Minimal Liquid Discharge: When 95 Percent Recovery Is the Right Answer
By Sean Alford, Permitting Plus
Minimal liquid discharge, usually shortened to MLD, means recovering most of the water from an industrial wastewater or desalination concentrate stream, typically 90 to 98 percent, and disposing of the small concentrate that remains by an approved route. It stops short of zero liquid discharge, and that is the point. The last few percent of water are the most expensive to recover, and for many plants they are not worth it.
For most industrial sites that are being pushed toward zero liquid discharge, our advice is to look hard at the 95 percent option first. These systems use membrane technology instead of evaporators for most of the work, it costs a fraction as much to run, and it satisfies the regulator in far more cases than owners expect.
Permitting Plus delivers brine management projects, project management, engineering, procurement, and construction through to commissioning, together with the permits they need, and the choice between a small permitted concentrate and full ZLD is the first decision on every one of them, for desalination concentrate and industrial wastewater alike. Our zero liquid discharge page covers the full-recovery end of that decision. This article covers the middle.
Table of Contents
What minimal liquid discharge means
The terms are worth separating, because vendors, desalination engineers, and regulators use them loosely, and because the technologies behind each are different.
| Term | Water recovered | What leaves the site as liquid | Main equipment |
|---|---|---|---|
| Conventional treatment and discharge | 0 to 75 percent | Treated effluent to a river, sewer, or the sea | Clarifiers, filters, reverse osmosis |
| Minimal liquid discharge (MLD) | 90 to 98 percent | A small, concentrated brine by an approved route | High-recovery reverse osmosis, brine concentrators, sometimes a small evaporator |
| Zero liquid discharge (ZLD) | Effectively 100 percent | Nothing. Only dry salt and residues leave | All of the above, plus evaporators and crystallizers |
The difference between MLD and ZLD is the thermal stage. Getting from 95 percent recovery to 100 percent means evaporating and crystallizing the last concentrate, and that step often costs as much in power and capital as everything before it combined. The 95 percent design keeps the membranes and drops most of the thermal equipment, which is why it is often the practical choice for industrial plants in water-stressed regions.
Where the small concentrate goes
It only works if there is a permitted route for the concentrate, the waste stream the membranes leave behind. That might be an existing outfall with headroom, a deep injection well, an evaporation pond in a dry climate, a sewer connection for a small enough volume, or a haul-away contract. The concentrate is small, a few percent of the original flow, which opens routes that the full flow could never use. The permit question is the same as for any brine, and we cover the options in our guide to brine disposal methods.
When MLD is the right answer
Our position is that it is the default for industrial water treatment and for desalination concentrate, and ZLD is the exception that has to be justified.
It is right when a small discharge is still permitted at the site, and water yield above 90 percent is realistic, when the wastewater is dominated by salts that membranes handle well, when electricity, the power for a thermal stage, is expensive or the site has no waste heat, and when the recovered water has a reuse inside the plant, as boiler or cooling tower make-up. A membrane treatment system built for elevated recovery efficiency can deliver 95 percent of the water at a fraction of the energy of ZLD.
It is wrong when the regulator mandates no liquid discharge by name, when there is no receiving water, well, pond, or sewer for even a small concentrate, or when the wastewater contains something, such as certain organics or metals, that no approved waste route will accept at any volume.
A worked example from a large brine project
On our project in Indonesia, a seawater desalination plant rejects tens of thousands of cubic meters of brine a day. The end design is full zero liquid discharge: the brine becomes salt and chlor-alkali chemicals, and nothing liquid returns to the sea.
But the decision was not automatic. Regulatory direction in the country, the marine setting of the outfall, the value of the recovered salt and chemicals, and the scale of the stream all pushed the answer to ZLD. Change any two of those, a smaller stream, a permitted outfall with headroom, no chemical buyers nearby, and the answer becomes MLD: recover the water, concentrate the brine as far as the membranes allow, and dispose of the remainder. Both cases were run before the route was chosen, and we do that on every project.
How MLD works
The plant is the front two-thirds of a ZLD plant: the same purification, the same membranes, and none or little of the thermal equipment. The treatment processes are shared; the process stops earlier, and the capital and running costs stop with it.
Purification first
Suspended solids, oil, hardness, and silica come out first, by clarification, chemical softening with lime or soda ash, and filtration. This purification stage is what lets the membranes run at high recovery without scaling. On our project in Indonesia the brine treatment stage removes calcium, magnesium, and sulfate with soda ash, caustic soda, and barium chloride, and an ion exchange polishing step follows. The same logic applies to industrial wastewater: get the scale-formers out, and the membranes can do far more of the work.
High recovery membranes
Standard reverse osmosis recovers 50 to 75 percent of a feed, and that recovery efficiency is the number the whole design turns on. Advanced designs push that to 90 percent and beyond by staging the membranes, softening between stages, and using brine concentrator membranes built for saline feeds. Electrodialysis reversal and ion exchange can push recovery further on some streams, and softening between stages is what keeps the water flowing when hardness would otherwise stop it. The power cost per cubic meter rises as recovery rises, but it stays well below evaporation until the brine is near saturation.
A small thermal stage, or none
Many of these systems stop at the membranes and send the concentrate to waste disposal. Some add a small evaporator to shrink the concentrate further before disposal, which cuts hauling or injection costs. Very few add a crystallizer; once you have a crystallizer, you have ZLD. Where a small evaporator is added, it is almost always a mechanical vapor recompression unit, and we explain what those cost to run in our article on mechanical vapor recompression.
What the recovered water is used for
Permeate from the membranes is clean enough for cooling tower make-up and, with polishing, for boiler feed. Plants that reuse the recovered water cut their intake as well as their waste discharge, and that double benefit is often what makes the business case. We go into the reuse side in our guide to industrial water reuse.
MLD technologies and solutions compared
The technologies used in minimal liquid discharge systems are the same ones used in the front end of ZLD systems. What changes is how hard each one is pushed.
| Technology | Role in an MLD system | Notes |
|---|---|---|
| Chemical softening and filtration | Purification ahead of the membranes | Sets the recovery ceiling |
| Ion exchange | Removes residual hardness between membrane stages | Small units, large effect on recovery |
| Reverse osmosis in stages | Bulk water recovery | 75 to 90 percent |
| Brine concentrator membranes | Push the concentrate further | To near saturation on some streams |
| Electrodialysis reversal | Concentrates salts electrically | Useful on silica-heavy water |
| Small evaporator | Optional, shrinks the concentrate before disposal | Only where hauling or injection is costly |
| Crystallizer | Not used | Adding one turns MLD into ZLD |
Vendors sell these technologies as packaged solutions, and the packaged solutions are often good. Comparing the technologies on paper is easy; comparing them on your water is the work. The decision that matters is made before the vendor is chosen: which solutions fit this wastewater at this site, and how far recovery should go. We run that comparison on every project we build, against operating reference plants rather than against a brochure. The right solutions for a textile mill and a power plant are rarely the same, even when the technologies overlap.
The recovered water loop
The best systems are designed as a loop, not a line. Water comes into the plant, is used, goes through treatment, and most of it comes back to be used again. The circuit closes when the recovered water replaces intake water in cooling towers, boilers, or process. The concentrate is the only thing that leaves the loop, and it leaves small.
Designing the loop means matching the quality of the recovered water to the use it will be put to, process by process. Cooling tower make-up tolerates some dissolved solids. Boiler feed does not, and needs polishing, usually ion exchange or a second reverse osmosis pass. Process water sits somewhere between, depending on the industry. Getting that match right is what turns treatment into a water supply.
MLD versus ZLD: the comparison owners should see
| Question | Minimal liquid discharge | Zero liquid discharge |
|---|---|---|
| Water recovery | 90 to 98 percent | Effectively 100 percent |
| Main energy user | Membrane pumps | Evaporators and crystallizers |
| Energy per cubic meter treated | Low to moderate | High |
| Capital cost | Moderate | High |
| Solid waste | Little or none; purification solids only | Salt cake, sold or landfilled |
| Liquid to dispose of | A small concentrate, by permit | None |
| Permit needed | Disposal permit for the concentrate, plus approval of the treatment facilities | Air, waste, and treatment permits, no liquid discharge permit |
| Upset case | Concentrate volume rises, disposal route absorbs it | Evaporator down means stored wastewater or a shutdown |
| Fit | Sites with any permitted concentrate route | Sites with none, or with a mandate |
Two rows in that table deserve attention. The upset case: an MLD plant has somewhere for the wastewater to go when equipment is down; a ZLD plant does not. And the solid waste: MLD makes almost no solid waste, while a ZLD plant has to sell or landfill its salt waste.
Energy is the deciding number
The energy difference between the two is not marginal, and energy efficiency is where the money is. Membranes move water with pressure and high efficiency. Evaporators move it with heat, and heating water to boil it, even with vapor recompression recovering most of the energy, takes many times the electricity per cubic meter.
On our project in Indonesia, electricity is the largest operating cost, and the concentration step is the largest energy item in the project. That is a valorization plant with products to sell against the energy bill. An industrial plant treating cooling tower blowdown has no products. For that plant, the difference between stopping at 95 percent and going to 100 percent may be the difference between a project that is approved and one that is not.
Where a site has waste heat, the calculation and the efficiency comparison shift. Recovered heat can run a small evaporator or a membrane distillation stage cheaply, and the case for going further toward zero improves. That is one of the first things we ask about.
Permitting a minimal discharge system
It is easier to permit than owners expect and harder than vendors suggest, because the environmental agency looks at the concentrate, not the recovery figure.
Easier, because the concentrate is small and the environmental review is scaled to it. A route that could never take the full wastewater flow can often take a few percent of it, and an existing permit may have the headroom.
Harder, because the concentrate is concentrated. Whatever was in the wastewater is now ten to fifty times stronger in the concentrate, and the permit for the disposal route will look at concentration, not only volume. Metals, organics, and salinity limits that were comfortable at full flow can be exceeded in the concentrate.
The permit file therefore has two parts: approval of the treatment facilities themselves, and a waste disposal permit for the concentrate at its new concentration. Where the facilities include a small evaporator, an air permit is added; where they include a crystallizer, the project has become ZLD and is permitted as one. We map both, with the agencies and the studies each needs, before the design is fixed. Where the concentrate goes to a pond, the pond is a facility with its own review; see our guide to evaporation ponds.
What an MLD treatment system includes
Owners sometimes hear “membranes” and picture a skid. A working treatment system of this kind at an industrial plant includes the intake and equalization tanks, the purification processes, the membrane trains with their cleaning systems, the ion exchange polishing for the recovered water, the concentrate storage and disposal connection, the controls and monitoring that prove recovery to the regulator, and the facilities to house all of it. On our project in Indonesia the equivalent front end includes primary and secondary purification, ion exchange, and the recirculation systems that return depleted brine to the process.
The water treatment technology, systems, and processes themselves are proven and modular. The engineering effort goes into the wastewater or desalination concentrate characterization, the recovery target, and the concentrate waste route. Get those three right and the equipment follows.
Mistakes we see
Jumping to ZLD. The proposal says zero liquid discharge because it sounds complete, and nobody priced the alternative.
Ignoring the concentrate’s chemistry. The disposal permit was assumed from the full-flow analysis, and the concentrate failed it.
Skimping on purification. The membranes were sized for 95 percent and scaled at 80 because hardness and silica were never removed and there was no ion exchange between stages.
Forgetting reuse. The recovered water went to drain instead of to the cooling towers, and the business case went with it.
Not running both cases. The owner never saw the two sets of costs side by side, and chose on a slogan. Both are legitimate solutions; only one fits a given site, and the site’s water decides which.
Ignoring the waste. Even a 95 percent design produces solids and sludge from purification and spent resin from polishing, and both need a waste route.
Frequently asked questions
What recovery does a minimal discharge design achieve?
Typically 90 to 98 percent of the wastewater volume, depending on the salinity and chemistry of the feed and how far the membranes can be pushed before scaling.
Is a small permitted concentrate accepted by regulators?
Where a small concentrate waste stream can still be disposed of by an approved route, yes. Regulators care that the concentrate goes to an approved route and meets that route’s limits. Where ZLD is mandated by name, a small discharge does not satisfy the mandate, and the environmental permit will say so.
Can an MLD system be upgraded to ZLD later?
Yes, and that is often the right plan. The purification and membrane technology are the same. Adding an evaporator and crystallizer later converts the system, and the waste changes from a liquid concentrate to dry solids, and designing the plot with that space reserved costs little.
What does the regulator want to see for MLD?
Three things. The wastewater analysis, the environmental setting, and the concentrate analysis, side by side, so the reviewer can see what the disposal route will receive. The route itself, with its permit and its limits. And the monitoring plan that proves the plant is doing what the file says: flow meters on the concentrate line, sampling at the disposal point, and records the agency can inspect.
How long does an MLD project take?
From first study to operation, usually one to two years for an industrial or desalination plant, driven more by the permit for the concentrate route than by the equipment. Where an existing permit has headroom, it can be faster. Where a new well, pond, or outfall is needed, the permitting for that route sets the schedule, and it should start before the equipment is ordered rather than after.
Which is cheaper, minimal or zero liquid discharge?
Minimal, in both capital and running cost and in energy efficiency, in almost every case. ZLD becomes cheaper only when the cost of disposing of the concentrate is very high, or when the recovered salt has real value.
In short
Minimal liquid discharge recovers most of the water with membrane systems and purification processes rather than evaporator technology, drops most of the thermal equipment, and satisfies the environmental regulator wherever a small waste concentrate can still be disposed of by an approved route. It should be priced against zero liquid discharge on every industrial and desalination project, running costs included, and it wins more often than not. The decision comes down to three things: whether any concentrate route is permitted, what energy costs at the site, and whether the wastewater chemistry lets the membranes run at high recovery.
If you are being pushed toward zero and want to know whether 95 percent would do, send us your wastewater analysis and your flows. Call (470) 714-8564 or request a consultation. Our zero liquid discharge page explains how we run the full comparison.
Talk to us about your wastewater
Send us the analysis, the flows, and a description of every stream that leaves the site, and we will tell you whether zero liquid discharge is required, what the regulator will ask, and how long it will take.