Friday, April 4, 2008
Clean drinking water - the goal of decontamination 
In my last post of the 101 series, we discussed how water contamination happens. In this post we'll talk about how to decontaminate water.
Decontamination really involves two main steps:
1. Figuring out what the contaminants are, and their levels of concentration:
As we talked about before, most contaminants cannot be seen by the human eye. You need to do a series of lab tests on the water to determine the types of contaminants and their concentrations. The concentration of the various contaminants is particularly important because you need to know how dangerous the water really is, and how powerful a cleaning process you need. The United States Environmental Protection Agency (US EPA), the European Union (EU), and the World Health Organization (WHO) (each country has their own such organization as well) all outline safe levels of contaminants for the human body. The thing is, many contaminants are naturally occurring in water, and they are not harmful upto a certain levels. It is beyond that level that the contaminant is dangerous for human consumption, and that's the point that a cleaning process needs to be used.
Water samples need to be lab tested to determine contaminants 
2. Using a series of cleaning steps or processes to remove the contaminants:
Once you have determined the nature of the contaminants and their concentrations, you need to find the right processes for each contaminant.
Decontamination generally involves the following overarching processes or a combination of processes (this is an oversimplified version, we can get into the details later):
A series of cleaning processes can get the water on the left to be clean like on the right 
a. Sedimentation: This is the best process for separating liquid and solids. Essentially you take the dirty water and let it sit for a while. Gravity kicks in, and all the heavier stuff will settle at the bottom (flotation, where lighter stuff floats on the top of the water and is skimmed off, is a subset of sedimentation).
Water sedimentation: notice how the heavier contaminants have settled to the bottom 
b. Decantation: Decantation is the process following sedimentation, where you pour the cleaner liquid (or let the liquid overflow) out, leaving behind the heavier stuff (sediments).
A Cambodian woman decants sedimented water 
c. Filtration: Filtration is the process where the most bad stuff gets taken out. Usually you pass the liquid through some sort of porous membrane (like cloth or other things). The bad stuff gets held back or stuck on one side of the membrane, while the cleaner water flows through to the other side. The efficacy of filtration depends on the quality of the filter, like how small the pores in the membrane are and how well it keeps the bad stuff on the other side.
Schematic diagram of a simple water filter. Water first flows through the sand, then the gravel, and finally a cloth filter. The sand and gravel take out the coarser contaminants; the cloth takes out the fine contaminants. 
d. Distillation: Distillation is the most effective water treatment process, but it is also the most expensive because of how much energy it uses. Essentially, you boil the water, collect the steam and cool it down. Its effective in removing metals (like arsenic, calcium and magnesium...which is why its good for your iron as it leaves no residue) and killing germs. However the removal of organic chemicals (derived from petroleum or such) have a variable removal rate. Chemicals that have boiling points lower than that of water, for example, will also vaporize with the water and contaminate it even in steamed form. For chemicals like these, advanced water cleaning processes are used.
Schematic diagram of distillation (in a lab). This can be done without the use of lab instruments as well, en masse. 
e. Disinfection: Disinfection is the process of killing germs. Sometimes the water is free of metals and the other grunge, and just has a bunch of germs which need to be killed. Or often, even the cleanest water that is exposed to air quickly gets contaminated by germs in the air or area surrounding the water. Disinfection is generally the last step of water treatment.
A woman disinfects her bucket of drinking water with chlorine. 
: Tim Norris
: Tom Sprague photo