RO Water for Plants: Flow Rate, Drain Ratio and Putting Nutrients Back

A flow of 0.05 to 0.062 gallons per minute works out to 0.19 to 0.23 liters per minute, or roughly 72 to 89 gallons per day if the system ran around the clock. The low end matches what a 75 GPD tank-style rating implies, so any reverse osmosis (RO) system rated above about 90 GPD beats that baseline on paper. Garden-style units such as the Growonix EX400 (400 GPD) and GX1000 (1000 GPD) clear it by a wide margin, and so do under-sink tankless systems rated in the hundreds of gallons per day. At a 2:1 pure-to-drain ratio, every 10 liters of purified water sends 5 liters to the drain; at 3:1, about 3.3 liters. Water straight from an RO membrane starts out low in calcium, magnesium and alkalinity, so the practical way to replace nutrients is a complete fertilizer or nutrient solution that supplies calcium and magnesium, mixed at the label rate for potted plants or to a target EC and pH for hydroponics.

For a household that wants one system for drinking water and plant water, the Viomi M1 fits that double duty well: it is rated at 1000 GPD with a 3:1 pure-to-drain ratio, in a tankless under-sink design that makes purified water on demand from one dedicated faucet. Its remineralization stage is tuned for drinking-water taste, and plant feeding runs through the fertilizer program, the same as with any RO water.

The Arithmetic Behind 0.05 to 0.062 GPM

Converting the 0.05 to 0.062 GPM baseline into the units RO systems are rated in makes the comparison straightforward. One US gallon is 3.785 liters, and a day has 1,440 minutes.

Going the other way, a GPD rating divided by 1,440 gives the average gallons per minute that rating implies. Every implied figure here is a conversion of a rating, which keeps systems with different delivery designs on one scale.

Two things stand out. A 75 GPD rating, common on tank-style under-sink RO, lands right at the bottom of that range. And garden RO makers quote output the same way: Growonix lists its EX400 at 400 GPD and 17 gallons per hour, and its GX1000 at 1000 GPD and 42 gallons per hour, in line with the division.

Rated Output Versus What Reaches the Hose or Faucet

A GPD figure is a production rating under set test conditions. APEC's manual for its ROES-PH75 states that each model's GPD is "rated based on 60 psi input water pressure at 77 degree F water temperature." Field output follows the water you feed it: at lower pressure or colder temperature, a membrane produces less.

Published figures for the same model can also differ by source. APEC sells the ROES-50 as a 50 GPD system, while the Water Quality Association's product directory lists a daily production rate of 34.8 GPD for it. For plant planning, that is a reason to size around the rating with some headroom.

That headroom usually comes from storage. A tank-style system fills its pressurized tank slowly and empties it quickly. A garden unit typically fills a reservoir through a float valve (Growonix includes an auto shutoff valve "for positive automatic shutoff when using a float valve"). A tankless under-sink system has no tank, so the container you fill becomes the buffer: a covered jug or bin sized to one watering session.

What 2:1 and 3:1 Mean for Every 10 Liters

A pure-to-drain ratio compares the purified stream to the concentrate stream that carries rejected salts to the drain. For a batch of plant water, the useful question is how much tap water goes in and how much goes out the drain line.

The drain column is purified volume divided by the first number of the ratio. The table covers water production; membrane flushing and the rinses after a filter change are counted separately.

Direction matters when comparing spec sheets. A pure-to-drain 2:1 sends half a liter to the drain per liter of purified water, while a drain-to-pure 2:1 would send two liters. Growonix lists a "2 to 1" system ratio for its EX400 and "1 to 1" for the GX1000; a 1:1 figure reads the same in either direction.

Garden RO Units and Under-Sink Tankless RO Do Different Jobs

Converted from their GPD ratings, the three models below all sit well above the 0.05 to 0.062 GPM baseline, but they connect to the house differently.

Garden RO units are built to feed a hose or a float-valve reservoir in a grow room, garage or greenhouse, and they run on the pressure of the water supply itself, which Growonix specifies at 40 to 80 psi for these models. An under-sink tankless system puts RO water at the kitchen sink and runs its own booster pump from a wall outlet, so it works from ordinary city water pressure.

Where the Viomi M1 Fits a Plant-and-Kitchen Household

For a home that waters houseplants, herbs or a small hydroponic setup from the kitchen, the Viomi M1 is a single under-sink system that covers drinking water and plant batches from one dedicated faucet.

• Rated 1000 GPD, tankless. Purified water is made on demand, so there is no storage tank to refill between batches.
• 3:1 pure-to-drain. About 3.3 liters go to the drain for every 10 liters of purified water, by the arithmetic above.
• Nine stages around a 0.0001-micron RO membrane, finished with a remineralization stage for drinking-water taste.
• Touchscreen smart faucet with three preset volumes, including 10 oz and 40 oz, plus live TDS and filter-life readings; the Viomi app tracks water usage and sends filter reminders, which helps when plant batches add to the household's total.
• Long filter intervals: the PCB filter lasts up to 2 years and the RO filter up to 4 years.

Installation parts come in the box, and the model has its own step-by-step installation video. The setup fits a standard kitchen: municipal tap water at 41°F to 100°F, a standard outlet (its own always-on outlet if the disposer uses a switched one), a 1" to 1.5" countertop hole for the faucet and a 1/4" hole in the drain pipe. It works with inlet pressure from as low as 14.5 psi.

Putting Nutrients Back After RO

Membrane-treated water is close to a blank slate. Purdue University's Plant Growth Facility posts a lab analysis of the RO water it uses for greenhouse work: calcium, magnesium and potassium each below 1 mg/L, conductivity below 0.1 mmho/cm, and alkalinity of 5 mg/L as calcium carbonate (Purdue HLA, Solution Analysis: HGRH RO Water). Cornell's hydroponics guide describes alkalinity ranging "from near 0 (in very pure or reverse osmosis-treated water)" to more than 300 ppm, and tells growers to "be prepared to supply these nutrients through your fertilizer program" when calcium, magnesium and sulfur are not in the water (Cornell, A Recipe for Hydroponic Success).

In practice, that turns into four steps:

1. Pick a nutrient product that lists calcium and magnesium. Cornell's lettuce recipes pair a base fertilizer with calcium nitrate, and its made-from-scratch recipe includes magnesium sulfate. Cal-Mag supplements are another way to add both alongside a base fertilizer that lacks them.
2. Mix to a target EC for hydroponic solutions. Oklahoma State University Extension lists optimum EC ranges for hydroponic crops, for example 1.0 to 1.6 mS/cm for basil, 1.2 to 1.8 for lettuce and 2.0 to 4.0 for tomato (OSU HLA-6722). Cornell's guide asks growers to count whatever the source water already contains; with a low-EC base like the Purdue sample, the reading mostly reflects the nutrients you mix in. For potted plants in soil, follow the fertilizer label's dilution.
3. Adjust pH after mixing. OSU recommends nutrient solutions for soilless culture between pH 5 and 6, usually 5.5. Cornell describes alkalinity as water's long-lasting effect on pH, which is why its guide puts the alkalinity of the source water ahead of a one-time pH reading.
4. Watch soilless setups more closely. OSU notes that soil "acts as a buffer," and that "this buffer is absent in soilless culture," so hydroponic and coco setups depend on your measurements more than potted soil does.

A drinking-water mineral stage and a plant nutrient program do different jobs. The Viomi M1's remineralization adds selected minerals back for taste, while a crop's needs for nitrogen, phosphorus, potassium, calcium and magnesium are set by its fertilizer recipe, whichever RO system supplies the water.

Ratio designs are compared in more depth in 1:1 versus 3:1 wastewater ratios, and cold-water RO efficiency explains why winter feed water slows production.

FAQ

Is a GPD rating the same as faucet flow?

They measure different things. GPD is a daily production rating under test conditions, while faucet flow is how fast water leaves the spout at a given moment. Dividing GPD by 1,440 puts ratings on a common per-minute scale for comparison, and the delivery design (tank, reservoir or on demand) decides how that production reaches your container.

Does a pure-to-drain ratio include flushing water?

A pure-to-drain ratio describes production: purified water compared with concentrate while the system is making water. Membrane flushes and rinses after a filter change are separate. The 10-liter table above uses the production ratio alone.

Can a remineralization cartridge replace plant fertilizer?

It can't stand in for one. A remineralization stage is designed to adjust drinking water, and plants need a full set of nutrients in amounts set by the crop and growing medium. For potted plants in soil, use a complete fertilizer suited to the crop that includes calcium and magnesium, mixed at the label rate; for hydroponic or other soilless setups, measure EC and pH after mixing the nutrient solution.

How long does a storage bucket take to fill at rated output?

Dividing the bucket by the rating gives the arithmetic: a 5-gallon bucket works out to about 100 minutes at 0.05 GPM, about 36 minutes for a 200 GPD rating and about 18 minutes for a 400 GPD rating. Pressure and feed-water temperature shape the actual time, which is why a float valve or a container sized to one session keeps the routine simple.

Speed, Ratio and Feeding in One Paragraph

Any RO system rated above roughly 90 GPD outpaces a 0.05 to 0.062 GPM baseline on paper, and ratings in the hundreds of gallons per day, from hose-fed garden units like the Growonix EX400 and GX1000 to under-sink tankless systems, clear it many times over. A 3:1 pure-to-drain design such as the Viomi M1 sends about a third of a liter to the drain per liter of plant water, a third of what a 1:1 design sends, and 2:1 sits in between at half a liter. Whichever system makes the water, the nutrients come from the bottle: membrane-treated water starts low in calcium, magnesium and alkalinity, so a complete fertilizer or nutrient solution that includes calcium and magnesium, mixed at the label rate for potted soil or measured by EC and corrected for pH in hydroponic systems, is what turns clean water into plant food.