Water 102: Water Distribution

Men, women, and children rush to get water from a well in Ethiopia (photo credit: Andrew Heavens)

Now that we've covered the processes involved in water treatment, the next stage is water distribution.

As stated in the Water 101: What is the Water Problem post, a huge part of the larger "water problem" is the transportation, storage, and distribution of clean water. In developed countries like the U.S, Canada, and most European countries, and urban parts of some developing countries, water is generally treated and distributed the same way. Its generally treated at a treatment plant, then distributed in pipes. The water usually comes disinfected and ready for drinking. People don't need to store the water, because the distribution system keeps the water clean and disinfected until you need it.* Once water comes in contact with contaminated areas or is exposed to the air, it gets contaminated.


Water flows out of a pipe in the UK countryside (photo credit: Martin Anataman)

*This may NOT be true of developing countries. Quite often there are water shortfalls, or the water distribution is so low that even in urban centers, families have their own boreholes that they pump water out of for use. They then regularly boil the water for drinking or order drinking water from wholesale providers.

In most developing countries, particularly in peri-urban, slum, semi-rural, and rural communities, water needs to collected from a central point (either from a tap, tubewell, borehole, river, lake, pond or other open water source), then carried to their homes. This is generally the work of women and children, who walk six miles (9.6 kms) to and from the water collection area. Water is heavy, weighing approximately 1 kg per litre or 8.34 lbs per gallon of water. Often the water collection and storage units are contaminated; if not, the pots or other collection units are open to the air and easily get contaminated. And finally when drinking the water, they dip their hands into the pots to get the water. Essentially, its very easy for water to get contaminated at any point.

In developing countries, people are responsible for the water collection, storage, and decontamination, which is why this is a major problem for them. The risk for illness and health issues resulting from lack of clean water or neck/head/back problems are very high. Therefore, besides availability of water, transportation and decontamination are major issues that need to be addressed when designing technology for developing countries.



L-R: A water truck carrying water into the periurban areas in Kenya (photo credit: Stig Nygaard); A line of colorful pots mark the queue for women and children in line to collect water in India (photo credit: McKay Savage).


Young women carry water back to their homes in Somalia (Copyright: Heather Arney / WaterPartners International)

Water 101: Decontamination, the basics

Clean drinking water - the goal of decontamination [1]

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]


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 [3]

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 [4]

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 [5]

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. [6]


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. [7]


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. [8]


Photo Sources:
[1]: Tim Norris
[2]: Juvertson
[3]: http://muriellascorneronmoringa.googlepages.com/
[4]: http://www.easttennesseewildflowers.com/Teacher_Resource.php
[5]: Tom Sprague photo
[6]: http://www.blithfieldeducationcentre.co.uk/kp/waterpages/make_a_water_filter.htm
[7]: www.chemistrydaily.com
[8]: http://www.usaid.gov/press/speeches/2005/ty050915.html