By the Petro Sep engineering team
Updated
What is a ZLD system?
A ZLD system is a train of pretreatment, membrane, brine concentration and solids equipment, sized together so that a stream enters as wastewater and leaves as water for reuse and salt.
A 2025 review describes a typical ZLD setup in four steps: pretreatment, pre-concentration, evaporation and crystallization. In equipment terms, a hybrid plant built around AQUA-SEP includes:
- Pretreatment. Filters, softening and antiscalant dosing, chosen for the feed. Petro Sep's RO presentation lists multimedia filters, microfiltration, antiscalants, water softening and sodium bisulfite among the options.
- Reverse osmosis (RO). A high-pressure pump forces water through the RO membranes and leaves almost all of the dissolved salts in the reject. The permeate is the first product water; the reject goes on to the brine stage.
- Brine concentration. In Petro Sep's drawing of the hybrid train, the RO reject collects in a vessel and is pumped through the AQUA-SEP VMD modules, with a condenser and a vacuum pump on the vapor side. In an evaporator and crystallizer train, this stage is a mechanical vapor compression (MVC) evaporator.
- Crystallization and solids. The saturated brine gives up its salt, which is separated from the remaining liquid; drying steps can follow. Petro Sep's energy data sheet runs the AQUA-SEP stage straight to solids separation.
How each stage works, ZLD against minimal liquid discharge (MLD), and why plants adopt ZLD are explained in the guide What is zero liquid discharge (ZLD)?
Which ZLD configurations does Petro Sep design?
Petro Sep's documents show four: AQUA-SEP on the pretreated feed, RO with AQUA-SEP on the reject, RO with AQUA-SEP stopping at a concentrated brine (MLD), and VMD for acids and bases, joined with a crystallizer and dryer to make acid hydrate crystals.
| Configuration | Train | Ends in | Fits |
|---|---|---|---|
| AQUA-SEP alone | Pretreatment → AQUA-SEP → solids separation → salt | Salt | Brines beyond RO's range: AQUA-SEP takes feeds of 25% salt and more |
| Hybrid ZLD | Pretreatment → RO → RO reject to AQUA-SEP with mechanical vapor compression → solids separation → salt | Salt | Streams RO can start on, including the reject of an existing RO plant |
| Hybrid MLD | RO → RO reject to AQUA-SEP → concentrated brine, collected in a pond for sale | Brine, with water recovery up to about 95% | Sites where the remaining brine has a legal outlet or a buyer |
| Acids and bases | VMD; for acid hydrate crystals, VMD integrated with a crystallizer and dryer in a hybrid ZLD system | Concentrated acids or acid hydrate crystals; concentrated bases or caustic soda solids | Chemical plants: succinic, adipic, propanoic and sulfuric acid; sodium hydroxide |
Membrane distillation reaches past RO because it does not push the feed through the membrane. The brine runs along one side of AQUA-SEP's hydrophobic, microporous hollow fibers with a vacuum on the other; the vapor pressure difference drives water vapor through the pores to the condenser, and almost all of the dissolved salts stay behind. RO, by contrast, needs more pressure the saltier its feed. More on the stage itself: AQUA-SEP and vacuum membrane distillation.
What is hybrid ZLD?
Hybrid ZLD shares the work between membranes and thermal steps: RO recovers the low-salinity water at low energy, and a stage built for high salinity, in Petro Sep's trains AQUA-SEP, takes the reject toward saturation before the salt is separated.
Evaporation and crystallization are the costly end of a ZLD plant. Tong and Elimelech (2016) write that ZLD "is constrained by high cost and intensive energy consumption", and trace "the evolution of ZLD from thermal- to membrane-based processes". In a 2025 review, Panagopoulos and Michailidis say the membrane step "lightens the burden on the following, more expensive steps."
RO alone stops early. The same review puts RO's limit at a feed salinity of 70 g/L and a recovery of up to 50%, and Petro Sep's RO presentation gives a recovery of about 40%. The review describes RO with membrane distillation as "a complementary system where RO is employed for initial water recovery, and MD further concentrates the brine to near saturation levels."
In Petro Sep's trains, AQUA-SEP is that second stage. Its brochure lists it as "Easily integrated in hybrid zero liquid discharge process", and Petro Sep's AQUA-SEP presentation describes integrating VMD with zero liquid discharge systems "up to the point of crystallization". Petro Sep's RO presentation says routing RO reject to an AQUA-SEP unit could raise water recovery "up to ~95%" while "Maintaining the performance of RO system & membranes".
The final recovery and the form of the solids depend on the stream. In the review's words, "there is no one-size-fits-all solution, since the makeup of brine effluents can differ a lot." The feasibility checklist below lists what decides it.
How much energy does a hybrid ZLD system use?
On Petro Sep's energy data sheet for a 100 GPM unit, the AQUA-SEP route uses 26 kWh/m³ of electricity and 129 kWh/m³ of utilities; an evaporator plus crystallizer uses 43 and 300.
| Route | Electrical energy (kWh/m³) | Utilities (kWh/m³) |
|---|---|---|
| Evaporator + crystallizer | 43 | 300 |
| AQUA-SEP | 26 | 129 |
The sheet gives no totals and does not break the utilities down. The sheet labels the reverse osmosis stage 5 kWh/m³. RO sits ahead of the evaporator and of one of the two AQUA-SEP branches; the other branch takes the pretreated feed straight to AQUA-SEP. The sheet's stage-by-stage labels are set out in the ZLD guide.
How does AQUA-SEP compare with an evaporator?
In Petro Sep's comparison, both recover water from brine and both use mechanical vapor compression, but AQUA-SEP's heat-transfer surfaces stay free of the scale that builds up on an evaporator's, and its modules can be replaced without a shutdown.
| Evaporator | AQUA-SEP VMD | |
|---|---|---|
| Duty | Recovers water from brine mixtures; can concentrate acid and base mixtures | The same |
| Materials | Must be corrosion resistant, such as duplex or super duplex stainless steel or Hastelloy C-276 | Membranes of plastic (PE, PP, PVDF and others); condenser in super duplex stainless steel or Hastelloy C-276 |
| Scale and operating cost | High operating cost: scale builds up on the heat-transfer surfaces, reducing energy efficiency and productivity | Low operating cost: heat-transfer surfaces free of scale |
| Vapor compression | Mechanical vapor compression (MVR) | Mechanical vapor compression (MVR) |
| Maintenance | High cost; frequent cleaning | Low cost; no cleaning downtime |
| Repairs | Expensive; the system must shut down | Low cost; modules are easy to replace, with no shutdown |
| Operating mode | Designed for one mode only: continuous or batch | The same design runs continuous or batch |
| Small plants | Not feasible, because of high capital cost | Feasible; also highly scalable |
Petro Sep's AQUA-SEP brochure adds that the system needs no high pressure and no chemical descalants.
What can a Petro Sep ZLD system handle?
AQUA-SEP takes feeds of 25% salt and more, concentrates brine to saturation and recovers 85 to 90%+ of the feedwater, in plants from 50 to 1,000 m³/day.
| Item | Value | Basis |
|---|---|---|
| Feed salinity | 25% salt and more | AQUA-SEP brochure |
| Brine produced | Saturation, 290,000 ppm for NaCl | Brochure's comparative example, from a 40,000 ppm feed |
| Product water | Under 10 ppm | Same example |
| Water recovery | 85 to 90%+ of the feedwater | AQUA-SEP brochure |
| Water recovery with RO ahead | Up to about 95%, with the brine collected for sale | RO presentation |
| Operating temperature | About 70 °C | AQUA-SEP presentation |
| Energy | 26 kWh/m³ electricity, 129 kWh/m³ utilities | Energy data sheet, 100 GPM unit |
| Plant size | 50 to 1,000 m³/day | AQUA-SEP brochure |
| Footprint | 100 m³/day in two skids: 5.5 × 5.5 × 5 m and 6.5 × 2.5 × 5 m | Brochure's comparative example |
| Membranes | Hydrophobic, microporous hollow fibers, in plastic (PE, PP, PVDF and others) | AQUA-SEP presentation |
| Operating mode | Continuous or batch, same design | AQUA-SEP presentation |
| Chemical descalants | None | AQUA-SEP brochure |
| Pretreatment | Required | AQUA-SEP brochure |
Petro Sep also has conventional pressure-driven membranes, including ultrafiltration and reverse osmosis, and its turnkey plants use membrane-assisted separation technologies including vacuum membrane distillation and ultrafiltration.
What should you have ready for a ZLD feasibility study?
Twelve facts: the stream's flow and full chemistry, its temperature, the recovery you need, and the site's disposal costs, energy and heat, space, rules and timeline.
Petro Sep's brochures list feasibility studies on complex fluid separation problems among its services. Bring these to the first conversation, and send a sample of the stream if you can.
| # | Have ready | Why it matters |
|---|---|---|
| 1 | Flow. Average and peak, in m³/day or US gallons per minute, and whether the stream runs continuously or in batches. | Flow sets the plant size. AQUA-SEP plants run from 50 to 1,000 m³/day, and the same VMD design can run continuously or in batches. In Petro Sep's comparison, small evaporators are not feasible because of their capital cost. |
| 2 | Salinity. Total dissolved solids (TDS), and how far it swings. | RO needs more pressure the saltier its feed, and a 2025 review puts its limit at 70 g/L. AQUA-SEP takes feeds of 25% salt and more. |
| 3 | Composition. A lab analysis of the main ions (sodium, potassium, calcium, magnesium, chloride, sulfate, carbonate and bicarbonate) and of any metals. | Brine effluents differ a lot, so there is "no one-size-fits-all solution" (Panagopoulos and Michailidis). The ions decide which salts form, and whether a plant can make separate, purer salts instead of a mixed solid. |
| 4 | Hardness and silica. Calcium, magnesium, sulfate and silica, called out from the analysis. | Calcium carbonate, calcium sulfate and silica are the most widespread scale-formers on membranes. They set the softening or antiscalant dosing the pretreatment needs. |
| 5 | Organics, oil, solids and oxidants. TOC or COD, oil and grease, suspended solids, silt density index (SDI), turbidity, and any free chlorine or chloramines. | Particles and organics foul RO membranes. Petro Sep's RO presentation suggests a multimedia filter when the SDI is above 3 or the turbidity above 0.2 NTU, and notes that thin film composite RO membranes do not tolerate chlorine or chloramines. |
| 6 | Temperature. Of the feed, with its range. | Membrane distillation is thermally driven, and AQUA-SEP operates at about 70 °C, so the feed's own heat counts toward the heat the stage needs. |
| 7 | Recovery target and end products. ZLD (salt) or MLD (a small concentrated brine), and what the recovered water will be used for. | MLD stops at about 95% water recovery with pretreatment and membranes; ZLD aims for "as much as 100%" and adds evaporation and crystallization, the more expensive steps. |
| 8 | Disposal today, and the salt. Where the wastewater or brine goes now (for example sewer, deep well injection or evaporation ponds) and what it costs; whether the salt could be sold or reused. | Petro Sep lists the "High cost and ability to dispose of highly concentrated brine" among the problems its brine systems address. ZLD salt can be marketed, recycled for industrial use, or handled in an environmentally sustainable manner. |
| 9 | Energy and heat. Electricity price, steam or fuel price, and any waste or low-grade heat on site, with its temperature and amount. | On Petro Sep's data sheet, utilities outweigh electricity on both routes: 300 against 43 kWh/m³ for an evaporator and crystallizer, 129 against 26 for AQUA-SEP. Membrane distillation can use low-grade or waste heat, and Petro Sep points to waste heat from power plants and refineries. |
| 10 | Space. Plot area, indoors or outdoors, and any height limit. | In the AQUA-SEP brochure's example, a 100 m³/day plant fits in two skids, 5.5 × 5.5 × 5 m and 6.5 × 2.5 × 5 m. |
| 11 | Regulatory driver and discharge limits. The permit, rule or company target behind the project, and the limits that apply. | It decides whether ZLD is required or MLD will do, since MLD leaves a brine that needs a legal outlet. In the United States, for example, EPA's 2024 rule for coal-fired power plants set "a zero discharge of pollutants limitation" for three wastewaters; a December 2025 rule extended its compliance deadlines. |
| 12 | Timeline. When the system must run, and any permit or shutdown dates. | It shapes the path from feasibility through conceptual design, FEED and detailed engineering to fabrication, construction and operation. |
How does a ZLD project with Petro Sep run?
From feasibility to operation: Petro Sep's AQUA-SEP presentation lists research, feasibility, conceptual design, FEED and detailed engineering, fabrication and construction, and project and operations management.
Petro Sep's 2024 General Presentation names two business models:
- Technology licensing: equipment sale, with membrane replacement and maintenance.
- BOOM (build, own, operate, maintain): a fee per litre or gallon processed, with a variable pricing model.
The engineering and build stages are described on turnkey EPCM, FEED and process engineering and skid-mounted process systems.
Where has Petro Sep built AQUA-SEP and brine systems?
Petro Sep's 2024 company profile and 2023 project photo list record AQUA-SEP and pretreatment systems in Saudi Arabia, China and the United States, from a 2002 seawater desalination skid to full-scale skids for RO reject treatment and brine concentration in China in 2008.
| Year | Location | As the documents describe it | Case page |
|---|---|---|---|
| 2002 | Saudi Arabia | AQUA-SEP, "Modular Skid for Seawater Desalination" | Saudi Arabia desalination pilot |
| 2007 | China | "Pretreatment UF Plant for RO Desalination (Aqua-Sep®)" | |
| 2007 | China | "Mining Water Pretreatment System (Aqua-Sep®)" | |
| 2008 | China | AQUA-SEP, "Full Scale Membrane Skids Installation for RO Reject Water Treatment & Brine Concentration" | China brine concentration plant |
| 2014 | China | AQUA-SEP, "Membrane Skid Testing for Industrial Waste & Brine Concentration" | |
| 2014 | Rhode Island, USA | AQUA-SEP, "Mobile Hazardous Water Treatment Plant Vacuum Skid" | Rhode Island mobile vacuum skid |
| 2015 | China | "Pretreatment Plant (Aqua-Sep®)": "Hazardous Mining Water Treatment Plant" | |
| 2015 | USA | AQUA-SEP, "Membrane Skid" and "Pilot Unit F.A.T." |
Petro Sep's documents list these applications for AQUA-SEP: desalination and RO reject; oil and gas produced water; mine tailings, mining wastewater and lithium recovery; acids and bases; and high-TDS toxic and industrial wastewater. See mining, oil and gas, chemical and petrochemical and industrial manufacturing.
Frequently asked questions
How much does a ZLD system cost?
What sizes do Petro Sep's ZLD systems come in?
Does Petro Sep supply the crystallizer?
Can we buy ZLD as a service instead of the equipment?
Can you guarantee zero liquid discharge?
Does a hybrid ZLD system need chemicals?
Does Petro Sep make its own membranes?
What is the difference between AQUA-SEP and a ZLD system?
Sources
- Petro Sep, AQUA-SEP® brochure.
- Petro Sep, AQUA-SEP® energy consumption comparison data sheet (100 GPM unit).
- Petro Sep, AQUA-SEP® vacuum membrane distillation presentation.
- Petro Sep, AQUA-SEP® and reverse osmosis presentation.
- Petro Sep, General Presentation (January 2024).
- Petro Sep, Company Profile (January 2024).
- Petro Sep, Project photo index (2023).
- Environmental Science & Technology (American Chemical Society), The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions (Tong and Elimelech, vol. 50, no. 13, pp. 6846 to 6855, 2016).
- Membranes (MDPI), Membrane Technologies for Sustainable Wastewater Treatment: Advances, Challenges, and Applications in Zero Liquid Discharge (ZLD) and Minimal Liquid Discharge (MLD) Systems (Panagopoulos and Michailidis, vol. 15, no. 2, article 64, 2025, open access).
- US Environmental Protection Agency, Fact sheet: Final Rule: Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Category (April 2024).
- US Environmental Protection Agency, Federal Register, Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source Category: Deadline Extensions (Final rule, 90 FR 61328, 31 December 2025).