What each part of a typical under-sink RO system does
- A feed-water valve supplies cold water to the treatment assembly.
- Prefilters protect the membrane from sediment or substances addressed by the particular filter design.
- The membrane separates the feed into product water and concentrate. RO can reduce many dissolved substances, but performance and claims are system-specific. [1]
- A flow control and drain connection carry concentrate away at the rate intended by the manufacturer.
- A pressurized storage tank holds treated water because membrane production is slower than faucet demand.
- A postfilter and dedicated faucet deliver stored product water. Some models add pumps, monitoring, or other stages, so the exact flow path comes from that product's manual.
How an RO system makes and stores water
Feed water enters
Cold water passes through the installed pretreatment stages. Feed pressure, temperature, and chemistry affect production.
The stream reaches the membrane
Pressure drives some water through the membrane while more of the rejected dissolved material remains in the concentrate stream. [1]
Concentrate drains
The rejected stream carries removed material toward an approved drain connection. RO therefore produces both treated water and wastewater.
Product water is stored
Treated water accumulates in the storage tank until pressure signals the system to stop production.
The faucet draws from storage
Opening the dedicated faucet releases stored water through any final polishing stage. The system then makes replacement water.
Read the certified claims, not just the product name
A specific RO model may be certified to
RO should not be assumed to
Reduce named substances listed on that model's certification. [3]
Remove every possible substance from every feed water.
Serve a defined point-of-use drinking-water goal.
Treat all household water unless specifically designed and installed as a larger point-of-entry system.
Work with its specified prefilters, membrane, and postfilters as one system.
Keep performing when replacement parts or maintenance are ignored.
Produce lower-dissolved-solids product water and a reject stream. [1]
Eliminate the need for a safe, approved drain connection.
What common RO symptoms point toward
| Symptom | Functions to check |
|---|---|
| Slow delivery | Storage pressure, prefilter restriction, membrane production, faucet, or kinked tubing |
| System drains continuously | Shutoff valve, tank pressure, membrane production, check valve, or flow control |
| Taste or odor returns | Filter age, sanitation, membrane performance, tank condition, or feed-water change |
| Leak under sink | Tubing insertion, fittings, housings, tank valve, faucet, or drain adapter |
RO production is different from faucet delivery. The membrane makes product water over time, while a storage tank allows a faster short draw at the faucet. Feed pressure, water temperature, pretreatment condition, membrane condition, and system design affect production. [1] Use the model's rated conditions before deciding that output is low.
Continuous draining, slow recovery, or an empty tank can come from more than one restriction, valve, pressure, or installation problem. Do not replace parts from the symptom alone. Check the manual, verify feed and tank conditions using the approved procedure, and keep the drain connection intact.
Plan for maintenance before choosing the system
- Replace each prefilter and postfilter according to the exact product instructions and actual feed conditions.
- Monitor product quality using the method appropriate to the system's certified reduction claims; taste alone is not a complete performance test. [2]
- Sanitize housings, tubing, tank, and faucet only by the manufacturer's approved procedure.
- Keep the concentrate drain connection secure, unobstructed, and separated from wastewater by the approved arrangement.
- Check under-sink fittings and tubing for moisture after filter service and after the cabinet has been disturbed.
- Retain the model, filter part numbers, membrane identity, service dates, and any measured feed and product results.