WASH 101: Chemical Alteration – Flocculation

April 1, 2019

Posted by Chip, Managing Director

Thanks for joining us again as we continue our conversation about potential solutions for the global water crisis. A quick reminder – if you’ve missed any part of our WASH 101 series – please go back and catch up with the conversation. We’ve talked about each part of the WASH acronym [Water, Sanitation, Hygiene], made sure we have a shared language, and looked into disparities for vulnerable populations.

As a quick reminder, here are the categories we’re working from:

  • Physical Intervention: including filtration, adsorption, sedimentation, boiling and distillation
  • Biological Intervention: including slow-sand, activated carbon, antimicrobial metals
  • Chemical Alteration: including chlorination, flocculation and iodination
  • Electromagnetic Radiation: including UV Light treatment
  • Sourcing: including rainfall, groundwater, underground aquifers, springs, and human-intervened (bottled, wells or municipal water).

This month, we’re continuing to look at Chemical Alteration. In today’s post, Chip will focus on using Flocculation to make dirty water clean.

First, a reminder that the primary challenge with using chemicals to make dirty water clean is that we don’t want to replace bad stuff [bacteria, parasites, etc.] with something else that is bad for human health. For example, dump a bunch of bleach into dirty water and you will kill off all the living organisms. But, if the concentration of bleach is too strong, that water is still going to make people sick. As my mother swears she said first, “Too much of anything is a bad thing.”

First, what problem do flocculants solve?

In Rwanda, the groundwater is usually a rusty-brown color because of the red (iron-rich) clay so prevalent in the environment. The fine clay particles are so small they remain suspended (floating throughout the water) in moving water and can take days to settle out of still water.

Not only does this clay affect the taste and texture of the water, but it also protects biological contagions from UV radiation, lowers the effectiveness of chemical treatments, and clogs filters.

The particulate problem is not always clay. It could be algae, mud, or any material that is light enough to stay suspended for long periods of time.

There are a few ways to deal with this sediment. You could just stop the water from moving (say, putting it in a bucket) and wait. We call this process sedimentation (or precipitation). But, depending on the weight of the particles you’re dealing with, this could take days, even weeks.

And, there are particles so light that they will never fully settle out on their own. If the particulates are close enough to the weight of a water molecule, or if the electrical charges of the material and the water repel each other… (this is complicated and involves knowledge of electromagnetic chemistry. So, let’s just take Science’s word for it)… they’ll never settle out on their own. Thus, a common early-stage solution in the purification process is to use coagulation and flocculation to get particulates out of the water.

So, what is flocculation?

Coagulation, in simple terms, just means clumping. Flocculation is the settlement of those clumps to the bottom, leaving clear water at the top. When we talk about flocculation, we’re usually referring to both processes since you can’t practically settle particulates out without making them heavier first.

There are a handful of chemicals that have flocculant properties, including iron and aluminum sulfates [alum]. In addition, salts of metals such as titanium and zirconium have been found to be highly effective coagulants. The most commonly used alum for drinking water is potassium alum, which was used in ancient Rome as far back as 2000 bce.

During flocculation, gentle mixing accelerates the rate of particle collision and encourages the creation of larger particulate clumps. The effectiveness of flocculation is affected by the concentration of the alum, mixing speeds, intensity, and time.


Flocculation can be an effective pre-treatment for water that has high levels of sediment. Using a flocculant as a pre-treatment in a filtration process can keep lots of particulates from ever reaching the filter, which causes less clogging and lowers the rate at which backwashing is needed. Flocculation also makes chlorine and UV radiation treatments more effective.


Flocculation is not a stand-alone treatment option.  Flocculation will not remove or destroy bacteria or viruses. So, while flocculation makes other forms of purification more effective, it’s only a first step.

Alum also shares many of the same downsides as chlorine, such as availability and variable exposure time. The amount of alum required to maximize flocculation is based on the original water’s turbidity, and additional measurements are needed to optimize mixing and settling times.

Most applications in the developing world recommend allowing flocculated water to sit overnight to maximize settlement. This requires families to have extra storage containers and somehow get a day ahead on water collection.

Finally, removing the flocculated clumps from the water without re-mixing the solution can also be challenging. Some systems use a strainer or filter membrane to capture clumps, some systems rely on the settled clumps to simply remain at the bottom of the container until the system is empty and the clumps can be rinsed out.

Let’s wrap up our conversation on Chemical Alteration with this truth… chemistry is awesome but chemistry can be tricky. While solutions using chemicals to purify water can be highly effective, in everyday life, there are often too many unknown variables to make these types of solutions user-friendly or fail-proof. But one thing I truly love about scientists – they never stop asking questions and so I know I’ll stay tuned, there is always more to come. 

Curiosity links:

We're making dirty water clean. But we can't do it alone.

Sign up for our email newsletter to learn more about how you can help.

Stay In Touch