Reverse Osmosis vs Carbon Filtration vs Gravity Filter: Which Actually Removes What
Three filtration technologies dominate the home water filter market. They work differently, cost differently, and remove different things. Here is the honest comparison — including what none of them remove, and the tradeoff no comparison article explains properly.
The three technologies at a glance
Reverse osmosis (RO): forces water through a semi-permeable membrane with 0.0001-micron pores, physically excluding dissolved contaminants. Requires water pressure (30–80 PSI), produces wastewater (3:1 ratio typically), removes 95–99% of TDS including PFAS, lead, fluoride, arsenic.
Activated carbon: adsorbs contaminants to a carbon surface. Removes chlorine, VOCs, taste, and odour effectively. Some carbon block formulations (NSF 53 certified) reduce lead. Does not remove fluoride, nitrates, or PFAS at certified levels.
Gravity filtration: water moves through filter media (ceramic, carbon, specialised resin) by gravity, no electricity or pressure required. Range from very effective (removing bacteria, protozoa, some VOCs) to limited depending on the specific filter media and what it has been independently tested for.
Contaminant removal chart
| Contaminant | RO (NSF 58) | Carbon block (NSF 53) | Carbon pitcher (NSF 42) | Gravity (Berkey, uncertified 2026) |
|---|---|---|---|---|
| Chlorine | Yes (pre-filter) | Yes | Yes | Claimed |
| Chloramine | Partial (pre-filter) | NSF 53 some | NSF 42 some | Claimed |
| Lead | Yes (95%+) | Yes (NSF 53) | No (Standard) / Yes (Elite) | Claimed, unverified post-2024 |
| PFOA/PFOS | Yes (NSF 58 certified) | Partial (2022+ NSF 53) | No | Uncertified |
| Fluoride | Yes (90–95%) | No | No | No |
| Arsenic | Yes (95%+) | Some (NSF 53 specific) | No | No |
| Nitrates | Yes (75–90%) | No | No | No |
| Bacteria | Yes (membrane size exclusion) | No | No | Yes (Black Berkey claims log-6) |
| Viruses | Yes (membrane size exclusion) | No | No | Claimed |
| TDS (calcium, magnesium) | Yes (removes 95–99%) | No | No | No (gravity retains minerals) |
| Sediment | Yes (pre-filter stage) | Yes (carbon block) | Partial | Yes |
Key insight: no single filter removes everything. RO comes closest but removes beneficial minerals. Gravity filtration is strongest for bacteria and no-install use cases but has the weakest current certification status for PFAS.
NSF standards mapped to technologies
| NSF Standard | What it certifies | Applies to |
|---|---|---|
| NSF/ANSI 42 | Chlorine, taste, odour, aesthetic effects | All filter types — the minimum |
| NSF/ANSI 53 | Lead, health-effect contaminants, some PFAS (2022+) | Carbon block, ZeroWater, some whole-house |
| NSF/ANSI 58 | RO systems — TDS, lead, fluoride, arsenic, PFAS | Under-sink and countertop RO systems |
| NSF/ANSI 401 | Emerging contaminants — pharmaceuticals, pesticides | Advanced carbon block, some RO |
| NSF/ANSI 372 | Lead-free materials in system components | All plumbed systems — material safety |
The certification verification rule: always check the manufacturer’s actual certification at info.nsf.org/Certified/dwtu/. “Tested to NSF standards” is not the same as “NSF certified.” Berkey’s situation — where years of “NSF-equivalent” marketing preceded regulatory action — is the clearest example.
Cost per gallon — honest figures
| Filter type | Example product | Unit cost | Annual filter cost | Annual gallons | Cost per gallon |
|---|---|---|---|---|---|
| Carbon pitcher (standard) | Brita Standard | $35 | $72 | 1,460 (4 fills/day x 1L) | $0.05 |
| Carbon pitcher (NSF 53) | Brita Elite | $45 | $52 | 2,920 (6 months filter life) | $0.02 |
| Ion exchange pitcher | ZeroWater (hard water) | $40 | $150 | 1,095 (3 fills/day) | $0.14 |
| Under-sink carbon (NSF 53) | Aquasana AQ-5300 | $200 | $100 | 5,000 | $0.02 |
| Under-sink RO (NSF 58) | APEC ROES-50 | $215 | $60 | 1,200 (50 GPD) | $0.05 |
| Gravity filter | Berkey Big Berkey | $395 | $130 | 3,000 (per filter pair) | $0.04 |
| Whole-house carbon | Aquasana EQ-1000 | $1,000 | $100 | 100,000 (10-year rating) | $0.01 |
The cheapest per-gallon cost is the whole-house carbon system — but only when amortised over 10 years. For most households, the Brita Elite at $0.02/gallon is the best-value entry point.
The decision framework in three questions
Question 1: What does your CCR or water test actually show?
- Chlorine taste only → activated carbon pitcher (NSF 42 or 53)
- Lead or PFAS → NSF 53 (lead only) or NSF 58 (PFAS — requires RO)
- Fluoride, arsenic, nitrates → NSF 58 required (RO)
- Bacteria (well water) → UV + sediment + carbon; or gravity filter with independently tested bacteria removal
Question 2: Can you install anything?
- Renter or no-install → pitcher or countertop gravity filter
- DIY-capable homeowner → under-sink carbon or RO
- Homeowner willing to hire plumber → whole-house systems unlock
Question 3: How much does mineral content matter to you?
- You want mineral-rich water → avoid RO (or add remineralisation stage)
- You want maximum TDS reduction → RO is the answer; accept flat taste or add remineralisation
When gravity filtration is the right answer
Gravity filters (Berkey, Alexapure, ProOne) are the correct choice when electricity and plumbing are unavailable — off-grid living, emergency preparedness, camping, or rentals where you want something that just sits on the counter and works without any connection. The best-maintained gravity filters with independently verified filter media (ProOne G2.0 is NSF 42, 53, and 401 certified as of 2026) offer real contaminant reduction for bacteria, chlorine, and some VOCs.
Gravity is not the right answer when PFAS or fluoride is the specific concern — no gravity filter is currently NSF 58 certified. For those contaminants, RO is the defensible choice.
Sources: NSF International standard descriptions; EPA PFAS National Primary Drinking Water Regulation (April 2024); APEC, Brita, ZeroWater, Berkey product specifications.