Reverse osmosis (RO) forces water through a semipermeable membrane to remove dissolved solids and impurities, producing clean drinking water. A common observation is that for every gallon of purified water created, a significant volume of water is sent down the drain. This water rejection is not a system flaw but a fundamental requirement rooted in the physics of the purification process itself. The system must intentionally discharge a portion of the incoming water to remain operational, ensuring the longevity of the membrane barrier.
The Necessity of Cross-Flow Filtration
The reverse osmosis process relies on cross-flow filtration, which differs fundamentally from standard dead-end filtration. Dead-end filtration pushes all source water straight through the filter, trapping contaminants until the filter clogs. Cross-flow filtration, in contrast, applies pressure to the water tangentially across the membrane surface.
As pure water molecules pass through the membrane, rejected contaminants (primarily dissolved salts and minerals) become increasingly concentrated. If this concentrated layer remains on the membrane surface, it quickly forms a hard scale or fouling layer. This accumulation drastically reduces the membrane’s efficiency and causes it to fail.
The “waste” water stream is a continuous flow that sweeps concentrated impurities away from the membrane face and directs them down the drain. This constant flushing maintains the differential pressure required for the process to work and ensures the membrane surface stays clean. Without this removal of concentrated solids, the system could not function reliably to produce high-purity water.
Practical Measures of Water Efficiency
The efficiency of a residential system is quantified by its reject ratio, comparing the volume of water sent to the drain (reject) against the volume of purified water (permeate) produced. Older systems commonly operate with a 4:1 ratio, meaning four gallons of water are sent to the drain for every one gallon of drinking water created. This ratio is necessary to maintain the proper pressure and flow to clean the membrane effectively.
The waste rate is controlled by a component called the flow restrictor, installed in the drain line. The flow restrictor creates the necessary back pressure on the membrane to force the water through, while simultaneously limiting the reject water flow rate. Matching the flow restrictor to the membrane size balances water production and membrane cleaning, preventing excessive water loss or premature fouling.
Modern, high-efficiency RO systems have significantly improved this ratio, often achieving 1:1 or better performance. Some advanced systems incorporate a non-electric permeate pump that uses the hydraulic energy of the wastewater to push purified water into the storage tank. This mechanism overcomes the tank’s back pressure, allowing the membrane to operate at optimal efficiency and reducing rejected water by up to 80 percent.
Options for Reusing the Reject Water
The rejected water has passed through the pre-filters, making it clean water, though it contains a higher concentration of salts and minerals. This water is not suitable for drinking or cooking but can be safely repurposed. Redirecting the drain line to a collection vessel allows for immediate reuse, conserving a significant volume of water over time.
Excellent uses for this mineral-rich stream include:
- Mopping floors.
- Washing vehicles.
- Cleaning driveways.
- Cleaning outdoor surfaces.
If a system is plumbed correctly, the reject water can be directed to the toilet tank for flushing purposes, which is a highly effective way to conserve potable water.
For gardening, the concentrated water should be used with caution due to its high Total Dissolved Solids (TDS) content, which can build up in the soil over time. It is suitable for hardy, established landscape plants, but should be avoided for highly sensitive or edible garden plants. Testing the TDS level of the reject water, and diluting it with fresh water if the concentration is high, can help prevent soil salinity issues.