Zero liquid discharge (ZLD): discharge nothing, recover everything

Zero-liquid-discharge water-treatment plant: incoming effluent, reverse osmosis and evaporator, water recycled in a closed loop and dry salt as the only residue — Zero liquid discharge (ZLD): recovering almost all the water and keeping only a dry-salt residue.

In certain situations — very strict discharge standards, no outlet, acute water stress — a site simply can no longer discharge liquid effluent. This is where zero liquid discharge (ZLD) comes in: recovering almost all the water and concentrating the salts down to a solid residue.

An ambitious concept, ZLD is not always realistic at 100%. A breakdown, without idealisation.

0liquid discharge (ZLD)

> 95%of water recovered

~7 vs ~10kWh/m³: MLD vs ZLD

30–40%cost of an MLD vs ZLD

1. What is ZLD?

Zero liquid discharge means discharging no liquid effluent: the water is recovered and recycled (often > 95%), while the salts and contaminants are concentrated down to a solid (dry salt), the only waste to remove. It is the most advanced level of discharge control.

Closed-loop diagram of zero liquid discharge: the effluent is treated, the water recycled (over 95%) goes back into the cycle and the only waste is dry salt — The principle of zero liquid discharge: water runs in a closed loop (recycled at over 95%) and is never discharged; only a dry-salt residue remains.

2. Why aim for ZLD?

Several factors drive towards ZLD:

Very strict discharge standards or the impossibility of discharging (no outlet).
Water stress: recycling water becomes vital.
Levies and taxation on discharges (including the PFAS levy).
Image and the requirements of clients.

3. The technology chain

A ZLD installation stacks several stages, from the least to the most energy-hungry:

Pretreatment: clarification, softening, removal of troublesome matter.
Membrane concentration: high-recovery reverse osmosis to extract as much water as possible at the lowest energy cost.
Evaporator / concentrator: reduction of the residual volume.
Crystalliser: production of the dry salt.

Diagram of the zero-liquid-discharge chain: pretreatment, high-recovery reverse osmosis, evaporator/concentrator, crystalliser, with recycled water going back into the loop and the concentrate reduced to dry salt — The ZLD chain: water is recovered at each stage (> 95%) while the concentrate is reduced to dry salt — reverse osmosis does the bulk of the work at low energy cost.

DIMM ion-exchange resin for pretreatment — Ion-exchange resin comes into play at pretreatment, upstream of the concentration.

4. MLD or ZLD? The comparison

Full ZLD is costly in energy and investment. Many sites prefer minimal liquid discharge (MLD): most of the water is recovered while keeping a very small residual discharge, for a much more reasonable cost. The table below clarifies the trade-off:

Criterion	ZLD — zero liquid discharge	MLD — minimal liquid discharge
Liquid discharge	None — only residue: dry salt	Residual, very small
Water recovery	Almost all	Up to ~ 95%
Technologies	High-recovery RO + evaporator + crystalliser	High-recovery reverse osmosis (limited evaporation)
Energy	High (~ 10 kWh/m³)	Lower (~ 7 kWh/m³)
Investment (CAPEX)	High (evaporator, crystalliser)	~ 30–40% of the cost of a ZLD
When to choose it	Discharge banned, protected zone, absolute constraint	High recovery at controlled cost

📊 Orders of magnitude from comparative studies: MLD achieves very high recovery for about a third of the cost of a full ZLD, by avoiding the most energy-hungry thermal stages.

5. Benefits and limits

Benefits: water very largely recycled, compliance even on the most constrained discharges, resilience. Limits: high CAPEX and energy consumption (evaporation is expensive), and salt management to organise. Hence the interest of maximising the membrane share before evaporation.

6. The central role of reverse osmosis

The more water is extracted by reverse osmosis (low energy) before evaporation (very energy-hungry), the more the overall cost drops. High-recovery osmosis is therefore the economic key of a successful ZLD/MLD project.

7. Who is ZLD for?

ZLD/MLD concerns sites with sensitive or impossible discharges: chemicals, pharmaceuticals, surface treatment, energy, textiles, as well as installations in a protected zone or subject to strict discharge standards.

Conclusion

Zero liquid discharge is the culmination of discharge control: recycling almost all the water and keeping only a solid residue. Ambitious and costly, it is often reasoned as MLD (minimal discharge), maximising the reverse-osmosis share to contain the energy bill. The right level is sized according to the discharge constraints and the site context.

The DIMM teams support industrial companies, in Belgium, France and the Netherlands, in the pretreatment and membrane concentration of discharge-reduction projects.

🔗 DIMM products & solutions

DIMM supplies the upstream building blocks of a controlled ZLD/MLD strategy — where most of the performance and cost is decided:

Industrial reverse osmosis (high recovery)
Industrial softeners and pretreatment
Filter media and cartridges
The full DIMM product range

Reference points

ZLD (Zero Liquid Discharge): no liquid discharge, water recycled (often > 95%), salts concentrated to a solid.
Typical chain: pretreatment → high-recovery reverse osmosis → evaporator/concentrator → crystalliser.
MLD (Minimal Liquid Discharge): a more economical alternative, with a very small residual discharge.
Limits: high CAPEX and energy (evaporation), dry-salt management.
Orders of magnitude (comparative studies): energy ~7 kWh/m³ (MLD) vs ~10 kWh/m³ (ZLD); CAPEX of an MLD ≈ 30–40% of that of a full ZLD.
Reverse osmosis maximises water recovery at low energy cost before evaporation. Information dated August 2026.