Reuse, Recovery & Stormwater
Minimum Liquid Discharge (MLD) Companies
MLD designers balancing high recovery with reduced CAPEX/OPEX vs. full ZLD, concentrate minimization at scale.
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Minimum Liquid Discharge Design: Concentration Factor Limits, Brine Management, and Economics
Minimum Liquid Discharge (MLD) maximises water recovery from a process or treatment stream while producing a manageable, reduced-volume liquid waste (brine, concentrate) rather than eliminating liquid discharge entirely (which is ZLD). MLD achieves recovery rates of 85 to 97 percent of feed water versus 50 to 80 percent for conventional single-pass RO, reducing concentrate volume by 3 to 6 times. Concentrate volume reduction enables smaller evaporation ponds, lower brine disposal costs, or improved economics for downstream ZLD evaporation if required. The key technological enablers are High Recovery RO (HRRO) and High Efficiency RO (HERO), which use scale inhibitors, pH adjustment (HERO: pH 10 to 11 to keep calcium carbonate supersaturation as soluble calcium carbonate complex), and fouling-resistant membranes to push recovery above 90 percent.
Brine management for MLD systems depends on the concentrate composition and volume. Options in increasing cost: direct discharge to sea or saline surface water (where permitted and within total dissolved solids and ionic strength limits); evaporation pond disposal (low operating cost but requires large land area and suitable climate); deep well injection (where permeable aquifer geology exists and UIC permits are obtainable in the US); and crystallisation or evaporation to produce dry salt for disposal or sale. MLD concentrate TDS is typically 50,000 to 150,000 mg per L; crystallisation to dry salt adds $3 to $15 per m3 of original feed water to the treatment cost, often prohibitive without valuable salt recovery (sodium sulphate, lithium, other commodities).
MLD economics depend on the value of recovered water versus the cost of concentrate disposal. In water-scarce regions (Middle East, India, US Southwest, Australia) where potable water costs $2 to $10 per m3, MLD economics are often favourable at 85 to 95 percent recovery. In industrial applications with trade effluent charges of $2 to $5 per m3, MLD dramatically reduces disposal costs. Capital cost premium for HRRO versus standard RO: 30 to 50 percent higher, reflecting additional pretreatment (softening, pH adjustment), additional membrane arrays, and scale inhibitor dosing systems. Operating cost addition: $0.10 to $0.40 per m3 of recovered water for scale inhibitors, pH chemicals, and additional energy. Payback period: 3 to 8 years in high-water-cost environments.
Frequently Asked Questions
What is the difference between MLD and ZLD?
Minimum Liquid Discharge (MLD) maximises water recovery while producing a small, manageable liquid brine stream - typically 3 to 15 percent of the original feed volume. The brine is then managed by evaporation pond, deep well injection, or sea discharge (where permitted). Zero Liquid Discharge (ZLD) eliminates all liquid waste, concentrating the brine to dry salt by evaporation and crystallisation. ZLD requires significantly higher capital ($2M to $50M additional per 1,000 m3 per day) and energy (50 to 80 kWh per m3 of brine evaporated by MVR evaporation) versus MLD. ZLD is appropriate where: brine discharge is prohibited (closed basin, inland location with no disposal option), water stress makes total recovery economically justified, or valuable minerals (lithium, sodium sulphate) in the brine offset evaporation cost. MLD is preferred when some liquid disposal route remains viable and the economics of total recovery are marginal.
What recovery rate is achievable with MLD systems?
Standard single-pass BWRO achieves 70 to 80 percent recovery from brackish groundwater. MLD-targeted HRRO (High Recovery RO) achieves 90 to 95 percent from the same feed by using: (1) Softening (lime or ion exchange) to remove calcium and magnesium before RO, allowing higher concentration factor without CaSO4 or CaCO3 scaling; (2) HERO (High-Efficiency RO) at pH 10 to 11: CO2 converted to CO3 which forms soluble CaCO3 complex, enabling recovery above 90 percent; (3) Chemical anti-scalant at high doses (10 to 50 mg per L); (4) Second-pass RO on first-pass concentrate (brine staging). For seawater, standard SWRO achieves 40 to 50 percent; SWRO with energy recovery and second-pass brine treatment achieves 55 to 65 percent. Maximum practical recovery is limited by osmotic pressure of the concentrate (at 95 percent recovery from 1,000 mg per L feed, concentrate TDS = 20,000 mg per L, osmotic pressure approximately 15 bar, within BWRO capacity).
When is MLD required by regulation?
MLD or ZLD requirements arise from: (1) Inland location with no access to surface water or sewer discharge (discharge to land prohibited in most jurisdictions for high-TDS concentrate); (2) Industrial permit conditions requiring zero process effluent discharge (common for mining, power plants, and chemical facilities in water-stressed US states like Texas, New Mexico, and California); (3) Total Maximum Daily Load (TMDL) limits on receiving water bodies that cannot accommodate additional TDS loading; (4) Groundwater protection zones where any discharge is prohibited; (5) India's ZLD mandate for industries in water-scarce areas and polluted river basins (Central Pollution Control Board rules requiring ZLD for textiles, sugar, distillery, pulp and paper, and tannery industries). EU Industrial Emissions Directive BAT Conclusions for several sectors specify minimum water discharge volumes, effectively requiring MLD in new plant designs.
How is MLD brine disposed of?
MLD brine disposal options by feasibility and cost: (1) Deep saline aquifer injection (Class I UIC permit in US, Environment Agency permit in UK): lowest operating cost ($0.50 to $2.00 per m3 of brine) if permeable geology exists; requires detailed hydrogeological study, well construction cost $500,000 to $3M, and long-term monitoring; (2) Evaporation pond: capital cost $500,000 to $5M per hectare (lined, instrumented), operating cost very low; requires arid climate (net evaporation above 1,500 mm per year), large land area (1 hectare per 50 to 200 m3 per day of brine depending on salinity and climate); (3) Co-disposal with municipal sewer brine: many utilities accept low-volume brines from small industrial users at additional trade effluent surcharges; only viable for concentrates within consent TDS limits; (4) Evaporation/crystallisation to dry salt: highest capital and energy cost, appropriate where salt has economic value or land/groundwater disposal is unavailable.
A semiconductor component manufacturer in Scotland required UPW but operated in an area where the local water company imposed a trade effluent consent restricting discharge TDS to below 1,000 mg per L. Conventional RO at 80 percent recovery produced a concentrate at 8,000 mg per L TDS that could not be discharged within consent. The alternative of full ZLD was estimated at 4.2 million GBP capital cost.
Designed an MLD system at 93 percent recovery: first-pass BWRO (80 percent recovery) followed by HERO second-pass RO (pH 10.5, lime softening to remove calcium before second pass, recovering further 65 percent of first-pass reject). The combined 93 percent recovery system produced 180 m3 per day of brine at 22,000 mg per L TDS, approximately 8 times lower volume than first-pass RO concentrate alone. The reduced brine volume was accommodated by deep well injection into a permitted saline aquifer 1.5 km from the site.
System capital cost was 1.8 million GBP versus 4.2 million GBP for ZLD, saving 2.4 million GBP. Annual freshwater savings of 2,200 m3 per day versus unrestricted abstraction alternative. Deep well injection permit obtained from the Environment Agency for 180 m3 per day brine, confirmed below the aquifer injection capacity. Trade effluent consent for residual process drainage (not RO concentrate) was maintained within the 1,000 mg per L TDS limit.
Questions to Ask Shortlisted Providers
- 1
What is the maximum recovery achievable for our specific feed water composition without anti-scalant or pH adjustment failures, and what evidence do you have from pilot testing at this recovery?
Maximum recovery is limited by the scaling potential of the concentrate stream at the target recovery. Calcium carbonate, calcium sulphate, barium sulphate, and silica each have specific saturation limits that constrain recovery. A claimed 95 percent recovery for high-sulphate brackish water may be achievable under ideal conditions but require precise pH control and anti-scalant management that is not demonstrated in a pilot. Ask for actual pilot plant data at the proposed recovery with your feedwater chemistry.
- 2
What is the brine TDS and ionic composition at the proposed recovery, and has a feasible disposal route been confirmed with the relevant regulatory authority?
Brine disposal is the critical path item for MLD projects. A techno-economically viable treatment system is valueless if no permitted disposal route exists. Confirmed disposal route options (deep well injection permit application, trade effluent consent for diluted brine to sewer, marine outfall consent) must be initiated in parallel with the treatment system design, not after it.
- 3
What is the energy consumption of the proposed MLD system at design recovery, and how does this compare with conventional single-pass RO plus trade effluent disposal?
MLD systems consume more energy per m3 of product water than single-pass RO (0.5 to 1.0 kWh per m3 additional for HERO second-pass, 0.5 to 2.0 kWh per m3 for MVR brine concentration stages). The total cost of ownership comparison must include the energy cost premium plus the trade effluent charge savings. In some cases, the energy premium makes MLD less economically attractive than paying trade effluent surcharges, particularly where water and effluent charges are low.
- 4
For HERO (High Efficiency RO) operation at pH 10 to 11, what are the chemical storage and handling requirements for lime or caustic, and have you confirmed COSHH and COMAH compliance for the proposed chemical storage volumes?
HERO requires pH adjustment to 10 to 11 before the second-pass RO. Using sodium hydroxide or lime at the required doses (typically 150 to 400 mg per L as NaOH) and at the required flow rate (typically 20 to 50 m3 per hr) creates significant caustic storage and handling requirements. Bulk NaOH storage above COMAH lower-tier threshold (50 tonnes) requires COMAH safety report. Lime slurry handling has different COSHH requirements from NaOH solution.
- 5
What is the projected scaling and fouling rate on the second-pass HERO membranes, and what is the design CIP frequency and protocol for this stage?
HERO second-pass membranes operate under conditions that conventional RO membranes were not originally designed for: high pH, high TDS concentrate, and elevated temperature from recirculation. Fouling rates and CIP requirements for HERO membranes differ from standard RO. Ask for reference plant data on CIP frequency and membrane service life achieved under HERO conditions specifically (not from standard RO installations operating at neutral pH).
What Drives Cost in This Category
Standard single-pass BWRO at 80 percent recovery: 150,000 to 500,000 GBP for 100 m3 per hr. Adding a second-pass HERO stage (softening, pH adjustment, additional membrane array): 200,000 to 600,000 GBP additional capital for recovery increase from 80 to 93 percent. MVR brine concentration from 93 to 97 percent recovery adds 500,000 to 2,000,000 GBP. Each increment of recovery above 90 percent costs progressively more per additional percent, as the concentrate TDS and osmotic pressure increase exponentially.
HERO second-pass requires lime softening (to remove calcium before high-pH operation) and caustic/lime for pH adjustment to 10 to 11. For a 50 m3 per hr second-pass HERO: lime 50 to 100 kg per hr (6,000 to 12,000 GBP per month at 100 to 150 GBP per tonne of Ca(OH)2) plus NaOH for pH adjustment 20 to 50 kg per hr (3,000 to 7,000 GBP per month). Annual chemical cost premium for HERO versus single-pass RO: 50,000 to 200,000 GBP depending on flow and pH requirements.
Deep well injection well construction: 500,000 to 3,000,000 GBP per well including hydrogeological study, drilling, well completion, and pump equipment. Operating cost: 50,000 to 150,000 GBP per year for pumping, monitoring, and regulatory compliance. Evaporation pond construction: 200,000 to 2,000,000 GBP per 1 hectare (depends on liner specification and instrumentation). Brine disposal infrastructure is often the dominant capital cost item in MLD projects, exceeding the treatment system capital at sites without a readily accessible disposal route.
For a site currently discharging 500 m3 per day of process effluent at 2,000 mg per L TDS, Mogden formula surcharge for TDS above consent may amount to 80,000 to 200,000 GBP per year. MLD at 93 percent recovery reduces discharge volume from 500 to 35 m3 per day (the residual brine), reducing trade effluent charges by approximately 90 percent. At 150,000 GBP per year savings, the MLD system capital of 1.5 to 3 million GBP has a 10 to 20 year payback, which may be borderline depending on other operating cost factors.
Key Regulations & Standards
Injection of MLD brine concentrate into deep geological formations (saline aquifers, depleted reservoirs) requires an Environmental Permit from the Environment Agency under EPR 2016 (mining waste and groundwater activities regime). The permit application must include a site-specific hydrogeological assessment demonstrating containment, non-interference with usable aquifers, and monitoring programme. EA groundwater protection zones (SPZ1, SPZ2, SPZ3) restrict injection in some areas. Permit determination timescale: typically 6 to 18 months.
Where MLD brine is diluted to within trade effluent consent TDS limits and discharged to sewer, trade effluent consent from the water company is required under WIA 1991 Section 118. Consent will specify maximum discharge TDS, flow rate, and specific ion limits (chloride, sulphate). Water companies may refuse trade effluent consent for brine if it would affect the sewerage system's capacity or the downstream wastewater treatment works' compliance with its own discharge permit.
IED BAT Conclusions for several sectors (textiles, chemicals, food processing) specify minimum discharge volumes and water use efficiency requirements. MLD is increasingly referenced in BAT guidance as the appropriate approach for sites in water-stressed areas where discharge prohibition effectively requires near-zero effluent. UK sites subject to IED must demonstrate compliance with applicable BAT Conclusions in their Environmental Permit applications, which for new applications in designated water-stressed areas may require MLD-level recovery.
EA Groundwater Protection: Policy and Practice (GP3) restricts disposal of concentrated brine to surface or near-surface strata above principal or secondary aquifers. Any MLD brine evaporation pond, surface application, or shallow disposal must be assessed against GP3 to confirm no significant risk to underlying aquifer. Source Protection Zone proximity, brine ionic composition, and soil permeability are the primary assessment factors. GP3 effectively prohibits surface disposal of high-TDS brine in most of England's significant groundwater areas.
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