An RO system recovery is the volume split between permeate and reject, while rejection is the membrane salt-removal figure, and this guide keeps the two apart: at 95% rejection a 200 ppm feed leaves a 10 ppm permeate, and the waste ratio you actually set on the controller decides how concentrated the brine gets. The feed-water math, permeate = feed TDS x (1 – rejection/100), is walked at 50%, 75% and 90% recovery so the reject concentration line is visible before you touch the system.
Key takeaways
- The Water Quality Analyzer is the feed-water screener: how to test water quality for hydroponics by reading pH off the 6.0 optimum, EC as the absolute salt reading, and hardness through the water hardness converter ppm CaCO3 bands.
- Penalties stack into a 0–100 score, the RO system calculator deri… How Water Quality Analyzer — Hardness, RO & Treatment Blueprint Helps You Master This Pick the water source type and read the feed: pH, EC, hardness in ppm CaCO3, temperature.
- Score each arm off the 6.0 pH optimum — every pH unit off costs ten, EC ten per mS, hardness one per 10 ppm, chlorine ten per ppm, metals and SAR a tail — and clamp the water quality score 0–100.
- Price the treatment: $/1000 L from the treatment base plus hardness, metals and chlorine removals, and size the pH buffer dose at |pH−6| × 1000 × 0.1 mL.
How Water Quality Analyzer — Hardness, RO & Treatment Blueprint Helps You Master This
- Pick the water source type and read the feed: pH, EC, hardness in ppm CaCO3, temperature.
- Score each arm off the 6.0 pH optimum — every pH unit off costs ten, EC ten per mS, hardness one per 10 ppm, chlorine ten per ppm, metals and SAR a tail — and clamp the water quality score 0–100.
- Classify the water: hardness bands soft/moderate/hard/very hard, TDS class off the meter scale, SAR class off the sodium hazard.
- Run the RO system arm: permeate = feed TDS × (1 − rejection/100), read against the 95% membrane guideline.
- Price the treatment: $/1000 L from the treatment base plus hardness, metals and chlorine removals, and size the pH buffer dose at |pH−6| × 1000 × 0.1 mL.
- Read the band — green above 80, yellow 60–80, red under 60 or metals over 50 ppb or effective chlorine over 2 ppm — and honor the >200 ppm alkalinity "RO Recommended" flag.
The Formula Behind the Result
Water quality score — stacked penalties off the 6.0 optimum → Q = 100 - left(10,|pH - 6.0| + 10,EC + 0.1,H + 10,Cl_{eff} + 0.001,M + 5,SARright), quad 0 le Q le 100Effective chlorine — residual after the treatment in line → Cl_{eff} = Cl cdot f(t), qquad f(text{carbon}) = 0.05, f(text{RO}) = 0.02, f(text{UV}) = 0.8, f(text{none}/text{softener}) = 1.0Hardness classification — ppm CaCO3 bands → text{Soft} le 60 < text{Moderate} le 120 < text{Hard} le 180 < text{Very hard}, quad 1,text{gpg} approx 17.1,text{ppm CaCO}_3RO permeate — membrane rejection on the feed TDS → P_{permeate} = TDS cdot left(1 - frac{r}{100}right), qquad r = 95%: 200 rightarrow 10,text{ppm}Treatment cost — base plus what the water forces you to remove → C = C_{base} + 0.01,H + 0.001,M + 0.1,Cl quad ($ / 1000,text{L})pH buffer — acid-equivalent dose off the optimum → B = |pH - 6.0| cdot 1000 cdot 0.1 quad (text{mL} text{per} 1000,text{L})TDS from EC — scale must be stated, EC is the absolute → TDS_{500} = EC times 500,qquad TDS_{700} = EC times 700 quad (text{meter-scale dependent})Safety bands — the bound the whole screener lives in → text{Green: } Q > 80,quad text{Yellow: } 60 le Q le 80,quad text{Red: } Q < 60 text{or} M > 50 text{or} Cl_{eff} > 2
Target Ranges: What the Numbers Should Be
- pH 6.0 is the nutrient-availability optimum: the archived pH-adjuster drove from the same 5.5–6.5 band, and pH is logarithmic so 5.0 is ten-times more acidic than 6.0.
- Hardness in ppm CaCO3 doubles as carbonate alkalinity that fights your pH; 1 gpg ≈ 17.1 ppm and 1 dH ≈ 17.9 ppm for the water hardness converter.
- RO at the archived 95% rejection turns a 200 ppm feed into a 10 ppm permeate; a 4-stage membrane strips salts and metals together, and the archived pH-adjuster called for RO once alkalinity passed 200 ppm.
- Treatment cost scales with what the water forces you to remove: +$0.01 per hardness ppm, +$0.001 per lead/copper ppb, +$0.10 per chlorine ppm on the $/1000 L reference.
- Carbon and RO leave only a 2–5% chlorine residual while UV alone leaves 80% — effective chlorine is what actually reaches the roots, and >2 ppm reads red.
Use the free Light Coverage Architect — Bar vs QB, Inter-Canopy & UV Blueprint to run the numbers on your own setup.
Use the free Climate Controller & HVAC Sizer — VPD, Dehumidifier & CO2 Blueprint to run the numbers on your own setup.
Use the free Flavor Brix Terpene Optimizer Chef-Grade Flavor Lab to run the numbers on your own setup.
Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.
Related guides
Recovery math: where the reject goes
Feed is 200 ppm TDS on a membrane rated at 95% rejection, so permeate is 200 x 0.05 = 10 ppm. Recovery is the permeate share of feed volume; every point higher concentrates the salt left in a smaller water volume.
- Permeate line: 200 ppm feed x (1 – 95/100) = 10 ppm at any recovery.
- At 50% recovery, 1 L of feed splits into 0.5 L of 10 ppm permeate and 0.5 L of reject carrying 195 mg of salt: 390 ppm.
- Push to 75% recovery and the reject volume shrinks to 0.25 L but still holds 192.5 mg: 770 ppm.
- At 90% recovery the reject is 0.1 L at about 1,910 ppm, where the scale and membrane-fouling risk starts to cost you in flush cycles.
- The 95% rejection rating is the salt ceiling; recovery is the dial you control, and the two never change together.
Frequently Asked Questions
References & Science
- NCSU CALS — Vapor Pressure Deficit resources
- Purdue Extension — hydroponics guide
- HortScience — hydroponic nutrient studies
Frequently asked questions
How do I calculate RO recovery rate correctly?
Recovery is permeate volume divided by feed volume. If the system feeds 1 L and produces 0.5 L of permeate, recovery is 50% and the rest is reject. Pair that with the membrane rejection figure: at 95% rejection a 200 ppm feed leaves a 10 ppm permeate. Recovery is the volume split, rejection is the salt split, and you need both before pricing waste water.
What is the most common recovery mistake?
Confusing recovery with rejection. Recovery is a volume ratio set by the controller, while rejection is the membrane salt-removal figure (here 95%, turning 200 ppm feed into 10 ppm permeate). Raising recovery to save water does not change rejection, but it concentrates the reject stream toward 770 ppm at 75% and about 1,910 ppm at 90%, so model the brine before dialing it up.
Which calculators handle RO recovery math?
The Water Quality Analyzer embedded on this page runs the RO arm, permeate = feed TDS x (1 – rejection/100), and reads it against the 95% membrane guideline with the alkalinity over 200 ppm RO Recommended flag. Run the permeate through the Nutrient Lab Pro before mixing so the feed numbers are real.
References
- Hoagland, D. R. & Arnon, D. I. (1950) — The Water-Culture Method for Growing Plants Without Soil
- Sonnenveld, C. & Voogt, W. — Plant Nutrition of Greenhouse Crops, Springer
- Resh, H. M. — Hydroponic Food Production (7th ed.), CRC Press
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