What is a good TDS reading for Reverse Osmosis water?

Water TreatmentMembrane Filtration Science

Reverse Osmosis (RO) System Efficiency Calculator

Calculate Reverse Osmosis (RO) membrane contaminant rejection efficiency, recovery percentage, and permeate product-to-waste ratios.

Water Treatment Field StandardsMembrane Filtration Science

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

Reverse Osmosis membrane rejection percentage is calculated as: [(Feed TDS - Permeate TDS) / Feed TDS] × 100. If tap water TDS is 350 PPM and RO product water is 18 PPM, the rejection rate is 94.9%. Modern residential RO systems typically produce 3 to 4 gallons of concentrate waste water for every 1 gallon of pure drinking water.

RO membranes reject 93% to 98% of total dissolved solids (TDS).
Membranes should be replaced when rejection efficiency drops below 85-90%.
Adding an ERP-500 permeate pump reduces drain wastewater by up to 80%.

Interactive Calculator

Adjust inputs for real-time results

Feed Water TDS (Tap / Well Water)PPM

PPM

Permeate Water TDS (Pure RO Water)PPM

PPM

Daily Drinking Water Usedgallons/day

gallons/day

System Drain Waste Ratio

Calculated OutputExcellent Filtration (>90%)

Membrane Rejection Rate (Salt Rejection)

94.9%% TDS rejected

Purified Water (Permeate): 5 Gal/Day
Drain Waste Water Generated: 15 Gal/Day (450 gal/mo)
System Recovery Rate: 25.0%
Salt Passage Rate: 5.1%

Note: A healthy TFC (thin-film composite) RO membrane rejects 93% to 98% of dissolved minerals and contaminants.

Optimizing RO Water Efficiency

Low water pressure (<50 PSI) causes high wastewater ratios; install a booster pump to maximize membrane rejection.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

A healthy RO system produces water with a TDS of 10 to 30 PPM, representing a 90% to 98% reduction from municipal tap water.