Quick answer: In a zero liquid discharge train, the evaporator and crystalliser are the thermal stages that remove the water membranes cannot. A brine concentrator or MVR evaporator concentrates the reject brine, and a forced circulation crystalliser then boils off the final water to leave a solid salt for disposal or reuse.
Key facts
- Thermal stages sit at the end of a ZLD train, after membrane pre-concentration has removed the bulk of the water at far lower energy cost.
- Mechanical vapour recompression (MVR) evaporators recycle the latent heat of the vapour, cutting energy use to roughly 20 to 40 kWh of electricity per m3 evaporated, well below simple thermal evaporation.
- Crystallisers are the most energy-intensive stage per m3 of water removed because they operate at the highest salinity and produce solids.
- Forced circulation is the workhorse configuration for high-salinity, scaling-prone ZLD brines in both concentrators and crystallisers.
- The choice and sizing of thermal equipment is set by the brine chemistry, the recovery target and the value placed on the recovered solids.
What role do evaporators and crystallisers play in ZLD?
Evaporators and crystallisers are the thermal heart of a zero liquid discharge system. They remove the water that membranes physically cannot, taking a concentrated brine all the way to a solid residue.
A typical ZLD train uses membranes such as reverse osmosis to remove the bulk water cheaply, because thermal energy is expensive. The evaporator then concentrates the membrane reject further, and the crystalliser finishes the job by evaporating the last water and precipitating salts. Splitting the duty this way minimises total energy use.
What is a brine concentrator?
A brine concentrator is an evaporator designed to take membrane reject brine and reduce its volume substantially before the crystalliser. It handles the middle of the concentration range, above what membranes can reach but below saturation.
Brine concentrators are commonly falling-film or forced-circulation evaporators, often with mechanical vapour recompression for energy efficiency. By shrinking the brine volume, the concentrator reduces the load on the more expensive crystalliser, improving overall economics.
How do falling film, forced circulation and MVR evaporators differ?
The three configurations differ in how they move and heat the brine and how they manage scaling. The right choice depends on brine salinity, scaling tendency and available energy.
- Falling film: brine flows as a thin film over heated tubes; efficient and low-fouling for cleaner, lower-salinity brines but less tolerant of scaling.
- Forced circulation: brine is pumped at high velocity through the heat exchanger to suppress scaling on surfaces; the standard choice for high-salinity, scaling-prone ZLD brines.
- MVR (mechanical vapour recompression): not a separate vessel type but an energy scheme; the vapour is compressed and reused as the heat source, sharply cutting external energy demand.
Many ZLD trains combine these, for example a falling-film brine concentrator with MVR followed by a forced-circulation crystalliser.
What does a crystalliser do in a ZLD train?
The crystalliser is the final thermal stage. It evaporates the remaining water from the near-saturated brine so that dissolved salts precipitate as crystals, which are then dewatered to a solid cake.
Forced-circulation crystallisers dominate ZLD service because they cope with high salinity and scaling. The crystalliser defines whether the plant achieves true zero liquid discharge, because it is the stage that converts the last liquid into solid. Its output is either a saleable salt or a waste requiring disposal, depending on purity.
How much energy do ZLD thermal stages use?
Thermal stages are the dominant energy consumers in a ZLD plant, and energy rises as salinity increases toward the crystalliser. This is why trains are designed to remove as much water as possible with membranes first.
The table below compares the main thermal options. Figures are approximate planning values that vary with brine chemistry, scale and energy scheme.
| Equipment | Typical role | Energy intensity | Strengths | Limitations |
|---|---|---|---|---|
| Falling film evaporator (MVR) | Brine concentration, lower salinity | Roughly 20 to 40 kWh/m3 electric | Energy-efficient, low fouling on cleaner brine | Less tolerant of high scaling |
| Forced circulation evaporator (MVR) | Brine concentration, high salinity | Roughly 25 to 45 kWh/m3 electric | Handles scaling and high TDS | Higher pumping energy |
| Forced circulation crystalliser | Final stage, produces solids | Highest per m3, often steam-driven | Achieves true ZLD, recovers solids | Most energy-intensive, complex operation |
| Steam-driven evaporator (no MVR) | Where cheap waste steam exists | High thermal demand | Simple, uses available heat | Poor economics without cheap heat |
When is each technology used?
Selection follows the brine chemistry and the recovery target. Membranes handle the dilute end, concentrators the middle and crystallisers the saturated end, with MVR added wherever electricity is cheaper than steam.
Follow this logic when configuring a thermal train.
- Maximise membrane recovery first to minimise the volume reaching thermal stages.
- Use a brine concentrator (falling film or forced circulation) to shrink the reject volume.
- Apply MVR where electricity price and duty favour recompression over raw steam.
- Use forced circulation where scaling and high salinity would foul a falling-film unit.
- Add a crystalliser to convert the final brine to solids and reach true ZLD.
- Select dewatering (centrifuge or filter) to match the required residue dryness.
What drives the selection of thermal equipment?
The main drivers are brine salinity and scaling tendency, the recovery target, the local energy price and the intended fate of the recovered solids. These interact, so equipment choice should follow a water and mass balance, not a default.
Scaling species such as silica, calcium and sulphate often govern the design, because they dictate whether forced circulation is needed and how the plant is operated to avoid fouling. Where the recovered salt has value, purity requirements can also shape the crystalliser and dewatering choice.
Netherlands and EU regulatory context
Thermal ZLD equipment is selected within the same regulatory frame that governs discharge and residues. In the Netherlands and EU this means the Water Framework Directive, the Industrial Emissions Directive and its BAT-BREF reference documents.
The classification of the final salt residue under EU and Dutch waste rules, including the ZZS framework for substances of very high concern, strongly affects the value of the whole train. A residue that can be reused is an asset; one classed as hazardous is a recurring cost. Energy-intensive thermal stages also carry a carbon and cost exposure under EU energy and climate policy, which increasingly favours MVR and waste-heat integration.
Common mistakes with ZLD thermal equipment
The most common mistake is oversizing thermal stages by underusing membranes upstream, which inflates both capital and energy cost. Getting the split right is the single biggest lever.
- Sending too much water to thermal stages instead of maximising membrane recovery.
- Ignoring scaling species, leading to fouling and unplanned shutdowns.
- Choosing falling film where forced circulation is needed for high-salinity brine.
- Neglecting waste-heat integration that could displace expensive electricity or steam.
- Overlooking the classification and value of the final solid residue.
- Sizing for average rather than peak brine load.
When to bring in an independent advisor
An independent advisor helps when thermal equipment quotes vary widely and the energy and residue economics are contested. Thermal stages are the most expensive part of a ZLD plant, so getting the configuration right matters most here.
Brine Consulting is independent and vendor-neutral. We do not sell evaporators or crystallisers, so our advice on configuration, sizing and the membrane-versus-thermal split is built around your brine and energy price, not a supplier’s product line.
Frequently asked questions
What is the difference between a brine concentrator and a crystalliser?
A brine concentrator is an evaporator that reduces brine volume in the mid-concentration range, above membrane limits but below saturation. A crystalliser is the final stage that evaporates the last water so salts precipitate as solids. The concentrator shrinks volume; the crystalliser produces the solid residue that makes ZLD complete.
Why use MVR in a ZLD evaporator?
Mechanical vapour recompression reuses the latent heat in the vapour by compressing it and returning it as the heat source. This sharply reduces external energy demand, often to roughly 20 to 40 kWh of electricity per m3 evaporated. MVR is favoured where electricity is cheaper than steam and no abundant waste heat is available.
Which crystalliser type is used in ZLD?
Forced-circulation crystallisers dominate ZLD service because they tolerate the high salinity and strong scaling tendency of concentrated brines. Brine is pumped at high velocity to keep salts from depositing on heat-transfer surfaces. This robustness matters more than peak efficiency at the saturated end of a ZLD train.
Why are evaporators and crystallisers so energy-intensive?
They remove water by boiling it, which requires the latent heat of vaporisation, and they operate at high salinity where boiling-point elevation raises the energy needed. The crystalliser is the most intensive because it works at saturation and produces solids. Maximising membrane recovery upstream is the main way to limit this energy.
Can the recovered salt be reused?
Sometimes. Where the crystalliser produces a sufficiently pure single salt, it may be reusable, but many ZLD residues are mixed salts of low value or are classified as waste. Whether reuse is viable depends on brine chemistry, crystalliser design and the classification rules that apply in the Netherlands and EU.
Sources and further reading
- European Commission, Industrial Emissions Directive (2010/75/EU) and BAT reference documents (BREF)
- European Commission, Water Framework Directive (2000/60/EC)
- Netherlands ZZS (Zeer Zorgwekkende Stoffen) framework, RIVM
- US EPA, effluent guidelines and steam electric power BAT literature
- Peer-reviewed literature on brine concentration and crystallisation (e.g. Desalination, Chemical Engineering Journal)
- Industry association technical references on evaporation and crystallisation
Talk to Brine Consulting for independent, vendor-neutral advice on configuring and sizing the evaporator and crystalliser stages of your ZLD train.
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes George Iacovou, a salicultural technologist with more than 23 years in crystallization, MVR systems and salt processing, and Prof. Basel Abusharkh, a specialist in brine valorization, ZLD and high-recovery desalination. Meet our experts.