iSpring CU-A4 Under-Sink Water Purifier Review

Designed for well water, this wallet-friendly pick reduces contaminants like lead, chlorine and heavy metals without stripping minerals. Its pre-filter and carbon block filters reduce other impurities like rust, dirt and silt, while a reverse osmosis membrane cuts out toxins and bacteria.


It also scores highly for its quick install and low price. However, it lags behind others in reducing emerging contaminants like pharmaceuticals and herbicides.

Reverse osmosis

Unlike regular water filters that physically screen out sediment particles, reverse osmosis systems use a membrane to filter down to the molecular level. This removes dissolved chemicals and salts that cannot be removed by a simple physical screening process. The resulting water is usually more pure than bottled water, with less sediment and chlorine.

A reverse osmosis system works by passing water through two or more pressure vessels that contain RO membranes. These separate the incoming water into clean and dirty water streams. The concentrate or reject from each stage is then pumped to the next stage. The last step in the filtration process is to add a carbon post filter to remove any remaining chemicals or impurities.

While there are many models of reverse osmosis systems, the APEC ROES-50 is one of the most popular and affordable options on the market. This system is NSF-certified and offers out-of-sight filtration with an on-demand filtered faucet dispenser. It uses four filters and can reduce a wide range of contaminants, including 93 percent of fluoride, 96 percent of iron, and 100 percent of lead.

The system can be easily installed in about an hour by someone with basic DIY skills. It has a large storage tank and comes with a faucet with an LCD display that shows TDS readings and a water status indicator. It also features an automatic shut off valve (ASOV) that relies on water pressure to indicate when the tank is full.

Carbon block

Carbon block water filters work by adsorption, which keeps particles like chlorine, mercury, dioxin, herbicides, solvents and pesticides stuck to the carbon’s surface rather than being pushed into it. This makes them more effective at removing contaminants like hydrogen sulfide (rotten egg smell) and improving taste, color and odor than other types of filters.

Unlike GAC filters that use loose carbon granules, which restrict water flow and reduce the ability to absorb contaminant matter, carbon block filters have a much more compact structure of finely powdered carbon granules. They also feature a binding agent that helps the granules stay in contact with each other and prevent channeling, which can diminish performance.

The pore structure of the carbon blocks is also more uniform than that of GAC, which means it has a higher capacity for contaminant reduction. This is especially true for volatile organic compounds, including pest repellents, herbicides, THMs, phenols, antibiotics and over-the-counter medications like ibuprofen or naproxen.

Carbon block filters are often used as a pre-filter or polishing filter in a reverse osmosis system because of their high adsorption capabilities and their ability to remove a wide range of contaminants. They’re also easy to maintain, as they don’t require any electricity or water pressure to operate. In fact, they’re more durable than many other types of water filters and can withstand the wear and tear that comes from long-term usage.


The iSpring CU-A4 is an easy-to-install under sink water filter that works well for most home environments. It features a 0.1-micron ultra filtration stage to remove chlorine treatment by-products, arsenic, lead and other impurities. The granular activated carbon and KDF filters prevent pollutants from accumulating in the system, while the CT filter improves water taste. Its simple push-to-connect fittings make it quick and easy to install and replace.

The Brondell Coral is another easy-to-install under sink water filter. Its granular and carbon block filters can reduce more than 50 contaminants from your drinking water. Its three-stage filtration process begins with a sediment filter to capture larger particles in your water. The next stage uses a pre-carbon filter to eliminate odor and bad taste, and the final stage reduces chlorine and other harmful contaminants. This under sink water filter also provides a convenient flow rate of 0.75 GPM.

The ClearlyFiltered under sink water filter is a more expensive option that includes two carbon filters and a reverse osmosis membrane. Its granular and carbon blocks reduce a wide variety of contaminants, including fluoride, chloramines, VOCs, and heavy metals. Its patented toxin-removal technology is ideal for families who want to protect their health and improve the quality of their tap water. Its high cost and maintenance costs, however, make it a less-than-ideal choice for budget-minded buyers.


Unlike single-stage water filters, multi-stage systems can filter larger quantities of water. They also reduce a wider range of contaminants. However, they can be more expensive than single-stage systems. Moreover, they may require more space under the sink. They also tend to have longer filter life.

A multi-stage under-sink system uses a series of pre-filters, reverse osmosis, and two final filters. It removes chlorine taste and odor and improves water flavor, which can help you enjoy healthier, better-tasting tap water. In addition, it can help protect you from illnesses caused by bacteria and other contaminants.

The first stage of this system uses a 5-micron pre-filter to remove sediment and dirt from the water. The second stage uses a submicron filter to remove bacteria and chemicals from the water. The third stage filters the water for a softer taste and odor. This water purification system is easy to install and requires minimal maintenance.

This under-sink system features a digital display that lets you know real-time total dissolved solids (TDS) levels at a glance. Its stainless steel dispenser faucet is a convenient feature that makes it easy to use and refill. Its final stage purifier chamber 78 includes a cartridge 92 that can house liquid water taste enhancers, medications or vitamins. It can even be used for storing water-based cleaning solutions or disinfectants.