Gravity water filters appeal to two very different groups of people. The first group wants off-grid capability: no power required, no plumbing, works anywhere. Campers, emergency preppers, cabin owners. The second group just wants a countertop option for filtered drinking water without any installation work at all.

Both groups are looking at the same small product category, but they’re making different trade-offs. And the question of NSF certification, which is straightforward in other filter categories, gets complicated here.

This review covers how gravity filters work, the NSF certification gap that exists in most of this category, and which products to choose based on what you actually need.

How Gravity Filters Work

Gravity filters are self-contained, two-chamber systems. You pour water into the upper chamber. Gravity slowly pulls it through filter elements into the lower chamber. You dispense from the lower chamber through a spigot.

No electricity. No water pressure connection. No plumbing. You can fill the upper chamber from a sink, a stream, a rain barrel, or any other water source.

The filter elements vary by product. Berkey uses carbon-based “Black Berkey” elements. Alexapure uses a ceramic-core carbon element. AquaTru Carafe uses a reverse osmosis membrane with a small electric pump to assist the process.

Flow rate is slow compared to pressurized faucet or under-sink filters. A Big Berkey fills the lower chamber over the course of several hours depending on how much water you pour in. For a countertop drinking water setup where you’re just pulling a glass at a time, this isn’t a practical problem. For filling a large pot quickly, you’ll wait.

The NSF Certification Gap

Most gravity filters don’t have NSF certification. This matters more for some buyers than others.

NSF certification protocols were largely developed around pressurized water systems. Gravity-fed systems operate at different flow rates and pressures, and some manufacturers have argued the standard protocols don’t apply well to their products. Whether or not that argument has merit, the practical result is that most gravity filters rely on manufacturer-commissioned third-party lab testing rather than NSF-certified testing.

Manufacturer-commissioned lab tests can produce real data. But they’re not the same as independent NSF certification. The manufacturer chooses the test conditions. The manufacturer publishes the results. That’s a different process than submitting to ongoing independent surveillance audits under NSF protocols.

For buyers in the US purchasing a filter for everyday tap water use, NSF certification is the reasonable standard to apply. For emergency preparedness or off-grid use where no NSF-certified option with sufficient capacity exists, manufacturer lab testing becomes the more relevant comparison point.

Know which situation you’re in before you decide how much the certification gap matters to you.

Big Berkey (2.25-Gallon)

The Big Berkey is the most widely used gravity filter in the US. The stainless steel two-chamber design holds 2.25 gallons and uses two Black Berkey filter elements by default, with space for up to four elements if you want faster flow.

Berkey claims their Black Berkey elements remove a wide range of contaminants including bacteria, viruses, heavy metals, pharmaceuticals, PFAS compounds, and chlorine. The specific figures cited are based on their own commissioned lab testing.

What the certification record actually shows:

Berkey is not NSF certified. Not NSF 42, not NSF 53, not NSF 58. In 2023, the EPA classified Berkey filters as a pesticide device because of their antimicrobial claims. That classification created complications with distribution, including a period where Berkey faced sales restrictions in some states.

The Big Berkey is widely used by people who are comfortable with the manufacturer-commissioned test data and value the off-grid capability. The community of Berkey users is large and vocal. The long-term track record matters: these systems have been used in the field for years, including in disaster relief contexts.

Black Berkey filter elements are rated at 3,000 gallons each. With two elements, a system is rated for 6,000 gallons before replacement. At the average household use of 1 to 2 gallons per day for drinking and cooking, that’s 8 to 16 years. Replacement elements cost around $100 to $130 per pair.

Cost: roughly $280 to $350 for the stainless steel Big Berkey with two Black elements.

Best for: Emergency preparedness, off-grid use, camping, or households comfortable with manufacturer-commissioned test data and for whom no-power operation is important.

Check current pricing (affiliate link, see disclosure above)

Alexapure Pro (2.25-Gallon)

Alexapure Pro is a direct competitor to Berkey in format and claimed capability. Stainless steel, 2.25-gallon capacity, single ceramic-core carbon filter element.

Alexapure claims over 99.9% reduction of more than 200 contaminants. Like Berkey, those figures come from manufacturer-commissioned lab testing, not NSF certification.

The single-filter design means slower flow rate than a dual-element Berkey. If you want faster throughput, Berkey’s dual-element setup has an advantage.

The main appeal of Alexapure is price. It typically runs $200 to $250, which is $50 to $100 less than a Big Berkey. If cost is the deciding factor and the two certifications are equivalent (both uncertified), Alexapure offers comparable claimed performance at a lower price.

Replacement filters run around $50 to $70 and are rated at approximately 5,000 gallons.

Cost: $200 to $250.

Best for: Emergency prep or off-grid use on a tighter budget than Berkey.

Check current pricing (affiliate link, see disclosure above)

AquaTru Carafe

The AquaTru Carafe is the outlier in this category. It uses a reverse osmosis process rather than gravity-fed carbon filtration. A small electric pump assists the RO process, and the filtered output collects in a glass carafe.

It requires an outlet. That disqualifies it for true off-grid or emergency-without-power use.

But it’s the only countertop option in this category with actual NSF certification. NSF 58 for the reverse osmosis standard, and NSF P473 for PFAS specifically. PFAS removal is documented through independent testing under NSF protocols, not manufacturer-commissioned lab tests.

What it removes:

• PFAS: NSF P473 certified
• Fluoride: RO membranes reduce fluoride, though check current AquaTru specs for specific claims
• Nitrates: RO membranes reduce nitrates (NSF 58 covers dissolved contaminants)
• Lead, arsenic, chromium: covered by NSF 58
• Chlorine, taste, odor: covered by pre-filter carbon stage

This is the treatment level you get from a pressurized under-sink RO system, in a countertop format that needs no plumbing. The trade-off is you need an outlet, and the output goes into a carafe rather than coming out of a tap.

Flow into the carafe is slow. You’ll fill the carafe and use from it rather than getting instant on-demand filtered water. The electric pump speeds up the process compared to pure gravity, but it’s still not instant.

Replacement filters are multi-stage and cost $50 to $80 depending on which stage. The pre-filter (stages 1-2) should be changed every 6 to 12 months depending on use. The RO membrane typically lasts 2 years.

Cost: $200 to $250.

Best for: Households who want certified PFAS removal in a countertop format, don’t mind having an outlet nearby, and want independent NSF documentation for what their filter removes.

Check current pricing (affiliate link, see disclosure above)

Propur Pitcher

Propur makes a smaller pitcher-format gravity filter. The ceramic-core filter in a 2.75-cup capacity unit is more suited for travel or single-person use than household filtration.

Like Berkey and Alexapure, Propur is not NSF certified. Claims are based on manufacturer-commissioned testing.

At the capacity and price point, it’s not a strong option compared to the main three. If you want something smaller for travel, a quality squeeze filter or personal straw filter designed for outdoor use (Sawyer, LifeStraw) is more practical and has NSF or ANSI 42-rated testing behind it.

AquaTru vs. Berkey: The Core Trade-Off

These are the two most common purchases in this category, and they serve genuinely different needs.

AquaTru Carafe wins on documented contaminant removal. NSF P473 for PFAS is the kind of independent certification that means something. If your municipal water has PFAS and you want a countertop solution you can verify, AquaTru is the right choice.

Berkey wins on off-grid capability and capacity. No power required, 2.25-gallon capacity, and a large existing user community. For emergency preparedness where electricity may be unavailable, Berkey is the practical choice.

The price is similar. The format is similar. The use case is what separates them.

Emergency Preparedness Considerations

Gravity filters are worth looking at specifically for emergency water supply, but there are limits to understand.

For tap water that loses municipal treatment during a boil water advisory or infrastructure disruption, a gravity filter is a reasonable layer of protection.

For water from truly unknown sources, like a stream, pond, or flood-affected well, the biological removal claims matter more. Berkey and Alexapure claim virus and bacteria removal based on their own lab tests. That’s not the same as NSF 55 Class A certification (UV systems) for purification. For true biological purification certainty from unknown sources, pair a gravity filter with chemical treatment or a UV purifier.

The best UV water purifiers covers UV disinfection for households who need pathogen removal with independent certification.

The Comparison

The Recommendation

For certified PFAS removal in a countertop format, AquaTru Carafe is the clear choice. The NSF 58 and P473 certifications mean the claims are verified. If you’re buying a filter to protect against PFAS and you want documentation to back it up, AquaTru is the only product here that provides it.

For off-grid capability or emergency preparedness where power is unavailable, Berkey is the practical standard. The lack of NSF certification is a real limitation, but Berkey’s long track record and large user community make it the most battle-tested gravity filter available. Alexapure is a reasonable budget alternative if cost is the deciding factor.

The one recommendation that applies to both categories: test your water before buying. Understanding what’s actually in your source water tells you which contaminants you’re trying to remove, which determines whether AquaTru’s certified RO coverage or Berkey’s off-grid carbon filtration is the right fit for your situation.

For contaminant testing guidance, NSF certification standards explained is a good starting point for understanding what the certifications actually mean and why the distinction between manufacturer-commissioned testing and independent NSF certification matters.