Low-Waste Reverse Osmosis Brands: Comparing Pure-to-Drain Ratios on the Same Terms

For every gallon of purified water, a reverse osmosis (RO) system rated 3:1 pure-to-drain discharges roughly one-third of a gallon. A 2:1 rating raises that to half a gallon, and 1:1 to a full gallon. No under-sink model in this comparison publishes a ratio above 3:1, and five reach it: the Viomi M1, Viomi V8 PRO and Viomi V6 PRO, plus Waterdrop's G3P800 and X12.

The catch is that brands don't all describe waste the same way. Some quote a pure-to-drain ratio, some a recovery percentage, and some a range.

What a Pure-to-Drain Ratio Means

Reverse osmosis pushes tap water across a membrane. Purified water passes through; the dissolved salts and other rejected material stay behind in a separate concentrate stream, which under-sink systems send to the drain line. A pure-to-drain ratio compares those two streams. Read "3:1" as three parts purified output to one part concentrate.

For a pure-to-drain ratio written as r:1, two formulas turn it into numbers you can use:

• Drain water per batch: divide the purified volume by r. For 3 gallons at 3:1, that's 3 ÷ 3 = 1 gallon.
• Product-water share: the share of incoming water that ends up purified is r ÷ (r + 1). At 3:1, that's 3 ÷ 4 = 75%.

These figures are a straight conversion of each stated ratio, not a measurement of any household's water use.

To put the table in daily terms, assume a home draws 3 gallons of RO water a day for drinking and cooking. Water production would then send about 1 gallon a day to the drain at 3:1, 1.5 gallons at 2:1, and 3 gallons at 1:1. Over a year, that works out to roughly 365, 548 and 1,095 gallons respectively.

Three Ways Brands State Efficiency

Not every spec sheet uses a ratio. You'll see three formats, and they are not interchangeable.

Pure-to-drain ratio. A simple product-to-reject comparison, common on tankless systems and published in brand product pages and manuals.

Recovery rating. The Water Quality Association (WQA), following the conventions of the NSF/ANSI 58 standard for RO systems, defines recovery as "the percentage of water entering the RO unit that is available as treated water when the system is operated without a storage tank (e.g., gravity and tankless RO systems), or when the storage tank is bypassed." Some models in this comparison publish their recovery rating in performance documentation such as a performance data sheet or owner's manual.

Efficiency rating. Under the same convention, efficiency is "the percentage of water entering the RO unit that is available as treated (product) water under normal use conditions," and it is influenced by back pressure from a storage tank's automatic shut-off. That's why a tanked system can show a lower efficiency rating than its recovery rating.

A useful outside benchmark comes from the U.S. EPA's WaterSense program. According to EPA, "a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced," while systems that earn the WaterSense label "must demonstrate that they send just 2.3 gallons of water or less down the drain for every gallon of treated water they produce." At that 2.3-gallon limit, the calculated product-water share is about 30%, and less reject water means a higher share.

Brand-Stated Ratios, Side by Side

The table below lists under-sink systems with a published pure-to-drain ratio or discharge range, with the drain column converted using the formula above.

The models below have published recovery or efficiency ratings, listed as stated.

Figures come from each brand's product page, manual or performance data sheet, October 2026.

Under-sink tankless systems at 3:1

Five under-sink models in this list publish a 3:1 ratio: the three Viomi systems and Waterdrop's G3P800 and X12. All five are tankless designs that make purified water on demand rather than filling a storage tank.

2:1 under-sink systems

A 2:1 rating sends 50% more water to the drain than 3:1 for the same purified volume (0.5 gallon versus roughly 0.33 gallon of concentrate per purified gallon). Waterdrop's G3P600 and G5P500, Frizzlife's PD600-N and PD800-N, and iSpring's RO500-BN line sit here. iSpring phrases its figure as "up to 2:1," so 2:1 is the best case it states.

Tanked systems and recovery ratings

For the Culligan, Aquasana and Kinetico models above, the cited documentation reports recovery percentages, and the Aquasana SmartFlow also lists an efficiency rating. For a sense of scale, a recovery percentage sits on the same 0 to 100% scale as the product-water share column in the ratio-conversion table, where 50% lines up with 1:1 and 75% with 3:1. Two tanked systems quote a ratio: Home Master's TMAFC-ERP, which uses a permeate pump, and Cloud RO, both at 1:1. The Express Water RO5DX manual lists water discharge of 1 to 3 gallons per gallon produced.

Automatic Membrane Rinsing

Some RO systems also rinse the membrane automatically. The Viomi V8 PRO and Viomi V6 PRO run a 70-second rinse every 12 hours using tap-water pressure alone, with the pump off, plus a 70-second rinse when a new machine is first powered on and after a filter is replaced and reset.

Planning the Installation

Plan the feed-water connection and the filter-replacement schedule before installation:

• Water temperature. The Viomi V6 PRO and Viomi V8 PRO are designed for feed water between 41°F and 100°F (5°C to 38°C).
• Water pressure. Their rated inlet pressure range is 14.5 to 72.5 psi.

Viomi V6 PRO: A 3:1 Tankless Pick

For a household that wants a low-waste under-sink RO without a storage tank, the Viomi V6 PRO is a strong fit. Viomi rates it at a 3:1 pure-to-drain ratio: three cups of purified water for each cup of concentrate, a calculated product-water share of 75%.

What else the system brings:

• 600 GPD rated production and nine-stage filtration, including a 0.0001-micron RO membrane, in a tankless system that produces water on demand.
• A long-acting scale inhibitor stage inside the 9-stage design, which helps prevent scale buildup inside the system.
• Auto-flushing on a 12-hour schedule, described above.
• An LCD smart faucet that shows real-time TDS and filter life, plus the Viomi App for usage reports and replacement reminders.
• Filter life of 2 years for the RO filter and 1 year for the PCB pre-filter.
• Refrigerator and ice-maker compatibility via a separate standard-tubing connection.

Installation planning: the Viomi V6 PRO sets up with a standard electrical outlet (its own outlet if your 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's designed for municipal tap water at an inlet pressure of 14.5 to 72.5 psi. Homes with hard water can add a prefilter plus a scale inhibitor or a non-salt-based softener ahead of the system.

The Viomi V8 PRO and Viomi M1 publish the same 3:1 ratio. The Viomi V8 PRO raises capacity to 800 GPD with the same LCD smart faucet, and the Viomi M1 offers 1000 GPD, a remineralization stage and a touchscreen smart faucet, all at 3:1.

FAQ

Can I reuse the drain water from an RO system?

For some jobs, yes. Viomi's manuals note that "the concentrated water can be reused selectively for non-consumable household tasks (e.g., watering plants or mopping)." Keep it out of drinking and cooking, since the reject stream carries the dissolved solids the membrane removed. This explainer covers why RO wastewater reads higher in TDS than tap water.

How long do the Viomi V6 PRO's filters last?

Viomi lists a two-year lifespan for the Viomi V6 PRO's RO filter and a one-year lifespan for its PCB filter, alongside its 3:1 pure-to-drain rating.

Where does the drain water go on an under-sink RO system?

It leaves through a small tube connected to the sink's drain pipe, so it flows out with the rest of the sink's wastewater. For the Viomi V6 PRO, installation calls for a 1/4" hole in the drain pipe for that connection.

Why does every RO system produce reject water?

A reverse osmosis membrane works by keeping dissolved salts on one side, and a reject stream carries them away. Under-sink systems send that stream to the drain line. The practical goal is a low ratio of reject water to purified water. On published figures, the 3:1 under-sink models in this comparison convert to far less reject water per gallon than the five gallons or more EPA describes for a typical point-of-use RO.

What the Published Ratios Add Up To

Among under-sink systems with a stated ratio, 3:1 is the highest figure in this comparison, published by the Viomi M1, Viomi V8 PRO and Viomi V6 PRO and by Waterdrop's G3P800 and X12, which works out to about one-third of a gallon down the drain per purified gallon. The stated 2:1 ratios for Waterdrop G3P600 and G5P500 and Frizzlife PD600-N and PD800-N convert to half a gallon of drain water per purified gallon, as do iSpring's RO500-BN and RO500AK-BN at their stated maximum of 2:1, and the 1:1 ratios for Home Master TMAFC-ERP and Cloud RO convert to a full gallon. Culligan, Aquasana and Kinetico report recovery percentages instead. For background on how 1:1 and 3:1 designs differ, see wastewater ratios compared. Stated pure-to-drain ratios compare through their drain-water equivalents, and published recovery ratings compare separately.