reversal with block rotation. By rotating pressure vessels between feed and concentrate positions, the system distributes fouling and scaling loads evenly across the membrane array, preventing localized scale buildup.
This design allowed the plant to increase recovery from ~65% to a consistent 88%, even under high-silica, high-SDI conditions. Cleaning intervals expanded by 2–4x, with CIP frequency reduced by 25–50%, moving from every 2–4 weeks to approximately 6–8 weeks. Importantly, this was achieved while lowering chemical intensity and operating at a reduced pH range compared to the legacy HERO system.
Permeate quality remained stable throughout operation, with conductivity consistently below 400 µS/cm, meeting boiler feed requirements and supporting reliable internal reuse across cooling and steam generation processes
The recovery increase translated directly into significant brine volume reduction, relieving pressure on the site’s evaporation ponds and extending their operational lifespan. By delaying the need for pond expansion, the retrofit reduced both capital risk and long-term environmental exposure.
FR-RO also introduced greater flow flexibility, allowing the plant to adjust production in real time to meet operational demand without adding new pretreatment infrastructure. Approximately 100 GPM of permeate is consistently routed to a second-pass RO for boiler feed, with the remaining flow supporting cooling tower operations and other internal uses. The improved internal water balance reduced freshwater intake and enhanced overall site resilience in arid desert operating conditions
Presenting Case Study at IWC and UltraFacility 2025
The results from this power plant retrofit were presented by the ROTEC team at the International Water Conference and UltraFacility Conference, where we shared operating data from high-recovery FR-RO systems under ZLD conditions. The presentations highlighted the impact of Flow Reversal and Block Rotation in the power sector, while also demonstrating how the same design principles are being applied across other industrial applications facing high salinity, scaling risk, and water reuse constraints.