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TDS Reduction Companies
TDS reduction via RO, NF, ED/EDR, capacitive deionization, and thermal processes for brackish and process water.
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TDS Reduction Technologies: Reverse Osmosis, Ion Exchange, and Nanofiltration for High-Salinity Water
Total dissolved solids (TDS) is the measure of all dissolved inorganic and organic constituents in water, expressed as mg/L (equivalent to ppm by mass). TDS comprises primarily ionic species: sodium, chloride, calcium, magnesium, sulphate, bicarbonate, potassium, and nitrate, plus silica, metals, and organics. WHO drinking water guideline: less than 600 mg/L TDS for palatability (greater than 1,000 mg/L noticeable salty taste; greater than 2,000 mg/L laxative effect). UK: no mandatory TDS standard in Water Supply (Water Quality) Regulations; conductivity less than 2,500 uS/cm (approximately 1,600 to 1,800 mg/L TDS equivalent) is the UK regulatory indicator. US EPA secondary standard: 500 mg/L TDS for aesthetics. High TDS sources: brackish groundwater (1,000 to 15,000 mg/L, common in arid regions, coastal areas, and some UK chalk aquifers); seawater (30,000 to 45,000 mg/L); agricultural return flow; industrial process water; brine intrusion into municipal water supply. TDS reduction is required for: desalination; industrial boiler makeup (scale prevention at TDS greater than 100 mg/L in high-pressure boilers); semiconductor and pharmaceutical UPW (ultra-pure water, TDS less than 0.01 mg/L); food and beverage product consistency.
Reverse osmosis (RO) is the most widely deployed TDS reduction technology. RO mechanism: hydraulic pressure (7 to 80 bar, depending on feed salinity) forces water through a semi-permeable membrane (polyamide thin-film composite, MWCO less than 100 Da); dissolved ions are rejected (Na+, Cl-, Ca2+, Mg2+, SO42-, NO3- rejection 95 to 99.8 percent); permeate (product water) has TDS typically 2 to 10 percent of feed. System recovery: brackish water RO (BWRO, TDS 1,000 to 10,000 mg/L, operating pressure 5 to 20 bar): 70 to 85 percent recovery typical; seawater RO (SWRO, TDS 30,000 to 45,000 mg/L, 50 to 80 bar): 40 to 50 percent recovery. Specific energy: BWRO 0.5 to 1.5 kWh/m3 product; SWRO with ERD 2.2 to 3.5 kWh/m3. Pre-treatment requirements: SDI15 less than 3.0 (ideally less than 1.5) at RO inlet; turbidity less than 0.1 NTU; free chlorine less than 0.1 mg/L (TFC membrane damage at greater than 200 to 1,000 ppm-hours); antiscalant dosing; pH adjustment. Post-treatment: remineralisation (lime or calcite contactors for corrosion control, Ca2+ and alkalinity addition); pH adjustment; disinfection.
Ion exchange demineralisation achieves near-zero TDS in applications requiring ultra-pure water. Two-bed demineralisation: strong acid cation (SAC, H+ form) removes cations (Na+, Ca2+, Mg2+, K+, replacing with H+); strong base anion (SBA, OH- form) removes anions (Cl-, SO42-, HCO3-, NO3-, SiO2, replacing with OH-); H+ + OH- yields H2O; effluent TDS less than 1 mg/L; conductivity less than 0.5 uS/cm. Mixed bed deionisation (cation + anion resin mixed): polishing after 2-bed; achieves less than 0.01 uS/cm (TDS effectively zero); used for UPW in electronics and pharmaceuticals. Regeneration: SAC resin with HCl (5 to 10 percent) or H2SO4; SBA resin with NaOH (4 to 8 percent); generates acidic and caustic waste streams requiring neutralisation before disposal. Electrodeionisation (EDI): membrane-based continuous deionisation combining ion exchange resin with selective membranes and electrical current for continuous regeneration without chemical addition; achieves 0.06 to 0.1 uS/cm conductivity; used for pharmaceutical and electronics UPW. Nanofiltration (NF, MWCO 200 to 1,000 Da): selective rejection of divalent ions (Ca2+, Mg2+, SO42-, 80 to 99 percent) while passing monovalent ions (Na+, Cl-, 30 to 70 percent); reduces TDS 40 to 70 percent from hard water supplies; used for softening and TDS reduction in European drinking water.
Frequently Asked Questions
What TDS level is acceptable in drinking water?
TDS acceptability thresholds: WHO Guideline Values (2022): less than 300 mg/L - excellent; 300 to 600 mg/L - good; 600 to 900 mg/L - fair; 900 to 1,200 mg/L - poor; greater than 1,200 mg/L - unacceptable. UK mandatory standard: no TDS MCL; conductivity less than 2,500 uS/cm (approximately 1,600 to 1,800 mg/L TDS) is the regulatory parameter per Water Supply (Water Quality) Regulations 2016. US EPA secondary maximum contaminant level (SMCL): 500 mg/L TDS (aesthetic, non-enforceable). Australian Drinking Water Guidelines: 500 mg/L as upper aesthetic value; greater than 1,000 mg/L laxative; greater than 2,000 mg/L health concern for sodium-sensitive individuals. Taste perception: TDS becomes noticeable to most consumers above 600 to 800 mg/L; specific ions (sulphate bitter taste, chloride salty taste, sodium salty) are more taste-relevant than total TDS. Most UK public water supplies are well within WHO guidelines (Thames Valley approximately 280 to 400 mg/L; Scottish Highlands approximately 30 to 80 mg/L; East Anglia chalk 350 to 600 mg/L). High TDS health impact: WHO notes very high TDS (greater than 2,000 mg/L) causes osmotic laxative effects; for most mineral species at concentrations in the TDS range, individual ion health standards are more limiting than TDS itself.
Can RO remove all dissolved salts?
RO membranes reject most dissolved salts but with varying efficiency depending on ion charge and size: High rejection (greater than 98 percent): divalent cations (Ca2+, Mg2+, Ba2+, Sr2+); divalent anions (SO42-, CO32-, PO43-); large organic molecules (greater than 200 Da molecular weight). Moderate to high rejection (95 to 98 percent): monovalent sodium (Na+), chloride (Cl-), potassium (K+), nitrate (NO3-), fluoride (F-); typical thin-film composite RO at standard operating conditions. Partial rejection (60 to 95 percent): very small neutral molecules (carbon dioxide CO2 passes freely through RO - must be removed separately by degassing); boron (H3BO3, neutral at pH less than 9.2, 40 to 75 percent rejection by standard SWRO membranes - requires high-pH or 2-pass RO for boron less than 0.5 mg/L); nitrate (95 to 98 percent rejection, some RO blends nitrate-high permeate to meet 50 mg/L limit); hydrogen sulphide (gas, passes freely). Not removed: dissolved gases (CO2, O2, H2S) pass through RO membranes; these require degassing towers or chemical treatment. Overall product TDS: BWRO typically achieves permeate TDS 10 to 100 mg/L from feed 1,000 to 3,000 mg/L (95 to 98 percent rejection); SWRO achieves 100 to 400 mg/L from seawater 35,000 mg/L.
What is the difference between nanofiltration and reverse osmosis for TDS reduction?
Nanofiltration (NF) and reverse osmosis (RO) are both pressure-driven membrane processes for TDS reduction but differ in selectivity and rejection characteristics: RO (TFC polyamide, MWCO less than 100 Da): rejects monovalent and divalent ions at greater than 95 to 99 percent; product TDS 5 to 10 percent of feed; operating pressure 5 to 80 bar; used for full desalination; produces very low TDS water (remineralisation often needed post-RO). NF (polyamide or polysulphone, MWCO 200 to 1,000 Da): rejects divalent ions (Ca2+, Mg2+, SO42-) at 80 to 99 percent but allows partial passage of monovalent ions (Na+, Cl-, K+ pass at 30 to 70 percent); operating pressure 3 to 15 bar; reduces hardness substantially while retaining some minerals; product TDS 40 to 70 percent below feed. Applications - NF: drinking water softening (removes hardness while maintaining mineral balance); colour and NOM removal from surface water; pharmaceutical intermediate water quality; selective salt removal where monovalent ion passage is acceptable. Applications - RO: full desalination of seawater or brackish water; UPW production; boiler makeup; when target TDS is less than 100 mg/L. Energy: NF typically 0.2 to 0.6 kWh/m3 (lower pressure vs RO 0.5 to 3.5 kWh/m3 for BWRO); NF is preferred when only partial TDS or hardness reduction is required to meet regulatory or process targets.
Is high TDS water safe to drink?
Health safety of high TDS water depends on which specific ions contribute to TDS: Sodium (Na+): no mandatory UK limit; WHO health-based value 200 mg/L as Na; DWI indicator parameter 200 mg/L Na+; at very high Na+ (greater than 500 mg/L), cardiovascular and blood pressure risks for hypertensive individuals; infant formula should not use water greater than 200 mg/L Na; Chloride (Cl-): WHO guideline 250 mg/L (taste); no health limit established at typical TDS concentrations; Sulphate (SO42-): WHO health-based guideline 500 mg/L; laxative effect at greater than 600 to 1,000 mg/L; taste becomes noticeable above 250 mg/L; Nitrate (NO3-): UK mandatory 50 mg/L (health-based, methaemoglobinaemia risk for infants); Fluoride (F-): UK 1.5 mg/L (mandatory, dental fluorosis above 1.5 mg/L, skeletal fluorosis risk at greater than 4 mg/L in lifetime exposure); Arsenic (As): UK mandatory 10 ug/L (carcinogen); naturally occurring in some UK aquifers at elevated levels contributing to TDS. A water with TDS of 2,000 mg/L predominantly from NaCl and CaCO3 is likely safe; the same TDS from arsenic, fluoride, and nitrate would be unsafe. WHO guidelines and UK regulations for individual parameters are health-protective even when TDS itself does not have a mandatory limit.
A ceramics manufacturer in the West Midlands operating three high-pressure steam boilers (90 bar, 500 degrees C) was experiencing accelerated tube corrosion and silica deposits on turbine blades. Borehole feedwater TDS was 1,450 mg/L (predominantly sodium sulphate and calcium bicarbonate) with silica at 28 mg/L. The boiler makeup quality target was TDS less than 5 mg/L and SiO2 less than 0.02 mg/L.
A two-pass BWRO system (Toray TM720 elements, 8-inch diameter, 80 percent recovery first pass, 85 percent recovery second pass) followed by a mixed-bed ion exchange polisher (Purolite C100 cation / Purolite A400 anion, hydrogen/hydroxide form) was installed. Antiscalant (Veolia Hydrex 4458) dosed at 4 mg/L before first-pass RO controlled calcium carbonate and silica scaling. Post-RO conductivity was 12 to 18 uS/cm; post-mixed-bed was consistently below 0.1 uS/cm. Silica post-mixed-bed was below 0.01 mg/L.
Boiler tube corrosion incidents fell from four per year to zero over the 30-month monitoring period. Silica deposition on turbine blades eliminated; scheduled turbine cleaning interval extended from 9 to 36 months, saving GBP 68,000 per cleaning event. Energy efficiency of the boiler system improved by an estimated 4 percent. Total system capital cost GBP 340,000; payback 2.4 years.
Questions to Ask Shortlisted Providers
- 1
What is the guaranteed rejection rate for the specific ions dominating our feed water TDS, tested at our design temperature and pressure?
RO rejection rates quoted at standard test conditions (25 degrees C, 15 psi, NaCl solution) differ materially from site-specific conditions; guaranteed rejection at actual feed conditions must be contractually specified.
- 2
What is the target permeate TDS and the recovery rate, and how much concentrate will need to be disposed of per m3 of product?
At 80 percent recovery, one m3 of product generates 0.25 m3 of concentrated brine; brine disposal cost (sewer consent, trade effluent charge, or evaporation) can equal or exceed membrane operating cost at some sites.
- 3
What antiscalant is recommended for our specific feed water chemistry, and what happens if antiscalant dosing fails?
Antiscalant failure causes irreversible membrane scaling within hours at high-TDS or high-silica feeds; the system must have antiscalant flow monitoring with automatic membrane bypass to prevent permanent membrane damage.
- 4
How does the system manage membrane cleaning, and what is the expected time and cost per clean-in-place cycle?
CIP frequency varies from monthly to annually depending on feed quality; CIP chemical cost (acid, caustic, biocide) per clean is GBP 500 to 3,000 for a medium system; this is a significant operating cost that must be budgeted.
- 5
What is the total specific energy consumption at design recovery and flux, and how does this change if feed TDS or temperature varies seasonally?
RO energy increases with TDS and decreases with temperature; sites with variable TDS (seasonal aquifer variation) or cold winters (ground water below 10 degrees C) see energy consumption 20 to 40 percent above the standard test condition figure.
What Drives Cost in This Category
Single-pass BWRO at 70 percent recovery from 2,000 mg/L TDS costs GBP 0.20 to 0.40 per m3 to operate; two-pass RO for TDS less than 10 mg/L from the same feed costs GBP 0.40 to 0.80 per m3; the additional pass adds 30 to 50 percent to membrane capital cost.
Discharge to sewer with trade effluent consent: GBP 1.50 to 3.50 per m3 concentrate (Mogden formula at elevated TDS); evaporation pond or ZLD crystalliser: GBP 5 to 20 per m3 concentrate; for 80 percent recovery from 200 m3/day feed, 40 m3/day concentrate cost can equal 50 to 80 percent of total RO operating cost.
RO membranes (8-inch spiral wound, Toray, DuPont, LG Chem) cost GBP 250 to 500 per element; a 200 m3/day system uses 6 to 12 elements; replacement at 5 to 7 years adds GBP 5,000 to 12,000 amortised capital cost per year.
SDI control below 3.0 requires multimedia filtration plus 5 micron cartridge filtration before RO; for high-turbidity feeds, UF pretreatment (SDI less than 1.0) adds GBP 50,000 to 200,000 capital but extends membrane life by 30 to 60 percent.
Key Regulations & Standards
Water Supply (Water Quality) Regulations 2016 (England): conductivity at the consumer tap must not exceed 2,500 uS/cm (approximately 1,600 to 1,800 mg/L TDS equivalent); post-RO remineralisation must restore sufficient alkalinity and calcium to prevent corrosive, low-pH, low-TDS water from attacking distribution pipework.
Water Industry Act 1991, Section 118: RO concentrate discharged to a public sewer requires trade effluent consent; elevated TDS, chloride, and sulphate in concentrate affect Mogden charge calculation; exceeding consent TDS or flow limits requires renegotiation with the sewerage undertaker.
Pressure Systems Safety Regulations 2000: RO membrane pressure vessels operating above 0.5 bar must be included in the written scheme of examination; annual inspection by a competent person; operating log maintained; pressure relief devices tested.
Where RO is used for drinking water production, all membrane elements, vessels, antiscalants, and dosing chemicals must hold DWI Regulation 31 approval; post-RO remineralisation chemicals (lime, calcite, carbon dioxide) must also be approved; RO product water must meet all WS(WQ)R 2016 parameters.