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Skid-mounted membrane module trains at a brine concentration plant.

VMD · Technology

Vacuum Membrane Distillation

Vacuum membrane distillation (VMD) is membrane distillation with a vacuum on the permeate side: a heated brine flows along a porous, water-repellent membrane, and the vacuum draws water through its pores as vapor, leaving the salts behind. Petro Sep builds it as AQUA-SEP®, a hollow-fiber VMD system for brines, the reject of reverse osmosis (RO) plants and high-salinity wastewater, taking feeds of 25% salt and more.

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What it solves

Performance you can size against.

What the process delivers. Every number is testable on your own feed.

Feed salinity
25%+ salt
Brines are concentrated up to salt saturation: 290,000 ppm for sodium chloride in the AQUA-SEP® brochure's example.
Total feedwater recovery
85 to 90%+
Up to about 95% overall when AQUA-SEP® treats the reject of an RO plant.
Energy, 100 GPM unit
26 + 129 kWh/m³
Electricity plus utilities on Petro Sep's data sheet, against 43 + 300 kWh/m³ for an evaporator and crystallizer.
Product water
Below 10 ppm
From a 40,000 ppm feed, in the AQUA-SEP® brochure's example.

How it works

How does vacuum membrane distillation work?

The vacuum creates the vapor pressure difference that drives the water across the membrane; a compressor reuses the heat of the vapor, and the brine is concentrated up to the point of crystallization.

A heated brine is pumped from a feed tank along hydrophobic, microporous hollow-fiber membranes, while the other side of the membrane is kept under vacuum. The vacuum creates a vapor pressure difference across the pores: water evaporates at the pore entrances and is drawn through as vapor, while liquid water and the dissolved salts stay in the brine. A compressor or blower moves the vapor to a heat exchanger, where it condenses as product water, and its heat is captured and reused to keep the circulating brine at temperature; a vacuum pump on the distillate tank holds the vacuum. The brine returns to the feed tank and is cycled again, growing more concentrated, up to the point of crystallization.

Guides

Performance envelope

Specs and operating range.

For preliminary sizing only. Production sizing is always validated against your specific feed.

Process
Vacuum membrane distillationHollow fiber, with mechanical vapor compression
Membranes
Hydrophobic, microporous, polymeric hollow fibersPlastic: PE, PP, PVDF and others
Feed salinity
25+% saltConcentrated up to salt saturation
Product water
< 10ppmFrom a 40,000 ppm feed, in the brochure's example
Total feedwater recovery
85 to 90+%Up to about 95% overall when AQUA-SEP® treats RO reject
Operating temperature
About 70°CThe brochure's example gives 70 °C as the maximum brine operating temperature
Pressure
Vacuum on the permeate sideNo high pressure required
Energy, 100 GPM unit
26 + 129kWh/m³ electricity + utilitiesAgainst 43 + 300 kWh/m³ for an evaporator and crystallizer, on Petro Sep's data sheet
Plant range
50 to 1,000m³/dayPurification plants in various ranges
Footprint, 100 m³/day plant
2 skids5.5 × 5.5 × 5 m and 6.5 × 2.5 × 5 mIn the brochure's example
Condenser materials
Super duplex SS or Hastelloy C-276Corrosion resistant
Pretreatment
Required
Descaling
No chemical descalants
Operating mode
Continuous or batchThe same system design runs either way

Explore further

Projects

How we compare

VMD vs the alternatives.

Where Vacuum Membrane Distillation wins, where it does not, and where the alternatives are honestly the better fit.

How water is separated

VMD (AQUA-SEP®)

Evaporates through a hydrophobic membrane under vacuum, at about 70 °C

Reverse osmosis

Pressed through a semi-permeable membrane by a high-pressure pump; the saltier the feed, the more pressure needed

Evaporator + crystallizer

Evaporated on metal heat-transfer surfaces, with mechanical vapor compression, then crystallized

Feed salinity

VMD (AQUA-SEP®)

25%+ salt, concentrated up to saturation

Reverse osmosis

About 6 to 7% TDS, the "max salt content for RO" on Petro Sep's chart

Evaporator + crystallizer

Through to moist salt

Water recovery

VMD (AQUA-SEP®)

85 to 90%+; up to about 95% overall on RO reject

Reverse osmosis

About 40% in Petro Sep's RO presentation; 50% at the RO stage of the data sheet's train

Evaporator + crystallizer

60% in the evaporator, then 99% in the crystallizer, on the data sheet

Energy, Petro Sep's 100 GPM data sheet

VMD (AQUA-SEP®)

26 kWh/m³ electricity + 129 kWh/m³ utilities

Reverse osmosis

5 kWh/m³ at the RO stage, the lowest of the three where the feed allows it

Evaporator + crystallizer

43 kWh/m³ electricity (25 in the evaporator, 18 in the crystallizer) + 300 kWh/m³ utilities

Scale and cleaning

VMD (AQUA-SEP®)

No chemical descalants; no cleaning downtime; modules replaced without shutting the system down

Reverse osmosis

Pretreatment critical against fouling and scaling; cleaning 1 to 4 times a year, depending on the feed

Evaporator + crystallizer

In the evaporator, scale builds up on the heat-transfer surfaces; frequent cleaning; repairs need a shutdown

Plant size and operating mode

VMD (AQUA-SEP®)

Highly scalable, small capacities also feasible; one design runs continuous or batch

Reverse osmosis

Widely used at full scale

Evaporator + crystallizer

For the evaporator, small capacities are not feasible, because of high capital cost; it can be designed for only one mode, continuous or batch

Place in a ZLD train

VMD (AQUA-SEP®)

Takes pretreated water or RO reject through to solids separation

Reverse osmosis

First stage after pretreatment; its reject goes on

Evaporator + crystallizer

After RO: evaporator, then crystallizer, then solids separation

Where this technology applies

Industries that use this process.

Vacuum Membrane Distillation fits some sectors better than others.

Oil, Gas & Energy

Process equipment, packaged plants, and membrane separation for upstream, midstream, downstream, and energy.

Learn more

Mining

Process plants and membrane separation for tailings, brines, lithium, and ZLD.

Learn more

Industrial Manufacturing & Environmental

Process equipment, packaged plants, and membrane recovery for manufacturing, water reuse, and emissions control.

Learn more

Engineering FAQ

Questions engineers ask.

The questions we hear weekly. If you have a different one, send it with your consultation request and we will answer it directly.

What is the difference between membrane distillation and vacuum membrane distillation?
VMD is one of the four main configurations of membrane distillation (MD). In all of them, water evaporates at a porous, water-repellent membrane and crosses it as vapor; in VMD a vacuum on the permeate side draws the vapor out, and it condenses outside the membrane module. The other configurations condense it in a cold water stream, on a cooled plate behind an air gap, or after a sweep gas carries it away.
Does Petro Sep supply other kinds of membrane distillation?
No. Petro Sep's membrane distillation technology is vacuum membrane distillation, built as AQUA-SEP®. Petro Sep also supplies conventional pressure-driven membranes: microfiltration, ultrafiltration, nanofiltration and reverse osmosis.
Why use a vacuum for high-salinity feeds?
Petro Sep's AQUA-SEP® brochure considers VMD potentially the most efficient MD configuration, and a better option than direct contact MD for high salt concentrations, because fouling in VMD is usually low and can be controlled through chemical or water wash. Reviews agree that VMD has the highest flux and the lowest conductive heat loss of the four configurations, and add the trade-offs: the membrane is more prone to wetting under vacuum, and the condenser and vacuum pump add cost.
Can a VMD system run on waste heat?
It runs at about 70 °C. The AQUA-SEP® brochure notes that MD could use low-grade and inexpensive heat sources, and Petro Sep's General Presentation names unused waste or low-grade energy from power plants and refineries as a way to reduce its energy further. AQUA-SEP® also reuses its own heat: the latent heat from the compressor or blower that moves the vapor is captured and reused in the system, for example to keep the circulating water at temperature.
How is a VMD system sized?
Through a feasibility study on the feed. AQUA-SEP® plants come in various ranges, from 50 to 1,000 m³/day; in the brochure's example, a 100 m³/day plant fits on 2 skids. The design is highly scalable, small capacities are also feasible, and the same system can run continuously or in batches. Petro Sep delivers turnkey projects, from research and feasibility through engineering and fabrication to operations management.
What pretreatment does the feed need?
Pretreatment is required: the AQUA-SEP® brochure lists it for AQUA-SEP® as for the alternatives it compares. Reactive or surface-active compounds in a feed can make a hydrophobic membrane wet, letting liquid into the pores; one review reports powdered activated carbon removing surfactants from a wastewater and mitigating wetting in MD.
How are fouling and membrane replacement handled?
AQUA-SEP® uses high-velocity hollow-fiber modules to keep scaling and fouling off the membrane surface, with no chemical descalants and no cleaning downtime. Membrane modules are easy to replace, and replacing them needs no system shutdown.
Can VMD treat RO reject?
Yes: the brochure names RO brine reject among the high-TDS applications it calls ideal for AQUA-SEP®, and adds that AQUA-SEP® can treat the highly concentrated brine rejected by conventional thermal desalination plants. Petro Sep's RO presentation says that routing an RO plant's reject to AQUA-SEP® could raise water recovery up to about 95%, maintain the performance of the RO system and its membranes, and allow the brine to be collected in a pond for sale.
What else can a VMD system concentrate?
Petro Sep's VMD presentation lists removing water from acids (succinic, adipic, propanoic and sulfuric acid) and bases (sodium hydroxide), and an example in which VMD recovered water for reuse as steam to produce bitumen. The AQUA-SEP® brochure adds fruit juices and milk, glycols, non-volatile acids, oil-in-water emulsions, streams that high temperatures would degrade, and textile wastewater contaminated with dyes.

Have a feed that VMD
might fit?

Send us a sample and a target outcome. Petro Sep conducts feasibility studies on complex fluid separation problems: if VMD fits your feed, we will tell you what to expect.

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