Rinsing Water Filter Cartridges: Getting the Air Out

by Gene Franks

When water filter owners ask how long they should rinse the filter cartridges in new water treatment units, or in units they are replacing the the filter cartridges in, they usually expect a simple answer and that’s what they usually get.  Actually, though, the answer can be quite complicated and in most cases there is not a pat answer.

We’re concentrating on carbon filters here, which are by far the most common in residential treatment units, but the same principles apply in varying degrees to other media, like calcite, ion exchange resins, or activated alumina.

Fines

If you have a carbon filter, you’ve probably noticed that a big blast of black stuff comes out of the faucet when you start up a new cartridge. This is called “carbon fines.” It’s just manufacturing left-overs,  small pieces of carbon that have to be washed out.  Some filters, you may also have noticed, put out virtually no fines. This is because the manufacturer has gone to the trouble to clean the carbon up well (often washing it with acid) to keep the “fines” from going into your RO membrane or your refrigerator.

The assumption is that when the fines have subsided and the water is running more or less clear, the cartridge is ready to use. Not so.  There are other considerations.  For example, brand new carbon filters can contain residuals of contaminants (like arsenic), and even NSF Standard 42 cartridges, which are certified to be safe, sometimes are labelled with the admonition to “place the cartridge in an appropriate housing and rinse for a minimum of 20 minutes before use.” Anyone who has tested a reverse osmosis unit after a cartridge change knows that you do not get a valid TDS (total dissolved solids) reading of membrane performance after the cartridge change.  This is because the new carbon postfilter, for up to a week after the cartridge change, is putting out “solids” that the meter can see but the human eye cannot. To be clear, the same “TDS throw” occurs in all new carbon filters, not just RO postfilters; it’s just that only RO units are routinely tested for TDS performance.

AirWhat air inside a carbon block cartridge looks like

A fact that water treatment professionals are aware of but that water filter users seldom consider is that new carbon filters are mostly air. What makes carbon such an amazingly effective filter medium, in fact, is not only what is there but what is not there. It’s the countless tiny air-filled pores inside the carbon particles that provide enormous amounts of surface area for chemical contaminants to cling to that make carbon so effective.   The so-called “40-40-20” rule has it that most carbon filters are 40% air-filled space between carbon particles, 40% air-filled inner pores, and 20% actual solid carbon. In fact, depending on the type of product and the manufacturing method, most carbon filters are said to be 70% to 90% air.

When a new cartridge is put into service, it can take days for the air to work out completely. That’s why users sometimes experience cloudy water (if air is causing the cloudiness, the water in a glass will clear from bottom to top) and why there sometimes appears be a scummy substance at the top of a glass of water from a new filter. The scum is air trapped under the “skin” at the top of the water column.  Both the cloudy color and the scum will go away with time, and it’s nothing to worry about.

Diminished Performance Because of Air

While fines and trapped air are aesthetic problems with filter startup that goes away fairly quickly, there is actually diminished performance from a new filter cartridge or carbon bed in a large filter caused by trapped air that lasts longer.   I sometimes tell customers with new products that the water will taste and look better after a week or so,  when the new filters have had a chance to “mellow in.”  Mellowing in is a low tech way of saying that everything will work better when water has had a chance to push the air out of the millions of tiny crevices within the carbon, thus allowing the water to come into intimate contact with the carbon itself.

Large industrial filters have to be soaked for long periods after rebedding to drive the air out the carbon.  Hot water, which speeds the process up, is also used.  Henry Norwicki et al. in a recent Water Conditioning and Purification article actually recommend a 72 hour soak for small filter cartridges:

There are two ways to replace the nano-spaced concentrated air: 72 hours submerged soaking in tap water or using hot water to remove trapped air. Water forms larger conglomerates by hydrogen bonding of water molecules. Conglomerates of hot water are smaller and can better penetrate adsorption spaces than larger, cold-water conglomerates. Replacing filter soaking water with fresh water and turning the filter vertically upside down is also beneficial. Draining helps remove air bubbles. When air in nanospaces is replaced by water, bubbles go into bulk volume between media particles. Simple draining removes these bulk water bubbles. Water inside particles, however, is not removed by draining. Soaking for 72 clock hours is necessary and extra time is acceptable.

We’re a long way from recommending that customers soak filter cartridges for 72 hours before using them, but it helps to know what’s going on inside the filter and be a bit forgiving if water is cloudy and doesn’t taste as good as you would expect with brand new filter cartridges.

Enlargement of granular carbon shows countless pores that adsorb contaminants. The surface area of the pores is exceptional. A single pound of activated carbon has more surface area in its pores than 100 football fields. When the carbon is new, these pores are filled with air that must eventually work its way out.