What is zero liquid discharge (ZLD)?

Tong and Elimelech (2016) define it as "a wastewater management strategy that eliminates liquid waste and maximizes water usage efficiency." Its outputs are reusable water and solids. ZLD systems first appeared in the 1970s at US power plants along the Colorado River, in response to the river's rising salinity.

Petro Sep uses the term for solvents as well as water: its General Presentation describes VOC-SEP® recovering "both chemicals and water to 'reusable levels'" as zero liquid discharge. The partial version, minimal liquid discharge (MLD), is compared below. Short definitions are in the glossary.

A 2025 review describes a typical ZLD setup in four steps (pretreatment, pre-concentration, evaporation and crystallization); this guide adds handling the salt as a fifth.

1. Pretreatment. Removes what would foul or scale the later stages: suspended solids, organics, hardness and silica. Petro Sep's RO presentation calls pretreatment "critical" for RO; Table 1 lists some of the options.
2. Reverse osmosis (pre-concentration). A high-pressure pump forces water through the membrane and leaves 95 to 99% of the dissolved salts in the reject. RO is the low-energy stage on Petro Sep's data sheet (5 kWh/m³), but the pressure it needs rises with salinity. A 2025 review puts RO's limit at a feed of 70 g/L and a recovery of up to 50%; Petro Sep's RO presentation gives a recovery of about 40%.
3. Brine concentration. Takes the RO reject toward saturation. In a thermal train this is an evaporator, such as the mechanical vapor compression (MVC) evaporator on Petro Sep's data sheet. Membrane distillation (MD) does the same job through a hydrophobic membrane: water vapor passes through the pores and the salts stay behind. In Petro Sep's trains, this is AQUA-SEP.
4. Crystallization. Takes the saturated brine past the point where salt comes out of solution and recovers nearly all of the remaining water. On Petro Sep's data sheet the MVC crystallizer is labelled 99% water recovery, and what leaves it is "moist salt."
5. Solids handling. Separates the salt crystals from the last of the liquid; a dryer can follow. The salt is then sold, reused or disposed of.

Petro Sep's energy data sheet draws two such trains for a 100 GPM unit:

• Evaporator and crystallizer train: saline water → pretreatment → reverse osmosis (5 kWh/m³; 50% water recovery) → MVC evaporator (25 kWh/m³; 60%) → MVC crystallizer (18 kWh/m³; 99%) → moist salt → solids separation → salt.
• AQUA-SEP train: the pretreated feed goes straight to AQUA-SEP, or through RO first, with the RO reject going to an AQUA-SEP stage with mechanical vapor compression. The AQUA-SEP block is labelled 26 kWh/m³ and 90% water recovery. Its concentrate goes straight to solids separation, so on the sheet it takes the place of both the evaporator and the crystallizer.

In the words of the 2025 review by Panagopoulos and Michailidis, the membrane step "lightens the burden on the following, more expensive steps." Tong and Elimelech trace the "evolution of ZLD from thermal- to membrane-based processes."

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." That is AQUA-SEP's role. Its brochure calls it "Easily integrated in hybrid zero liquid discharge process", and Petro Sep's RO presentation says RO reject can be routed to an AQUA-SEP unit, which could raise water recovery "up to ~95%". Petro Sep's configurations are on the hybrid ZLD page.

On Petro Sep's data sheet, RO uses 5 kWh/m³; the evaporator and crystallizer after it use 43 kWh/m³ of electricity plus 300 kWh/m³ of utilities.

AQUA-SEP is lower on both lines. The data sheet does not break its utilities figure down. A 2020 study in Water Research puts mechanical vapor compression at 20 to 25 kWh of electricity per m³ of feed, to concentrate a saline stream from 0.1 to 1.0 M NaCl.

Under "Less Energy", Petro Sep's AQUA-SEP brochure lists a "Lower operating temperature, more transfer surface area, reuse of heat energy". Petro Sep's General Presentation adds that in VMD the latent heat from the blowers, compressors or pumps that move vapors and liquids is "captured and reused for other purposes within system". Membrane distillation can also use low-grade or waste heat (see the FAQ).

• Salinity and chemistry. The saltier the feed, the more pressure RO needs, until it reaches its limit. Hardness and silica, which form scale, shape the pretreatment the train needs.
• Recovery target. MLD stops at about 95% with membranes. ZLD gets the rest from evaporation and crystallization, which use far more energy than RO on Petro Sep's data sheet.
• Energy and heat. On the sheet's evaporator and crystallizer route, utilities (300 kWh/m³) outweigh electricity (43 kWh/m³). Capturing unused waste or low-grade heat can cut the energy membrane distillation draws.
• Materials. Petro Sep's comparison says evaporators must be built of corrosion-resistant materials such as duplex or super duplex stainless steel or Hastelloy C-276. AQUA-SEP's membranes are plastic (PE, PP, PVDF and others), with a super duplex or Hastelloy C-276 condenser.
• Scaling and cleaning. In the same comparison, evaporators carry a high operational cost from scale on their heat-transfer surfaces, which "leads to reduced energy efficiency and lower productivity", and they need frequent cleaning. AQUA-SEP membrane modules can be replaced with no system shutdown.
• Size. For evaporators, "Small capacity scale is not feasible due to high capital cost"; VMD is "Highly scalable, but small capacity scale is also feasible."
• Disposal. Petro Sep lists the "High cost and ability to dispose of highly concentrated brine" among the problems to address.
• Ownership. Petro Sep's 2024 General Presentation also lists build, own, operate, maintain (BOOM), priced per liter or gallon processed.

Capex and opex depend on all of these, so this guide gives no price ranges. A feasibility read on a sample is the way to a number.

Petro Sep lists both ZLD and MLD as applications for AQUA-SEP. The review attaches a condition to MLD: it suits manufacturers "contingent upon the fulfillment of legal and environmental prerequisites". In practice, the remaining brine needs a legal outlet.

• Rules. Petro Sep's General Presentation names "Tougher Disposal Regulations". In the United States, EPA's 2024 rule for coal-fired power plants set zero discharge limitations for three wastewaters (see the FAQ).
• Water. Plants in arid regions or with limited fresh water "increasingly adopt ZLD/MLD technologies to optimize water recovery" (Panagopoulos and Michailidis).
• Disposal cost and harm. Brine is costly to dispose of, and Petro Sep's RO presentation warns that RO reject returned to the sea harms marine life.
• Value. Recovered water goes back into the process, and Petro Sep counts the "Recovery of valuable waste streams" among the drivers.

Petro Sep's documents list these applications for AQUA-SEP:

• Desalination and RO reject: the reject of RO desalination plants and the concentrated brine of thermal desalination plants.
• Oil and gas: produced water from tight oil, shale gas and coalbed methane, including recovering water for reuse as steam to produce bitumen.
• Mining: mine tailings, mining wastewater and lithium recovery.
• Chemical plants: water removal from acids (succinic, adipic, propanoic, sulfuric) and bases (sodium hydroxide), up to caustic soda solids, or acid hydrate crystals when VMD is paired with a crystallizer and dryer in a hybrid ZLD system.
• Industrial wastewater: high-TDS toxic wastewater, and industrial wastewater with high salts and inorganic chemicals.

• Cost and energy. Tong and Elimelech: the technology "is constrained by high cost and intensive energy consumption."
• Scaling. Calcium carbonate, calcium sulfate and silica are common scale-formers on membranes, and evaporator heat-transfer surfaces scale too, so the train needs pretreatment matched to the feed.
• Mixed salts. Pure, saleable salts take design; otherwise the product is a mixed solid (see the FAQ on salt).
• Waste and emissions. Tong and Elimelech list "greenhouse gas emission and generation of solid waste" among ZLD's potential environmental impacts.
• When MLD is enough. Where a small brine stream can legally be disposed of, MLD leaves out evaporation and crystallization.

AQUA-SEP is a hollow fiber vacuum membrane distillation system. The feed runs along hydrophobic, microporous hollow fibers with a vacuum on the other side; water vapor passes through the pores to a condenser and almost all of the dissolved salts stay behind. Petro Sep integrates VMD with zero liquid discharge systems "up to the point of crystallization" (AQUA-SEP presentation). Its General Presentation marks "Crystallization In Feed Tank" at the salty end of AQUA-SEP's range, beside a photo of a saturated reject that "creates salt crystals".

AQUA-SEP in a ZLD train, from Petro Sep's documents:

• Feed: brines of 25% salt and more.
• Brine: concentrated to saturation; the brochure's comparative example takes a 40,000 ppm feed to saturation (290,000 ppm for NaCl).
• Product water: under 10 ppm in the same example.
• Recovery: 85 to 90%+ total feedwater recovery; RO plus AQUA-SEP could reach up to about 95%.
• Operation: about 70 °C, with a maximum brine temperature of 70 °C in the example; no chemical descalants; membrane modules can be replaced with no system shutdown.
• Size: plants from 50 to 1,000 m³/day; in the example, 100 m³/day fits in two skids, 5.5 × 5.5 × 5 m and 6.5 × 2.5 × 5 m.
• Pretreatment: required.

Petro Sep works turnkey, from research and feasibility through FEED, detailed engineering and fabrication to construction and operations management. Systems, sizes and options are on the ZLD systems page.

Send a sample or the data, with:

• flow rate, TDS and the main ions, including hardness and silica;
• organics, oil and suspended solids;
• temperature, and any waste heat on site;
• the recovery you need, and your disposal route and its cost;
• energy price, space, the regulatory driver and the timeline.