Reverse Osmosis

Reverse osmosis is one of the most thorough point-of-use treatment options, typically installed under the kitchen sink.

Short answer: RO systems push water through a semi-permeable membrane, removing a broad range of dissolved contaminants when properly certified and maintained. They're point-of-use (usually one tap), not whole-house, and require periodic membrane and filter changes.

What RO can address

A broad range of dissolved solids and contaminants, depending on certification
Nitrate, arsenic, and PFAS when the specific system is certified for them
Improved taste from reduced dissolved solids and salinity

Delaware uses for RO

RO is especially relevant as a point-of-use option after a Delaware private-well test shows nitrate, elevated dissolved solids, or chloride/salinity. That matters in agricultural Sussex and Kent, where shallow Columbia-aquifer wells can need nitrate-specific treatment, and in coastal Sussex, where saltwater intrusion must first be investigated rather than hidden with a filter. RO can reduce salts at one tap; it does not repair a failing well or resolve the cause of regional salinity.

What to know before buying

Confirm NSF/ANSI certification for the specific contaminant you're addressing — certification is claim-specific, not universal.
RO systems produce wastewater; check the system's efficiency ratio if that matters to you.
A storage tank buffers output since RO produces water slowly.
Systems require periodic pre-filter, membrane, and post-filter replacement.

RO design for a Delaware home

Start with the result you are trying to change. A private-well owner in agricultural Kent or Sussex may be looking for nitrate reduction; a coastal Sussex owner may be looking for lower chloride or dissolved solids; a public-water customer may be responding to a certified PFAS or lead concern. The system must be certified for that specific claim. A TDS meter can show a change in dissolved solids but cannot prove that nitrate, PFAS, or a particular salt was removed.

RO needs adequate feed pressure, a clean prefilter, a correctly sized storage tank, an air-gap or drain arrangement, and a sanitary faucet connection. Hardness, iron, manganese, sediment, and chlorine can shorten membrane life, so pretreatment may be more important than buying a larger membrane. If the home is on a well, check pump cycling and pressure before assuming the RO is defective.

What it costs to operate

Prefilters are typically replaced several times a year depending on use and incoming water quality.
The membrane is a periodic replacement part; actual life depends on pressure, hardness, chlorine exposure, and feed-water loading.
Postfilters and storage-tank sanitation matter because RO treats a single drinking-water branch, not every fixture.
Retest after installation when the original concern is health-related or regulated.

Common mistakes

Assuming any RO system automatically removes every contaminant
Skipping membrane replacement, which degrades performance over time
Installing RO as a whole-house solution when it's designed for point-of-use

RO checklist before installation

Record the raw-water result and the exact contaminant claim the system must address.
Confirm feed pressure during peak household demand, not only when no fixture is running.
Check whether iron, manganese, hardness, sediment, or disinfectant requires pretreatment.
Verify the system's efficiency, drain connection, storage capacity, and replacement parts.
Plan a post-installation test when the system is being used for nitrate, PFAS, lead, or salinity reduction.

RO is a focused solution, not a substitute for investigating the source. A coastal Sussex well with rising chloride may need a well professional and a water-supply plan even if an under-sink system improves the drinking tap. Likewise, an agricultural Kent or Sussex nitrate result should be confirmed and monitored rather than treated as a permanent number from one sample.

When point-of-use is the better choice

RO is often most practical when the concern is limited to drinking and cooking water. It avoids treating every shower, toilet, and hose bib, reduces equipment size, and makes it easier to verify performance at the drinking tap. Whole-house RO can create substantial reject water, require larger storage and pumps, and change the chemistry of water used throughout the plumbing. Choose it only after a documented need and design review.

Keep replacement dates visible, label the treated faucet, and do not assume a refrigerator filter has the same certified claim as the RO system.

Official sources

NSF/ANSI 58 (Reverse Osmosis Drinking Water Treatment Systems)
U.S. EPA point-of-use treatment guidance

Does RO help with salty well water?

RO can reduce dissolved salts at the point of use, but for whole-house salinity concerns from saltwater intrusion, see Salty Well Water for a fuller discussion of causes and options.

Does not endorse a specific manufacturer or product claim.

Last reviewed: 2026-07-11. Report a correction.