WASH 101: Physical Interventions – Sedimentation and Adsorption

September 10, 2019

Posted by Chip, Managing Director

At this point in this series, you may be questioning the rigidity of these solution categories. As we briefly mentioned in our post on antimicrobial metals, there are a few interventions that can make just as much sense in one category as another. This is particularly noticeable in this month’s post, because, we’re about to focus on physical interventions including sedimentation and adsorption. We have already talked a little bit about both sedimentation and adsorption in previous posts. [Sedimentation overlaps with our discussion of flocculation; and we already touched on adsorption when we discussed activated carbon.]

Which leads us to our standard reminder: 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.

Now, we’re 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 adsorption, boiling 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).

Sedimentation

Gravity is not just a reality we deal with; it can also be a tool we harness. We know gravity pulls particles toward the earth. And we know, especially in liquids and gasses, that lighter particles will move to allow heavier particles to travel towards the earth. [Think about Newton’s apple. As the apple falls from the tree, the air moves aside to allow gravity to pull it closer and closer to the earth].

What does gravity have to do with clean water?

Because of gravity, solids within a liquid will naturally fall out of suspension in a process called sedimentation. Sedimentation is often the first step in lowering turbidity, but why is that important?

Turbidity refers to the loss of transparency in water because of suspended particulates. Water that is cloudy, discolored, or very opaque is said to have high turbidity.

Drinking water filled with floaties is gross. But beyond the yuck factor, turbidity affects how successful other interventions can be to make dirty water clean.

Previously, we discussed how high turbidity can make UV radiation or chemical additives like chlorine less effective. And when we get to next month’s discussion of filtration, we’ll go into more detail about the difficulties that turbidity can cause.

To mitigate these complications, many water purification strategies will start by allowing the process of sedimentation to occur, lowering the turbidity of the water that will be treated. The methodology is pretty simple. Moving water helps keeps particulates in suspension. So, if you stop the water from moving (like by gathering some in a bucket). And then wait, a good quantity of the sediment will sink to the bottom naturally and the clarity of the water will improve. By carefully pouring or scooping out the clearer water from the top, you can leave most of the sediment behind.

As we discuss in our flocculation article, once all the large particulates have settled out, it’s much easier to treat the remaining water with an additional intervention.

The benefits of sedimentation are pretty clear.

First, it can happen in nature without any human help. Lakes, ponds, and other “still” water sources will naturally allow sediment to fall out of the water over time, so by carefully extracting water from near the surface of these water sources, humans can capture fairly clear water to begin with. When that isn’t possible, sedimentation only requires a water-tight container and time. This combination makes sedimentation a cheap, easy first intervention to make dirty water clean.

The downsides of sedimentation keep water not so clear.

The amount of time required for sedimentation to be effective varies based upon the particulates. So, the process can take anywhere from hours to weeks. In addition, when water is scarce, you might not have the luxury of allowing water to sit for extended periods of time without using it. And, no matter how long you wait, some particulates will never settle out on their own. Finally, sedimentation is not a stand-alone intervention. It can be a great first step, but since it does not remove or destroy microorganisms living in the water and hardly ever lowers chemical concentrations.

In the end, sedimentation is only ever the first step.

Sedimentation reading list for those with burning questions…

 

Adsorption

First, let’s clear up any confusion. There is a difference between adsorption and absorption. Now, absorption is when something permeates past the surface. So, those absorbent paper towels soak up messes by bringing the liquids into the dry hollow spaces formed by the fibers of the towel. On the other hand, adsorption is when something sticks to the surface of a material. Usually, this is because of the chemical composition of the material.

When we’re talking about solutions to make dirty water clean, we’re talking about adsorption.

Similar to surface tension, adsorption is a consequence of surface energy, which is really fascinating.  Consider this: in a bulk of material, the internal atoms of that material are surrounded by other atoms, so all of their bonds are utilized. But the atoms on the edges have surfaces that are available for bonding. This creates more energy at the surface than is found internally. If you don’t think that’s cool, check to make sure you have a pulse.

These open bonds on the surface are what create the tension that allows you to float a paperclip on top of a glass of water, or build up a dome of water drops on top of a penny. [If you’re curious, you can find more information about surface tension and water here.]

Open bonds are unsatisfied, always looking to bond with other molecules. This is what we take advantage of when we’re using an adsorbent material. The free surface bonds grab nearby particulates in the water, thus satisfying their open bond. For the purpose of water purification, adsorptive materials are used to capture microbes, particulates, metals and chemicals. This is done by passing water across the surface of the adsorptive material.

There are a variety of materials that have an adsorption effect.

We touched on adsorption when we discussed activated carbon because carbon molecules are wonderfully adsorbent for lots of the stuff we don’t want in our water. Adsorption is also a principle utilized in flocculation where particulates are clumped together in order to help them settle out as sediment. Finally, sand filter systems utilize the adsorptive properties of silica to capture particulates and microbes.

The benefits of adsorption include:

Adsorption is reliable and predictable, we usually know the capacity and life span of the adsorptive material we’re using. In addition, most adsorptive materials are readily available and inexpensive to purchase. Also, adsorption works well with others. Adsorptive materials are used as one step in a longer process with lots of success. Finally, there are additional benefits to each of the different adsorptive materials. So, finding the right one for your purpose just takes a little research. You can follow the links provided or read more of our WASH 101 Series as a start to your research.

But there are always downsides…

The downsides of adsorption are pretty simple. First, adsorption alone is not usually enough to remove everything from your water. It’s just one step [but an effective step]. And secondly, ratios matter. Every adsorptive material has a saturation point (when all the free surface bonds are filled up). We need to know where that is in order to design effective solutions.

Adsorption reading list for the intensely curious…

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