Cooling Tower Blowdown

VSEP^® for Power Plant Cooling Tower Blowdown

High‑recovery RO that resists scaling, reduces ZLD thermal load, and recycles high‑quality water for cooling‑tower makeup.

Cooling towers serve as the thermal workhorses of a power plant, but their blowdown stream becomes heavily concentrated with dissolved solids, treatment chemicals, corrosion products, and biological load. This makes discharge costly and often restricted—especially at inland facilities with strict TDS or salinity limits.

VSEP® (Vibratory Shear Enhanced Processing) offers a fundamentally different RO approach, using vibration‑induced shear to maintain a clean membrane surface. This enables production of high‑quality permeate for reuse without the extensive pretreatment required by conventional spiral‑wound RO and significantly reduces brine volume sent to the evaporator/crystallizer in ZLD service.

Primary outcomes

• Maximize water reuse: High‑quality permeate for cooling‑tower makeup.
• Reduce ZLD thermal load: Higher RO recovery decreases the volume entering the evaporator and crystallizer, lowering both capital and operating costs.
• Handle near‑saturation brine: VSEP’s high shear enables operation on feeds containing gypsum and other sparingly soluble salts while allowing solids to precipitate and exit as slurry, functioning effectively as a membrane crystallizer.
• Lower total cost: Less chemical pretreatment, smaller thermal equipment, and fewer outages.

 

How a Natural Draft Cooling Tower Works (Quick Primer)

Large power plants commonly use natural‑draft, hyperbolic cooling towers. Their tall “hourglass” (hyperboloid) geometry creates a chimney effect: warm, moist air inside the tower becomes buoyant and rises; cooler ambient air is drawn in at the base, enabling evaporative cooling, no fans required. This design is favored for very large, steady heat loads typical of thermal and nuclear stations.

How Blowdown Volume & Frequency Are Determined

1) Mass balance foundation

Cooling‑tower water balance is commonly expressed as:
Makeup (M) = Evaporation (E) + Blowdown (B) + Drift (D). Evaporation removes essentially pure water, concentrating dissolved solids in the recirculating loop; blowdown controls that concentration.

• Rule of thumb for evaporation: ≈ 1% of circulation flow for every 10°F (≈5.6°C) of cooling across the tower.

2) Cycles of Concentration (CoC)

CoC = (TDS in circulating water) / (TDS in makeup). For a given CoC, an idealized relationship is:
B ≈ E / (CoC − 1) (ignoring drift/leaks for simplicity). Higher CoC save water and reduce blowdown, but increase scaling and corrosion risk, so the achievable CoC depends on makeup quality and inhibitor program.

Example: If E = 100 gpm and CoC = 5, then B ≈ 100 / (5–1) = 25 gpm blowdown. (Actual values must account for drift, leaks, and site‑specific chemistry.)

3) Why blowdown is hard to discharge

Blowdown often carries high TDS/TSS, scaling salts (e.g., gypsum/ CaSO₄·2H₂O), corrosion inhibitors, antiscalants, biocides, and biofouling byproducts. Inland power plants face strict discharge permits or no‑discharge requirements, making onsite treatment, reuse, or ZLD necessary.

VSEP as the Primary Membrane Step in a ZLD Process

A typical ZLD process for blowdown includes membranes upfront to recover as much reusable water as possible, followed by thermal steps (brine concentrator and crystallizer) to handle the remaining brine and solids. VSEP enables much higher recoveries on blowdown streams than spiral‑wound RO, directly reducing thermal system size and cost.

Typical VSEP ZLD Process:

1. Equalization + Screening (minimal pretreatment)
   Stabilize flow and remove gross solids.
2. VSEP® High‑Recovery RO
   • Vibratory shear disrupts concentration polarization and scale formation, enabling high recovery even on near‑saturation feeds (e.g., gypsum).
   • Solids can precipitate in situ and are carried out as a concentrated slurry—VSEP operates like a membrane‑crystallizer, producing clear permeate and a small volume of concentrate.
3. Thermal Finishing
   • Evaporator/Brine Concentrator → Crystallizer sized for the reduced brine volume delivered by VSEP.
4. Permeate Reuse
   • Recycle permeate as cooling‑tower makeup, reducing freshwater demand and chemical cost.

Why this matters: Conventional spiral‑wound RO on blowdown typically needs elaborate pretreatment (softening, pH conditioning, antiscalants, sometimes seed slurries) and still runs at modest recovery, pushing a larger load to the evaporator. VSEP’s vibration tolerates high scaling indices and supports substantially higher recovery with simpler front‑end—cutting both capex and opex in the thermal steps.

VSEP vs. Spiral‑Wound RO (Head‑to‑Head)

Conventional spiral‑wound RO struggles with cooling‑tower blowdown due to scaling salts, biological load, and antiscalant interactions. As a result, pretreatment becomes complex and recovery remains limited. VSEP’s vibration‑induced shear overcomes these constraints.

 

Challenge	Spiral‑Wound RO (Conventional)	VSEP® Vibratory RO
Feed near saturation (e.g., gypsum)	High risk of rapid scaling; often requires extensive pretreatment and still limited recovery; frequent cleanings/outages.	Vibratory shear suppresses scale deposition; operates at high recovery with minimal pretreatment; can precipitate solids during filtration (membrane‑crystallizer behavior).
ZLD integration	Lower recovery → larger evaporator/crystallizer, higher energy use.	Smaller brine volume → downsized thermal equipment, lower energy and capex.
Chemical program	Heavier reliance on antiscalants and softening, increases complexity and cost.	Simplified chemistry, fewer additives, consistent runs.
Uptime & maintenance	Frequent cleanings due to fouling/scaling on difficult feeds.	Longer run times between cleanings; easier solids management.
Footprint	Larger when recovery is constrained and thermal gear must be oversized.	Compact overall system thanks to higher membrane recovery and smaller thermal train.

How VSEP Works: Torsional Oscillation & High Shear at the Membrane

VSEP® (Vibratory Shear Enhanced Processing) operates on a completely different physical principle than conventional spiral‑wound RO. Instead of forcing water through a stationary membrane and allowing a boundary layer of solids to accumulate, VSEP uses torsional oscillation to keep the membrane surface continually refreshed.

Torsional Oscillation

At the core of every VSEP unit is a patented system that oscillates and resonates the membrane stack in a torsional, twisting motion, similar to the way a tuning fork vibrates. This oscillation creates rapid acceleration and deceleration at the membrane surface. The movement is extremely small in amplitude but extremely high in energy density, allowing the system to generate powerful shear without moving the feed water through complex flow channels.

Shear at the Membrane Surface

As the membrane vibrates, it creates intense shear waves across its surface. This shear performs several critical functions:

• Prevents boundary-layer buildup: Solids, crystals, and biological matter cannot accumulate because the shear continuously sweeps the surface clean.
• Suppresses scaling: High-shear conditions disrupt the nucleation and growth of sparingly soluble salts—such as gypsum—that normally scale and destroy RO membranes.
• Allows solids to precipitate and be carried away: Instead of forming a hard scale, solids remain suspended and are removed in the concentrated reject stream. In this way, VSEP behaves like a membrane‑crystallizer, enabling treatment of near‑saturation blowdown brines.
• Enables high recovery without elaborate pretreatment: Because the membrane surface remains clean and free of scale, VSEP can operate at recovery rates far beyond what spiral RO can achieve on the same challenging water.

Why This Matters for Cooling‑Tower Blowdown

Natural cooling‑tower blowdown contains high TDS, scaling salts, corrosion inhibitors, and organic/biological load. Traditional RO membranes foul quickly unless the plant adds softening, chemical conditioning, or extensive pretreatment.

VSEP’s torsional oscillation overcomes these limits by maintaining a continuously renewed membrane surface, even under extreme brine conditions, making it ideal as the primary membrane step in a ZLD system. High recovery from VSEP means a smaller evaporator, lower energy consumption, and dramatically lower lifecycle cost.

Where VSEP Fits in the Power Plant Water Cycle

• Makeup water reduction: Recycle permeate directly to the cooling‑tower basin.
• Compliance & stewardship: Meet inland discharge restrictions and salinity limits; avoid challenges of direct sewer/surface‑water disposal, which often require pretreatment and tight permits.
• Chemicals optimization: By removing TDS and troublesome ions, VSEP permeate can reduce dosage of inhibitors and biocides in the tower loop.

Case Study #1 – California Power Plant

A power plant in the Central Valley had a ZLD system installed to treat cooling tower blowdown. The process involved a HERO® RO system as a primary stage. The concentrate from that was sent to a brine concentrator and then finally to a crystallizer. The existing RO system was producing too much reject water and the brine concentrator could not handle the capacity. In 2008, New Logic installed a VSEP RO system to filter the primary RO system reject to volume reduce it by another 50% so that the brine concentrator could handle the reject volume from the VSEP.

The reject from the existing spiral RO was supersaturated and VSEP operates continuously to reduce the reject TDS from 13,000 mg/L to 200 mg/L. the VSEP permeate is sent back to the cooling tower as makeup water.

Case Study #2 – California Power Plant

In 2014, New Logic commissioned a VSEP RO system at a biomass power plant in Northern California. The client had been storing cooling tower blowdown and stormwater in a pond and was then hauling this for disposal. The VSEP treated this water recovering 90% that is recycled as makeup water. The reduced hauling costs by 90%.

Case Study #3 – Texas Power Plant

In 2011, a VSEP system was commissioned at a Texas natural gas-fired power plant to treat cooling tower blowdown. A 6-modules system installed treats 160 gpm and recovers 90% of the volume as clean water that can be recycled to the cooling tower or used as boiler feed to make steam.

Typical VSEP Cooling Tower Blowdown Performance Examples:

	Flux Rate	Recovery	Feed Solids	Reject Solids
Project 1	15 gfd	80%	0.14%	2.21%
Project 2	25 gfd	90%	0.09%	1.24%
Project 3	21 gfd	82%	0.82%	4.65%
Project 4	35 gfd	94%	0.18%	3.21%
Project 5	39 gfd	84%	0.78%	4.88%

Pilot Testing and Performance Projections

New Logic offers pilot testing, including full‑scale demonstrations, to evaluate performance for specific projects. Results will vary depending on feedwater characteristics as shown above. In-house bench testing is also available. Pilot testing is required to validate full-scale system design. Pilot testing is supervised by membrane engineers with decades of experience. Pilot testing allows:

• Verification of filtrate quality
• Measurement of flux, recovery, and fouling behavior
• Accurate operating cost projections
• Optimization for reuse or ZLD configurations

New Logic partners with evaporator companies and can coordinate a full ZLD pilot test using full-scale VSEP equipment and an evaporator pilot system. Such a pilot was recently completed at a power plant in Mississippi.

Full-Scale VSEP Pilot for Cooling Tower Blowdown

Get a Site‑Specific Cost Analysis

System performance depends on TDS, solids loading, chemistry, and target recovery rates. Every facility is different. Actual performance and system design depends on:

• Treatment requirements
• Recovery target
• Brine composition
• Chemical additives
• Type of membrane targeted

For a performance projection for your specific project, contact a VSEP engineer: info@vsep.com

Learn more:

• Review VSEP® Technology: https://www.vsep.com/technology/
• Explore Pilot Testing Options: https://www.vsep.com/technology/pilot-testing/
• Biomass CTBD Case Study: Burney Power

Patented VSEP^® is manufactured by New Logic Research since 1987 and is used globally for industrial wastewater treatment, including commercial laundries, landfills, rendering, food processing, and PFAS‑impacted streams. VSEP is widely used in permit-driven discharge applications. This page has been reviewed by New Logic’s senior engineering staff. Date of last review: February 2026

 

New Logic Research, 2527 Aviation Way, Minden, NV 89423