Quick answer: Zero liquid discharge desalination treats the concentrated brine from a desalination plant so that no liquid reject is discharged. It concentrates the brine further and then crystallises it into solid salt, recovering additional water and eliminating brine disposal. It is more energy-intensive than conventional discharge and is chosen where brine disposal is restricted.
Key facts
- Conventional seawater desalination recovers roughly 40 to 50 per cent of the feed as fresh water and rejects the rest as brine at roughly twice the feed salinity.
- Brine disposal to sea, sewer or deep wells is increasingly restricted because of local salinity, density and ecological impact near the outfall.
- Zero liquid discharge eliminates the liquid brine entirely, while minimal liquid discharge (MLD) recovers most water but leaves a small residual stream at far lower energy cost.
- The ZLD chain concentrates the brine (membrane and thermal stages) then crystallises it, with energy rising steeply toward the crystalliser.
- Brine valorisation, recovering salts or minerals as products, can offset cost but rarely pays for ZLD on its own with current markets.
What is zero liquid discharge desalination?
Zero liquid discharge desalination is the treatment of desalination brine so that no liquid is discharged from the plant. The brine is concentrated and then crystallised, leaving only solid salt and recovered water.
Conventional desalination produces fresh water plus a concentrated brine reject that must go somewhere. ZLD removes that reject by driving it to dryness, turning a disposal problem into a solid residue and recovering extra water in the process. It is the highest-recovery, highest-energy end of brine management.
Why is desalination brine a problem?
Desalination brine is a problem because it is a large, concentrated, dense stream whose disposal can harm local ecosystems and is increasingly restricted by regulators. The volume is comparable to the fresh water produced.
Brine discharged to the sea sinks and spreads near the outfall, raising local salinity and affecting benthic life. Discharge to sewer or inland waters is often not permitted at this salinity. Deep-well injection and evaporation ponds need specific geology or land. As these routes narrow, higher-recovery approaches including MLD and ZLD become more attractive.
What is the difference between MLD and ZLD?
Minimal liquid discharge (MLD) recovers most of the water and leaves a small concentrated residual stream, while zero liquid discharge eliminates the liquid entirely by crystallising the final brine. The difference is the last increment of recovery, which is the most expensive.
The table below frames the decision between conventional discharge, MLD and ZLD.
| Approach | Water recovery | Liquid reject | Relative energy and cost | Typical fit |
|---|---|---|---|---|
| Conventional discharge | Standard plant recovery | Full brine reject discharged | Lowest | Where brine disposal is permitted and cheap |
| Minimal liquid discharge (MLD) | High | Small residual stream | Moderate to high | Where disposal is restricted but a small reject is allowed |
| Zero liquid discharge (ZLD) | Highest, near total | None (solid residue only) | Highest | Where no liquid discharge is permitted or brine has no outlet |
MLD often captures most of the benefit of ZLD at materially lower cost, so it deserves explicit evaluation rather than defaulting to full ZLD.
How does the ZLD desalination treatment chain work?
The chain concentrates the brine in stages and then crystallises it, so each stage handles a higher salinity than the last. Cheaper membrane concentration comes first, expensive thermal crystallisation last.
Follow this general sequence for a ZLD desalination train.
- Start from the desalination reject brine, already concentrated by the primary process.
- Further concentrate the brine with high-recovery membranes such as brine-concentration RO or electrodialysis where feasible.
- Pretreat to remove scaling species (calcium, sulphate, silica) that would foul thermal stages.
- Concentrate thermally in a brine concentrator, often with mechanical vapour recompression.
- Crystallise the near-saturated brine to precipitate salts and reach zero liquid discharge.
- Dewater the solids and recover the condensed water for reuse.
What is brine valorisation and salt recovery?
Brine valorisation is the recovery of salts, minerals or other products from desalination brine so that the residue has value instead of only cost. It ranges from bulk sodium chloride to higher-value minerals such as magnesium or lithium compounds.
Selective recovery can produce a saleable salt and improve the ZLD business case. In practice, product purity, market access and processing cost limit how much value can be captured, so valorisation usually offsets rather than fully funds ZLD with current markets. It is strongest where a specific, recoverable mineral is present at useful concentration.
How do you decide between discharge, MLD and ZLD?
The decision turns on whether liquid discharge is permitted, the cost and availability of disposal routes, the value of recovered water, and local energy price. These factors together set which approach is defensible.
- Regulation: if no liquid discharge is permitted, ZLD or near-ZLD is forced.
- Disposal availability: where outfalls, wells or ponds are unavailable, recovery rises in value.
- Water value: in water-scarce settings, recovered water strengthens the case for high recovery.
- Energy price: high electricity or steam cost pushes toward MLD over full ZLD.
- Residue fate: whether the salt is saleable, inert waste or hazardous changes the economics.
A structured techno-economic comparison, rather than a default to ZLD, gives the most defensible answer.
Netherlands and EU regulatory context
In the Netherlands and EU, desalination and brine discharge fall under the Water Framework Directive and, for industrial plants, the Industrial Emissions Directive with its BAT-BREF documents. These govern what may be discharged and where.
Discharge permits are issued through Rijkswaterstaat for direct discharge to state waters or the Waterschappen for indirect discharge, and the ZZS framework restricts substances of very high concern. While large-scale seawater desalination is less common in the Netherlands than in water-scarce regions, industrial desalination and brine streams still face these controls, and tightening limits are a key driver toward higher recovery and ZLD.
Common mistakes in ZLD desalination projects
The most common mistake is defaulting to full ZLD when MLD would meet the drivers at much lower cost. The last increment of recovery is disproportionately expensive.
- Choosing full ZLD without evaluating MLD as an alternative.
- Overestimating brine valorisation revenue in current markets.
- Underestimating scaling from calcium, sulphate and silica in thermal stages.
- Ignoring the energy and carbon cost of thermal crystallisation.
- Overlooking residue classification and disposal cost.
- Sizing on average rather than peak brine flow and salinity.
When to bring in an independent advisor
An independent advisor is valuable when the choice between discharge, MLD and ZLD carries large capital and energy consequences and vendor proposals differ. The right answer depends on site specifics, not a standard package.
Brine Consulting is independent and vendor-neutral. We do not sell desalination or brine equipment, so our comparison of discharge, MLD and ZLD, and any valorisation case, is built around your brine, permit and energy price rather than a supplier’s product line.
Frequently asked questions
What is zero liquid discharge desalination?
Zero liquid discharge desalination treats the concentrated brine from a desalination plant so no liquid is discharged. The brine is further concentrated and then crystallised into solid salt, recovering additional water. It eliminates brine disposal entirely but is the most energy-intensive brine management approach and is chosen where discharge is restricted.
What is the difference between MLD and ZLD in desalination?
Minimal liquid discharge recovers most of the water and leaves a small concentrated residual stream, while zero liquid discharge eliminates the liquid entirely by crystallising the final brine into solids. MLD avoids the most expensive crystallisation stage, so it often captures most of the benefit at materially lower energy and cost.
Can you recover salt or minerals from desalination brine?
Yes, through brine valorisation, which recovers salts or minerals such as sodium chloride, magnesium or lithium compounds. Selective recovery can yield a saleable product and offset cost. In practice, purity, market access and processing cost limit the value captured, so valorisation usually offsets rather than fully funds ZLD in current markets.
Why not just discharge desalination brine to the sea?
Because concentrated brine is denser than seawater, it sinks and spreads near the outfall, raising local salinity and harming benthic life. Regulators increasingly restrict this, and inland or sewer discharge is often not permitted at brine salinity. Where disposal routes are unavailable, higher-recovery approaches such as MLD or ZLD become necessary.
Is ZLD always the best option for desalination brine?
No. ZLD is the highest-recovery but most energy-intensive and costly option. Where a small liquid reject is permitted, MLD often meets the same drivers at far lower cost. The best option depends on discharge rules, disposal availability, water value and energy price, which is why a structured techno-economic comparison is needed.
Sources and further reading
- European Commission, Water Framework Directive (2000/60/EC)
- European Commission, Industrial Emissions Directive (2010/75/EU) and BAT reference documents (BREF)
- Rijkswaterstaat and Unie van Waterschappen, guidance on discharge permitting (Netherlands)
- Netherlands ZZS (Zeer Zorgwekkende Stoffen) framework, RIVM
- US EPA, guidance on desalination concentrate management
- Peer-reviewed literature on desalination brine management, MLD and ZLD (e.g. Desalination, Water Research, Science of the Total Environment)
Talk to Brine Consulting for an independent, vendor-neutral comparison of discharge, MLD and ZLD options for your desalination brine.
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Prof. Basel Abusharkh, a specialist in brine valorization, ZLD and high-recovery desalination, and George Iacovou, a salicultural technologist with more than 23 years in crystallization, MVR systems and salt processing. Meet our experts.