When does ZLD actually make sense — and when is MLD enough?

There is a pattern in how industrial water projects go wrong. A discharge limit tightens or a permit renewal looks uncertain, someone senior says the words “zero liquid discharge”, and the project is scoped around ZLD before anyone has established whether the site actually needs it. Two years later the plant works exactly as designed and the operating cost is a permanent line item that nobody modelled properly.

ZLD is a genuinely excellent technology. It is also thermal, and thermal processes are expensive to run. The question worth asking early is not whether ZLD would solve the problem — it almost always would — but whether Minimum Liquid Discharge solves enough of it.

What each one actually delivers

MLD uses advanced filtration to recover the maximum practical volume of water without going thermal. In our systems that means conventional reverse osmosis, closed-circuit reverse osmosis (CCRO) and high-efficiency reverse osmosis (HERO), and it achieves around 85% water recovery from a wastewater stream. It is a cost-effective alternative to expensive technologies like evaporation, and it still meets many environmental limits.

ZLD goes further by adding evaporation and crystallisation after pre-treatment. Contaminants are reduced to close to zero, and the water leaving the evaporator is crystal clear at less than 100 ppm total dissolved solids — near-100% recovery, and clean enough to reuse directly in cooling towers and boiler feed. What leaves the plant is a solid rather than a liquid.

So the real comparison is not “85% versus 100%”. It is: what does that final 15% cost to recover, and what is it worth to you?

Four conditions that push you toward ZLD

In our experience the decision turns on a small number of factors, and they are rarely about the water chemistry.

• Discharge is prohibited or effectively capped. If the permit does not allow liquid discharge at all, or the volume allowance blocks the expansion you are planning, MLD leaves you with a concentrate you still cannot get rid of.
• Water is genuinely scarce. In a water-stressed region the recovered volume has a real replacement cost, and the last 15% is worth more than it looks on paper.
• The stream is high-salinity. ZLD is often considered the final stage of treatment for high-salinity industrial wastewater, and RO-based recovery hits osmotic limits well before it runs out of contaminants.
• The solids have value. If crystallisation separates a salt you can sell or reuse in your own process, part of the operating cost is offset. In alcohol production and yeast-containing lines, for example, the output can carry over 20% organic carbon and up to 17% potassium.

If none of those apply, MLD deserves a serious look before the thermal option is committed to.

The thing that changes both cases: pre-treatment

This is where most of the economics actually sit, and it is consistently underestimated. Pre-treatment is one of the crucial stages in ZLD technology, and for complex input streams it can be more complex than the evaporator stage itself. It is also where the cost of everything downstream is set.

The arithmetic is simple: every unit of load removed cheaply upstream is a unit that does not have to be boiled off. Depending on the feed, DAF, RO, UF, CCRO and HART reduce that load, and the result is fewer evaporators, lower capital cost and lower energy consumption. A pre-treatment design tuned to the actual contaminant profile routinely changes the size of the plant that has to be bought.

A useful test: if a supplier quotes ZLD without first asking for a detailed stream analysis, they are sizing the evaporator from a template rather than from your effluent.

Evaporator choice is the second lever

If you do go thermal, the evaporation method matters as much as the decision to evaporate. Multi-effect evaporators (MEE), mechanical vapour recompression (MVR), multi-stage flash (MSF) and forced-circulation evaporators differ substantially in energy consumption, initial investment, operating cost, maintenance burden and tolerance of scaling.

Choosing the right evaporator affects the overall efficiency of the ZLD process, energy usage, initial investment, operating cost and maintenance cost — which is a long way of saying it affects everything. This is one reason we keep all four options open rather than offering a single evaporation method: a supplier with one technology will design the process to suit the technology.

A practical sequence

The order we would suggest, and the order we work in:

• Characterise the stream properly — volume, composition, variability, and where it comes from in the process.
• Establish the real constraint: is it the permit, the water cost, the expansion plan, or a corporate ESG target?
• Model MLD first. If ~85% recovery clears the constraint, the thermal stage may be unnecessary.
• If it does not, design pre-treatment to minimise the thermal duty before sizing any evaporator.
• Select the evaporator type against energy, maintenance and available heat sources — not against catalogue availability.
• Decide what happens to the solids before you build, because that determines whether the output is a product or a disposal cost.

The honest summary

ZLD is the right answer when discharge is genuinely constrained, water is genuinely scarce, or the solids are genuinely worth something. In those cases it does something no other approach does, and it does it reliably — running 24/7 with automatic CIP and rinsing, without operator intervention.

When those conditions are not present, MLD delivers most of the benefit for materially less energy, and the difference shows up on the operating budget every year for the life of the plant. Recommending the smaller system is not a lack of ambition; it is the same engineering judgement that decides which evaporator to use.