ENV · Minimum liquid discharge
Minimum liquid discharge and water recovery
Around 85% recovery from a wastewater stream using membranes alone — a cost-effective alternative to thermal evaporation for sites that can still discharge a small, concentrated volume.
The starting point
What problem does it solve?
- “Our fresh water bill keeps rising”
- “We can't get an increase in our abstraction licence”
- “Effluent disposal costs more every year”
- “We need to expand production but water caps us”
- “ZLD was quoted and it is out of budget”
Your solution can be MLD.
Minimum liquid discharge is a unit used in treatment plants to limit the amount of effluent discharged to the environment. It uses advanced filtration to recover the maximum amount of liquid from wastewater, and it achieves around 85% water recovery from wastewater streams.
Compared with ZLD, MLD is the more common choice because it costs less while still meeting many environmental laws and restrictions. It is the right answer for the large majority of sites that can legally discharge a small, concentrated volume and simply need to stop discharging a large, dilute one.
The technologies are conventional reverse osmosis, closed circuit reverse osmosis (CCRO) and high-efficiency reverse osmosis (HERO). Which one applies depends on the scaling and fouling behaviour of your specific stream, not on which is newest.
Why it pays
Benefits
Around 85% recovery
Around 85% water recovery from wastewater streams, which on most sites is enough to release the abstraction constraint that was blocking expansion.
A cost-effective alternative to evaporation
MLD avoids the capital and energy cost of an evaporator train. Where a small concentrated discharge is permitted, that difference is usually decisive.
Three membrane routes, not one
Conventional RO where the stream behaves; closed circuit RO where recovery has to be pushed without proportionally increasing scaling risk; HERO where silica and organic fouling are what actually limit you.
Compliance with room to move
A properly designed MLD system meets many environmental laws and restrictions while leaving headroom for the consent to tighten further before ZLD becomes necessary.
A stepping stone to ZLD
An MLD line built with the right pre-treatment becomes the front end of a ZLD plant later. Designing it that way costs little now and saves a rebuild if regulation moves.
Recovered water fit for real duties
Permeate quality is normally suitable for cooling tower make-up and process water; where boiler feed or ultrapure quality is needed, ion exchange and EDI polish it.
Process
How it works
MLD is a recovery strategy built from membrane stages, and the engineering work sits in two places: protecting the membranes and dealing with the concentrate. Both are decided by the feed analysis rather than by the target recovery figure.
We design the line so that each stage removes what would otherwise limit the next one, and we are explicit about the concentrate route from the first meeting — because a recovery figure with no plan for the reject is not a design.
Process diagram
Pre-treatment → primary RO → high-recovery stage → concentrate management
Reserved for a static MLD process diagram showing pre-treatment, primary RO, CCRO or HERO high-recovery stage, permeate to reuse, and concentrate routing with the optional link to evaporation.
Static process diagram — final artwork to be supplied.
Characterise and pre-treat
Suspended solids, organics, hardness, silica and the specific scaling species present decide the pre-treatment. Options range from dissolved air flotation and media filtration through ultrafiltration and ceramic or silicon carbide membranes to adsorbent systems where colour, metals or recalcitrant COD are present.
Primary reverse osmosis
Conventional RO recovers the bulk of the water at low energy cost. Its recovery limit is set by the first species to reach saturation — usually silica, calcium sulphate or calcium carbonate.
High-recovery stage
Closed circuit RO recirculates concentrate within a closed loop, raising recovery without the long residence time at high concentration that causes scaling in a conventional array. High-efficiency RO operates at elevated pH, which keeps silica soluble and biological fouling suppressed. Selection depends entirely on which species limits you.
Permeate polishing and reuse
Permeate goes to cooling tower make-up, process water or boiler feed. Where boiler feed or ultrapure quality is required, ion exchange, mixed bed or electrodeionization follow.
Concentrate management
The remaining 15% or so is a small, concentrated stream. Where discharge is permitted it goes to consent. Where valuable salts are present or discharge closes entirely, it feeds an evaporation and crystallisation stage and the plant becomes ZLD.
Applications
Where it is used
Any site where fresh water is capped, effluent disposal is expensive, or production growth is blocked by the water balance.
Industries
- Power generation — cooling tower blowdown recovery
- Oil, gas and petrochemical — refinery cooling water and effluent reuse
- Chemical — high-TDS process and neutralisation streams
- Textile — dye-house and washing water reuse
- Pulp and paper — process water recycling and white water recovery
- Food and beverage — recovery from cleaning and blowdown streams
- Pharmaceutical — reuse in non-critical duties
- Mining and steel — process water recycling and tailings water recovery
- Fertilizer — plant-wide water cycle recovery
- Municipal — treated effluent reuse for irrigation and industrial supply
- Agriculture — reuse of treated effluent for irrigation
Typical applications
- Cooling tower blowdown recovery
- RO reject recovery ahead of a smaller ZLD train
- Brackish water treatment for inland supply
- Rinse and wash water recycling
- Water footprint reduction programmes and ESG targets
- Debottlenecking a site whose abstraction licence is capped
- Reducing trade effluent charges on volume
Equipment
Related products
Ion exchange resins
Over 200 types — cation, anion, mixed bed, chelating, adsorbent and chromatography media, with resin screening and process design support.
Read moreAnalyzer & metering systems
TOC, oil-in-water, pH, conductivity, dissolved oxygen and CEMS packages with sampling systems and shelters.
Read moreLiquid ring vacuum pumps
Vacuum and compression duties across wastewater, chemical, paper and power applications, including explosion-protected models.
Read moreSteam turbines
50 kW to 180 MW plus energy recovery sets that convert surplus steam pressure into power.
Read moreEngineering services
Engineering support for this technology
Selecting a process is an engineering decision, not a purchasing one. We run feasibility and pre-feasibility studies, characterise the stream, compare the routes on capital and operating cost, and act as owner's engineer while somebody else builds it if that suits you better.
Evidence
Projects
Sample layout. Reference projects are being cleared for publication with the clients concerned. The structure below is final; the content is placeholder.
SampleWater reuse retrofit — sample card
Card layout for a completed water reuse project: stream data before and after, recovery achieved, scope Glorinda held, and the client's own words.
SampleZLD package — sample card
Card layout for a zero liquid discharge package: feed characterisation, evaporator type selected, solid output route, and commissioning duration.
SampleOwner's engineer mandate — sample card
Card layout for an advisory mandate: the client's decision, the options assessed, and what changed in the tender as a result.
FAQ
Questions engineers ask
Taken from the questions procurement teams, plant managers and consultants actually send us.
What recovery can I actually expect?
Around 85% from wastewater streams is the figure we design to. The achievable number for your stream depends on which species reaches saturation first — usually silica, calcium sulphate or calcium carbonate — and on what pre-treatment removes before the membranes see it.
How is MLD different from ZLD?
MLD recovers around 85% using membranes and discharges a small concentrated stream. ZLD recovers close to 100% by adding evaporation and crystallisation and discharges nothing liquid at all. MLD is considered more common because it is less expensive while still meeting many environmental laws and restrictions.
What is the difference between CCRO and HERO?
Closed circuit RO recirculates concentrate in a closed loop, so the membranes spend less time exposed to peak concentration and recovery can be pushed without proportionally increasing scaling risk. HERO operates at elevated pH, which keeps silica soluble and suppresses biological fouling. If silica limits you, HERO is usually the answer; if scaling kinetics limit you, CCRO usually is.
Can I upgrade an MLD plant to ZLD later?
Yes, and it is worth designing for. The concentrate from an MLD line is exactly the feed an evaporator wants. Sizing the pre-treatment and the concentrate handling with that in mind costs little now and avoids a rebuild if your consent tightens.
How do I justify the investment?
Three lines usually move: fresh water purchase, effluent disposal charges, and — where abstraction is capped — the production you cannot otherwise add. On constrained sites the third is normally much larger than the other two combined.
Conversion
Take this further
Send us the stream data, the discharge limit or the equipment list you are working from. An engineer reads every enquiry and replies with the questions that actually decide the solution — not a brochure.