A reverse osmosis system in San Diego is a drinking water system: it sits under the kitchen sink and feeds one tap with water pushed through a fine membrane. This page explains how it works, what EPA says it can remove, the water it sends to the drain, and how it fits with a softener.
Water Softener San Diego is an independent referral service, not an installer, plumber or equipment seller. When you send the form, your details are passed to an independent local water treatment pro, who can contact you and prepare a quote.
How reverse osmosis works
EPA’s WaterSense program describes reverse osmosis as the process by which pressure forces water through a semi-permeable membrane. Two streams come out: treated water, called permeate, and reject water, called concentrate or brine, which carries what the membrane held back to the drain. A point-of-use RO system is connected to a single fixture, such as under the kitchen sink, and feeds its own faucet or the main kitchen tap.
An under-sink system has filter stages around the membrane and may have a small storage tank so a glass fills quickly. The exact set of stages differs by model; the maker’s documents list them.
What EPA says RO can remove
According to EPA, RO systems can potentially remove water contaminants such as lead, volatile organic compounds, PFAS, arsenic, bacteria and viruses. “Potentially” is the important word: what a given unit removes depends on its design and its certification. The standard for reverse osmosis drinking water treatment systems is NSF/ANSI 58. Ask which contaminants a unit is certified to reduce under it.
PFAS and the federal rule
PFAS is one reason to look at RO. EPA’s final rule set enforceable limits of 4.0 parts per trillion for PFOA and for PFOS. On May 18, 2026, EPA proposed keeping those two limits while letting water systems request two more years, to 2031, to comply, and proposed rescinding the limits for PFHxS, PFNA, HFPO-DA (GenX) and the mixture hazard index. Those changes are proposals, not final. Your water provider’s yearly report is the place to see what it measures.
The water RO sends to the drain
RO uses more water than it delivers. EPA says a typical point-of-use RO system generates five gallons or more of reject water for every gallon of treated water, and some inefficient units generate up to 10 gallons. Systems that earn the WaterSense label must send 2.3 gallons or less down the drain per gallon treated.
| Unit | Reject water per gallon treated (EPA) |
|---|---|
| Typical point-of-use RO | Five gallons or more |
| Some inefficient units | Up to 10 gallons |
| WaterSense labeled | 2.3 gallons or less |
The drain figure is worth asking about. Ask any installer whether the unit they propose is WaterSense labeled, and what its rated reject water is.
Want RO at the kitchen tap? Tell us about your kitchen and a local pro can contact you about a quote.
RO and a softener together
The City of San Diego says its water typically averages about 16 grains per gallon of hardness. RO at one tap does not soften the rest of the house, and a softener does not filter drinking water. That is why the two can be paired: a softener on the main line for scale, spots and soap, and RO under the kitchen sink for drinking and cooking. Ask the installer how the RO unit will be fed and whether the softened supply affects it, following the maker’s instructions.
A softener adds some sodium to the water. If that matters to you, ask your doctor, and ask the installer whether the kitchen RO tap is the right place for drinking water in your home. See water softener installation for the softener side.
What an under-sink install involves
- A connection to the cold water supply under the sink
- A drain connection for the reject water
- A faucet for the RO water, or a connection to an existing tap
- Room under the sink for the unit and any storage tank
- A first flush of the system, as the maker’s instructions say
Look under your sink before anyone visits: a garbage disposal, a water heater line or cleaning supplies can all compete for the space. A refrigerator with a water and ice dispenser can also be fed from the RO unit; ask the installer whether your layout allows it.
RO or a simpler filter?
RO is not the only way to filter drinking water, and it is not always the one you need. If your reason is chlorine taste and odor, a filter certified to NSF/ANSI 42 deals with aesthetic effects of that kind and sends no water to the drain. If your reason is lead, a filter certified to NSF/ANSI 53 for lead reduction is another route. RO earns its place when you want a broad range of contaminants reduced at one tap, the list EPA gives: lead, volatile organic compounds, PFAS, arsenic, bacteria and viruses.
| Your reason | Options to compare |
|---|---|
| Chlorine taste and odor | A filter certified to NSF/ANSI 42, or RO |
| Lead | A filter certified to NSF/ANSI 53 for lead, or RO certified for lead |
| PFAS or arsenic | RO certified under NSF/ANSI 58 for the contaminant, or a filter certified for it |
| A broad range at the kitchen tap | RO under NSF/ANSI 58 |
The basic advice on choosing home water treatment sums it up: decide what contaminant you want to remove or reduce, then make sure the unit you are considering is certified to remove that one. The water filtration page goes through the other filter types.
When an RO unit needs attention
An RO unit that has been running for a while can tell you it needs service: the flow from its faucet slows, the storage tank empties sooner, or the water starts to taste different. Those are reasons to check the maker’s schedule for the filters and membrane and, if the schedule is up to date, to ask a pro to look at it. A leak under the sink is a reason to shut the unit’s supply valve and ask for help.
- Slower flow at the RO faucet
- Water that tastes different from before
- A storage tank that runs out sooner
- Water under the sink or a drip at a fitting
Whole-house reverse osmosis
RO for the whole house exists, but it is a bigger installation than an under-sink unit, and the installer sizes it after a water test. If whole-house RO is proposed, ask what it adds over a softener and a whole house water filter with RO at the kitchen tap, and how much water it sends to the drain.
Upkeep
An RO unit’s filters and membrane are changed on the maker’s schedule. This site does not give an interval, because it depends on the unit and the water. Ask for the schedule and the part numbers in writing, so the running cost of two units can be compared.
Questions to ask before you buy
- Is the unit certified to NSF/ANSI 58, and for which contaminants?
- Is it WaterSense labeled, and what is its reject water per gallon?
- Where will it go under my sink, and where will the faucet sit?
- How will it be fed if there is a softener?
- What does the maker’s schedule say for filters and the membrane?
For other ways to filter drinking water, see water filtration. For a plan that also covers hardness and anything a test turns up, see water treatment. This site prints no prices; a quote depends on the unit, its stages, the faucet, the drain connection and the filters on the maker’s schedule.
Questions people ask
What does reverse osmosis remove?
EPA says RO can potentially remove contaminants such as lead, volatile organic compounds, PFAS, arsenic, bacteria and viruses. Check which ones a unit is certified to reduce under NSF/ANSI 58.
How much water does an RO system waste?
EPA says a typical point-of-use unit sends five gallons or more to the drain per gallon treated, some up to 10. WaterSense labeled units send 2.3 gallons or less.
Do I need a softener if I have RO?
RO treats one tap. City water is hard, about 16 grains per gallon, so the rest of the house still gets hard water unless a softener is fitted.
Does RO soften water?
RO treats drinking water at one tap. For hardness across the house, the equipment is a softener, covered by NSF/ANSI 44.
How often are RO filters changed?
On the maker’s schedule; it depends on the unit and the water.
Is RO better than a regular filter?
It depends on what you want to reduce. For chlorine taste and odor a filter certified to NSF/ANSI 42 may be enough; RO under NSF/ANSI 58 covers a wider list at one tap but sends water to the drain.
Can RO feed my refrigerator?
It can, where the layout allows a line from the RO unit to the refrigerator. Ask the installer when they look at your kitchen.
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