RO Wastewater Myths

Reverse osmosis pushes water against a membrane. Most of it carries the rejected dissolved solids to the drain as concentrate; a smaller share passes through as permeate — the water you drink.

The ratio describes that split. A 3:1 system sends three gallons to the drain for every gallon produced.

Older tank systems with no pump do commonly run 3:1 or 4:1. Modern tankless units with recirculation or a permeate pump can approach 1:1 by reusing concentrate to pressurize the system.

A poor ratio is often a sign of low water pressure or a cold supply rather than an inherent design limit. Production falls in winter because membranes slow down as temperature drops.

If you pay for water and sewer, the concentrate is a real operating cost: at 3:1, four gallons of supply produce one gallon of drinking water.

The second cost is membrane life. Running with low pressure or a clogged pre-filter makes the membrane work harder and shortens its useful life.

Replacing a $40 pre-filter on schedule is usually cheaper than replacing a $90 membrane early.

A permeate pump uses the pressure of the concentrate stream to pressurize the storage tank, which raises recovery, reduces drain flow, and delivers better tank pressure at the faucet.

It is most useful on tank systems with low supply pressure or a long run to the drain. On a tankless unit the equivalent function is usually built in.

If your system runs well and your water is cheap, a pump is optional. If the tank takes forever to refill, it is the first upgrade to consider.

Measure recovery at a normal operating temperature, not on the coldest morning of the year.

Check that the flow restrictor matches the membrane rating; a mismatched restrictor quietly changes both recovery and membrane life.

Confirm the drain line has a proper air gap or backflow prevention so nothing can siphon back toward the membrane.