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How to Choose a Whole House Reverse Osmosis System

Scott Winfield
Written by Scott Winfield
Last Updated on

If you are looking for a reliable water filtration system for your home, you’ve likely come across reverse osmosis units in your research.

That’s because these systems use one of the most effective water treatment technologies on the market. In this article, we’ll teach you how to choose the right RO unit for your needs.

Whole House Reverse Osmosis System

Let’s start by explaining how they function.

What Is Reverse Osmosis?

Reverse osmosis is an extensive water purification process that removes dissolved solids and other contaminants from the water by applying pressure to it through a semipermeable membrane.

The membrane allows water to pass through while blocking impurities, resulting in clean, filtered water. It has small holes throughout it that are microscopic and invisible to the naked eye. The pores are large enough for ions and microscopic particles but nothing else.

Water contaminants, especially dirt particles and dissolved solids like sand, silt, and clay, are much bigger than hydrogen and oxygen molecules.

So, when contaminated water passes through the membrane, the larger molecules are blocked, while hydrogen and oxygen pass through it.

The process effectively separates pure water from any other molecule inside it. The filtration cycle ends with the contaminants and water particles that the device couldn’t filter being flushed from the system through a line connected to a drain or a wastewater container.

Once the process is over, you should have clean water in your reservoir and the pipes leading to your faucets.

There’s a bit more to the process, but this is the gist, and we think it’s just enough to help you understand how reverse osmosis works.

Criteria for Picking a RO Unit

Picking a RO Unit

The market is saturated with hundreds of reverse osmosis models. Although many of them have the label “best RO filter,” it’s crucial to know your specific household needs and consider several factors before making a purchasing decision.

These include contaminant-removal capacity, micron size, filtration media, GPD, price, and additions.


Filtration systems are rated based on their ability to remove contaminants from the water. However, your water may be contaminated with common pollutants or rare viruses and bacteria.

Some filters are exceptional at removing one specific contaminant type but average or poor at eliminating others. The same principle applies to reverse osmosis units.

RO membranes are excellent at eliminating solids from your water, but they fall short when it comes to chemical contaminants and bacteria. This is because non-solid contaminants are usually small enough to pass through the membrane along with the water molecules.

If you live in an area where your main concern is sediment particles or hard minerals like calcium and magnesium, then most reverse osmosis models would be a great solution to your problem.

However, we recommend a multi-stage filtration system if you have issues with other water contaminants.

Most reverse osmosis filters come with a multi-stage filtration setup. This passes the water through different filtration cartridges designed to remove specific contaminants. So when you’re searching for the right product for you, keep a close eye on the media involved in the purification process to discern whether the product removes the impurities you have a problem with.

The most common filtration media paired with a RO filter is activated carbon.

Activated Carbon

If chlorine or other pharmaceuticals, herbicides, or pesticides contaminate your water, your best bet would be to find a reverse osmosis filter with an activated carbon filter.

An activated carbon block filter effectively absorbs chemical contaminants and removes unpleasant tastes and odors from your water.

Thanks to their compatibility, these filters are a common addition to most reverse osmosis systems. The ability of RO units to remove dissolved solids pairs well with the chemical removal capabilities of activated carbon filters.

Today, it’s harder to find a RO model without an activated carbon filter, but there are still some with just RO membranes. We recommend using a model with the activated carbon filter due to the mentioned benefits.

Micron Size

The micron size determines with what efficacy the RO device removes contaminants from the water.

In the section above, we explained that, typically, reverse osmosis units do not remove bacteria from water, which depends on the membrane’s micron size.

Bacteria and viruses are very small microorganisms that slip through most membranes with no issue. However, RO devices with astonishingly small micron screens are better at stopping these contaminants.

RO membranes usually range from 0.5 to 5 microns. The 5-micron models remove around 90% of sediment particles.

On the other hand, the 0.5-micron screens effectively remove bacteria from the water and 99% of sediment and mineral particles. Certain bacteria are smaller than 0.5 microns but are not that common.

Even though they’re rare, you may also find a good RO model with a micron rating of 0.2 and even 0.1 that can treat these bacteria. We generally recommend opting for the more common and affordable 0.5-micron models, which typically meet the needs of most users, but if you aren’t sure which one you need, the best course of action would be to test your water by sending a sample down to a local water testing lab.

You’d have reason to get a 0.2- or a 0.1-micron filter only if the results come back positive for any bacteria or microorganism smaller than 0.5 microns.

Disadvantages of Small Micron Filters

Reverse osmosis filters are notorious for producing three to four times more wastewater than clean water. The quantity varies depending on the product, but if a reverse osmosis system produces one gallon of clean water for every two gallons of wastewater, consider it effective.

Removing contaminants from water using a semipermeable membrane sounds straightforward. Still, the truth is that these contaminants are usually too infused with water particles to be easily separated. This leads to a large quantity of water getting removed along with the impurities.

Larger, 5-micron filters don’t produce as much wastewater as the 0.5-micron variety, but they allow many more contaminants to pass through, so there’s a tradeoff.

Smaller micron filters also reduce the water pressure in your home. Water passing through a very fine filter at high speeds creates too much pressure and may damage the membrane.

So, for the smaller micron membrane to filter the water effectively without any damage to the pipes or the filtration unit, the flow rate will be slower than with a 5-micron filter.


The gallons per day (GPD) capacity of reverse osmosis units determines how many rooms can be connected to the system without the water pressure suffering.

Small one-floor households with up to 3 people are expected to use around 200 – 400 gallons of water per day. This means that a product like the iSpring RCB3P, filtering 300 GPD, would be powerful enough to keep up.

On the other hand, larger houses will be better off with something like the USWaterSystems Defender since the GPD of this unit starts at 2,000 gallons and goes as high as 8,000 gallons.

Additionally, the water tank is another factor that needs to be considered.

Even a low GPD unit like the iSpring may be used in larger homes if paired with a water tank. This way, the water will be filtered and stored in the tank before it’s needed. People can then draw from the already present water in the tank while the RO system filters more simultaneously.

We suggest buying a model that manages between 500 and 1,000 GPD per day and pairing it with a water tank that holds 200 or 300 gallons. This will give you a good price-per-flow-rate ratio.


Whole-house reverse osmosis filters cost anywhere from $500 to $20,000.

For example, the iSpring we mentioned has a price tag of $669.99. But, if you get all of the add-ons and different features that the Defender comes with, you’ll be saddled with a bill that’s just under $13,000, and that’s far from being the most expensive unit on the market.

Realistically, we would say that you can get a high-quality unit that fulfills your needs for close to 10k or maybe even less.

Take the Defender as an example. It comes with a lot of additional gear, but you don’t need all of it. If you get the 250-gallon tank, the permeate flush option, and the 2k GPD flow rate, you wind up with a system that costs under $10,000.

The system we just described is powerful enough to handle most households’ needs, proving that you don’t need to go overboard when it comes to gear and cost.

However, we don’t recommend getting a cheap system and cutting corners. A higher-quality unit costs a bit more, but it’ll last you longer and be more effective.

Additional Systems

As mentioned, if you’re up against bacteria and chemical contaminants in your water, you’ll need to use a small micron filter with an activated carbon block.

What we neglected to mention was the issue of hard water. Hard water contains large amounts of calcium, magnesium, and other minerals. Reverse osmosis membranes are good at removing these types of dissolved sediments, so there shouldn’t be an issue if the water isn’t too rich in minerals.

However, if the water in your area is particularly hard, then problems may arise. The minerals will clog the device or cause much more wastewater due to the membrane being unable to remove the minerals effectively. If you have an issue with hard water, pair your RO system with a water softener.

Water softeners are purpose-built to soften hard water using an ion exchange process. They neutralize the hard minerals by replacing them with sodium ions.

Using a water softener protects the RO system from clogging or damage from the minerals, so we highly recommend doing this if it’s within your budget.

Lets Recap

We know the info we just shared is a lot to process, but we had to give you some general info on how RO systems work and the factors you need to consider when buying a unit.

Let’s quickly recap the most important information.


Reverse osmosis membranes are great at removing dissolved sediments but struggle with chemical contaminants.

We’d recommend testing your water to determine whether you have any pesticides, herbicides, pharmaceuticals, or other common chemical pollutants. If the tests return positive for such contaminants, then you can’t solely rely on the RO membrane.

We recommend getting a filter with both a RO membrane and an active carbon filter if you need  to neutralize sediment particles and chemical contaminants.

Micron Size

If you have a problem with bacteria, then you need a filter with a 1-micron or even a 0.5-micron membrane.

However, if your water supply doesn’t have virus and bacteria content, then a 2-micron or 5-micron filter will give you a much better flow rate and water pressure.


Small houses with only one floor can make do with a system that produces 300 or 400 GPD, but larger houses might need a device that filters 1000 GPD or more.

The best option, in our opinion, is to pair the system with a water tank if it doesn’t already have one. In that case, you won’t have to worry about the GPD because the water doesn’t go directly to your pipes.

Using the tank, you can get a 400 GPD RO unit and always have access to clean water. The system will fill the tank back up gradually over the day.


Choose a RO device designed to handle the issues you’re facing. It’s okay to look for the best deal, but only from the filters suitable for your household. If the RO device is substandard, you’ll spend your money without solving your problem.

Additional Filters

And finally, while RO devices remove minerals from the water, a water softener is an excellent additional water filtration system that you should consider if you have exceptionally mineral-rich water in your area.

A water softener will remove hard minerals from your water effortlessly, allowing the RO filter to work more effectively and minimizing the risk of clogging.

If you’re ready to start browsing units, check out our guide on the best RO systems.

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Scott Winfield
Scott Winfield
My name is Scott Winfield and researching and writing about water filters and other strategies to purify water has become my full time passion in recent years. I'm glad that you found our site and you can look forward to authoritative and well researched content here to help you get the best in water.
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