Whether you are building, operating or just buying a water filter, filtration is an essential part of most water treatment processes. Filtration is used in the counter top/faucet filter all the way up to municipal water treatment facilities serving tens of millions of people. Even in emergency and survival situations filtering with a shirt or other cloth is often the first thing recommended for water treatment. A shirt isn’t adequate on its own by a long shot, but it is better than un-filtered/untreated water. Used in conjunction with other water treatment steps, filtration makes the rest of the disinfection process significantly easier and cheaper.
We filter water primarily as part of the disinfection of water. Disinfection is the inactivation and removal of pathogenic organisms. Filtration is part of the removal portion of disinfection. (The other part being settling or clarification).
Filters work to physically remove contaminants from the water. They do this by passing the water through a filter media. The media presents a barrier to solids in the water. They literally collide into each other and become trapped. The media can be made of almost anything. As long as it has the ability to let water through and preventing solids from passing through. The pore size (size of space between the media) dictates the performance of a filter. The smaller the pore size the more that gets removed from the water. That sounds like smaller is better, but small pore sizes reduce the rate you can filter water and the total volume of water you can filter. It is always a trade off between ability to remove contaminants from the water and the ability to filter larger volumes of water.
Below is a diagram of pore sizes and which contaminants can be removed at a given pore size. Filters with smaller pore sizes tend to be more expensive. They require precise manufacturing techniques and maintenance.
This trade off lead to the development of chemically assisted filtration. The filter media and the water itself is treated with chemicals. The water is treated with chemicals to make the particles in the water larger called floc (large groups of particles stuck together). At the same time the filter media is treated with a chemical to make the media attract and trap the particles in the water. Most chemically assisted media, has a electro-static charge opposite the one in the floc. Typically the media has a positive charge and the floc has a negative charge. Now instead of waiting for the particles to collide with the filter media, the floc is attracted onto the surface of the media (adsorbtion) and into the filter media itself (absorbtion). These types of filter media are said to be activated. Below is a diagram outlining the difference between straight filtration and chemically assisted filtration.
The two most common filter types are charcoal filters and activated carbon filters. They are essentially they same thing, but activated carbon is chemically assisted and charcoal is not. When the chemical assistance in activated carbon filters is used up it becomes inactivated and has similar performance to a traditional charcoal filter. Each contaminant in water needs a different activation chemical on the filter media for its best removal from the water. The activation chemicals used to remove iron and manganese are not the same for removing organic material. This is why knowing what is in your water, is always so important.
Every filter gets plugged eventually, there is a limit to the amount of solids any filter can handle. Now this process isn’t completely bad. As the filter get plugged the pore size decreases (gets filled up). This makes the filter performance increase with use. In fact for most filters the first bit of filtered water is the worst filtering job that specific filter will do. If you can afford to, monetarily and water supply wise, I recommend throwing away first batch from any new/cleaned filter media or cartridge. Allow for the filter to ripen, let it reach better filtering levels before you use the water. It makes the filter effluent safer to drink. That is until the filter stops allowing water through or the activation of the media is used up.
At this point the filter is not only useless it become dangerous. This is from a few things, filter breakthrough and sloughing. Sloughing is when trapped particles break away and rejoin the water. Remember you have just concentrated the contamination into one place. With the example of the PurifiCup (see my review here), it can filter approximately 150 cups, on the 151st cup you are exposing the water to contamination 150 times greater then is already in the water. Filter breakthrough is exactly like it sounds, the water pressure literally breaks/makes a hole in the filter media. This effectively bypasses any/all filtration and can significantly increase sloughing.
At the end of a filter run, there are only two things possible. A washing of the filter media or a total replacement of the filter media. After washing the filter media, some media can be reactivated.
Here your options are limited by the media type. Some filter media cannot be washed easily and others cannot be reactivated. Replacement of filter media is always an option, but it is the most expensive at all scales of water filtration.
Filters are washed by pushing water backward through the media and scouring off with water and/or air all the trapped particles. This is called a backwash. Backwashing regularily keeps the media performing at its peak, and if done properly can extend filter life indefinitely. Almost every large water filter has the ability to backwash. It saves significant amounts of money. After a backwash chemical re-activation can occur if the media allows for it and you have to chemicals and know how.
Filtering water seems easy at first glance. As you read above there can be a lot more to it than just running water through sand. When done properly filtration can remove almost all of the suspended solids in a water source. You have nothing to lose and everything to gain with proper filtration of water you plan on drinking.