WASH 101: Physical Interventions – Filtration

October 8, 2019

Posted by Chip, Managing Director

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Ladies and Gentlemen, we have finally made it to the final category of clean water solutions. And yes, we saved our favorite for last: Filtration.

But before we get to that, one last time, all together: You’re joining 20 Liters in an ongoing conversation. We’ve already covered each part of the WASH acronym [WaterSanitationHygiene], made sure we have a shared language, and looked into disparities for vulnerable populations.

We have been working our way through a discussion about various solutions to make dirty water clean. We’ve divided the solutions into a few categories:

Physical Intervention: including filtration, sedimentation and adsorptionboiling and distillation
Biological Intervention: including antimicrobial metalsactivated carbon bio-sand filters
Chemical Alteration: including chlorination & flocculation
Electromagnetic Radiation: including UV Light treatment
Sourcing: including rainfall, groundwater, underground aquifers, springs, and human-intervened (bottled, wells or municipal water).

A quick warning about impartiality: 20 Liters is NOT impartial when it comes to filtration. 20 Liters is strongly committed to filtration as a solution to make dirty water clean. We champion point-of-use filtration as the under-utilized, often overlooked solution that most communities actually need.

So, let’s start with a couple of definitions, so that we can talk implementation.


The word filtration covers a lot of territory. In the most general terms, filtration is using a medium (some physical thing) to separate particulates from a liquid. Any material that creates or can be used to create a semi-permeable barrier can be a filter medium.

We can get a bit more technical by distinguishing between a surface filter and a depth filter.

A surface filter uses a porous material to act like a sieve, the most common example here is a coffee filter that keeps coffee grounds trapped, while the coffee passes through. Common materials for surface filters [in the context of clean water] would include hollow fiber membrane filters, reverse-osmosis filters, and pleated filters.

A depth filter uses a bed of granular material to create channels of a certain size that block particulates that are too large. Think of the bio-sand filter we discussed a while back. Common materials for depth filters [again, in the context of clean water] would include ceramic, sand, diametric earth, and activated carbon, among others.

With that general understand of filtration; let’s zoom in. What is the argument for using water filters to save the world?

First, we have to agree that even under-developed, rural or nomadic populations are getting water from somewhere. Granted, the water source could be far away, insufficient in volume, unsafe to consume, create hardships and disparities, and all sorts of other bad stuff. But, at the macro level, the population wouldn’t be able to stick around if there were no access at all.

Consider this axiom: where there is no water, there is no life. The inverse is also true. So, if people are living there, there must be water.

Once we can agree that there is some kind of water source somewhere, we can move to the next logical step. There is water, but that water is likely unsafe for human usage. So, let’s clean it up.

Simple enough. We’ve already covered a wide variety of solutions to make dirty water clean. But if you go back and look at the negatives of each of the other solutions, you’ll see a pattern. We’ve often been left with questions about reliability. For example, did the water get enough exposure to enough UV light or antimicrobial metal? Is the bio-layer alive? Did this intervention actually work to remove everything dangerous? Answering these questions usually involves a petri dish or microscope.

We want to be sure that we’re making dirty water clean, without the need for ongoing complex testing. We want to know that if the water is flowing, it’s clean.

Which brings us to filtration, which operates on a much simpler principle than many other solutions: if it’s too big, it didn’t get through.

Let’s use the example of a 20 Liters SAM3 Household Filter. We utilize 0.1-micron hollow-fiber membrane filter in our SAM3 Filter. That’s a fancy way of saying that the filter that we use is filled with looped coils of hollow fibers [straws] that have 0.1-micron openings. Only things that are smaller than 0.1 micron are going to be able to get from one side of the filter to the other.

So, how big are the contaminants in the water that we want to remove to protect human health? Bacteria range from 0.2 to 3 microns in size. Worm cysts are typically 2 to 50 microns in size. Very simply, they are too big to pass through the filter.

Now, depending on the filter you are using, not every potential contaminant will be too big to get through. Going back to the example of the SAM3, viruses are usually between 0.004 and 0.1-micron in size. So, they can get through the filter. But there is often fairly high local immunity to water-borne viruses – and it comes down to the difference between having a common cold versus typhoid or cholera.

So, you might be thinking, use a smaller filter! Hollow-fiber membrane cartridges like the one we use comes in sizes as small as 0.003-micron. But, the trade-off is flow-rate. The smaller the filter membrane size, the slower the flow, especially in gravity-fed systems like ours. If you’re hooking a membrane filter up to your pressurized household plumbing, you can utilize that 22psi of pressure to force water through. We want to avoid pumps that need electricity or filters that have moving parts, so we found a sweet spot that balances the flow rate with the removal size.

Reliability is the primary reason 20 Liters loves filters. But it isn’t the only reason.

Filters are scalable. You can build a filter system to serve a single person or to serve an entire city. Looking at the 20 Liters model, we have both household systems that serve approximately 5 people each day and community systems that serve up to 400 people each day.

Filters are long-lasting. Or at least, they can be. For instance, the filters 20 Liters uses are rated for 10 years of use at 22psi [garden hose pressure]. But, our filters are used in gravity-fed systems and are never put under that level of pressure. And so, we expect that they can work for longer than 10 years.

Filters are not without challenges.

Most importantly, filters require maintenance. At the most basic level, filters block particulates that build up on the “dirty” side of the filter. This build up will clog the filter. When this happens, the system needs to be back-flushed. Those using the filters need to be trained and have necessary equipment to periodically service their filters.

In addition, depending on the design of the filter system, there can be risks of contaminating the “clean” side of the filter or re-contaminating water after filtering it. Properly designed filters should minimize these risks. But again, training is the best way to ensure that filters work the way they are intended to work.

These two challenges are the reason that 20 Liters asks filter recipients to contribute sweat-equity before receiving a filter by dedicating nearly 10 hours to training on how to maintain their system and how to avoid re-contaminating water after filtration.

The final challenge with filtration is logistical. Most filters and filter materials are not easily accessible in rural, developing communities. So, it takes a whole lot of planning and energy to develop and support supply chains, to get filter technologies to where they are needed.

To meet that challenge, 20 Liters relies on volunteers both in the U.S. and in Rwanda to make components of the filters. In the U.S., volunteers assemble component parts, that are shipped to Rwanda. In Rwanda, volunteers take the component parts and assemble the final filters that are provided to households, health clinics, and schools.

For those of you who want to know more about filtration, you can follow the links below.  Or send an email to info@20liters.org with your questions!


Filtration reading list (for nerds):

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