Zero liquid discharge is the most misunderstood item on a plating shop’s capital plan. It is usually discussed as a treatment technology, alongside precipitation and filtration, when it is nothing of the kind. It is a disposal strategy: instead of meeting a discharge limit, you stop discharging altogether, and everything that would have left in the water leaves as a solid instead.
That distinction decides the economics. Treatment removes pollutants from water you then discharge. ZLD removes the water, at a cost that scales with how much water there is and how much salt is in it. Which is why the most important design question in a ZLD project is not which evaporator to buy. It is how little water you can get away with sending to it.
What zero liquid discharge means in practice
A ZLD plant takes the treated effluent from a conventional plating wastewater plant — water that would otherwise go to sewer or surface water — and reduces it to two outputs: water clean enough to return to the process, and a solid waste that goes to licensed disposal.
Nothing leaves as liquid. That is the whole point, and it is why ZLD is adopted when there is no discharge route available at all: a site with no sewer connection, a catchment where the local limit cannot be met at reasonable cost, a region where water abstraction is restricted, or a customer or authority that requires it outright.
The stages, and what each one is for
ZLD is a train, and each stage exists to make the next one cheaper.
1. Conventional pre-treatment
Everything covered in how electroplating wastewater treatment works still happens first: segregation, cyanide destruction, chromium reduction, precipitation, clarification, filtration. By the end of it, the metals are out.
What is left is water carrying dissolved salts — sodium, sulfate, chloride, and whatever the treatment chemicals added. Metals are no longer the problem. Volume is.
2. Reverse osmosis — the stage that pays for itself
RO pushes the treated water through a membrane, producing two streams: permeate, clean enough to go back to the rinse line, and concentrate, which carries the salts and goes on to the next stage.
This is the stage that most often justifies the project on its own, before evaporation enters the picture. Permeate returning to rinsing is water you do not buy, do not soften, and do not heat. In a shop with high water costs it can pay back independently of the ZLD requirement, and it is worth designing for reuse whether or not you go all the way to zero discharge.
RO recovery is limited by osmotic pressure and by fouling. Standard RO membranes concentrate to roughly 5–8 percent dissolved solids; beyond that the pressure required climbs steeply and membrane life falls. Getting further usually means high-pressure RO or a different concentration technology.
3. Evaporation — the stage that costs
Past the point where membranes stop being economic, the water has to be boiled off. This is the heart of a ZLD plant and the source of almost all of its operating cost.
The modern answer is MVR — mechanical vapour recompression. Instead of heating with steam and throwing the vapour away, an MVR system compresses the vapour it just produced and uses the compressed vapour to heat the next batch of liquid. The latent heat is recycled rather than discarded, which is what makes it far more efficient than a conventional thermal evaporator.
Even so, it is energy-hungry. A well-designed MVR evaporator typically consumes something in the range of 20 to 40 kWh per cubic metre of water evaporated, and the figure is very sensitive to the design and to the boiling point elevation of the liquor — concentrated brines boil at higher temperatures and take more work. Compare that with the tens of kilowatt-hours a plating line spends per hour on everything else combined, and the shape of the problem becomes clear: this cost is per cubic metre, so it is a volume problem before it is a technology problem.
4. Crystallisation and solids handling
The evaporator drives the brine towards saturation. A crystalliser then forces the dissolved salts out as solid, which is dewatered and sent to licensed disposal.
One thing worth knowing before you commit: the crystallised product from mixed plating effluent is usually a mixed salt. It is not a saleable by-product and in most jurisdictions it is not recyclable — it is a waste stream with a disposal cost. Some processes can recover specific salts where the waste is segregated and single-source, but a mixed effluent generally cannot.
Why volume is the whole game
Put the numbers side by side and the design priority writes itself.
| Stage | Cost driver | Lever you control |
|---|---|---|
| Pre-treatment | Chemical dose, sludge volume | Segregation; keeping chelated and concentrated streams out of the main flow |
| Reverse osmosis | Membrane area, pressure, cleaning frequency | Feed quality — RO performance is set by what the clarifier let through |
| Evaporation | Volume of water to evaporate, and its salt load | How much water you sent to it in the first place |
| Crystallisation and disposal | Mass of solid waste | Drag-out, rinse efficiency, chemical dose |
Every stage downstream of the rinse tanks is sized by what the rinse tanks send it. That means the cheapest cubic metre in a ZLD plant is the one that never enters it, and the engineering that produces that saving is unglamorous: counterflow rinsing, drag-out reduction, drip time, spray rinses, and returning RO permeate to the rinse line instead of using fresh water.
This is the same point that governs the sizing of any plating wastewater plant — see how to size a plating wastewater treatment system — but under ZLD it is amplified, because every cubic metre you were going to discharge anyway now also has to be boiled. A shop that halves its rinse flow halves the evaporator, the RO plant, the chemical consumption and the energy bill together. There is no other decision in the project with that leverage.
When ZLD is the wrong answer
It is worth being direct about this, because ZLD is sometimes specified by habit rather than by necessity.
ZLD rarely pays for itself on energy savings or water savings alone. It is justified by a constraint: no discharge route, a limit that cannot be met, a water abstraction restriction, or a requirement imposed by a customer, a lender or a regulator. Where a site has a workable discharge route and meets its limits, the money is almost always better spent on reducing water use and improving the existing treatment plant.
It is a demanding plant to operate. RO membranes foul and need cleaning; evaporators scale, and scaling is the most common cause of ZLD plants underperforming. The performance of the whole train depends on the pre-treatment being run properly every day, not on the quality of the evaporator.
It does not make waste disappear. The pollutants leave as solid waste that still has to be disposed of under hazardous waste rules, and the mixed salt cake is a cost, not a product.
Where ZLD is genuinely required, none of that changes the answer — but it does mean the project should be approached as a water-minimisation project with an evaporator attached, rather than an evaporator purchase with some tanks in front of it.
What a ZLD project needs to be specified properly
- Actual flow to be evaporated — after every water reduction measure, not the current discharge figure
- Dissolved solids load and composition — the salt load sets the evaporator duty, and the chemistry sets the scaling risk
- Water reuse target — how much permeate can genuinely be returned to the process, and where
- Energy available and its cost — MVR needs electrical capacity, and the operating cost is a function of the tariff
- Waste disposal route and cost for the salt cake, agreed before the plant is built
- Operating labour and skill — this is a plant that needs attention
- Redundancy expectations — a ZLD plant with no bypass and a failed evaporator has nowhere to put its water
The equipment we build for this stage of a plant draws on the same filtration and metal recovery equipment used across our wastewater systems. If you are facing a discharge limit that treatment alone cannot reach, the useful first conversation is about your water balance rather than your evaporator.
Frequently asked questions
- Is zero liquid discharge the same as zero discharge?
- Effectively, for a plating shop. ZLD means no liquid effluent leaves the site — water is recovered for reuse and the pollutants leave as solid waste. It is not “no waste”; the solid still has to be disposed of.
- What does ZLD cost to run?
- The dominant operating cost is the energy to evaporate water, typically in the range of 20–40 kWh per cubic metre for a well-designed MVR system, plus membrane replacement, chemicals and labour. Because the energy cost is per cubic metre, the running cost is set by how much water the plant has to evaporate — which is why reducing rinse flow matters more than choosing between evaporator designs.
- Can I do ZLD without an evaporator?
- Not usually to true zero discharge. Reverse osmosis can recover a large fraction of the water and return it to the process, but it produces a concentrate that still has to go somewhere. Evaporation and crystallisation are what close the loop.
- Is the salt from a ZLD plant saleable?
- Generally no. Mixed plating effluent produces a mixed salt that is normally disposed of as waste rather than sold. Recovery of specific saleable salts requires segregated, single-source waste streams, which most job shops do not have.
- Does ZLD replace my existing wastewater treatment plant?
- No — it sits behind it. Precipitation, clarification and filtration still have to remove the metals first, and the ZLD stages only work if that pre-treatment is performing. A ZLD plant fed with poorly treated water will foul and scale.
- Will ZLD pay for itself?
- Rarely on savings alone. It is normally justified by a constraint — no discharge route, an unmeetable limit, or a regulatory or customer requirement. Where a discharge route exists and limits are met, water reduction and better treatment usually give a better return.