Quick answer: Zero liquid discharge means no wastewater leaves the site: the water is recovered for reuse and the dissolved salts leave as a solid. The usual train is pre-treatment, reverse osmosis to concentrate the brine as far as membranes allow, evaporation to take it the rest of the way, and a filter press or centrifuge to turn the crystal slurry into a dry cake. RO does the cheap work; evaporation does the expensive work; the cake is what you actually have to haul.
What ZLD actually is
A conventional plant treats wastewater until it is clean enough to discharge. A ZLD plant treats it until there is nothing left to discharge. Every litre either goes back into the process as recovered water or leaves as water vapour, and everything that was dissolved in it — salts, metals, hardness — ends up as a solid.
Regulation is the usual driver. In the United States the 2024 supplemental steam electric rule established a zero discharge of pollutants limitation for flue gas desulfurization wastewater, bottom ash transport water and combustion residual leachate at coal-fired power plants. Elsewhere, ZLD is imposed on textile, tannery, pharmaceutical and chemical parks where the receiving water can no longer accept salt, or it is chosen voluntarily where fresh water is scarce and the recovered water is worth more than the treatment costs.
The standard train, stage by stage
1. Pre-treatment: protect the membranes
Whatever will scale or foul the RO has to come out first. That means suspended solids, oil, organics and the hardness and silica that would precipitate as the brine concentrates. Chemical softening with lime and soda ash is common; it produces its own sludge, which is dewatered on a chamber filter press alongside the final salt. A plant that skips this stage pays for it in membrane cleaning and evaporator downtime.
2. Reverse osmosis: the cheap part
RO removes water from the brine by pressure, and pressure is far cheaper than heat. The catch is osmotic pressure, which rises with salinity. Standard seawater elements are rated to about 80 bar, and above roughly 70–80 g/L total dissolved solids the osmotic pressure of the brine approaches that limit and recovery stops. So RO can typically recover 50–75% of the water, depending on the feed, and hands over a concentrate of several tens of grams per litre. High-pressure RO and multi-stage designs push this further; the concentrate volume they leave behind is what sets the size and cost of everything downstream.
3. Evaporation: the expensive part
The remaining brine is boiled. Evaporating one cubic metre of water takes about 2,260 MJ of latent heat, which is around 630 kWh of thermal energy if none of it is recovered. Brine concentrators recover most of it: mechanical vapour recompression compresses the vapour and reuses its heat to boil more brine, so the electrical demand is a fraction of the raw latent heat. Even so, evaporation costs several times more per cubic metre than RO, which is why every litre RO can take first is worth taking.
The brine concentrator stops at a slurry that is still pumpable, typically at or just below the saturation point of the dominant salt. A crystallizer or a solar pond finishes the job: the water flashes off and the salts crystallize.
4. Solids dewatering: the part that leaves the site
The crystallizer produces a slurry of salt crystals in saturated mother liquor, usually 20–40% solids. It has to be dewatered before disposal, and the mother liquor has to go back to the crystallizer, not to drain. Three points decide the equipment:
- Corrosion. Saturated chloride and sulfate brines at elevated temperature attack carbon steel and many stainless grades. Polypropylene plates and cloth are unaffected, which is one reason filter presses are common here; wetted metal parts are specified in duplex stainless or lined.
- Mixed salts. Industrial ZLD rarely produces a single clean salt. The cake is a mixture of sodium chloride, sodium sulfate, calcium and magnesium salts and whatever trace metals were in the feed. It is usually classified as hazardous or restricted waste and disposed of by weight, so cake dryness matters.
- Cake washing. Where the salt is to be sold or landfilled as non-hazardous, the mother liquor trapped in the cake carries the impurities. A filter press allows the cake to be washed in place before it is blown dry and discharged, which a centrifuge cannot do.
A membrane filter press takes crystal slurry to a cake that is typically 80–90% solids for well-formed crystals, and the squeeze stage shortens the cycle. The softening sludge from stage 1, by contrast, behaves like any metal hydroxide sludge and dewaters to 30–40% in a chamber press.
Where the water goes and where the cost goes
| Stage | What it removes | Relative cost per m³ | By-product |
|---|---|---|---|
| Pre-treatment | Solids, oil, hardness, silica | Low | Softening sludge (filter press) |
| Reverse osmosis | 50–75% of the water as permeate | Low to moderate | Concentrate to evaporator |
| Brine concentrator | Most of the remaining water | High | Saturated brine |
| Crystallizer | Last of the water | Highest per m³ | Crystal slurry |
| Dewatering | Mother liquor from crystals | Low | Salt cake for disposal or sale |
The lesson in that table is simple: the cheapest cubic metre is the one that never reaches the evaporator. Better pre-treatment lets RO run to higher recovery, and higher RO recovery shrinks the evaporator, the crystallizer and the energy bill together.
Minimal liquid discharge: the honest alternative
Many plants that ask for ZLD actually need minimal liquid discharge. If a small volume of concentrated brine can be sent to a licensed evaporation pond, a deep well, a brine line or a third-party treater, the crystallizer disappears from the flowsheet and with it a large share of the capital and energy. RO plus a brine concentrator, or RO alone with a high-recovery design, reaches 90–98% water recovery at a fraction of the cost of true zero discharge. Ask the regulator what the permit actually requires before designing for zero.
What to send for a ZLD proposal
A full water analysis (TDS, chloride, sulfate, sodium, calcium, magnesium, silica, COD, oil), the daily flow, the required reuse water quality, the available energy (steam, electricity, price of each), and the disposal route for solids. From that we can estimate RO recovery, evaporator load, the dry salt production per day and the dewatering equipment for both the softening sludge and the salt cake.
Related: industrial RO water purification systems, heavy metal wastewater treatment, how much sludge a wastewater plant produces, and industrial wastewater treatment equipment.
Frequently asked questions
What does zero liquid discharge mean?
No liquid effluent leaves the site. Water is recovered for reuse or evaporated, and all dissolved solids are removed as a solid cake. The term is regulatory as much as technical: it describes the permit condition, not a specific machine.
Why is RO used before the evaporator in a ZLD plant?
Because removing water by pressure is far cheaper than removing it by heat. RO takes out 50–75% of the water first, so the evaporator and crystallizer are sized for a fraction of the original flow. RO cannot finish the job on its own because osmotic pressure rises with salinity until it exceeds what the membranes can withstand.
What limits RO recovery in ZLD?
Osmotic pressure and scaling. Standard membranes are rated to roughly 80 bar, which is reached at about 70–80 g/L TDS, and sparingly soluble salts such as calcium sulfate and silica precipitate on the membrane long before that unless they are removed in pre-treatment.
Is ZLD salt cake hazardous waste?
Often, because it is a mixture of salts that carries the trace metals and organics from the feed. Classification depends on local rules and on what is in the water. Washing the cake in a filter press reduces the mother liquor it carries and can change the classification.
What is minimal liquid discharge?
A design that recovers 90–98% of the water and sends a small volume of concentrated brine to an approved outlet instead of crystallizing it. It avoids the crystallizer, which is the most expensive stage per cubic metre, and is the right answer for many sites that first ask for ZLD.
Which dewatering equipment is used for ZLD solids?
Filter presses and centrifuges for the crystal slurry, filter presses for the softening sludge. Filter presses are favoured where the brine is corrosive, where the cake must be washed, or where the highest cake solids is needed to cut disposal weight.
Send your water analysis, flow and energy costs and we will lay out the recovery stages and size the dewatering for both sludge and salt: request a ZLD flowsheet and sizing.
References: US EPA — Steam Electric Power Generating Effluent Guidelines, 2024 Final Rule, US EPA — 2024 Steam Electric Final Rule Fact Sheet.
This article is part of our guide to wastewater sludge dewatering.